Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Robert J. Diaz
Human development has eroded Chesapeake Bay's health, resulting in an increase in the extent and severity of hypoxia (≤2 mg O2 l-1). The Bay's hypoxic zones have an adverse affect on community function and secondary production of macrobenthos. The production of macrobenthos is important as these fauna link energy transfer from primary consumers to epibenthic predators and demersal fish, and serve as the foremost pathway that carbon is recycled out of the sediment. Additionally, bioturbation, an essential macrobenthic function that causes the displacement and mixing of sediment particles, increases the quality of marine sediments. In the marine environment bioturbation is primarily mediated by macrofauna which are susceptible to perturbations in their surrounding environment due to their sedentary life history traits. The effect of hypoxia on macrobenthic production was assessed in Chesapeake Bay and three of its tributaries (Potomac, Rappahannock, and York rivers) from 1996 to 2004. Each year, 25 random samples were collected from each system and macrobenthic production estimated using Edgar's allometric equation. Efforts were then focused on the Rappahannock River, a sub-estuary of Chesapeake Bay known to experience seasonal hypoxia, to assess changes in macrobenthic production and function. During the spring, summer, fall, and following spring of 2007 and 2008, samples were collected each season in each year, and DO concentrations were measured continuously at two sites in 2007 and two in 2008. A benthic observing system (Wormcam) was also deployed in 2009 from early spring to late fall to assess the impact of hypoxia on bioturbation. Wormcam transmitted a time series of in situ images and water quality data in near real-time. Results from the previous projects was used to develop a continuous-time, biomass-based model, including phytoplankton, zooplankton, and macrobenthic state variables. The primary focus aimed at predicting the effect of hypoxia on macrobenthic biomass. Z', a sigmoid relationship between macrobenthic biomass and DO concentration, was derived from macrobenthic data collected from the 2007 and 2008 field experiments. Annual fluctuations in macrobenthic production were significantly correlated with DO. Hypoxia led to a 90% reduction in daily macrobenthic production relative to normoxia, and production at hypoxic sites was composed primarily of smaller, disturbance-related annelids. The reduced production resulted in an annual biomass loss of approximately 7320 to 13,200 metric tons C, which equated to a 6 to 12% annual displacement of the Bay's total macrobenthic productivity due to hypoxia. Macrobenthic production differed across seasons, and sediment reworking rates were significantly higher during normoxia, indicating a change in the functional role of the macrobenthic community. Hypoxia was found to significantly reduce bioturbation through reductions in burrow lengths, burrow rates, and burrowing depth. Although infaunal activity was greatly reduced during hypoxic and near anoxic conditions, some individuals remained active. The biomass-based model was successfully calibrated and verified, using independent data, to accurately predict B annually. Simulation analysis of the DO formulation showed B strongly linked to DO concentration, with fluctuations in biomass significantly correlated with the duration and severity of hypoxia.
© The Author
Sturdivant, S. Kersey, "The Effects of Hypoxia on Macrobenthic Production and Function in the lower Rappahannock River, Chesapeake Bay, USA" (2011). Dissertations, Theses, and Masters Projects. Paper 1539616867.