Influence of suspended oyster cultures on nitrogen regeneration in a coastal lagoon (Thau, France)

The present study was designed to quantify the potential influence of oyster cultures (Crassostrea gigas,Thunberg) on dissolved inorganic nitrogen (DIN) renewal in a shallow coastal lagoon (Thau, France), where nitrogen is the factor limiting primary production. The approach was based on a comparativein situanalysis of the seasonal patterns of nitrogenous nutrient exchanges at both the water sediment and oyster cultures water interfaces. The experiments were performed at a station under the influence of oyster cultures (UC) compared with benthic exchanges in a station outside their direct influence (OC) which was used as the reference site for benthic fluxes. In contrast to what it is usually reported for shallow ecosystems, I found evidence of a key influence of suspended oyster culture units (OCU) on the regeneration in the water column of nitrogen available to the phytoplankton, with benthic remineralisation being only of secondary importance. The results also showed that the greatest impact of oyster cultures was on DIN renewal (e.g. nitrogen transformation) in the water column, not on particulate organic nitrogen transfer to the sediment. In the sector studied, it also appeared that nitrogen recycling by both sediments and OCU could satisfy the nitrogen requirements of the primary producers except in summer when other internal nitrogen sources were necessary. Conversely, during most seasons, pelagic primary production could not meet the demand for particulate organic nitrogen of the cultured assemblages (oysters and epibiota). This emphasises the importance to the diet of cultured assemblages of other nitrogen sources such as heterotrophic populations of bacteria, cillates or flagellates.