Natural abundance of ¹⁵N in particulate nitrogen and zooplankton in the Chesapeake Bay
Montoya, Joseph P.
Horrigan, S. G.
McCarthy, J. J.
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Samples of dissolved inorganic nitrogen (DIN), particulate nitrogen (PN), and several species of zooplankton were collected at a series of stations in the main channel of the Chesapeake Bay, USA, during cruises in spring and fall 1984. The spatial and temporal variation in the natural abundance of ¹⁵N (δ¹⁵N) in each of these pools, in combination with measurements of the concentrations of DIN, PN, plant pigments, and the rates of biologically-mediated transformations of nitrogen, provide a number of insights into the dynamics of the nitrogen cycle in the Chesapeake Bay. During both spring and fall δ¹⁵N of surface layer PN showed no consistent Bay-wide pattern of distribution. Instead, the overall gradient of DIN concentrations along the axis of the Bay appears to be less important than local processes in determining the distribution of ¹⁵N in PN. The relationship between δ¹⁵N PN and δ¹⁵N of dissolved pools indicated that phytoplankton uptake was the dominant process acting on DIN in spring, but that microbially-mediated transformations of nitrogen dominated in fall. During both seasons δ¹⁵N of particulate and dissolved pools suggested that phytoplankton consume both NO₃ and NH₄⁺ roughly in proportion to concentration. The δ¹⁵N of the zooplankton species sampled generally increased with trophic level. The δ¹⁵N of the copepod Acartia tonsa was higher than that of PN by 4.2 ± 2.3 ‰ (X ± SD) in spring and 3.3 ±1.0 ‰ (X±SD) in fall. Similarly. δ¹⁵N of the ctenophore Mnemiopsis leidyi was higher than that of A. tonsa by 2.0 ± 2.6‰ (X±SD) in spring and 3.3 ±1.0‰ (X±SD) in fall. A reversal of the usual relationship between A. tonsa and M. leidyi occurred near the southern end of the Bay during spring, where δ¹⁵N of the copepod was greater than that of the ctenophore by as much as 4.9‰. In general, spatial variability of δ¹⁵N of all 3 of these trophic levels (PN, copepods, and ctenophores) was greater in spring than in all, suggesting that phyto- and zooplankton have a greater direct influence on the estuarine nitrogen cycle during spring. A comparison of the 2 transects conducted on each cruise demonstrates that δ¹⁵N of the PN and A. tonsa, but not that of M. leidyi, can change markedly on a time scale of roughly a week. Such changes clearly indicate that repeated sampling may be essential in studies of the natural abundance of ¹⁵N in dynamic planktonic systems such as that in the Chesapeake Bay.