Modeling water quality and cyanobacteria blooms in Lake Okeechobee: I. Model descriptions, seasonal cycles, and spatial patterns

Lake Okeechobee is a shallow subtropical lake that is critically important for south Florida agriculture and the Everglades. In summer, the lake typically experiences strong blooms of cyanobacteria including toxin producing Microcystis aeruginosa. To understand the dynamics of these blooms and water quality in the lake, a coupled hydrodynamic-biogeochemical model based on the Regional Ocean Modeling System (ROMS) has been developed. The biogeochemical model was constructed to simulate major biogeochemical processes including nitrogen (N) and phosphorus (P) cycles, phytoplankton growth, zooplankton grazing, and microbial loop, among others. A three-year (2018-2020) simulation was carried out and calibrated with available in situ and remote sensing data for key physical and biogeochemical parameters. Although model and data generally agree in spatial patterns and seasonal cycles, significant discrepancies exist including exact timings of the blooms and dissolved inorganic and organic P concentrations. Model results indicate that Lake Okeechobee typically exhibits a two-layer circulation in summer with surface and bottom currents generally moving in the opposite directions. This feature couples with diurnal cycles of atmospheric forcing (winds and heating/cooling) and diel vertical migration (DVM) of Microcystis to strongly affect not only the spatial patterns of cyanobacteria blooms but also the bloom intensity in summertime. Horizontally, both model results and remote sensing images indicate that cyanobacteria distributions are concentrated in the central and northern lake during summer and in western lake in spring and fall, in responses to the prevailing winds. Consistent with previous laboratory and observational studies, model results also suggest that, among the two main nutrients N and P, nitrogen is likely the primary limiting nutrient for phytoplankton growth along the northwestern coast where dissolved inorganic nitrogen is typically depleted in summer. In the central and southeastern lake, nutrient concentrations are relatively higher, and light and winds are likely the main factors limiting phytoplankton blooms. In addition, surface winds and water temperature are important in regulating the seasonality of phytoplankton blooms. The model, however, is limited by the uncertainties of key biogeochemical parameters including the specifics of Microcystis vertical migration, and sediment-water interactions including nutrient fluxes and sediment transport. Nevertheless, with further development, this model can be useful for forecasting water quality and phytoplankton blooms and to assist in water management decision-making in the future.