COS 75-1 - Fish and phosphorus in a eutrophic wetland: Contrasting bioturbation effects from the species to the ecosystem level

Abstract: The Everglades Stormwater Treatment Areas (STAs) are wetlands constructed to reduce surface-water phosphorus (P) through retention in sediment and vegetation prior to discharge to the Everglades Protection Area. Aquatic animals complicate the retention process by resuspending P through bioturbation as part of their normal behavior. This internal P loading can alter nutrient dynamics and add an element of uncertainty to predicted STA responses and management decisions. We quantified the influence of fish bioturbation on P dynamics at the individual and system-levels in an effort to improve system-level planning. Bioturbation rates were experimentally measured for three common fish species in the STAs, Largemouth Bass (Micropterus salmoides), Blue Tilapia (Oreochromis aureus) and Sailfin Catfish (Pterygoplichthys spp.) in a series of two-week long enclosure experiments. In each enclosure, we measured water column and sediment nutrients, and suspended sediment over the course of the experiment to calculate species-specific relationships between fish biomass and P resuspension resulting from bioturbation. Using these experimentally determined resuspension rates and fish population estimates based on 6 years of sampling, we scaled up the rates to estimate system-wide bioturbation effects. We found that water column P concentrations increased with higher biomass for all three fish species. Only Blue Tilapia and Sailfin Catfish caused sediment resuspension however, indicating that Largemouth Bass were increasing water column nutrients through other pathways (likely excretion based on results from a companion study). Blue Tilapia increased water column P through bioturbation at rates of 0.75 mg TP/ kg wet weight, while Sailfin Catfish caused 20x more bioturbation resulting in nutrient increases at a rate of 16.96 mg TP/ kg wet weight. The difference in these bioturbation rates is linked to difference in the functional traits of the two fish species; Blue Tilapia are largely herbivorous, while Sailfin Catfish are benthic detritivores and burrowers. However, Blue Tilapia are the most common fish species in the Everglades STAs, with an estimated biomass more than 25x higher than Sailfin Catfish. As a result, our scaled-up bioturbation estimates indicate that Blue Tilapia and Sailfin Catfish resuspend similar amounts of P from the sediment to the water column and are both responsible for decreasing the efficiency of P retention by sediment. This suggests that management leading to reductions in bioturbating fish density may improve STA P-removal efficiency to the benefit of downstream Everglades wetlands.