Mixing Patterns and Plankton Biomass of the St. Lawrence Great Lakes under Climate Change Scenarios

This study is part of an assessment of potential effects of climate change on the St. Lawrence Great Lakes. Its purpose is to investigate potential future lake mixing patterns and primary production. Nested physical and biological models were applied to seasonal mixed layer depth, heat content, primary productivity, and to algal biomass measured as particulate chlorophyll. Two independent second generation General Circulation Models provided scenarios for future conditions of cloud cover, air temperature, humidity, and winds. The climate variables were used to force heat balance and surface mixed layer models for Lakes Superior, Michigan, Huron, Erie, and Ontario. Physical models of heat balance and mixed layer dynamics were coupled with a model of primary biological production and growth of phytoplankton. Simulated climate conditions were for time periods centered at 1975, 2030, 2050, and 2090. Climate projections from both GCMs lead to elevated mixed layer and bottom temperatures in all five lakes by as much as 5°C during this century. Both GCMs point to longer duration of thermal stratification in the five lakes, stronger stability of stratification, and deeper daily mixing depths during peak thermal stratification. For Lake Erie, no striking differences in algal biomass are likely according to climate projections of either model, but for the other lakes, either the duration of nutrient limitation of algal growth is projected to increase, or light limitation caused by deeper mixing is projected to limit the development of algal biomass.