Factors influencing changes in the nearshore benthic community on the Canadian side of Lake Ontario

The nearshore benthic communities on the Canadian side of Lake Ontario were surveyed in 1991 at depths of 5 and 20 m, and compared to data obtained in 1981 at the same depths and locations. The comparison was made to determine if the nearshore community reflected known reductions in nutrient concentrations over the 10-year period. The study also was designed to determine the extent to which natural environmental factors such as depth, substrate, fetch, and the likelihood of upwelling alter responses of benthic communities to improvements in water quality. In general, both 5 and 20-m communities reflected the reductions in nutrient concentrations and were indicative of the improved conditions. In 1981 at 5 m, worms were the most dominant group, accounting for 79% of the total fauna. Chironomidae (9%), Gastropoda (4%), Amphipoda (3%), and Sphaeriidae (2%) were subdominant. In 1991 at 5 m, the dominance of worms in the total fauna was reduced to about 15%, while chironomids increased to 34%, amphipods to 27%, gastropods to 18%, and sphaeriid darns to 4%. Similar to 5 m, the fauna at 20 m in 1981 were dominated by worms (65%), while chironomids (10%), amphipods (15%), and sphaeriids (8%) were less dominant. In 1991 at 20 m, the fauna remained dominated by worms but their proportion of the total numbers declined to 48%. At the same time, the proportion of chironomids increased to 20%, amphipods decreased to 7%, gastropods increased to 3%, and sphaeriids increased to 21%. With this data set a variety of indices of benthic community composition were calculated based on lowest-practical-level identifications. Relationships between changes in these indices and habitat features were explored. Upwelling and substrate type modified the response of the benthic community at 20 m, and are thus important factors to consider in future designs examining whole lake changes in benthic community composition. In contrast changes in benthic community indices over time at 5 m were unrelated to the type of substrate, fetch or upwelling. As a result, future surveys of 5-m benthos require data from only one substrate type and do not require stratification of study designs by fetch or upwelling. Zebra mussels were found along the western shoreline of Lake Ontario in this 1991 survey in low abundances (averaged 730 indiv/m(2)), as well as sporadically along the northern shore. At these low abundances, zebra mussels had no significant effects on changes in benthic community composition.