Thermal habitat characteristics for warmwater fishes in coastal embayments of Lake Ontario

Along 40 km of the Lake Ontario shoreline near Toronto, Ontario, small coastal embayments (4.38–848 × 103 m2) have been constructed or modified by lake-infilling to restore warmwater fish habitat. We describe how the thermal regimes of these embayments differ from those of small inland lakes, how embayment bathymetry alters the degree of exchange with Lake Ontario, and predict how embayment thermal regimes affect warmwater fish growth. The accumulated growing-degree days, calculated from a 14 °C baseline (∑ GDD14), of seven South-Central Ontario inland lakes and an embayment disconnected from Lake Ontario ranged from 844 to 1157 GDD. Compared to the inland lakes and disconnected embayment, the coastal embayments connected to Lake Ontario achieved fewer ∑ GDD14 and had a greater range, 390–1047, reflecting differences in their degree of exchange with and the cooling effect of Lake Ontario. The thermal regime of coastal embayments differs most from inland lakes during May–late-July. During early summer, mean embayment depth explains over 50% of the variability in ∑ GDD14, with deeper embayments cooling more from lake exchange than shallow embayments. After mid-summer, the cooling embayments are warmed by exchanges with Lake Ontario. This late-summer warming is insufficient to compensate for the cooling effect of the Lake earlier in the summer. Bioenergetic simulations of bluegill, Lepomis macrochirus, growth indicate that most of these embayments are too cool for adequate summer growth of YOY sunfish. Nevertheless, small coastal embayments that are shallow with suitable controls on exchange with the Lake can provide acceptable warmwater fish habitat.

Research Highlights
► Small coastal embayments are cooler than small inland lakes.
► Coastal embayments are cooled by the input of water from the Great Lakes.
► Cooling in the coastal embayments is greatest in the spring through mid-summer.
► The cooling is sufficient to reduce the growth potential of YOY warmwater fish.
► Shallow embayments are warmer than deep embayments.