An Alternative to Proposed Phosphorus TMDLs for the Management of Lake Okeechobee

Abstract

ABSTRACT The U.S. Environmental Protection Agency and the Florida Department of Environmental Protection recently proposed total maximum daily loads (TMDL) of phosphorus for Lake Okeechobee of 198 t yr−1 and 140 t yr−1, respectively, with a goal of limiting algal blooms in the near-shore areas of the lake. We evaluated the reasonableness of these regulations by examining evidence of past changes in the lake, factors determining water quality, current phosphorus targets for the pelagic water, factors determining total phosphorus levels in the lake, expected benefits of the proposed TMDLs, and alternative management options. Available data are insufficient to demonstrate the degree of any recent eutrophication of the lake. Movement of the sediments in this shallow lake, perhaps by hurricane generated currents, casts doubt on reconstructions of lake history through sediment cores. We estimated presettlement phosphorus loading of 377 t yr−1, indicating that the lake has been eutrophic for a hundred or more years. The pelagic region of the lake is dominated by resuspended sediments that increase the phosphorus concentration and reduce light transmission. Thus, algal populations are limited by light and not phosphorus. As a result, there is no correlation between phosphorus and plankton chlorophylls and only a very weak correlation (r = 0.26) between Secchi depth and chlorophyll a. Living plankton account for only 15% of the total suspended solids in the pelagic region. Phosphorus targets for Lake Okeechobee have been based on conditions in the near-shore region along the western marshes. A concentration of 40 mg m3 of TP in the near-shore region would limit the frequency of algal blooms (chlorophyll a> mg m−3) to 15% of the year, the lowest bloom frequency observed in the closest pelagic stations in the past 28 years. Models indicate mat this frequency can be achieved with a pelagic TP concentration of between 72 and 92 mg m−3, though the TMDL models use 40 mg m−3 as their pelagic target TP concentration. Phosphorus levels in the pelagic region are not correlated with phosphorus inputs, so in spite of recent decreases in phosphorus loading to the lake, phosphorus concentrations continue to rise. This seems to be due to a reduction in phosphorus retention by the lake since 1973. The reason for the loss of phosphorus retention is not known, though there are indications that rising water levels and/or declining calcium concentrations might be involved. The data do not demonstrate that stringent phosphorus TMDLs will improve the lake. On the other hand, there is evidence that high water levels are responsible for reduced water quality in the lake and loss of macrophytes. A planned reduction in lake stage in 2000 made significant improvements in plant coverage, water clarity, and fish habitat without any change in the phosphorus regime. Water level controls seem to have more promise as a management tool for Lake Okeechobee than TMDLs for phosphorus.