Phosphorus dynamics in Delavan Lake Inlet, southeastern Wisconsin, 1994

Removal of rough fish from Delavan Lake Inlet in southeastern Wisconsin transformed the inlet from a shallow, turbid-water system with few macrophytes to a clear-water system with an abundance of macrophytes and increased phosphorus concentrations. To understand the changes in the phosphorus dynamics, the U.S. Geological Survey and the U.S. Army Corps of Engineers, in cooperation with the Town of Delavan, constructed a detailed phosphorus budget for the inlet for April through September 1994. The budget included inputs from the drainage basin, atmosphere, sediments (as a function of pH and the fraction of the day under anoxic conditions), and ground water. Laboratory studies estimated phosphorus flux rates under various conditions. Field studies documented ambient conditions in the inlet and fluxes of phosphorus into and out of the inlet. Phosphorus released from the sediments was estimated by coupling the flux rates, estimated in the laboratory, with ambient conditions in the inlet.

The detailed phosphorus budget indicated that the increase in phosphorus concentrations was caused primarily by elevated pH resulting from increased photosynthetic activity of the macrophytes and a high release of phosphorus from the sediments. The release of phosphorus from the sediments was the largest source of phosphorus to the inlet in the spring and summer of 1994 and in other years of low to near normal runoff; however, in years of high runoff, phosphorus input from the inlet's drainage basin was the largest source of phosphorus. A less-detailed phosphorus budget constructed for the period from February 1993 to September 1994 demonstrated that, over the entire year, runoff from the drainage basin was the dominant source in the phosphorus budget. During April-September 1994, the input of phosphorus from the inlet may especially affect the summer productivity in Delavan Lake because almost 80 percent of the phosphorus loading during this time was in the form of dissolved orthophosphate.