MACROPHYTE COMMUNITIES AND THEIR IMPACT ON BENTHIC FLUXES OF OXYGEN, SULFIDE AND NUTRIENTS IN SHALLOW EUTROPHIC ENVIRONMENTS

The impact of macrophyte communities on benthic fluxes has been analyz ed in three shallow coastal environments: Etang du Prevost (Mediterran ean coast of France), characterized by the large floating macro-alga U lva rigida; Certes fishponds (Bassin d'Arcachon), covered by Ruppia ci rrhosa; and the inner intertidal mud-flat in the Arcachon Bay (French Atlantic coast), which has extensive Zostera noltii meadows. In these bodies of water, primary production is dependent primarily on the domi nant seagrasses and macroalgae that are also responsible for the large quantity of organic matter deposited on the sediment surface. In 1993 and 1994, fluxes of oxygen, sulphide and nutrients were measured in e arly and late summer, which were selected in order to represent the pr oduction and decomposition phases of the dominant macrophytes. Experim ental work was undertaken to measure: (1) standing crop of dominant ma croalgae and rooted phanerogams and the elemental and macromolecular c omposition of plant biomass; (2) benthic fluxes of oxygen, sulphide, n itrogen and phosphorus using incubation of multiple dark and light ben thic chambers; (3) water-sediment profiles of free-sulphide in sedimen t cores with rooted phanerogams (Ruppia) as well as with floating seaw eeds (Ulva). At the selected sampling sites, in addition to external ( tides) and/or internal (sediment reactivity) factors, we observed diff erences in benthic fluxes which were clearly related to growth pattern s and structure of the macrophyte communities. The Z. noltii meadows w ere stable and characterized by slow growth and almost constant biomas s. In the more sheltered sampling station in the Certes fishponds, R. cirrhosa beds showed a summer decrease due to extensive epiphyte growt h. During the decomposition phase, significant fluxes of free-sulphide occurred inside the dark benthic chambers, probably due to the metabo lism of the epiphytic layer. In the Etang du Prevost, U. rigida achiev ed high biomass levels, even though the macroalgal beds exhibited a pa tchy distribution due to wind action and the hydrodynamics of the lago on. In the decomposition phase, which was coincident with the annual d ystrophic crisis the rapid decomposition of Ulva led to high fluxes of free sulphide. The shift from the production to decomposition phase r esulted in substantial changes in nutrient recycling only in the macro -algal-dominated system. During the growth period dissolved inorganic nitrogen and phosphorus were kept at low levels due to macrophyte upta ke. In contrast during the decomposition phase when the macroalgal bio mass was mineralised, nitrogen and phosphorus were rapidly recycled. T he same processes did not occur in the Certes fishponds probably becau se of the greater internal buffering capacity linked either to plant m orphology/physiology or to the properties of the sediment.