The magnitude of phytoplankton release of photosynthetically produced dissolved organic carbon (PDOC), the size distribution and chemical nature of the released substances, and the quantity and composition of PDOC that were immediately assimilated by heterotrophic bacteria were investigated in a meso- to oligotrophic, mesohumic forest lake by means of the 14 CO 2 technique. The percentage extracellular release (PER) was comparatively high (up to ˜40%). The bulk of the released substances was of high mol. wt (>5000 dalton), with a minor contribution from low mol. wt (<1000 dalton) compounds. Heterotrophic bacteria preferentially assimilated the high molecular weight components of PDOC, and a polysaccharide of ˜6000 dalton with a very short turnover time was a key substance in the carbon flow from autotrophic phytoplankton to heterotrophic bacteria. The shortcomings of the 14 CO 2 method to assess phytoplankton release of PDOC is critically evaluated and discussed.
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Ingvar Sundh (1989) studied this question.