Immediately after labeling the samples, they were transfered to deck-board incubator tanks supplied with flowing sea-surface water providing mixed-layer temperature conditions, and screened with neutral density material to simulate the in situ light level. The experiments were incubated for 3-4 hrs and terminated by filtration through Whatman 47 mm GF/F glass fiber filters (precombusted at 450° C for 1 hr) and rinsed with 50 ml of GF/F filtered sea water. The filters were placed into individual polyethylene envelopes, dried at 60° in a gravity drying oven, and stored in an air tight container with silica gel desiccant. On selected samples in each profile, 500 ml of the filtrate was retained for NH+ and NO- extraction and subsequent isotopic analysis in order to determine the degree of isotope dilution which occurred during the course of the experiments. The method of Brzezinski (1987) incorporating solid phase extraction proceedures (J.T. Baker, 3 ml, C18 cartridges) was used to extract NH+. In the case of NO-, reagents were added to the filtrate to form an analine compound (Horrigan et al. 1990) which was then extracted using the solid phase techniques employed for NH+. If necessary, prior to extraction, unenriched carrier NO- or NH+ was added to these samples to bring their mass to ~40µg of N for isotopic analysis. The final eluate was collected and stored in sealed 3 ml, combusted, Pyrex glass vials (1 hr @ 450° C). In the laboratory, shortly before isotopic analysis, the eluate was reduced to a volume of ~0.5 ml by evaporation on a hot plate at ~80° C, and transfered by pipette to a combusted Whatman 25 mm GF/F glass fiber filter and dried in a gravity oven at 60° C. With the the isotopic data obtained from the NO- and NH+ extraction samples, uptake rates corrected for isotope dilution and NH+ remineralization and 1° nitrification rates were calculated (Horrigan et al. 1990).
N enrichment of the samples was determined by mass spectrometry in our laboratory at Harvard University, using methods and instrumentation described previously (Nevins and McCarthy 1982). Particulate N from each sample was obtained manometrically from the mass spectrometer sample preparation system and these data were used in the subsequent rate calculations. NH+ uptake rates were calculated using concentration data obtained from the nM method of Brzezinski (1987), used with solid phase extraction (J.T. Baker, 3 ml, C18 cartridges). Other uptake rates were calculated using data from the ship-board auto analyzer system (C.Garside) obtained from discrete samples from the Niskin bottles.
Because of the strong diel pattern observed in NO- uptake through out the period of observations, it would be inappropriate to use the discrete rates directly in estimates of new production (eg. an f ratio). We are preparing a manuscript (McCarthy, Nevins and Garside) which will present the results of an analysis which uses these data to model NO- uptake on an integrated daily basis.