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THEES

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300i

 

LIBRARY
Michigan State
University

 

 

 

This is to certify that the

thesis entitled
Movement, Growth, and Density of Stagnicola
emarginata (Lymnaeidae) in Higgins Lake, Michigan in
Relation to Limnological Variables: Implications for
Control of Cercarial Dermatitis

presented by

James Barrett Saxton

has been accepted towards fulfillment
of the requirements for

Master of Science . Fisheries & Wildlife
degree 1n

 

Major professor

Date {ﬁt-Ni 1?] (306/

0-7639 MS U is an Afﬁrmative Action/Equal Opportunity Institution

 

 

 

PLACE IN RETURN BOX to remove this checkout from your record.
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DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6/01 c:/C|RC/DateDue.p65-p.15

 

MOVEMENT, GROWTH, AND DENSITY OF S TAGNICOLA Eli/IARGINA TA
(LYMNAEIDAE) IN HIGGINS LAKE, MICHIGAN IN RELATION TO
LWOLOGICAL VARIABLES: IMPLICATIONS FOR CONTROL OF CERCARIAL

DERMATITIS

By

James Barrett Saxton

AN ABSTRACT OF A THESIS
Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of
MASTER OF SCIENCE
Department of Fisheries and Wildlife
2001

Thomas M. Burton, P.h.D.

ABSTRACT

Stagm‘cola emarginata (Sowerby), the blue snail, is one of the most common
intermediate hosts for T richobilharzia stagm'colae (Talbot), a parasite the causes cercarial
dermatitis (swimmer’s itch). Because relatively little was known about the biology of the
blue snail, the objective of this study was to document the prevalence of infected S.
emarginata at four stations over time and to better describe its growth, density, and
movements in Higgins Lake, Michigan, in relation to water depth, water temperature,
dissolved oxygen, and food resources. The percentage of snails shedding II stagnicolae
cercariae increased with snail size and was similar between years. The Columbine Lane
site had signiﬁcantly more snails shedding T. stagm’colae than the other three sites.
Densities of S. emarginata varied spatially and temporally and greatest densities were
associated with relatively shallow water, higher concentrations of benthic algae, and
lower concentrations of benthic organic matter. Snails in water greater than 1 m deep
were signiﬁcantly larger than those in shallower water. No relationship between snail
density and dissolved oxygen concentrations or water temperature were detected. The
blue snail migrated towards deep water in late July. Growth of S. emarginata was greatest
at 20°C in light conditions that simulated the summer environment. The results of this
Study have important implications for the control of swimmer’s itch in Michigan lakes. S.
emarginata in water greater than 1 m deep may be a signiﬁcant source of cercariae and
densities of S. emarginata may increase if nutrient pollution is not controlled. Because
abundance of S. emarginata is known for several water depths from May through August,

maxrmum densities can now be targeted for molluscicide application.

ACKNOWLEDGMENTS

I thank the members of my graduate committee, Dr. Thomas Burton, Dr. Richard
Merritt, Dr. Patrick Muzzall, and Dr. Richard Snider for their constructive advice and
criticism of this manuscript. I especially thank Dr. Burton for invaluable advice and
support throughout my graduate studies.

Kevin Cluley and Mel Asher spent many hours assisting in the ﬁeld with
collection of the organisms and the limnological data for this study. Thanks to Mike
Sergeant for serving as my diving partner for surveys and for his many insights. Special
thanks also to Dr. Don Uzarski and Craig Stricker, for advice and training in limnological
methods and statistical assistance. Thanks also to those warm individuals living on
Higgins Lake, especially Liz Wade and Ned Wickes, who made my stay at the lake
enjoyable and educational. I also want to thank those who inﬂuenced me in past
education, including Dr. Kenneth Krieger and Dr. Kenneth Baker. I gratefully
acknowledge the numerous lake associations (Higgins Lake Property Owners
Association, the Lake Leelanau Association, the Walloon Lake Trust, and the Walloon
Lake Association) and the Michigan Department of Natural Resources for their monetary
contribution to this project. Thanks also to Senator George McManus and Representative
John Gernaat for their assistance in obtaining funds.

Thanks to my parents who have introduced me to the wonders of nature and have
always given me their full support. A ﬁnal thanks to my wife, Amanda, who has tolerated
many hours of my complaints and helped me maintain some sanity throughout the

writing of this thesis. Her support throughout my graduate studies is greatly appreciated.

iii

TABLE OF CONTENTS

List of Tables

List of Figures

Chapter One: Prevalence of T richobilharzia slagnicolae (Talbot)
in Stagnicola emarginata (Sowerby)
1. Introduction
2. Description of Study Site
3. Methods
4. Results

5. Discussion

Chapter Two: Distribution of Stagm'cola emarginata (Sowerby)
in Relation to Limnological Variables
1. Introduction
1.1 Objectives
2. Methods
2.1 Establishment of Sampling Sites

2.2 Limnological Data Collection

iv

Page

vii

14

17
17
21
22
22

23

2.3 Laboratory Analysis
2.4 Statistical Analysis
Results

3.1 Snail Density and Size
3.2 Dissolved Oxygen

3.3 Water Temperature
3.4 Chlorophyll a

3.5 Organic Matter

4. Discussion

Chapter Three: Movement of Stagm'cola emarginata (Sowerby)

1.

Introduction

1.1 Objectives

. Methods

. Results

Discussion

Chapter Four: Growth of Stagm'cola emarginata (Sowerby)

1.

2.

Introduction

Methods

. Results

Discussion

25

27

27

27

34

36

38

4O

43

52

52

53

53

55

61

64

64

65

66

69

Summary and Conclusions

Appendices

Literature Cited

71

74

85

Table 1.

Table 2.

Table 3.

Table 4.

Table 5.

Table 6.

Table 7.

Table 8.

Table 9.

Table 10.

Table 1 1.

Table A-1.

LIST OF TABLES

Prevalence of T. stagnicolae in S. emarginata collected in
northern Michigan. Modiﬁed from Keas and Blankespoor
(1997). Higgins Lake data from Blankespoor (1992) and
Nyman (1998).

Number of S. emarginata shedding T. stagnicolae at four
monitoring stations in 1999.

Number of S. emarginata shedding T. stagnicolae at four
monitoring stations in 2000.

Percentage of S. emarginata shedding T. stagnicolae in six
size classes collected from Higgins Lake in 1999 and 2000.

Number of S. emarginata collected per shell length for
three sampled water depths.

Mean dissolved oxygen concentrations with percent
saturation and standard errors at three water depths.

Mean AFDM organic matter concentrations with standard
errors at three water depths.

Total S. emarginata in each category for trial 1 of the
movement study conducted in Higgins Lake, June 9-23,
2000.

Total S. emarginata in each category for trial 2 of the
movement study conducted in Higgins Lake, July 21 -
August 4, 2000.

Total S. emarginata in four quadrants and predominant
wind direction for two movement trials. Data include all
snail size classes. Predominant wind direction data from
National Oceanic and Atmospheric Administration.

Mean change in S. emarginata size in four aquaria at
winter, spring, and summer temperature and light
conditions.

Snail density data collected over 2000 at 0.0 - 0.5 m water
depth.

vii

10

14

28

34

41

56

57

59

60

75

Table A-2.

Table A-3.

Table A-4.

Table A—5.

Table A-6.

Snail density data collected over 2000 at 0.51 - 1.0 m water
depth.

Snail density in 2000 at >1 m water depth.

Limnological data collected in 2000 at 0.0 - 0.5 m water
depth.

Limnological data collected in 2000 at 0.51 - 1.0 m water
depth.

Limnological data collected in 2000 at > 1 m water depth.

77

79

8O

82

84

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Figure 12.

Figure 13.

Figure 14.

LIST OF FIGURES

Higgins Lake, Michigan showing locations of studies
monitoring prevalence of T. stagnicolae in S. emarginata, snail
movement, and snail densities in relation to limnological
variables.

Prevalence of S. emarginata shedding T. stagm'colae in Higgins
Lake, 1999.

Prevalence of S. emarginata shedding T. stagnicolae in Higgins
Lake, 2000.

Prevalence of S. emarginata shedding T. stagnicolae in 1999
and 2000.

Coring device used to collect sand samples for food availability
analyses.

Frequency distribution of S. emarginata size classes collected
in 2000 at three water depths. Size class 2 = 5.0 - 9.9 mm, 3 =
10.0 — 14.9 mm, and 4 = 15.0 — 19.9 mm. Bars equal standard
errors and * is signiﬁcant at p<0.05.

S. emarginata density versus sampling date at three water
depths. Bars equal standard errors.

S. emarginata density versus sampling date at 0.0 - 0.5 m water
depth for three size classes. Bars equal standard errors.

S. emarginata density versus sampling date at 0.51 - 1.0 m
water depth for three size classes. Bars equal standard errors.

S. emarginata density versus sampling date at > 1 m water
depth for four size classes. Bars equal standard errors.

Dissolved oxygen versus sampling date at three water depths.

S. emarginata density versus dissolved oxygen at three water
depths.

Mean daily water temperature versus sampling date at three
water depths.

S. emarginata density versus mean daily water temperature at
three water depths.

ix

ll

12

13

24

29

30

31

32

33

35

36

37

38

Figure 15.

Figure 16.

Figure 17.

Figure 18.

Figure 19.

Figure 20.

Figure 21.

Figure 22.

Figure 23.

Figure 24.

Figure 25.

Mean benthic chlorophyll a concentration versus sampling date
at three water depths. Bars equal standard errors.

S. emarginata density versus benthic chlorophyll a
concentration at three water depths.

Mean benthic AFDM organic matter concentration versus
sampling date at three water depths.

S. emarginata density versus benthic AFDM organic matter
concentration at three water depths.

Experimental design of S. emarginata movement study
showing quadrant relationship to the shoreline. Center circle
(0.5 m radius) used for release of snails. Movements recorded
when snails moved out of center circle (distance greater than
1.0 m from center plot).

Mean daily wind speed versus number of S. emarginata
changing categories in a twenty-four hour period for the
movement experiment.

Number of S. emarginata in each of four quadrants throughout
trial 1 of the movement study.

Number of S. emarginata in each of four quadrants throughout
trial 2 of the movement study.

Mean difference between initial and ﬁnal measurements in wet
mass of S. emarginata for three treatments. Bars equal standard
errors.

Mean difference between initial and ﬁnal measurements in
shell length of S. emarginata for three treatments. Bars equal
standard errors.

Mean diﬂ‘erence between initial and ﬁnal measurements in
maximum shell width of S. emarginata for three treatments.
Bars equal standard errors.

39

4o

42

43

58

60

61

68

68

69

CHAPTER ONE
PREVALENCE OF TRICHOBHJIARZIA STAGNICOLAE (TALBOT) IN

STA GNICOLA EMARGHVA TA (SOWERBY)

Introduction

The common beach or blue snail (Stagm'cola emarginata (Sowerby)) is a
ﬁ'eshwater pulmonate snail belonging to the family Lyrnnaeidae which is primarily
distributed from Maine west to Minnesota and in the southern Canadian Interior Basin
(Burch 1988). In Michigan, the distribution includes numerous lakes in the northern part
of the lower-peninsula and the southern portion of the upper-peninsula (Wall 1968).
Within these lakes the organism is generally found in exposed littoral regions, often
wave-swept beaches with a low angle of repose (Baker 1928; Harman 1972). Because of
its characteristic distribution and its predisposition to certain types of avian parasitic
infections, S. emarginata also is among the more common intermediate hosts of
T richobilharzia stagnicolae (Talbot), a trematode blood ﬂuke parasite belonging to the
Schistosomidae family that causes cercarial dermatitis, commonly known as swimmer’s
itch (Burton et al. 1999; Cort et al. 1937; Cort et al. 1940; Brackett 1940).

The life cycle of T. stagm‘colae involves one of a large number of aquatic birds or
mammals as the primary host and one of several species of snails as the intermediate
host. Adult ﬂukes live in blood vessels of the digestive tract of ﬁnal bird or mammal
hosts until reproduction is completed. Newly hatched eggs move through the intestinal
wall and are excreted with feces into the surrounding water (Blankespoor 1991). Once in

the water, eggs hatch into a larval stage called a miracidium, which seeks out S.

emarginata or another species of snail intermediate host. Once the miracidium comes
into contact with the snail, it penetrates the skin and seeks out the liver. The parasite then
develops into mother sporocysts, which mature over time to produce daughter sporocysts.
These daughter sporocysts continue to develop and, when conditions are correct, produce
cercariae, which move out of the snail and seek the ﬁnal avian or mammalian best so that
the life cycle can be completed (Blankespoor 1991; Burton et al. 1998). Swimmer's itch
is an inﬂammatory immune response produced as a result of accidental penetration of
cercariae into the skin of a human (Cort et al. 1940). This disease is characterized by pus-
ﬁlled pimples that cause a painful and irritating itch which may last for several days to
over a week (Burton et al. 1998).

A substantial amount of behavioral information is known about cercariae of T.
stagnicolae. Brackett (1940) concluded that these cercariae swim towards light and
during summer in northern Michigan lakes, they emerge from their snail intermediate
hosts at approximately 4:30 AM. He also concluded that emergence from snail hosts may
be triggered by sudden changes in ambient temperature, both in dark and lighted
conditions, and that emergence was inhibited when the snail was malnourished. Brackett
(1940) also found that cercariae of T. stagnicolae vigorously swim towards shadows
when placed in a lighted container, indicating a search response for their ﬁnal avian
hosts.

Data are also available describing the prevalence of T. stagnicolae in their snail
intermediate hosts over both time and space in northern Michigan lakes (Table 1). It is
well documented that the incidence of T. stagm’colae in S. emarginata varies anywhere

from 1.1% to 10.3% from studies conducted in Douglas Lake, Lake Leelanau, and

 

 

 

 

 

Lake Site Year of Collection Prevalence (% Infected)
Douglas Lake Phragmites Flats 1936 4.8
1957 1.1
1994 1.9
Nutting Bay 1935 - 1936 1.9
1994 4.8
North Fishtail Bay 1935 - 1936 10.3
1994 2.3
Higgins Lake 1992 1.5
1998 1.4
North Lake Leelanau 1990 1.2
South Lake Leelanau 1990 2.1

 

 

Table 1. Prevalence of T. stagm’colae in S. emarginata collected in northern Michigan.
Modiﬁed from Keas and Blankespoor (1997). Higgins Lake data from Blankespoor
(1992) and Nyman (1998).

Higgins Lake, Michigan (Cort et al. 1937; Cort et al. 1960; Keas and Blankespoor 1997;
Nyman 1998). Parasitic infections of S. emarginata also vary both by sampling site and
by year (Cort et a1. 1937; Cort et a1. 1940). Broad trends were apparent as they found that
snails were most heavily infected with parasites near the end of July and into early
August, and they attributed this to warmer water temperatures during this portion of the
summer.

Although this literature is valuable for understanding cercarial dermatitis in
Michigan lakes, the data are generally outdated and the literature states that prevalence of
schistosome infections in S. emarginata varies both spatially and temporally. Also, little
work has been completed in Higgins Lake, Michigan. Therefore, the goal of this study
was to document the seasonal prevalence of T. stagnicolae in S. emarginata at several

sites in a northern Michigan lake. Based on the literature, I hypothesized that the

percentage of S. emarginata shedding T. stagm'colae cercariae would vary both among

sites and at any one site from May through August.

Description of Study Site

The study was conducted in Higgins Lake, a 4130 hectare cold-water lake located
in Roscommon County in the northern portion of the lower peninsula of Michigan
(Figure 1). The lake, which is Michigan’s tenth largest inland lake (Jones 1990), was
formed approximately 11,000 years ago as a result of a melting glacial ice block ﬁ'om the
Pleistocene era which was isolated by marginal moraines at the current north and south
ends of the lake (Schultz and Fairchild 1984). As a result of the geological processes that
formed Higgins Lake, most of the lake’s 8940 hectare watershed is glacial outwash plain
while the soils are a mixture of gravel, sand, and clays (Limno-Tech, Inc. 1992).
Averaging 13.7 meters in depth, the deepest part of the lake is nearly 41 meters deep and
it holds 5.64 x 108 cubic meters of water (Schultz and Fairch 1984). The water body
has a hydraulic residence time of 12.4 years (Limno-Tech, Inc. 1992) and is characterized
by a shoal area that encompasses nearly one-third of the entire area of the lake (Jones
1990). Higgins Lake is also primarily groundwater fed as approximately 50% of the
water volume within the lake comes from beneath the surface of the earth (Jones 1990).
Higgins Lake discharges via the Cut River to Houghton Lake and is part of the Muskegon
River drainage basin.

Because of the two state parks located on both the north and south ends of the
lake and the large number of permanent and summer homes located around the lake,

Higgins Lake is one of the most heavily used inland lakes in Michigan (Jones 1990).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   
 

jig“ Ci} 9
Point Detroit

 

    

renewing;

0 Monitoring Site a“: V camshengg

I Snail Movement Site

A Limnological Data Site _____ :3: E A ' ‘ .. (,

”l" sci W12. like

Figure 1. Higgins Lake, Michigan showing locations of studies monitoring prevalence of
T stagnicolae in S. emarginata, snail movement, and snail densities in relation to
limnological variables.

Adding to this popularity is a Michigan Department of Natural Resources public access
site on the west side of the lake, three township parks scattered around the shoreline, and
approximately seventy-ﬁve public roads ends (Limno-Tech, Inc. 1992). The water body
has traditionally been classiﬁed as an oligotrophic lake, but because of the tremendous
recreational demand placed on the lake in the summer months, Higgins Lake could be
classiﬁed as mesotrophic water body, depending on the speciﬁc limnological variable
quantiﬁed (Limno-Tech, Inc. 1992).

Cercarial dermatitis seems to be a concern for many residents and visitors to
Higgins Lake, as well as many other lakes in northern Michigan. Operators of resorts
have reported that at times their beaches had to be closed because of the concern for
contracting the disease and they also feel that this disease limits the appeal of their
facility to visitors (Wall 1968). As a response to this concern, residents on Higgins Lake
operate a hotline where individuals infected with the disease are encouraged to call to
report incidents of swimmer’s itch. This hotline yielded over 400 calls in 1999 and the
numbers have increased since then. Although these calls do not represent a scientiﬁcally
valid measurement of number of persons who have contracted the condition and cercarial
dermatitis can be confused with other diseases, it is reasonable to assume that swimmer’s
itch is a major problem in Higgins Lake for at least a portion of the individuals that

recreate in the lake from June through August.

Methods
Four sites (Saginaw Avenue, Point Detroit, Columbine Lane, and Rosewood

Avenue) were chosen in Higgins Lake for intensive bimonthly monitoring from the

middle of May through the middle of August, 1999 and 2000 (Figure 1). These sites were
selected because previous research indicated that a relatively high percentage of S.
emarginata were infected with T. stagnicolae at these particular locations (Burton et al.
1999; Nyman 1998). Two of these sites were located in the northern sub-basin of the lake
while the other two sampling sites were located in the southern portion of the lake. Each
of these sites was relatively similar, characterized by a predominately sand substrate with
a small number of scattered cobbles. An additional site (Birch Lodge) was also examined
only in 2000 (Figure 1).

At each sampling area, S. emarginata and Physa integra (another snail known to
host the dermatitis producing parasite T richobilharzia physellae) were collected along an
approximate 100 m long stretch for two hours or until at least 400 snails were gathered.
To locate snails, a diving mask or a glass-bottom bucket was utilized and collection was
accomplished by directly picking snails off of the substrate or by using a cooking strainer
mounted to a wooden dowel rod. Once collected, organisms were placed in a bucket and
brought to the laboratory where identiﬁcation of the organism was conﬁrmed according
to Burch (1988). Snails collected from the monitoring sites were then measured with a
metric ruler and sorted into size classes based on shell length. The size classes were as
follows: one (0.0 — 4.9 mm), two (5.0 — 9.9 mm), three (10.0 — 14.9 mm), four (15.0 —
19.9 mm), ﬁve (20.0 — 24.9 mm), and six (25.0 — 29.9 mm).

Once measured, snails collected from the four monitoring stations were isolated in
1 oz. plastic cups and exposed to white ﬂuorescent light for at least two hours to
stimulate emergence of cercariae (Blankespoor and Reimink 1998). Each of the plastic

cups was then observed under a dissecting microscope to determine the presence of

cercariae. Cercariae were identiﬁed under 10-40X magniﬁcation using a compound
microscope and procedures developed by Schell (1970). Snails collected in 2000 from the
Birch Lodge site were extracted from their shells and examined for T. stagnicolae

cercariae under 10-40X magniﬁcation (Cort et al. 1940).

Results

A total of 554 P. integra were assayed in 1999 and 2000, and 4 (0.72%) were
shedding T. physellae cercariae in the light box assay. The majority of P. integra were
collected during May and early June when water temperatures were low and therefore
when few residents swim in Higgins Lake. Additionally, only 3.0% of snails collected
throughout 1999 and 2000 were P. integra. Thus, P. integra were deemed unimportant
for transmission of cercarial dermatitis in Higgins Lake.

A total of 7,166 S. emarginata were collected from the monitoring sites and
assayed in 1999, of which 64 (0.89%) were shedding T stagnicolae (Table 2) in the light
box assay. In 2000 at the monitoring stations, a total of 10,633 S. emarginata were
examined and 60 (0.56%) were shedding the parasite that causes cercarial dermatitis
(Table 3) in the light box assay. At the Birch Lodge site, 5,898 S. emarginata were
dissected to determine presence of T. stagm’colae cercariae and 6 (0.10%) were infected.
Although these results seem to indicate a higher prevalence in 1999, there were no
signiﬁcant differences in percentage of snails shedding T. stagm'colae in the light box
assay between 1999 and 2000 at the four monitoring stations (x2=1.94, p=0.16). Data
collected ﬁom May 2000 were not included in this analysis because no snails were

collected in May of 1999. The highest prevalence in each year was at the Columbine

 

 

 

 

Point Detroit
No. No. Percentage
Collection Date Collected Infected Infected
3-Jun-99 210 0 0.00%
l4-Jun-99 259 0 0.00%
28-Jun-99 395 0 0.00%
12-Jul-99 347 6 1.73%
26-Jul-99 300 2 0.67%
9-Aug-99 321 0 0.00%
TOTAL 1832 8 0.44%
Saginaw Avenue
No. No. Percentage
Collection Date Collected Infected Infected
4-Jun-99 292 0 0.00%
lS-Jun-99 299 1 0.33%
29-Jun-99 3 13 0 0.00%
13-Jul-99 283 1 0.35%
27-Jul-99 309 1 0.32%
10-Aug-99 3 18 2 0.63%
TOTAL 1814 S 0.28%
Columbine Lane
No. No. Percentage
Collection Date Collected Infected Infected
7-Jun-99 203 2 0.99%
21-Jun-99 315 l 0.32%
7-Jul-99 276 10 3.62%
l9-Jul-99 385 10 2.60%
2-Aug-99 301 8 2.66%
16-Aug-99 214 8 3.74%
TOTAL 1694 39 2.30%
Rosewood Avenue
No. No.‘ Percentage
Collection Date Collected Infected Infected
8-Jun-99 206 0 0.00%
22-Jun-99 308 0 0.00%
8-Jul-99 325 2 0.62%
22-Jul-99 318 7 2.20%
3-Aug-99 349 1 0.29%
17-Aug-99 320 2 0.63%
TOTAL 1826 12 0.66%

 

 

 

Table 2. Number of S. emarginata shedding T. stagm'colae at four monitoring stations in
1999.

 

 

 

 

 

 

Point Detroit
No. No. Percentage
Collection Date Collected Infected Infected
10-May-00 303 0 0.00%
23 -May-00 341 0 0.00%
5-Jun-00 438 0 0.00%
l9-Jun-00 450 1 0.22%
5-Jul-00 485 0 0.00%
17-J ul-OO 485 3 0.62%
2-Aug-00 485 2 0.41%
14-Aug-00 485 1 0.21%
TOTAL 3472 7 0.20%
Saginaw Avenue
No. No. Percentage
Collection Date Collected Infected Infected
1 l-May-OO 285 0 0.00%
24-May-00 324 0 0.00%
6-Jun-00 427 1 0.23%
20-Jun-00 447 0 0.00%
6-Jul-00 483 2 0.41%
18-Jul-00 300 0 0.00%
3-Aug-00 485 O 0.00%
TOTAL 2751 3 0.11%
Columbine Lane
No. No. Percentage
Collection Date Collected Infected Infected
15-May-00 13 l 0 0.00%
30-May-00 1 1 1 0 0.00%
12-Jun-00 463 2 0.43%
26-Jun-00 236 4 1.69%
10-Jul-00 308 4 1.30%
24-Jul-00 477 3 1 6.50%
7-Aug-00 326 5 1 .53%
TOTAL 2052 46 2.24%
Rosewood Avenue
No. No. Percentage
Collection Date Collected Infected Infected
l6-May-00 1 l3 0 0.00%
3 l-May-OO 254 2 0.79%
l4-Jun-00 23 l 0 0.00%
27-Jun-00 481 0 0.00%
1 l-Jul-OO 309 l 0.32%
25-Jul-00 485 1 0.21%
8-Aug-00 485 0 0.00%
TOTAL 2358 4 0.17%

 

Table 3. Number of S. emarginata shedding T. stagnicolae at four monitoring stations in
2000.

10

Lane site. Prevalence at the other three sites was relatively low (Tables 2 and 3).

In 1999, few snails assayed in June were shedding T. stagnicolae in the light box
assay and the greatest percentage of shedding snails occurred in July (Table 2 and Figure
2). At Point Detroit, the majority of S. emarginata which were shedding cercariae
occurred in mid to late July and none of the assayed snails were shedding cercariae
before July 12. Prevalence at the Rosewood Avenue monitoring station peaked in late
July while the prevalence of shedding snails peaked in middle August at Columbine
Lane. The prevalence of S. emarginata that were shedding T. stagnicolae remained

relatively small from June through August at Saginaw Avenue (Table 2 and Figure 2).

 

 

 

 

 

 

 

 

 

4% ,
- + - Point Detroit A .‘
+ Saginaw Avenue .' x ,r'
3% .1 ' ' it - - Columbine Lane ‘s‘ I;
g j - C - Rosewood Avenue ‘3 .‘o
m 3 '
i 2% a
a: .
m
(H
o
o .
o :
g 1% j
3 ..
d: 1
0% 3
20-May 4-Jun 19-Jun 4-Jul l9-Jul 3-Aug l8-Aug 2-Sep

Date

Figure 2. Prevalence of S. emarginata shedding T stagm'colae in Higgins
Lake, 1999.

11

The year 2000 was characterized by a different pattern in prevalence as the
percentage of shedding snails remained relatively constant ﬁ'om May through August at
three of four monitored sites, including sites at Point Detroit, Saginaw Avenue, and
Rosewood Avenue (Table 3 and Figure 3). Conversely, cercarial shedding rates from
snails peaked in July of 2000 at the Columbine Lane station just as in 1999 (Figures 2
and 3). When all stations were combined and averaged over two-week periods, the
percentage of S. emarginata shedding cercariae peaked in July, just as in 1999 (Figure 4).
In 2000, Columbine Lane had a higher prevalence of shedding snails ﬁ'om May through
August (2.17%) and prevalence peaked on July 24 at 6.5 %, which was greater than for

any other site or sampling date in 1999 or 2000 (Tables 2 and 3). At no time after

 

 

 

 

 

7% ,
2 — -o - Point Detroit A
0 j . ‘
2 5/0 . . -' ‘
Q . —I- Saginaw Avenue - ‘.
r20 5% --*--Columbine Lane ,'
g j - O - Rosewood Avenue
E 4% 3 ,'
m . ' '
“a . 1 " ‘
o 3 A 1 '. 2
o ' ‘
c: l ' ‘
% 2‘V ‘ " ~.
0 .
«>3 :
l-
a.

 

 

 

1% j
0% ‘
8-May 22-May 5-Jun l9-Jun 3-Jul l7-Jul 31-Jul 14-Aug
Date

Figure 3. Prevalence of S. emarginata shedding T. stagm'colae in
Higgins Lake, 2000.

12

Prevalence of Shedding Snail

June 26, 2000, did prevalence fall below 1.3% at Columbine Lane and in 1999
prevalence never fell below 2.6% aﬁer July 7. Including both years, Columbine Lane had
signiﬁcantly more S. emarginata infected with T. stagm'colae than the other three

monitoring stations (x2=41.86, p<0.01). There was no signiﬁcant difference in prevalence

of shedding snails at the other three stations during this same time period (x2=0.18,

 

4.0%

L4L

3.5% 5 "

3.0%5 "
‘ +1999
- - I - '2000

 

2.5% E

 

 

 

 

2.0% f
1.5% f
1.0%

0.5% E

 

0.0% g

 

-0.5% 3

 

 

 

-1.0% ‘
May May Jun Jun Jul Jul Aug Aug

1-15 16-31 1-15 16-30 1-15 16- 1-15 16-30

3 1
Date

Figure 4. Prevalence of S. emarginata shedding T. stagm'colae in
1999 and 2000.

p=0.67).

stagm'colae also increased (Table 4). Snails with a shell length between 20.0 and 24.9

As size of S. emarginata increased, the percentage of snails shedding T.

13

mm were the largest size class of snails collected in large enough numbers to be able to
detect rate of cercarial shedding with statistical conﬁdence. 1.75% of snails in this large
size class were shedding cercariae in the light box assay. 0.89% of snails with a shell
length between 15.0 and 19.9 mm long and 0.29% of snails between 10.0 and 14.9 mm
long were shedding cercariae. No S. emarginata less than 10.0 mm long ever shed
cercariae. Too few snails greater than 24.9 mm long were collected to detect percent
shedding. These data suggest that snails less than 10.0 mm long were either not infected

with the parasite or that they do not shed cercariae until they grow to a larger size.

 

 

 

Size Class No. Examined No. Infected Percentage Infected

0.0 - 4.9 mm 12 0 0.00%
5.0 — 9.9 mm 438 0 0.00%
10.0 - 14.9 mm 7,566 22 0.29%
15.0 - 19.9 m 10,265 91 0.89%
20.0 — 24.9 mm 1,200 21 1.75%
25.0 - 29.9 mm 14 0 0.00%
Total 19495 134 0.69%

 

 

 

Table 4. Percentage of S. emarginata shedding T. stagnicolae in six size classes collected
ﬁom Higgins Lake in 1999 and 2000.

Discussion

Over both years, the mean percentage of S. emarginata infected with T.
stagm'colae was 0.69% according to the light box assay and there was no signiﬁcant
difference between 1999 and 2000 (Tables 2 and 3). 0.10% of S. emarginata were
infected with T. stagnicolae in 2000 at the Birch Lodge site. This prevalence at these ﬁve
sites is slightly lower than reported in other studies (Table I) conducted in Higgins Lake

(Blankespoor 1992; Nyman 1998) and in other northern Michigan lakes (Keas and

14

Blankespoor 1997; Cort et al. 1960; Blankespoor 1984). However, these results should be
expected because studies by other authors did not examine infections of S. emarginata
throughout May and early June, when the percentage of infected snails tends to be
extremely low. Thus, it appears that the prevalence recorded for Higgins Lake in this
study is very similar to other studies.

The rate of cercarial shedding by S. emarginata peaked during late July and early
August at the four monitoring stations (Figures 2, 3, and 4), supporting the hypothesis
that the percentage of snails infected with T. stagnicolae would vary from May through
August. These results are consistent with other studies that documented an increase in the
percentage of S. emarginata infected with parasites in July (Cort et al. 1937; Cort et al.
1940). An increase in the prevalence of T. stagnicolae is generally associated with an
increase in water temperature, accelerating development in larval stages of the parasite
(Brackett 1940). Indeed, water temperatures in Higgins Lake increased from 9 — 11°C in
mid-May to > 21 — 24°C in mid-July (See Figure 13, Chapter 2).

Consistent with the hypothesis that the percentage of snails infected with T.
stagm'colae varied spatially, S. emarginata were signiﬁcantly more infected with T.
stagm'colae at the Columbine Lane site than at the other three monitored stations (Figures
2 and 3 and Tables 2 and 3). The reasons for this are unclear. Although not directly
measured in this study, one hypothesis may be that ﬁnal avian hosts of the parasite ﬁnd
this area of Higgins Lake more satisfactory than other examined areas, increasing the
likelihood that snail intermediate hosts become infected. Also, because the outlet for the
Cut River is relatively close to this site, water ﬂow in the lake may be bringing a large

number of miracidia to the area, infecting a higher percentage of S. emarginata.

15

As the size of S. emarginata increased, the percentage that were shedding T.
stagm'colae cercariae also increased, and no snails smaller than 10.0 mm shed cercariae
(Table 4). One explanation for this may be that parasite development time in the snail
results in shedding only after small snails have grown to lengths greater than 10 mm.
Alternatively, parasitic infections in many types of snails leads to an increase in growth
rates in intermediate host organisms (Esch and Fernandez 1994).

These data have implications for controlling cercarial dermatitis in Michigan
lakes. A majority of S. emarginata shed T. stagm'colae during July and early August.
Thus, swimmers susceptible to cercarial dermatitis are evidently more likely to contract
the disease during this portion of summer. This agrees with reports of swimmer’s itch
reported to the swimmer’s itch hotline for Higgins Lake. However, the paper by
Lindblade (1998) for Walloon Lake suggests that reports of swimmer’s itch for this lake
peak in early July. Even though not directly measured, it also appears that percentage of
snails infected with T. stagm'colae is correlated with water temperature, at least to some
degree. Therefore, an intense warming trend may lead to an outbreak of cercarial

dermatitis within a lake (Brackett 1940; Cort et a1. 1937).

16

CHAPTER TWO
DISTRIBUTION OF STAGNICOLA EAIARGHVA TA (SOWERBY) IN RELATION

TO LIMNOLOGICAL VARIABLES

Introduction

Numerous studies have documented the complete life cycle of T. stagnicolae
including morphological characteristics of each life stage, length of time required for
each stage to develop, and the incidence of snails infected with the parasite (Brackett
1940; Cort et al. 1940). The effects of temperature on emergence of cercariae, swimming
power of larval stages of the parasite, and activities of cercariae in normal habitats have
been described (Brackett 1940). Cort (1950) described effects of cercariae on the liver of
snail intermediate hosts.

Relatively little is known about life history traits and behavior of the snail S.
emarginata. To better understand occurrence and distribution of cercaria in lakes and
occurrence of cercarial dermatitis in swimmers, abundance, distribution, and movements
of S. emarginata should be better documented. Such documentation is a necessary step in
effectively developing methods to control snail intermediate hosts of the parasite that
causes swimmer’s itch (Brackett 1940; Laman et al. 1984).

The amount of calcium carbonate in water has been described as the variable that
most inﬂuences distribution and abundance of freshwater snails in temperate regions.
Boycott (1936) and Macan (1950) found a signiﬁcant positive correlation between
concentrations of calcium and abundance of snails in European lakes. Okland (1983)

found this same relationship in Norway and concluded that taxonomic diversity of

17

gastropods declined signiﬁcantly in lakes with extremely low calcium concentrations.
These conclusions were based on lakes in a homogenous region and did not explain
variation in abundance and distribution of several species of snails in the same lake.

Variables that may inﬂuence distribution and abundance of gastropods in an
individual lake include size and type of substrate, water depth, temperature and dissolved
oxygen ﬂuctuations, and availability of food resources, among other things (Lodge et al.
1987). Little attention has been paid to the effects of these variables on S. emarginata.
The majority of studies have instead focused on the entire Lymnaeidae family rather than
on S. emarginata speciﬁcally. Each of these variables is covered in more detail below.

Distribution and abundance of snails may be inﬂuenced by size and type of
substrate in lakes (Harman 1972). Clampitt (1973) attempted to determine characteristics
of substrates that most inﬂuenced distribution of several pulmonate snails, including S.
emarginata in Douglas Lake, Michigan. He concluded that S. emarginata exhibited a
complex and varied pattern of distribution in regards to substratum and was found on
both cobbles and sand. Because these conclusions were based on a small number of ﬁeld
observartions, Clampitt (1973) performed laboratory studies to conﬁrm that S.
emarginata was a generalist in regard to preferred substratum size. These results were
similar to earlier ﬁndings by Baker (1928) who concluded that blue snails were usually
found in exposed littoral regions clinging tightly to rocks or buried in sand or muck.
Therefore, substrate type and size may not be an eﬂ‘ective indicator of distribution and
abundance of S. emarginata in a lake.

Water depth may inﬂuence distribution and abundance of S. emarginata. Burton

et al. (1999) were unable to determine whether signiﬁcant diﬂ‘erences existed in the

18

distribution of blue snails at three depths in Higgins Lake, Michigan because of small
sample sizes and high variance in data. Their data were suggestive, however, because S.
emarginata was more common in water depths of 60 and 90 cm than at 30 cm. Laman et
a1. (1984) believed that the population density of Lymnaeidae snails in a northern
Michigan lake varied spatially and temporally and was inﬂuenced by water depth.
However, these individuals did not focus speciﬁcally on density and distribution of S.
emarginata. The inﬂuence of water depth on snail distribution has also been documented
in laboratory studies (Boag 1981).

In a study examining diversity of the aquatic biota on a wave swept sandy beach
in a northern Michigan lake, Moﬂ‘ett (1943) determined that S. emarginata were
relatively rare compared to other species of snails and were most common in depths
between 2.4 and 3.0 m. Moffett (1943) suggested that the blue snail was unable to cope
with the intensity of disturbances commonly present in wave-swept shoals of lakes, but
were able to survive in deeper water in reduced numbers where disturbance was less.
Moﬁ‘ett’s (1943) results supported the general conclusions of Cheatum (1934) who stated
that S. emarginata numbers tended to be greatest in depths greater than one meter.
Cheatum (193 4) measured the distribution of blue snails qualitatively at one point in time
rather than over a long period of time. While these studies suggest that density of S.
emarginata varies with depth, the quantitative relationship between depth and snail
distribution needs to be determined more deﬁnitively both spartially and temporally in
relation to other limnological variables such as temperature, substrate, and food

availability.

19

The abundance and distribution of S. emarginata in a lake may be inﬂuenced by
ambient water temperature and dissolved oxygen levels (Lodge et a1. 1987). Cheatum
(1934) conducted laboratory and ﬁeld studies on respiration of pulmonate snails common
to northern Michigan and concluded that snails, including the blue snail, were inﬂuenced
by amounts of dissolved oxygen in water. McMahon (1983) also concluded that
distribution of many species of prosobranch snails was inﬂuenced by the amount of
oxygen in water. Some snails even utilize macrophytes to move to the surface to acquire
adequate oxygen (Lodge et a1. 1987).

The amount of oxygen dissolved in water is, in part, a ﬁinction of temperature
(Home and Goldman 1994), so water temperature may inﬂuence oxygen availability to S.
emarginata. Water temperature may also inﬂuence reproductive success, growth, and
other life history traits of snails (Russell-Hunter 1978), and snails may move to
temperatures which are more favorable for development. Van der Schalie and Ben'y
(1973) concluded that high ambient temperatures may limit geographic distribution of
snaﬂs.

The amount and type of food resources available may inﬂuence distribution and
abundance of snails (Lodge et al. 1987). Little is known speciﬁcally about food
preferences of S. emarginata. Based on laboratory and ﬁeld studies, Kesler et a1. (1986)
concluded that ﬁlamentous green algae was one of the most common food resource
utilized by Lymnaeidae snails. Lindblade (1998) suggested that higher levels of algae
could result in higher populations of snails in a given area. Others documented that
Lymnaeidae snails utilize periphyton, including bacteria, fungi, and other organic matter

(Barnese et al. 1990; Bovbjerg 1968; and Lodge 1986). Weber and Lodge (1990)

20

concluded from a ﬁeld study that more S. emarginata colonized rocks with periphyton
than colonized rocks without periphyton. Therefore, literature about food preference in
snails is inconsistent and focuses on the entire Lymnaeidae family rather than speciﬁcally
on S. emarginata.
theetiaaas

Because of deﬁciencies and inconsistencies in literature in regards to factors that
inﬂuence abundance and distribution of S. emarginata in lakes, the overall goal of this
study was to document the snail’s distribution in relation to depth and to identify other
limnological variables that inﬂuence abundance and distribution of S. emarginata.

Speciﬁc objectives were to:

1. Document the density of different sizes of S. emarginata ﬁ'om mid-May

through mid-August at three water depths

2. Determine the degree to which limnological variables identiﬁed ﬁom the
literature, including water depth, oxygen levels, food availability, and

water temperature, inﬂuence abundance and distribution of S. emarginata

Based on the literature, I hypothesized that density of S. emarginata would differ
between three measured depths and would change from May through August. I also
hypothesized that greater densities of S. emarginata in summer months would be
associated with higher oxygen levels, lower water temperatures, and increased food

ICSOUI’CCS.

21

Methods
Establishment of Sampling Sites

In order to correlate limnological variables with abundance and distribution of S.
emarginata, two transects were established perpendicular to the shoreline near Birch
Lodge during the summer of 2000 (Figure 1). This area was located along the western
portion of the northern sub-basin of Higgins Lake in an area that supported a greater
density of snails than other areas of the lake (Chapter 1; Burton et al. 2000). The
sampling area was characterized by a sandy substrate from shoreline to approximately 90
m out from this point and then was dominated by sand covered with silt as the water
depth increased beyond this point. Depth increased gradually from the shoreline outwards
at the rate of about 0.5 m for each 10 m horizontal distance.

Along the transects, three water depths were used to categorize the snail
population: 0.0 - 0.5 m, 0.51 - 1.0 m, and greater than 1 m. HOBO temperature data
loggers were placed in sealed plastic chambers and attached to cinder block anchors at
each of these depths. These data loggers measured water temperature at one-hour
intervals. They were placed in the lake on the ﬁrst snail sampling date (May 12, 2001)
and were recovered after the last sampling date (August 9, 2001). Each HOBO was
examined weekly to ensure that they were working properly and that their location had
not been changed by swimmers.

At water less than 1 m deep, glass-bottomed buckets were used initially to
examine distribution and density of the snails to ensure that each sampling area contained
at least one hundred blue snails. The sampling area needed for obtaining one hundred

snails varied ﬁ'om 2 x 2 m to 10 x 10 m plots. Once the dimensions of an adequate

22

sampling area were established, a metric tape rule and metal stakes were used to mark the
corners of the sampling area. Brightly colored masonry line was attached to the stakes
and used to deﬁne the boundaries of the sampling areas. All S. emarginata on the
substrate surface in the sampling area were collected using a glass bottomed-bucket and a
strainer attached to a wooden dowel rod. Snails “on the line” were included in the
sample. Sampling in any given area was terminated when no new S. emarginata were
discovered in a ﬁve-minute period. An additional sampling station located ten meters
away ﬁ'om the ﬁrst area at the same water depth was marked out and sampled so that
variances in the measured parameters could be determined. To avoid re-sampling the
same area, areas that had been previously sampled were referenced to land stnrctures and
were not sampled again for at least ﬁve weeks.
Limnological Dar; Collection

Dissolved oxygen, chlorophyll a, and organic matter concentrations were
measured on each snail sampling date. A YSI Model 51B dissolved oxygen meter was
used to document the amount of oxygen in the water. To do this, the meter was calibrated
according to the manufacturer’s recommendations and the probe was placed in water just
above the substrate in the middle of the sampling area. The probe was agitated for at least
one minute before the oxygen concentration was recorded. All measurements were
recorded between 8:30 and 11:00 AM.

Using a 1.5 m long piece of schedule 30 PVC pipe, a wooden broom handle, a
wood screw, and a disc of plastic with a diameter (3.81 cm) the inside diameter of the
PVC, a coring device was constructed to collect a known volume of sand (45.6 cm3) for

chlorophyll and organic matter analyses (Figure 5). To do this, the open end of the coring

23

device was gently placed on the substrate surface and the broom handle, which was
attached to the plastic disc with a screw, was slowly pulled upwards, creating a vacuum
to extract a 4 cm long core for each sample. This technique was utilized to randomly
collect three chlorophyll samples and three separate organic matter samples for each
sampling area. Three samples of each parameter were collected to minimize eﬂ‘ects of
patchiness in the sampling area. Once collected, all six sand cores were transferred into
six separate plastic bags and placed in a cooler. As soon as possible after initial

collection, sand samples were ﬂown in the dark to prevent degradation.

 

 

 

 

K; Schedule 30 Pvc Pipe

Broom Handle

 

 

 

'l'-1‘—— Plastic Disk

. ”'ma- ama'w.

 

 

V

Figure 5. Coring device used to collect sand samples for food availability analyses.

24

The same general techniques were used at water depths greater than 1 m, except
that SCUBA techniques were employed to collect S. emarginata. Sampling stations were
prepared in the same manner, with the exception that small diameter PVC pipes were
used to anchor four comers of the sampling area to increase visibility of the corners. Each
S. emarginata was collected directly from the substrate by the diver and placed in ﬁne
meshed bags. Again, sampling stopped when no new blue snails were seen in a ﬁve-
rninute period. Dissolved oxygen was collected with the dissolved oxygen meter using
the same method except that the probe was suspended ﬁom the side of the boat.
Chlorophyll and organic matter samples were collected in the same manner, but a 2.5 m
long piece of PVC was used as a corer rather than the shorter corer used in shallower
water. This additional length in the PVC pipe prevented water from entering the top of
the apparatus, destroying the vacuum needed to collect sand cores.

Laboratog Anglysis

 

In the laboratory, all S. emarginata were counted, measured with calipers, and
placed in size classes based on shell length as follows: class one (0.0 — 4.9 mm), class
two (5.0 - 9.9 mm), class three (10.0 - 14.9 mm), class four (15.0 -— 19.9 mm), and class
ﬁve (20.0 — 24.9 mm).

To estimate food availability for S. emarginata, chlorophyll a was quantiﬁed as a
surrogate of algal abundance (American Public Health Association et al. 1985). To
prevent degradation of pigments, all collected chlorophyll samples were quantiﬁed within
thirty days after collection using ﬂuorometry as proposed by American Public Health
Association et a1. (1985) using a Turner Designs ﬂuorometer. One modiﬁcation to this

procedure, which is speciﬁc for planktonic chlorophyll measurements, was required. The

25

modiﬁcation consisted of extracting chlorophyll a from the sand by soaking the sand in
90% buﬂ‘ered acetone in a ﬁ'eezer for twenty-four hours. After twenty-four hours, the
sand was shaken vigorously for thirty seconds. The acetone and chlorophyll solution was
ﬁltered with a Whatman #4 ﬁlter to remove the sand and used to determine chlorophyll a
concentrations. Because phaeophyton and other pigments found in natural communities
of algae affect determination of chlorophyll a concentrations, samples collected before
July 12 were acidiﬁed (American Public Health Association et al. 1985). Chlorophyll
samples collected on or after July 12 were analyzed using a Turner Designs 436 nm
excitation ﬁlter installed in the ﬂuorometer to negate the additive effects of phaeophyton
on chlorophyll a concentrations. The ﬂuorometer was calibrated using standardized
methods (American Public Health Association et al. 1985) before initial samples were
analyzed and immediately after the ﬁlter was installed. Chlorophyll a concentrations
measured with the ﬂuorometer were converted to mg chlorophyll a m'2 by the following
equation:
Chlorophyll a_(mg m3) X volume of extrgc;(l_.)
0.0011395 rn'2 (area of substrate collected with core sampler)

The amount of organic matter, an additional food source for the blue snail
(Barnese et al. 1990; Bovbjerg 1968; and Lodge 1986), was quantiﬁed by drying the
three remaining sand samples to a constant weight at 100 °C prior to ashing at 500 °C in
a Fisher Isotemp Model 186 mufﬂe ﬁrmace for 24 hours. Loss on ignition was used as a
measurement of the ash-free dry mass (AFDM) of organic matter (American Public
Health Association et al. 1985). AFDM of organic matter was converted to mg m'2 by the

following equation:

26

 

organic m_atter lost on igm'tion (mg)
0.0011395 m'2 (area of substrate collected with core sampler)

Statistical Analysis

All statistical analyses were performed according to the guidelines of Younger
(1998) and Neter et al. ( 1996) utilizing SAS version 8.0. Scatter plots and residual plots
were used to assess normality in the data. Data were considered signiﬁcant at 0t=0.05.
Results

Min and Density

The 0.0 - 0.5 m water depth was sampled on twelve dates with two plots sampled
per date (24 plots) from May 12 through August 9 and a total of 2,736 snails were
collected (Table 5). A total of 2,973 S. emarginata were collected from the 0.51 - 1.0 m
water depth on 1] dates (22 plots) from May 22 through August 9. Water greater than
one meter in depth was sampled a total of six times (12 plots) from May 26 through
August 1, yielding a total of 2,531 S. emarginata (Table 5). In water greater than 1 m
deep, mean water depth sampled was 2.1 m. Depth ranged ﬁom 1.4 to 2.4 m.

Including snails from all dates and depths, S. emarginata between 10.0 and 14.9
mm long comprised over 70% of collected snails (Figure 6). Snails between 5.0 and 9.9
mm long were the next most common size class of snails collected. Snails with shell
length between 15.0 and 19.9 mm were even less common while S. emarginata greater
than 20 mm in shell length were rare and comprised about 1% of the total number of blue
snails collected. No organisms with a shell length less than 5 mm were collected during

this study. Snails ﬁrst start shedding cercariae when they are longer than 9.9 mm

27

(Chapter 1; Burton et al. 2000). Thus, a signiﬁcant number of snails were too small to

shed cercariae at any point in time.

S. emarginata between 5.0 and 9.9 mm represented 20% to 25% of collected
individuals at the shallowest two water depths but only 4.2% in water greater than one
meter deep (Table 5). Snails with shell lengths between 15.0 and 19.9 mm made up
approximately 9% of snails collected in depths less than 0.5 m and only about 5% of

snails collected from water between 0.51 and 1.0 m deep. In water greater than a meter

 

 

 

 

 

 

 

 

Water Depth Snail Size Number of Snails Percent Frequency

0.0 - 0.5 m 0.0 - 4.9 mm 0 0.0%
5.0 - 9.9 mm 498 18.2%

10.0 - 14.9 mm 2001 73.1%

15.0 - 19.9 mm 237 8.6%

20.0 - 24.9 mm 0 0.00%

0.51 - 1.0 m 0.0 - 4.9 mm 0 0.0%
5.0 - 9.9 mm 766 25.7%

10.0 - 14.9 mm 2060 69.2%

15.0 - 19.9 mm 147 4.9%

20.0 - 24.9 mm 0 0.0%

> 1 m 0.0 - 4.9 mm 0 0.0%
5.0 - 9.9 mm 108 4.2%
10.0 - 14.9 mm 1744 68.9%
15.0 - 19.9 mm 677 26.7%

20.0 - 24.9 mm 2 0.0%

 

Table 5. Number of S. emarginata collected per shell length for three sampled water

depths.

28

 

deep, however, snails between 15.0 and 19.9 mm long made up 27% of snails collected.
Because of this, larger S. emarginata were signiﬁcantly more common in depths greater
than one meter than in the other two water depth categories examined in this study
(F=52.34, p<0.001), suggesting that snails at depths greater than 1 m may be particularly
important as vectors of swimmer’s itch.

Densities of S. emarginata varied anywhere from 1.3 snails m'2 in early June to
over 18 snails m'2 in the later portion of July with the mean density of blue snail collected
during 2000 being 6.4 snails m". The greatest mean density of snails per unit area over

the season were found at the 0.51 - 1.0 m water depth category with 8.2 snails m”. The

90%

 

B 0.0 - 0.5 m
B 0.51-1.0 m

7W0 Z'I-Z-Z .>1m

80%

 

 

 

60%

50%

40%

Percent Frequency

30%

20%

10%

 

0%

2 3 4
S. emarginata size class

Figure 6. Frequency distribution of S. emarginata size classes collected in 2000 at
three water depths. Size class 2 = 5.0 - 9.9 mm, 3 =10.0 -14.9 mm, 4 =15.0 -
19.9 mm Bars equal standard errors and * is signiﬁcant at p<0.05.

29

next highest mean seasonal density of snails occurred at water depths greater than 1 m
and averaged 5.2 snails m'2 while 5.0 snails m’2 occurred in the shallowest water depth
category. Thus, there was no signiﬁcant difference in snail density at the two shallowest
depths. Even though there was a signiﬁcant difference in density of S. emarginata at
three measured depths over the summer (F=7.37, p<0.001), the major diﬂ‘erence was
between the deepest depth examined and the two shallow depths.

The greatest density of S. emarginata at depths less than 1 m occurred ﬁ'om early

June through early July (Figure 7). The density of the blue snail was relatively low before

 

 

—0——0.0-0.5m I
. --- 0.51-1.0m '
16 i” --*-->lm

 

 

 

ﬂ
0
ALLA
I

 

S. emarginata Density (Number m'z)

 

 

 

4
J J I I i 1
ﬁ 1 r r [ f r T I 1 f rrrrrrrr F r r l r I r

4-May 19-May 3-Jun 18-Jun 3-Jul l8-Ju 2-Aug l7-Aug
Date

 

Figure 7. S. emarginata density versus sampling date at three water
depths. Bars equal standard errors.

30

i; “m

 

 

 

late May/early June (Figure 7). The number of snails in a given area decreased from late
June/early July through mid-August (Figure 7). At water depths greater than 1 m,
conversely, the density of S. emarginata increased after mid-June and peaked in late July
at approximately 18.2 snails m", only to fall after this date (Figure 7).

At the 0.0 - 0.5 m water depth, S. emarginata between 10.0 and 14.9 mm long
were the most common size class during the summer (Figure 8). The density of snails in
this size class increased in mid to late June and then decreased gradually over the rest of
the summer, although there was a reduction in the density of S. emarginata on June 15. In
early May, there were more snails between 5.0 and 9.9 mm long than in the other two

size classes, although this was no longer true by early June (Figure 8). The density of this

 

 

 

 

 

    

 

 

 

16
j +5.0-9.9mm
F 141‘ -I-10.0-14.9mm
'a I .. . -
E 12‘} i- 15.0 19.9m
5 i I
3,104— I, i
a? ‘ I l i
g s 4 I i I \
e . |
3 6T I, 1 I \
9 ‘ / 1 ’ \
e *_ I
95 I
2:_ k’ ~c\_
I t. ___,..—
O -—II—-I-—-P-*?’s r s . s s r s
4-May 24-May l3-Jun 3-Jul 23-Jul 12-Aug

Date

Figure 8. S. emarginata density versus sampling date at 0.0 - 0.5 m water
depth for three size classes. Bars equal standard errors.

31

size class of snails peaked on June 13 and then declined after this date. The density of
15.0 — 19.9 mm long S. emarginata remained relatively constant from May through
August at 0.0 — 0.5 m water depths (Figure 8).

At 0.51 - 1.0 m water depths, the largest snails (15.0 — 19.9 mm long) remained at
a relatively constant density except for a slight peak in early July while density of 5.0 —
9.9 mm long snails peaked on May 25 and gradually declined thereafter (Figure 9). The
density of S. emarginata between 10.0 and 14.9 mm long increased from May to July 7,

after which the density of these snails decreased. There was always a higher density of S.

 

 

14
—o—5.0 - 9.9 mm
12 I —I— 10.0 -149 m
I --*--15.0-19.9mm

 

 

 

ﬂ
0
m
\

S. emarginata Density (Number m'z)
A

 

 

 

 

9-May 24-May 8-Jun 23-Jun 8-Jul 23-Jul 7-Aug 22-Aug

Date

Figure 9. S. emarginata density versus sampling date at 0.51 - 1.0 m
water depth for three size classes. Bars equal standard errors.

32

emarginata between 10.0 and 14.9 mm long than in the other two size classes after June
8, while blue snails between 5.0 and 9.9 mm dominated in late May (Figure 9).

In water deeper than one meter, densities of S. emarginata in the smallest and
largest size classes stayed relatively constant over the sampling season with very small
numbers of individuals collected. Snail densities in the 10.0 — 14.9 mm and 15.0 — 19.9
mm size classes increased until July 26, after which density of the blue snail decreased
(Figure 10). Snails between 10.0 and 14.9 mm in shell length were always more common
in this water depth than were snails in other size categories. S. emarginata between 15.0
and 19.9 mm long were always the next most abundant size class after early June (Figure

10). Thus, almost all snails in water greater than 1 m deep were approaching or in the size

 

 

 

 

 

 

 

14
ll —o-—5.0-9.9mm
.g‘ 121 -I-10.0-14.9mm I!
g mi --*--15.0-19.9mm / “
g : -~ i ~ 20.0 - 24.9 mm ,’ l
e 1 ’ ‘
8* l l
3.? j l,’ 1‘
E 6‘1" ll ‘o"‘ ‘
S : ’ "x “ ‘1
u < I 0' ‘
{.0 4-- I '0" ‘I‘
E j I! ",‘ I“
E . I v' ‘1‘
I); 24v ”! "’o' ‘
l ______ I’ .—
1 i _. _____
mm. .;. . . is

 

18-May 28-May 7-Jun 17-Jun 27-Jun 7-Jul l7-Jul 27-Jul 6-Aug
Date

Figure 10. S. emarginata density versus sampling date at > 1 m water
depth for four size classes. Bars equal standard errors.

33

classes known to shed cercariae (Chapter 1).
Dissolved Oggen
Dissolved oxygen concentrations ranged from 10.6 mg/L at its highest to 8.4

mg/L at its lowest and were always near saturation or slightly under saturated (Table 6).

 

 

 

 

 

 

Water Sampling Mean Dissolved Percent Standard
Depth Date Oxygen (mg/L) Saturation Error
0.0 - 0.5 m 12-May-00 9.3 90 0.30
17-May-00 10.2 95 0.05
25-May-00 10.4 100 0.15
13-Jun-00 9.0 97 0.10
15-Jun-00 9.3 98 0.10
28-Jun-00 10.6 115 0.05
12-Jul-00 9.5 108 0.05
l9-Jul-00 8.8 104 0.10
20-Jul-00 8.8 104 0.05
26-Ju1-00 8.9 103 0.00
28-Jul-00 8.9 103 0.05
9-Aug-00 8.6 100 0.10
0.51 - 1.0 m 22-May-00 9.9 96 0.00
25-May-00 10.4 100 0.10
8-Jun-00 9.9 103 0.05
15-Jun-00 9. 1 99 0.00
22-Jun-00 9.4 102 0. 15
28-Jun-00 10.6 115 0.00
7-Jul-00 9.5 110 0.05
1 1-Jul-00 9.2 109 0.00
19-Jul-00 8.7 103 0.00
28-Jul-00 8.7 103 0.00
9-Aug-00 8.4 97 0.00
> 1 m 26-May-00 10.4 105 0.15
9-Jun-00 9.2 100 0.05
21-Jun-00 8.8 99 0.10
29—Jun-00 9.5 107 0.05
26-Jul-00 8.6 100 0.00
1-Aug-00 8.7 100 0.05

 

Table 6. Mean dissolved oxygen concentrations with percent saturation and standard
errors at three water depths.

34

The concentration of dissolved oxygen fell from May through August at all three water
depths, although changes were very gradual (Figure 11) and appeared to simply reﬂect
increases in temperature since warmer water holds less oxygen at saturation (Home and
Goldman 1994). The mean dissolved oxygen concentration from May through August for
the shallowest through the deepest water depth categories, respectively, were: 9.3 mg/L,
9.4 mg/L, and 9.2 mg/L. As a result, there was no signiﬁcant difference in dissolved
oxygen concentrations over the summer at the three measured depths (F=0.57, p=0.5703)
and dissolved oxygen concentrations were always well above any level known to cause
stress for aquatic organisms (McKee and Wolf 1963).

Including all three water depths from May through August, density of S.
emarginata was not correlated with the concentration of dissolved oxygen in water

(p=0. 1053) as indicated by a correlation coefﬁcient of 0.21. Snail densities in water

 

 

 

 

 

 

 

 

12
l
3 e
A10 ‘- 0 . J ' a I 1'
El ° W I e
5 8“ 2M 3
a 4
0
w
5
E :

u-é 4.1»— 900-05111
a 1 I0.51-l.0m
2.. A>lm

l
0‘::;::;s:;::;a:;¢:;::;::
29-Apr l4-May 29-May l3-Jun 28-Jun 13-Jul 28-Jul 12-Aug 27-Aug
Date
Figure 11. Dissolved oxygen versus sampling date at three water
depths.

35

between 0.51 and 1.0 m deep were signiﬁcantly correlated with oxygen concentrations
(p=0.0047). Snail densities in water less than 0.5 m deep were not signiﬁcantly correlated
with dissolved oxygen concentrations (p=0.4967). Higher densities of S. emarginata in
water greater than 1 m deep were not signiﬁcantly correlated (p=0. 1700) with dissolved

oxygen levels (Figure 12).

 

 

 

 

  

 

 

 

 

20 ,
‘?E 13 5 e
E 16r 0.51m-l.0m
S : '0-0-0-5 m ' y=3.4032x-23.586
514 -~ 9 2
2‘ I0.51 - 1.0m I I R =0.336
g 12 i” 1> 1 m '
o 10 {~
s . .
.§ :
U 6 TI .
§ 4 1 I . :8
L75 :L O - Q
: O ‘ A A A
0‘ . . i . i . . s
8 9 10 11
Dissolved Oxygen (mg/L)

Figure 12. S. emarginata density versus dissolved oxygen at three
water depths.

Water Tern erature

Water temperatures recorded by HOBO data loggers varied from May through

August. The lowest mean daily temperature recorded was 9.3 °C in water between 0.51
and 1.0 m deep and the highest temperature recorded (24.5 °C) was in water less than

0.51 m deep (Figure 13). From May through August, mean daily water temperature for

36

30

 

25

20

 

 

Mean Daily Water Temperature (°C)
52

 

 

 

 

 

 

. 90.0-0.5m
5 :. l0.51-1.0m
‘ A>1m
0*
3-Ma l8-May 2-Jun 17-Jun 2-Jul 17-Jul l-Aug l6-Aug
Date

Figure 13. Mean daily water temperature versus sampling date at three
water depths.

both the shallowest and middle water depths was 18.5 °C while the mean temperature in
water greater than 1 m deep was 18.9 °C. Mean daily water temperatures at all three
depths increased from May through August until mid-July, after which temperatures fell,
only to recover in early August (Figure 13). Thus, there was no signiﬁcant difference in
water temperature between the three measured depths from May through August (F=0.06,

p=0.9463).

37

There was no signiﬁcant correlation between density of S. emarginata and mean
daily water temperature (p=0.6234) as indicated by the correlation coefﬁcient of -0.07

(Figure 14). This was also true when depths were considered individually.

 

 

 

 

 

 

 

 

20 .
'E : IO.51-l.0m
.2 16 ET I A) I m
S 14 i . I n
:1 12 3- I ' I
v, 1 l I .
8 10 :-
Q . I e
S 8 ~ I
s 6. I
'50 “i I
E 4 O ‘ o. 3
s i I o ‘ 8
VS 2 _ . A t A'.
4 A ‘ ‘
O fer i ......... i . r . . ﬁs s s r i r
10 15 20 25

Mean Water Temperature (°C)

Figure 14. S. emarginata density versus mean daily water temperature
at three water depths.

Chlorophyll a
Chlorophyll a concentrations ranged from 2.5 mg m2 in late May to 22.5 mg m'2
when concentrations peaked near late June and early July (Figure 15). Chlorophyll a
concentrations tended to increase from May through August at all three water depths
(Figure 15). Water greater than 1 m deep had a signiﬁcantly lower chlorophyll a
concentration (F=25.37, p<0.001) and averaged 5.5 mg m'2 from May through August.

Water between 0.51 and 1.0 m deep had the highest mean chlorophyll a concentration

38

 

 

 

 

 

 

 

 

 

25
« —o—0.0-0.5m
I I --- 0.51-1.0m
20 T 1 ‘xl --*-->1m
(4" ~ i i
a - I i
‘ l
E 15.- 1
s * .
a : ,I
8 « ,
8 10 ‘t j
E . I
U .
52..
0 . . i . s i . s i . . l f T i . . i .
4-May 19-May 3-Jun 18-Jun 3-Jul l8-Jul 2-Aug 17-Aug

Date

Figure 15. Mean benthic chlorophyll a concentration versus sampling
date at three water depths. Bars equal standard errors.

(12.2 mg m'z) and the shallowest water depth averaged 9.5 mg m'2 chlorophyll a from

May through August.

Considering all water depths, greater densities of S. emarginata were signiﬁcantly
correlated (p=0.003 8) with higher chlorophyll a concentrations (Figure 16). This
relationship was not signiﬁcant in water between 0.0 and 0.5 m deep (p=0.5128) and
water between 0.51 and 1.0 m deep (p=0.2139). Snail densities in water greater than 1 m

deep were signiﬁcantly positively correlated with chlorophyll a concentrations

(p=0.0005).

39

 

 

 

 

 

 

  
 

 

 

1 ‘ 00.0 - 0.5 m
"A 181' o l0.51-1.0m
g 16 5- >1... . 1> 1 m
.o 3 y=2.2951x-7.4135
S 14 i” R2=0.7221 . I I
a j I 9' I I
a 12 1' . ,
m i I O I
5 10 «1 '
C3 : '. , 0.51 -l.0m
S 3 L- .' y=0.l922x+6.1275
'3 i I . I R2=0.0762
go 6 j“ ' an
‘ I “- ~- -
, 4 - . 9 8‘ y-0.0494x+4.5165
‘4 ~ 9A A I 0 0 R2=00016
2 ‘ , e 0 I9. 9 -
‘“ A e
. g .
0 ‘ I r r r
0 5 10 15 20 25
Chlorophyll a (mg m'z)

Figure 16. S. emarginata density versus benthic chlorOphyll a

concentration at three water depths.

Organic Matter

The amount of organic matter present on the substrate in the form of AFDM

varied from 224 mg m'2 to 439 mg m’2 (Table 7). AFDM organic matter concentrations

remained relatively constant from May through August at all three water depths (Figure

17). Water between 0.0 and 0.5 m deep had a mean AFDM organic matter concentration

of 315 mg m'2 from May through August and water between 0.51 and 1.0 m deep had a

mean AFDM organic matter concentration of 323 mg m'z. Organic matter present in

water greater than one meter deep averaged 287 mg m'2 from May through August. There

40

was no signiﬁcant difference in organic matter concentrations ﬁom May through August

at the three measured depths (F =2.35, p=0.0982).

 

 

 

 

 

 

Water Sampling Mean Standard
Depth Date AFDM (mg m'z) Error
0.0 m - 0.5 m 12-May-00 267 22.70
l7-May-00 332 29.36
25-May-00 340 29.41
13-Jun-00 235 20.96
1 5-Jun-00 273 22.49
28-Jun-00 297 36.07
12-Jul-00 324 1 1.29
19-Jul-00 271 16.45
20-Jul-00 349 28.08
26-Jul-00 400 29.37
28-Jul-00 366 23.46
9-Aug-00 324 59.75
0.51 m - 1.0 m 22-May-00 299 45.96
25-May-00 305 46.87
8-Jun-00 271 32.96
lS-Jun—OO 272 18.17
22-Jun-00 270 25.55
28-Jun-00 329 34.29
7-Ju1-00 207 24.60
1 l-Jul-OO 359 28.22
19-Jul-00 370 22.55
28-Jul-00 439 42. 19
9-Aug-00 432 35.99
> 1 m 26-May-00 281 29.08
9-Jun-00 322 45.77
21-Jun-00 297 41.36
29-Jun-00 229 28.01
26-Jul-00 224 16.32
l-Aug-OO 343 29.46

 

Table 7. Mean AFDM organic matter concentrations with standard errors at three
water depths.

41

 

 

 

 

 

 

 

 

600 4
. 00.0-0.5m
1 . -
500+ 0.51 1.0m I
: A>1m I
A 400-1 I I .' o
”E . o I A o o 9" I
. ’ I ‘ o
E) 3001 ," ' ‘ ' ~ A
. I
A 0
g 1 . t ‘l i A A 9
Q . I , e
200 j» I A
100 3~
0L
24-Apr 14-May 3-Jun 23-Jun 13-Jul 2-Aug 22-Aug
Date

Figure 17. Mean benthic AFDM organic matter concentration versus
sampling date at three water depths.

From May through August, there was a highly signiﬁcant negative correlation
(-0.43) between density of S. emarginata and the amount of organic matter on the
substrate of the lake (p<0.001). This relationship was signiﬁcant at the 0.0 — 0.5 m
(p=0.0271), the 0.51 - 1.0 m (p=0.0002), and the > 1 m (p=0.0354) water depths (Figure

13).

42

 

 

 

 

 

 
 
   
  

 

 

 

20
‘ O 0.0 - 0.5 m
13 “f ‘9 IO.51- 1.0m
‘1 16 J- A> l m
4g 14 3- °
:3 I I I
- >1m
a 12 4+ I
2: 3 y = -0.07lx + 25.591 ‘
g 10 ;_ R2=0.37l4 \
9 §
5’. 8 ‘r ‘
a . 0.0-0.5m \‘ 0.51-1.0m
6 r- y = -0.0307x + 14.654 ' y: -0.0368x+20.385
< I
E 4 1 R2=0.2031 a “\\ . R2=0.5136
VS , N I
2 -_ ‘ 9 P: 9 »\ .
. A ‘A ‘ ‘ . ~
0 q 1 1 1 ‘ 1
O 1 00 200 300 400 500 600
AFDM (mg m'z)

Figure 18. S. emarginata density versus benthic AFDM organic matter
concentration at three water depths.

Discussion

S. emarginata between 10.0 and 14.9 mm long were most common from May
through August at each of the three measured depths (Figure 6 and Table 5), indicating
that this is the most common size of snail in this portion of the lake. According to other
studies (Chapter 1; Burton et a1. 1999, 2000), S. emarginata were considerably larger at
these same depths in other areas of Higgins Lake, especially towards the end of June and
the ﬁrst of July when snails between 15.0 and 19.9 mm long were most prevalent. The
reasons for this are unclear, but these results may be a function of food availability in this

area of the lake. Chlorophyll a concentrations were not quantiﬁed in other portions of the

43

lake, so little can be said to support this argument. Also, other areas of the lake have
considerably more cobbles as part of the substrate. Because increased predation pressure
tends to decrease foraging time in snails (Weber and Lodge 1990), cobbles present in
other portions of the lake may be acting as a refuge for snails ﬁ'om their predators. This
refuge would allow more time for snails to forage for food and to increase the amount of
energy allocated for growth.

Larger S. emarginata were signiﬁcantly more prevalent in water greater than 1 m
deep than those snails collected in shallower water (Table 5). One explanation for this
phenomenon may be that larger snails have stronger shells and can better resist predation
than can smaller snails (Brown and Devries 1985). Thus, larger snails may have an
advantage over smaller snails in water depths greater than 1 m because they are better
able to resist predation by ﬁsh or crayﬁsh predators than are smaller snails in shallower
water (Slootweg 1987; Turner et a1. 2000). This hypothesis agrees with the model
proposed by Lodge et a1. (1987), who found that predation is the most important factor
structuring distribution of freshwater snails in permanent water bodies. This relationship
is diﬂicult to document in this study, however, because the depth distributions of sunﬁsh
and crayﬁsh, the main predators of many snails (Lodge et al. 1987), were not known.

The ﬁnal possible explanation for the fact that S. emarginata were signiﬁcantly
larger in water greater than 1 m deep may be that this depth of water may have a higher
proportion of snails infected with a parasite, which tends to increase grth rates of
gastropods (Esch and Fernandez 1994). Although Burton et al. (1999) reﬁtted this by
concluding that more snails infected with schistosomes were present in the shallowest

depth of water (30 cm), they concluded that their results may have been an artifact of

44

sample size and this data did not include other types of parasitic infections. Because
Burton et al. (1999, 2000) and the results from Chapter 1 indicate that S. emarginata
were increasingly more heavily infected with schistosome parasites as the size of snail
increased, their results (along with these data) do imply that water greater than 1 m deep
may harbor a disproportionably higher prevalence of S. emarginata infected with T.
stagnicolae. Attempts were made to document this relationship in this study, but only six
assayed snails were infected with this parasite and therefore little can be concluded from
these results.

These data have important implications for the control of swimmer’s itch in
Higgins Lake. There were signiﬁcantly more large snails in water greater than 1 m deep
than in shallower water and large snails are more likely to shed cercariae of schistosome
parasites (Burton et al. 1999, 2000; Chapter 1), suggesting that snails in water greater
than 1 m deep are a signiﬁcant source of cercariae. Efforts to control swimmer’s itch
have often emphasized treatment of shallow beach areas with copper sulfate (Howard et
a1. 1964; Novy et a1. 1973). Therefore, managers may need to focus on controlling S.
emarginata in water greater than 1 m deep more than on snails in a shallower water
depths to control the disease.

Consistent with the proposed hypothesis, densities of S. emarginata from May
through August varied by water depth. Overall, the lowest density of S. emarginata was
present in water less than 0.5 m deep (Figure 7). These data were consistent with the
results of Nyman (1998), who concluded that densities of S. emarginata were lowest in
water 0.3 m deep and greatest in water depths of 0.6 and 0.9 m. One possible explanation

for these results may be related to the amount of disturbance in the shallowest water

45

depth. Lodge et a1. (1987) concluded that waves on exposed lakeshores may reduce snail
numbers because of increased habitat disturbance. Lodge and Kelly (1985) also
concluded that Lymnae peregra, a member of the same family of snails that includes S.
emarginata, exhibited low resistance to habitat disturbance and were not present in areas
of lakes with high disturbance regimes. Moﬁ‘ett (1943) concluded that S. emarginata was
not present in large numbers in shoal areas of Douglas Lake, which were dominated by a
high number of waves.

The greatest mean summer density of S. emarginata was in the 0.51 - 1.0 m depth
category (Figure 7), indicating that this depth was most suitable for this particular
organism. This depth of water also supported the highest standing crop of chlorophyll a
(Figure 15) and AFDM of organic matter (Figure 17 and Table 7), although diﬂ‘erences in
AFDM organic matter were not signiﬁcant. Another explanation for this relationship may
be that predation pressure was lowest in this area, although this was not documented in
this study.

Densities of S. emarginata varied both spatially and temporally, supporting the
hypothesis that densities of S. emarginata would change from May through August at
each of three water depths. The peak in density at the two shallowest depths occurred in
June and early July (Figures 8 and 9). The density of blue snails in water greater than 1 m
deep peaked towards the end of July (Figure 10). These data suggest that a large group of
snails present in the shallowest two water depths during June and early July may have
moved to water greater than 1 m deep in late July. This movement coincided with a
decrease in water temperature (Figure 13). A similar pattern was described by Cheatum

(193 4), who concluded that S. emarginata underwent an annual movement in spring and

46

autumn to avoid hazards in the littoral zone in the winter and to take advantage of
resources in shallower water in the summer. This phenomenon was investigated in this
study and will be reported in chapter three.

These results imply that treatments to kill snails to control cercarial dermatitis in
Higgins Lake should occur in late June and early July in water less than 1 m deep and in
mid to late July in water greater than 1m deep. Additionally, water greater than 1 m deep
should be treated throughout the summer because this is the depth of water where S.
emarginata large enough (larger than 9.9 mm long) to consistently shed cercariae are
found (Figure 6). Since peak density of snails tends to be greatest in water between 0.51
and 1.0 m deep from May through August, this is the depth where managers should apply
molluscicides most heavily.

The densities of S. emarginata in this study tended to be greater than those
reported by Nyman (1998) for Higgins Lake and by Clampitt (1973) for Douglas Lake,
Michigan. The lowest density 1 observed was 1.3 snails m'z, while Nyman (1998) found
anywhere from 0.37 — 0.70 snails rn'2 and Clampitt (1973) did not ﬁnd any S. emarginata
in mid-May. I consistently found densities of the blue snail greater than 4 snails m'2 in
this study while this was the maximum density found by Clampitt (1973). The difference
in snail densities between the three studies indicates that the habitat analyzed by Cheatum
(1973) and Nyman (1998) was not conducive to a high density of S. emarginata, as was
the case at the one site examined in Higgins Lake.

The concentration of dissolved oxygen (D.O.) in the lake was relatively high and
at or near saturation from May through August (Table 6). These D.O. values were similar

to those recorded by Limno-Tech, Inc. (1992). The concentration of dissolved oxygen

47

decreased slightly over time (Figure 11), perhaps reﬂecting the fact that the amount of
oxygen in saturated water is in part a ﬁrnction of water temperature (Home and Goldman
1994), which increased from May through August at all three water depths (Figure 13).

There were no signiﬁcant differences in DD. concentrations between the three
measured depths from May through August. D.O. concentrations did not signiﬁcantly
correlate with density of S. emarginata (Figure 14), disputing the hypothesis that higher
densities of snails would be associated with higher D.O. concentrations. These data
suggest that these organisms were not inﬂuenced greatly by the oxygen concentrations
observed in Higgins Lake. Cheatum (1934) concluded that S. emarginata decreased
intervals between breathing periods when water warmed because there was less available
oxygen, and that they were able to survive 62 days in an experimentally induced
desiccation state. Therefore, the relatively small variability in dissolved oxygen
concentrations observed in this study from May through August was not great enough to
correlate with a response in the density of S. emarginata.

Mean daily water temperatures recorded by HOBO data loggers increased from
May through mid-July at all three water depths (Figure 13), which correlated with
warming air temperatures during this same period of time. There was no signiﬁcant
diﬁ‘erence in water temperature between the three water depths, which could be explained
by the fact that the lake tends to stratify at water depths between 12 and 15 m (Limno-
Tech 1992), which is much deeper than depths examined in this study. Therefore,
temperature differences in water depths examined in this study would not be expected to

be diﬂ‘erent in the well-mixed epilirnnion of the lake.

48

Reﬁiting the hypothesis that abundance of S. emarginata would be negatively
correlated with water temperature, snail densities did not signiﬁcantly correlate with
mean daily water temperatures (Figure 14). There was no signiﬁcant difference in mean
daily water temperatures between the three water depths (Figure 13), suggesting that
water temperature differences were not great enough to cause a response in densities of S.
eamrginata. However, the data do indicate that snails were affected by water temperature
to some degree because densities of snails in water between 0.0 and 1.0 m deep decreased
when mean daily water temperature fell in late July (Figure 13). This relationship may
indicate that S. emarginata migrate and that this movement may be triggered by a change
in water temperature (Cheatum 1934).

Algal biomass, recorded as chlorophyll a, increased ﬁ'om May through August at
all three water depths (Figure 15). This relationship was likely due to the fact that
productivity in a water body tends to increase as temperatures increase because of higher
metabolic rates (Home and Goldman 1994). There was a signiﬁcant difference in
chlorophyll a concentrations between the three measured depths. Water greater than 1 m
deep had the lowest mean concentration and water between 0.51 and 1.0 m deep had the
highest concentration from May through August. This suggests that the algal community
in water greater than 1 m deep may have been light inhibited because light extinction
does increase as a function of water depth (Horne and Goldman 1994). Algae in water
less than 0.5 m deep were likely disturbed by increased wave action in the lake (Home
and Goldman 1994), causing a decrease in algal biomass.

Greater chlorophyll a concentrations were overall signiﬁcantly associated with

greater densities of S. emarginata (Figure 16), although the relationship was not

49

signiﬁcant in water less than 1 m deep. This relationship supported the original
hypothesis and indicated that S. emarginata may be inﬂuenced by algal biomass, which is
a food source for the snail (Kesler et al. 1986). Densities of S. emarginata were highly
correlated with chlorophyll a concentrations in water greater than 1 m deep (Figure 16),
implying that algae may be increasingly more important for the blue snail as water depth
increases.

These results have great implications for controlling swimmer’s itch in Higgins
Lake. Because increased algae biomass is associated with nitrogen and phosphorus
pollution, increases in nutrient levels may lead to increased algae in the lake (Dillon and
Rigler 1974). Thus, managers should focus on controlling pollution in Higgins Lake
because greater densities of S. emarginata are associated with a greater algal biomass.
Greater densities of the blue snail provide more intermediate snail hosts for T.
stagm'colae, the parasite that causes cercarial dermatitis.

Organic matter in the form of AF DM increased from May through August (Table
7 and Figure 17), suggesting an accumulation due to settling of organic sediments from
the water column (Home and Goldman 1994). There was no signiﬁcant difference in
organic matter concentrations between the three water depths, likely due to a relatively
small difference in water depth examined in this study.

There was a signiﬁcant negative relationship between density of blue snails and
organic matter on the lake substrate (Figure 18), refuting the hypothesis that greater
densities of S. emarginata would be associated with higher levels of organic matter. This
relationship was unexpected because S. emarginata utilize detritus as a staple in their

diets (Barnese et al. 1990; Bovbjerg 1968; and Lodge 1986). The negative relationship

50

between snail density and organic matter could indicate that snail grazing pressure is
great enough to reduce organic matter biomass at high snail densities. The negative
relationship could also be explained by the fact that blue snails are often found on wave
swept beaches (Baker 1928) where, because of disturbances caused by waves, organic
matter does not accumulate (Horne and Goldman 1994). This hypothesis is supported by
the fact that S. emarginata were never found in depths greater than 9 m while SCUBA
diving, where the lake substrate is dominated by ﬁne organic matter (Limno-Tech, Inc.

1992)

51

CHAPTER THREE

MOVEMENT or STA GNICOLA EMARGHVA TA (SOWERBY)

Introduction

Many groups of animals undergo regular movements within their environment.
Salmon migrate between saltwater and freshwater to mature and complete reproduction
(Moyle and Cech 2000), phytoplankton regularly move vertically through the water
column to avoid predators and to absorb light for photosynthesis (Home and Goldman
1994), zooplankton move to collect prey and avoid predation (Dini and Carpenter 1991),
and Chaoborus midges migrate to obtain food resources (Roth 1968). These migrations
are inﬂuenced by a large number of variables, including food acquisition, predator
avoidance, and physiochemical changes. Regular movements have been documented in
snail populations, although data presented in these studies were not quantitative and were
based on anecdotal evidence (Cheatum 1934; Brackett 1940). These studies also did not
focus on S. emarginata but rather on several other species of snails in the same family.
Other studies shed doubt on movements in snail populations (Morrison 1932).

Cheatum (1934) concluded that snails move during spring and autumn in Douglas
Lake, Michigan. He attributed movements towards deeper water in autumn to avoidance
of hazards in the littoral zone as water temperatures decrease and migration to shallow
water in spring to the snail seeking food resources. These conclusions were drawn from
several studies completed during winter where Cheatum (1934) ﬁlled aquaria with water
at room temperature and placed several species of snails in the tanks. He then measured

water temperature and placed the aquaria on a window ledge. After two days, aquaria

52

were taken from the window ledge and water temperature was recorded. He noticed that a
majority of snails had moved to the bottom of the tank, which corresponded with a
decrease in water temperature. Cheatum (1934) also stated that when he observed snails
in the lake, snails were facing towards the center of the lake as water temperatures
decreased. When water temperatures began to increase, snails oriented themselves
towards the shoreline. Cheatum (1 93 4) attributed this regulated movement to respiration
requirements of snails and to water temperature changes.
Objectives

Because of the importance of S. emarginata as a host of T. stagm'colae and its
implications for better understanding cercarial dermatitis, the speciﬁc objective of this
study was to document movement of S. emarginata in Higgins Lake. Based on the
literature, I hypothesized that S. emarginata would move towards shallow water in spring

and deep water in autumn.

Methods

To determine movement patterns of S. emarginata, a circular area was established
in water approximately 0.50 m deep in Higgins Lake near the Earl Avenue road end,
located along the northeast shoreline of the lake (Figure 1). The substrate of this area,
dominated by sand, cobbles, and small numbers of macrophytes, is typical of other areas
in the lake and is characterized by a low angle of repose. Although not directly measured,
this area appeared to have S. emarginata densities that were typical of other areas in the

lake (Chapter 1).

53

To construct the circle, a wooden broomstick was colored with surveyor’s paint
and driven into the substrate approximately 50 m from the shoreline. A metric tape rule,
metal stakes, and brightly colored masonry line were then used to establish a 0.5 m radius
circle around the broom pole. Four quadrants were constructed so that one quadth faced
the shoreline, one faced towards deep water, and the other two quadrants faced in a

direction parallel to the shoreline (Figure 19). To ensure an adequate size range of snails,

QUADRANT 4

QUADRANT 1

 

Water

 

Figure 19. Experimental design of S. emarginata movement study showing quadrant
relationship to the shoreline. Center circle (0.5 m radius) used for release of snails.
Movements recorded when snails moved out of center circle (distance greater than 1.0 m
from center plot).

54

S. emarginata were randomly collected ﬁom several areas of Higgins Lake and placed
into size classes based on shell length: class three (10.0 — 14.9 mm), class four (15.0 -
19.9 mm), and class ﬁve (20.0 — 24.9 mm). Thirty snails per size class were painted with
a different color of Testor’s paint (90 total snails) and placed in a small volume of water
until the paint had dried.

All S. emarginata were then placed in a bucket and transported to the lake where
they were randomly released within 0.5 m of the center of the circle. Every 24 i 2 hours
after this point, a metric tape rule was anchored to the broomstick with a nail so that
distance the snails had traveled could be measured by category: in the center circle, 1 — 2
m from center, 2 — 3 m from center, 3 — 4 m ﬁ'om center, 4 — 5 m from center, 5 — 6 m
from center, 6 — 7 m from center, 7 — 8 m from center, 8 — 9 m from center, 9 — 10 m
from center, and > 10 m from the center circle. The number of snails belonging to each
size class was recorded for each quadrant to determine directional movement. This
procedure was completed for a trial running from June 9 through June 23 and for another

trial running from July 21 through August 4.

Results

The number of snails in the center circle decreased through both studies (Tables 8
and 9). Snails were greater than 10 m from the center circle during the last seven days of
the ﬁrst trial and no snails were greater than 10 m from the center at any time during the
second trial, so S. emarginata in the ﬁrst trial traveled greater distances than in the
second trial (Tables 8 and 9). At the end of the ﬁrst study, there were more snails 2 to 4 m

from the center area than in the other remaining categories. This pattern was not observed

55

 

 

 

o v o a m a 0 ~ m _ m ~ 5 w ooucz—r mN
o v o w o a o _ N g 2 w w oougnuNN
m o m m b w 2 o— 2 2 w OCASHAN
m v v m v m 2 2 C m— _ H ecu—STAN
m m m m m v N— N~ : E N~ 27.57%
N v n v v m 2 2 3 2 2 oo.=:—.-w_
N v w v v m a a h _ 3 v g COLE—(h ~
N h m v o h o_ C 2 : GOA—363
~ N v _ v m 2 m— VN "N 00.5.72
N ~ 9 N w o _ a _ 0N VN 0015:. 1v ~
m _ m m h 2 ON cm ASA—=72
N _ v m h h _ m _ m m 0019:. -N _
_ N m 2 3 cm ooénné ~
m o m on 09572
ca ooéa—rm
Ec—A Ec—ua Emu» Earn Shiv Swim Emrv Evin EnuN ENu— 53:00 3:—
62 62 .92 62 dz .92 62 62 .oz 62 5 .92

 

 

Table 8. Total S. emarginata in each category for trial 1 of the movement study
conducted in Higgins Lake, June 9 — 23, 2000.

56

 

 

 

N N 2 N m 8 2 «N 2 m2 8-2.23.4
2 N 2 m 2. m 222 4N 2 N2 8-85m
2 N N 2. m m 2 MN 22 82 8-w_2<-N
2 2 N m 2V 2 NN ON 4N 8-82.2-2
2 2 2 N m 2 2 22 NN 2m 8-2222- 2 m
2 N N 2V 2: N2 2N NN. 8-22.2-8
2 N 2 2. a N2 NN 8 8.23.8
2 2 N N 2 N2 3 823.8
2 N o 2 v2 3 823.8
2 N m 22 m2 8 8.23-8
2 N 2V 2 m2 8 8-2.22-3
N N a N2 8 8-23-3
2 2. 22 «N 8-23-8
m 2- 8 8-2=2-NN
8 8-22.2-2N
as? 2.52-22 :23 2.222-2- 55. a: an... Era an-N .52 .880 8.5
.32 az az .oz .32 .222 .oz 62 .872 .oz 2.2 .2272

 

 

Table 9. Total S. emarginata in each category for trial 2 of the movement study

conducted in Higgins Lake, July 21 - August 4, 2000.

57

in the second trial because relatively few blue snails were farther than 4 m from the
center circle (Tables 8 and 9).

Because there was a relatively large diﬁ‘erence in numbers of snails that changed
categories between both trials (Tables 8 and 9), mean daily wind speed and predominant
wind direction were retrieved from the Houghton Lake weather station
(http://www.noaa.gov), slightly south of Higgins Lake. A signiﬁcant relationship existed
(p<0.01) between the number of snails changing categories in a twenty-four hour period

and mean daily wind speed (Figure 20). Because predominant wind was from the east,

 

   
 

 

 

 

 

 

 

 

45 ,
a 0 3 ’
s 4 3* ’ , x
=8 35 3- I ’
on : v
a 1 o o l 4
g 30 j” p 9
. . I .
g 8 25 EL I ' ’ Trrall
§ '8 : I I , ' y= 1.5488x+ 1.984
a) .
go :3; 20 5. , R’=0.4344
. I
E 4
g 15 ~j~ ’
‘8 3 Tria12
r.- 10 jr _
g 1 y — 1.3048x + 3.5671
g 5 :_ R2=0.322 OJune 9-23
2 9'9 IJuly21-Aug4
OJ—v—r—r—I—I-v—r—rO-ir---%----§-r-.§-T--
0 5 10 15 20 25 30
Mean Daily Wind Speed (km/hr)

Figure 20. Mean daily wind speed versus number of S. emarginata
changing categories in a twenty-four hour period for the movement
experiment.

58

northeast, or southeast during a major portion of trial two and many snails accumulated in

the leeward quadrant (quadth three) during this same trial (Table 10), wind direction

 

Trial 1 Jun 9 - 23, 2000
Quadrant l Quadrant 2 Quadrant 3 Quadrant 4 Predominant

 

Date (East) (South) (W est) (North) Wind Direction

9-Jun-00 23 24 23 l 8 SW
lO-Jun-OO 21 24 22 19 SW
1 1-Jun-00 22 23 21 24 SE
lZ-Jun-OO 25 23 20 22 E
l3-Jun-00 27 22 20 22 E
l4-Jun-00 24 25 20 22 SW
15-Jun-00 25 23 21 21 SW
16-Jun-00 23 21 26 18 SW
l7-Jun-00 25 22 20 20 NW
18-Jun-00 25 24 17 22 S
l9-Jun-00 25 23 18 23 W
20-Jun-00 24 24 21 21 SE
21-Jun-00 28 23 21 18 W
22-Jun-00 25 25 22 19 W
23-Jun-00 25 23 22 20 S

 

 

rial 2 Jul 21- Aug 4, 2000
Quadrant 1 Quadrant 2 Quadrant 3 Quadrant 4 Predominant

 

 

Date (East) (South) (West) (North) Wind Direction
21-Jul-00 25 21 26 18 NW
22-Jul-00 20 22 30 19 NW
23-Jul-00 18 19 3 1 23 W
24-Jul-00 1 8 20 3 l 21 SE
25-Jul-00 16 20 32 20 S
26-Jul-00 16 20 3 l 20 S
27-Jul-00 13 2 1 33 20 SE
28-Jul-00 13 21 36 20 E
29-Jul-00 8 22 38 20 E
30-Jul-00 10 23 44 19 NE
3 l-Jul-OO 8 25 44 17 SE
l-Aug-OO 9 22 44 l 8 NE
2-Aug-00 8 1 8 44 21 NW
3-Aug-00 5 20 45 19 N
4-Aug-00 6 19 47 1 8 S

 

 

Table 10. Total S. emarginata in four quadrants and predominant wind direction for two
movement trials. Data include all snail size classes. Predominant wind direction data
from National Oceanic and Atmospheric Administration.

59

did inﬂuence numbers of S. emarginata in each quadrant. This was also true in trial one
because wind direction varied by date, just as did the number of snails in each quadrant
(Table 10).

There was no signiﬁcant difference in snail size between the four quadrants after
one day in either trial (t=2.57, p=0.642). There was also no signiﬁcant difference in snail
size between the four quadrants after seven days (t=3.14, p=0.784) or at the end of the
experiment (t=l .64, p=0.241) in both trials. Numbers of S. emarginata in each of four
quadrants were not signiﬁcantly different (t=1.98, p=0. 175) at the end of trial one (Table

10 and Figure 21). In trial two, the quadrant facing deeper water had signiﬁcantly more S.

Quad 1 - Toward Shore (East)
30

 
 
 
  

 

- 9 - 24 Hours After Release
--l--7 DaysAﬁerRelease
+14 Days After Release

 

 

 

Quad 4 (North)

 

Quad 2 (South)

 

Quad 3 - Toward Deep Water (West)

Figure 21. Number of S. emarginata in each of four quadrants throughout trial 1 of the
movement study.

60

Quad 1 - Toward Shore (East)
50 n—

 

- 0- 24 Hours After Release
. - - I - - 7 Days After Release
1* + 14 Days After Release

 

 

 

 

 

 

Quad4(North)-L + 551% %=%%+1'Quad2(South)

 

 

Quad 3 - Toward Deep Water (West)

Figure 22. Number of S. emarginata in each of four quadrants throughout trial 2 of the
movement study.

emarginata than did the ﬁrst quadrant (t=5.6 1, p<0.01), which was facing the shoreline
(Table 10 and Figure 22). There was no signiﬁcant difference in snail numbers between

the second and fourth quadrants during trial two (t=2.01, p=0.098).

Discussion

There was a signiﬁcant difference in the number of S. emarginata between the
ﬁrst and third quadrants in trial two (Figure 22). Blue snails showed a preference towards
the quadrant facing deep water in late July and early August, corresponding with results

of Cheatum (1934), who concluded that snails exhibit a regular movement in spring and

61

early autumn as a response to changes in dissolved oxygen levels and water temperature.
Brackett (1940) attributed snail movement to acquisition of previously unusable areas in
spring. Because of these data and patterns observed in Chapter 2, S. emarginata seem to
move towards deep water in late July/early August, consistent with my original
hypothesis.

There was no signiﬁcant difference between numbers of snails in the four
quadrants at the end of the trial conducted ﬁ'om June 9 through June 23 (Figure 21).
Cheatum (1934) concluded that blue snails move towards shallow areas of the littoral
zone when water thaws in spring. Thus, S. emarginata may move in spring, but
movement was not detected in this study because snails had already moved to shallow
water by June 9.

These data may explain results from other studies. Burton et al. (1999, 2000)
concluded that Lake Leelanau and Walloon Lake saw a reduction in S. emarginata
numbers in shallow areas during late July and early August. Assuming that movements of
snails in Higgins Lake are similar to those in Lake Leelanau and Walloon Lake,
researchers may have seen reduced snail numbers in shallow areas because of a
movement pattern in the snail population.

S. emarginata movement is inﬂuenced by wind (Table 10 and Figure 20). Laman
et al. (1984) found that when wind speeds increased, snails became unattached from the
substrate and were consequently moved by water movement. Although this correlation
was signiﬁcant at water 0.5 m deep, the relationship may not exist in deeper water.
Because the maximum wavelength that a wave can obtain is partially a ﬁrnction of water

depth (Horne and Goldman 1994), snails in water greater than 0.5 m deep may be less

62

inﬂuenced by wind because deep water inhibits the depth to which a surface wave can
penetrate the water column. Conversely, snails in water less than 0.5 m deep may be
more aﬂ‘ected by air movements.

Each of these results has implications for controlling swimmer’s itch in lakes.
Because S. emarginata exhibit regular movements during autumn and early spring, it
would be unwise for managers to apply molluscicides during this time unless they knew
exact depths at which large numbers of snails were located. Winds also affect movements
of S. emarginata. Thus, management should consider wind speed when applying copper

sulfate because snails may be blown into or out of areas treated for swimmer’s itch.

63

CHAPTER 4

GROWTH OF STA GNICOLA EAlARGINA TA (SOWERBY)

Introduction

Water temperature greatly inﬂuences behavioral and physiological traits of
aquatic organisms. Biochemical reactions in aquatic organisms are extremely sensitive to
water temperature, as all enzymes ﬁJnction best at an optimal temperature and any
deviation from this temperature may cause inefﬁciencies in the organism (Hickman and
Roberts 1994). When ambient temperatures are lowered in poikilotherms, metabolic
processes slow and organisms have less energy for normal activities (Hickman and
Roberts 1994). Increases in temperature cause proteins to become denatured, destroying
the efﬁcacy of proteins and possibly leading to negative effects on the organism. Within
an optimal temperature range, lowered ambient temperatures may negatively affect
grth and reproduction of the organism (Hickman and Roberts 1994).

Little is known about grth of S. emarginata in the natural environment. Burton
et al. (1999, 2000) and Chapter 1 have performed intensive studies in three northern
Michigan lakes and have concluded that shell lengths of S. emarginata vary over time at
the same site and between sites on the same date. The majority of S. emarginata in
Higgins Lake are between 10.0 and 24.9 mm in length (Burton et a1. 1999, 2000; Chapter
1; Chapter 2). Cort er al. (1940) concluded that S. emarginata live ﬁ'om thirteen to
sixteen months, although this conclusion was based on anecdotal evidence and was not
measured quantitatively. Brackett (1940) concluded that Lymnaeidae snails live for a

year or less. With these data and mathematical calculations, S. emarginata grow, on

64

average, from 0.63 to 1.9 mm per month. Because of this tremendous variation in grth
rate of S. emarginata, the objective of this study was to determine the degree to which
diﬂ‘ering environmental conditions alter grth of S. emarginata and to better understand
the life span of the snail. Based on the literature, I hypothesized that S. emarginata would

grow fastest in summer conditions and slowest in winter conditions.

Methods

During mid-August 1999, buckets of sand, cobbles, and a large number of S.
emarginata were collected ﬁ'om several areas around Higgins Lake (Figure 1). Once
collected, an artiﬁcial stream ﬁlled with de—chlorinated tap water chilled to water
temperatures in Higgins Lake was ﬁlled with the sand, cobbles, and snails. All items
were stored in this stream to prevent desiccation while the remaining portion of the
experiment was assembled.

Four 10-gallon aquaria were ﬁlled with nine gallons of de-chlorinated tap water
and 25 mm of sand and two equally sized cobbles taken from the artiﬁcial stream. All
four tanks were placed in one of three environmental chambers: “winter” conditions with
a temperature of 4°C and light cycle of 9 hours of light and 15 hours of darkness,

“spring” conditions with a temperature of 12°C and light cycle of 12 hours of light and

12 hours dark, and “summer” conditions with a temperature of 20°C and light cycle of 14
hours of light and 10 hours of dark. Thus, there were a total of four aquaria for each of
three treatments. Air stones were connected to an air pump and placed into each of

twelve tanks to ensure an adequate dissolved oxygen concentration throughout the study.

65

Ten S. emarginata of an approximate equal size were collected from the artiﬁcial
stream and both maximum shell width and shell length were measured with calipers.
Each S. emarginata was dried with paper towels and distinctively marked with Testor’s
paint. Snails were then weighed with a Mettler balance to quantify wet mass and placed
into one of the aquaria. This procedure was completed for all twelve tanks so that forty
snails were exposed to each of three treatments.

Throughout the sixty-day experiment, consistent water levels were maintained in
all tanks by adding de-chlorinated tap water as needed. Once sixty days had passed, shell

length, maximum shell width, and wet mass were recorded for all 120 snails.

Results

Because S. emarginata in “winter” conditions (4°C; 9 L: 15 D) had the smallest
increase in shell length and maximum shell width, these snails grew at the slowest rate.
On average, snails in “winter” conditions gained approximately 0.7 mm in shell length,
0.13 mm in maximum shell width, and 0.3 g during the experiment (Table 11). Snails in
“summer” conditions (20°C; 14 L:10 D) gained approximately 3 mm in shell length and
0.6 mm in maximum shell width (Table 11). S. emarginata exposed to “spring”
conditions (12°C; 12 L:12 D) grew slower than in “summer” conditions and faster than
“winter” conditions in all three grth measurements (Table 11). Both “spring” and
“summer” conditions had, on average, a smaller increase in wet mass than in the “winter
treatment” (Figure 23).

There was a signiﬁcant difference (F=4.20, p<0.01) in maximum shell length

changes between “summer” and the other two conditions after the sixty-day experiment

66

(Figure 24). Maximum shell width change was signiﬁcantly greater (F=7.36, p<0.01) in

“summer” conditions (Figure 25) and wet mass changes were signiﬁcantly greater

(F =4. 1 1, p<0.01) in “winter” conditions than in the other two treatments (Figure 23).

 

 

 

 

 

Mean Change in Mean Change in Mean Change in
Length (mm) SE Width (mmL SE Wet Mass (g) SE
Winter Conditions
(4°C; 9 hr. light: 15 hr. dark)
Tank 1 0.22 0.07 0.02 0.01 0.40 0.01
Tank 2 0.71 0.25 0.03 0.01 0.50 0.01
Tank 3 0.16 0.08 0.10 0.04 0.30 0.04
Tank 4 1.67 0.73 0.38 0.09 0.10 0.01
Spring Conditions
(12°C; 12 hr. light: 12 hr. dark)
Tank 1 0.85 0.02 0.53 0.08 0.27 0.01
Tank 2 0.46 0.01 0.08 0.01 0.04 0.01
Tank 3 0.28 0.01 0.11 0.01 0.02 0.01
Tank 4 0.47 0.01 0.10 0.01 0.03 0.01
Summer Conditions
(20°C; 14 hr. light: 10 hr. dark)
Tank 1 3.03 0.11 0.72 0.14 0.08 0.01
Tank 2 4.10 0.12 0.66 0.20 0.05 0.01
Tank 3 1.67 0.07 0.39 0.12 0.05 0.01
Tank 4 2.21 0.07 0.64 0.14 0.00 0.00

 

Table 11. Mean change in S. emarginata size in four aquaria at winter, spring, and
summer temperature and light conditions.

67

 

A

{G
2
ES

3 0.15

Shell Length (mm)

 

0.45 .
0,40 ElWinter Conditions - 4°C; 9L:15D

‘ I Spring Conditions - 12°C; 12L: 12D
Summer Conditions - 20°C; 14L: 10D

 

0.35
0.30
0.25
0.20

 

 

 

 

0.10
0.05
0.00 :

 

 

 

 

 

Treatment

Figure 23. Mean difference between initial and ﬁnal measurements in wet
mass of S. emarginata for three treatments. Bars equal standard errors.

 

 
  
 
  

 

l

D Winter Conditions - 4°C; 9L: 15D
I Spring Conditions - 12°C; 12L: 12D
ﬂ Summer Conditions - 20°C; 14L210D

 

 

0.5 §~

 

 

 

0.0 ‘

 

Treatment

Figure 24. Mean difference between initial and ﬁnal measurements in shell
length of S. emarginata for three treatments. Bars equal standard errors.

68

 

 

   

 

 

 

 

 

0.8
D Winter Conditions - 4°C; 9L:15D
A 0.7 ‘
E I Spring Conditions - 12°C; 12L212D
E 0'6 _ Summer Conditions - 20°C; 14L210D
1:
g 0.5 -
=
,2 0.4 -
U)
S 0.3 :
0.1 -
0.0
Treatment
Figure 25. Mean difference between initial and ﬁnal
measurements in maximum shell width of S. emarginata for
three treatments. Bars equal standard errors.
Discussion

Consistent with the hypothesis, there were signiﬁcant differences between the
three treatments in all three measured variables (Figures 23, 24, and 25 and Table 11). S.
emarginata in “summer” conditions grew longer and wider at the fastest rate and snails
exposed to “winter” conditions grew at the slowest rate, both likely a result of water
temperature differences causing changes in metabolic rates of the snails (Hickman and
Roberts 1994).

Snails in “winter” conditions added signiﬁcantly more mass than did S.
emarginata in the other two conditions (Figure 23). This result may be due to food
quality in the tanks because the algal community in “winter” were likely different than in

the other two conditions due to differences in light and water temperature (Home and

69

Goldman 1994) or to snails putting most of their energy into increases in weight rather
than to increases in shell length.

Based on mean grth rates from this experiment, S. emarginata would add
approximately 8 mm year'1 in shell length. Because the most common size of S.
emarginata found in Higgins Lake had a shell length between 10 and 14.9 mm long
(Chapters 1 and 2; Burton et al. 1999, 2000), one could conclude that blue snails live
slightly over one year. However, one must be cautious in extrapolating results ﬁ'om
laboratory studies to ﬁeld conditions. It is likely that food quality in laboratory aquaria
were different than in the lake because of use of artiﬁcial lights, less water movement
than would occur on the wave-swept lake shore, and greater probability of nutrient
limitation in laboratory aquaria. Even so, grth rates measured in the laboratory do
support other estimates (Cort el al. 1940; Brackett 1940) that blue snails live slightly
more than one year in lakes.

Based on these data and on anecdotal evidence gained from ﬁeld studies, it
appears that adult S. emarginata reproduce and deposit eggs on hard substrates
throughout June. Eggs hatch throughout June and early July. By the early portion of
August, newly hatched S. emarginata are approximately 4.0 to 10.0 mm long. In late
September and early October, large numbers of adult snail shells are seen on the
shoreline of the lake. Development of young snails continues during this same time
period and throughout winter and spring months. In June, adult snails reproduce and the
life cycle is initiated once again. Thus, it appears that S. emarginata live from late June to

early July through September and October of the following year.

70

SUMMARY AND CONCLUSIONS

The overall goal of these studies was to better understand the biology of
Stagm'cola emarginata because of its role as a major host of T richobilharzia stagm’colae,
one of the parasites that causes cercarial dermatitis (swimmer’s itch) in northern
Michigan lakes. From May through August of 1999 and 2000, studies were performed in
Higgins Lake, Michigan to determine variation in the percentage of shedding snails over
time and space, to identify limnological variables that inﬂuence abundance and
distribution of S. emarginata, and to determine whether S. emarginata exhibit regular
movement during portions of the year. Grth of snails in several different
environmental conditions was also examined in the laboratory.

The overall percentage of S. emarginata shedding T. stagnicolae cercariae
according to the light box assay was 0.69% over both summers, although prevalence
varied spatially and temporally. The greatest prevalence occurred during July and August
of both years, and the Columbine Lane site had signiﬁcantly more shedding snails
according to the light box assay than did the other three sites. Larger snails were more
likely to be shedding T. stagnicolae cercariae, and snails less than 10.0 mm long never
shed cercariae.

At the Birch Lodge site, snails with a shell length between 10.0 and 14.9 mm
were most common, and snails were largest in water greater than 1 m deep. The density
of S. emarginata varied by water depth from May through August and the greatest
density of blue snails was in water between 0.51 and 1.0 m deep. The lowest mean

density from May through August was in water less than 0.5 m deep. S. emarginata were

71

most dense between early June and early July in water less than 1 m deep and in late July
in water greater than 1 m deep, indicating a movement towards deep water.

Water temperature increased from May to early July and was not signiﬁcantly
correlated with density of S. emarginata. Dissolved oxygen concentrations were also not
correlated with density of S. emarginata and tended to decrease slightly ﬁom May to
early July as temperatures increased. Percent saturation of dissolved oxygen was at or
near 100% from May through August. At all three water depths, density of blue snails
was positively correlated with benthic chlorophyll a and there was a negative association
between density of S. emarginata and benthic organic matter, whose concentrations
stayed relatively constant from May through August.

Movement experiments conducted in the lake, along with density changes at three
depths from May through August, documented the fact that S. emarginata do move
towards water greater than 1 m deep in late July. Movement was likely initiated by a drop
in water temperature and was inﬂuenced by wind speed and direction. Diﬁ‘erent sized
snails did not prefer to move in one particular direction. Growth experiments showed that
changes in both shell length and maximum shell width of snails were positively
associated with increasing water temperatures and changes in wet mass were negatively
associated with increasing water temperatures. Extrapolations from the growth
experiment indicate that S. emarginata live slightly over one year.

These results have implications for controlling swimmer’s itch in Higgins Lake.
Because most snails were shedding cercariae in July and early August, this should be a
time to focus on controlling the disease. Water greater than 1 m deep had signiﬁcantly

larger snails than shallower water and larger snails were more likely to shed T.

72

stagm'colae cercariae, so water greater than 1 m deep may be a signiﬁcant source of
cercariae. Densities of S. emarginata varied over time and space, so managers should
target maximum densities of snails for molluscicide control. Greater densities of snails
were associated with higher concentrations of algae, indicating that nutrient pollution
should be controlled to inhibit increases in algal biomass. By doing so, managers may be

able to alleviate the swimmer’s itch problem in Higgins Lake to a certain degree.

73

APPENDICES

74

 

 

Appendix A - 1. Snail density data collected over 2000 at 0.0 - 0.5 m water depth.

 

o o Nm 2 2e 8 2. x 2. 823.8
22 2 N: N. NN2 :2 N x N 823.8
o 222 N2: 2 N2 2 E o x o 823.8
22 2 Na N N e 2. x 2. 823.2
22 m 9. o 2m 8 2. x 2. 823.2
22 o 8 o 82 E m .2 m 8-2222-N2
o N 82 m o: E m .2 m 8-22.2-2
o N N22 2. 8 a m x N 8-82-8
o 2. MN v 222 e N. .2 N 8-82-8
o 2 Na 2.N 222.2 a o x 22 8.5.2.2
22 N2 2.2: 2 2.2.2 225.6 8.82-2
o N mm 2.2 2N :2 N x N 8.5.2
o 2.2 22.22 mm NNN 2: 2. x 2. 8.3.2
o N 8 NN N2 2 a o 22 22 8.32.8
c N 2.2. 8 m2 2 E o x o 8-32-2
o N N2 N2. No 8 m x N 842222-:
22 m N 2». 2e :2 m x m 8222-:
o 222 8 ON 82 E n x m 84222-2
22 N2 N2 NN 22.2 E m x m. 8-32-2

o22.225 m 3.6 22.2.5 2. 3.26 2222225 N 3.20 £25 N 2820 2222.25 .oz 85 3.5

85 62 32m 62 82m .oz 85 .oz 23....- 222E235

 

 

75

 

 

 

Appendix A - 1. (cont'd)

22 NN 2.2 2 2.: :2 222 x 222 8.223;
22 NN 2.2: o 22 e 2: x 222 8N3;
o N no N 8 e o .2 o 8-23-8
o m 222 2 No a o 22 o 8-22278
o N oN2 o 82 a N x N 8-22.2-8
madam m mud—U n—maﬂm v and—U maﬁa—m m and—U maﬁa—m N aha—U m—maﬂm .cz 05m unan—
on2m 52 82m 52 32m 52 32m .oz 2.82. 22.22.222.225

 

l|\‘l

 

76

 

 

 

Appendix A - 2. Snail density data collected over 2000 at 0.51 - 1.0 m water depth.

22 o 8 22 8 E2. .2 2222. 8-22.2-2
o N NN N 82 E 2. N 2. 8-22.2-2 2
22 2 2.22 2 2.22 2222.222. 8.23.:
o N N22 2: NN2 :2 N N N 8.2::
22 N2 NN N N22 8 N .2 N 8.2::
o N N2 N2 NN2 a 2. N 2. 8-22222-NN
o v E N2 N2 8 2. N 2. 8.82-8
o N NN2 22 92 222 2. N 2. 8-222;-NN
o N N2 2 N2 NN2 a 2. N 2. 8-22227NN
o 2 2.N ON N: E N N N 8-82-2
o 2. NN NN N2 2 E N N N 8.222272
22 N N2. No N2 2 E N N N 8.83
o 2 24 NN No E N N N 8-82-N
o 22 NN N2.2 2N2 :2 2. N 2. 8-32-3
o N 8 NN2 2.2 E 2. N 2. 8.8.2.3
o N 2.N 82 NN2 :2 N x N 8-32.2-8
22 N2 8 No No2 :2 N x N 8NN2>2NN

madam m mud—U nag—m v nun—U nan—um M 330 n—maﬂm N nun—U nan—um .62 ONE 36A—

oN2N .oz 85 .oz 35 .oz 85 .oz .58. 2.2.2.2255

 

77

 

 

 

Appendix A - 2. (cont'd)

 

22 N2 NN2 2. 2.N2 e N .2 N 8.223;
22 N 2N2 N 2N2 2: N .2 N 8-8.2.22
22 N2 8 N NN2 :2 o N o 8-2_22.-NN
o N 2: 2 N NN2 :2 o .2 o 8-2222-NN
o N 82 N N: e N .2 N 8-22.2-2
madam m and-U m_maﬂm v ans—U mﬂaﬂm m 320 madam N :30 max—um .cz 05m unﬂa—
oN2N .oz 35 52 32m 62 35 .oz 232; 25:22:20

 

 

78

 

 

Appendix A - 3. Snail density data collected over 2000 at >1 m water depth.

 

o 2.222 82 2 SN 2252 N 222 8.3-2.2
22 N2 NN o 22. 222 N N N 8-35.2
o 82 NNN o 5. E N N N 8-22.2-8
N NN2 N8 22 NN2. :2 N N N 8-2.22-8
22 NN 2N2 2. NN2 :2 o N o 8-82-8
22 NN NN2 o NN2 e o N o 8-82-8
22 N2 222 N NN :2 N N N 8-82-2N
22 N2. 2.N2 N ON E 2: x 222 8.232. N
o 2: NN N2 2222 2252.22: 8-83
22 N2 2222 N2 22.2 2252.82 8-82-22
22 N 2.2 22. N2.N :2 2: N 2: 8-32-8
22 N2 NN NN N2: E 2: N 2: 8-32-8
madam m aux—U madam v nun—U madam m nun—U madam N 366 madam .02 ONE cana—
oN2N .oz :5 .oz 35 52 32m .2272 25.; 2.2.2.2325

 

 

79

Appendix A - 4. Limnological data collected over 2000 at 0.0 - 0.5 m water depth.

 

 

 

 

Dissolved Chlorophyll a
Date Oxygn (mg/L) (mg m' ) AFDM (mgm'z)

12-May-00 9.0 6.01 197
8.23 328

7.12 289

12-May-00 9.6 6. 18 295
7.12 199

6.46 297

17-May-00 10.1 5.56 297
5.99 325

4.87 236

17-May-00 10.2 3.73 414
4.01 420

4.89 302

25-May-00 10.5 3.67 412
4.61 223

5.33 328

25-May-00 10.2 3.88 345
4.77 312

4.20 419

13-Jun-00 9.] 11.12 210
10.79 184

12.44 249

13-Jun-00 8.9 13.49 203
10.14 328

9.50 236

15-Jun-00 9.2 5.66 299
6.13 256

7.66 256

15-Jun-00 9.4 5.95 197
5.56 363

6.72 266

28-Jun-00 10.6 6.78 214
9.45 236

10.88 217

28-Jun-00 10.5 8.00 423
9.45 322

5.90 371

 

80

Appendix A - 4. (cont'd)

 

 

 

Dissolved Chloroph 11 a AFDM
Date Oxygen (mg/L) (mg m' ) (mg m")
12-Jul-00 9.5 8.19 311
9.19 298
10.45 355
12-Jul—00 9.4 l 1.49 319
12.19 362
13.12 299
l9-Jul-00 8.9 12.49 305
12.45 217
10.79 295
19-Jul-00 8.7 1 1.49 311
12.49 229
12.78 268
20—Jul-00 8.8 13.49 229
14.19 399
15.13 355
20-Jul-00 8.7 9.08 402
10.46 312
9.17 399
26—Jul-00 8.9 12.50 375
12.46 299
13.45 455
26-Jul-00 8.9 9.80 501
12.79 411
10.79 359
28-Jul-00 8.8 14.79 322
13.49 401
14.19 322
28-Jul-00 8.9 14.12 311
14.76 455
13.15 384
9-Aug-00 8.7 12.19 315
13.14 236
10.13 197
9-Aug-00 8.5 10.45 554
11.79 442
10.79 197

 

81

 

Appendix A - 5. Limnological data collected over 2000 at 0.51 -1.0 m water depth.

 

 

Dissolved Chlorophyll a AFDM

Date Oxygen (mg/I) (mg m' ) (mg ml)
22-May-00 9.9 6.85 341
5.96 302
6.00 295
22-May-00 9.9 5.98 453
6.75 105
8.49 295
25-May-00 10.5 4.72 328
1.65 519
1.89 289
25-May-00 10.3 2.49 198
2.00 221
2.99 277
8-Jun-00 9.8 5.91 276
7.57 249
5.73 414
8-Jun-00 9.9 7.99 164
8.46 256
8.76 264
15-Jun—00 9. 1 18.23 249
19.99 236
22.67 289
15-Jun-00 9.1 18.50 256
16.20 247
15.79 355
22-Jun-00 9.5 5.46 249
6.79 374
7.49 210
22-Jun-00 9.2 7.49 302
8.46 210
7.99 276
28-Jun-00 10.6 25.40 394
20.49 256
21.49 276
28-Jun-00 10.6 18.49 452
19.46 ' 352
18.47 244

 

 

 

82

Appendix A - 5. (cont'd)

 

 

 

Dissolved Chlorophyll a AFDM
Date Oxyan M (mg m') (mgm’z)
7-Jul-00 9.4 18.50 282
20.49 124
19.46 190
7-Jul-00 9.5 17.13 155
18.19 244
19.47 244
11-Jul-00 9.2 16.80 309
15.49 445
18.18 399
11-Jul-00 9.2 13.91 344
17.82 257
15.19 401
19-Jul-00 8. 7 15 .46 304
12.50 322
16.49 388
19-Jul-00 8.7 10.49 355
12.79 455
12.49 397
28-Jul-00 8.7 14.94 282
10.79 401
12.46 559
28-Jul-00 8.7 12.99 377
13.99 502
11.72 511
9-Aug-00 8.4 10.74 445
9.79 335
11.44 322
9-Aug-00 8.4 10.99 544
9.45 446
9.16 499

 

83

 

Appendix A - 6. Limnological data collected over 2000 at > 1 m water depth.

 

 

Dissolved Chlorophyll a AFDM
Date Oxygen (mg/l.) (mg m' @g ml)

26-May-00 10.5 2.45 361
3.45 249

3.15 328

26-May-00 10.2 1.91 236
4.18 335

3.13 177

9-Jun-00 9.2 1.57 368
3.98 460

3.76 295

9-Jun-00 9. 1 3 .78 344
2.47 122

3.00 344

21-Jun-00 8.9 4.12 401
3.49 276

4.99 249

21-Jun-00 8.7 4.98 184
5.50 230

5.01 440

29-Jun-00 9.4 6. 12 243
7.13 256

7.12 266

29-Jun-00 9.5 6.18 243
6.19 92

6.99 276

26—Jul-00 8.6 10. 13 177
9.45 212

8.50 217

26-Jul-00 8.6 10.92 199
8.45 253

7.99 288

l-Aug-OO 8.7 5.13 355
6.47 312

5.79 246

l-Aug-OO 8.6 8.13 311
6.45 459

5.99 377

 

 

 

s4

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