Nitrification, denitrification, and nitrate ammonification in sediments of two coastal lagoons in Southern France

Seasonal and diurnal variations in sediment-water fluxes of O-2, NO3-, and NH4+ as well as rates of nitrification, denitrification, and nitrate ammonification were determined in two different coastal lagoons of southern France: The seagrass (Zostera noltii) dominated tidal Bassin d'Arcachon and the dystrophic Etang du Prevost. Overall, denitrification rates in both Bassin d'Arcachon (<0.4 mmol m(-2) d(-1)) and Etang du Prevost(<1 mmol m(-2) d(-1)) were low. This was mainly caused by a combination of low NO3-, concentrations in the water column and a low nitrification activity within the sediment. In both Bassin d'Arcachon and Etang du Prevost, rates of nitrate ammonification were quantitatively as important as denitrification. Denitrification played a minor role as a nitrogen sink in both systems. In the tidal influenced Bassin d'Arcachon, Z. noltii was quantitatively more important than denitrification as a nitrogen sink due to the high assimilation rates of the plants. Throughout the year, Z. noltii stabilized the mudflats of the bay by its well- developed root matrix and controlled the nitrogen cycle due to its high uptake rates. In contrast, the lack of rooted macrophytes, and dominance of fl eating macroalgae, made nitrogen cycling in Etang du Prevost more unstable and unpredictable. Inhibition of nitrification and denitrification during the dystrophic crisis in the summer time increased the inorganic nitrogen flux from the sediment to the water column and thus increased the degree of benthic-pelagic coupling within this bay. During winter, however, benthic microalgae colonizing the sediment surface changed the sediment in the lagoon from being a nitrogen source to the over-lying water to being a sink due to their high assimilation rates. It is likely, however, that this assimilated nitrogen is liberated to the water column at the onset of summer thereby fueling the extensive growth of the floating macroalgae, Ulva sp. The combination of a high nitrogen coupling between sediment and water column, little water exchange and low denitrification rates resulted in an unstable system with fast growing algal species such as phytoplankton and floating algae.