EXCHANGES OF DIN AND DOC BETWEEN SALT MARSH SEDIMENTS AND OVERLYING TIDAL WATER.

Berry, B. E., I. C. Anderson, and R. L. Wetzel.

Salt marshes vary greatly in their abilities to serve as sources or sinks of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), dissolved organic carbon (DOC), and dissolved oxygen (DO) to overlying tidal water. In order to better understand what processes and conditions regulate these exchanges we performed seasonal measurements of DIN, DON, DOC, and DO exchange between salt marsh sediments and tidal water using in situ chambers. In addition, we measured sediment and water column chlorophyll in the chambers and DIN concentrations in adjacent creek water at one hour intervals over 24 hour periods. Specific objectives of these studies were to: (1) use chambers to perform a small scale study on exchanges between the vegetated (Spartina alternifllora) salt marsh and tidal water, (2) measure seasonal variability of DIN and DOC exchanges between the sediment and overlying water, (3) assess relationships between observed sediment/water DIN exchanges and DIN concentrations in creek water, and (4) perform experiments at Phillips Creek on the Eastern Shore of Virginia (Virginia Coast Reserve), where parallel studies examining microbial nitrogen transformations within sediments are being conducted.

The results thus far indicate that the vegetated salt marsh imports DIN from the overlying water in the spring, summer, fall, and early winter, with no net DIN exchange in mid-winter (Figure 1). Salt marsh sediments exported DOC in the summer and exhibited either no net exchange or slight uptake in the fall and winter. The tidal creek, on the other hand, generally exported DIN. Figure 2 illustrates data collected in March 1994, when the export of DIN out of the creek is observed to be greatest. There appears to be no significant exchange of NH4+ via the tidal creek, however, concentrations of NO2- + NO3- increase dramatically on the falling tide. A negetative relationship between NO2- + NO3- concentration and salinity exists; i.e., as NO2- + NO3- concentration increases the salinity of the creek water decreases, suggesting that dilution is occurring. Since there are no other permanent freshwater connections (e.g. streams) between the upland and the tidal creek at this Phillips Creek site, we hypothesize that this dilution is occurring via a groundwater source of fresh water.