A Phosphorus Budget for the Lake Mendota Watershed

A phosphorus (P) budget was calculated for the agriculture-dominated Lake Mendota watershed located in Dane and Columbia Counties, Wisconsin, USA. P inputs included fertilizer for agricultural crops and lawns, dietary supplements for dairy cattle, and natural inputs such as dry and wet deposition. Outputs included agricultural crops, livestock and livestock products, and hydrologic export to Lake Mendota. The total P input to the watershed ($1,307,000\ {\rm kg}\ \text{year}^{-1}$) and total output ($732,000\ {\rm kg}\ \text{year}^{-1}$) are large relative to the average of 34,000 kg P washing into the lake each year, indicating that the P flux that eutrophies Lake Mendota is a very minor component of the total watershed P budget. Using the formula inputs - outputs = change in storage, we found that 575,000 kg P accumulated in the watershed in 1995. This estimate was corroborated by long-term soil P concentration data, which showed an average annual increase in soil P of over$450,000\ {\rm kg}\ \text{year}^{-1}$. Future management programs designed to reduce P inputs to Lake Mendota will be compelled to cope with the large amount of P being stored in the watershed.