Early spring phosphorus limitation of primary productivity in a NW Mediterranean coastal zone (Gulf of Lions)

Evidence of phosphorus Limitation of algal C- and N-uptake in a NW Mediterr anean coastal area (Gulf of Lions) was obtained from a field survey of inor ganic and organic N, P and C and from bioassays carried out during the late winter-early spring 1998. Dissolved inorganic nitrogen (Sigma DIN = NO3 NO2 + NH4) and phosphorus (DIP) distributions showed a clear DIP depletion in the inorganic fractions available for primary production. While below th e 150 m depth, the mean Sigma DIN to DIP ratio was close to the typical Med iterranean ratio of 22, while values found in the upper layer (0 to 150 m) were about 3 times higher (68.4:1 on average). In this upper layer, N:P (19 .9:1) and C:P (159.7:1) ratios in the particulate organic matter were highe r than the Redfield ratio and also indicated P depletion in this fraction. In the dissolved organic pool, P depletion was higher than in the particula te organic pool, since the mean C:N:P ratios were 1674:75:1 in the photic l ayer. Dissolved organic forms of C and N represented the bulk (ca 94 and 86 %, respectively) of the total organic matter, while ca 31% of the organic P was in particulate fraction. The apparent imbalance between N and P in the inorganic fraction was partly attributed to an imbalance in the correspond ing nutrient utilization by the phytoplanktonic community, and partly due t o the influence of the Rhone River. Additions of small amounts of DIP to su rface samples led (1) to a decrease in C-uptake (congruent to 30 %) during the first 24 h incubation, (2) to a rapid increase in chlorophyll biomass a nd (3) to stimulate nitrate uptake (congruent to 60 %), suggesting DIP limi tation of new production and of algal biomass during the spring 1998 in the Gulf of Lions.