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

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ABSTRAff: Pigment distribution and photosynthesis versus irradiante (P-E) responses in the rnixed layer (ML) and in the chlorophyil maximurn (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 from the Afncan continental shelf, entrained around a cyclonic eddy about 100 km width. Chiorophyii a and P-E pararneters (a, P,) showed reqonal differences refleding changes in the water column structure and phytoplankton species composition. The deep CM shoaied from -100 to -10 m as the African shelf was approached, and there was a clear offshore-onshore transition in the verticaiiy integrated chlorophyli distribution and P-E responses related to the upwelling front. CM oceanic samples presented high a (0.020 to 0.042 mg C mg-' chl h-' [pmol m-' S-']-') and photomhibition. indicating adaptation to low irradmnce. Differences between CM and ML in P-E responses decreased in the vicinity of the upwelling untu they eventuaiiy became indistinguishable in a well-rnixed station on the continental sheif. Island-generated eddies affected mainly the productivity and chlorophyli distributions at the deep CM. Nutrient input in the eddy center resulted in an increase of a in the CM to the leve1 of the upwellmg samples (-0.053 mg C mg-' chl he' [wol m-' S-']-'). On the basis of the mesoscaie variability in the physical structure of the water column and the vertical distribution of chlorophyil, the CTZ was divided into 4 subregions with characteristic photosynthetic pararneters. Average integrated production in these areas vaned from -100 mg C m-' d-' m the most oligotrophic subregion to more than 500:: mg m-2 d-l .be upwe--ly zuiie. T;ie raiige vaA*'iioii &le p~otosymi'ne~c pararneiers in this CTZ was of the sarne magnitude as ranges observed in basin-scale studies of the Atlantic. Integrated produdion estimates are also in the lower and higher extremes of the observed values in the ocean. Our results highlight the importante of addressing the variability of the photosynthetic parameters in coastal upweiiing-open ocean transition zones in order to model primary production at regional scales.