Climate Change Effects on Water Temperature and Dissolved Oxygen in Five North Carolina Lakes

Abstract

A deterministic, year-round, one-dimensional water quality model, MINLAKE95, was used to investigate climate change effects on water temperatures and dissolved oxygen (DO) in five North Carolina lakes. The model was applied in daily timesteps over periods of several years. Past (recorded) weather conditions and a projected climate scenario under a doubling of atmospheric carhon dioxide were used as model inputs. Many of the lakes in the southeastern United States are matl~made reservoirs. Three such reservoirs atld two shallow natural lakes were modeled. The reservoirs are in the Piedmont and Mountain regions of North Carolina and are from 20 m to 65 m deep. The natural lakes are in the Coastal Plain and less than 4 m deep. All five lakes are major water bodies with long hydraulic residence times and surface areas ranging from 11 to 58 km2• Standard errors of simulation relative to point measurements were 2.0DC for water temperature and 1.4 mg L-J for dissolved oxygen. Measurements were available as profiles against depth at a single station in each lake. Each lake was modeled under past lxC02 climate conditions (1961- 79) and under a projected 2xC02 climate scenario. To illustrate the climate change effect, plots of isotherms and DO-isopleths in a depth versus time coordinate system were prepared. Mean annual and extreme year values were - plotted. To quantify the climate change effect further, extreme values of temperature and DO values in the surface layer and the bottom layer of each lake were tabulated, as well as periods of anoxia, lake volumes affected by anoxia, and periods and lake volumes with DO < 2' mg/l or DO < 3 mg/I. The .climate change effects were most apparent in maximum surface temperature increases of 2.8 to 3 DC, maximum bottom temperature increases of 1.6 to 2.9DC, and a 20% increase in evaporation in all five lakes. Low DO levels in the three reservoir hypolimnia were projected to be extended by up to 48 days in the 2xC02 climate scenario. Implications for fish habitat are that only the most temperature tolerant of the warm~water fishes would find suitable habitat in the natural 'lakes under the 2xC02 climate scenario. Cool~water fishes might survive at the intermediate (thermocline) depths of reservoirs where DO levels are sufficient and maximunl temperatures tolerable. Cold-water fishes would not find suitable habitat, except in refugia, e.g. due to groundwater inflows.