Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Rochelle D. Seitz


Hypoxia, low dissolved oxygen, is an important environmental stressor in estuarine systems. In this dissertation, I examine the effects of hypoxia on the macrobenthic communities of the York and Rappahannock Rivers, Chesapeake Bay, USA, and in particular its effects on the infaunal clam Macoma balthica. A survey of the macrobenthic community was performed using box-coring before and after hypoxia in 2003 and 2004 in both rivers. Hypoxia was associated with a change in the macrobenthic community towards smaller, shorter-lived, opportunistic species; and a substantial decrease in biomass. M. balthica recruited into all areas of the river but suffered local extinction in hypoxic areas, demonstrating that these areas represent sink habitats. I developed an enzyme-linked immunosorbent assay to quantify fecundity in M. balthica and used it determine the effect of hypoxia on clam fecundity. In laboratory experiments performed in 2005 and 2006, M. balthica migrated toward the sediment surface and decreased egg production in response to hypoxia. In field caging experiments, performed during the summers of 2005 and 2006, episodic hypoxia caused a three-fold increase in the rate of predation on M. balthica , suggesting that the behavioral responses of M. balthica to hypoxia make it more vulnerable to predation. These results further suggested that hypoxia may change the functional response of epibenthic predators to M. balthica from a stabilizing type III to a destabilizing type I or II. Using the results of the previous experiments, I constructed a density-independent model of the M. balthica population, which predicted that increasing the spatial extent and duration of hypoxia could cause the population to decline toward extinction. A second model, which incorporated density-dependent predation, predicted that, under mild hypoxic conditions, trophic transfer of biomass from M. balthica to predators could be enhanced, but that increasing the severity of hypoxia would decrease trophic transfer. The model further predicted that increasing hypoxia would decrease the resilience of the M. balthica population to disturbance, making functional extinction of the population more likely. This body of work underscores the negative effects of hypoxia on the levels of the individual, the population, and the ecosystem.



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