Responses of epiphytic aquatic macroinvertebrates to hypoxia

Abstract

Different species of aquatic macrophytes have strongly contrasting effects on the oxygen dynamics of the waters they inhabit. We conducted laboratory experiments to test the hypothesis that macroinvertebrates inhabiting stands of the floating-leaved water-chestnut (Trapa natans), which causes severe hypoxia, are more resistant to low oxygen concentrations (0.5-1 mg L-1) than those that inhabit submerged aquatic vegetation (Vallisneria americana and Myriophyllum spicatum), which do not cause hypoxia. Chironomids and amphipods associated with T. natans were more resistant to hypoxia than those associated with submerged plants, in support of our hypothesis. Gastropods showed the opposite pattern. Survival was significantly related to exposure time as well in these 3 groups. Ostracods from the 2 habitats were equally resistant to hypoxia, with high survival regardless of exposure time. Almost all zygopterans died when exposed to hypoxia. Different kinds of macroinvertebrates have distinct responses to hypoxia, so different kinds of hypoxic macrophyte beds may support distinct assemblages of macroinvertebrates. Specifically, the spatial and temporal extent of hypoxia will be critical, and shifts in macrophyte species composition caused by species invasions or other reasons may have differential effects on macroinvertebrates by causing different oxygen regimes.