Seasonal and interannual variability in algal biomass and primary production in the Mediterranean Sea, as derived from 4 years of SeaWiFS observations

Because the Mediterranean has been subject for several decades to increasing anthropogenic influences, monitoring algal biomass and primary production on a long-term basis is required to detect possible modifications in the biogeochemical equilibrium of the basin. This work was initiated thanks to a 4-year-long time series of SeaWiFS observations. Seasonal variations of algal biomass ( estimated using a previously developed regional algorithm) and primary production were analyzed for the various regions, and compared with those estimated using the CZCS sensor (1978-1986). Also, interannual variations could be assessed for the first time. The seasonal cycles of algal biomass generally reveal a maximum in winter or spring, and a minimum in summer. Some conspicuous differences with CZCS observations (e.g., in the Northwest Basin, reduction of the deep convection zone, earlier start of the spring bloom, quasi-absence of the vernal bloom) likely result from environmental changes. Interannual variations in algal biomass are noticeable all over the basin, including in the very oligotrophic waters of the Eastern Basin. The seasonal evolution of primary production is predominantly influenced by that of algal biomass in the Western Basin (with, in particular, a spring maximum). In the Eastern Basin, the seasonal courses of PAR and biomass tend to compensate each other, and primary production varies weakly along the year. The annual values computed over the 1998-2001 period for the Western Basin (163 +/- 7 gC m(-2) yr(-1)) and the Eastern Basin (121 +/- 5 gC m(-2) yr(-1)) are lower (by 17 and 12%, respectively) than those previously derived (using the same light-photosynthesis model) from CZCS data.