Rapid, storm-induced changes in the natural abundance of 15N in a planktonic ecosystem, Chesapeake Bay, USA

Abstract

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 15N (δ15N) in the major dissolved and planktonic pools of nitrogen suggested that the δ15N of PN was largely determined by isotopic fractionation during uptake of NH4+ by phytoplankton. Averaged over the transect as a whole, the δ15N of the herbivorous calanoid copepod Acartia tonsa was 4.1‰ higher than that of the PN, while the δ15N of the carnivorous ctenophore Mnemiopsis leidyi was 6.4‰ 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 δ15N of the major dissolved and planktonic pools of nitrogen in the bay. In combination with ancillary physical, chemical and biological data, these changes in δ15N provided estimates of the isotopic fractionation factor for NH4+ uptake by phytoplankton (α = 1.0065 −1.0080) as well as the turnover time of nitrogen in Acartia tonsa (6.0–9.6 days). Despite the changes in δ15N observed during this cruise, the relative distribution of 15N between trophic levels was preserved: during the second transect, the difference in δ15N between Acartia tonsa and PN was 3.6‰ and the difference in δ15N between Mnemiopsis leidyi and PN was 7.3‰. These results demonstrate that the natural abundance of 15N can change dramatically on a time scale of days and that time-series studies of the natural abundance of 15N can be a useful complement to studies using tracer additions of 15N to document nitrogen transformations in planktonic ecosystems.