Dreissena population status in nearshore Lake Ontario

Dreissenid mussels are ecosystem engineers in the Great Lakes, affecting benthic and water column communities and production. We surveyed mussel populations at four Canadian and three U.S. locations in summer 2008 to update population status and examine correlations with water column data. We measured mussel length, density, shell-free dry mass (SFDM), condition index, and phosphorus content of both shells and mussel tissue. The water column variables of chlorophyll a, turbidity, and total phosphorus (TP) were correlated with each other lake-wide, but exhibited only a few correlations with mussel metrics within seasons or shorelines. Quagga mussels (Dreissena rostriformis bugensis) represented similar to 99% of the mussel community in nearshore collections. Mussel length declined in a west-to-east direction and increased with depth in both U.S. and Canadian nearshore waters. Mussel density declined west-to-east in U.S. water, but exhibited no difference among sites in Canadian waters. Mussel condition index and phosphorus concentrations were correlated and increased west-to-east within the U.S. nearshore. There were significant declines in both tissue and shell P content with season in U.S. mussels, but no clear patterns in Canadian mussels. We estimated there were 9.7 x 10(12) mussels (mean = 3402.9/m(2)) in the Lake Ontario nearshore totalling 1.2 x 10(5) mT of mussel tissue which could filter the entire Lake Ontario nearshore volume (0-20 m depth = 30.9 km(3)) in roughly 1 to 7 days. It appears that mussel density has declined since the last large surveys of 5 or 10 years ago (Canadian nearshore or U.S. nearshore, respectively), however the data were either only slightly supportive of, or showed no support for, food limitation or goby predation as the most parsimonious explanation for the decline in mussel abundance. (C) 2012 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.