Benthic invertebrate assemblage changes in an urban bay of Lake Ontario: 1990 to 2012

Abstract Invasions by dreissenid mussels and the round goby have altered the makeup of benthos in many areas of the Great Lakes complicating the use of benthic invertebrate composition as an indicator of environmental conditions. The Ontario Ministry of the Environment, Conservation and Parks (MECP) has been monitoring the composition of benthos at nearshore stations since the 1990s. The interactive influences of environmental change and invasive species have been evident in areas of the Toronto Waterfront such as Humber Bay (HB), an area of intensifying urbanization. Dreissenid mussels were first detected in HB in 1994, became abundant in 2000, and were followed by the round goby in 2005. Both invasions overlapped with increasing population density in the watershed and efforts to reduce pollutant and nutrient inputs. Benthic invertebrate diversity increased post-dreissenids, with densities of gastropods and amphipods increasing during peak mussel abundance (2000–2003). Dreissenid abundance declined after 2005, and snails (Valvatidae) disappeared, corresponding with increasing goby abundance. Abundance of sphaeriid clams declined over the monitoring period. Water clarity increased and the chlorophyll a level declined in the mid-1990s, before dreissenids were abundant at the site. Trophic conditions inferred from invertebrate composition shifted from oligotrophic in 1990 to mesotrophic and back to oligotrophic in 2012. There was little change in sediment quality other than a subtle increase in clay and fine silt at the primarily silty site. The trajectory of changes in the benthos was more strongly correlated with the dreissenid and goby invasions than changes in environmental quality.