Concentrations and fluxes of organic carbon substrates in the aquatic environment.

Data concerning concentrations and fluxes of dissolved organic compounds (DOC) from marine and lacustrine environments are reviewed and discussed. Dissolved free amino acids and carbohydrates comprised the main fraction in the labile organic carbon pool. Dissolved free amino acids in marine waters varied between 3-1400 nM and those of fresh waters between 2.6-4124 nM. Dissolved free carbohydrates varied between 0.4-5000 nM in marine systems and between 14-111 nM in fresh waters, The turnover times of both substrate pools varied in marine waters between 1.4 hours and 948 days and in fresh waters between 2 hours and 51 days. Measurements of stable 12/13C-ratio and 14C-isotope dating in ocean deep water samples revealed DOC turnover times between 2000-6000 years. Studies on carbon flows within the aquatic food webs revealed that about 50% of photosynthetically fixed carbon was channelled via DOC to the bacterioplankton. Excreted organic carbon varied between 1-70% of photosynthetically fixed carbon in marine waters and between 1-99% in fresh waters. The labile organic carbon pool represented only 10-30% of the DOC. The majority (70-90%) of the DOC was recalcitrant to microbial assimilation. Only 10-20% of the DOC could be easily chemically identified. Most of the large bulk material represented dissolved humic matter and neither the chemical structure nor the ecological function of the DOC is as yet clearly understood.