Legacy Phosphorus

Big Green Lake, Wisconsin’s deepest natural inland lake, has long been known and valued for recreational opportunities and scientific study. The lake’s primarily agricultural watershed supports productive grain, dairy, and canning crop industries, and encompasses two growing cities. However, since the 1950’s, Green Lake has experienced marked declines in water quality. Currently, the lake and four of its seven tributaries are on the Clean Water 303(d) impaired waters list for total phosphorus and/or total suspended solids.

Despite widespread implementation of conservation practices since the 1980’s, watershed managers have not observed reductions of phosphorus and sediment loads to Green Lake sufficient to improve its trophic state. This begs a better understanding of processes in upland areas that may be delaying improvement, particularly the presence of legacy phosphorus in watershed soils, streams, wetlands, and lake sediments. As legacy P is moved, recycled, and redeposited over time it acts as a continuous P source to water bodies, effectively veiling the positive benefits of conservation practices. This makes it extremely challenging to identify whether current or historical land management are having more of an effect on water quality. Taking a holistic look at watershed phosphorus use and accumulation over time can begin to decouple the impact of current and historical actions and give watershed managers valuable information to guide future planning.