During 1994 net sediment-water fluxes of oxygen, ammonium and inorganic phosphorus as well as sediment profiles of organic matter, nitrogen, phosphorus and iron were determined in three shallow eutrophic environments. Investigations were conducted monthly from March to December at five stations in the Sacca di Gore (Po River Delta, Italy). In the late summer, samples were collected from a single site in the Prevost lagoon (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 incubation 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 activity. In terms of organic matter and nutrient levels, sediments from the Sacca di Gore and from the Prevost lagoon, both colonised by the floating macroalga Ulva rigida C. Agardh, were similar. Sediments from the inner sheltered site in the Bassin d'Arcachon, invaded by the rooted macrophyte Ruppia cirrhosa (Pet.) Grande, showed the highest total N and P content (363 +/- 157 mu moI N cm(-3) and 15 +/- 2 mu mol P cm(-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 sequestering capacity of the system and/or efficient coupling between bacterial nutrient regeneration and assimilation by the plant roots. In addition the outer tidal stations in the Bassin d'Arcachon, invaded by rooted macrophytes, were low in pore water nutrients. A different trend was evident in the Prevost lagoon where the concentrations of exchangeable 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 the key role of the organic nitrogen in the recycling of nitrogen in these systems. The hypothesis that iron is a key factor in controlling phosphorus release is discussed since the Sacca di Gore, which is subject to dystrophic crises, is richer in iron than the Bassin d'Arcachon, which is a more buffered system.
