Effect of omnivorous fish on the production of labile and refractory dissolved organic carbon by zooplankton excretion in a simulated eutrophic lake

Experiments using six large, outdoor, ponds were conducted from 2 August 1993 to 10 September 1993 to examine whether the presence of omnivorous fish can affect the production of labile (DOCL) and refractory dissolved organic carbon (DOCR) by zooplankton excretion in a shallow eutrophic lake. Addition of goldfish (Carrasius auratus) to duplicate ponds at two densities (high density: 14.0 g C l−1; low density: 1.6 g C l−1) resulted in lower densities of large zooplankton (>100 μm) and higher chlorophyll-a concentrations than in reference ponds without goldfish. Rapid density increases in numbers of large zooplankton in reference ponds were significantly correlated with decreases in chlorophyll-a concentration, and were followed by increases in total dissolved organic carbon (DOCT). While no relationship between density of large zooplankton and chlorophyll-a concentrations in high- or low-density ponds was observed due to reduced feeding pressure on large zooplankton by goldfish, DOCT concentrations remained at levels 30–220% higher than those in reference ponds after the 15th day. Although DOCT and DOCL concentrations in apparent steady state in high- and low-density ponds were significantly higher than those in reference ponds (P<0.05), no significant difference in DOCR concentration was observed. Estimates suggest that the enhanced DOCT production rates are mainly attributable to excretion by small zooplankton (100 μm>), which may also enhance DOCT production in eutrophic lakes.