Benthic nutrient fluxes and limited denitrification in a sub-tropical groundwater-influenced coastal lagoon

Benthic processes in estuarine systems can alleviate nutrient over-enrichment by removing nitrogen (N) via denitrification, or further intensify eutrophication by supplying N back to the water column. The main objectives of this study were to (1) examine the magnitude and seasonality of benthic fluxes and denitrification, and (2) determine the relative importance of benthic versus submarine groundwater discharge (SGD) N fluxes in Little Lagoon, a shallow, sub-tropical, groundwater-influenced coastal lagoon in the North Central Gulf of Mexico. In the summer, when low water column nitrate (NO₃⁻) concentration (3000 μM), rates of net N₂ fixation (61.7 ± 4.5 SE μmol N m−2 h−1) exceeded rates of net N₂ efflux (51.6 ± 4.9 SE μmol N m−2 h−1). Unlike many estuarine systems where denitrification is coupled to nitrification, sulfidic sediments caused denitrification to primarily be supported by water column NO₃⁻ in Little Lagoon. Gene copy numbers indicated that markers for N₂ fixation, sulfate reduction, and dissimilatory nitrate reduction to ammonium were more abundant than markers for denitrification and nitrification. SGD N fluxes were up to 42 times larger than the benthic rates, and both the benthos and SGD provided dissolved organic nitrogen to the water column. This study presents evidence of limited benthic N fluxes in supplying N to the water column compared to input associated with SGD.