RELATIONSHIP BETWEEN BENTHIC FLUXES AND MACROPHYTE COVER IN A SHALLOWBRACKISH LAGOON

The relationship between macrophyte cover and benthic fluxes of oxygen , nutrients and sulphide has been examined in a shallow fishpond with a nearly homogeneous meadow of Ruppia cirrhosa (Petagna) Grande (Bassi n d'Arcachon, western France). In 1993 and 1994, benthic fluxes were m easured in early and late summer. These periods were selected to repre sent the production and decay phases of Ruppia in order to determine t he effect on benthic processes. Benthic fluxes of elements were measur ed by means of multiple dark and light benthic chambers in the presenc e or absence of community components. In summer 1994, at the end of th e incubation period, profiles of acid volatile sulphide (AVS) and chro mium reducible sulphur (CRS) were measured also in the 0-5 cm sediment horizon in cores withdrawn from the dark benthic chambers and from th e sediment outside the chambers. Oxygen production and consumption wer e closely related to macrophyte cover, whilst the contributions of pla nkton and microphytobenthic communities were less significant. In the water column, dissolved inorganic nutrients were almost totally deplet ed, while dissolved organic nitrogen attained concentrations up to 200 mu M. In late summer, Ruppia biomass underwent a significant decay du e to the build up of a thick epiphyte layer, mostly around floating le aves. The epiphyte slime was rich in labile organic matter, the decomp osition of which led to a significant oxygen uptake as well as to sulp hide production. Therefore, we postulate that epiphyte growth can caus e disturbance in the aquatic system keeping dissolved sulphide at very high levels. Biogeochemical reactions, such as precipitation of iron sulphide, can exert a control lowering the amplitude of such disturban ces.