Benthic Macroinvertebrate Communities of Southwestern Lake Ontario Following Invasion of Dreissena

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1 J. Great Lakes Res. 20(2): Internat. Assoc. Great Lakes Res., 1994 Benthic Macroinvertebrate Communities of Southwestern Lake Ontario Following Invasion of Dreissena Timothy W. Stewart 1 and James M. Haynes 2 Center for Applied Aquatic Science and Aquaculture Department of Biological Sciences State University of New York College at Brockport Brockport, New York Abstract. Changes in benthic macroinvertebrate communities inhabiting natural cobble and artificial reef substrates in southwestern Lake Ontario were quantified following invasion of zebra mussels (Dreissena polymorpha) and "quagga" mussels (Dreissena sp.). Post-Dreissena invasion data ( ) were compared with preinvasion data (1983) from the same sites. In , Dreissena comprised 79% and 93% of macroinvertebrates collected at cobble and artificial reef sites, respectively, replacing the amphipod Gammarus fasciatus as the numerically dominant taxon at both sites. Total abundance of non-dreissena macroinvertebrates was significantly greater at both sites in than in Taxa showing the greatest increases in abundance at the cobble site included the annelids Manayunkia speciosa, Spirosperma ferox, and unidentified tubificids; the gastropods Helisoma anceps, Physa heterostropha, Stagnicola catascopium, Valvata tricarinata, Goniobasis livescens, and Amnicola limosa; the amphipod Gammarus fasciatus; and the decapod Orconectes propinquis. At the artificial reef site, significant population increases of Physa heterostropha, Valvata tricarinata, Goniobasis livescens, Amnicola limosa, Gammarus fasciatus and the trichopteran Polycentropus were observed. No taxon was less abundant in than Comparisons of macroinvertebrate community similarity in 1983 and by Morisita's Index, excluding Dreissena, indicated that previously established taxa did not change substantially between 1983 and at either site. Although many factors may have contributed to the changes observed, our results support theories that Dreissena is facilitating energy transfer to the benthos through pseudofecal / fecal deposition, and that mussel colonies are providing additional habitat for other invertebrate taxa. INDEX WORDS: Artificial reefs, benthic macroinvertebrates, Dreissena polymorpha, Lake Ontario, zebra mussel, energy transfer, food web. 1 Current address: Department of Biological Sciences, Bowling Green State University, Bowling Green, OH Author to whom reprint requests should be addressed

2 Introduction The first sighting of the zebra mussel (Dreissena polymorpha) in the Great Lakes occurred within Lake St. Clair in June 1988 (Hebert et al. 1989). Since then, the "quagga" mussel, believed to be a second species of Dreissena, has also invaded the Great Lakes (May and Marsden 1992). At present, Dreissena is colonizing portions of all the Great Lakes, with exception of Lake Superior. Spread of Dreissena through the Great Lakes has generated concern among ecologists for several reasons. Of particular importance is the ability of adult Dreissena populations to 1) completely cover and change the physical structure of hard substrates (Lewandowski 1976, Stanczykowska 1977); and 2) reduce limnetic phytoplankton biomass by filter feeding (Reeders and Bij de Vaate 1990, MacIsaac et al. 1992, Leach 1993), likely impacting species dependent upon this food resource (Mackie 1991, Schloesser and Kovalak 1991). Concern about the impact of Dreissena upon other taxa (Hebert et al. 1991, Mackie 1991) has prompted studies of effects on a diverse array of faunal and floral taxa (MacIsaac et al. 1991, Wu and Culver 1991, Wormington and Leach 1992, Leach 1993). Since Dreissena and other benthic macroinvertebrate taxa are fairly sessile, often occupy the same substrates (Sebestyen 1938), and in some instances may consume similar foods (Hebert et al. 1991, Mackie 1991), changes in previously established benthic macroinvertebrate communities following Dreissena invasion are possible (Mackie et al. 1989, Reeders and Bij de Vaate 1990, Hebert et al. 1991). Investigations of Dreissena impacts on some Great Lakes benthic macroinvertebrate taxa have been conducted (Mackie 1991, Hunter and Bailey 1992, Dermott et al. 1993, Griffiths 1993). However, scarcity of pre-invasion data has made assessment of Dreissena impacts on many pre-established macroinvertebrate taxa difficult. Quantitative analysis of macroinvertebrate communities occupying natural cobble and artificial reef sites in southwestern Lake Ontario by Bader (1985), followed by invasion of these sites by Dreissena in 1990 (Haynes, personal observation), provided an opportunity to investigate benthic macroinvertebrate community changes resulting from Dreissena colonization. Study Area The study sites were situated km west and km offshore of Olcott, New York, in the southwestern region of Lake Ontario (Fig. 1). Physical characteristics of the cobble site (latitude = 43 20'9" N; longitude = 78 44'48"W) were representative of the benthic environment naturally found along this nearshore region of Lake Ontario. The gradually sloping lake bed was covered with gravel and small rocks, though some scattered boulders as large as 0.25 m in diameter were present. The artificial reef site (latitude = 43 20'9" N; longitude = 78 45'30" W; Fig. 1) was located 0.5 km west of the cobble site. The artificial reef was constructed in of siltstone and shale. Particle sizes on the reef ranged from small pebbles to boulders approximately 0.5 m in diameter. Although six discrete piles of material were placed in the area, all samples were collected from two reef sections located along the 7 m contour (Fig. 1). These sections, approximately 30 meters in length, are connected to each other by cinderblocks. Materials and Methods This study was designed to replicate that of Bader (1985), who quantified abundances of benthic macroinvertebrate taxa at cobble and artificial reef sites in 1983, 7 years before establishment of Dreissena populations. The present study duplicated sampling methods employed in the 1983 study and included additional sampling methods for Dreissena, which required different sampling methodology. Samples were randomly collected by SCUBA divers at five stations along 30 m transects at cobble and artifi-

3 cial reef sites on 12 July and 21 September 1991, and 15 May and August Sampling depths ranged from 5-7 m below the water surface (Fig. 1). Macroinvertebrate taxa (with exception of Dreissena) were collected by sampling for a 3-minute period at each sampling location (Bader 1985) with a dome suction sampler (Gale and Thompson 1975), which enclosed an area of m 2 and was equipped with a 0.5 mm mesh collecting bag. The dome suction sampler was made of a serrated steel band, effective in penetrating cobble site substrate and providing a seal between sampler and substrate. Although the serrated band prevented establishment of a tight seal between the band and hard artificial reef substrate, perhaps allowing some organisms to escape, this design was used to maintain consistency between 1983 (Bader 1985) and sampling methods. Because the dome suction sampler lacked power needed to remove enough Dreissena from substrates to obtain accurate representations of population sizes, plot sampling (Stanczyckowska 1977, Lewandowski and Stanczyckowska 1986, Hebert et al. 1991) was used to obtain abundance estimates of Dreissena. A square frame enclosing an area of m 2 was placed on cobble and artificial reef substrates, adjacent to the dome sampler, at three of the five sampling stations. Rocks forming the surface of the enclosed area were manually placed in a 0.5 mm mesh collecting bag. On the dive boat, Dreissena were removed from rocks by scraping with a hard-bristle brush (Piesik 1983).

4 Samples were initially preserved in 3-5% buffered formalin with rose bengal dye (200 mg/l). Within 24 hours, this solution was replaced with 70% ethanol (Clesceri et al. 1989). In the laboratory, macroinvertebrates were separated from other debris under a stereoscopic microscope (5x power). Subsampling (Elliott 1971) was used to obtain abundance estimates of Dreissena. Dreissena were uniformly mixed in a dissecting pan, and 500 undamaged individuals were removed (beginning at the upper left region of the pan and proceeding from left to right). This procedure improved the likelihood of unbiased selection of individuals for enumeration (Elliott 1971). The average mass/individual was calculated after the 500 mussels were blotted dry and weighed. Abundance estimates (# individuals/m 2 ) from each replicate sample were determined based on the average mass/individual and the total mass of zebra mussels in the sample. Based on weights of three sets of 500 mussels selected from the same sample in a preliminary trial, an index of precision (Elliott 1971) demonstrated that use of weights was suitable in estimating Dreissena abundance if a standard error equivalent to 5% of the mean is tolerated. Abundance estimates of other taxa were usually obtained by direct counts of individuals after identifications were made under stereoscopic (Nematoda, Turbellaria, Hirudinea, Mollusca, and Arthropoda with exception of Chironomidae) or compound (other Annelida, Chironomidae) microscopes. A damaged turbellarian, annelid, or arthropod was included in counts if the animal's head was present. Damaged mollusks were included in counts if the animals were attached to their shells. On four occasions (one cobble site sample on 12 July 1991 and three cobble site samples on 20 August 1992), sub-sampling of oligochaetes was employed due to their high abundances. Oligochaete subsampling involved counting all oligochaetes present in the sample, thoroughly mixing the animals, then proceeding to identify to the lowest taxonomic level every second individual encountered as we proceeded from left to right through this assemblage (Elliott 1971). Subsequent to identification of individuals, the total count for each taxon would be doubled to provide abundance estimates of each taxon in the sample. Abundances of Hydracharina and Unionidae, quantified in but not in 1983, are not included here. Bader's (1985) neglect of these taxa is understandable since the size ranges of these organisms make accurate abundance estimates by dome sampling difficult. Abundance estimates of additional taxa are reported in Stewart (1993), along with detailed descriptions of field and laboratory methods. Mean macroinvertebrate abundance estimates, standard errors, and ranges (95% confidence limits) were calculated for artificial reef and cobble site taxa on each sampling date. Differences in abundance estimates between 12 July 1983 and 12 July 1991; 10 September 1983 and 21 September 1991; 11 May 1983 and 15 May 1992; and 31 August 1983 and August 1992 were determined by two-sample t-test after log +1 transformation of abundance estimates (Elliott 1971). Changes in composition of macroinvertebrate communities between 1983 and were further analyzed by Morisita's Community Similarity Index (Brower and Zar 1977). This index is based on the probability that two individuals, randomly selected from two different communities, will belong to the same taxon. Mean abundance estimates for each taxon on each sampling date were used to calculate the degree of similarity between cobble site communities (1983 and ), artificial reef communities (1983 and ), cobble site and artificial reef communities (1983), and cobble site and artificial reef communities ( ). Morisita's Index was calculated including and excluding abundance estimates of Dreissena. Species diversity was measured by counting the number of taxa collected at each site on each sampling date. Mann-Whitney U-tests (Elliott 1971) were used to assess changes in community composition and number of taxa collected in relative to 1983.

5 Results Relative Abundance of Benthic Macroinvertebrate Taxa Relative abundance estimates of taxa whose abundance estimates comprised at least 1% of all benthic macroinvertebrates collected at cobble and artificial reef sites are shown in Table 1. Dreissena dominated cobble (79%) and artificial reef (93%) macroinvertebrate communities in Approximately 99% of Dreissena collected in were zebra mussels, Dreissena polymorpha, while the remaining 1% were "quagga" mussels, Dreissena sp. (L. Ben Motten, State University of New York College at Brockport, unpublished data). In contrast, the amphipod Gammarus fasciatus was numerically dominant on both cobble (55%) and artificial reef (78%) sites in If Dreissena is disregarded (Table 1), it is evident that evenness of other numerically important taxa remained high and may have even increased since 1983, especially at the cobble site. While taxa of numerical importance in 1983 remained so in , some taxa of little numerical importance in 1983, such as the oligochaetes

6 Stylaria lacustris, Potamothrix vejdovskyi, and Spirosperma ferox, and the gastropod Amnicola limosa, were of increased importance in Changes in Benthic Macroinvertebrate Abundance Abundance estimates of taxa collected at cobble and artificial reef sites in 1983 and are provided in Tables 2 and 3, respectively. Estimated abundance (mean ± SE) of Dreissena at the cobble site in ranged from a low of 5,919 ± 489/m 2 in July to a high of 20,773 ± 315/m 2 in August (Table 2). Estimated abundance of Dreissena at the artificial reef site ranged from a low of 19,486 ± 5,056/m 2 in May to a high of 55,508 ± 11,627/m 2 in July (Table 3). Total abundance of benthic macroinvertebrates at the cobble site was significantly greater (p < 0.01) on all sampling dates (range = 8,309 ± 997 to 25,743 ± 1,240/m 2 ), relative to sampling dates from the same months in 1983 (range = 261 ± 91 to 1,159 ± 107/m 2 ; Table 2). If Dreissena was not included in cobble counts, abundance increases since 1983 (range = 1,316 ± 170 to 4,595 ± 745/m 2 ) were still observed during three of the four months (p < 0.05).

7 Taxa more abundant (p < 0.05) at the cobble site in on a minimum of two dates relative to the same dates in 1983 were the polychaete Manayunkia speciosa, the tubificid Spirosperma ferox, unidentified (primarily immature) tubificids, and Dreissena; the gastropods Helisoma anceps, Physa heterostropha, Stagnicola catascopium, Valvata tricarinata, Goniobasis livescens, and Amnicola limosa; the amphipod Gammarus fasciatus and the decapod Orconectes propinquis (Table 2). Total abundance of benthic macroinvertebrates at the artificial reef site was greater (p < 0.01) in (range = 20,859 ± 5,363 to 58,018 ± 11,954/m 2 ) than 1983 (range = 127 ± 41 to 1,866 ± 577/m 2 ) on all four dates (Table 3). Excluding Dreissena from comparisons, abundance increases (p < 0.05) also were observed on all dates (range = 1,373 ± 307 to 5, /m 2 ) relative to 1983.

8 Taxa more abundant (p < 0.05) in than 1983 on two or more dates were Dreissena, Physa heterostropha, Valvata tricarinata, Goniobasis livescens, Amnicola limosa, Gammarus fasciatus, and the trichopteran Polycentropus (Table 3). Eight taxa collected at the cobble site in 1983 were not collected in , while 17 taxa collected at the cobble site in were not collected on similar dates in 1983 (Table 2). Four taxa collected at the artificial reef site in 1983 were not collected in , and 20 taxa collected at the artificial reef site in were not collected on similar dates in 1983 (Table 3). No taxon exhibited a significant population decline between 1983 and on more than one sampling date at either site (Tables 2 and 3).

9 Changes in Benthic Macroinvertebrate Diversity and Community Composition Changes in benthic macroinvertebrate community composition at the cobble and artificial reef sites, according to Morisita's Index (MI), are presented in Table 4. When Dreissena was included in calculations of the index, little community similarity was observed between and 1983 for the cobble site (MI range = ) or the artificial reef site (MI range = ). Excluding Dreissena from the calculations revealed significantly greater similarity (p < 0.05) between and 1983 for the cobble (MI range = ) and the artificial reef (MI range = ) communities. Community similarity at the artificial reef site between and 1983 was especially high on three of the four sampling dates (Table 4). Similarities in Morisita's Index values obtained when Dreissena was not included in community comparisons support the observations (Table 1) that numerically important taxa at both sites in 1983 remained numerically important in Same year comparisons between cobble and artificial reef site communities (Table 4), including Dreissena, suggested that both communities were similar to each other in (MI range = ) and in 1983 (MI range = ). Excluding Dreissena from calculations of Morisita's Index, the cobble and artificial reef communities were less similar (p < 0.05) to each other in (MI range = ) than in Post-Dreissena population increases of oligochaetes at the cobble site, relative to the artificial reef site (Tables 2 and 3), contributed to compositional divergence of the two communities in relative to 1983 (Table 4).

10 Although oligochaete populations also increased at the artificial reef site between 1983 and , changes were less dramatic than at the cobble site where substrate more favorable to burrowing taxa was more prevalent. The overwhelming dominance of Dreissena at both sites in (Table 1) made it otherwise appear that the two sites had nearly identical community compositions (Table 4). The number of taxa collected at cobble and artificial reef sites was greater (p < 0.05) in than in 1983 (Table 5). The number of taxa collected at the cobble site in ranged from 27 to 32 taxa per sampling date, while no more than 22 taxa were collected in Numbers of taxa at the artificial reef site ranged from 19 to 26 taxa in In contrast, no more than 15 taxa were collected on a sampling date in Discussion Our data suggest that Dreissena has thus far had a positive impact on benthic macroinvertebrate communities at cobble and artificial reef sites in southwestern Lake Ontario. Population changes of some taxa are similar to those reported by other researchers who have studied impacts of Dreissena on benthic macroinvertebrate communities in Lakes Erie and St. Clair. Dermott et al. (1993) found total macroinvertebrate abundance (excluding Dreissena) on northeastern Lake Erie bedrock substrates to be greater on bedrock heavily colonized by Dreissena polymorpha than on uncolonized bedrock. Among specific taxa, gastropods and Gammarus fasciatus were significantly more abundant on colonized than on uncolonized substrates. Griffiths (1993) found total abundance and species richness of benthic macroinvertebrates increased in both northwestern and southeastern regions of Lake St. Clair (even if Dreissena was excluded from abundance estimates) following colonization of the southeastern region of the lake by Dreissena. Among taxa increasing in total and relative abundance were turbellarians; the tubificids Potamothrix moldaviensis and Spirosperma ferox; the gastropods Gyraulus sp., Physa gyrina, Goniobasis livescens, and Amnicola sp.; Gammarus sp. and Polycentropus. At both of our sites in , Physa heterostropha, Goniobasis livescens,

11 Amnicola limosa, and Gammarus fasciatus were more abundant than in 1983 (Tables 2 and 3). Spirosperma ferox and Polycentropus were more abundant in than in 1983 at cobble and artificial reef sites, respectively (Tables 2 and 3). Populations of turbellarians, Potamothrix moldaviensis, and Gyraulus parvus were relatively unchanged at either site. We may have underestimated populations of P. moldaviensis due to difficulty encountered in identifying this taxon. Consequently, many P. moldaviensis individuals may have been categorized by us as unidentified Tubificidae. Many factors potentially have contributed to benthic macroinvertebrate community changes observed in our study. Dreissena may be facilitating transfer of nutrients to the benthos by filter-feeding and subsequently depositing feces and pseudofeces (Wiktor 1963, Stanczykowksa et al. 1976, Reeders and Bij de Vaate 1990, Leach 1993). Wiktor (1969) and Izvekova and Lvova-Katchanova (1972) have discussed the importance of Dreissena feces and pseudofeces in diets of detritivorous benthic macroinvertebrates. Saturated with bacteria and digestive enzymes, pseudofeces not only have high nutritive value, but may be easily digested and assimilated (Izvekova and Lvova- Katchanova 1972). Facultative or obligatory detritivores (Harman and Berg 1971, Hynes 1974, Caspers 1980, Klemm 1985) showing population increases at cobble or artificial reef sites were Spirosperma ferox and other tubificids, Physa heterostropha, Valvata tricarinata, Gammarus fasciatus, and Polycentropus (Tables 2 and 3). Dusoge (1966), Wiktor (1969), and Lewandowski (1976) found abundance and biomass of many benthic invertebrates in Europe, including oligochaetes and chironomids, to be greatest among clumps of Dreissena where feces and pseudofeces accumulate. Griffiths (1993) suspected that population increases in Potamothrix moldaviensis and Spirosperma ferox in Lake St. Clair resulted from deposition of feces and pseudofeces by Dreissena. Enhanced substrate complexity also may have contributed to increased abundance and diversity of macroinvertebrates at our study sites. By creating an interstitial network that may increase refugia available to other benthic organisms, Dermott et al. (1993) suggested that Dreissena was responsible for increases in Gammarus sp. observed on bedrock substrates colonized by Dreissena. Griffiths (1993) likewise attributed population increases of hirudineans, gastropods, Gammarus sp., Polycentropus, and the chironomid Polypedilum in Lake St. Clair to increased substrate heterogeneity provided by Dreissena. Of these taxa, only hirudineans and Polypedilum failed to show significant population increases at cobble or artificial reef sites in our study between 1983 and (Tables 2 and 3). Dreissena indirectly creates benthic habitat as well. Filter-feeding improves water clarity through removal of suspended particles (Stanczykowska 1984, Hebert et al. 1991). The resulting increase in the euphotic zone, in combination with increased transfer of nutrients to the benthos by Dreissena, may promote growth of benthic macrophytes (Reeders and Bij de Vaate 1990) and perhaps benthic algae as well. Positive relationships between benthic algae and populations of nematodes, naidid oligochaetes, hirudineans, gastropods (Gyraulus sp., Helisoma sp., Physa sp., Valvata sp., Goniobasis sp., Amnicola sp.), Gammarus sp., ephemeropterans, and Chironomus have been reported from the Great Lakes in the past (Cook and Johnson 1974, Barton and Hynes 1978). Griffiths (1993) believed that increased densities of submerged vascular plants and benthic algae following colonization of Lake St. Clair by Dreissena contributed to the observed increase in macroinvertebrate populations. The filamentous alga Cladophora was present at our study sites in 1983 (Bader 1985) and in , and macroinvertebrates, especially Gammarus fasciatus, were associated with it. Some macroinvertebrate taxa may benefit by consuming or parasitizing Dreissena. The decapod Orconectes limosus was observed to consume large numbers of Dreissena under ideal water temperature conditions in Poland (Piesik 1974). Populations of O. propinquis were far more abundant at the cobble site in than in 1983 (Table 2), although direct consumption of Dreissena by this species has not yet been documented. Dreissena in Europe have also been preyed upon by hirudineans (Lewan-

12 dowski 1976) and the oligochaete Chaetogaster sp. (Piesik 1983). It is interesting that Chaetogaster limnaei, a known commensal of gastropods (Sankwathri and Holmes 1976, Fernandez et al. 1991), was not collected at our sites until after Dreissena became established (Tables 2 and 3). Beedham (1970) described a symbiotic relationship between the chironomid larva Metriocnemus and Dreissena, in which chironomids lived within and upon shells of living Dreissena, possibly feeding upon mucal body secretions. This genus was not collected in our study, but other chironomids may be taking advantage of Dreissena in a similar way. Our study failed to show that Dreissena is negatively affecting other benthic macroinvertebrate taxa of southwestern Lake Ontario (Tables 2 and 3). These results are encouraging in light of concern that Dreissena has generated since it invaded North America. Dreissena is known to settle on the bodies of decapods (Sebestyen 1938), impairing movement and occasionally piercing the decapod exoskeleton with byssal fibers (Lamanova 1971). Goniobasis livescens (Stewart, personal observation) is also colonized. Fortunately, decapods discard mussels upon molting, and most other taxa are too small and short-lived to become heavily colonized. No decapod collected in our study was observed to be carrying Dreissena. Studies conducted within the Great Lakes do suggest Dreissena is threatening bivalves of the family Unionidae by settling on their shells and possibly inhibiting their ability to feed, respire, and reproduce (Hebert et al. 1989, Hebert et al. 1991, Mackie 1991, Schloesser and Kovalak 1991, Hunter and Bailey 1992). There is additional concern that removal of suspended organic matter by Dreissena will also imperil unionids dependent upon this food source (Hebert et al. 1991). While the dome suction sampler was unable to collect large unionids in our study, both large and small unionids were observed at the cobble site in , and few zebra mussels were attached to them. Unionids collected by hand at the cobble site included Elliptio complanata, Lampsilis radiata, and Ligumia sp. While most macroinvertebrate population changes observed in our study may be attributable to Dreissena, these changes may also reflect changes in water quality (Cook and Johnson 1974, Sly 1991), habitat (Haynes and Makarewicz 1982) and thermal regimes (Barton 1986) that were unrelated to the Dreissena invasion. Phosphorus abatement programs have contributed to declines in total phosphorus concentrations throughout Lake Ontario since the mid-1970s (Stevens and Neilson 1987, Great Lakes Fishery Commission 1992), but assessing the effects that phosphorus abatement has had and will continue to have on benthic macroinvertebrate populations is problematic. Johnson and Mac-Neil (1986) attributed declines in the abundances of some oligochaete, sphaeriid bivalve and isopod taxa in the Bay of Quinte to the reductions in phosphorus loading to Lake Ontario. Barton (1986) observed declines in total benthic macroinvertebrate abundance in areas undergoing rapid deeutrophica-tion, but noted that species diversity often increased under such conditions. Increased overall abundance of benthic macroinvertebrates, including at least one taxon (Spirosperma ferox) known to inhabit nutrient-rich habitats (Klemm 1985), suggest nutrient deposition by Dreissena has more than compensated for oligotrophication processes in southwestern Lake Ontario benthic environments between 1983 and Increased water clarity resulting from declining phosphorus concentrations, in combination with Dreissena filter-feeding, also may lead to a deeper and warmer epilimnion (Mazumder 1990) which may increase benthic production. Since the artificial reef was only a year old when its macroinvertebrate fauna were first sampled in 1983 (Bader 1985), many taxa may not have had sufficient opportunity to colonize the site or to attain stable population densities by that time. It is also conceivable that natural processes of primary succession have altered physical characteristics of the artificial reef, making the structure more or less favorable over time for certain taxa. Despite these possibilities, community changes at the long established cobble site appeared more extensive than at the artificial reef (Table 4), suggesting that age of study site cannot explain changes measured in our study.

13 Many benthic macroinvertebrate populations are irregularly distributed (Elliott 1971) or vary greatly in seasonal abundance. Since any sampling is likely to miss rare taxa, and the ecological requirements of the taxa not collected in 1983 or (Tables 2 and 3) are diverse (Simpson and Bode 1980, Clarke 1981), the absence of many rare taxa is likely due to sampling anomalies or chance. However, the absence of Musculium partumeium at the artificial reef site in (Table 3) is understandable since this species prefers silty or muddy substrate rather than rock (Mackie et al. 1980, Clarke 1981). In summary, Dreissena was the overwhelmingly dominant benthic macroinvertebrate taxon at cobble and artificial reef sites in southwestern Lake Ontario by Nevertheless, overall abundance of other benthic macroinvertebrates, and the number of taxa collected, were greater in both habitats following establishment of Dreissena populations. By occurring in large clumps and filter-feeding intensively, Dreissena may benefit other macroinvertebrate taxa by increasing the complexity of benthic substrate and by increasing the flow of energy to benthic environments. Thus an increase in benthic photosynthesis (due to increased water clarity caused by nutrient reduction programs and Dreissena biofiltration), along with biodeposition by Dreissena, may be keeping benthic food resources at high levels, despite overall declines in fertility of the Lake Ontario ecosystem. This study failed to provide evidence that Dreissena has induced population declines in any taxon collected at cobble or artificial reef sites in It appears that invasion of Dreissena and other recent environmental changes have created conditions more favorable for most benthic macroinvertebrate taxa in nearshore regions of southwestern Lake Ontario. Acknowledgments Drs. J.K. Buttner, J.C. Makarewicz (State University of New York College at Brockport, NY), and Mr. D.B. MacNeill (New York State Sea Grant, Brockport, NY), reviewed an early draft of the manuscript. Mr. T. Belling (Niagara County Department of Planning and Industrial Development, Lockport, NY) and the Niagara County Sheriff's Department provided boats, dive gear, and other support. We thank B. Drury, B. Wright, and dive-team members for help in collecting samples. A dome suction sampler was borrowed from J. Ноmа (Icthyological Associates Inc., Ludlowville, NY). Dr. J. McNamara and Mr. N.J. Frisch (SUNY Brockport) assisted with data analysis and graphic design, respectively. Annelid identifications were verified by Drs. J. K. Hiltunen (Great Lakes Fishery Laboratory, Ann Arbor, MI), D. J. Klemm (United States Environmental Protection Agency, Cincinnati, OH), and D. A. Strayer (Institute of Ecosystem Studies, Millbrook, NY). A. Bader (New York City Department of Environmental Protection, Grahamsville, NY) confirmed that identifications of taxa collected in were consistent with those collected in We also thank R. Cleland, M. Keleher, S. Miller, K. Nolan, A. Pulver, L. Serafin, and other Brockport students for their help in the field and lab.