Effects of Gammarus lacustris (Crustacea : Amphipoda) on plankton community structure in an alpine lake

The amphipod Gammarus lacustris is generally considered as a herbivore or a detritivorous scavenger, but we and others have observed it preying on pel agic zooplankton in fishless alpine lakes of the Canadian Rockies. We teste d the hypothesis that G. lacustris predation alters the plankton community structure of alpine lakes by manipulating G. lacustris density in large (28 00 L) in situ enclosure experiments. Compared with control enclosures (no G ammarus), Daphnia middendorffiana abundance was reduced 49, 85, and 100% in the low, medium, and high (40, 200, and 400 individuals.m(-2)) Gammarus de nsity enclosures, respectively. Hesperodiaptomus arcticus density was not s ignificantly related to G. lacustris density. Comparison of mesocosm result s with lake data showed that G. lacustris predation on D. middendorffiana i n the lake was lower, possibly because a pelagic deepwater refugium allows Daphnia to escape predation. The results show that G. lacustris predation c an affect the plankton community structure in small fishless alpine lakes.