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

Mathematics – Logic

Scientific paper

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Scientific paper

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 15 N ( 15 N ) in the major dissolved and planktonic pools of nitrogen suggested that the 15 N of PN was largely determined by isotopic fractionation during uptake of NH 4 + by phytoplankton. Averaged over the transect as a whole, the 15 N of the herbivorous calanoid copepod Acartia tonsa was 4.1 higher than that of the PN, while the 15 N 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 NH 4 + into the surface layer of the bay. This perturbation in the estuarine nitrogen cycle resulted in marked changes in the 15 N of the major dissolved and planktonic pools of nitrogen in the bay. In combination with ancillary physical, chemical and biological data, these changes in 15 N provided estimates of the isotopic fractionation factor for NH 4 + uptake by phytoplankton ( = 1.0065 -1.0080) as well as the turnover time of nitrogen in Acartia tonsa (6.0-9.6 days). Despite the changes in 15 N observed during this cruise, the relative distribution of 15 N between trophic levels was preserved: during the second transect, the difference in 15 N between Acartia tonsa and PN was 3.6 and the difference in 15 N between Mnemiopsis leidyi and PN was 7.3 . These results demonstrate that the natural abundance of 15 N can change dramatically on a time scale of days and that time-series studies of the natural abundance of 15 N can be a useful complement to studies using tracer additions of 15 N to document nitrogen transformations in planktonic ecosystems.

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