Changes in phytoplankton ecophysiology across a coastal upwelling front

Abstract

The abundance, taxonomic composition and patterns of macromolecular synthesis of phytoplankton were determined across an upwelling-induced thermal front in the central Cantabrian Sea (southern Bay of Biscay) during July 1993. Enhanced levels of phytoplankton biomass, diatom abundance and photosynthetic rate were measured on the coastal side of the front. Relative carbon (C) incorporation into proteins increased noticeably 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 oceanic side of the front were adapted to the prevailing growth-limiting conditions by maintaining the synthesis of functionally essential molecules—proteins—rather than the synthesis of storage compounds. As a result, the carbon to nitrogen uptake ratio varied from ˜5.7 in offshore waters to 8.0 in the nearshore region. Our results suggest that the taxonomic and physiological changes in phytoplankton assemblages as a response to upwelling may result in an increase in the synthesis of organic C relative to the upward flux of nitrate.