Experimental evidence of a low-oxygen refuge for large zooplankton

We tested the hypothesis that hypoxic zones in the metalimnion and hypolimnion of lakes can provide a refuge against fish predation for large zooplankton. Experiments were run in a large indoor mesocosm system (Plon Plankton Towers). We compared mortality rates of Daphnia pulicaria due to free-ranging fish in mesocosms with either oxic or hypoxic hypolimnia. In the presence of fish Daphnia moved down below the thermocline. Under hypoxic conditions their distribution peaked in the upper hypolimnion at a concentration of approximately 1 mg O(2) L(-1). In oxygen-saturated hypolimnia Daphnia were distributed evenly. The mortality rate of Daphnia in the hypoxic treatment was only one third of that in the oxic treatment. The hypoxic habitat provided a refuge, as Daphnia tolerated lower oxygen concentrations than did fish. However, there may be demographic costs associated with living in low-oxygen conditions. Hence, the importance of a hypoxic refuge under natural conditions will most likely depend on the trade-off between predation risk and cost of living in hypoxic waters.