RAPID, STORM-INDUCED CHANGES IN THE NATURAL ABUNDANCE OF N-15 IN A PLANKTONIC ECOSYSTEM, CHESAPEAKE BAY, USA

Samples of dissolved inorganic nitrogen (DIN), particulate nitrogen (PN), and two species of zooplankton were collected during two north-south transects of the Chesapeake Bay in the autumn of 1984 (27-28 September and 3-5 October). During the first transect, the natural abundance of N-15 (delta-N-15) in the major dissolved and planktonic pools of nitrogen suggested that the delta-N-15 of PN was largely determined by isotopic fractionation during uptake of NH4+ by phytoplankton. Averaged over the transect as a whole, the delta-N-15 of the herbivorous calanoid copepod Acartia tonsa was 4.1 parts per thousand higher than that of the PN, while the delta-N-15 of the carnivorous ctenophore Mnemiopsis leidyi was 6.4 parts per thousands higher than that of the PN. In the interval between the two transects, storm-induced mixing of the water column resulted in the injection of NH4+ into the surface layer of the bay. This perturbation in the estuarine nitrogen cycle resulted in marked changes in the delta-N-15 of the major dissolved and planktonic pools of nitrogen in the bay. In combination with ancillary physical, chemical, and biological data, these changes in delta-N-15 provided estimates of the isotopic fractionation factor for NH4+ uptake by phytoplankton (alpha = 1.0065-1.0080) as well as the turnover time of nitrogen in Acartia tonsa (6.0-9.6 days). Despite the changes in delta-N-15 observed during this cruise, the relative distribution of N-15 between trophic levels was preserved: during the second transect, the difference in delta-N-15 between Acartia tonsa and PN was 3.6 parts per thousands, and the difference in delta-N-15 between Mnemiopsis leidyi and PN was 7.3 parts per thousands. These results demonstrate that the natural abundance of N-15 can change dramatically on a time scale of days, and that time-series studies of the natural abundance of N-15 can be a useful complement to studies using tracer additions of N-15 to document nitrogen transformations in planktonic ecosystems.