The shoreline and sublittoral areas of Lake Simcoe were surveyed in October 2005 as part of a baseline inventory of current conditions, and as part of an exploratory analysis to determine if there were threshold nutrient levels above which nutrients were a major controlling factor in the distributions (abundances) of benthic organisms. Along the shore environment, there was no generalized relationship between nutrients and indices of benthic community composition below water-borne total phosphorus concentrations of ~ 0.05 mg/L, indicating that other factors were more important at those lower levels of phosphorus. Benthic community composition did not vary with water quality in the sublittoral zone, but did vary with sediment quality (total Kjeldahl nitrogen, total phosphorus). The benthic community had higher relative abundances of caddisflies and mayflies at sublittoral stations with TKN levels at or near the provincial low-effect level of 550 mg/kg, and had higher relative abundances of more tolerant taxa such as worms, amphipods, and snails at locations with TKN levels in excess of the provincial severe-effect level of 4,800 mg/kg. The relationship between composition and TKN in sediments was not obvious below concentrations of ~ 300 mg/kg, indicating that other factors were more important at those levels of TKN. This study was the first to document an inland (non-Great Lake) colonization by the exotic amphipod Echinogammarus ischnus. This species was found along most of the shoreline with the exception of shorelines within Cook's Bay, along Sibbald Point Provincial Park and at the far end of Kempenfelt Bay. Where found, E. ischnus were generally numerically dominant members of the benthic community.
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