Dynamics of biodegradable DOC produced by freshwater plankton communities

The net production of dissolved organic carbon (DOC) and DOC biodegradabili ty were followed during experimental diatom blooms in freshwater enclosures . DOC accumulated at almost constant rates of 4.4 muM d(-1) during the incr ease and the decay of the blooms. A high production of 188 PM occurred in a n episodic event over 5 d in 1 of 4 enclosures. Two independent methods to measure biodegradable DOC (BDOC) showed perfect agreement and inorganic nut rients neither affected the degradation rates nor the amount of DOC degrade d. The accumulating DOC was initially recalcitrant (RDOC) and about 25 muM RDOC was produced during the increase in phytoplankton biomass. When the pl ankton communities declined, the BDOC increased, concomitant with the conti nued DOC accumulation: about 60 to 82 % of the new DOC was biodegradable wh en 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 f rom 75 to 120 d. The biodegradable pool was totally dominated by semi-labil e components. The high chemical resistance to degradation controls the long turnover times and the accumulation of DOC.