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Surface aggregations of the diazotrophic cyanobacterium Trichodesmium were encountered at nearly every station along a 300-km transect in the northwestern Gulf of Mexico during July 2000. 15 N 2 -fixation rate measurements for all treatments and locations ranged from 1.3 X 10 -4 nmol N trichome -1 h -1 to 2.6 X 10 -2 nmol N trichome -1 h -1 , with a mean of 3.4 X 10 -3 ± 7.6 X 10 -4 nmol N trichome -1 h -1 (mean ± SE, n = 45). Trichodesmium C-fixation, measured by the incorporation of 13 C, ranged from 1.3 X 10 -2 nmol C trichome -1 h -1 to 4.5 X 10 -1 nmol C trichome -1 h -1 , with a mean of 1.2 X 10 -1 ± 9.3 X 10 -2 nmol C trichome -1 h -1 (mean ± SE, n = 44). Both 15 N 2 -fixation and 13 C-fixation rates attenuated with decreasing light intensity from a maximum at 50% surface intensity. Areal Trichodesmium 15 N 2 -fixation rates, based on a 10-h N 2 -fixation day, enumerated vertical trichome abundance and corrected for estimated in situ light intensity ranged from 47.2 μmol N m -2 d -1 to 118.6 μmol N m -2 d -1 , with a mean of 84.5 ± 17.7 μmol N m -2 d -1 (mean ± SE, n = 4). The light natural abundance isotopic signatures of dissolved inorganic nitrogen, particulate nitrogen, and zooplankton nitrogen within these surface populations are consistent with our limited biological rate measurements and indicate that recently fixed N is moving into the food chain to promote secondary production in these nutrient-impoverished waters. Although our sample size was relatively small, natural abundance isotope data indicate that as much as 60% of macrozooplankton C and N was derived from Trichodesmium in the oligotrophic regions of the Gulf of Mexico.

In May 1992, surface seawater samples were collected along an equatorial transit (130 to 100°W) and analyzed for total hydrogen ion concentration (expressed as spectrophotometric pHT) total dissolved inorganic carbon (coulometric CT), and total alkalinity (potentiometric AT and spectrophotometric AT). This data set, which presents a striking view of the chemical signature of tropical instability waves, is also unusual in that its “overdetermination” of the CO2-system in seawater includes both potentiometric AT data and the first spectrophotometric AT data collected at sea using a double-wavelength, one-step acid addition method. Our data set indicates that spectrophotometric AT measurements (ATspec) are both precise (±1.8 μmol/kg) and accurate; the mean observed difference between ATspec and AT obtained potentiometrically in this study (ATpot) is 1 μmol/kg. As only two of the three measured parameters are required to characterize the CO2-system in seawater, this analytical redundancy of our analyses (CT, AT, pH) afforded an opportunity to evaluate, in terms of thermodynamic consistency, a recent calibration of m-cresol purple, a pH indicator. Using various carbonic acid dissociation constants, measured parameters were combined in pairs (CT-AT, pH-CT, and pH-AT) to calculate a third parameter for comparison with the shipboard measurements. Depending on the selected set of carbonic acid dissociation constants, the average offset between directly measured and predicted values of AT and CT was as small as ±1 μmol/kg. The results of this study indicate that the present 25°C mCP calibration, paired with the 25°C combined dissociation constants of Hansson and Mehrbach, produces accurate predictions of AT and CT. Extensive replication of the shipboard spectrophotometric measurements made it possible to examine directly the sensitivity of derived parameters to variations or errors in input AT and pH. In accord with earlier theoretical treatments of this question, the results presented demonstrate that pH is imprecisely predicted from AT and CT, whereas imprecision in measured pH—at the level typical of spectrophotometric measurements (±0.0004)—contributes negligibly (±0.3 μmol/kg) to imprecision in derived AT and CT. In view of the high precision of pHTspec measurements and both the precision and reliability of CT measurements (supported by the use of certified SIO reference materials) pHTspec and CTcoul constitute a particularly useful pair of parameters for shipboard study of the oceanic carbon dioxide system. The results indicate, as well, that the AT-pHTspec pairing is of particular interest in ocean regions where simple AT vs salinity relationships are observed. In light of recent advances, the role of pH measurements in CO2-system characterizations should be re-evaluated. Spectrophotometric measurements of pH have much to contribute in documenting the oceans' evolving response to anthropogenic C02.

During a recent NOAA JGOFS Equatorial Pacific cruise all four analytical parameters of the carbonate system were measured—pH, total alkalinity (TA), total carbon dioxide (TCO2), and the fugacity of carbon dioxide (fCO2). The measurements made during leg 2 on surface waters have been used to examine the internal consistency of the carbon dioxide system in these waters. The internal consistency of the measurements was examined by using various inputs of the measured parameters (pHTA, pHTCO2, pHfCO2, fCO2TA, fCO2TCO2 and TATCO2) to calculate the components of the CO2 system. The results indicate that the measurements have an internal consistency of ±0.003–0.006 in pH, ±5–7 μmol kg−1 in TA, ±5–7 μmol kg−1 in TCO2 and ±6–9 μAtm in fCO2 if reliable constants are used for the dissociation of carbonic acid in seawater. These results indicate that our present understanding of the thermodynamics of the carbonate system in seawater is close to the present accuracy in measuring the various parameters of the system (±0.002 in pH, ±4 μmol kg−1 in TA, ±2 μmol kg−1 in TCO2 and ±2 μAtm in fCO2).

Spectrophotometric open-ocean seawater pH measurements are simple, fast and precise. The sulfonephthalein indicator m-cresol purple (mCP) is recommended for open-ocean surface-to-deep pH measurements. The salinity- and temperature-dependence of such measurements (293⩽T⩽303 and 30⩽S⩽37) is given as: pHT(sw) = 1245.69T + (2.11 × 10−3)(35 − S) + log(R − 0.00692.2220 − R0.1330) on the total hydrogen ion concentration scale ([H+]T = [H+]f + [HSO4−]), in units of mol·(kg-soln)−1. R is the ratio of indicator absorbances at molar absorptivity maxima (i.e. R + 578A/434A). The at-sea analytical precision of the technique, evaluated on two recent NOAA cruises, is approximately 0.0004 pH units.