Production and bioavailability of autochthonous dissolved organic carbon: effects of mesozooplankton

Abstract

A phytoplankton bloom and decay sequence was created in 2 laboratory containers and mesozooplankton was added to one container before the peak of algal biomass. Each day for 22 d, the net production of autochthonous dissolved organic carbon (DOC) was measured and on 5 occasions the degradation kinetics and the total pool of biodegradable DOC (BDOC) were assayed in experi- ments lasting 230 d. Net accumulation of new DOC was 235 and 280 µM in the containers with and without zooplankton, respectively. The best description of microbial DOC degradation was a 2-pool model and 1st order exponential decay. Without mesozooplankton present, the degradation experi- ments showed accumulation of a large pool of labile BDOC characterised by decay coefficients >0.2 d -1 . The least labile pools in the 2 containers had similar coefficients (average 0.02 d -1 ). The amount of newly produced recalcitrant DOC (RDOC) accounted for about 12% of new DOC. The differences observed with respect to degradation kinetics and net DOC production are explained by food web interactions and nutrient limitation. The presence of mesozooplankton resulted in high bacterial production keeping labile BDOC at low concentrations. In the container without meso- zooplankton, the bacterial uptake capacity was reduced, probably by a combination of protist grazing and nutrient limitation. Consequently, about 75 µM BDOC with a half-life of less than 3 d accumu- lated during the experiment. Mineralisation of the accumulated dissolved organic matter (DOM) during microbial degradation in a nutrient replete environment was measured as the decrease in DOC and net mineralisation/immobilisation of inorganic N and P. The mineralisation of DOC was accompanied by low mineralisation of N and P and even immobilisation of phosphate during degra- dation of DOM produced in the container with mesozooplankton present. Bacterial production of DON and DOP is believed to result in a recalcitrant DOM pool enriched in N and P, and the activity of mesozooplankton seems to enhance this scenario.