Extracellular Organic Carbon (EOC) Released by Phytoplankton and Bacterial Production

Parts of the pelagic carbon cycle were investigated during ten diel cycles in five Danish lakes and one coastal area. The study included simultaneous measurements of primary production, phytoplankton release and bacterial assimilation of extracellular organic carbon (EOC), bacterial production and bacterial assimilation of dissolved free amino acids. The primary production, EOC release and assimilation were measured with the '4Cmethod and a particle size fractionation. The gross release of EOC ranged from 5 to 46% of the diel primary production and the major part of the released products were assimilated by bacteria. It was shown that diel time-course incubations should be used as opposed to short-time incubations. The bacterial production was estimated by means of two methods: 1) The frequency of dividing cells (FDC) and 2) 'H-thymidine incorporation into DNA. An analysis based on primary production and amino acid assimilation in the freshwater samples showed that the FDC-method tended to overestimate the bacterial production, whereas the thymidine-method probably represents the true order of magnitude with a trend toward underestimation. In the lakes the assimilation of EOC contributed substantially (>80%) to the bacterial production in three cases, moderately (38-50%) in three cases and was of less (<38%) importance in one case. Due to the presence of picophytoplankton (<1him) reliable results were not obtained at the coastal station.