Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


The lower Rappahannock River, a subestuary of the Chesapeake Bay, is affected by seasonal low dissolved oxygen events that are intermittent and vary in intensity. Overall, macrobenthic abundance and species number were low, decreasing towards the channel and upriver, where dissolved oxygen concentrations were lowest. Mortality of macrobenthos in early summer was attributed to hypoxia. Species abundances and number declined with the onset of hypoxia, and did not recover in the deep channel until the fall. Tolerance of macrobenthos to hypoxia appeared to be species-specific. Recruitment of hesionid polychaetes occurred in summer during normoxia, but mortality followed during hypoxia, suggesting that macrobenthic assemblages in affected areas may be largely structured by the relationship between the timing of low dissolved oxygen events and species life histories. Laboratory experiments with dominant polychaete species (Streblospio benedicti and Lomia medusa) were conducted to determine their tolerance and behavior to hypoxia, anoxia, and hydrogen sulfide. Under severe hypoxia, S. benedicti survived for two weeks without significant mortality at two experimental dissolved oxygen concentrations (14.5 and 7% air saturation). In anoxia, all worms died within 55 hrs. Larvae survived severe hypoxia without displaying behavioral ill effects for at least 4 days. The behavior of S. benedicti was modified; feeding and burrowing activities ceased, and worms came out of their tubes onto the sediment surface. L. medusa showed higher tolerance of anoxia, surviving up to 5 days. Survival under prolonged hypoxia was significantly reduced. The behavior of L. medusa was also modified by hypoxia and anoxia. Worms ceased feeding and protruded head and abdominal tip out of their tubes. Worm occurrences on the sediment surface at time of death were treatment-dependent. Survival of both species was not significantly reduced when worms were exposed to anoxia with moderate concentrations of hydrogen sulfide. These results are discussed in terms of species distributions, presence of hydrogen sulfide in sediment, and mechanisms of resistance to sulfide toxicity.



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