Seasonally non-uniform responses to climate change in temperate lakes

Climate change effects on physical, chemical and biological processes of lakes are of great concern to society. Most work on contemporary climate change in lakes has focused on trends in seasonally averaged lake temperatures, frequently focusing on the summertime or stratified periods. Such approaches may mask heterogeneity in lake temperature trends across depth or seasons that are of critical importance to understanding the ecological implications of future climate change.

Here we analyze a long-term dataset (1984-2014) of bi-weekly collected water temperature data in 6 temperate U.S. lakes to examine how temperature trends vary across season and depth. Being insulated from energy fluxes at the surface for most of the summer, deep-water trends were more seasonally consistent and more muted than surface trends; bottom waters of all lakes warmed at a rate of 0.1-0.5 °C/decade, excluding one lake which had cooling bottom waters (-0.2 °C/decade) caused by a change in water clarity. Surface temperature trends were heterogeneous across seasons, with the strongest warming occurring in surface waters in the late summer and early fall (rates up to ~1 °C/decade). Paradoxically, early spring surface water temperatures cooled during the same period by an average of -0.2 °C/decade. This strong seasonality in surface water temperature trends matched seasonality in air temperature trends, suggesting air temperature trend seasonality might be used to extend these results to unexamined lakes. Understanding this seasonality and cross-depth heterogeneity of temperature trends in lakes will be important to understanding the ecological implications of future climate change.