NITRIFICATION, DENITRIFICATION, AND NITRATE AMMONIFICATION IN SEDIMENTS OF 2 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 nitr ate 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, denitrificat ion 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 quantitativ ely as important as denitrification. Denitrification played a minor ro le as a nitrogen sink in both systems. In the tidal influenced Bassin d'Arcachon, Z. noltii was quantitatively more important than denitrifi cation as a nitrogen sink due to the high assimilation rates of the pl ants. Throughout the year, Z. noltii stabilized the mudflats of the ba y by its well- developed root matrix and controlled the nitrogen cycle due to its high uptake rates. In contrast, the lack of rooted macroph ytes, and dominance of fl eating macroalgae, made nitrogen cycling in Etang du Prevost more unstable and unpredictable. Inhibition of nitrif ication and denitrification during the dystrophic crisis in the summer time increased the inorganic nitrogen flux from the sediment to the w ater 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 h igh assimilation rates. It is likely, however, that this assimilated n itrogen is liberated to the water column at the onset of summer thereb y fueling the extensive growth of the floating macroalgae, Ulva sp. Th e combination of a high nitrogen coupling between sediment and water c olumn, little water exchange and low denitrification rates resulted in an unstable system with fast growing algal species such as phytoplank ton and floating algae.