Characterisation of water temperature variability within a harbour connected to a large lake

Many coastal embayments in the Laurentian Great Lakes have highly variable temperatures due to pronounced movements of the thermocline in the nearshore zone. As an example, we document the diverse thermal regimes in Toronto Harbour, which is also the site of some of the most extensive fish habitat restoration activities in Lake Ontario. Toronto Harbour is characterised by considerable thermal variability as a result of diurnal heat fluxes and large amplitude movements of the thermocline of Lake Ontario. During the ice-free period from April-November 2013, an array of benthic and surface temperature loggers were deployed to obtain the spatio-temporal distribution of water temperatures. Complementary measurements of stratification were made in Lake Ontario at a site 5 km offshore. The dominant periods of short-term thermal variability were 12, 17, and 24 h, reflecting both diurnal heat fluxes and inertial oscillations of Lake Ontario's thermocline. The thermocline in Lake Ontario was observed to oscillate by as much as 15 m, around a mean depth of 9 m, which is comparable to the mean depth of Toronto Harbour. Cold intrusions were found to quickly flow from the lake into the harbour and lead to rapid drops in temperature (e.g., as much as 15 °C in less than 4 h). Such “cold shock” events may be associated with a variety of negative effects on many aquatic organisms, especially warm-water fishes. We consider the potential impacts of the observed temperature variability on cool and warm-water fish species that are the target of restoration activities.