PATTERNS OF MACROMOLECULAR-SYNTHESIS BY NATURAL PHYTOPLANKTON ASSEMBLAGES UNDER CHANGING UPWELLING REGIMES - IN-SITU OBSERVATIONS AND MICROCOSM EXPERIMENTS

A coastal station located in the southern Bay of Biscay was sampled on six occasions between 12 July and 10 September 1991. The vertical dis tribution of the patterns of photosynthetic carbon incorporation into proteins, polysaccharides, lipids and low molecular weight metabolites , as well as the biochemical composition of particulate matter were de termined on each cruise. In addition, the same variables were monitore d during microcosm experiments in which sub-surface phytoplankton asse mblages were incubated under different rates of increasing irradiance simulating differences in upwelling intensity. During the first three experiments, phytoplankton cells did not show any growth response to t he simulated upwelling conditions. When phytoplankton growth was slow or absent and nutrients were still available, the relative synthesis o f proteins was high, suggesting that phytoplankton cells tended to mai ntain the synthesis of proteins rather than storage products under adv erse growth-limiting conditions. Sub-surface phytoplankton assemblages had the potential for growth in response to upwelling. Several diatom blooms developed during the last two experiments showing enhanced lev els of protein and polysaccharide specific synthesis rates (SSR) and a marked increase in the protein to carbohydrate (P:C) compositional ra tio. Parallel sea-truth observations carried out during an upwelling p ulse indicated that phytoplankton assemblages under natural conditions underwent similar physiological changes to those found in the experim ental microcosms under simulated upwelling. In general, the most remar kable increases in chlorophyll a (Chl a) concentration and macromolecu lar SSR took place in those microcosms showing higher rates of increas ing irradiance. Variations in the patterns of photosynthate partitioni ng and the P:C ratio were also related to the intensity of the advecti ve process. These results emphasize the importance of the fine variabi lity of the physical held in modulating the physiological responses of phytoplankton to upwelling.