The potential influence of climate change on offshore primary production in Lake Michigan

This paper examines the potential influence of climate change on the primary productivity of Lake Michigan. Two general circulation models (GCMs) provided physical information on projected regional climate for the years 2030, 2050, and 2090. A 30-year record of meteorological data and limnological observations, from 1961 to through,1990, was used to define present, baseline conditions for the lake. GCM output was used to develop scenarios of future thermal characteristics, mixing patterns, and surface irradiance, which were then used to drive primary production calculations. Mean annual primary production for the base period was 116 g C/m(2). Under base conditions thermal stratification of the lake occurred on 13 June and extended 135 days until 26 October. Conditions projected for 2090 showed the mean date of stratification beginning as early as 5 April and remaining for 225 days until 20 November. Estimated mean annual primary production under these conditions totaled 113 g C/m(2), a decrease of 3% from the mean base value. Under the most extreme conditions of maximum projected cloud cover for 2090, primary production in that year could fall to 101 g C/m(2), a decrease of 13% from the base mean, or down 22% from maximum base production calculated under minimum base cloud cover conditions. The projected decrease may be attributed to physical/chemical constraints imposed on spring primary production by altered climate conditions. Early stratification would shorten the period of winter-spring mixing, during which time nutrients from the sediment are transported to the productive euphotic zone. The spring bloom was projected to diminish if early stratification capped the nutrient supply, and increased cloud cover reduced light input for photosynthesis. To a lesser extent fall production could also be reduced by the extension of the stratified period. Altered physical/chemical conditions influenced by a changing climate will be an important factor to consider in assessing future water quality conditions, primary production and the food web dynamics of the Great Lakes.