MilllWllllelNlWNUINN)lllHWWIHHIWI 125 839 .THS LIBRA R Y Michign State University This is to certify that the thesis entitled ENTRAINMENT OF ZOOPLANKTON BY A ONCE THROUGH COOLING SYSTEM ON WESTERN LAKE ERIE presented by Mark Andrew Simons has been accepted towards fulfillment of the requirements for Master ofi Scieneeegree in liahexieeLand Wildlife /Mfl (#6; Major professor 0-7 639 ENTRAINMENT OF ZOOPLANKTON BY A ONCE THROUGH COOLING SYSTEM ON WESTERN LAKE ERIE BY Mark Andrew Simons 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 1977 c4 “3'3 b; ABSTRACT ENTRAINMENT OF ZOOPLANKTON BY A ONCE THROUGH COOLING SYSTEM ON WESTERN LAKE ERIE By Mark Andrew Simons The entrainment of the zooplankton passing through the once-through cooling system of the Monroe Power Plant on western Lake Erie was studied from November, 1972 to October, 1973. The plankton were sampled in the intake region, discharge canal, thermal plume and lake at a total of 7 stations. They were sampled at bimonthly intervals in the morning, afternoon and evening. An 8.1 liter Van Dorn water sampler was used to collect the animals which were concentrated using a 75-u plankton net and preserved in 5% formalin. One-way analysis of variance and Tukey's test of paired comparisons were conducted for each sampling date on the density and biomass of the major taxa including 3 categories: Cladocera, Copepoda, and Rotifera. Linear regression analysis was used to assess the relation between depth and distribution of the major taxa. Zoo- plankton diversity was also determined. Entrainment rates were greatest during summer months but time of day appeared not to effect the entrain- ment rate. Patchy spatial distributions confounded spatial comparisons of density on any particular date but annual mean density and biomass were depressed 35 to 50% regardless of the time of day. Neither chlorine application nor temperature (as it relates to power plant operation) appeared to be important causative factors. Zoo- plankters, which were uniformly mixed from top to bottom by condenser passage, appeared to recover vertical distribution similar to that in Mark Andrew Simons the intake as cooling water passed down the discharge canal. Diversity appeared not to be affected by passage. There was no evidence that entrainment effects on zooplankton persisted beyond the thermal plume. ACKNOWLEDGEMENTS I would like to express my appreciation to Dr. Richard A. Cole for his guidance and patience through the preparation of this manu- script. To my graduate committee, appreciation is also expressed for their advice and support in reviewing this manuscript. I would like to thank Dave Kenaga, Don Nelson, and James WoJcik for their assistance with rigorous fieldwork. I would also like to thank Julin Lu for his aid in identification and for the use of some of his data. This study was supported from a grant from the U.S. Environmental Protection Agency to the Institute of Water Research at Michigan State University. Much of the equipment was made possible through a grant from the Detroit Edison Company. To my family and friends I am extremely grateful for their support and encouragement throughout this endeavor. ii TABLE OF CONTENTS LIST OF TABLES . LIST OF FIGURES INTRODUCTION . . . . . METHODS. Study Area . . . . . . . . . Hydrology . . . . . . . . . . . Zooplankton Sampling . Laboratory and Statistical Analysis RESULTS . . . . . . . . . . ZOOplankton Abundance and Size . The Intake and Discharge Canals The Discharge - Plume Diversity in the Cooling System DISCUSSION . LITERATURE CITED . . . . . APPENDIX . . . . iii . ll . l3 . l3 . 13 . 25 .26 . 30 . 35 . 37 Table 10 ll LIST OF TABLES Amounts of river and lake water (m3) used to determine the contribution of zooplankton at Station 18 in the intake canal, water passage times through the discharge canal and total chlorine on the dates sampled. The distribution of mean total chloride (mg/liter) and mean temperature (DC) at all stations, June ll through October 1, I973. Mean annual density (numbers/liter) at all stations throughout the water column and near the surface. Mean annual biomass (pg/liter) at all stations throughout the water column and near the surface. Mean density (numbers/liter) for the major taxa at the three daily sampling periods. Mean biomass (pg/liter) for the major taxa at the three daily time periods. Mean size/individual for the major taxa at the three daily time periods. Comparison of mean annual density (numbers/liter) between the intake station (18) and the upper discharge station (12) at the three daily time periods. Mean annual size/individual (ug) throughout the water column and near the surface. Comparison of mean density, biomass, and size among the lake stations and lower discharge station lh at 0.5 m below the surface. Samples included in the comparison were collected from April to November because no winter samples were taken at stations 3, h, and 5. Diversity index values for zooplankton taxa captured in the cooling system. iv 10 1h 16 l7 19 20 21 23 27 28 LIST OF TABLES (Cont'd): Table A1 A2 A3 Ah A5 Depth in meters at the various stations. Percent composition of the taxa encountered in the study. Tukey's multiple range test for zooplankton density. Tukey's multiple range test for zooplankton biomass. Percent change in zooplankton numbers (over day 0) in aquaria after transplanting from the cooling system. hO hh h? LIST OF FIGURES Map of the study area. Temperature (C) profile in the study area from November, 1972 through October, 1973. Mean annual density of major taxa throughout the sampling period (numbers/liter). Density at all depths for the three time periods showing a plot of the mean densities and r values Regression values are calculated from samples taken from all time periods in a particular data set. vi 15 2h INTRODUCTION Prodigious quantities of zooplankton can be transported through once-through cooling systems of steam-electric stations. The demand for cooling water has been doubling every ten years in densely populated areas, like that around the western end of Lake Erie, As energy demanding trends continue, increasing numbers of zooplankton will be transported through cooling systems. Such passage through cooling systems could affect the health and survival of the entrained animals (McNaught, 1972; Coutant, 1970). Zooplankton serve as a trophic linkage between algae and fish populations; therefore changes in zooplankton abundance may be reflected in altered fish population abundances. The purpose of this research was to quantify the passage of zooplankton through a once-through cooling system on western Lake Erie and assess the impact of the passage on zooplankton abundance in the receiving waters of western Lake Erie. Concern about the effects of entrainment mostly arise from the rapid elevation of temperatures, usually from 5 to 15 C, as water passes through the cooling system, and the length of time the animals remain exposed to elevated temperatures (Coutant, 1970). This problem seems most pressing in the summer when lethal temperatures are most likely to be reached in cooling systems. However, the impact of elevated temperature also may be compounded by other environmental stresses including mechanical damage (McNaught, 1972; Coutant, 1970), the application of chlorine as a biocide (U.S.A.E.C., 1972) and increased vulnerability to predatory fishes concentrated in the thermal discharge (Neill and Magnuson, 197A). I examined this problem at the Monroe Power plant by measuring (1) the numbers and biomass of important taxa, (2) their size distribu- tion, (3) their capacity to orient to normal vertical distributions and (h) their species diversity. Sampling was conducted at several points in the cooling water source, the cooling system and the thermal plume that mixed with the receiving water of western Lake Erie. METHODS StudygArea The study was conducted at the Monroe Power Plant, which is located on the western shore of Lake Erie at the mouth of the Raisin River. The plant has the potential to discharge 85 m3/sec of cooling water. The plant began Operating in June, 1971 with the first of four, 800 megawatt units. The fourth unit began operating in May, l97h. At the time of this study in 1972-73, two to three units operated sporadically. Because of start-up Operational difficulties, power generation and heated water output were less stable than expected for the completed power plant. The magnitude of power generation and thermal discharge varied with the number of functioning units and the output of each unit. Figure 1 shows the cooling system, lake environs around the Monroe Power Plant and the sampling stations used in this study. The cooling water is drawn from two sources: the lake and the Raisin River. The proportion of water coming from both sources is important because the Raisin River water quality differed from that of the lake, partly because it was loaded with municipal and industrial waste. The western basin of Lake Erie is usually very turbid from suspended matter produced by wave action, runoff, and dense algal growths. The intake is located about 650 m upstream from the mouth of the Raisin River and is designed to take most of the river flow before 3 IMAUMEE RIVER IiUFKDN,z*' FIHlER DETROIT K RIVER . snrm I5 . STATION :6 Figure 1. Map of the study area. the balance is made up from the lake. On the dates sampled, the flow in the river varied from 38 m3/sec to 63 m3/sec in the spring, to late summer discharges near h m3/sec. Because of seasonal fluctuations, the river may comprise nearly all or at least a large percentage of the pumped volume in the spring and a very small to negligible part in the late summer and fall. Because the river and lake differ biologically, this variation in flow contribution influences the composition of zoo- plankton in the cooling system. Stations 17 and 9 were chosen for sampling from the intake region (Figure 1) because the water at those two locations generally could be representatively sampled with relatively few samples. In contrast, the water in the short intake canal was incompletely mixed and complexly distributed; discrete proportions of lake and river water could vary in position and amount from day to day. The river contributes almost all of its flow to the cooling system and lake water makes up any deficit. In order to compare water at station 12 in the upper discharge canal to the mixture of waters from the two sources, an "artificial" station was calculated for the intake by proportioning water flow from the river and lake (Table 1). Daily USGS measures of river discharge were used along with information from the Detroit Edison Company on the known amounts of water pumped into the discharge canal. After the water passes through the intake canal (which is 100 m long), it goes through the condensers, then on to the discharge canal via a concrete overflow canal. Water passes through the condensers in 7 seconds at a rate of nearly 2 m/sec. 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On the dates sampled, the average rate of flow reached as high as 58 cm/sec near the upper end of the discharge canal (Table 1). However, the velocity is not uniform because high velocity (1 m/sec) water from the overflow canal enters the discharge canal on the west side and forms an eddy of slower water on the east side. This creates wide variation in the exposure time of zooplankton to the heated effluent. During passage through the 2000 m discharge canal to the lake (about 7.5 to ll.h hours on the average) less than 10% of the heat is lost (Table l) . Temperatures have been elevated as high as 16 C above ambient during condenser passage, but 10 C is expected to be closer to the normal operational elevation. Temperature elevations varied from date to date during this study but the maximum rise encountered on the dates sampled was 11 C above ambient (Figure 2). The plume from the thermal discharge extended from the mouth of the canal over an extensive (as indicated by a 100 elevation above adjacent waters), shallow, sandy shoal. The location of the plume varied with the direction and velocity of the wind (Figure 1). It drifted from about h km south of the mouth of the discharge canal to about 1 km.north of the mouth of the Raisin River. Station 15 was located in the plume at a point along the central axis where the temperature was about one-half the difference between the lake temperature and the temperature in the upper discharge canal. For example, when Lake Erie was 10 C, and the upper discharge canal JANUARY TEMPERATUR N VEMBER STATION [7 -————' RIVER STATION 9 LAKE ---------- DISCHARGE STATIONS 8-|2-ond ‘4 —-—-—-— PLUME STATIONS I50nd|6 Temperature (C) profile in the study area from November, 1972 through October, 1973. III-mu. ......... 0 2 3 -- I til-dun...- - R m 2 m I It lllllllllllllll R IIIIIIIIIIIII Lint-III". IIIIIII / E «I... 2 Or I r I T l B Oil-Inn}..-- / I WW 2 ”MIIIIIIF.I 1.1/IIIL IIIIII C 5 0 5 U B m T 5 I- ll! 3 2 1| 1 WI 4 A O m rIIIIIIUMI‘III'In-L IIIIII .HI IIIIII 3 I I . - w... .-..-m «w I: i T / III I w % U-UITIII / IIIIIL U 5HIIIV1III I A m I P a M. r .4. E 3 .-.. 2 b lllllllll "an." lllll m LJIVIIMMHIHIIIII T 5 vlllII-IIHUIIIIII a ........ $- ........ I I. H .3 .4 E 2 AIM... I N O IIIIIIII I...- U 2 J S b .3334: IRE-”.56 Izsmss 123456 mm .52 z. Iaawo mam-Wm: 2_ Frame 03.502 ZOOZMMPud 02.2% 02.285. zoozmwhnz 02.sz Figure 2 . was 18 C, station 15 was about 1h C. Station 16 was located along the central axis near the plume edge at temperatures about 1 C above ambient. Hydrology Chloride was measured in order to confirm mixing ratios in the intake and estimate what portion of the water at station 16 was indeed discharge water. Most of the time, station 16 was composed almost entirely of lake water according to the chloride index (Table 2). The discharge water was partially diluted by lake water by the time it had reached station 15, and at station 16 the chloride in the discharge water was mixed back to ambient concentrations in the lake water. Since station 16 and station 17 have similar chloride values, both were considered to be lake water which should behave similarly in terms of zooplankton abundances. Variation between the two were expected to be a measure of spatial variation of zooplankton in the lake. Zooplankton Sampling Three sets of zooplankton samples were collected over a two to three day period in the morning, afternoon and evening every two months from November, 1972 through October, 1973. The morning sampling began at approximately 0800 hrs., to match the time that the plant chlorinated its condensers (0730 hrs.). The afternoon sampling began when the sun approached its peak at about 1300 hrs. The evening sampling was initiated promptly after sunset. During the summer months, the cooling system was also chlorinated during this evening period. 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At those times, all replicates were taken from the surface (Table Al). One incomplete set Of samples was taken on January 25, when stations 1 and 2 were omitted because of inclement weather By sampling during different times Of the day, we incorporated diurnal variability into estimates Of the entrainment and mass move- ment Of organisms through the cooling system. Sampling every two months included seasonal changes in the relative river and lake contributions tO the system and seasonal changes Of ambient temperatures. Laboratory and Statistical Analysis Zooplankton samples were Obtained using an 8.1 liter Van Dorn water sampler. The samples were concentrated using a #25 Wisconsin plankton bucket and preserved in 5% formalin. In the laboratory they were diluted to a known concentration, and a l—ml aliquot was placed in a Sedgewick-rafter counting cell. Using a binocular zoom sc0pe, I counted organisms, measured (using a Whipple micrometer), identified them tO species when possible, and then calculated population density, biomass, and indices Of species diversity. Individual ZOOplankter volumes were estimated by using linear measurements Of length and width to calculate the volume Of a common geometric figure which best approximated the shape Of the animal. Dry weights were calculated from the volume by assuming that the organism's specific gravity was 1.0 and it was 90% water (Cummins and Waycheck, 1971). Analysis Of variance among stations (combining depth) was conducted for each sampling date on the density and biomass Of the major taxa 12 including three categories: Cladocera, Copepoda, and Rotifera. This was followed by Tukey's test Of paired comparisons (SOkal and Rohlf, 1969). The data were corrected for heterogeneity in variance by loglo transformation. Linear regression analyses were also used to assess the relation between depth and distribution Of the major taxa. Differences in coefficients measured at different stations were used to indicate the relative ability Of zooplankters tO redistribute in relation to depth as they passed through the cooling system. The zooplankton diversity was determined using the species diversity index as described by Pielou (1969): H II II MU) (NJ/N)loglo(NJ/N) j 1 where N = total number or total weight Of all species together, N = the J number or weight Of the jth species and S = species. RESULTS Zooplankton Abundance and Size The Intake and Discharge Canals Zooplankton abundance varied from nearly negligible quantities Of copepods, cladocerans and rotifers in winter to greatest concentrations in late summer and early fall (Table 3; Figure 3). Biomass varied over the year more than density because the small rotifers were relatively numerous in winter. The three most abundant species were Bosmina sp., Cyclops vernalis and Daphnia retrocurva (Table h; Figure 3). Bosmina sp. was abundant in June, August,and September while C. vernalis and D. retro- curva were abundant only in June and August. Based on these sampling results, nearly all zooplankton entrainment occurs between April and November. Abundances varied less dramatically over the short-term periods when the cooling system was sampled morning, afternoon, and evening (Figures 3), Even so, the abundances found at different times of the day commonly varied up tO 100% or more Of the mean. However, there was no consistant relationship between abundance and the time Of the day sampled. Mean densities for morning, afternoon, and evening samples reveal no important differences that are related tO the time Of the day (Table 5). The greatest mean density, in the afternoon, was only 35% more than the lowest mean density, in the evening. 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Am.m.m.m Aa.m.m.a am.m.m.m im.m.o.m 1m.m.m.m 1m.m.:.m unmeasom mH m.m :H o NH m.m m NH mom-m9 acapaam .ammmmepemhaa eav momaRSm one name use assaoo swam: one psoemsoaep macaumpm aaa pm ammaaa\mav mmaaoan access new: .: magma 17 Table 5. Mean density (numbers/liter) for the major taxa at the three daily sampling periods. Taxa Morning Afternoon Evening Rotifera 3A2.9 h86.0 289.3 Cladocera, 136 . o 132. 2 126 .7 D. retrocurva h6.3 37.5 h0.7 Bosmina sp. 72-7 66.7 55.7 Adult Copepoda ' 311.3 h26.7 371.7 Adult 0. vernalis 212.3 315.5 308.9 Nauplii 169.5 lhl.3 90.h Total Copepoda h80.9 567.8 h62.1 Total Mean 959 . 8 1186 . O 878 . 1 18 independently from each other from one time Of day to the next without any indication of trends. The total zooplankton biomass estimates also show only minor, short-term temporal differences, although in this instance, evening samples had the highest biomass (Table 5). Accordingly, the mean individual size was greatest in evening samples, particularly in C. vernalis and D. retrocurva (Table'T). The short-term variation in density and biomass indicated nO consistant effects from chlorine application in the morning and evening. If chlorine had been an important factor, the morning and evening zooplankton concentrations in the upper discharge canal at station 12 should have been consistantly lower than afternoon concentrations (Table53). Because mean water velocities would carry water to station 8 in h tO 6 hours, afternoon samples would tend to be lower than morning and evening samples if chlorine were an important factor. The subtle spatial differences that appear in the comparisons Of annual means (Table 3) rarely show up as significant (a = 0.05) differences on the individual dates sampled; probably because of the relatively low sampling intensity for the spatial variability that exists. Lake concentrations at intake station 17 averaged about twice as great as river concentrations at station 9 (Table 3) and the differences were significant (a = 0.05) on several dates for most of the taxa (Table A3). The statistically identified differences among stations in the intake and discharge canals may have been caused by naturally patchy distribu- tions in the study area rather than by entrainment effects. However, significant (a = 0.05) differences for total zooplankton density and biomass were determined 3 out of 9 times when animals were common and intake abundances always ranked higher. This indicates that although 19 Table 6. Mean biomass (pg/liter) for the major taxa at the three daily time periods. Taxa Morning AfternOOn Evening Rotifera h2-5 57-3 37.7 Cladocera 553.9 h37.6 556.7 D. retrocurva h59.2 369.3 h56.7 Bosmina Sp. 71.2 61.7 15.5 I Adult Copepoda 993.9 122h.3 1315.9 Adult C. vernalis 639.9 830.7 100b.7 Nauplii 16.0 12.0 7.2 Total Copepoda 1009.9 1236.2 1323.1 Total 1606.3 1731.1 1917.5 20 Table 7. Mean size/individual for the major taxa at the three daily time periods. Taxa Morning Afternoon Evening Rotifera 0.12 0.12 0.13 Cladocera h.1 3.3 h.h D. retrocurva 9.9 9.8 11.3 Bosmina sp. 0.98 0.93 0.82 Adult Copepoda 3.2 2.9 3.5 Adult C. vernalis 3.0 2.6 3.3 Nauplii 0.09 0.08 0.08 Total Copepoda 2.1 2.2 2.9 Total 2.1 1.9 2.5 21 Table 8. Comparison in mean annual density (numbers/liter) between the intake station (18) and the upper discharge station (12) at the three daily time periods. Morning Afternoon Evening Station Station Station 18 12 18 12 18 12 Rotifers 70.7 35.8 90.2 h9.7 h7.8 h0.h Cladocerans 26.6 15.6 36.2 8.h 21.5 1h.7 Daphnia retrocurva 11.0 7.5 15.1 3.3 7.0 h.0 Bosmina sp. l3.h 7.8 13.8 3.6 6.6 7.1 Adult Copepoda 107.9 62.9 11h.2 89.h 72.3 57.5 Adult cyclops vernalis 55.2 25.6 55.8 51.5 h5.h 35.5 Nauplii 37.0 20.5 3h.8 22.5 15.9 10.h Total Copepoda lhh.9 83.h 1h9.0 111.9 88.2 67.9 Total 2A2.2 13h.8 275.h 170.0 157.5 123.0 22 patchy distributions increased the variability, there were consistant depressions in abundances as a consequence Of passage through the cooling system. Among the most abundant species, significant differences (s = 0.05) were about equally represented by higher and lower concentra- tions in the discharge canal compared to the intake. The incidence Of statistical difference seemed not to be related to absolute water temperature or the elevation at the condenser. The mean annual density Of all taxa appeared tO decrease from 27 to 55% in passage from the intake to the upper discharge canal (Table 3). Biomass also seemed to decrease similarly (Table A) so that the mean size Of animals remained about the same. In passage from the upper to the lower discharge canal the mean annual density of most taxa increased slightly while mean annual biomass changed very little. Therefore, mean annual sizes Of zooplankters seemed to decrease at least slightly among most zooplankter groups as water passed through the discharge canal (Table 9) oApparent changes in mean size were most pronounced in the cladocera which decreased h0% in mean size during the passage. Although there was no indication that Obvious vertical migration occurred within any Of the taxa investigated, based on day and night comparisons, both cladocerans and copepods exhibited depth related distributions (Figure A). Both groups were more abundant near the bottom than near the surface at three Of the four stations (17, 9, l2 and 8) where depths were randomly sampled over 5 m from top tO bottom. Other stations were too shallow tO sample similarly. The time of day did not seem to influence this distribution (Figure A). NO depth- related distribution was Observed in the upper discharge canal (station 12). Whatever disrupted the distributional pattern in the upstream 23 Am.av am.av aa.avw.a Am.ava.m am.mvm.m aw.avm.m am.avm.m Am.avm.m ampoa . . . . . . . . . . . . . . mooamaoo Am av A: me am avm m Aa mvm m Aa mvm m Am avm m an mam m A: avm m aaaoa A00; AND-v :beo. CLOQQO. CLO-v00. ACLU-VOL”. AOHQQO. AQOQHH. 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AHH.VNH. dammwpom 0H mH :H w NH ma 0 PH mxme acapapm .AmmmmnpsmAdm Gav momahzm 039 ammo cam QSSHOO Moan: mnp psonmsoanp Awnv H850a>m©ca\ouwm Amanda com: .m manna O :a mUOaaom mEap HHO Boa“ :Oxmp mOHQEmm Scam Oopmafioamo mam mosam> OOammmhmom a wad moauamcop cams osu mo voam d meaSOSm mpoahom mEap mmth may you mnpmmp Ham um hammoon .pom dump amasoaphmm .mmsaw> .: OROMam m .5765 N_ cozoammamw. ._.m303aoza\mmam zamz m.mma m.mmm a.mmm m.aom m.mom m.mmm a.mmm ampoa a.mm m.aaa m.maa m.mom a.mam m.mmm m.maa aeommaoo aaaoa a.mm m.oaa m.mma a.mm a.ma m.mm m.mm ammooemao m.ma m.ma m.m m.m m.m a.m m.m mammapom mm<20am m.mam m.aom m.m:m a.amm a.mmm m.mom a.mam aaaoa m.aoa m.ma a.moa a.a:a m.maa m.aaa m.aaa aeomomoo aapoa m.ma m.am m.mm m.mm o.a: m.am a.ma ammooeaao m.mm m.moa m.aoa m.am m.ma m.ma m.mm mammapom aeamzmo m a m NH ma ma wH mama mma< mama pmoomnm< madam amcmo mmamzomam acapaam .m mom .: .m mmOapmum pm swamp who: mmagamm ampmas OO mmomomn amQSm>Oz Op aaaa< 50am monomaaoo who: domaammaoo mSp OH pmosaoca moamamm .oommasm map soamn E m.o pm :a :anmpm mmamnomaw amsoa pom mOOapmpm mama map MOQfim mmam O cm .mmmaoan .hvamcmp cmma mo nomaamQEOo .oa mapme 28 Table 11. Diversity index values for zooplankton taxa captured in the cooling system. Station 17 9 18 12 8 1h 15 16 6/11 0.53 0.59 .53 0.57 .59 0.59 .66 .65 6/12 0.70 0.51 .59 0.73 .78 0.69 .h8 .6h 6/13 0.68 0.35 .51 0.57 .72 0.65 .61 .51 8/9 O.h8 0.69 .6h 0.h0 .h5 0.hl .29 .A6 8/10 0.57 0.85 .78 0.53 .83 0.59 .71 .76 8/8 0.92 0.h9 .h5 0.50 .h3 0.35 .37 .hl 9/30 0.89 0.76 .89 0.76 .80 0.77 .83 .79 9/29 0.81 0.83 .83 0.93 .96 0.96 .92 .89 9/28 0.8h 0.76 .92 0.87 .95 0.89 .80 .85 29 day period at specific stations were Often as great as differences Observed among stations at any particular time. There were no consistant trends in diversity related to the time of sampling. Short-term vairations appear to be caused primarily by spatial variability among samples at each station and may secondarily be effected by patchy distributions associated with different water masses. There is no indication that entrainment had any effect on diversity. DISCUSSION There are a number of ways in which zooplankton abundance and size distributions could be altered by power plant Operation at the Monroe site. They could respond to (1) water quality in the river water as river and lake water mixed, (2) the toxic effect Of chlorine application at the condenser, (3) mechanical damage during condenser passage, (h) thermal "shock" caused by rapid temperature change and prolonged exposure to elevated temperature, and (5) increased vulner- ability tO predation caused by thermal shock or increased concentra— tions of predators in the discharge canal. Trends were recognizable in the mean annual averages of all the major taxonomic groups investigated but limited statistical confidence can be placed on what may have been a subtle net decrease in density as zooplankton passed from the intake to the upper discharge canal. A mean annual average Of about h0% of all zooplankton seemed to disappear from the water column sampled somewhere between the intake and the upper discharge canal. The effect was not size related. This loss, if it is real, probably has a negligible affect on the lake ecosystem. However, explanations for any loss may be pertinent to future power plant design or Observations at other sites. A possible explanation for the apparent loss is that the chloride tracing technique for evaluating mixing in the intake inaccurately estimates the proportions of river and lake water drawn by the power 30 31 plant. If the river impact has been underestimated, then the Observed h0% depression is an artifact Of the sampling design. However, such a sampling artifact should be reflected in the abundance ratios of specific taxa. For example, rotifers were relatively more abundant than copepods in the river water compared to lake water. If the underestimation Of river water contribution to the mix were the only factor, then rotifer densities should decrease 1eSs than copepod densities. Just the Opposite appeared in the data. Whatever may have caused the decreased zooplankton density in the discharge canal did so before the water reached the upstream station in the discharge canal. The plankton pass through condensers at about 2 m/sec and then into a concrete overflow canal which carries them at about 0.75 m/sec to the upper discharge canal only minutes after condenser passage. Potential predators, the fish populations, have never been sampled in the overflow canal but velocities seem tOO high to maintain very high fish densities. Chlorine was not applied in the afternoon but density differences between that time and chlorination times are not enough to implicate chlorine as the cause. Even if chlorine were involved it would not destroy the bodies Of individual zooplankters. Settling of dead, dying or "shocked" animals could have decreased densities in the sampled water column, although this probably would not happen in the rapidly flowing water Of the overflow canal. Settling may have occurred in the upper discharge canal soon after water emptied into it from the overflow canal. Velocities in the discharge canal averaged less than 0.15 m/sec and may have been much less than that near the bottom. Perhaps some Of the animals actively swam toward the 32 bottom as soon as they passed into the upper discharge canal. McLaren (1963) and Gehrs (197A) found that certain zooplanktonic species swam toward bottom when they were exposed tO higher temperatures. Passage through to the upper discharge canal appeared to uniformly mix any re- maining zooplankton evenly throughout the water column. Changes in the abundance and size distribution Of zooplankton may have occurred after the plankton drifted beyond station 12 in the upper discharge canal. Although the density changed slightly as the plankton passed down the discharge canal, the mean size of zooplankton appeared to decrease consistantly among the taxa. These changes could have resulted because (1) larger animals, more than smaller ones, were eaten, settled out, or emigrated from the water column, (2) small animals immigrated or were born into the populations, or (3) there was some combination Of size-related mortality and recruitment. Among copepods, the mean size of adults remained fairly constant and apparent average increases in the density of nauplii caused a decrease in copepod mean size. However, it seems unlikely that the number of nauplii would increase as much as indicated by birth alone. Assuming life expectancies Of 2 weeks and a mean fecundity per individual of about 10 young, (Ceiling and Cambell, 1972; Eckstein, 196A; and Munro, 197k) I would expect only a 2 to 3% increase from birth during the 8 to 11 hr passage. Either nauplii are moving up from the bottom or the distribu- tion is a chance consequence Of patchiness. Nauplii were particularly abundant at the mouth Of the discharge canal on one date. Among the Cladocera and the Rotifera, regeneration rates also are likely to cause little change in density over a few hours, but densities seem to increase 20 to 25% as the water passes through the cooling 33 system. This could be caused by animals moving up from the bottom after they have recovered from the impact Of condenser passage. Apparently these taxa are capable Of recovering their ability to orient to depth related differences within their environment within a few hours of the time that they passed through the condenser. But, because mean sizes Of cladocerans, in particular, seemed to decrease as the plankter passed down the canal, either larger animals settled or moved out of the water column or they were consumed by predators. Kenaga and Cole (1975) indicated that certain fish species caught in the vicinity Of the power plant tend to be size-selective feeders. Kelly and Cole (1976) also found parallel size changes in benthic organisms inhabiting the discharge canal. Although apparent size changes may be the chance outcome of variability, it is also possible they reflect the effect of predation in the discharge canal. If a size related selection process is operating in the cooling system it was too subtle to have any impact on the measured diversity. Diversity indices have been promoted as a means of identifying community- wide influences of man-caused perturbations (Wilhm and Dorris, 1968). None of the diversity estimates made during this study indicate that passage through the cooling system had any consistent impact on community structure. There was no indication from the chloride data that substantial quantities of lake water were being recycled by power plant operation even though measurements Of plume temperatures indicated that wind frequently forced the plume waters northward toward the intake. Neither samples of zooplankton or chloride taken over three day periods revealed any trends in changing concentration that would imply cumulative 3h effects from recycling. Apparently the mixing of discharge waters and the receiving waters in Lake Erie effectively diluted any subtle impact from entrainment. In summary, although there were some subtle indications that plant operation caused transitory disorientation and may have slightly altered distributions or susceptibility to predators, there was no indication of consequential or persistent effects beyond the thermal plume. Literature Cited Coutant, C.C. 1970. Biological aspects of thermal pollution. I. Entrainment and discharge canal effects. CRC Crit. Rev. Environ. Contr. 1(3):3h1. Cummins, K.W. and J.C. wuycheck. 1971. Caloric equivalents for investigations in ecological energetics. Int. Assoc. of Theor. and Appl. Limnol. Pub. No. 18. 158pp. Eckstein, H. 196k. Utersuchungen uber den ein fluss des Rheinwassers auf die limnologie des Schlussee h. Arch. Hydrobiol. Suppl. 28:119—182. I§_Geiling and Campbell, 1972. Gehrs, Carl W. 197k. Vertical movement of zooplankton in response to heated water in Thermal Ecology. Proceedings of a symposium held at Augusta, Georgia, May 3-5, 1973. pp. 285-290. Ceiling, W.T. and R.S. Campbell. 1972. The effect of temperature on the de- velopment rate of the major life stages of Diaptomus pallidus (Herrick). Limnol. and Oceanog. l7(2):30h-307. Kelly, J.E. 1976. The distribution and abundance of benthic macroinvertebrates near the western shore of Lake Erie. Technical Report No. 32.7, Inst. of Water Research, Michigan State University. 77pp. Kenaga, D.E. and R.A. Cole. 1975. Food selection and feeding relationships of yellow perch Perca flavescens (Mitchell), white bass Monrone chrysops (Refinesque), freshwater drum Aplodinotus grunniens (Rafinesque) and goldfish Carassius auratus (Linneaus) in western Lake Erie. Technical Report No. 32.5, Inst. Water Research, Michigan State University, East Lansing, Michigan. 50pp. MacLaren, I.A. 1963. Effects of temperature on growth of zooplankton, and adaptive value of vertical migration. J. Fish. Res. Ed. Can. 20(3): 685-727. McNaught, D.C. 1972. The potential effects of condenser passage on the en- trained zooplankton at Zion station. In Review of Recent Technical Infor- mation Concerning the Adverse Effects of Once-through Cooling on Lake Michigan. Prepared for the Lake Michigan Enforcement Conference, Sept. 19-21, 1972, Chicago, Ill. by Thomas A. Edsall and Thomas G. Yocum. Munro, I.G. 19TH. The effect of temperature on the development of egg, naupliar and copepodite stages of two species of copepods, C C10 8 vicinus (Uljanin) and Eudiaptomus gracilis (Sars), Oceolog. (Brit.) 18:355-367- Neill, W.H. and J.J. Magnuson. l97h. Distributional ecology and behavioral thermoregulation of fishes in relation to heated effluent from a power plant at Lake Monona, Wisconsin. Trans. Am. Fish. Soc. 103(k):663—710. 35 36 Pielou, E.C. 1966. An_Introduction 32_Mathematical Ecology. Wiley, New York. 286pp. Sokal, R.R. and F.J. Rohlf. 1969. Biometry. W.H. Freeman and Co., San Francisco, California. 776pp. [J.S. Atomic Energy Commission. 1972. Draft detailed statement on the environ- mental considerations related to the proposed issuance of an operating license to the Consumers Power Company, Inc., for the Palisades Nuclear Generating Plant. IN Review of Recent Technical Information Concerning the Adverse Effects of Once-through Cooling in Lake Michigan. Prepared for the Lake Michigan Enforcement Conference, Sept. 19—21, 1972, Chicago, Ill. By Thomas A. Edsall and Thomas Yocum. ‘Wilhm, J.L. and T.C. Dorris. 1968. Biolgoical Parameters for water quality criteria. Bio-Science. l8:h77. 37 Table A1. Depth in meters at the various stations. Station Depth 17 O 3 6 h 2 9 3 6 h 2 o 12 2 O 6 h 3 8 O 3 h 6 2 1h S B S B 8* 15 S S S S S 16 S S S S S *S = surface; B = bottom 38 TableIUB. Percent composition of the taxa encountered in the study. Taxa Percent ROTIFERA KerateZZa cochlearis 6.3 Karatella quadrata <1.0 Brachionus sp. Brachionus calyciflorus 5.3 Brachionus havanensis