Hypolimnetic Iron:Phosphorus Ratio Considerations in the Management of Lake Desair, Wisconsin

Abstract

Lake Desair exhibits hypereutrophic conditions and severe summer algal bloom development. A monitoring program was conducted in 2013 to examine phosphorus (P) and chlorophyll dynamics in relation to internal and tributary P loading. The lake strongly stratified and concentrations of both Fe and P increased substantially in the anoxic hypolimnion during the summer. Internal P loading, estimated by mass balance, was high at 10 mg/m2 d and accounted for 32% of the annual P load to the lake. However, physical entrainment of hypolimnetic P into the epilimnion was not detected due to a high hypolimnetic Fe:P ratio (> 5:1 wt:wt), which resulted in complete precipitation and adsorption of P during chemical oxidation rather than algal uptake. Internal P loading, although high, probably does not contribute to the epilimnetic P budget of this lake. Instead, tributary SRP loading from the agriculturally-dominated Northwest Creek watershed is likely the primary P source to the epilimnion and cyanobacterial blooms in the lake. SRP concentrations accounted for over 40% of the load and, therefore, represented a directly available P source for phytoplankton uptake. Thus, best management practices in the watershed should be considered to lower bio-available P concentrations that cause cyanobacteria activity and growth.