The littoral zone of lake okeechobee as a source of phosphorus after drawdown

Abstract

Following a period of prolonged drought or intentional lake level drawdown, large littoral areas that once contained submersed aquatic vegetation (SAV) are reinundated when lake levels rise. A complete assessment of the contribution made by decomposing SAV to the in-lake phosphorus (P) concentration is important in both the management of Lake Okeechobee and understanding basic P processes. The P contribution to the open waters of Lake Okeechobee from a rapid inundation of exposed SAV was calculated by four methods: cores of field-desiccated SAV, cores of lab-desiccated SAV in the presence and absence of sediments, in situ decomposition, and sequential macrophyte harvesting. P releases, given such an episodic event, were similar among the four methods, ranging from 116±48 to 384±528 mg/m² in the absence of sediment. When SAV is in contact with sediment, which is the realistic field situation, the amount of P released was four times less (30±14 mg/m²) than in the absence of sediment. The calculated P releases would result in total P concentration increases in the lake from 2 to 15 µg/liter (upper 95% CI=2–25 µg/liter) in the absence of sediment; only 1 µg/liter increase was predicted when SAV released P in contact with sediment. Thus it is unlikely that a significant rise in total P concentrations in the limnetic zone of the lake would occur from the export of P released during the desiccation of SAV in the littoral-marsh zone during a drawdown.