Seawater samples were spiked with H2O enriched in O-18, such that the 18/16 oxygen mass ratio in the sample water was about twice the natural level. Gross production during a 14 hour bottle incubation generates O2 with the same enriched ratio, thereby enhancing the 18/16 ratio in the large original dissolved O2 pool. The tagged O2 mixes with the large O2 pool before being respired, thereby minimizing changes in the isotopic composition of the measured O2 pool associated with respiration. Thus, the O-18 enrichment of the dissolved O2 is taken to be a proportional measure of gross production.
Water samples were drawn before dawn, spiked with 0.2 ml of H2O(18) and incubated in 100 ml quartz bottles for 14 daylight hrs at the depth of collection. A drifting buoy was used for sample deployment. The incubated samples, as well as unincubated samples from each depth were processed to strip and collect dissolved gases. This stripping process was accomplished by introducing approximately 50 ml. of seawater into an evacuated two chamber container; the degassed water remained in a lower chamber and the stripped gases rose to an upper glass ampoule. The ampoule was flame sealed and returned to the lab where the 18/16 ratio of the dissolved O2 was measured with an isotope ratio mass spectrometer as the per mil difference relative to a laboratory standard (referred to as 'del' measurements).
The gross O2 production, denoted [O2]p, was calculated as: [O2]p = {[(del)f - (del)i]/[(del)p - (del)f]}*[O2]i (eqn 1) The parameters (del)i and (del)f are the 'del' values of the dissolved O2 measured respectively before and after incubation. (del)p is the isotopic composition of the O2 produced during the incubation, calculated knowing the volumes (V) of the H2O(18) spike and the sample, the mole fraction of O-18 in the spike (0.980) and the mole fraction of O-18 in natural seawater (0.002). That is, (del)p = {[(X)incub/(X)ref] - 1}*1000, where (X)incub is the mole fraction of O-18 in the spiked sample water, itself calculated as: (Vspike/Vbottle)*0.978 + 0.002; and (X)ref is the separately determined O-18 mole fraction in the laboratory standard. [O2]i in equation 1 is the initial O2 concentration, determined via Winkler titration by the Oceanographic Data facility. Equation 1 may be derived from a more intuitive equation which expresses the final isotopic composition (del)f as a weighted average of the isotopic compositions of the initial O2 and the O2 added during production: (del)f = {[O2]i*(del)i + [O2]p*(del)p}/{[O2]i + [O2]p}.
Integrated values of productivity were calculated using the histogram method. The euphotic region (surface to the 1% light level, Knudson et al.) was divided into intervals of uniform productivity associated with O-18 gross production measurements. The summation interval was defined as the depth interval bounded by the two mid points of three adjacent sampling depths. For example, if depths 4, 12, 20 and 30 meters were sampled, the productivity measured at 20m was taken to represent the interval 16 to 25 meters. The productivity of the shallowest sample represents the interval from the surface to the mid point with the next deepest sample, i.e., from 0 to 8 meters. The deepest sample represents the interval from the last mid point to the 1% light level.
Net O2 production was determined as the difference in the measured dissolved O2 concentrations of sea water, measured before and after a 24 hour light/dark incubation by Winkler titrations. High precision in the Winkler determinations, +/- 0.1% umole O2/L, was achieved both by using an automated titrator (Radiometer, model ABU93) and by averaging four replicate measurements for each water sample.
Eight replicate water samples were drawn into quartz bottles from a Go-Flo flask containing water collected from the euphotic region before dawn (sample volumes about 100 ml, known to 0.01ml). Winkler titrations were performed on four of the replicates immediately, and the results averaged to establish the initial O2 concentration. The remaining four samples were incubated for 14 daylight hours at the depth of collection (attached to a drifting buoy), and further incubated for 10 night hours in a darkened, ship board incubator, maintained at constant temperature by flowing surface water. Winkler titrations were then performed on the incubated samples and the results averaged. Net production was calculated as the difference between the final and initial O2 concentrations.
Integrated values of net O2 productivity were calculated using the histogram method. The summation intervals were defined as described above for the integrated gross production, e.g., with boundaries set midway between sampled depths. Productivity was integrated over the euphotic region, to the 1% light level (Knudson et al.). In cases where data was sparse (stations 19 and 20 and, in general, for depths near the 1% light level), the net productivity was augmented using computed values of respiration rates and extrapolated gross O2 productivities: net prod = gross prod - 24 * resp rate.