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BENTHIC COMMUNITY RESPONSES TO HYPOXIC CONDITIONS IN THE RAPPAHANNOCK RIVER, VIRGINIA Mary Elaine Smith Old Dominion University, 1994 Director: Dr. Daniel M. Dauer The effects of seasonal low dissolved oxygen conditions upon benthic macroinvertebrate communities were studied in the lower Rappahannock River, a tributary of the Chesapeake Bay. Benthic communities were sampled during March, June, August, and September 1993 at five equidistant depths (5, 10, 15, 20, and 25 m) extending upwards from the deepwater Virginia Benthic Biological Monitoring Station LE3.4 located in the deep basin just inside the mouth of the Rappahannock River. Infaunal species diversity, richness, biomass, and density were measured along with the vertical depth distribution of organisms within the sediment. Significant sediment differences were found between the shallowest, 5 m station, and all deeper sampling depths. Benthic communities displayed a systematic response to the temporal and spatial extent of the hypoxic water mass. Following summer low dissolved oxygen conditions, infaunal communities at the shallow and intermediate depths were dominated by surface-dwelling opportunistic fauna. Several lines of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. evidence indicate a continuous degradation of the communities inhabiting the two deepest sampling depths, including depressed levels of infaunal biomass and diversity preceding summer hypoxia, a year-long dominance by opportunistic fauna, and the lack of infaunal organisms inhabiting sediment depths greater than 5 cm. Comparison of these deepwater communities with an upriver station not subjected to low dissolved oxygen stress confirms this. Long-term monitoring of these deepwater communities can provide little information regarding the relative contributions of natural and anthropogenic forces to seasonal low dissolved oxygen in the lower Rappahannock River. The creation of an additional mid-water monitoring station is suggested, which in combination with the present deepwater station may be more indicative of changes in benthic community health due to Chesapeake Bay restoration efforts or changing water quality conditions. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Benthic Community Responses to Hypoxic Conditions in the Lower Rappahannock River, Virginia