Changes in the Dreissenid Community in the Lower Great Lakes with Emphasis on Southern Lake Ontario

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1 J. Great Lakes Res. 25(1): Internat. Assoc. Great Lakes Res., 1999 Changes in the Dreissenid Community in the Lower Great Lakes with Emphasis on Southern Lake Ontario Edward L. Mills 1,*, Jana R. Chrisman 1, Brad Baldwin 2, Randall W. Owens 3, Robert O Gorman 3, Todd Howell 4, Edward F. Roseman 5, and Melinda K. Raths 5 1 Department of Natural Resources Cornell Biological Field Station 900 Shackelton Point Road Bridgeport, New York Biology Department Bewkes Hall St. Lawrence University Canton, New York U.S. Geological Survey Biological Resources Division Great Lakes Science Center Lake Ontario Biological Station 17 Lake Street Oswego, New York Ontario Ministry of Environment and Energy 125 Resources Road Etobicoke, Ontario M9P 3V6 5 Department of Fisheries and Wildlife Michigan State University 13 Natural Resources Building East Lansing, Michigan ABSTRACT. A field study was conducted in the lower Great Lakes to assess changes in spatial distribution and population structure of dreissenid mussel populations. More specifically, the westward range expansion of quagga mussel into western Lake Erie and toward Lake Huron was investigated and the shell size, density, and biomass of zebra and quagga mussel with depth in southern Lake Ontario in 1992 and 1995 were compared. In Lake Erie, quagga mussel dominated the dreissenid community in the eastern basin and zebra mussel dominated in the western basin. In southern Lake Ontario, an east to west gradient was observed with the quagga mussel dominant at western sites and zebra mussel dominant at eastern locations. Mean shell size of quagga mussel was generally larger than that of zebra mussel except in western Lake Erie and one site in eastern Lake Erie. Although mean shell size and our index of numbers and biomass of both dreissenid species increased sharply in southern Lake Ontario between 1992 and 1995, the increase in density and biomass was much greater for quagga mussels over the 3-year period. In 1995, zebra mussels were most abundant at 15 to 25 m whereas the highest numbers and biomass of quagga mussel were at 35 to 45 m. The quagga mussel is now the most abundant dreissenid in areas of southern Lake Ontario where the zebra mussel was once the most abundant dreis- * Corresponding author. elm5@cornell.edu 187

2 188 Mills et al. senid; this trend parallels that observed for dreissenid populations in the Dneiper River basin in the Ukraine. INDEX WORDS: Quagga mussel, Dreissena, zebra mussel, Lake Erie, Lake Ontario. INTRODUCTION Two species of dreissenid mussels were introduced into the Great Lakes in the late 1980s (Griffiths et al. 1991, Mills et al. 1993), one was Dreissena polymorpha Andrusov, 1897 (herein referred to as zebra mussel) and the other was given the working name of quagga later identified as Dreissena bugensis (Rosenberg and Ludyanskiy 1994, Spidle et al. 1994). The quagga mussel was first sighted at Port Colborne in Lake Erie in 1989 (Mills et al. 1993). By 1993, the distribution of quagga mussel extended from Lake St. Clair eastward to Quebec City on the St. Lawrence River. The quagga most likely originated from a population in the Black Sea and Dneiper River drainage in the former Soviet Union (Spidle et al. 1994, Mills et al. 1996) and its release into Great Lakes waters is linked to discharge of ship ballast water (Mills et al. 1994). North American populations of quagga mussel were once thought to have a lower thermal maximum than zebra mussel (Mills et al. 1993, Spidle et al. 1995); however, Mitchell et al. (1996) demonstrated that these populations, similar to Ukrainian populations, were not thermally separated. In fact, Ukrainian populations of quagga mussel have a higher thermal maximum than zebra mussel and areas once dominated by zebra mussel are now dominated by the quagga mussel (Mills et al. 1996). In Lake Erie s eastern basin, dense colonies of quagga mussel have infested profundal areas up to depths of 55 m (Roe and MacIsaac 1997). Although some meiofaunal species have benefitted from the presence of D. bugensis in the profundal zone, burrowing amphipod Diporeia hoyi numbers have declined sharply (Dermott and Kerec 1997). Water temperatures in eastern Lake Erie s profundal zone rarely exceed 5 C yet recent evidence indicates that gonadal development and spawning by quagga mussel occurs at these low tempertures (Roe and MacIsaac 1997). In this paper, differences in spatial distribution and population structure of dreissenid populations in the lower Great Lakes are examined. More specifically, the range expansion of quagga mussel westward from eastern Lake Erie toward Lake Huron is examined and the shell size, density, biomass, and depth distribution of zebra mussel and quagga mussel in southern Lake Ontario between 1992 (Mills et al. 1993) and 1995 are compared. METHODS Sampling Locations and Gear Dreissenid mussels were sampled in spring, summer, and fall (1992 to 1995) at locations in Lakes Huron, St. Clair, Erie, and Ontario and the St. Clair, Detroit, and St. Lawrence rivers (Fig. 1) using bottom trawls, a ponar dredge, a benthic sled, and a benthic egg pump. A summary of waterbodies sampled, months of collection, number of locations, depths sampled, and gear used to collect dreissenids is shown in Table 1. At sites where a ponar dredge (523 cm 2 ) was used to collect dreissenids, five replicate samples of benthic invertebrates were collected, washed through a 0.6-mm mesh sieve, and preserved in 10% formalin. Mussels were identified to species and enumerated in the laboratory. Samples of mussels obtained by the benthic sled, egg pump, and 5.5-m, 10-m, and 12-m headrope bottom trawls were qualitative and included a few hundred to several thousand individual dreissenids from each location. Mussels collected with these devices were frozen until processing, then thawed in the laboratory where they were sorted by species, counted, and shell length measured to the nearest millimeter. Density and Biomass of Zebra and Quagga Mussel in Southern Lake Ontario Besides determining relative densities, wet weight (kg) of zebra and quagga mussel was determined at four sites in southern Lake Ontario (Olcott, Thirty Mile Point, Rochester, Smoky Point) in April and October, 1992 and LORAN was used to locate trawl sites and samples were collected with a 12-m headrope bottom trawl at 10-m depth intervals from 12 to 85 m. On occasion, trawl collections were made between 95 and 130 m. All trawl hauls were made on soft substrate and tow times were usually 10 min. Catches in shorter tows were adjusted by simple proportion to weight per 10 min trawl. If the wet weight of Dreissena in the trawl was < 0.2 kg, the entire sample was saved;

3 Quagga Mussel in Lower Great Lakes 189 FIG. 1. Sampling locations of zebra (Z) and quagga (Q) mussels in the Great Lakes and the St. Lawrence River in 1993 ( ), 1994 ( ), and 1995 ( ). Solid symbol = zebra and quagga mussels absent; open symbol = zebra and/or quagga mussels present (asterisk represents the first sighting of the quagga mussel). otherwise, only a subsample grab of the total catch was retained. Wet weight of individual dreissenids was estimated from shell length by converting length into dry weight (g) and then converting dry weight to wet weight by use of the following equations (Mills et al., unpublished). Zebra mussel: dry weight = e^( lnl ) (1) wet weight = dry weight (2) Quagga mussel: dry weight = e^( lnl ) (3) where L = shell length in mm. wet weight = dry weight (4) Total numbers and wet weight biomass of each dreissenid species caught was determined using the following equation: (# or kg in subsample/wet weight of subsample). (wet weight of total catch). Data Analysis Shell Length To determine which species, quagga or zebra mussel, was the largest among five sites in Lake Erie, eight sites in Lake Ontario, and one site in the St. Lawrence River, the length-frequency distributions were first tested for normality using the Pro Univariate statement (SAS Institute, Inc. 1989) to determine the appropriate test. As 22 of the 28 distributions tested were not considered normal, a nonparametric test was employed. The Mann-Whitney U test (in which the data are ranked) was used to determine which species was the largest at the various sites. Tukey s Studentized Range test was used to assess multiple comparisons in mean shell length of the two dreissenid species between years for Lake Ontario (1995 quagga vs zebra, 1995 quagga vs quagga, 1995 zebra vs zebra, and 1992 quagga vs zebra). Regression analysis (Zar 1984) was used to determine if shell length of the two dreissenid species differed at each depth zone in southern Lake Ontario.

4 190 Mills et al. TABLE 1. Waterbody, month of collection, number of locations, number of depths, and collection gear used to collect zebera and quagga mussels, Number of Year Waterbody Months of collection of locations Depths (m) a Collection gear 1993 St. Clair River April, July, September 1 2 Ponar Lake St. Clair April, July, September 2 5, 5 Ponar Detroit River April, July, September 2 1, 7 Ponar Lake Erie April, July, September October 7 9, 10, 10, 11, 11, 11, 12 Ponar Niagara River May, July, October 1 3 Ponar 1994 Lake Ontario Six Mile Creek to Hamilton April May, August 4 18, 19, 23, 23 Ponar Oakville to Toronto April May, August 3 9, 15, 21 Ponar Presquile to Amherst Island April May, August 3 3, 21, 25 Ponar Bay of Quinte May, August 2 4, 5 Ponar St. Lawrence River April May, August 3 4, 5, 11 Ponar 1995 Lake Huron August 5 6, 9, 11, 17, 20 Ponar Lake St. Clair October 3 1, 3, 7 Ponar Lake Erie Western basin October 1 2, 3, 5, 7, 10 Ponar, Bottom trawl b Egg pump c Eastern basin October 5 15, 19, 20, 20, 21 Bottom trawl d 22, 25, 26, 29 Lake Ontario Olcott to Mexico Bay April June, September October 8 15, 25, 35, 45, 55, 65 Bottom trawl e 75, 85, 95, 110, 130 St. Lawrence River Cape Vincent November 1 9 Benthic sled Total number of sites 51 a average over surveys to nearest meter b 5.5-m foot rope, 8-mm, stretch measure, cod end (E.F. Roseman, Department of Fisheries and Wildlife, Michigan State University, 13 Natural Resources Building, East Lansing, MI 48824, personal communication) c (Stauffer 1981) d 10-m head rope, 9-mm, stretch measure, cod end (Culligan et al. 1992) e 12-m head rope, 9-mm, stretch measure, cod end (O Gorman et al. 1991)

5 Quagga Mussel in Lower Great Lakes 191 Dreissenid Community Structure Differences in quagga mussel density (number. 10 min trawl) and biomass (kg 10 min trawl) across three depth ranges (15 25 m, m, and m) were tested in southern Lake Ontario. These three depth ranges were selected based on thermal profiles of Lake Ontario observed during 1972 (Almazan and Pickett 1980) and were chosen to reflect different temperature regimes bottom dwelling mussels could be exposed to from July through October. In general, the depths m were above the thermocline from the end of July through fall turnover, m were above the thermocline only during fall turnover, and m were never above the thermocline. Water temperatures in the m depth range reached 7 C briefly during fall turnover (Almazan and Pickett 1980; USGS, unpublished data). Density and biomass for each dreissenid species was transformed by log(x+1) and an ANOVA was performed to test for differences between dreissenid species at increasing depth within and between years. Tukey s Studentized Range test was used to assess which depth ranges were different. Both ANOVA and Tukey s Studentized Range test were adapted from SAS (SAS Institute, Inc. 1989). Comparisons were considered statistically significant at p < RESULTS Geographic Distribution Quagga mussels were not observed in the Detroit nor the St. Clair rivers. A single quagga mussel was detected in fall samples from a location in eastern Lake St. Clair in However, further sampling at the Lake St. Clair site in 1995 did not produce any quagga mussels, nor were any quagga mussels found northward at sites in Lake Huron. The pattern of colonization of zebra and quagga mussels differed along an east to west gradient in Lakes Erie and Ontario. In 1995, quagga mussels were present at all locations sampled in both the western and eastern basins of Lake Erie (Fig. 1). Quagga mussels comprised 13% of the dreissenid population by number in the western basin whereas eastern basin dreissenids were 78 to 100% quagga mussels (Table 2). In Lake Ontario, quaggas were not observed along the northwest shore (9 to 21 m depth) between Oakville and Toronto but were observed along the northeast shore (3 to 25 m depth) between Presqu`ile Bay and Wolfe Island. Along the south shore of Lake Ontario, the most abundant dreissenid shifted from quagga to zebra mussels progressively from west to east. Dreissenids at locations from Smoky Point westward (Olcott, Thirty Mile Point, Hamlin, Rochester, and Smoky Point) were mostly quagga mussels whereas dreissenids at locations east of Smoky Point (Fair Haven, Nine Mile Point, and Mexico Bay) were mostly zebra mussels (Table 2). Shell Length To determine which of the two dreissenid species might attain the largest body size in the Great Lakes, mean shell lengths of quagga and zebra mussels at each of 14 sites in Lakes Erie and Ontario and the St. Lawrence River in 1995 were compared. In Lake Erie, there was no clear pattern among the four sites tested where both dreissenid species were present. Mean shell length of quagga mussels was longer than that of zebra mussels at two sites (p < 0.01), shorter than that of zebra mussels at two sites (p < 0.01), and not statistically different (p = 0.37) at the Sturgeon Point site (Table 2). However, in Lake Ontario, mean shell length of quagga mussels was significantly greater (p < 0.01) than that of zebra mussels at seven of eight sites tested, and in the St. Lawrence River the quagga was significantly the longer (p < 0.01) of the two species. Not only were quagga mussels larger on average than zebra mussels at 10 of 14 sites in the lower lakes, the largest individuals in the random samples taken for length-frequency measurements were quagga mussels at all locations. These results suggest that quagga mussels have the potential to be the larger of the two forms in the Great Lakes (Table 2). The relationship between mean shell length and depth for zebra and quagga mussels was examined from collections made in Lake Ontario at 10-m intervals from 15 to 85 m in 1992 and Samples were pooled across four sites (Olcott, Thirty Mile Point, Rochester, and Smoky Point) and two seasons (spring and fall) but samples from each year were analyzed separately. No relationship was found between mean shell length and depth for either species in either year slopes of the shell length-depth linear regressions were significantly different from zero (ANOVA, p > 0.05). The same data sets were used, but pooled across depths, to examine changes in mean shell length between 1992 and 1995 and found that zebra mussels and quagga mussels were significantly larger in 1995 (p < 0.01).

6 192 Mills et al. TABLE 2. Mean shell lengths of zebra (Z) and quagga (Q) mussels collected April to November from Lake Erie, Lake Ontario, and St. Lawrence River in All shell lengths for each dreissenid species were pooled for the range of depths indicated (asterisk indicates species with significantly larger shell length, p < 0.01; N = number of zebra and quagga mussels in subsample of trawl; SE = standard error). Length (mm) Location Species N % Q () b Depth range (m) Mean () c SE Min Max Lake Erie Western Basin Locust Point Z* 2, Q Eastern Basin Seneca Shoals Z Q* Sturgeon Point Z Q Silver Creek Z* Q Dunkirk Z Q* Barcelona Z a Q Lake Ontario Olcott Z (36) (10.8) Q* (12.6) Thirty Mile Point Z (39) (9.9) Q* 1, (14.0) Hamlin Z Q* Rochester Z (23) (9.9) Q* (12.8) Smoky Point Z (33) (4.0) Q * (12.8) Fair Haven Z Q* Nine Mile Point Z (2) (10.3) Q (12.0) Mexico Bay Z Q* St. Lawrence River Cape Vincent Z (10) (19.2) Q* (29.2) a no statistical test done, N = 0 for zebra mussel b % quagga mussel in 1992 (after Mills et al. 1993) c mean length of quagga and zebra mussels in 1992 (after Mills et al. 1993) Dreissenid Density and Biomass in Southern Lake Ontario Dreissenid mussels were collected at 10 different depths in southern Lake Ontario in April and October. The mean density and biomass of quagga and zebra mussels per 10 min trawl tow was determined at each depth at four locations (Olcott, Thirty Mile Point, Rochester, and Smoky Point) in 1992 and 1995 (Fig. 2). In 1992, quagga mussels were observed at all depths < 85 m and average densities ranged from 1 to over 2,700 individuals per 10 min trawl. Mean densities of quagga mussels increased from 1992 to 1995 at all depths except at 75 and 85 m. However, the biomass of both dreissenids in-

7 Quagga Mussel in Lower Great Lakes 193 FIG. 2. Percent biomass (A) and number (B) of quagga mussels based on a 10-min tow with bottom trawls (about 0.73 ha area swept with each tow) along 10-m depth contours in Lake Ontario in 1992 (Mills et al. 1993) and For each depth and above each bar are mean biomass (panel A) and mean number (panel B) determined from April and October samples pooled at four sites (Olcott, Thirty Mile Point, Rochester, and Smoky Point). creased at all depths between 15 and 85 m. The average biomass of all Dreissena per 10 min tow summed across depths 15 to 85 m increased dramatically from 0.8 kg/10 min trawl in 1992 to 145 kg/10 min trawl in 1995 (Fig. 2). In 1992 and 1995, the index of total dreissenid density differed significantly among three depth ranges (15 to 25 m, 35 to 55 m, and 65 to 85 m) (ANOVA, 1992, p = ; 1995, p = ). In 1992, quagga mussel numbers did not differ among three depth ranges (ANOVA, p = ) whereas zebra mussel numbers did differ significantly between the two deepest depth ranges (Tukey, p < 0.05). By 1995, the relative density of both dreissenid species did not significantly differ between the two shallowest depth ranges, but m differed with m and m differed with m (Tukey, p < 0.05). Density per 10 min trawl tow of zebra and quagga mussels significantly increased from 1992 to 1995 (ANOVA, p = ). While total dreissenid mean biomass differed with depth in both years (ANOVA, 1992, p = ; 1995, p = ), the biomass of each species examined separately did not differ with depth in 1992 (ANOVA, zebra mussels, p = ; quagga mussels, p = ). By 1995, biomass of zebra mussels differed between the m and m depth ranges and biomass of quagga mussels differed between the m and m and between m and m depth ranges (Tukey, p < 0.05). The relative change in biomass for both dreissenid species at each depth range significantly increased from 1992 to 1995 (zebra mussels, p = ; quagga mussels, p = ). The index of dreissenid mussel biomass in Lake Ontario indicates that zebra mussel and quagga mussel biomass increased over the study period and that areas of the lake bottom dominated by zebra mussel biomass in 1992 were dominated by quagga mussel biomass in For example, quagga mussel biomass in 1992 averaged by depth for sites at Olcott, Thirty Mile Point, Rochester, and Smoky Point was lower than zebra mussel biomass at all depths from 15 to 75 m. By 1995, zebra mussel biomass exceeded quagga mussel biomass only at 15 m, while at all other depths, 51 to 88% of the dreissenid biomass was quagga mussel (Fig. 2). The mean biomass of quagga mussel increased dramatically with depth from 15 to 45 m in 1995 compared to 1992, reaching a maximum of 55.3 kg/10 min tow at 35 m, whereas zebra mussel biomass was 2 to 17 times lower over the same depth range. DISCUSSION Geographic Distribution of Quagga Mussel These findings indicate that the demographics of quagga mussel colonization differs in Lakes Erie and Ontario. In Lake Erie, quagga mussels dominate the dreissenid community in the eastern basin and have recently colonized the western basin. Few or no quagga mussels have been observed upstream of western Lake Erie indicating that northward expansion of quagga mussels is progressing slowly. In contrast to Lake Erie, the highest proportion of quagga mussels in southern Lake Ontario occurred in the west and declined eastward; the percentage of

8 194 Mills et al. quagga mussels caught in bottom trawls declined precipitously east of Smoky Point (Table 2). In Lake Ontario, the proportion of quagga mussels has increased since 1992 along the heavily colonized southwest shore (Fig. 2) and in the eastern basin (Nine Mile Point) where quagga have increased from 2% of the dreissenid population in 1992 (Mills et al. 1993) to 11% in 1995 (Table 2). Changes in the Size of Dreissenid Mussels In Lake Erie, shell lengths of quagga mussels in 1995 were larger than zebra mussels at all sites except those in the western basin and at one site in the eastern basin (Mills et al. 1993; Table 2). Quagga shell length increased 30% in the eastern basin of Lake Erie despite documented reductions in phytoplankton biomass (MacIsaac et al. 1992, Holland 1993, Leach 1993, Nicholls and Hopkins 1993, Madenjian 1995). In the eastern basin of Lake Erie (off Van Buren and Brockton), the mean shell length of quagga mussels was 8.6 mm in 1992 (Mills et al. 1993) and 12.3 mm in 1995 (based on mean of five locations, Table 2). In contrast, the mean shell length of zebra mussels in Lake Erie s eastern basin decreased (13.3 to 11.6 mm) over the same time period. These changes are consistent with MacIsaac (1994); shell length of small zebra mussels transplanted from the western basin to the eastern basin of Lake Erie decreased by 7%, whereas shell length of small transplanted quaggas increased by 1%. Similarly, Nalepa et al. (1996) found that mean shell length of zebra mussels declined along with dramatic declines in chlorophyll between 1990 and 1994 in Lake St. Clair. In southwestern Lake Ontario, where dreissenid infestation was high and food resources low (Mills et al. 1995), mean shell length of quagga mussels exceeded that of zebra mussels by as much as 15% even though zebra mussel shell length increased between 1992 and A Species Shift toward Quagga Mussel In this study, bottom trawls towed along depth contours were used to integrate dreissenid densities over large areas. This approach did not give true estimates of dreissenid density but it did provide relative changes in density and biomass of quagga and zebra mussels over the study period. Consequently, it is reasonable to conclude that the quagga mussel is increasing in importance in southern Lake Ontario. Catches of quagga mussels in 1995 were higher in areas of southwestern Lake Ontario that were dominated by zebra mussels in Such a species shift is supported by evidence that the trawl index of quagga mussel densities exceeded zebra mussel densities at all depths > 15 m in 1995 whereas this was not true in 1992 (Fig. 2). Similarly, dreissenid biomass has also shifted toward the quagga mussel. In 1992, zebra mussel biomass exceeded quagga mussel biomass at depths of 15 to 65 m whereas in 1995 the biomass of zebra mussels exceeded that of quagga mussels only at 15 m. The apparent species shift toward quagga mussels in Lake Ontario is consistent with species shifts that occurred among Ukrainian dreissenid populations in the Dneiper River basin. In shallow Ukrainian reservoirs, D. polymorpha has been largely replaced by D. bugensis (Mills et al. 1996). This study supports earlier evidence (Stanczykowska and Lewandowski 1993, Mitchell et al. 1996, Mills et al. 1993) that quagga mussels have broad thermal tolerance and can inhabit a wide range of depths. Quagga mussel in Lake Ontario were observed to depths of 85 m and at bottom water temperatures ranging from 4 to 15 C. At depths of 25 to 55 m, however, the quagga mussel encountered 12 to 15 C water for only a short time period during fall turnover (based on bottom water temperatures collected by USGS, Oswego, NY). In the deep waters of eastern Lake Erie where quagga mussels dominate, near-bottom temperatures rarely exceed 7 to 8 C during the summer and only approach 10 to 11 C during fall turnover (Schertz et al. 1987). In contrast, Lake Ontario D. polymorpha, which dominated in shallower water (15 m), were exposed to maximum bottom water temperatures ranging from 12 to 18 C from late July to fall turnover. The dreissenid species shift toward the quagga mussel in both Lakes Erie and Ontario suggest that quagga mussels can outcompete zebra mussels when seston is reduced; this should be reflected in a higher net energy balance (Hilbish et al. 1994) or scope for growth (Warren and Davis 1967) for quagga mussels under low food levels. When seston is limited, quagga mussels may maintain higher ingestion and/or assimilation rates, and/or lower metabolic rates and thus grow faster, reach larger sizes, and maintain higher fecundity than zebra mussels. According to Walz (1978a, c), metabolic costs increases with size of zebra mussels and a decline in available food affects the growth and survival of large zebra mussels more than small ones. This pattern, however, may not be true for quagga

9 Quagga Mussel in Lower Great Lakes 195 mussels and may be the reason they do better under low food. Ecological Considerations Insight on the species shift to Dreissena bugensis in the lower Great Lakes can be gained from the extensive work on sympatric species of marine mussels, some of which have been invasive species. For example, along the western coast of Europe, where Mytilus edulis and the larger M. galloprovincialis co-occur, Gardner (1994) has argued that there is strong directional selection in favor of the M. galloprovincialis genotype based on its greater growth, survivorship, fecundity, strength of attachment, and resistance to parasitic infestations. Consistent with this argument, Hilbish et al. (1994) found greater growth and feeding rates of M. galloprovincialis over M. edulis in the field. In South Africa, M. galloprovincialis invaded and colonized extensive intertidal regions and within 20 years became the dominant mussel, comprising about 74% of the intertidal mussel biomass (van Erkom Schurink and Griffiths 1990, Hockey and van Erkom Schurink 1992). The success of this invading species, and its ability to displace native mussels, has been attributed to several factors including rapid growth, ability to grow in dense mussel beds, and high fecundity. The fact that the quagga mussel attains larger shell size than the zebra mussel suggests that the quagga mussel possesses bioenergetic and reproductive advantages over the zebra mussel. Because fecundity is usually directly related to body size in iteroparous marine bivalves (Calow 1983) and in zebra mussels (Walz 1978b, Sprung 1991, Sprung 1995), it is likely that the larger-sized quagga mussel produces more gametes per individual than do zebra mussels. Moreover, since physiologicallystressed marine mussels produce fewer and smaller eggs (Bayne et al. 1978), it is possible that zebra mussel fecundity is further limited under low seston conditions. Finally, reproduction by quagga mussels does not appear to be constrained in the colder hypolimnetic waters of the eastern basin of Lake Erie. Gonadal development and spawning of quagga mussels can occur in waters 4.8 C but it is unclear whether such activity contributes to local recruitment (Roe and MacIsaac 1997). This study provides the first evidence of a species shift within the dreissenid community to quagga mussels in the lower Great Lakes. In the eastern basin of Lake Erie, D. polymorpha is now rare and, in Lake Ontario, quagga mussels dominate in areas of lake bottom that once were mostly zebra mussels. If these trends continue, the quagga mussel might be expected to eventually colonize the upper Great Lakes and other waters as well. At present, it is possible only to speculate on what factors underlie the shift to quagga mussels in Lakes Erie and Ontario but it is clear that further work on exploitative competition, recruitment, and growth is required to better understand the population dynamics of these two sympatric dreissenids. ACKNOWLEDGMENTS This study was supported by funds from New York Sea Grant and Cornell University. We especially wish to thank Christine Mayer, Veronica Ludyanskiy, Robert Haas (Michigan DNR), Shawn Sitar, Kristi Sitar, vessel crews of the New York State Department of Environmental Conservation for Lakes Erie and Ontario, crew of the Kaho, crew of the R/V Channel Cat, Don Einhouse, and Cliff Schneider for their assistance in collecting mussels. Dr. Charles McCulloch provided valuable advice for statistical analyses. We also wish to thank two anonymous reviewers who made valuable contributions to improving the manuscript. Contribution 177 of the Cornell Biological Field Station and contribution 1044 of the USGS, Great Lakes Science Center. REFERENCES Almazan, J. A., and Pickett, R. K Water Temperature. In IFYGL Atlas Lake Ontario Summary Data, ed. C.F., Jenkins, pp NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, Michigan. Bayne, B. L., Holland, D.L., Moore M.N., Lowe, D.M., and Widdows, J Further studies on the effects of stress in the adult on the eggs of Mytilus edulis. J. Mar. Biol. Ass. U.K. 58: Calow, P Life-cycle patterns and evolution. In The Mollusca, Volume 6, ed. W.D. Russell-Hunter, pp New York: Academic Press. Culligan, W.J., Cornelius, F.C., Einhouse, D.W., Zeller, D.L., and Zimar, R.C annual report of the Bureau of Fisheries Lake Erie Unit to the Lake Erie Committee and the Great Lakes Fisheries Commission, April Publication of the New York State Department of Conservation. Dermott, R., and Kerec, D Changes in the deepwater benthos of eastern Lake Erie since the invasion of Dreissena: Can. J. Fish. Aquat. Sci. 54:

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