Scenarios of water quality: projected effects of phosphorus management in a changing climate

Background/Question/Methods

Changes in climate, land use, management practices and other factors will affect lake water quality in coming decades. We developed a family of models for projecting future water quality using a 33-year database (1975-2008) for the Yahara River chain of lakes (Mendota, Monona, Waubesa, and Kegonsa) near Madison, Wisconsin. The database includes complete phosphorus loading budgets for the lakes as well as water quality variates such as P concentrations and Secchi transparency. The models compute posterior probabilities of summer water quality (TP and Secchi) conditional on management scenarios. We used the models to address two questions. (1) Will proposed reductions of phosphorus inputs from land to the lakes improve the water quality from eutrophic to mesotrophic? (2) How will a trend toward more extreme precipitation events affect water quality?

Results/Conclusions

Improvements in summer water quality due to P management were different among lakes. The largest benefits are projected for Mendota, the uppermost lake of the chain with the greatest mean depth and longest water residence time. Monona, with an intermediate mean depth and residence time, also shows some improvements in water quality. Waubesa and Kegonsa, the lower lakes with relatively shallow mean depths, water residence times of only a few months and high P loadings from the inflowing Yahara River, remain eutrophic under most conditions. Presence of the grazer Daphnia pulicaria significantly reduces TP and increases Secchi during summer in all conditions we considered. However, a shift toward more extreme precipitation events, a trend recently observed and projected to continue due to climate change, could undermine any gains in water quality from P management.