Continental-Scale Projections of Potential Climate Cbange Effects on Small Lakes in the Contiguous U .8. Vol. 2 Effects of Projected Future Climate Conditions on Lake Water Temperatures and Dissolved Oxygen

This study is concerned with projections of climate change effects on lakes, especially small lakes with surface areas up to 10 km2 and depths up to '24m in the cold regions of the contiguous U.S. For this study, we have chosen lake parameters which are most directly influenced by climate and which in tum have much influence on aquatic lifeforms, water quality and water uses. The two main parameters studied herein are lake water temperature (T) and dissolved oxygen (DO) concentration. In the process, we have also obtained projections on evaporative water losses from lakes, ice covers on lakes and sediment temperatures below lakes. Potential changes of fish habitat (as constrained by T and DO) in lakes have also been estimated. To make such a broad study, we had to develop and apply process-oriented, simulation models which link atmospheric conditions to lake water conditions. Before the models were applied at the continental-scale in this report, the model formulations and assumptions were reviewed to examine what geographically variable parameters had to be introduced. The models were used on 27 different types of lakes. The lakes' chosen differed by surface area, maximum depth and transparency as measured by Secchi depth. These three parameters are known to have a crucial influence on lake water temperatures and DO concentrations. The Secchi depth was related to transparency as well as trophic state of a lake. This is a major assumption which will not hold true in lakes which show turbidity from inorganic suspended sediments. Secchi depth was related to mean annual phytoplankton chlorophyll-a concentration in a lake. This made it possible to estimate photosynthetic oxygen production without specification of nutrient inputs from the watershed. Lakes were also treated as having constant volume and long hydraulic residence times.