Participants:

Sediment Nitrogen Fluxes and Microbial Nitrogen Cycling in the Sediments of Chesapeake Bay
Jeffrey Cornwell

Jeffrey Cornwell’s laboratory has been involved in the study of sediment biogeochemical processes in a number of Chesapeake and other estuarine systems (see vita). At Horn Point, the application of mass spectrometry to the measurement of N2 and O2 fluxes (see Kana et al. 1994; 1998) has provided a direct, simple alternative to other denitrification techniques (Cornwell et al. 1999). In the Chesapeake Bay, we have applied these techniques to wetland sediments, the mainstem of the Chesapeake Bay, and a number of tidal river systems (the Choptank River, Patuxent River, Back River/Baltimore Harbor, Pocomoke River) with great success.

In this Biocomplexity project, sediment sampling is carried out at two stations on the tidal Choptank River and four stations in the mainstem of the Chesapeake Bay (Figure 1; Table 1). Three of these stations have been studied for the NSF LMER/Proteus Program for studies of sediment-water exchange (Cowan et al.), sulfate reduction (diPasqule et etl., 1998), iron sulfide minerals (Cornwell and Sampou 1995) and DOC/DON fluxes (Burdige and Homstead, 1994), with additional information on sediment accretion, nutrient and trace metal profiles at the deep mid-ba site(Cornwell et al. 1996). We have added one site at the shallow flanks of the bay to provide one mid-bay site that remains aerobic throughout the summer.

Station Location	Characteristics
Upper Choptank	high nitrate, low productivity, tidal freshwater/oligohaline, mud
Lower Choptank	moderate nitrate, high productivity, mesohaline, mud
Upper Chesapeake	high nitrate, low productivity, oligohaline, mud
Mid Chesapeake Deep	seasonally anoxic, high productivity, mesohaline, mud
Mid Chesapeake Shallow	high productivity, mud, mesohaline
South Chesapeake	moderate productivity, sand/mud, polyhaline

The sediment program has gone well, with successful experiments conducted with sediments from all sites. A mid bay core is shown in Figure 2. The rates of denitrification are similar to our previous rates for the Choptank River; we have been surprised by the high rates throughout the Chesapeake in April and in the north and shallow mid bay sites in summer. Denitrification in the upper Choptank and northern Chesapeake utilizes overlying water nitrate; the other sites rely on nitrification.

References
Burdige, D.J. and Homstead, J., 1994. Fluxes of dissolved organic carbon from Chesapeake Bay sediments. Geochimica et Cosmochimica Acta, 58: 3407-3423.
Burdige, D.J. and Zheng, S., 1998. The biogeochemical cycling of dissolved organic nitrogen in estuarine sediments. Limnology and Oceanography, 43: 1796-1813.
Cornwell, J.C. and Sampou, P.A., 1995. Environmental controls on iron sulfide mineral formation in a coastal plain estuary. In: M.A. Vairavamurthy and M.A.A. Schoonen (Editors), Geochemical Transformations of Sedimentary Sulfur. American Chemical Society, Washington, DC, pp. 224-242.
Cornwell, J.C., Stevenson, J.C., Conley, D.J. and Owens, M., 1996. A sediment chronology of Chesapeake Bay eutrophication. Estuaries, 19: 488-499.
Cornwell, J.C., Kemp, W.M. and Kana, T.M., 1999. Denitrification in coastal ecosystems: environmental controls and aspects of spatial and temporal scale. Aquatic Ecology, 33: 41-54.
Cowan, J.L.W. and Boynton, W.R., 1996. Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns, controlling factors and ecological significance. Estuaries, 19: 562-580.
Kana, T.M. et al., 1994. Membrane inlet mass spectrometer for rapid high-precision determination of N2, O2, and Ar in environmental water samples. Analytical Chemistry, 66: 4166-4170.
Kana, T.M., Sullivan, M.B., Cornwell, J.C. and Groszkowski, K., 1998. Denitrification in estuarine sediments determined by membrane inlet mass spectrometry. Limnology and Oceanography, 42: 334-339.
Marvin-DiPasquale, M.C. and Capone, D.G., 1998. Benthic sulfate reduction along the Chesapeake Bay central channel. I. Spatial trends and controls. Marine Ecology Progress Series, 168: 213-228.

Figure 1. Map of ECRAB stations.

Figure 2. Mid Bay Deep Cores, Spring 2001. These cores show a characteristic oxidized surface layer indicative of iron oxide minerals. In mid-summer, the sediment-water interface is anaerobic and iron monsulfide minerals and pyrite predominate.

Figure 3. Fluxes of N2-N in the initial two whole system experiments. Error bars represent one standard deviation of triplicate cores.