Science Serving Maryland's Coasts

Ben Henry, Middlebury College

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Project Title: 

Influence of Microphytobenthos on Denitrification And Nutrient Fluxes in Shallow Coastal Systems: A Modeling Approach


The sediments of shallow coastal estuaries play important roles in storing, transforming, and removing nutrients from coastal ecosystems, and using quantitative models to accurately predict the response of sediment nutrient fluxes to environmental changes is crucial to environmental regulation. Microphytobenthos (MPB), photosynthetic microalgae inhabiting the sediment, are widespread in shallow areas and substantially alter the oxygen and nutrient dynamics of photic (illuminated) sediments, but past efforts to model ecosystem-scale sediment nutrient fluxes have largely neglected their effects. This study presents a sediment nitrogen model that computes seasonal cycles of diagenesis, nitrification, and denitrification, and includes a layer of MPB on the sediment surface that performs photosynthesis to characterize the effect of light exposure. Model outputs are validated against data from sediment core incubations at two sites in Chesapeake Bay. The model reproduces observed oxygen dynamics well; sediments exposed to light take up less oxygen during midsummer peaks in oxygen consumption than sediments in the dark, and become net sources of oxygen during spring and fall. Measured sediment nitrogen fluxes are more variable, and the model reproduces seasonal patterns less effectively, but overall both measured and modeled illuminated sediments supply less nitrogen to the water column than dark sediments. This model can be applied to water quality management to help develop recommendations for total maximum daily load of nitrogen to coastal systems.