Natural abundance of 15N in particulate nitrogen and zooplankton in the Chesapeake Bay

Abstract

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 I5N (Sl5N) in each of these pools, in combination with measurements of the concentrahons 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. SL5N 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 I5N in PN. The relationship between 6 1 5~ of PN and 615N 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. 615N of particulate and dissolved pools suggested that phytoplankton consume both NOS and NH: roughly in proportion to concentration. The 6l5N of the zooplankton species sampled generally increased with trophic level. The S 1 5~ of the copepod Acartia tonsa was higher than that of PN by 4.2 ? 2.3 Om (X 2 SD) in spring and 3.3 f 1.0 9m (X f SD) in fall. Similarly. 615N of the ctenophore Mnemiopsis leidyi was higher than that of A. tonsa by 2.0 f 2.6% ((R f SD) in spring and 3.3 f 1.0% (X f 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 6 1 5~ of the copepod was greater than that of the ctenophore by as much as 4.9Ym. In general, spatial variability of S'=N 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 6 1 5~ 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 I5N in dynamic planktonic systems such as that in the Chesapeake Bay.

Extensive bloom of a N2-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean

“…The value of about 3 % difference in 6I5N signature between bulk suspended material and the smallest zooplankton fraction (Fig. 5) is very similar to the average trophic shift of ca 3.5%0 typically found in marine and other ecosystems (Minagawa & Wada 1984, Montoya et al 1990, 1992, which implies that nitrogen derived either directly or indirectly from N2 fixation was a major component of the diet of the small zooplankton. Our isotopic data indicate that the bloom had persisted long enough for the isotopic signature of N2 fixation to propagate into the food web.…”

Ectosymbiotic behavior of Cancer gracilis and its trophic relationships with its host Phacellophora camtschatica and the parasitoid Hyperia medusarum

“…The δ 15 N values of Phacellophora camtschatica in Puget Sound are similar to those of other scyphomedusae (Montoya et al 1990, Brodeur et al 2002, however its δ 13 C values are lower than those of other scyphomedusae (Malej et al 1993, Brodeur et al 2002. The stable isotope ratios of hydromedusae in this present report are slightly depleted in 13 C and enriched in 15 N compared with those reported previously for hydromedusae from the Bering Sea (Brodeur et al 2002).…”

“…The stable isotope ratios of Cancer spp. megalopae caught with the zooplankton net were intermediate between the ectosymbiont megalopae and juveniles, indicating that the diets of ectosymbiotic megalopae and free-living planktonic megalopae differ.The natural abundance of stable isotopes in estuarine zooplankton is known to vary significantly with location and season (Simenstad & Wissmar 1985, Montoya et al 1990, and caution has to be taken when comparing stable isotope ratios in food-chain studies (Peterson & Fry 1987). The δ 15 N values of Phacellophora camtschatica in Puget Sound are similar to those of other scyphomedusae (Montoya et al 1990, Brodeur et al 2002, however its δ 13 C values are lower than those of other scyphomedusae (Malej et al 1993, Brodeur et al 2002.…”

Nitrogen isotopic composition of marine and freshwater invertebrates

“…birds, and 17 fishes showed that marine animals have 6I5N values which average 6 to 10%0 higher (I5N enriched) than those for terrestrial or freshwater organisms (Schoeninger & DeNiro 1984). The low sample sizes (98 measurements) and narrow fauna1 range (66 vertebrate species) of this previous study limits conclusions about the generality of broad-scale environmental differences in source variability of 6I5N [see Owens (1987) and Gearing (1988) Yoshioka et al (1989), Montoya et al (1990), Rau et al (1990, 1991), Toda & Wada (1990), and Mihuc & Toetz (1994, in addition to those studies listed in France (1995).…”