BIOCHEMICAL-COMPOSITION OF DISSOLVED ORGANIC-CARBON RELEASED FROM NATURAL COMMUNITIES OF LAKE PHYTOPLANKTON

The net products of the dissolved organic carbon released from in situ phytoplankton (PDOC) were collected after diel tracer incubations of natural plankton communities with (HCO3-)-C-14. The molecular size distribution of the dissolved (PDOC)-C-14 was determined with gel filtration chromatography, and high and low mol. wt. peaks were subsequently fractionated into major biochemical groups. To reveal general relationships between the taxonomical composition of the phytoplankton and the biochemical composition of released (PDOC)-C-14, experiments were carried out in three lakes of widely differing trophic conditions and taxonomical phytoplankton composition. The "PER" (= the percentage of primary production that appeared in the dissolved phase) was highly variable within the lakes, ranging from 4.9-29% in the eutrophic, 8.2-28% in the mesotrophic and 17-37% in the oligotrophic lake, respectively. "PER" showed a weak general tendency to be positively correlated with the contribution of chrysophycean flagellates to the phytoplankton biomass. The size distribution of the net (PDOC)-C-14 was highly variable within and between lakes, but always contained both high (> 10,000 dalton) and low (< 1,000 dalton) and sometimes intermediate mol. wt. substances. In the low mol. wt. components, amino acids, other organic acids and carbohydrates were all sometimes quantitatively important, but in most cases, the amino acids made the smallest contribution. The high and intermediate mol. wt. components were in most situations dominated by polysaccharides, and overall, the net (PDOC)-C-14 was often dominated by carbohydrates. Linear regression analysis revealed few direct relationships between the taxonomic composition of phytoplankton and the biochemical composition of the net (PDOC)-C-14. Weak positive correlations were found between the contribution of low mol. wt. compounds in (PDOC)-C-14 and the biomass percentage of cyanobacteria in phytoplankton, as well as between the contribution of high mol. wt. substances and the biomass percentage of diatoms.