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Seawater CO2 measurements are being made with increasing frequency as interest grows in the ocean's response to changing atmospheric CO2 levels and to climate change. The ultimate usefulness of these measurements depends on the data quality and consistency. An inter-laboratory comparison was undertaken to help evaluate and understand the current reliability of seawater CO2 measurements. Two seawater test samples of different CO2 content were prepared according to the usual method for the creation of seawater reference materials in the Dickson Laboratory at Scripps Institution of Oceanography. These two test samples were distributed in duplicate to more than 60 laboratories around the world. The laboratories returned their measurement results for one or more of the following parameters: total alkalinity (AT), total dissolved inorganic carbon (CT), and pH, together with information about the methods used and the expected uncertainty of the measurements. The majority of laboratories reported AT and CT values for all their measurements that were within 10μmolkg-1 of the assigned values (i.e. within ±0.5%), however few achieved results within 2μmolkg-1 (i.e. within ±0.1%), especially for CT. Results for the analysis of pH were quite scattered, with little suggestion of a consensus value. The high-CO2 test sample produced results for both CT and pH that suggested in many cases that CO2 was lost during analysis of these parameters. This study thus documents the current quality of seawater CO2 measurements in the various participating laboratories, and helps provide a better understanding of the likely magnitude of uncertainties in these measurements within the marine science community at the present time. Further improvements will necessarily hinge on adoption of an improved level of training in both measurement technique and of suitable quality control procedures for these measurements.

Spawning market squid lay embryo capsules on the seafloor of the continental shelf of the California Current System (CCS), where ocean acidification, deoxygenation and intensified upwelling lower the pH and [O2]. Squid statolith geochemistry has been shown to reflect the squid's environment (e.g., seawater temperature and elemental concentration). We used real-world environmental levels of pH and [O2] observed on squid-embryo beds to test in the laboratory whether or not squid statolith geochemistry reflects environmental pH and [O2]. We asked whether pH and [O2] levels might affect the incorporation of element ratios (B:Ca, Mg:Ca, Sr:Ca, Ba:Ca, Pb:Ca, U:Ca) into squid embryonic statoliths as (1) individual elements and/or (2) multivariate elemental signatures, and consider future applications as proxies for pH and [O2] exposure. Embryo exposure to high and low pH and [O2] alone and together during development over four weeks only moderately affected elemental concentrations of the statoliths, and uranium was an important element driving these differences. Uranium:Ca was eight-times higher in statoliths exposed to low pHT (7.57-7.58) and low [O2] (79-82 μmol∙kg-1) than those exposed to higher ambient pHT (7.92-7.94) and [O2] (241-243 μmol∙kg-1). In a separate experiment, exposure to low pHT (7.55-7.56) or low [O2] (83-86 μmol∙kg-1) yielded elevated U:Ca and Sr:Ca in the low [O2] treatment only. We found capsular effects on multiple elements in statoliths of all treatments. The multivariate elemental signatures of embryonic statoliths were distinct among capsules, but did not reflect environmental factors (pH and/or [O2]). We show that statoliths of squid embryos developing inside capsules have the potential to reflect environmental pH and [O2], but that these "signals" are generated in concert with the physiological effects of the capsules and embryos themselves. [ABSTRACT FROM AUTHOR]