Description: We show the distribution of nutrients, oxygen, total dissolved inorganic carbon (CT) and total alkalinity (AT) along three sections close to the Canary Islands, between 18°W and the African coast during Meteor 37/2 cruise (January 1997). From the thermohaline properties of Eastern North Atlantic Central Water (ENACW), Mediterranean Water (MW), Antarctic Intermediate Water (AAIW) and North Atlantic Deep Water (NADW), a mixing model has been established based on the water mass description. It can explain most of the variabilities found in the distribution of the chemical variables, including the carbon system, and it is validated through the use of conservative chemical variables like ‘NO.’ From nutrients, oxygen, AT and CT, the chemical characterisation of the water masses was performed by calculating the concentration of these variables in the previously defined thermohaline end-members. The relative variation of nutrient concentrations, resulting from the regeneration of organic matter, was estimated. Close to the African shelf-break, a poleward undercurrent conveying as much as a 11% of AAIW was observed only in the southern section (28.5°N). From the chemical and thermohaline properties of the end-members, a comparison with data from other oceanic regions was made in respect to conservative chemical variables (‘NO’). In addition, a north–south gradient in the ventilation pattern of water masses is observed from the residuals of the model.

Description: Seawater was sampled from different depths in the North Atlantic Ocean (Canary Islands region) and distributed among three different labs for the determination of titration alkalinity. Analysis was performed by potentiometric methods, involving titration in a closed cell, titration in an open cell and a two end-point acid addition method. The precision, which is the sample reproducibility taken from the mean standard deviation for replicate measurements, was between 0.45 and 0.90 µmol kg(-1) for the individual labs. Accuracy, here taken as the deviation for the values of a lab from the mean of all three, was mostly below 1 µmol kg(-1) and never exceeded 0.1% of the sample value. Mean standard deviation for all labs and all samples was 0.87 µmol kg(-1), once the individual methods were calibrated using certified reference material (CRM). Without CRM calibration, the mean standard deviation would increase to 2.8 µmol k(-1). The conclusion is that current high precision methods for alkalinity measurements calibrated with CRMs are able to reach similar accuracy as the measurement of total dissolved inorganic carbon by coulometry and therefore allow for the precise determination of the oceanic carbon dioxide system by using the two measured parameters.

Description: The ‘International Intercomparison Exercise of fCO2 Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO2) systems, one discrete fCO2 system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO2 measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation of final fCO2 were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO2 in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO2 measurements can be made in good agreement (±3 μatm) with underway fCO2 data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO2 system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO2 results. Calculation of fCO2 from high-quality total dissolved inorganic carbon (CT) and total alkalinity (AT) measurements does not yield results comparable in accuracy and precision to fCO2 measurements.

Description: Redfield stoichiometry has proved a robust paradigm for the understanding of biological production and export in the ocean on a long-term and a large-scale basis. However, deviations of carbon and nitrogen uptake ratios from the Redfield ratio have been reported. A comprehensive data set including all carbon and nitrogen pools relevant to biological production in the surface ocean (DIC, DIN, DOC, DON, POC, PON) was used to calculate seasonal new production based on carbon and nitrogen uptake in summer along 20°W in the northeast Atlantic Ocean. The 20°W transect between 30 and 60°N covers different trophic states and seasonal stages of the productive surface layer, including early bloom, bloom, post-bloom and non-bloom situations. The spatial pattern has elements of a seasonal progression. We also calculated exported production, i.e., that part of seasonal new production not accumulated in particulate and dissolved pools, again separately for carbon and nitrogen. The pairs of estimates of `seasonal new production’ and `exported production’ allowed us to calculate the C : N ratios of these quantities. While suspended particulate matter in the mixed layer largely conforms to Redfield stoichiometry, marked deviations were observed in carbon and nitrogen uptake and export with progressing season or nutrient depletion. The spring system was characterized by nitrogen overconsumption and the oligotrophic summer system by a marked carbon overconsumption. The C : N ratios of seasonal new as well as exported production increase from early bloom values of 5–6 to values of 10–16 in the post-bloom/oligotrophic system. The summertime accumulation of nitrogen-poor dissolved organic matter can explain only part of this shift.