Effects of seasonal hypoxia on macrobenthic production and function in the Rappahannock River, Virginia, USA

Since colonial times, anthropogenic effects have eroded Chesapeake Bay’s health, resulting in an increase in the extent and severity of hypoxia (≤2 mg O2 l−1), adversely affecting community structure and secondary production of macrobenthos in the Bay and its tributaries. The influence of hypoxia on macrobenthic communities is well documented, but less well known is the regulatory effect of hypoxia on macrobenthic production. Changes in macrobenthic production were assessed in the lower Rappahannock River, a sub-estuary of Chesapeake Bay, in an area known to experience seasonal hypoxia. During the spring, summer, fall, and following spring of 2007 and 2008, 10 samples were collected each season for a total of 80 samples, and macrobenthic production was estimated using Edgar’s allometric equation. Additionally, from early spring to late fall, dissolved oxygen concentrations were measured continuously at 2 of the 10 sites in 2007, and 2 of the 10 sites in 2008; in conjunction, the macrobenthic community was assessed through bi-weekly grab samples at these sites. Hypoxic sites had as much as 85% lower macrobenthic production compared to normoxic sites, and macrobenthic production at hypoxic sites was associated with primarily smaller, disturbance-related annelids. Macrobenthic production differed across seasons, and estimated sediment reworking rates were significantly higher during normoxia, indicating that the functional role of the macrobenthic community changed during hypoxia.