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

Abstract 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 (DOC L ) and refractory dissolved organic carbon (DOC R ) 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  μ m) 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 (DOC T ). 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, DOC T concentrations remained at levels 30–220% higher than those in reference ponds after the 15th day. Although DOC T and DOC L concentrations in apparent steady state in high- and low-density ponds were significantly higher than those in reference ponds ( P R concentration was observed. Estimates suggest that the enhanced DOC T production rates are mainly attributable to excretion by small zooplankton (100  μ m>), which may also enhance DOC T production in eutrophic lakes.