NATURAL ISOTOPIC COMPOSITION OF DISSOLVED INORGANIC NITROGEN IN THE CHESAPEAKE BAY

The natural abundances of 15N in the dissolved inorganic pools of nitrogen in the Chesapeake Bay were measured in the spring and fall of 1984. Changes in the δ15N of NH4+ and the combined pool of (NO3- + NO2-) reflected both seasonal and short-term changes in the estuarine nitrogen cycle. In the spring, oxidation of NH4+ at the head of the bay in the region of the turbidity maximum and in localized regions throughout the bay, led to elevated values of δ15N in the NH4+ pool. The δ15N of the (NO3- + NO2-) pool tended to increase toward the south, enabling an estimate of the isotopic fractionation factor for the consumption of NO3- to be derived; the estimate (1·0070), is similar to literature values of the fractionation factor for NO3- uptake by phytoplankton, supporting previous research suggesting that phytoplankton uptake is the major sink for NO3- in the bay. Denitrification led to elevated values of δ15N in the (NO3- + NO2-) pool in deep water. Over the course of the summer, the δ15N of NH4+ increased throughout the bay. A significant correlation was found between the δ15N of the NH4+ pool and the concentration of NO2- both above and below the pycnocline during the fall cruise, suggesting that the increase in the δ15N of the NH4+ pool was due to the oxidation of NH4+. In the fall, changes were also observed in the δ15N of both the NH4+ and (NO3- + NO2-) pools which were consistent with the occurrence of NH4+ oxidation. From these changes, a fractionation factor for NH4+ oxidation between 1·0120 and 1·0167 was derived, which is similar to values reported in the literature. © 1990.