Oxygen fluxes involving the benthic micro- and macrophytic components in the Thau Lagoon under pre-anoxic conditions
Résumé
Measurements of oxygen fluxes at the water-sediment interface (transparent and opaque enclosures) were made on bare sediments inhabited by microphytes on the one hand, and on sediments diversely colonized by macrophytes (macroalgae and seagrasses) on the other hand. Five stations, typical of different biotopes of a Mediterranean shallow lagoon were studied from May to July 1993 in order to observe an anoxic event which usually occurs at that time of the year. Average diurnal respiration of benthic communities ranged from 40 mg (or 1.25 mmol) O2 m2 h−1 in bare sediments (31 % of the lagoon area) to 76 mg (or 2.37 mmol) O2 m−2 h−1 in sediments with a medium coverage of macroalgae (37 % of the total area) and, finally, to 100 mg (or 3.12 mmol) O2 m−2h−1 in the denser macrophytic area (32 % of the total area). The highest diurnal gross production was observed in the zone colonized by macroalgae and seagrasses, and especially in corridors between shellfish-cultivation tables (300 mg O2 m−2 h−1, or 9.37 mmol O2m−2 h−1, equivalent to 113 mg C m−2 h−1). Overall, during this period, net bottom oxygen production was close to nil in ca. 30 %, and positive in 70 % of the lagoon area. The average net oxygen production for the whole lagoon in summer is thus in the order of 100 mg O2 m−2h−1. In 1993, at the end of July, an anoxic event was avoided due to a period of strong wind.