Effects of periodic hypoxia on mortality, feeding and predation in an estuarine epifaunal community.

The York River Estuary, a tributary of the Chesapeake Bay, USA, experiences periodic low oxygen stress (hypoxia), yet epifaunal species form dense communities there. We studied hypoxia tolerance of common epifaunal species in the York River by exposing sessile and mobile epifauna to high and low oxygen concentrations in laboratory aquaria. Mortality in hypoxia varied among species, ranging from 0% to 100%, with trends of decreased tolerance by mobile species relative to sessile species. While most species tested experienced some mortality after being exposed to hypoxia (at 1 mg O(2)/l or 0.5 mg O(2)/l) for 5 days, many species had a median lethal time (LT(50)) in hypoxia greater than 1 week (3 of 6 species at 1 mg O(2)/l and 6 of 14 species at 0.5 mg O(2)/l), the maximum duration of typical hypoxic episodes in the York River, suggesting that hypoxia may cause little mortality for some species in this system. However, hypoxia had sub-lethal effects on behavior in all species tested. Epifaunal animals responded to hypoxia with behaviors that moved them higher in the water column or by entering resting states until hypoxia passed. Feeding and predation by a variety of taxa (the hydroid Obelia bicuspidata, the mud crab Neopanope sayi, juvenile blue crabs Callinectes sapidus, the flatworm Stylochus ellipticus, and the nudibranch Doridella leucolena) decreased during hypoxia, despite varying mortality responses to low oxygen stress, suggesting that short hypoxic episodes may create predation refuges for prey species. At least one highly tolerant species (O. bicuspidata) showed substantially decreased growth in hypoxia. Although relatively high tolerance of hypoxia by many estuarine epifaunal species limits serious disturbance during brief hypoxic episodes, hypoxia's greatest impact on York River epifaunal communities might be through its indirect effects on behavior and predation.