Discrete samples for dissolved nutrient analyses were collected at each depth using 30-L Go-Flo sampling bottles mounted on the JGOFS trace-metal clean rosette. Duplicate (n = 2-3) samples from each depth were transferred to 50-mL rinsed, polyethylene (sterile) tubes and manually analyzed for ammonium (NH4+) and urea (CO[15NH4]2) according to Solorzano (1969) and Price & Harrison (1987), respectively. Ammonium samples were immediately inoculated with the phenolic reagent for stabilization, and stored refrigerated for generally <1 day before analysis; ammonium samples were never frozen and always analyzed at sea aboard the R.V.I.B Nathaniel B. Palmer. Urea samples were stored frozen, and allowed to thaw at room temperature before analysis either ashore at McMurdo Station (Process 1 and 2) or at sea (Process 3 and 4). For both nutrients, the absorbance of each sample was determined a minimum of three times using a spectrophotometer (Beckman DU 640 or Beckman DU 7400) equipped with a 10-cm cell to improve sensitivity. Five to six duplicate standards, made from combusted NaCl and spanning the ambient concentrations of interest, were run during each day of analysis.
References Cited Price, N.M., and P.J. Harrison. 1987. A comparison of methods for the measurement of dissolved urea concentrations in seawater. Mar. Biol. 92: 307-319. Solorzano, L. 1969. Determination of ammonia in natural waters by the phenolhypochlorite method. Limnol. Oceanogr. 14: 799-801.
Discrete samples for nitrogen uptake rates were collected at each depth using 30-L Go-Flo sampling bottles mounted on the JGOFS trace-metal clean rosette. Samples from each depth were transferred into 1.2-L (Process 2) or 2.3-L (Process 1,3,4) polycarbonate bottles, and additions of 15NH4Cl, Na15NO3, CO[15NH4]2, and Na15NO2 (98-99 atom %; Cambridge Isotopes) were made to the separate incubation bottles at "tracer levels" (0.1 microgram-at N/L for 15NH4 , 15NO2 and CO[15NH4]2 ) and <10% of the ambient concentration for 15NO3. All enrichments and manipulations were conducted using trace-metal clean techniques within laminar-flow (HEPA) clean working areas as described by Fitzwater et al. (1982). Inoculated bottles were hermetically heat-sealed within polyethylene bags and incubated for 24 h (12 h during Process 2) in separate Plexiglas deck incubators (together with 14C samples) at in situ temperature, with PPFD attenuated to the PPFD of collection using with neutral density film (CinemillsCorp.). Samples for 15N analysis were collected by filtration (ca. 80 mm Hg) onto precombusted Whatman GF/F filters (2.5 cm), and immediately frozen for later analyses of 15N enrichment and particulate N (PN) concentration ashore with a Europa Scientific Robo-Prep Tracermass mass spectrometer. Absolute uptake rates, estimated from 15N accumulation in the particulates, were calculated using equation 3 of Dugdale and Wilkerson (1986). Uptake rates have not yet been corrected for isotopic dilution (Glibert et al., 1982) or potential losses of the 15N label to the DON pool during incubations (Bronk et al., 1994).
References Cited Bronk, D.A., P.M. Glibert and B.B. Ward. 1994. Nitrogen uptake, dissolved organic nitrogen release, and new production. Science 265: 1843-1846. Dugdale, R.C., and F.P. Wilkerson. 1986. The use of 15N to measure nitrogen uptake in eutrophic oceans; experimental considerations. Limnol. Oceanogr. 31: 673-689. Fitzwater, S.E., G.A. Knauer, and J.H. Martin. 1982. Metal contamination and its effect on primary production measurements. Limnol. Oceanogr. 27: 544-551. Glibert, P.M., F. Lipschultz, J.J. McCarthy and M.A. Altabet. 1982. Isotope dilution models of uptake and remineralization of ammonium by marine plankton. Limnol. Oceanogr. 27: 639-650.