Biosynthesis of macromolecular and lipid classes by phytoplankton in the Northeast Water Polynya

The patterns and environmental correlates of macromolecular synthesis were investigated during 1993 in the Northeast Water Polynya, a region of variable ice cover and hydrodynamic regime on the continental shelf of northeast Greenland (77" to 81' N). Allocation of photosynthate to major macromolecular classes (protein, lipid, and polysaccharides), like phytoplankton biomass and primary production, showed strong spatio-temporal variability. Time series observations in the central part of the polynya and correlational analysis over all stations provided physiological evidence of nutrient deficiency in surface populations as nitrate and silicate were depleted. Lipid, rather than polysaccharide, synthes~s was favoured as nutrient concentrations decreased, and C/N assimilation ratios inferred from protein labelling reached very high values ( > l 5 by weight). C/N composition ratios increased in parallel but remained lower (<10) than the assimilation ratios. Lipid synthesis shifted towards neutral lipids and glycolipids, and away from phospholipids, as nutrient concentrations declined to their observed minimum. At stations with a dominance of large (mainly diatom) phytoplankton more of the lipid synthesis was directed to glycolipids and less to phospholipids and neutral lipids than at those dominated by small (mainly flagellate) phytoplankton, but relative allocation to total lipids and other macromolecular classes was not significantly different between the 2 types of stations. Although both nitrate and silicate became depleted, changes in silicate were most closely related to the physiological changes observed, consistent with a predominance of diatoms at the more productive stations.