Abstract
Lake Okeechobee is one of the largest freshwater lakes in the United States. As a eutrophic lake, it has frequent algal blooms composed predominantly of the cyanobacterium genus Microcystis. Many of the algal blooms are associated with the resuspension of a thixotropic benthic mud containing legacy nutrients. Since Lake Okeechobee has an area of 1732 km2 (40–50 km radius) and a mean depth of only 2.7 m, there is sufficient fetch and shallow water depth to allow frequent wind, wave, and current generated events, which cause sediment resuspension. Three types of mud exist in the lake including an immobile dark-colored, consolidated mud, a brownish-colored mud, which is poorly consolidated and mobile, and a dark-colored thixotropic, highly mobile mud that is a mixture of organic matter and clay-sized minerals. Altogether, these muds contain an estimated 4.6 × 106 kg of total phosphorus and commensurate high amounts of labile nitrogen. The thixotropic mud covers most of the lakebed and contains the suitable nutrient ratios to trigger algal blooms. A bioassay analysis of the thixotropic mud compared to the consolidated mud showed that it produced up to 50% more nutrient mass compared to the consolidated mud. The thixotropic mud does not consolidate, thus remains mobile. The mobility is maintained by the dynamics of the algal blooms and bacterial decay of extracellular secretions (transparent exopolymer particles) that bind sediment, transfer it to the bottom, and undergo bacterial digestion causing gas emissions, thus maintaining the organic/sediment matrix in suspension. Despite major efforts to control external nutrient loading into the lake, the high frequency of algal blooms will continue until the muds bearing legacy nutrients are removed from the lake.
Highlights
The occurrence and movement of labile forms of nutrients, mainly phosphorus (P)and nitrogen (N), in the environment have considerable impacts on many natural ecosystems
Based on the difficulty in resuspending the consolidated sediments and the maps showing the changes in bottom sediment thicknesses (Figure 5), there may be a third type of mud on the lake bottom, which has a density lying between the consolidated mud and the thixotropic mud
This conclusion is supported by Canfield et al [16], who clearly stated that the restoration of Lake Okeechobee was “mission impossible” without the removal of the legacy nutrients that occur in the thixotropic mud
Summary
Nitrogen (N), in the environment have considerable impacts on many natural ecosystems. Water quality issues within Lake Okeechobee, the Greater Everglades Ecosystem, Bioavailable the limiting nutrient and, as such,the it is important nutrient and the majornitrogen dischargeisrivers to the east and west including. Agricultural runoff best management practices were implemented to control soluble and particulate P, and the meanders were placed back into the surface-water system of the Kissimmee River partly to help assimilate P. Despite all of these control measures, there has been a massive accumulation of P in the sediments of Lake Okeechobee and recycling of P from these bottom sediments into its water column.
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