Phosphorus Release from Sediments in Shawano Lake, Wisconsin

Abstract

The release of phosphorus (P) from lake sediments to the water column is important to lake water quality. Previous research on sediment P release has largely been in deeper, stratified lakes where hypolimnetic anoxia can lead to very high sediment P release rates. Recent studies suggest that sediment P release may also be important in large shallow lakes. Sediment P release in shallow lakes is poorly understood, and it is important that lake managers have a better understanding of how it influences lake nutrient budgets. This research developed a P budget for Shawano Lake, a large shallow lake in north central Wisconsin, and used the sediment P release to explain the difference between measured loads and summer P increases in the lake. Laboratory derived P release rates from intact sediment cores taken from littoral areas of Shawano Lake exhibit mean P release rates that are high under anoxic conditions (1.25 mg m-2 d-1) and lower, although still significant, under oxic conditions (0.25 mg m-2 d-1). When compared to the other components of the summer P budget, internal sediment P load accounted for 71% (46% to 81% range) of the summer P budget. This P release could be explained with an oxic P release rate of 0.31 mg m-2 d-1 (0.10 to 0.49 mg m-2 d-1 range). Sediment P release was clearly important to P concentrations in Shawano Lake as it accounted for the largest source of P in the monitored summers. This internal P source is a vital P pathway to the lake and should be considered when attempting to manage P cycling in Shawano Lake and other shallow lakes.