BIOGEOCHEMICAL COMPOSITION OF MEDITERRANEAN WATERS OUTSIDE THAU LAGOON

Physicochemical characteristics (temperature, salinity, dissolved oxyg en), nutrients (NO3-, NO2-, NH4+, soluble reactive phosphorus, Si), di ssolved organic matter with bacterial cell counts (DON, DOP, DOG, BACT ) and particulate matter (POC, PON, Chl a) were measured weekly from J anuary 1993 to March 1994 in Mediterranean surface waters, sampled 2.5 km offshore in Thau Lagoon (Sete, France). Waters outside the lagoon displayed salinity reductions below 29 which corresponded to flood per iods of the Rhone River and/or to winds oriented in a southeasterly di rection. Levels of nutrients were increased by the influence of the Rh one river plume. Concentrations of NO3- and NH4+ were rather close to the theoretical dilution line, while SRP and Si seemed to be rapidly t aken up along their dilution gradient, especially in spring. The influ x of NO3- enriched waters into the lagoon in October showed that the R hone river plume can potentially fertilize this lagoon. The flux of nu trient from the Rhone River led to Chl a maxima above 4 mu g l(-1) in spring. the concentration of DOC and BACT reached peak values at the d ecay of the spring bloom and decreased to their lowest level afterward s, suggesting that primary production was an important source of DOC b ut also that DOC was repackaged in the microbial loop rather than bein g transported to deeper layers. Yearly averaged atomic C:N and C:P rat ios in DOM were 15 and 1000, respectively, which probably compelled ba cterioplankton to compete with phytoplankton for NH4+ and SRP. No infl uence of the Rhone River was observed during summer. This period was c haracterized by SRP, NO3- and NO3- concentrations below the limits of detection and by the lowest levels in DOC, DON, BACT and Chl a. Howeve r, NH4+ and DOP concentrations remained, respectively, above 0.3 and 0 .15 mu M, and the summer period corresponded to oxygen supersaturation (up to 122%). These observations suggest the dominance of picophytopl anktonic communities with low biomass and high productivity associated with a heterotrophic component providing NH4+ directly to phytoplankt on, and phosphorus dominated by organic forms. (C) 1997 Academic Press Limited.