Primary production, carbon export and nutrients availability in western and eastern Mediterranean Sea in early summer 1996 (MINOS cruise)

The distribution of primary production (PP), particulate carbon export from the photic zone to deeper layer, and nutrient concentrations are investigated in the Mediterranean Sea (MS) during May–June 1996. A decrease in integrated primary production, particulate carbon export and nutrient availability towards the eastern part of the Mediterranean sea was observed while integrated chlorophyll a remains rather constant. This pattern may be explained both by the adaptation of phytoplanktonic organisms to low light conditions and by a more efficient nutrient diffusion from the deeper layer in the east related to the position of the nutricline and density gradient. Integrated primary production ranging from 350 to 450 mgC m−2 day−1 in the west decreases toward the east to a value of about 150 mgC m−2 day−1. The latter value may appear as a limit for primary production rates under strong oligotrophic conditions. Particulate carbon export represents 4.0+2.9% of integrated primary production. Up to 90–95% of primary production is then sustained by internal recycling of organic matter. Evidence of a limitation of production by phosphate was obtained from differences between depth of nitracline and phosphacline and by enrichment experiments. The wide range of oligotrophic conditions in the Mediterranean Sea provides a case study for links between C, N and P-cycles.