Water quality changes following nutrient loading reduction and biomanipulation in a large shallow subtropical lake, Lake Griffin, Florida, USA

Lake Griffin received discharges for decades from muck farms developed on former floodplain wetlands, leading to hypereutrophic conditions. Management actions included wetland restoration of farmland to reduce nutrient discharges, and harvesting of gizzard shad to remove nutrients in fish biomass and reduce nutrient recycling from sediments. Despite a reported susceptibility to wind-driven sediment resuspension, there have been substantial improvements in water quality in Lake Griffin, including decreases in nutrient concentrations, chlorophyll- a , and cyanobacterial biovolume, and increases in water transparency. Water quality improvements in Lake Griffin were substantially greater than occurred in ten comparison lakes. External nutrient load reduction was the primary factor contributing to water quality improvement, although there was evidence of an effect of shad harvesting, including correlations between shad catch per unit effort and nutrient concentrations, and an estimated effect of biomass removal and recycling reduction accounting for about 40% of the external load during the harvest period. Net production of total nitrogen in the lake was strongly related to external total phosphorus loading, indicating phosphorus limitation of nitrogen fixation. The response of Lake Griffin indicates that the combination of external nutrient load reduction and biomanipulation can result in sustained improvements in water quality in shallow subtropical lakes.