Budgeting internal and external nutrient loads in a shallow freshwater bay: Disentangling bioturbator-mediated and net ambient nutrient fluxes

Shallow embayments are particularly vulnerable to eutrophication and harmful algal blooms (HABs). Most management efforts focus on reducing external nutrient (i.e., nitrogen [N] and phosphorus [P]) loading from landscape runoff, yet seasonal HABs persist. When external loads are lessened, the internal loading of nutrients may contribute to continued eutrophic conditions. We investigated internal loading, including the contribution of bioturbator-mediated sediment-surface water nutrient exchange, in Sandusky Bay, OH, USA, the largest embayment in Lake Erie. Sediment-surface water nutrient fluxes are driven by oxygen conditions, geochemical processes, and microbial nutrient processes. Invertebrate bioturbation can modify nutrient fluxes by transporting oxygenated surface water through burrows, into normally anoxic sediment, which can alter abiotic nutrient flux rates (i.e., via phosphorus sorption) and promote microbial nutrient processing (i.e., nitrogen transformations). We measured benthic invertebrate densities monthly across Sandusky Bay and quantified bioturbation-driven and ambient nutrient flux rates through intact sediment core experiments. We assessed the contribution of bioturbation to net ambient nutrient fluxes and compared internal loads (ambient and bioturbation-mediated) to external loads. Net ambient phosphorus and nitrogen fluxes varied across time, space, and oxygen conditions. Invertebrate bioturbation contributed to net internal fluxes, and internal phosphorus loads were similar in magnitude to external phosphorus loads while internal nitrogen loads were much smaller than external loads in Sandusky Bay during mid-summer. We demonstrate that invertebrates contribute greatly to internal fluxes and that dynamic bioturbation and ambient nutrient fluxes at the sediment–water interface of shallow bays contribute to internal loading in freshwater aquatic ecosystems.