Influence d'un élevage ostréicole sur les flux de nutriments et d'oxygène dans un écosystème lagunaire

The impact of suspended oyster culture (Crassostrea gigas, Thunberg) on oxygen and nutrient fluxes has been studied in situ, in a coastal lagoon (Thau, France), during a seasonal cycle. On the first plan of the multiple factorial correspondences analysis (MCA), seasons were well discriminated. The fluxes were maximum in summer and minimum in winter. However, this seasonal pattern was not only linked to the water temperature, as autumn and spring (similar temperatures of about 12 degrees C) were distinct in the second factorial plan (2.3). Oxygen uptake by the oyster cultures varied between 0 mu mol m(-2) h(-1) (January) and 11 823 +/- 377 mu mol m(-2) h(-1) (July). Ammonia and nitrate-nitrites were released into the water column respectively at a rate of 2905 +/- 327 mu mol m(-2) h(-1) and 891 +/- 88 mu mol m(-2) h(-1) in the summer and 0 mu mol m(-2) h(-1) and 177 +/- 97 mu mol m(-2) h(-1) in the cold season. During the summer, the nitrate-nitrites flux was about 20 % of the total dissolved inorganic nitrogen production. Phosphate release was low except for two periods during which an important release was measured; in May (1686 +/- 44 mu mol m(-2) h(-1)) and in November (2691 +/- 800 mu mol m(-2) h(-1)). No linear relation between water temperature and phosphate flux was found. In Than Lagoon, oyster cultures (oysters and epibiota) by producing 2 x 10(7) mol-N y(-1) play a central role in nitrogen renewal in the water column.