Patterns in water quality on Canadian shores of Lake Ontario: Correspondence with proximity to land and level of urbanization

Spatially detailed surveys were conducted in 2008 over 97 km of the Canadian coastline of Lake Ontario to describe patterns in water quality correlated with natural and anthropogenic features. Features of variability were used to infer lake-wide, regional, and location specific influences on nearshore water quality. Levels of solids, nutrients, major ions, Escherichia coli, and dissolved organic carbon were higher and more variable among tributary, embayment, and shoreside (0 to 1.2 m depth) areas compared with the open nearshore (~3 m depth to 5 km offshore). A recurring pattern was a sharp drop in these features away from the shoreline with little or subtle decline beyond 1 to 3 km into the nearshore. Conductivity and UV-induced fluorescence of organic material were used to detect discharges to the nearshore. Land-based influence on water quality was greatest in the urbanized Greater Toronto Area and the least over the less developed areas. Nearshore regions received inputs from the adjacent shoreline affecting water quality, the spatial extent of which was difficult to generalize, ranging from 10s of metres to several kilometres in breadth. Despite this, concentrations of total phosphorus, soluble reactive phosphorus, dissolved inorganic nitrogen, and silicate, when averaged over the entire nearshore, were typically similar to the open lake reflecting the broad distribution of the nearshore relative to the areas most influenced by adjacent lands. The development of environmental management to address water resource problems in the nearshore should proceed with awareness of the strong spatial dimensions that define nearshore water quality.

► Shipboard sensors were used to map water quality in the nearshore of Lake Ontario.
► Variability in water quality typically became uniformly low beyond 1-2 km from shore.
► Nutrients, major ions, solids and E.coli levels were highly variable at the shore.
► Mixing areas of shore discharge and runoff where common but widely variable in extent.
► Differences in land-based effects on the nearshore at a regional scale were observed.