P. Souchu et al., BIOGEOCHEMICAL COMPOSITION OF MEDITERRANEAN WATERS OUTSIDE THAU LAGOON, Estuarine, coastal and shelf science, 44(3), 1997, pp. 275-284
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.