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 19 93 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. Additi on of goldfish (Carrasius auratus) to duplicate ponds at two densities (hig h density: 14.0 g C l(-1); low density: 1.6 g C l(-1)) resulted in lower de nsities of large zooplankton (>100 mu m) and higher chlorophyll-a concentra tions than in reference ponds without goldfish. Rapid density increases in numbers of large zooplankton in reference ponds were significantly correlat ed with decreases in chlorophyll-a concentration, and were followed by incr eases in total dissolved organic carbon (DOCT). While no relationship betwe en density of large zooplankton and chloraphyll-a concentrations in high- o r low-density ponds was observed due to reduced feeding pressure on large z ooplankton by goldfish, DOCT concentrations remained at levels 30-220% high er than those in reference ponds after the 15th day. Although DOCT and DOCL concentrations in apparent steady state in high- and low-density ponds wer e significantly higher than those in reference ponds (P < 0.05), no signifi cant difference in DOCR concentration was observed. Estimates suggest that the enhanced DOCT production rates are mainly attributable to excretion by small zooplankton (100 mu m >), which may also enhance DOCT production in e utrophic lakes. (C) 1999 Elsevier Science Ltd. All rights reserved.