Temporal changes in the benthic invertebrate community of Lake Simcoe, 1983–2008

The Lake Simcoe ecosystem has been subjected to multiple stressors during the past century, including changes in nutrient loads and the introduction of non-native species. To evaluate the effects of these changes on the Lake Simcoe offshore benthic community, we collected benthic samples in 2008 at sites allowing statistical comparisons with detailed surveys in 1983 and qualitative comparisons with others focussing more on nearshore collections in 2005 and 1926. Total abundance of benthic invertebrates declined significantly from 1983 to 2008, and correspondence analyses revealed major changes in community composition. Typical of the overall pattern, three of the major benthic taxa (Nematoda, Oligochaeta and Ostracoda) declined from 1983 to 2008, particularly at depths greater than 10 m. Chironomids showed little change in abundance from 1983 to 2008 but their biomass increased because body length nearly doubled during this period. Oligochaete abundance declined but biomass remained the same due to increases in body length from 1983 to 2008. Ostracod abundance and biomass both declined because body size distributions did not change from 1983 to 2008. In contrast to total invertebrate abundance, that of dreissenid mussels, amphipods, isopods, gastropods and sphaeriids increased and these taxa also occupied greater depths (up to 30 m) in 2008 compared with historical surveys. Our results suggest a strong effect of both water quality changes and dreissenid establishment in shaping the contemporary Lake Simcoe benthic invertebrate community.