Thermal mixing patterns in Lake Ontario as revealed by novel year-round observations of thermal stratification

Year-round records of thermal stratification in the Great Lakes are rare, and there are few observations of thermal stratification during winter. In this paper we analyze temperature data from 13 temperature logger chains and from over 130 benthic acoustic receivers that were deployed across Lake Ontario for two years. The timing and duration of the fall overturn correlates with the local average water depth, and shallow sites (<50 m depth) overturn up to a month before deep sites (> 100 m depths). Likewise, in spring the shallow sites warm faster. Lake Ontario has partial ice cover, so wind driven mixing stirs the water column throughout winter and inverse thermal stratification is largely absent. The depth-averaged winter water temperatures vary between 0 – 4^oC, with the coldest temperatures (near 0.1^oC) found in the shallow Kingston basin, and warmest temperatures (near 4^oC) at sites near the 244 m deep Rochester Basin. Lake Ontario appears to be a warm monomictic lake, rather than having a dimictic mixing pattern – there is no sustained ice cover or inverse stratification that inhibits vertical mixing in winter. Winter is a poorly understood season for many aquatic processes, including fish bioenergetics, fish distribution, biochemical processes, invertebrate distribution and production. Moreover, lack of knowledge of winter has hampered the use of correct initial conditions for running large lake hydrodynamic models. We discuss the implications of these 2-year observations of thermal stratification in Lake Ontario for interpreting fish habitat usage and fish reproductive phenology and for fisheries management.