Multiple stressors influencing the lakebed ecology of nearshore eastern Georgian Bay

Abstract

Spanning two contrasting geological landforms, the heterogeneous landscape of eastern Georgian Bay comprises an archipelago of embayments, channels, and reefs that support a diverse benthic macroinvertebrate community. The exposed bedrock of the outer coastline contrasts with the mostly depositional silts of the inner coastline embayments bordering the Canadian Shield. Abundance of benthic algae and composition of benthic invertebrates were sampled over four regions and 50 km of coastline from Severn Sound to Shawanaga Island from 2014 to 2015, to assess where invasive dreissenid mussels and anthropogenic activity are impacting water quality or habitat conditions. Low surface cover and biomass of Dreissena, an absence of benthic macro-algae, and the lack of relationship between mussel occurrence and periphyton chlorophyll a on hard substrate of the more calcium-rich waters of the outer nearshore where dreissenids are found, suggests little effect of mussels on lakebed bio-physical structure. Further inshore, where dreissenids are largely absent, composition of benthic invertebrates in depositional sediments was influenced by physical makeup of substrate and the ubiquitous gradient in water quality from inshore to offshore associated with mixing of the drainage from watersheds on the Canadian Shield into the Lake Huron basin. Round goby (Neogobius melanostomus) was widely distributed over the outer coastline where visual sampling was conducted over hard substrate. The composition of oligochaetes on soft sediment is suggestive of anthropogenic enrichment at some sites. Collectively, the inherent physical–chemical habitat variability of the coastline accounts for much of the variability in the benthos.