Legacy Phosphorus in Lake Okeechobee (Florida, USA) Sediments: A Review and New Perspective

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.

The Effects of Climate Variability on Florida’s Major Water Resources

“…Additionally, the over-pumping of groundwater has resulted in saline intrusion in the surficial aquifers along the northeastern coastal areas of the state [160]. Most estimates of the associated sea level rise range from 0.45 m to 1.37 m for the increase in sea level by the end of this century [164][165][166]. This will complicate matters further when the rise in sea level increases the groundwater table of the Surficial Aquifer System in the future [164,166].…”

“…Most estimates of the associated sea level rise range from 0.45 m to 1.37 m for the increase in sea level by the end of this century [164][165][166]. This will complicate matters further when the rise in sea level increases the groundwater table of the Surficial Aquifer System in the future [164,166].…”

Legacy Phosphorus and Ecosystem Memory Control Future Water Quality in a Eutrophic Lake

“…Here, we define ecosystem memory (sensu, Ogle et al., 2015) as the influence of past ecosystem states on the rates and patterns of future responses to change. In lakes, many processes may contribute to ecosystem memory and associated responses to nutrient load reductions, such as slow flushing in lakes with long hydrologic residence times (Hotchkiss et al., 2018), internal loading of nutrients from large sediment pools (Carleton & Lee, 2023; Missimer et al., 2020; Søndergaard et al., 2007), and biological feedback mechanisms that promote persistently high algal biomass, despite decreasing nutrient loads (Scheffer et al., 2001). While specific biophysical processes may be well described, it remains a challenge to understand how their interactions control water quality metrics that emerge at the ecosystem scale, such as seasonal patterns of phytoplankton biomass, water clarity, and formation of deep‐water anoxia.…”

Watershed Response to Legacy Phosphorus and Best Management Practices in an Impacted Agricultural Watershed in Florida, U.S.A.

“…Lake Okeechobee in south-central Florida (Figure 1) is a large shallow lake (area-1730 km 2 ; maximum length-57.5 km; maximum width-46.6 km; average depth-2.7 m; largest depth-3.7 m; volume-5.2 km 3 when the stage is approximately 4.7 m (NGVD); residence time-approximately 3 years) that has experienced cultural eutrophication for at least the last five decades primarily from P loading from its large watershed (encompassing 13,964 km 2 ) [31][32][33][34]. Beef cattle pastures and dairy-related activities have historically been the dominant land uses in its watershed [35,36].…”

“…The deadline to achieve this TMDL was 2015, which was subsequently extended to 2035. Siders et al [33], in a re-evaluation study of TMDL targets based on an enhanced water quality dataset for the lake, concluded that the original target of 40 µ g/L for the in-lake TP concentration is still appropriate. Lake Okeechobee's ability to assimilate phosphorus into its sediments has diminished over the last five decades, and the lake is estimated to turn from a P sink to a permanent source as early as 2035 [53].…”