Hypoxia in Chesapeake Bay, 1950-2001: Long-term change in relation to nutrient loading and river flow

A 52-yr record of dissolved oxygen in Chesapeake Bay (1950-2001) and a record of nitrate (NO3-) loading by the Susquehanna River spanning a longer period (1903, 1945-2001) were assembled to describe the long-term pattern of hypoxia and anoxia in Chesapeake Bay and its relationship to NO3- loading. The effect of freshwater inflow on NO3- loading and hypoxia was also examined to characterize its effect at interannual and longer time scales. Year to year variability in river flow accounted for some of the observed changes in hypoxic volume, but the long-term increase was not due to increased river flow. From 1950-2001, the volume of hypoxic water in mid summer increased substantially and at an accelerating rate. Predicted anoxic volume (DO < 0.2 mg l(-1)) at average river flow increased from zero in 1950 to 3.6 x 10(9) m(3) in 2001. Severe hypoxia (DO < 1.0 mg l(-1)) increased from 1.6 x 10(9) to 6.5 x 10(9) m(3) over the same period, while mild hypoxia (DO < 2.0 mg l(-1)) increased from 3.4 x 10(9) to 9.2 x 10(9) m(3). NO3- concentrations in the Susquehanna River at Harrisburg, Pennsylvania, increased up to 3-fold from 1945 to a 1989 maximum and declined through 2001. On a decadal average basis, the superposition of changes in river flow on the long-term increase in NO3- resulted in a 2-fold increase in NO3- loading from the Susquehanna River during the 1960s to 1970s. Decadal average loads were subsequently stable through the 1990s. Hypoxia was positively correlated with NO3- loading, but more extensive hypoxia was observed in recent years than would be expected from the observed relationship. The results suggested that the Bay may have become more susceptible to NO3- loading. To eliminate or greatly reduce anoxia will require reducing average annual total nitrogen loading to the Maryland mainstem Bay to 50 x 10(6) kg yr(-1), a reduction of 40% from recent levels.