The Lake Okeechobee Water Quality Model (LOWQM) Enhancements, Calibration, Validation and Analysis

Abstract The Lake Okeechobee Water Quality Model (LOWQM) was enhanced to more accurately simulate sediment-water phosphorus (P) dynamics by separating the organic P (OP) into four classes (readily degradable, moderately degradable, non-degradable and dissolved), and to more accurately simulate algal dynamics by representing the phytoplankton community with the three distinct major algal groups (cyanobacteria, diatoms and green algae) observed in the lake. The model was calibrated and validated to observed water column nutrient data, sediment nutrient measurements and biovolume data for cyanobacteria, diatoms, and green algae. Model predictions were consistent with experimental observations and indicated that net sediment inorganic P (IP) loads were twice the external TP loads and net sediment inorganic nitrogen (IN) loads were 0.64 times the external total N loads. However, because of organic nutrient and algal settling the lake sediments are an overall nutrient sink. Sensitivity analysis indicated that total algal carbon, algal groups and chlorophyll a were very sensitive to changing algal parameters, parameters affecting light, temperature and supply of IP to the water column. Nutrients were less sensitive for two reasons: 1) algae represent a small fraction of the total nutrient mass, 2) the large pools of sediment nutrients, with long turnover times, buffer changes in the water column. Sensitivity analysis pointed to three potential management options to improve lake water quality: dredging, chemical treatment of sediments and external load reduction. These options were previously considered in a large sediment management feasibility study, which concluded that the last option-load reduction-was the most viable.

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