EFFECTS OF THE DIATOM-EMILIANIA HUXLEYI SUCCESSION ON PHOTOSYNTHESIS,CALCIFICATION AND CARBON METABOLISM BY SIZE-FRACTIONATED PHYTOPLANKTON

Changes in species composition, photosynthesis, calcification and size -fractionated carbon metabolism by natural phytoplankton assemblages w ere monitored in three mesocosms under different nutrient conditions d uring May 1993. In the 3 enclosures, the decline of the diatom-dominat ed assemblages was followed by the development of a bloom of the cocco lithophorid Emiliania huxleyi. Highest growth of E. huxleyi was observ ed in the mesocosm with a high N:P ratio, suggesting this species is a good competitor at low phosphate concentrations. The transition from diatom- to E. huxleyi-dominated assemblages brought about a sharp redu ction of the phytoplankton standing stock and carbon-specific photosyn thetic rate. The relative contribution of the smaller size fraction to total photosynthesis increased as the succession progressed. Calcific ation rate and E. huxleyi cell-specific calcite production were highes t during the early stages of development of the E. huxleyi bloom. Dist inct changes in the patterns of C-14 allocation into biomolecules were noticed during the diatom-E. huxleyi succession. The diatom-dominated assemblage showed high relative C-14 incorporation into low molecular weight metabolites (LMWM), whereas proteins and, specially, lipids ac counted for the largest proportion of carbon incorporation in the E. h uxleyi bloom. The patterns of photoassimilated carbon metabolism prove d to be strongly dependent on cellular size, as protein relative synth esis was significantly higher in the smaller than in the larger size f raction, irrespective of the nutrient regime and the successional stag e. These results are discussed in relation to the ecological and physi ological features of small phytoplankton.