Primary Productivity &Carbon Assimilation Patterns in Tropical Marine Phytoplankton

Abstract

Primary productivity values measured at 3 stations in the Lakshadweep Sea (Laccadive Sea) varied from 0.44 to 2.11 mg Cm^-3h^-1 and were characterised by a distinct subsurface maxima. Over a diel cycle in the Minicoy Lagoon,the rates varied from 0.22 to 12.80 mg Cm^-3h^-1 and closely paralleled the diurnal changes in solar irradiance levels. Studies on the distribution of assimilated carbon into the major end products of photosynthesis showed that the synthesis of polysaccharides is favoured at higher light intensities. There was a steady decline in the levels of polysaccharides synthesized with a decrease in levels of light available for photosynthesis. The synthesis of lipid/LMWM (low molecular weight metabolites) was also favoured at higher light intensities. A highly significant relationship between the percentage of lipid/LMWM synthesized and the productivity rates per unit chl <i>a</i> suggests that the appearance of newly fixed carbon within this fraction is strongly dependent on the rates of photosynthesis at any given time. A higher proportion of protein synthesized was observed under conditions of low light intensity. Regression analysis between the percentage of protein synthesized and the productivity rates per unit chl <i>a</i> showed an inverse relationship. Changes in the nature of the end products of photosynthesis in relation to available light in 2 clones of marine phytoplankton, viz. <i>Nitzschia closterium</i> and <i>Pleurosigma</i> sp., were in general agreement with those observed in natural populations.