Restoration of Lake Okeechobee, Florida: mission impossible?

Abstract

Legally mandated eutrophication restoration goals for Lake Okeechobee (FL) are unachievable, therefore assigning managers a "mission impossible." Since the 1970s, restoration efforts have focused on reducing pelagic total phosphorus (TP) to 40 mg/L. A total daily maximum load (TMDL) of 140 metric tons (t)/yr was adopted by the Florida Department of Environmental Protection in 1999 (effective date 2015) to restore the lake's balance of flora and fauna. Phosphorus (P) loads (1975-2018) averaged 516 t/yr with no significant change over time, yet average TP significantly increased from 51 mg/L (1974-1977) to 146 mg/L (2015-2019). Greater TP values in 2019 were due to Hurricane Irma and an early June storm event. Annual P-loads and pelagic TP were not significantly correlated. Instead, TP was strongly correlated with turbidity (R 2 ¼ 0.85), which is generated by wave-driven resuspension of P-rich unconsolidated sediments. Since 1973, >13,000 t of TP has been added to Okeechobee's sediments that have accumulated over the past century due to the lowering of water levels and the construction of the Herbert Hoover Dike. Prior to settlement, high water levels allowed turbid lake waters to flood large areas of adjacent wetlands, where suspended sediments were removed from the lake. With the minimization of this self-cleansing mechanism after construction of the Herbert Hoover Dike, P-rich fine sediments accumulated, and periodic hurricanes disrupted consolidated sediments. Unconsolidated sediments are easily resuspended into the water column, raising TP. Efforts to reduce Okeechobee's pelagic TP through reductions of P-loads alone will not work due to sediment accumulation and resuspension.

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

“…Three types of mud occur in Lake Okeechobee: consolidated mud, partially consolidated mud, and a fluid mud layer that occurs in variable thicknesses in time [16]. The consolidated mud is dark-colored, layered, contains shell lag deposits, and has heterogeneous properties [4].…”

“…The total estimated volume was 201 × 10 6 m 3 . Subsequent surveys of the mud thickness in Lake Okeechobee were conducted in 1988, 1998, and 2006 [16,[28][29][30] (Figure 5). The estimated average mud thickness was 66, 74, and 51 cm, respectively [20].…”

“…However, P is a key nutrient and contaminant within this ecosystem since its excessive loading leads and amplifies the nitrogen limitation (i.e., secondary nitrogen limitation) and thus is the focus of this research. [15,16]. Phosphorus has 80º40' Figure 2.…”

“…Bioavailable nitrogen is the limiting nutrient and, as such, it is an important nutrient in Lake Okeechobee [12][13][14]. However, P is a key nutrient and contaminant within this ecosystem since its excessive loading leads and amplifies the nitrogen limitation (i.e., secondary nitrogen limitation) and thus is the focus of this research [15,16]. Phosphorus has indeed led to water quality issues within Lake Okeechobee, the Greater Everglades Ecosystem, and the major discharge rivers to the east and west including the St. Lucie Canal/River continuum and the Caloosahatchee River, respectively.…”

“…In addition, 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. These sediments are considered to be legacy P sources [16,19]. The planned removal of organic sediments containing P inside the Hoover Dike was also implemented during some drought periods [20].…”