PHYTOPLANKTON DYNAMICS ASSOCIATED WITH A GEOSTROPHIC FRONT - ECOLOGICAL AND BIOGEOCHEMICAL IMPLICATIONS

Using phytoplankton pigments as biomarkers, we investigated the relati onship between the physical forcing and the resulting biological, ecol ogical and biogeochemical properties of the geostrophic front of the E astern Alboran Sea. (1) Typical frontal sites present biomass levels a veraging 60 mg chl a m-2 (Up to 100 Mg m-2), whereas the adjacent zone s (typical Atlantic and Mediterranean) are characterized by an average integrated chlorophyll biomass of 20 mg chl a M-2. (2) The phytoplank ton biomass at front is diatom-dominated and differs markedly from the adjacent zones (typical Atlantic and Mediterranean), flagellate- and cyanobacteria-dominated. Therefore, high biomasses at the front do not result from purely physical accumulation but rather from local produc tion. (3) The chlorophyll and diatom biomasses increase from the left to the right side of the Atlantic jet, which supports the hypothesis o f a cross-frontal secondary circulation allowing a diatom bloom develo pment. (4) Using assumptions on the carbon/chlorophyll ratio and growt h rates for the different phytoplankton taxa, we evaluated the specifi c productions: diatoms account for 67% of the production at front and only about 10% at adjacent zones. (5) High concentrations of phaeopigm ents are only found at frontal stations, which points out the peculari ties of the food web at the frontal site, compared to adjacent areas. (6) The observations made during this study give a precise picture of that frontal system: autotrophic new production and exportation are en hanced. The implication of this frontal system on the carbon budget at a regional scale may be important.