PHOTOSYNTHESIS AND CARBON METABOLISM BY SIZE-FRACTIONATED PHYTOPLANKTON IN THE SOUTHERN NORTH-SEA IN EARLY SUMMER

Productivity and physiological parameters of three size fractions of phytoplankton were measured in the southern North Sea in June 1989, in the period after the major Phaeocystis blooms had occurred. The central region was characterized by low densities of a dinoflagellate-dominated plankton assemblages, but the German Bight had much higher phytoplankton biomass and productivity, which was due to increases in the numbers of >5 mu m phytoplankton. The productivity of small nanoplankton and picoplankton was relatively constant throughout the region. The physiological conditions of the three phytoplankton size fractions were assessed by measuring the incorporation of C-14 labels into metabolic intermediates and by determining photosynthesis/ irradiance curves. In 24 h incubations, most of the label in all size fractions was found in protein; however, there were differences between size fractions with more of the total label being in protein and less in low molecular weight metabolites for the picophytoplankton than for the >5 mu m phytoplankton. Similarly, there were differences in photosynthetic parameters of the three size fractions, with values of both P-m(B),and a(B) which were always lowest for >5 mu m phytoplankton and highest for the picoplankton. There was good agreement between values of P-m(B) and the synthesis ratio of protein to low molecular weight metabolites.