PRODUCTION OF LABILE AND REFRACTORY DISSOLVED ORGANIC-CARBON BY ZOOPLANKTON EXCRETION - AN EXPERIMENTAL-STUDY USING LARGE OUTDOOR CONTINUOUS-FLOW THROUGH PONDS

Experiments using six outdoor experimental ponds equipped with a conti nuous flow-through system were conducted from 6 to 31 August 1992 to d etermine whether zooplankton grazing activity can be a significant sou rce of labile (L-DOC) and refractory dissolved organic carbon (R-DOC) in a eutrophic shallow lake. During large variations of chlorophyll a concentration caused by active zooplankton grazing pressure, distinct increases in total dissolved organic carbon (T-DOC) concentrations wer e observed in all ponds. Subsequently, pond T-DOC concentrations fluct uated at levels higher than those in inflow water, which remained almo st constant throughout the experimental period. The T-DOC fluctuations were closely related to changes in zooplankton density. The correlati on between L-DOC production rates and zooplankton densities in the pon ds was highly significant (r = 0.74, P < 0.01). Incubation experiments with filtrates of pond water showed that R-DOC, defined as T-DOC rema ining after 100 days of incubation in the dark at 20 degrees C, was pr oduced during active zooplankton grazing. The proportion of R-DOC in T -DOC was almost constant, irrespective of the N:P loading ratios (5, 1 0, and 20 by weight) in inflow waters.