Mesoscale variability in phytoplankton biomass distribution and photosynthetic parameters in the Canary-NW African coastal transition zone

Pigment distribution and photosynthesis versus irradiance (P-E) responses i n the mixed layer (ML) and in the chlorophyll maximum (CM) were examined in the coastal transition zone (CTZ) between the NW African coastal upwelling and the Canary Islands during August 1993. The sampling included 2 island- generated eddies in the lee of the archipelago and an upwelling filament fr om the African continental shelf, entrained around a cyclonic eddy about 10 0 km width. Chlorophyll a and P-E parameters (alpha, P-m) showed regional d ifferences reflecting changes in the water column structure and phytoplankt on species composition. The deep CM shoaled from similar to 100 to similar to 10 m as the African shelf was approached, and there was a clear offshore -onshore transition in the vertically integrated chlorophyll distribution a nd P-E responses related to the upwelling front. CM oceanic samples present ed high alpha (0.020 to 0.042 mg C mg(-1) chi h(-1) [mu mol m(-2) s(-1)](-1 )) and photoinhibition, indicating adaptation to low irradiance. Difference s between CM and ML in P-E responses decreased in the vicinity of the upwel ling until they eventually became indistinguishable in a well-mixed station on the continental shelf. Island-generated eddies affected mainly the prod uctivity and chlorophyll distributions at the deep CM. Nutrient input in th e eddy center resulted in an increase of a in the CM to the level of the up welling samples (similar to 0.053 mg C mg(-1) chl h(-1) [pmol m(-2) s(-1)]( -1)). On the basis of the mesoscale variability in the physical structure o f the water column and the vertical distribution of chlorophyll, the CTZ wa s divided into 4 subregions with characteristic photosynthetic parameters. Average integrated production in these areas varied from similar to 100 mg C m(-2) d(-1) in the most oligotrophic subregion to more than 5000 mg C m(- 2) d(-1) in the upwelling zone. The range of variation in the photosyntheti c parameters in this CTZ was of the same magnitude as ranges observed in ba sin-scale studies of the Atlantic. Integrated production estimates are also in the lower and higher extremes of the observed values in the ocean. Our results highlight the importance of addressing the variability of the photo synthetic parameters in coastal upwelling-open ocean transition zones in or der to model primary production at regional scales.