Lethal Limits and Sublethal Effects of Hypoxia on the Amphipod Gammarus pseudolimnaeus

The amphipod Gammarus pseudolimnaeus is an important prey species for trout in certain tailwater fisheries below hydropower dams. The effects of low dissolved oxygen (DO) on this species were investigated in laboratory experiments. The duration of survival in anoxia and in lethally low DO concentrations differed among adult female, adult male, and juvenile individuals. Adult females were the group most sensitive to both anoxia and hypoxia; for example, LC50 in hypoxia was 2.00, 1.28, and 1.05 mg/L, respectively, for females, males and juveniles (48 h, 15°C). Juveniles were generally less sensitive to hypoxia but more sensitive to anoxia than were adult males. LC50 increased with increasing temperature but changed little with duration of exposure after 24 h. The rate of oxygen consumption was a hyperbolic function of DO without a distinct critical level; oxygen consumption was depressed 10% at 5.7 mg/L and 50% at 1.3 mg/L (15°C). Hypoxia induced the separation of amplexing pairs at 2 ppm and inhibited the reunion of separated pairs at 5 ppm (15°C). Current US government (US Environmental Protection Agency) criteria for DO appear to be sufficient for protection of this species, but these criteria may often not be met below hypolimnetic-release hydropower dams.