CHANGES IN PHYTOPLANKTON ECOPHYSIOLOGY ACROSS A COASTAL UPWELLING FRONT

The abundance, taxonomic composition and patterns of macromolecular sy nthesis of phytoplankton were determined across an upwelling-induced t hermal front in the central Cantabrian Sea (southern Bay of Biscay) du ring July 1993. Enhanced levels of phytoplankton biomass, diatom abund ance and photosynthetic rate were measured on the coastal side of the front. Relative carbon (C) incorporation into proteins increased notic eably on the oceanic side, taking values of up to 64%, whereas changes in the relative C incorporation into lipids and low-molecular-weight metabolites followed an opposite trend. Phytoplankton cells on the oce anic side of the front were adapted to the prevailing growth-limiting conditions by maintaining the synthesis of functionally essential mole cules-proteins-rather than the synthesis of storage compounds. As a re sult, the carbon to nitrogen uptake ratio varied from similar to 5.7 i n offshore waters to 8.0 in the nearshore region. Our results suggest that the taxonomic and physiological changes in phytoplankton assembla ges as a response to upwelling may result in an increase in the synthe sis of organic C relative to the upward flux of nitrate.