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

Abstract

Evidence of phosphorus limitation of algal C- and N-uptake in a NW Mediterranean coastal area (Gulf of Lions) was obtained from a field survey of inorganic and organic N, P and C and from bioassays carried out during the late winter-early spring 1998. Dissolved inorganic nitrogen (ΣDIN = NO3 + NO2 + NH4) and phosphorus (DIP) distributions showed a clear DIP depletion in the inorganic fractions available for primary production. While below the 150 m depth, the mean ΣDIN to DIP ratio was close to the typical Mediterranean 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 higher than the Redfield ratio and also indi- cated P depletion in this fraction. In the dissolved organic pool, P depletion was higher than in the particulate organic pool, since the mean C:N:P ratios were 1674:75:1 in the photic layer. 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 corresponding nutrient utilization by the phytoplanktonic community, and partly due to the influence of the Rhone River. Additions of small amounts of DIP to surface samples led (1) to a decrease in C-uptake (o30%) dur- ing the first 24 h incubation, (2) to a rapid increase in chlorophyll biomass and (3) to stimulate nitrate uptake (o60%), suggesting DIP limitation of new production and of algal biomass during the spring 1998 in the Gulf of Lions.