Dynamics of biodegradable DOC produced by freshwater plankton communities

The net production of dissolved organic carbon (DOC) and DOC biodegradability were followed during experimental diatom blooms in freshwater enclosures. DOC accumulated at almost constant rates of 4.4 muM d(-1) during the increase and the decay of the blooms. A high production of 188 PM occurred in an episodic event over 5 d in 1 of 4 enclosures. Two independent methods to measure biodegradable DOC (BDOC) showed perfect agreement and inorganic nutrients neither affected the degradation rates nor the amount of DOC degraded. The accumulating DOC was initially recalcitrant (RDOC) and about 25 muM RDOC was produced during the increase in phytoplankton biomass. When the plankton communities declined, the BDOC increased, concomitant with the continued DOC accumulation: about 60 to 82 % of the new DOC was biodegradable when the experiment was terminated after 19 d. It is suggested that a change in the quality of produced DOC took place over the course of the bloom. The turnover time of the BDOC pool was measured in batch cultures and ranged from 75 to 120 d. The biodegradable pool was totally dominated by semi-labile components. The high chemical resistance to degradation controls the long turnover times and the accumulation of DOC.