Effects of low dissolved oxygen on zooplankton predation by the ctenophore Mnemiopsis leidyi

The occurrence of low dissolved oxygen (DO), caused by vertical stratification and excess nutrient inputs, is an important and widely occurring physical feature in aquatic systems. Because some gelatinous species, such as the lobate ctenophore Mnemiopsis leidyi are more tolerant of low DO concentrations than their prey and competitors, hypoxia may have profound effects on trophic interactions. Predation, clearance and digestion rates of ctenophores feeding on zooplankton (primarily Acartia tonsa) were measured at 1.0, 2.0, 3.0 mg l–1 and air-saturated (approximately 7 mg l–1) DO. Clearance of zooplankton by large ctenophores (mean 22.5 ml, range 7 to 46 ml) was greater at low DO concentrations than under normoxic conditions. In contrast, consumption of zooplankton by small (mean 2.9 ml, range 1 to 10 ml) M. leidyi did not differ among DO levels. Similarly, ctenophore digestion rates were unchanged at oxygen concentrations as low as 1 mg l–1. Jumping frequency of A. tonsa copepods decreased significantly with decreasing DO concentration (1.0, 2.0, 3.0 mg l–1 and air-saturated). Such changes in prey behavior in low DO could affect both encounter and capture rates, potentially making less-tolerant prey more vulnerable to predation in hypoxic waters. Gelatinous species, which are more tolerant of hypoxia than fishes, may be able to inhabit regions of low oxygen that are avoided by zooplanktivorous fishes with high oxygen requirements. This could lead to dominance of gelatinous predators in areas affected by severe hypoxia and might alter energy pathways in these systems.