BENTHIC OXYGEN RESPIRATION, AMMONIUM AND PHOSPHORUS REGENERATION IN SURFICIAL SEDIMENTS OF THE SACCA-DI-GORO (NORTHERN ITALY) AND 2 FRENCH COASTAL LAGOONS - A COMPARATIVE-STUDY

During 1994 net sediment-water fluxes of oxygen, ammonium and inorgani c phosphorus as well as sediment profiles of organic matter, nitrogen, phosphorus and iron were determined in three shallow eutrophic enviro nments. Investigations were conducted monthly from March to December a t five stations in the Sacca di Gore (Po River Delta, Italy). In the l ate summer, samples were collected from a single site in the Prevost l agoon (French Mediterranean coast) and three stations in the Bassin d' Arcachon (French Atlantic coast). In the Sacca di Gore, water-sediment exchanges of O-2, NH4+ and PO43- were estimated by means of core incu bation in the dark. Benthic fluxes for the French lagoons were in part determined experimentally using benthic chambers and in part from the literature. In general in the Sacca di Gore the highest oxygen uptake and nutrient release rates were found at the central sites, affected by macroalgal growth. At the sampling site adjacent to the freshwater inlet, sediment-water exchanges were principally influenced by tidal a ctivity. In terms of organic matter and nutrient levels, sediments fro m the Sacca di Gore and from the Prevost lagoon, both colonised by the floating macroalga Ulva rigida C. Agardh, were similar. Sediments fro m the inner sheltered site in the Bassin d'Arcachon, invaded by the ro oted macrophyte Ruppia cirrhosa (Pet.) Grande, showed the highest tota l N and P content (363 +/- 157 mu moI N cm(-3) and 15 +/- 2 mu mol P c m(-3) as average values in the top IO cm of sediment), but were low in pore water ammonium and orthophosphate probably due to the high seque stering capacity of the system and/or efficient coupling between bacte rial nutrient regeneration and assimilation by the plant roots. In add ition the outer tidal stations in the Bassin d'Arcachon, invaded by ro oted macrophytes, were low in pore water nutrients. A different trend was evident in the Prevost lagoon where the concentrations of exchange able inorganic phosphorus and ammonium were appreciable (0.28 +/- 0.07 mu mol P cm(-3) and 2.4 +/- 1.4 mu mol N cm(-3) as average values in the top 10 cm of sediment). High amounts of dissolved organic nitrogen were found in the pore water at all the sites investigated showing th e key role of the organic nitrogen in the recycling of nitrogen in the se systems. The hypothesis that iron is a key factor in controlling ph osphorus release is discussed since the Sacca di Gore, which is subjec t to dystrophic crises, is richer in iron than the Bassin d'Arcachon, which is a more buffered system.