Tributary discharge, lake circulation and lake biology as drivers of water quality in the Canadian Nearshore of Lake Ontario

A combination of data from shipboard and deployed sensors, nutrient and major ion concentration data, and tributary discharge and current data was used to assess water quality patterns on the Canadian nearshore of Lake Ontario in 2008 and the factors that impact these patterns. Mixing areas at tributary mouths were characterized by high spatial and temporal variability in water quality. Conductivity at sensors offshore from tributaries was variably correlated with discharge, with the correspondence most evident during high spring flow. Tracers of land runoff were positively correlated with tributary discharge at three of five study locations. The spatial distribution of water quality was strongly influenced by the predominating alongshore circulations, with gradients frequently observed parallel to the shoreline away from tributary mouths. Upwelling, evident from temperature bands along the coastline, periodically created onshore–offshore gradients in chlorophyll a and dissolved inorganic nitrogen. Lake whiting, starting in July, further enhanced the effect of upwelling by accentuating differences in water clarity, suspended solids, and ionic composition between surface and bottom strata. In general, turbidity decreased and water clarity increased with lake depth. Gradients in water clarity and chlorophyll a suggestive of dreissenid mussel feeding were uncommon. Interacting land- and lake-based factors contributed to heterogeneous water quality over the coastline of Lake Ontario highlighting needs for water resource assessment methods sensitive to the temporal and spatial scales of problems as well as the connectivity between the land and the lake.

► Drivers of water quality in the Canadian nearshore of Lake Ontario were examined.
► Variable water quality at tributary mouths was a pervasive feature of the nearshore.
► Spatial patterns were strongly shaped by predominating alongshore circulation.
► Upwelling resulted in complex onshore–offshore gradients and changeable conditions.
► Interacting land and lake-based factors create heterogeneous nearshore water quality.