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We examined the effect of light on the oxidation of NH4+ to NO2 by oceanic and estuarine bacteria. Inhibition of NH4+ oxidation by light was more pronounced in the oceanic isolates. The estuarine bacteria, isolated from Chesapeake Bay, produced NO2− at a rate faster than the oceanic isolates. We conclude that photoinhibition of NH4+ oxidation may be less important in estuarine environments than in oceanic ones.

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.

Primary production, defined as fixation of [14C]bicarbonate, occurs in the waters under the Ross Ice Shelf, Antarctica. About 1.5 g C·m−2 is fixed annually. This amount is sufficient to support the observed macrofaunal population and may be due to the activity of chemoautotrophic nitrifying bacteria.

Stimulation of heterotrophic bacterial growth by inorganic nitrogen (nitrate and ammonium) was observed in natural assemblages of marine bacteria grown in continuous culture with unsupplemented sea water as primary medium. In the presence of nitrogenous supplements, bacterial numbers increased approximately 3-fold. These results indicate that re-evaluation of the role of heterotrophic bacterioplankton in the pelagic nitrogen cycle may b e necessary. INTRODUCTIONIn the classical view of the pelagic marine nitrogen cycle, heterotrophic bacteria have been portrayed as net mineralizers, producing ammonium from dissolved organic nitrogen (DON). Assimilation of dissolved inorganic nitrogen (DIN) as regenerated ammonium or upwelled nitrate is thought to be due mainly to phytoplankton. Recently, several lines of evidence have suggested that heterotrophic bacteria might be significant consumers of the inorganic N, in competition with phytoplankton (which are generally N-limited in the marine pelagial). Indirect evidence comes from an inability to balance a nitrogen budget without invoking DIN uptake by bacteria (Laws et al. 1985). Additional evidence comes from DIN uptake studies using specific inhibitors for prokaryotes and eukaryotes, respectively (Wheeler & Kirchman 1986) and 13NH4+ followed by size-fractionation (Fuhrman et al. 1988). Further, laboratory studies have demonstrated growth of marine bacteria using nitrate as a nitrogen source (Brown et al. 1972), and field and mesocosm studies suggest that bacteria may limit autotrophic production by competing for nitrate (Parker et al. 1975, Parsons et al. 1981.We studied DIN uptake by natural assemblages of bacteria grown in seawater cultures. We also asked whether DIN additions, made in the second stage of a ' Contribution No. 617