The effect of future nutrient scenarios on a eutrophic lake: Insights from aquatic ecosystem simulations

Background/Question/Methods

Lake Mendota, Wisconsin, USA has been subject to anthropogenic eutrophication for more than a century. A phosphorus (P) budget was estimated for the watershed of Lake Mendota to assess the effects of recent nutrient management on P accumulation in the watershed soils. We estimated how nutrient management programs and legislation have affected the budget by comparing the budget for 2007 to a budget calculated for 1995, prior to implementation of the programs. Four scenarios were investigated to determine potential impacts of additional nutrient management tactics on the watershed P budget and P loading to Lake Mendota. Concurrently, to determine the influence of exogenous drivers and starting conditions on phytoplankton dynamics, we simulated the response of dominant phytoplankton groups to different nutrient loading and nutrient initial conditions using a one-dimensional coupled hydrodynamic-biogeochemical model, also for Lake Mendota, Wisconsin. High-frequency in situ automated sensor data and long-term manual observational data from Lake Mendota, Wisconsin, USA, collected by the North Temperate Lakes Long Term Ecological Research program, were used to parameterize, calibrate, and evaluate model predictions. We focused specifically on short-term (< 1 month) predictions of phytoplankton biomass over one season.

Results/Conclusions

Since 1995, overall inputs to the watershed decreased from 1,310,000 to 853,000 kg P/yr (35% reduction) and accumulation decreased from 575,000 to 279,000 kg P/yr (51% reduction). Changes in P input and accumulation were attributed primarily to enhanced agricultural nutrient management, reduction in dairy cattle feed supplements and an urban P fertilizer ban. Elimination of chemical P fertilizer input has the greatest potential to reduce watershed P accumulation and establishment of riparian buffers has the greatest potential to prevent P loading to Lake Mendota. For the in-lake aquatic ecosystem simulations, we found that the initial condition of water column phosphorus concentration was more important to the timing and magnitude phytoplankton response than was nitrogen concentration, and that phytoplankton biomass was less sensitive to the magnitude of loading than to springtime loading, represented by increased nutrient concentrations at the start of the simulation. Together, these results indicate that minimizing the loss of accumulated nutrients stored in watershed soil, e.g. by implementation of riparian buffers, could result in significant benefits to in-lake water quality.