Biogeochemical Composition of Mediterranean Waters Outside Thau Lagoon

Physicochemical characteristics (temperature, salinity, dissolved oxygen), nutrients (NO3−, NO2−, NH4+, soluble reactive phosphorus, Si), dissolved organic matter with bacterial cell counts (DON, DOP, DOC, BACT) and particulate matter (POC, PON, Chla) were measured weekly from January 1993 to March 1994 in Mediterranean surface waters, sampled 2•5 km offshore in Thau Lagoon (Sète, France). Waters outside the lagoon displayed salinity reductions below 29 which corresponded to flood periods of the Rhône River and/or to winds oriented in a southeasterly direction. Levels of nutrients were increased by the influence of the Rhône river plume. Concentrations of NO3−and NH4+were rather close to the theoretical dilution line, while SRP and Si seemed to be rapidly taken up along their dilution gradient, especially in spring. The influx of NO3−enriched waters into the lagoon in October showed that the Rhône river plume can potentially fertilize this lagoon. The flux of nutrient from the Rhône River led to Chlamaxima above 4 μ g l−1in spring. The concentration of DOC and BACT reached peak values at the decay of the spring bloom and decreased to their lowest level afterwards, suggesting that primary production was an important source of DOC but also that DOC was repackaged in the microbial loop rather than being transported to deeper layers. Yearly averaged atomic C:N and C:P ratios in DOM were 15 and 1000, respectively, which probably compelled bacterioplankton to compete with phytoplankton for NH4+and SRP. No influence of the Rhône River was observed during summer. This period was characterized by SRP, NO3−and NO2−concentrations below the limits of detection and by the lowest levels in DOC, DON, BACT and Chla. However, NH4+and DOP concentrations remained, respectively, above 0•3 and 0•15 μ M, and the summer period corresponded to oxygen supersaturation (up to 122\%). These observations suggest the dominance of picophytoplanktonic communities with low biomass and high productivity associated with a heterotrophic component providing NH4+directly to phytoplankton, and phosphorus dominated by organic forms.