Benthic primary production and bacterial denitrification in a Mediterranean eutrophic coastal lagoon

Microphytobenthos and macroalgal mats are simultaneously present in eutrophic lagoons and are expected to have different direct and indirect effects on nitrogen related processes through uptake and inhibition or stimulation of microbial activity. To assess the relative contribution of different primary producer communities and heterotrophic processes to benthic nitrogen cycling, we studied nitrogen uptake and bacterial denitrification in the eutrophic Sacca di Goro lagoon (northern Italy). Benthic fluxes of oxygen and dissolved inorganic nitrogen (DIN), and rates of nitrification-coupled (Dn) and water–nitrate (Dw) denitrification were measured every 30–45days for one year at two shallow sites. Station Giralda is close to the main freshwater inlet, has turbid waters and muddy-organic and bioturbated sediments with microphytobenthos (MPB). Station Gorino is brackish with muddy-sand sediments which are covered by macroalgal mats of the genus Ulva. Here, sediment patches with and without macroalgae (MA) were simultaneously studied. Sediments with MPB were net heterotrophic and regenerated large amounts of ammonium to the water column. At Gorino, sediments with MA were net autotrophic through the year, and DIN fluxes were mainly controlled by macroalgal uptake. On an annual basis, denitrification rates were three fold higher at Giralda (2.27±0.06molNm⁻²yr⁻¹) than at Gorino (0.83±0.01molNm⁻²yr⁻¹), due to higher nitrate in the water column and nitrification in surface sediments. At Gorino, denitrification was one order of magnitude lower than DIN uptake by macroalgae (10.39±1.30molNm⁻²yr⁻¹). Nevertheless, the differences between denitrification rates in sediments with and without MA were unexpectedly negligible, showing that the denitrification capacity was not suppressed by macroalgal competition. Results from this study suggest that in eutrophic lagoons nitrogen cycling seems less affected by MPB compared to more oligotrophic coastal waters and that most of the available DIN flows through benthic macroalgae. However, Ulva is only a temporary N-sink and most of its nitrogen pool can be either rapidly recycled or exported by tidal currents to the open sea.