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LIBRAé

Michigan State
Univctsit)’

AN INVESTIGATION OF SOME ASPECTS OF
THE INTRA-LAKE DISTRIBUTION OF
CHYDORUS SPHAERICUS (CLADOCERA:CHYDORIDAE)

BY

Donald Joseph Wagner

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Zoology

1978

ABSTRACT
AN INVESTIGATION OF SOME ASPECTS OF

THE INTRA-LAKE DISTRIBUTION OF
CHYDORUS SPHAERICUS (CLADOCERA:CHYDORIDAE)

BY

Donald Joseph Wagner

Chydorus sphaericus was associated in very high densities with

 

the macrophyte_Myrigphyllum heterophyllum, and very low densities with
Scirpus subterminalis. Standardizing Chydorus numbers by surface areas
(calculated from marl-free dry weights using regressions I determined)
of the plants showed that the differential distribution is not simply
an artifact of variation in surface area between macrophyte species.

Tests using plastic plants indicated that the Scirpus shape is favored

over the Myriophyllum shape. The effect of new surface area (which

 

myriophyllum continually generates while Scirpus does not) was tested

by comparing cleaned plants to naturally marled controls; little effect
was seen. Finally, observations of relatively predator-free aquaria
indicated that predation may play a secondary role to that of the macro-
phyte itself. The alternatives of variation in food resources, chemical

exudates, and predation as potential causes of the observed distribution

are discussed.

ACKNOWLEDGMENTS

I would like to thank the members of my guidance committee,
D. J. Hall, R. W. Hill, E. E. Werner, and K. W. Cummins, for the
many stimulating experiences they have provided me during my
graduate career. I am very grateful to those who helped me in the
field: Leni Wilsmann, Dennis Laughlin, John Dacey, Joe Arruda, and
especially my diving buddy Nancy Winters Werner. These and many
others, in particular Larry Crowder and Don Howard, have been most
generous with their comments on this work. Special thanks goes to
Dr. R. G. Wetzel for the use of his boat, grappling hook, and map
of Lawrence Lake, and for his discussions of many questions I had
about periphyton.

Finally, I want to express my gratitude to my wife, Stellie,
for the criticism, encouragement, support, and inspiration she so

lovingly provided when each was needed.

ii

TABLE OF CONTENTS

LIST OF TABLES. . . . . . . . . . . . . . . . . .
LIST OF FIGURES . . . . . . . . . . . . . . . . .
INTRODUCTION. . . . . . . . . . . . . . . . . . .
STUDY SITE. . . . . . . . . . . . . . . . . . . .
GENERAL METHODS . . . . . . . . . . . . . . . . .
EXPERIMENTS, WITH RATIONALE, TECHNIQUES, RESULTS,
AND INTERPRETATION . . . . . . . . . . . . .
Surface Area . . . . . . . . . . . . . . . .
Substrate Morphology . . . . . . . . . . . .
The Effect of New Surface. . . . . . . . .
Aquaria Observations . . . . . . . . . . . .
GENERAL DISCUSSION. . . . . . . . . . . . . . . .
LIST OF REFERENCES. . . . . . . . . . . . . . . .
APPENDIX A: DISTRIBUTION OF CHYDORIDS IN LAWRENCE
MICHIGAN, SUMMER 1976 . . . . . . . . . . .

APPENDIX B: DATA FOR COMPUTATION OF r, rm, AND rs

iii

PAGE

iv

10

12
12
17
21
26
33

47

53

56

TABLE

B-lo

LIST OF TABLES
PAGE

Surface area (Y) to marl-free dry weight (X)
regressions for several macrophytes. . . . . . . . . . . . 15

Mean density (#lmz ) of C_yh dorus sphaericus
on various macrophytes. . . . . . . . . . . . . . . . . . . l6

 

Comparison of real and artificial substrates
using Spearman's Rank Correlation Coefficient (rs). . . . . 20

Analysis of observed and expected (1,2,3) rank orders

of macrophytes by density (#lfmz ) of thdoruss sphaericus

using Spearman' 8 Rank Correlation Coefficient. Critical
value (n=4;a=.05)=1. . . . . . . . . . . . . . . . . . . . 24

Comparison of mean densities (#/ m2) of Chzdorus
sphaericus on Myrigphyllum (M) and Scirpus (S) plants

 

in relatively predator-free aquaria, using the Mann-
mlitnEY U-Test O I O I O O O O O O O O I I O O O O I O O O O 29

Rates of population increase (r) and rates of increase
of density of Chydorus sphaericus on Myriophyllum (rm )
and on Scirpus (rS ) plants in several aquaria. . . . . . . 31

Egg ratio (E = # eggs/mature female) for Chydorus
sphaericus in two plant beds during July and August, 1977. 35

Mean proportions of the eleven most abundant chydorids
of species 1 associated with macrophyte species j
(x1j /§x ij’ wherex xij = mean number of animals of

species idlg plant species 1).. . . . . . . . . . . . . . 55

Relevant data necessary for computation of growth rates
of population sizes and densities of Chydorus sphaericus

on Myriophyllum (M) and Scirpus (S) plants in several
aquaria. . . . . . . . . . . . . . . . . . . . . . . . . . 56

iv

FIGURE

1.

2.

LIST OF FIGURES
PAGE
Bathymetric map of Lawrence Lake, Michigan. . . . . . . . . 7
Bathymetric map of the southwestern corner of Lawrence
Lake. Crosshatching indicates the approximate distribution

of Scirpus, while stippling indicates the approximate
distribution of Myriophyllum. Contour intervals in meters. 8

 

Density (#lm2 plant surface area) of Chydorus sphaericus

and invertebrate predators in two plant beds during the

summer of 1977. Symbols represent individual observations

and lines connect means. . . . . . . . . . . . . . . . . . 40

Cumulative size-frequency distributions of Chydorus
sphaericus in Myriophyllum (--) and Scirpus (---) beds.

 

 

A. For the period 19 July - 2 August. B. For the period
19 August - 6 September. Sizes to the left of the heavy
vertical lines are juveniles. . . . . . . . . . . . . . . . 44

INTRODUCTION

The Chydoridae are nearly microscopic crustaceans inhabiting the
littoral and benthic regions of lakes, ponds, and sluggish streams
worldwide. It is a large family, consisting of a total of about 185
species ranging in length as adults from about 0.28 to 6.0 mm, with
most species smaller than 1.5 mm. In general, they are adapted for
moving over and feeding on submerged surfaces, such as macrophytes and
rocks, or in softer sediments. Fryer (1963, 1968) and Smirnov (1966,
1968, 1971 a & b) suggest that various species are adapted to their
modes of life with remarkably subtle morphological adaptations.

The earliest taxonomic work is attributed to O. F. Mfiller, who

named, among others, Chydorus sphaericus in 1785 (Smirnov, 1971b). Yet

 

even in this field, there is much ongoing research, both in description
of species (Frey 1965, Megard 1967, Flfissner and Frey 1970, Flassner
and Kraus 1977) and in allocation of species to genera (Fryer 1968,
Smirnov 1971b, Frey 1976).

Most ecological work with chydorids has been descriptive in nature,
for example, species lists of various locales (e.g., Smyly 1958,
Smirnov 1963, Anderson et a1. 1977). In contrast, there have recently
been attempts to make broad generalizations about distribution and
control of diversity and abundance (DeCosta 1964, Straskraba 1965,
Whiteside and Harmsworth 1967, Harmsworth and Whiteside 1968, Whiteside
1970). Also, paleolimnologists have begun to make use of techniques

1

2

pioneered by Frey (1958) based on the fact that chydorid carapaces

are preserved very nicely in lake sediments (Megard 1964; Goulden 1964,
1966; Whiteside 1970). Unfortunately, interpretation has been hampered
by the lack of detailed ecological information on individual species
(Frey 1969).

In the last 15 years, some workers have begun to tackle the prob-
lems of quantitative sampling, producing studies of two general types.
In the first of these,cnueor more lakes in a region have been examined
in the attempt to define assemblages of species associated with fairly
broad habitat subdivisions, i.e., limnetic, benthic, or plant surfaces
(Fléssner 1964, Pennak 1966, Quade 1969, Lang 1970). Others have ex-
amined population trends and dynamics (Goulden 1971; Keen 1973, 1976;
Whiteside 1974).

There has been very little published on the affinity of chydorids
for various macrophytes, although some authors, such as Quade (1969),
do report applicable data. However, Quade's data are such that, while
they indicate dominant species on each plant, comparisons between
macrophyte species are not easily made. This is because his results
are expressed as proportions of total chydorid fauna collected on all
plants of a given species with no attempt to standardize for the amount
of plant material collected. I have chosen to explore this aspect of
the ecology of the chydorids more closely.

In a preliminary study on Lawrence Lake, Barry County, Michigan,

I developed a method for sampling individual macrophytes (see General
Methods) and obtained data on the distribution of many chydorid species
(Appendix A). Many species were found in greatest density (numbers per

gram plant tissue) on_Myriophy11um heterophyllum, some seemed to prefer

other macrophytes, and most were present only in low numbers, if at all,

on Scirpus subterminalis or the lily pads Nuphar variegeta and Nymphaea

 

 

odorata. Chydorus sphaericus was chosen for more detailed study because

 

 

it is the most abundant in the lake (see also Keen 1973, 1976) and has
the most consistent distribution among the macrophytes over the entire
summer.

Chydorus sphaericus (O. F. Mfiller, 1785) is the most widely dis-

 

tributed chydorid in the world (Smirnov 1971b). It is a small crusta-
cean (adult size about 0.5 mm maximum), with wide tolerance limits in
terms of oxygen concentration, temperature, and pH (Fryer 1968). The

morphology of g, sphaericus has been detailed by Fryer (1968) who showed

 

that it is capable of utilizing a variety of substrates provided that
they are not perfectly smooth and provide purchase for the first pair
of legs. It is not found in the benthos of Lawrence Lake (personal
observation), nor in the plankton, although it may be found in the
limnetic zone of some lakes under conditions of blue green algal blooms
which supply a substrate (Fryer 1968). In 1976, I found it in very low

densities on Scirpus subterminalis Torr., a plant that accounts for

 

70-80% of the total macrophyte biomass in Lawrence Lake (Rich et al.
1971). In contrast, the highest densities of Chydorus were always

supported by Myriophyllum heterophyllum Michx. (similar findings were

 

reported by Fléssner 1964, Pennak 1966), a minor constituent of the
macrophyte assemblage of the lake (Rich et a1. 1971). Myriophyllum
and Scirpus are also very dissimilar stucturally; the former is very
complex, with whorls of finely divided leaves extending from the stem,
while the latter is grass—like in appearance.

In this study, I examine various aspects of the distributional

dichotomy of Chydorus sphaericus between these two very different

 

plants. I begin with a simple physical explanation and proceed to

more complex biological alternatives.

STUDY SITE

Lawrence Lake (Figure l) is a small (4.9 ha), oligotrophic, marl
lake 2.1 km east of Hickory Corners in southern Barry County, Michigan.
For the most part, the littoral zone is composed of a shallow marl
bench gradually sloping to about 1.5 m depth, after which the bottom
falls sharply to about 7 m. The maximum depth is 12.6 m. The dominant
macrophyte over much of the littoral zone is Scirpus subterminalis
(Rich et a1. 1971).

Studies of many facets of the limnology and biology of this lake
are available, including the history (Rich 1970), general limnology
(Wetzel et a1. 1972, Wetzel 1975), macrophytes (Wetzel 1969, Rich etal.
1971, Wetzel and Manny 1972, Hough and Wetzel 1975), periphyton (Allen
1971, wetzel and Allen 1972), fish communities (Werner et a1. 1977,
Hall and Werner 1977), and previous work on the Chydoridae (Keen 1973,
1976).

The study area (Figure 2) was located in the southwestern corner
of the natural lake basin, next to an abandoned marl excavation site
(Rich 1970). In contrast to most of the littoral zone, the bottom here
slopes gradually to a depth of about 4 meters and supports a variety
of macrophytes (see maps in Rich et a1. 1971). The Scirpus bed extends
from about 1 to 3 m depth in a C-shape around the western part of the

area. The Myriophyllum bed was in 2.5 to 4 m of water, with a small

 

band of Najas flexilis (Willd.) Rostk. & Schmidt separating it from

 

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Contour intervals

Crosshatching indicates the approximate distri-

Bathymetric map of the southwestern corner of Lawrence
bution of Scirpus, while stippling indicates the approx-

imate distribution of Myriophyllum.

Lake.
in meters.

Figure 2.

 

the Scirpus. Sampling (see next section) was restricted to plants
2.5 to 3.5 m deep. The bottom was composed of a soft mixture of marl

(mostly CaCO3) and organic sediments.

GENERAL METHODS

Individual plants (or bunches of leaves in the case of Scirpus)
were collected while diving with SCUBA. A 143 u mesh plankton net
with a one quart collecting jar attached was submerged and held near
the base of the plant. The plant stem was gently pinched to break it
and the plankton net was raised around the plant. This method excludes
animals in the sediments from the sample. The contents of the net
(i.e., the plant and associated fauna) were washed into the collecting
jar and transported to the laboratory.

The contents of each jar were emptied into a sieve made with 143 u
Nitex screening, and the plant was washed thoroughly with a stream of
water. The sieve was then submersed in carbonated water for several
minutes to anesthetize the animals; samples were preserved in 4% sucrose
formalin (Haney and Hall 1973).

The washed plants were rinsed with 0.2 N HCl to remove carbonate
deposits (Wetzel 1960) and oven dried at 100-1050C for at least 24
hours. Dry weights were determined with a microbeam balance to the
nearest 10 pg. Macrophytes were identified using Fassett (1957).

Preserved animal samples were placed in 90 mm disposable petri
dishes with a grid etched on the underside and examined at 12-50 X
with a dissecting microscope. The entire sample was examined for
Chydorus sphaericus, and often all chydorids and invertebrate predators

10

11

were noted. Chydorids were classified to species level if possible
(to genus in the case of small Along and Alonella spp.), and predators
to various levels using Edmondson (1959), Pennak (1953), and

Smirnov (1971b).

In several field experiments (see Substrate Morphology, The Effect
of New Surface), samples of experimental and control plants were
collected for analysis of periphyton. These samples were preserved
with Lugol's iodine, but unfortunately most of them later developedwflun:

appeared to be bacterial blooms, and had to be discarded.

EXPERIMENTS, WITH RATIONALE, TECHNIQUES, RESULTS, AND INTERPRETATION

Surface Area

 

The simplest way to explain the observation that more Chydorus

sphaericus are found on some macrophyte species than others is that it

 

is a surface area phenomenon. That is, the animals are evenly dis-
tributed over the available surfaces, and it is the difference in sur—
face area between plants that produces the effect. If this is so, when
the numbers of animals found on each plant are standardized by the
surface area of the plant, there should be no significant differences
in density of animals between plant species. Other studies (Pennak
1966, Lang 1970, Whiteside 1974, and others) have recognized this pos-
sibility, but because of the difficulties associated with determination
of plant surface areas (cf. Wohlschlag 1950, Mrachek 1966) these
investigators have preferred to develop sampling methods that standard-
ize the area of lake bottom sampled or restricted their studies to a
single macrophyte species.

In this study, regression lines relating marl-free dry weight
(an easily obtained quantity) to an estimate of each plant's surface
area were developed. The method used for estimating surface area was
that of Harrod and Hall (1962). Between 20 and 40 plants of each
species were collected while diving or with a grappling hook. Each
plant was rinsed in 0.2 N HCl to remove marl, rinsed in water, and
dipped in acetone. After allowing the acetone to evaporate, surface

12

13

dry weights (i.e., weight of plant with dry surface, but with hydrated
cells) were obtained. The plant was then immediately dipped in a 50%
solution of the surfactant Teepol 610R (Shell Chemical Corp., N. Y.),
shaken for 20 sec to remove the excess, and weighed again for a Teepol
wet weight. The difference between the Teepol wet weight and the
acetone dry weight is the weight of Teepol covering the plant, which is
proportional to the surface area of the plant. The Teepol was rinsed
from the plant which was then oven dried (loo—105°C for 24 hr) and
finally weighed for a marl-free dry weight. (The plastic plants used
in a later experiment were similarly treated, except that they were
air dried rather than dipped in acetone or oven dried.)

The amount of surface area covered by a given weight of Teepol
varies depending on a number of factors, including the microstructure
of the surface (i.e., presence of hairs, grooves, etc.that can trap
liquid). It was assumed in this study that such differences were
negligable, and a single proportionality constant relating weight
of Teepol to surface area was determined. Leaves of the lily pad

Nuphar variegeta Engelm., which could be cut into easily measured

 

pieces were used. These were treated as above, producing a mean
value of 23 g Teepol/m2 surface area (02 = 1.5). (Pieces of acetate
stock were used to determine the proportionality constant for the
plastic plants of 16 g Teepol,/m2 surface area (02 = 9.4).) Multi-
plication of these proportionality constants by the weight of Teepol
covering a plant gives an estimate of the total surface area of the
plant.

Regression equations of the relationship between marl-free dry

weight and surface area were calculated for several species of

14

macrophyte, and are presented in Table 1. It can be seen that in spite
of the complex technique and the many weighings and estimations in-
volved, these relationships are strongly linear. The observed results
are strong enough to show that determination of marl-free dry weight
and appropriate conversion to surface area produces accurate estimates
of the latter, at least in a relative sense, and that comparison of
numbers using these relationships are valid.

Using these regression lines, the density of Chydorus sphaericus
was determined for several macrophytes on a number of sampling dates.
These results are presented in Table 2.

While it is apparent that the rank order of the various macro-
phytes is relatively constant (with the exception of the single night
sample and the fall sample), the comparison of most interest in this

study is between Myriophyllum and Scripus, and statistical treatment

 

is limited to these. Depending on the number of samples, the densities
in Table 2 were compared using the Mann-Whitney U-Test of means (Sokal
and Rohlf 1969), or Fisher's Exact Probability Test applied to pop-
ulation medians (Bradley 1968). In all cases the density of Chydorus

sphaericus was greater on Myriophyllum than on Scirpus. It should be

 

 

mentioned that although Fisher's Exact Test gave fairly large proba-
bilities for the observed differences not being real, this value

(p = 0.17) is the lowest obtainable with the small number of samples
(2) involved. Also, all 1977 sampling dates were combined and a Mann-
Whitney U-Test (one-tailed) was performed on the pooled data. The
result is that for the entire season Myrigphyllum plants supported a
higher density of Chydorus than did the Scirpus with a < 0.001.

I conclude that through the summer and into the fall, in Lawrence

15

 

 

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17

Lake, the density of Chydorus sphaericus is greater on_Myriophyllum

 

 

plants than on Scirpus. The remainder of this study consists of a
number of attempts to explain the causes of this differential

distribution.

Substrate Morphology

 

Quade (1969) has reported some success in correlating an assemblage
of particular species of chydorids with a particular plant form (e.g.,
fine-leaved submersed, floating leaved, etc.). Fryer (1968) elucidated
the functional morphology of a number of species of chydorids and showed
that individual species are adapted to various microhabitats to differ-

ent degress. For example, he showed that Alonella exigua and especially

 

Graptoleberis testudinaria were particularly adept to clinging to the

 

underside of horizontally oriented surfaces. While he described

9, sphaericus as a generalist, I felt that the differences in density

 

of this animal on different macrophytes might be most easily explained
as being due to differential ability to cling to and feed from sub-
strates of various geometrical configurations. Specifically, the
surfaces of Scirpus plants are essentially entirely vertical in
orientation, while the finely-divided leaves of Myriophyllum extend
horizontally from the vertical stem.

Accordingly, an in situ experiment was devised to test this
hypothesis indirectly, assuming that the density differences reflect
differences in suitability between various substrates. In order to
test only for differences in the shape of the substrate, artificial
plants were used. It was assumed that differences in predatory regime,

chemical attractants or repellants, etc., would be negligable if the

18

same materials were used for construction of all artificial plants

and the entire experiment were set up in a small, uniform area. Food
supply, however, presents a potentially confounding variable. Studies
on orientation of glass slides (Castenholz 1961; reviews in Cooke 1956,
Slédeékova 1962, Wetzel 1964; and Wetzel, personal communication) have
shown that horizontally oriented surfaces support up to six times as
much material as vertically oriented ones, including much detritus and
unattached material. Presumably this relationship holds for different
kinds of natural surfaces as well, but no one seems to have studied
this. Since detritus is one food source for chydorids (see General
Discussion) this would seem to favor higher densities of Chydorus on

Myrigphyllum plants, with their many horizontal leaves. Thus, in this

 

experiment, higher densities of Chydorus on_Myriophyllum-type than
on Scirpus-type plants could be due to either the orientation of the
surface itself or to secondary effects on food supply.

Plastic aquarium plants (Penn-Plax, Inc., Gardencity, N. J.),

consisting of Myriophyllum-like leaf whorls on plastic tubing were used

 

to simulate this substrate. By cutting off the leaflets, Naiagflike
plastic plants were made. Finally, by cutting some of the plastic
tubing into thin strips, a simulated Scirpus plant was made.

Four individual plastic plants of each type were randomly placed
in a 3 X 4 grid (approximately 0.5 m between each plant) in a uniform
Chara spp.- 3152 bed at 3.5 m depth in the study area. The plants were
anchored with small pieces of brick, and small labelled pieces of
styrofoam were attached with thread to the tops to provide the proper
‘vertical orientation and for identification. The experiment was set up

cum 1 July, 1977. Allowing time for colonization of the surfaces, two

19

of each type were randomly selected and sampled on 19 July and again on
2 August. Concurrent duplicate samples of real Myriophyllum, Najas,
and Scirpus were taken for comparison.

If substrate morphology (in the sense of leaf shape and orienta-
tion) is an important determinant of the distribution of Chydorus, then
the ranking of densities of these animals on the plastic plants should
be similar to that on the analogous real plants. An expected ranking
was generated from the results of analysis of the real plants, and was
compared to the observed ranking of the plastic plants using Spearman's
Rank Correlation Coefficient (Siegel 1956). Equality or inequality
signs within rankings were determined by comparison of medians using
Fisher's Exact Test (Bradley 1968). Results are presented in Table 3.

There is no statistically significant correlation between the rank
orders of real and plastic plants, i.e., the animals are treating the
real and plastic substrates differently. Since plastic Scirpus plants
supported a higher density of Chydorus than plastic Myriophyllum, it
seems reasonable to conclude that the shape of a natural Myriophyllum
plant is not the feature responsible for the regularly observedchydorid
density. These results are also contrary to the expected result if
quantity of food were a major force in determining Chydorus distribu-
tion, since the total amount of detritus was greater on the y153_-
phyllum-type plastic plant (personal observation). An alternative
explanation might be that there exist qualitative differences in the
periphyton associated with different experimental and control sub-
strates. I had hoped to analyze samples in this regard, but diffi-
culties with preservation techniques (see General Methods) made this

immossible. The following section, while it suffers from the same

20

 

 

 

 

 

 

 

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21

shortcoming, deals with one aspect of the quality of food resources.

The Effect of New Surface

 

One of the qualities of a surface that is important to colonizing
bacteria and algae has to do with the age or condition of the surface
with regard to previous colonizers. When using artificial substrates,
the time of exposure is tailored to the objectives of the study in order
to allow for a succession of periphytic organisms up to a mature
assemblage that is similar to naturally occurring ones. This same
process occurs on new growth of natural plants.

It is possible that herbivorous animals like_ghydorus sphaericus
prefer particular stages in the succession. The results of the previous
section seem to refute this; although the several different plastic
plants were of equal ages, and presumably supported similar periphyton
assemblages, there did exist significantly different Chydorus densities
between them. However, although the ages of the plastic plants were
equal, the stages of development of the periphyton may have differed,
owing to dissimilar colonization rates of surfaces that differ in
orientation. The unfortunate lack of periphyton samples for analysis
precludes a definitive statement on this point.

When the substrates to be compared are living organisms of
different species, further complications are introduced into any
consideration of the qualities of the attached flora. For example,
during regular sampling, it was noticed that, due to their different
life habits, Myriophyllum and Scirpus present surfaces of very different
natures to periphytic organisms. Scirpus, at the depths involved in

this study, is a perennial plant. Much of the total surface is heavily

22

encrusted with a marl-epiphyte complex (Wetzel 1975); new growth is not
extensive and seems to be complete by early to mid-July, after which
the new shoots also become increasingly encrusted (Keen 1976). In
contrast, Myriophyllum is an annual, and continuously grows at its
apices through the summer. Older (deeper) portions of the plant
develop an increasingly heavy marl-epiphyte complex.

The possible significance of this difference is indicated in an
observation made by Lang (1970): he found that on Myriophyllum plants

in Lake West Okoboji, Iowa, the density of g, sphaericus (and most

 

other chydorids) was greatest on the top 10 cm and decreased rapidly
on the lower sections.
An experiment was set up to test the direct or indirect role of

new surface on the distribution of g; sphaericus in Lawrence Lake.

 

Two uniform areas in each plant bed (Myriophyllum and Scirpus) 0.25 m2

in extent (0.5 m square) were selected and marked. One area of each
bed was designated the control with no manipulation. In the other
(experimental) area, the surfaces of all plants were manually cleaned
of their marl-epiphyte coatings, care being taken not to injure the
plants. This was much more easily and completely accomplished with the
Scirpus plants, but even the more delicate and complex experimental

Myriophyllum plants were markedly cleaner than the controls. The

 

experiment was set up on 9 August, 1977; duplicate samples of control
and experimental plants of both species were taken on 12, 15, 19, 23,
and 30 August and 6 September.

If the amount of new surface alone is an important factor in

controlling the distribution of Chydorus, the expected ranking of

23

plant species by density of animals would reflect the proportion of

new or clean surface on the plant:

Experimental Myriophyllum (ME) 2 Experimental Scirpus (SE)

 

> Control Myriophyllum (MC) > Control Scirpus (SC). (1)

 

Another possibility makes use of information from the previous
section, where plastic Scirpus plants supported higher densities of

Chydorus than did plastic Myriophyllum. Then, if both the amount of

 

new surface and the substrate morphology are important, the expected

ranks, by density of Chydorus would be:

SE > ME > MC > SC. (2)

As time passed and the marl-epiphyte complex built back up, it was

expected that the observed ranking would revert back to:

MC 3 ME > SC 2 SE. (3)

Results are presented in Table 4. Signs of inequality were
obtained using Fisher's Exact Test for differences between medians,
and the correlation between the observed and expected rank orders was
tested using Spearman's Rank Correlation Coefficient (Siegel 1956).

The results show little correlation between the observed and the
first two expected rankings, and a very rapid (within 6 days)Iattainment
of high correlation with the third, indicating that the state of the
marl-epiphyte complex does not account for the distribution of Chydorus.

In general, any Myriophyllum plant always supported a greater density

 

of Chydorus than any Scirpus plant. It was not until three weeks after

the start of the experiment that the experimental Myriophyllum plants

 

 

.mamufiom Houusoo now “mammaom Housmafiuonxm «mm “Esaahsmoflamz Houuaoo no: nasaamsmoafimz Hmucoafluoaxm um: a

 

 

 

 

24

 

 

 

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mo Ams\*v huawcop an mmuznaouoms mo muwvuo xamu Am.m.~v wouooaxo cam po>uwmno mo mwmzamn< .q manna

25

supported a higher density than the controls (Fisher's Exact

Probabilty = 0.21), even though by then the plants were very similar
in appearance and presumably in the assemblage of bacteria and algae
they supported. The differences between the controls and experimental
Scirpus plants were expected to be the most drastic, yet only once were
there any significant differences (19 August, p = 0.17), and then the
controls ranked ahead of the experimental plants.

Thus, cleaning the marl-epiphyte complex from the surface of these
plants had little effect on the density of Chydorus other than a slight
reduction. It seems, not surprisingly, that the plants act as more than
just inert surfaces, and are somehow involved in determining the distri-
bution of Chydorus through some biological means. Unlike those in the
previous section, this experiment does not control for equivalence of
food organisms, chemical exudates, or predation pressure. The roles of
food and chemical excretions by the macrophytes or their associated
algae in determining the distribution of Chydorus may be complimentary
rather than antagonistic (see General Discussion). Although suchinfor-
mation is essential to a complete understanding of the situation, it is
not included in the present study.

Alternatively, it is possible that the results of the present
experiment are not due to differences in food quality (or chemical
excretions) at all, but instead are imposed upon the chydorid population
by predators. That is, predators of various sorts may be feeding on
Chydorus more heavily in Scirpus than in Myriophyllum beds, and are
unaffected by the amount of marl present. Before further analysis of
the role of food can be justified, some basic elucidation of the

relative importance of predation is necessary. The next section details

26

some observations relevant to this topic.

Aquaria Observations

 

In an effort to get first-level information about the importance
of predation relative to food and/or chemical effects, relatively
predator-free laboratory situations were set up for observation. Five
replicate, large (29 gal; 74 cm X 30.5 cm X 40.5 cm) aquaria were
erected in the laboratory facility at the Experimental Ponds site atthe
Kellogg Biological Station. The first three aquaria were filled with
water obtained from Lawrence Lake, while the last two were filled with
water from an on-site reservoir. All the water was filtered through a
74 u sieve to remove invertebrates. The aquaria were continuously,
lightly aerated, and exposed to a 14 hour light-10 hour dark cycle.

(Myriophyllum and Scirpus plants were collected from Lawrence Lake.
No attempt was made to keep the animals associated with these plants;
in fact, it was hoped that as many as possible would remove themselves
during collection. The plants were not handled vigorously however,
since as much natural algae and bacteria as possible were desired to
inoculate the aquaria, helping to insure food conditions as natural as
possible.

Although there were varying numbers of potential invertebrate
predators (hydras, some Chaetoggster spp.) and potential competitors

(other chydorids, notably Camptocercus rectirostris Schoedler,

 

Graptoleberis testudinaria Fischer, and small Alona spp.; copepods;
snails) in the aquaria, the numbers were small relative to nature in
most cases. The one aquarium that had large numbers of hydras (#2)

produced results not notably different from the others (see below).

27

Therefore, the effect of animals other than Chydorus sphaericus was

 

considered negligable.

Six Myriophyllum and six Scirpus plants were anchored in each

 

aquarium with small pieces of brick. The aquaria were then left
undisturbed for 1 to 3 weeks to allow the plants to adjust and the
food organisms to recover and build up their populations. Next 600-

1,000 g, sphaericus were seperated from laboratory cultures and added

 

to the aquaria. Allowing 2—4 days for the animals to adjust, thepdants
were then examined through the side of the aquarium using a dissecting
microscope at 6X, and the number of Chydorus seen on each plant were
recorded. These observations were repeated 3-5 days after initial
observations.

Tests with a very small aquarium, in which the entire volume was
visible with the microscope, indicated that 46-51% of the Chydorus
actually on a plant were seen with this method, with little difference
in visibility between the macrophyte species used. While generally
only 7-15% of the total number of Chydorus in a aquarium were seen,
this was usually more than 95 individuals, which I consider an adequate
sample size. The major exceptions were the second observations of
aquarium #4 (95 individuals, approximately 2% of total Chydorus) and #5
(79 individuals, approximately 34% of total Chydorus , but these too are
acceptable samples. Thus, this technique provides results that are
considered representative samples.

At the termination of each setup, the plants were removed and the
aquarium drained through a 143 u sieve. The plants were rinsed as
before, with all Chydorus collected on the sieve. The animals so

collected were anesthetized, preserved, and enumerated as before (see

28

General Methods). Plants were also treated as before, so that all
observations of Chydorus could be expressed as numbers per square meter
of plant surface. Winter buds produced in the last aquaria were not
included because they were not included in the plants used to estab-
lish the dry weight - surface area regressions, and because Chydorus
were never seen on them.

These animals are motile, and those added were capable of sampling
all environments within the aquarium in the time before observations
were begun. Thus, we may regard the relative density of Chydorus
observed on each plant as a measure of the relative amount of time
spent on that plant by an average individual. Further, we may use the
average density as a measure of the relative attractiveness of a macro-
phyte to an average chydorid. The nature of this attraction is
unknown, but, as discussed previously, is probably either the food
associated with each plant or some chemical attractant (or repellant).

The densities of Chydorus on each species of macrophyte were
compared using the Mann-Whitney U-Test (one-tailed)(Sokal and Rohlf
1969). Means for each plant species on each day of observation were
compared, as were the overall means for each plant species across all
aquaria, summed separately over the first and second observations. The
means and results of comparisons are presented in Table 5.

Of the total of ten comparisons (two observations in each of five
aquaria), only two yielded means that were not significantly different
between macrophyte species at the 0.10 level. The overall comparisons
were highly significantly different (a < 0.005). Thus, just as in the
lake, Myriophyllum plants in the relatively predator-free aquaria

harbor greater densities of Chydorus than do Scirpus plants. However,

29

Table 5. Comparison of mean densities (#l’mz) of Chydorus sphaericus
on Myriophyllum (M) and Scirpus (8) plants in relatively
predator-free aquaria, using the Mann-Whitney U-Test.

 

 

 

 

 

Aquar. lst Observation 2nd Observation

# Plant density U-value signif. density U-value signif.
1 2' 33? 36 0.005 “13;: 36 0.005
2 I: 1333 27 0.10 {fig 29 0.05
3 I: 1;; 29 0.05 ' 33(2) 31 0.025
4 g :3: 16 n.s. 38; 21 n.s.
5 '8' :2; 29 0.05 (1‘3; 31 0.025
a: :2: .3222: 3:; .8233:

 

the fact that two of the laboratory comparisons were not significantly
different (at the 0.1 level), plus the fact that the magnitude of the
differences between macrophytes are lower (Mann-Whitney U-Test,

0.005 > a > 0.001) in the lab than in the natural setting (see Tables
2 and 5) suggest that predation may have a greater effect in the field.
Another analysis of the data was performed, producing further
insight. By comparing the changes in density of Chydorus within plant

species between the two observations in each aquarium, to the change
in population size in the entire aquarium, one can get some idea of how
the attractiveness of a macrophyte changes as the population grows.
The chydorid population in each aquarium has a growth rate (or
in the case of #5, a decline rate). Assuming this growth to be

exponential, we can calculate, for each aquarium : (next page)

30

r = (ln Nt - 1n N0) / t, where:

r = population growth rate (number X individual.l X day-1),
N0 = initial population size (number of chydorids added),
Nt = final population size, and

t = number of days between addition of chydorids and

termination of aquarium.

Similarly, a rate of change of density of Chydorus on each plant

species in each aquarium can be calculated. For Myriophyllum:

 

H
II

(1n NMt - 1n NMO) / t', where:

r = rate of change of density of Chydorus on Myriophyllum,

 

NM = average density of Chydorus on Myriophyllum plants on
first observation date,

NM = average density of Chydorus on Myriophyllum plants on
second observation date, and

t' = number of days between observations.

An analogous rate of density change (rs) can be calculated for
Scirpus.

Note that, due to the fact that individuals can move around, it is
impossible to calculate a population growth rate for animals associated
with a particular macrophyte. Rather, by using density of Chydorus as
a measure of attraction to a plant species, the rate of change of
attractiveness of a plant is calculated. If either rm or rS is greater
than r for an aquarium, this indicates that the average chydorid is

spending a greater proportion of its time on Myriophyllum or Scirpus

31

as the population as a whole changes in size. Notice also that,
since the aquarium itself contributes to the total substrate surface
area available to the animals, there is no inherent reason why rm and
rS should be directly or inversely related.

The various growth rates for each aquarium are presented in Table

6 (see Appendix B for the data used in these computations). Aquarium #4

Table 6. Rates of population increase (r) and rates of increase of
density of Chydorus sphaericus on Myriophyllum (rm) and on
Scirpus (rs) plants in several aquaria.

 

 

 

 

 

Aquarium r rm rS
1 0.189 0.191 -0.019
2 0.269 0.050 0.059
3 0.015 0.125 0.092
5 -0.134 -O.123 -0.135

 

was omitted because the mean chydorid densities were not significantly
different between plants (see Table 5), although this aquarium other-

wise does not seem exceptional. Notice that rm > r in 2 of 4 compari-
sons, rs > r only once, and rm > rS three times. Thus there is a trend

for Myriophyllum to be incresingly preferred over Scirpus, under condi-

 

tions of both population growth and decline. One possible explanation
of these trends is that bacterial growth rates are greater on HZEEQT
phyllum than on Scirpus, producing a generally greater source of food
on the former. This possibility is examined in more detail in the
General Discussion.

In any event, the conclusion that can be drawn from these aquaria

studies is that the general dichotomy of distribution of Chydorus

32

between these macrophytes occurs under relatively-free laboratory
conditions as well as in the lake. This supports the contention that
the animals are responding primarily to some food (or chemical) differ—
ence. However, the more equivocal nature of the lab results indicates

that predation may have an additional effect in the lake.

GENERAL DISCUSSION

This study was undertaken in an attempt to delineate the factors

responsible for the observed distribution pattern of Chydorus sphaericus

 

within Lawrence Lake. It has shown that uneven distribution between
macrophytes cannot be explained simply by variation in surface area
between macrophyte species, nor does the gross morphology of the sub-
strate seem to be directly involved. However, there is a suggestion
from the latter experiments that variation in food quality may be
important. This could be because different surface orientations support
different kinds of periphytic communities (Cooke 1956, Castenholz 1961,
Sladeckova 1962, Wetzel 1964).

Further experiments with artificially cleaned natural surfaces
suggested that a dominant role in determining Chydorus distributions
is played by the macrophyte itself, either directly through excretions
or indirectly through qualitative differences in periphyton or through
differences in predatory regime between plant beds. Experiments with
relatively predator-free aquaria supported this viewpoint: distribution
of Chydorus within aquaria was similar to that in the lake, but the

dichotomy between Myriophyllum and Scirpus was not so consistant,

 

suggesting that predation may enhance the distributional trends in the
lake. A discussion of information related to this idea and possible
mechanisms for the action of these three factors follows.

Chydorus feeds by scraping small particles off of submerged
33

34

surfaces; it cannot filter feed (Fryer 1968, Smirnov 1971b). The
nature of the food is not well known, but it includes attached algae
and detritus. It is most likely that the nutritional value of the
latter is due to its bacterial flora; Saunders (1969) states that
detritus is more refractory to digestion by zooplankton than are bac-
teria. Conversely, the bacteria associated with a detrital particle
varies, depending on the particle's source, condition, and so on
(Wetzel 1975). A large amount of detritus does not necessarily repre-
sent a good food source for Chydorus if the material is highly refrac-
tory. This may explain the rather anomalous results of the Substrate
Morphology experiments, i.e., the fact that although the plastic

"Myriophyllum" plants had much more detritus than plastic "Scirpus",the

 

former had fewer Chydorus (per square meter) than the latter.

Bacteria do not necessarily need to be ingested with a detrital
particle to be useful to Chydorus: Smirnov (1971b) cultured these
animals very successfully on bacteria alone. In a lake, many macro-
phytes and algae excrete organic compounds (Wetzel 1969, Fogg 1971,
Wetzel and Manny 1972, Hough and Wetzel 1975) that can be used by
attached algae and especially bacteria for growth (Wright and Hobie
1966, Allen 1969, Wetzel and Allen 1972). While it seems that excretion
rates and epiphyte assemblages differ between emergent, floating leaved,
and submerged macrophytes (Allen 1971, Wetzel 1975), there is little
comparative data within these groups. Hough and Wetzel (1975) reported

that Scirpus subterminalis has lower excretion rates for several organic

 

compounds than does Najas flexilis (which supports the second highest

 

density of Chydorus after Myriophyllum - see Table 2). It is also

suggestive that Myriophyllum appears to be especially leaky with regard

 

35

to some inorganics, notably potassium (Wetzel, personal communication).

It is possible, then, that_Myriophyllum plants support a better epiphyt-

 

ic flora than do Scirpus plants, and so the former represent better
feeding sites for Chydorus. It must be stressed that much of this is
circumstantial and requires more information on excretion of dissolved
organics by various macrophytes as well as the epiphytic flora's
response.

Furthermore, the suitability of different food sources for the
growth and reproduction of Chydorus must be assessed before this scheme
can be adequately evaluated. In this regard, one might make use of the
fact that some Cladocera have been shown to respond to varying food
levels through egg production (Green 1956, Hall 1964, Kerfoot 1975).
Chydorids are limited in their response in that they can carry a maximum

of two eggs (except Eurycercus lamellatus). Even so, egg ratios

 

(# eggs / mature female) were calculated for the Chydorus in each plant
bed during July, 1977 (the period of population steady state) and late

August (the period of decline) (Table 7).

Table 7. Egg ratios (E = #eggsl’mature female) for Chydorus sphaericus
in two plant beds during July and August, 1977.

 

 

 

 

 

Plant Bed Time Total Indiv. # Mature Total # E
Period Examined Females Eggs
late July 100 67 72 1.08
Myrigphyllum
late Aug. 104 72 70 0.97
late July 22 14 14 1.0
Scirpus

late Aug. 14 10 4 0.4

 

36

Comparison of these results shows that the only noticeable
difference is a lowered value in August in Scirpus. This indicatesthat
variation in food supply among the two plant beds may be responsible
for the observed density differences in August. However, it does not
explain the existence of difference in density of Chydorus in July,
nor the population decline in both plant beds concurrently. It should
be pointed out that visiblity-selective predation (Zaret 1972) of egg
bearing females could also produce these differences in egg ratios if
the predatory regime differs between plant beds.

Future studies to further explore this problem might include the
following. First, macrophytes of several species could be grown in
single species stands under predator-free conditions in laboratory
and/or field situations and the response of the Chydorus populations
(as well as the food organisms) determined. This would provide stronger
evidence on the question of macrophyte-food source effects. Alter—
natively, one could take samples of periphyton concurrently with the
animal samples, and analyze the epiphyte composition and biomass.
Statistical treatment of these results could produce associations
between the animals and specific food resources. Laboratory populations
could be cultured on different foods based on these results to quantify
more precisely the benefits accruing to selective feeders. Finally, a
series of field studies using radioactive tracers could provide much
insight into natural production rates of bacteria and epiphytes associ-
ated with particular macrophytes , and into utilization of these food
organisms by the chydorids. All such experiments would be most meaning-
ful, at least initially, in the absence of predators.

The effect of dissolved organic substances on aquatic animals are

37

only slightly known (Beauchamp 1952 (in Zaret 1972), Katona 1973,
Griffiths and Frost 1976, Poulet and Marsot 1978), but it is apparent
that they are potentially very important in a variety of ways. In the

present context, iquyrigphyllum has higher excretion rates than

 

Scirpus (as already discussed), Chydorus might simply be reacting posi-
tively to this. A similar phenomenon has recently been reported for a
marine harpacticoid copepod (Hicks 1977). However, the presence in
Lawrence Lake of large amounts of monocarbonates that strongly adsorb
organic compounds and remove them from the water (Wetzel and Allen 1972)
makes such a scheme unlikely. Furthermore, even if it were possible,

it is unlikely that the dissolved material itself has any direct value
to the animals (Saunders 1969, Seepers 1977). Rather, it would seem
that the ultimate attraction would be the food with which such dissolved
organics would be associated.

As has already been stated, the aquaria studies performed in this
study suggested a subordinate role for predation in producing the
observed distribution of Chydorus. That food and predation could act
in a complimentary fashion is suggested by several sets of results
reported above, that could be explained by either alternative. While
there is some limited evidence that predation by invertebrates (Goulden
1971) or vertebrates (Keen unpublished manuscript) may regulate popula-
tion sizes of chydorids within a lake, there are no data directly
pertinant to the question at hand. The following discussion of the
potentiality of differing predation regimes between plant beds is culled
from very limited data from this and other studies on a variety of
topics by other investigators working in Lawrence Lake.

Potential invertebrate predators of g, sphaericus include

38

tanypodine midges, some caddisflies, odonates, hydras, and oligochaete

worms of the genus Chaetogaster (Roback 1969, Goulden 1971, personal

 

observation). In many cases, these predators were recorded during
analysis of samples in the present study. These data were used to
prepare graphs of population density of Chydorus and density of all
invertebrate predators in each plant bed over the summer of 1977

(Figure 3). It can be seen that, in August, there were more predators
in Scirpus than in Myriophyllum (a = 0.05, Mann-Whitney U-Test). This
coincides with the period of maximum decline of the Chydorus population,
and may be a contributing factor to the distribution of the chydorids.

It is by no means the complete story, however, since the decline of

 

Chydorus in Myriophyllum is still unexplained.

Furthermore, in the present study, of the many ($100) tanypods
examined, only two had Chydorus in their guts. Rarer predators also
contained few Chydorus. Finally, the single night sample taken showed
relatively higher densities of Chydorus in Scirpus and lower in Myrigf
phyllum than normal, while typical values were obtained the next day
(see Table 2). This suggests that the normal daytime pattern was
established after visual predators (fish) began to feed. Keen (unpub-
lished manuscript) attributes the population declines he observed in
Lawrence Lake chydorids to fish predation.

Potential vertebrate predators of Chydorus include many small fish
of various species (Costa and Cummins 1972, L. Wilsmann and E. Werner
personal communication). In Lawrence Lake, the primary predators are
small (<50 mm) bluegills (Lepomis macrochirus) and shiners of the genus
Notropis (L. Wilsmann personal communication). Werner et a1. (1977)

found the shiners to be limited to the shallow Scirpus bench, which

39

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40

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DENSITY INVERTEBRATE PREDATORS (x 0.01)

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41

excludes them from the present study area, and the bluegill to be the
dominant species in the deeper littoral. Hall and Werner (1977)

explored the distribution of the bluegills further and found that, in
late June, the small (<50 mm) ones are in dense Scirpus and Chara beds
at several meters depth. By late July, these fish are getting larger
and moving into the water stratum above the Scirpus beds, which would

include the more open Myriophyllum areas. Finally, R. G. Werner (1967)

 

noted that the bluegill fry in Crane Lake, Iowa, return to the littoral
zone in early August. This pattern has also been noted in Lawrence Lake
(Hall and Werner 1977, L. Wilsmann personal communication). Thus, these
small fish are almost constantly found in the Scirpus beds while larger
bluegills, which would eat fewer of the small Chydorus, are associated

more with the_Myriophyllum beds.

 

Evidence that bluegill predation is actually different between
plant beds is indirect. Use can be made of evidence that the small
bluegills feed selectively on larger Chydorus (from comparison of the
size distribution of Chydorus in fish guts with that in concurrent
resource samples - data and samples supplied by L. Wilsmann). Whether
these fish are actively seeking only larger Chydorus and neglecting the
smaller ones or just cannot see the smaller prey as well is not known.
In either case, the effect is the same: the proportion of small
Chydorus in the guts is noticably smaller than the proportionof the same
sized individuals in the resources, and the maximum size of Chydorus
in the guts is larger than the maximum found in the resource samples.

If predation by smaller bluegills is significantly more severe in
Scirpus beds than in Myriophyllum, we would expect to see differences

in the size distribution of Chydorus in the two locations, with a

42

greater proportion of large individuals in the latter. Cumulative
size-frequency distributions of the Chydorus in both plant beds in July
and late August are presented in Figure 4. The largest difference
occurred in late August, when there was a reduction of juveniles in the
Scirpus beds, but even this difference is not significant (x2 test).
Thus, there is no evidence that predation is strongly involved
in producing the observed distribution of Chydorus among macrophytes.
This is not unexpected because the results previously discussed(Aquaria
Observations) would lead one to expect that any such effects would be
subtle and difficult to detect. Conclusive studies need to be designed
specifically for this question and should include a more refined anal-
ysis of the distribution and feeding ecology of the various potential
predators coupled with continual monitoring of the chydorids. Specifi-
cally, one must determine the distribution of the predators more
precisely, including movements on a diel and seasonal basis. Next,data
on the rates of predation by the various predators would be required,
also including diel and seasonal patterns and (especially for fish)
the effect of foraging in different plant beds, i.e., can bluegills
feed as effectively from a Myriophyllum plant as from a Scirpus plant?
Finally, these predation rates, combined with the density of predators
in various places at various times, must be compared to data on the
distribution and growth rates of the Chydorus population, to see if the
predators could be responsible for the decline of the chydorids. In
addition, there might also be indirect effects to account for, such as
fish eating invertebrate predators and thereby decreasing overallpreda-
tion pressure on the chydorids.

In conclusion, the most probable primary reason for the different

 

43

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45

densities of Chydorus sphaericus between plant beds in Lawrence Lake

 

is that it is a response to different food resources. Predation of
various types may enhance these effects. The significance of thisstudy
is that it is the first of its kind undertaken with the Chydoridae and
that it has shown that a full understanding of the intra- (and possibly
inter-) lake distribution of these animals will require much more study
of a variety of related organisms as well: the macrophytes and the con-
dition of their associated bacteria and algae, and the distribution and
feeding ecology of potential predators. Since chydorids are a basic
link in many aquatic food webs, and since they are a part of a process
that recycles detritus directly back into the consumer components of
such webs (Smirnov 1971b), knowledge such as that gained in this study
and future offshoots is potentially important for a more complete
understanding (and perhaps management) of carbon flow in lakes.
Finally, studies of the ecology of individual chydorid species
will provide a basis for explorations of the coexistence of related
species. While it may not seem unusual to have 185 different species
of chydorids inhabiting the littoral regions of the world, it is
remarkable that Keen (1976) found 21 species in Lawrence Lake and not
infrequently I found 10 or more species on individual macrophytes.
Presumably the complexity of the structure of the littoral, with a
number of macrophyte and inanimate substrates, provides many oppor-
tunities for specialization, while the diversity and abundance of food
resources could also provide additional means for avoiding competition.
It may also be that these populations seldom escape predatory or
climatic constraints long enough to allow competitive exclusion

(Hutchinson 1961). The solutions to these and similar questions must

46

await the development of more basic ecological information on the
members of the Chydoridae, which will require increasingly creative

and sophisticated approaches.

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APPENDICES

APPENDIX A

DISTRIBUTION OF CHYDORIDS IN LAWRENCE
LAKE,MICHIGAN, SUMMER 1976.

53

Table A-1.

54

Mean proportions of the eleven most abundant chydorids_of
species 1 associated with macrophyte species j (xij / E x1.j
9

where = mean number of animals of species iI’g plant

xij
species j).
Key to Animal Species

Eurycercus lamellatus
Camptocercus rectirostris
Acroperus harpae
Graptoleberis testudinaria
small Alona spp.

Alona affinis

Pleuroxus procurvus
Pleuroxus denticulatus
Chydorus sphaericus
Alonella nana

Alonella exigua/excisa

 

 

 

 

 

 

 

 

 

KVNIQHSMUow>

 

Key to Plant Species (number samples)

Myrigphyllum hetergphyllum (n83)
Najas flexilis (n=2)
Potamggeton pectinatus (n=2)
Scirpus subterminalis (n=2)
Potamogeton praelongus (n83)
Nymphaea odorata (n=2)

Nuphar variegeta (n=2)

 

 

 

 

 

 

 

I" \lO‘Lfl-bWNO-fi
II I! u 11 II II II

Actual total number of animals of
species 1 collected on that date.

9 July

Plant Species (j)

22 July

Plant Species (j)

26 Aug.

Plant Species (j)

\IO‘U‘I-L‘UJNH \lO‘U‘I-L‘UNI-I

NO‘UIwaI-a

55

 

 

 

 

 

 

 

 

 

Table A-1.
Animal Species (i)
A B C D E F G H K L M
.64 .89 .19 .35 .22 .19 .06 .65 .79 .29
.15 .35 .12 .10 .23 .21 .10 .69
.34 .17 .60 .43 .42 .05 .03 .16 .35
.15 .06 .09 .18 .03 .13 .05 .14
.04 .04 .02 .17 .05 .02 .41 .08 .03 .01 .34
.02 .01 .01
.01 .01 .01 .11 .02 .01
130 27 926 76 403 14 39 36 1792 18 49
A B C D E F G H K L M
.73 .59 .29 .71 .05 .59 .55 .83 .41 .08
.13 .26 .26 .05 .18 .41 .24 .36 .08 .25
.03 .06 .28 .08 .56 .25 .10 .03 .20 .21
.02 .01 .04 .09 .01
.10 .03 .12 .07 .11 .35 .05 .39 .45
.01 .02 .03 .05 .02
.01 .01 .14 .01
21 26 180 43 120 10 79 14 521 4 70
A B C D E F G H K L M
1.0 .61 .52 .37 .04 1.0 .05 1.0 .91 .15
.10 .08 .16 .10 .04 .21
.28 .13 .69 .55 .01
.20 .50 .07
.07 .05 .01 .10 .03 .62
.01 .01
.02 .06 .03 .19 .02
3 25 181 6 54 l 35 9 445 0 36

Table B-1.

APPENDIX B

DATA FOR COMPUTATION
OF r, rm, AND rs.

Relevant data necessary for computation of growth rates
of population sizes and densities of Chydorus_§phaericus
on_Myrigphyllum (M) and Scirpus (8) plants in several
aquaria.

 

 

 

 

Aquarium Total Chydorus Total Densities (#/m2) Days Between
# Begin End Time(d) Plant Obs 1 Obs 2 Observations
M 845 1813
1 1000 3772 7 S 201 186 4
M 1368 1675
2 1000 5028 6 S 883 1120 4
M 177 330
3 600 705 11 S 58 92 5
M 184 362
4 1000 4719 12 S 224 209 5
M 787 431
5 1000 230 11 S 363 185 5

 

 

 

56