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

Abstract

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 °C) were distinct in the second factorial plan (2.3). Oxygen uptake by the oyster cultures varied between 0 μmol m −2 h −1 (January) and 11 823 ±377 μmol m −2 h −1 (July). Ammonia and nitrate-nitrites were released into the water column respectively at a rate of 2905 ± 327 μmol m −2h −1 and 891 ± 88 μmol m −2 h −1 in the summer and 0 μmol m −2 h −1 and 177 ± 97 μmol 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 μmol m −2 h −1) and in November (2691 ± 800 μmol m −2 h −1). No linear relation between water temperature and phosphate flux was found. In Thau Lagoon, oyster cultures (oysters and epibiota) by producing 2 × 10 7 mol-N y −1 play a central role in nitrogen renewal in the water column.