Development of phosphorus load reduction goals for seven lakes in the Upper Ocklawaha River Basin, Florida

Abstract

External phosphorus loading targets have been developed for seven lakes in the Upper Ocklawaha River basin (UORB), and for six of these lakes the loading targets have been used as a basis for total maximum daily loads (TMDLs). External phosphorus loads for the seven lakes were estimated from 1986–2005. Three of the lakes that have received partial reductions in external loads have shown proportional reductions in phosphorus concentrations. Natural background phosphorus concentrations for the lakes were determined through a weight-of-evidence approach combining existing concentrations in reference lakes and modeling of natural background conditions in the basin. Target phosphorus concentrations were established by allowing a 10% degradation from natural background water transparency, as specified in Florida water quality standards. Data collected from the UORB lakes were analyzed to determine the relationship between phosphorus concentrations and water transparency. Target phosphorus concentrations ranged from 14–32 μg/L, which range from 15–100% of existing concentrations in the lakes. Recommended external phosphorus load reductions for the lakes range from 0 to 85%. We estimated that meeting the target phosphorus concentrations in the lakes with poorest water quality would reduce mean chlorophyll a concentrations up to 80%, substantially reduce algal bloom frequencies, and more than double mean Secchi transparency.