EUTROPHICATION HISTORY OF CHESAPEAKE BAY RECONSTRUCTED FROM FLUXES OF RHENIUM, MOLYBDENUM, AND URANIUM TO SEDIMENTS

High phytoplankton productivity and seasonal water-column stratification produce summer depletion of dissolved oxygen in Chesapeake Bay bottom waters below depths of 5-10 m. Some researchers have proposed that anthropogenic eutrophication has progressively exacerbated this problem. Others have argued that the time series of water-column monitoring data available is too short, and the potential influence of natural climatic variability is too poorly constrained to make it possible to clearly delineate the human role in the problem.

To determine the longer-term history of seasonal oxygen depletion and associated redox changes in Chesapeake Bay, we have measured authigenic concentrations of the redox-sensitive trace metals rhenium (Re), molybdenum (Mo), and uranium (U). Concentrations in 4.5-m and 20.7-m sediment cores from the northern bay and mid-bay, respectively, were converted to fluxes using age models constructed from radiocarbon and other radioisotopic data. After 1800 A.D., fluxes of Re at the northern site increased by 3-5 times, and Mo and U fluxes both increased about 5-7 times. Fluxes of all three elements increased simultaneously at this site, showed more significant variability before the agricultural period than at the mid-bay site, and showed secondary increases in the early 1900s that may be associated with dam construction on the Susquehanna River. Fluxes at the mid-bay site increased during approximately the same time period but only by 1.5-3 times. Smaller increases in fluxes at the mid-bay site are consistent with water-column monitoring data that typically show temporally and spatially less extreme oxygen depletion in this part of the bay than at the northern site.

Patterns seen in redox-sensitive metals are mirrored by proxies of increased diatom productivity, abundances of benthic foraminifera tolerant of low-oxygen conditions, and isotopically heavier nitrogen composition of organic matter indicating increased denitrification. This combined approach using high-resolution geochronology, redox-sensitive metal fluxes, and other environmental proxies can be applied to other coastal settings where there is a need for a better understanding of the human contribution to eutrophication.