Phosphorus Dynamics in Big Round Lake, Wisconsin

Abstract

Big Round Lake (BRL) is a shallow (mean depth = 3 m), polymictic lake (area = 1,014 acres) located in Georgetown, Polk County, Wisconsin. It is a eutrophic lake with cyanobacteria blooms and listed as an impaired water. The Straight River (SR) flows through BRL and is the main source for phosphorus (P) within the forest and agriculture dominated watershed. A study was conducted from May to October in 2019 to develop a P budget to determine how internal P loading affects it by monitoring lake chemistry, P flux from sediment, and velocity loading from the SR watershed. During periods of stratification, hypolimnetic anoxia occurred (<1 mg/L) and total phosphorus (TP) increased in the water column above the sediment-water surface (max = 0.146 mg/L) suggesting the occurrence of internal P loading. Surface chlorophyll exceeded 200 μg/L after the bottom anoxia development suggesting potential entrainment of P and uptake by cyanobacteria for growth. Sediment cores indicated an increase of soluble reactive phosphorus (SRP) release under anoxic conditions compared to oxic conditions (0.89 and 0.36 mg/m2 d). The estimated P budget of external load was about 400 kg/summer (65%) and internal P loading was 219 kg/summer (35%) suggesting both need to be managed. Decreasing fertilizer usage, erosion control, and best management practices can reduce the load of external P, while chemical application of aluminum sulfate (alum) can decrease internal P loads, and together they can limit P and decrease cyanobacteria blooms.