THE EFFECTS OF SPATE-INDUCED DISTURBANCE, PREDATION AND ENVIRONMENTAL COMPLEXITY ON MACROINVERTEBRATES IN A TROPICAL STREAM

1. The combined effects of fish predation, substrate complexity and flow on benthic macroinvertebrates inhabiting riffles was investigated in a Hong Kong stream. Predation was manipulated using fish inclusion/exclusion cages containing complex (= many refuges) or simple (= few refuges) substrates. Experiments were undertaken during the winter dry season, when disturbance due to flow events was minimal, and repeated during the summer monsoon, when the stream experienced spates of varying intensity and duration. 2. Predation by the fish, Oreonectes platycephalus, significantly reduced the abundance of macroinvertebrates, especially chironomids and mayflies. Because chironomids (mostly Chironominae) were the dominant cage colonists, there was also a reduction in total macroinvertebrate density. 3. Predator impacts were significantly lessened during the wet season, when macroinvertebrate densities increased considerably, but significant reductions in the densities of vulnerable taxa and total macroinvertebrate abundance were nevertheless apparent. 4. Substrate complexity (the presence of prey refuges) had no significant effect on the ability of predators to reduce prey abundance. 5. Detritus accumulated in cages during the latter part of the study, and densities of most taxa were correlated with detrital standing stocks. 6. The results of this experiment indicate that biotic interactions such as predation may be suppressed during periods of spate-induced disturbance, although they can still influence benthic communities significantly. However, the effects of predation are highly taxon specific and may vary among streams in response to changes in predator and prey species composition, or the severity and duration of spates.