Epifaunal communities thrive in an estuary with hypoxic episodes

We characterized the abundance and species composition of sessile and mobil e epifaunal assemblages in the York River, a tributary of the Chesapeake Ba y, U.S., during the summer hypoxia seasons in 1996 and 1997. We collected c ommunities on artificial substrates in two areas of the river that have his torically experienced different exposure to hypoxia. Despite frequent hypox ic stress, epifauna formed dense communities in both areas. Dominant specie s comprised a range of phyla and included the polychaetes Polydora cornuta and Sabellaria vulgaris, the bryozoans Membranipora tenuis and Conopeum ten uissimum, the tunicate Molgula manhattensis, the barnacle Balanus improvisu s, the anemone Diadumene leucolena, and the hydroids Ectopleura dumortieri and Obelia bicuspidata. Common mobile species included the nudibranchs Crat ena kaoruae and Doridella obscura, the amphipods Melita nitida and Paracapr ella tenuis, the polychaete Nereis succinea, and the flatworm Stylochus ell ipticus. We found few differences in species composition between the two ar eas;, even though one area usually experienced lower oxygen concentrations during hypoxic events, suggesting that hypoxia does not exclude any epifaun al species in the York River. We did find differences between the two study areas in percent cover and abundance of some species. While tunicates, hyd roids, and anemones were equally abundant in both areas during both study y ears, bryozoans and the polychaete S. vulgaris were more abundant in the ar ea with generally higher oxygen, suggesting that they may be less tolerant of hypoxic stress. The polychaete P. cornuta was more abundant in the area that usually had lower oxygen. These results suggest that many epifaunal sp ecies have high hypoxia tolerance, and most epifaunal species found in the lower York River are able to survive in hypoxic areas. Epifaunal species ar e not necessarily more susceptible to hypoxia than infaunal species in the York River. Epifaunal communities in areas with brief hypoxic episodes and moderate hypoxia (0.5-2 mg O-2 l(-1)) can persist with Little change in spe cies composition, and with few changes in abundance, as oxygen concentratio ns fall.