Search

The California Current System is thought to be particularly vulnerable to ocean acidification, yet pH remains chronically undersampled along this coast, limiting our ability to assess the impacts of ocean acidification. To address this observational gap, we integrated the Deep-Sea-DuraFET, a solid-state pH sensor, onto a Spray underwater glider. Over the course of a year starting in April 2019, we conducted seven missions in central California that spanned 161 glider days and >1600 dives to a maximum depth of 1000 m. The sensor accuracy was estimated to be ± 0.01 based on comparisons to discrete samples taken alongside the glider (n = 105), and the precision was ±0.0016. CO2 partial pressure, dissolved inorganic carbon, and aragonite saturation state could be estimated from the pH data with uncertainty better than ± 2.5%, ± 8 μmol kg−1, and ± 2%, respectively. The sensor was stable to ±0.01 for the first 9 months but exhibited a drift of 0.015 during the last mission. The drift was correctable using a piecewise linear regression based on a reference pH field at 450 m estimated from published global empirical algorithms. These algorithms require accurate O2 as inputs; thus, protocols for a simple predeployment air calibration that achieved accuracy of better than 1% were implemented. The glider observations revealed upwelling of undersaturated waters with respect to aragonite to within 5 m below the surface near Monterey Bay. These observations highlight the importance of persistent observations through autonomous platforms in highly dynamic coastal environments.

Ion-sensitive field effect transistor-based pH sensors have been shown to perform well in high frequency and long-term ocean sampling regimes. The Honeywell Durafet is widely used due to its stability, fast response, and characterization over a large range of oceanic conditions. However, potentiometric pH monitoring is inherently complicated by the fact that the sensors require careful calibration. Offsets in calibration coefficients have been observed when comparing laboratory to field-based calibrations and prior work has led to the recommendation that an in situ calibration be performed based on comparison to discrete samples. Here, we describe our work toward a self-calibration apparatus integrated into a SeapHOx pH, dissolved oxygen, and CTD sensor package. This Self-Calibrating SeapHOx is capable of autonomously recording calibration values from a high quality, traceable, primary reference standard: equimolar tris buffer. The Self-Calibrating SeapHOx's functionality was demonstrated in a 6-d test in a seawater tank at Scripps Institution of Oceanography (La Jolla, California, U.S.A.) and was successfully deployed for 2 weeks on a shallow, coral reef flat (Lizard Island, Australia). During the latter deployment, the tris-based self-calibration using 15 on-board samples exhibited superior reproducibility to the standard spectrophotometric pH-based calibration using > 100 discrete samples. Standard deviations of calibration pH using tris ranged from 0.002 to 0.005 whereas they ranged from 0.006 to 0.009 for the standard spectrophotometric pH-based method; the two independent calibration methods resulted in a mean pH difference of 0.008. We anticipate that the Self-Calibrating SeapHOx will be capable of autonomously providing climate quality pH data, directly linked to a primary seawater pH standard, and with improvements over standard calibration techniques.

Over the last century, unexploded ordnances have been disposed of in marine shelf systems because of a lack of cost-effective alternatives. Underwater unexploded ordnances have the potential to leak 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5-triazine (RDX), commonly used chemical munitions, and contaminate local waters, biota, and sediments. The rate at which this contamination occurs in the environment is relatively unknown, and the cost- and time-prohibitive nature of sampling across sites makes mapping difficult. In the present study we assessed the efficacy of ethylene-vinyl acetate (EVA) for sampling relatively soluble munitions compounds over a range of environmental conditions (i.e., changes in temperature and salinity) and optimized the composition of the passive sampling polymer. The EVA sampler was able to successfully detect ambient concentrations of lingering munitions compounds from field sites containing unexploded ordnances. The sampler affinity for the munitions in terms of an EVA-water partition coefficient was greater than the standard octanol water values for each target compound. Partitioning of compounds onto EVA over the natural ranges of salinity did not change significantly, although uptake varied consistently and predictably with temperature. Increasing the vinyl acetate to ethylene ratio of the polymer corresponded to an increase in uptake capacity, consistent with enhanced dipole-dipole interactions between the munitions and the polymer. This sampler provides a cost-effective means to map and track leakage of unexploded ordnances both spatially and temporally. Environ Toxicol Chem 2018;37:1990-1997. © 2018 SETAC.

Analysis of a global hydrographic data product showed a clear pH-dependent discrepancy between pH on the total scale measured spectrophotometrically (pHspec) using purified meta-Cresol Purple (mCP) and pH calculated from total alkalinity and dissolved inorganic carbon. However, this was based mainly on US cruises, and three recent Japanese cruises do not show this pH-dependent discrepancy. One potential explanation is that purified mCP batches obtained from different institutions lead to significantly different pHspec. Here, we tested this hypothesis by comparing the performance of purified mCP obtained from four different institutions. We demonstrate that consistent pH of ±0.0012 (95% C.I.) can be achieved regardless of the institution when impurities are properly removed. However, there was at least one batch from three of the four institutions that had significant pH-dependent errors that were as large as −0.008. The presence of impurities that led to pH-dependent errors was identified using HPLC and, for 8 out of the 9 cases, by spectrophotometry (although issues still remain for the latter). We conclude that pH-dependent errors due to impurities that remain after the purification process are, by themselves, too small to account for the differences observed between the recent set of cruises. Identifying the source of this difference should be a top priority. This study also highlights the importance of establishing robust quality assurance and quality control protocols to ensure consistent behavior with previously published equations to compute pH. We recommend a centralized system where one or a handful of institutions distribute purified mCP for the community, as this distribution approach will lead to lower prices and simplify quality assurance.

High-resolution records of porewater sulfate concentrations are critical to understanding the modern biogeochemical sulfur cycle, particularly the connection between microbial metabolic activity, ambient geochemistry, and feedbacks on global carbon cycling and climate. To date, the nature of sulfate measurements requires extraction of fluids or sediments from the field, often leading to significant disturbances in the systems studied. Further, the resulting data may have limited spatial resolution (due to volume restrictions of porewater sampling), hindering the ability to reconstruct key biological and geochemical processes. Here a novel passive sampler that is seeded with barium oxalate is optimized for the in situ sampling of sulfate to improve both the fidelity and the spatial resolution of sulfate profiles that may be obtained. Simulated sediment studies showed that consistent profiles could be resolved in both 2 and 6 h deployments that were in good agreement with traditional porewater reconstructio...

A recent analysis of full water column hydrographic data revealed a pH-dependent discrepancy between spectrophotometrically measured pH using purified meta-cresol purple and pH calculated from dissolved inorganic carbon (DIC) and total alkalinity (TA). The discrepancy (pHspec – pHTA,DIC) is approximately −0.018 and 0.014 at pH 7.4 and 8.2, respectively. This discrepancy has a wide range of implications for marine inorganic carbon measurements, such as establishing robust calibration protocols for pH sensors operating on profiling floats. Here, we conducted a series of lab based experiments to assess the magnitude of pH-dependent errors for spectrophotometric pH measurements in seawater by directly comparing its performance to pH measured by an Ion Sensitive Field Effect Transistor (ISFET) pH sensor known to have Nernstian behavior. Natural seawater was titrated with high CO2 seawater while simultaneously measuring pH using spectrophotometry and an ISFET sensor over a large range in pH (7–8.5) and temperature (5–30 °C). The two pH measurements were consistent to better than ±0.003 (range) at all temperatures except at 5 and 10 °C and very low and high pH, where discrepancies were as large as ±0.005. These results demonstrate that pH-dependent errors in spectrophotometric pH measurements can be rejected as the cause of the pH-dependent discrepancy between pHspec and pHTA,DIC. The cause of this discrepancy is thus likely due to our incomplete understanding of the marine inorganic carbon model that could include errors in thermodynamic constants, concentrations of major ions in seawater, systematic biases in measurements of TA or DIC, or contributions of organic compounds that are not accounted for in the definition of total alkalinity. This should be a research priority for the inorganic carbon community.

Long Island Sound (LIS), is an urban estuary on the US east coast that undergoes seasonal hypoxia in its western and central regions. Approximately 90% of the water exchange between LIS and the adjacent continental shelf, the Mid Atlantic Bight, occurs through its eastern boundary. Recent estimates of organic carbon (OC) export from LIS have shown that this value varies appreciably, both seasonally and inter-annually. In this study, the spatial, tidal, and seasonal variability were resolved by measuring dissolved and particulate organic carbon (DOC and POC) concentrations and currents at a cross-section in eastern LIS near the estuary mouth. Fluxes were extrapolated from these high-resolution observations collected in May, August, and November 2016, and August 2017. In August of 2016 (a low flow year), an OC flux of 41 × 106 kg C month−1 resulted from a net import of DOC and an export of POC. In August of 2017 (an average flow year), there was an OC flux of 30 × 106 kg C month −1 due to both DOC and POC export, indicating significant seasonal and inter-annual variability. The seasonally weighted-average total OC flux for 2016 was −41 × 106 ± 9 kg C yr−1 (net import). Results show that spot sampling in tidal estuaries can lead to spatial and tidal bias in concentrations of DOC (6–15%) and POC (31–18%) under high and low river flow conditions, respectively. This uncertainty is recommended for spot sampling in future OC studies and flux estimates in LIS.