Many lakes in agricultural watersheds are eutrophic and suffer from increased sedimentation, due to excessive runoff of nutrients and sediment. In recent decades, widespread improvements in agricultural practices have occurred. Especially prominent among these is a shift towards reduced tillage, which often leads to decreased exports of phosphorus (P) and sediment from agricultural landscapes. However, we know very little about how downstream lakes respond to shifts to more sustainable agricultural practices. We use long-term data to test the hypothesis that improvements in agricultural practices in the watershed of a eutrophic reservoir (Acton Lake, Ohio) lead to decreased phytoplankton biomass. 
Results/Conclusions

The use of conservation tillage in this watershed has increased from about 15% in 1992 to about 60-75% of cropland in 2007. In addition, the use of P fertilizers has decreased and animal operations (e.g., hog farms) are managed better. During this period, concentrations of P and suspended sediment in streams entering Acton Lake have declined by about 5-6% per year, when standardized for stream discharge; thus concentrations have declined to about 50% of initial values. P and sediment loading to the lake is greatly influenced by stream discharge, which depends on precipitation; however, discharge-standardized loading has declined substantially over this period. Despite this substantial decline in P loading, phytoplankton biomass has steadily increased: chlorophyll increased by about 50% from 1994-2007 (about 2.4 ug chlorophyll per liter per year). We attribute the increase in phytoplankton to two causes. First, the decline in suspended sediment inputs from streams has led to a 75% decrease in suspended sediment concentrations in the lake, leading to increased light availability for phytoplankton. In addition, the biomass of sediment-feeding fish (gizzard shad) is high; translocation of P from sediments to water by these fish sustains high phytoplankton productivity, especially in summer. Our results support the notion that eutrophic reservoirs in agricultural watersheds may be quite resilient against decreases in non-point P inputs because of concomitant reductions in sediment loading, and because of high rates of internal P fluxes mediated by sediment-feeding fish.