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Ocean Acidification (OA) is negatively affecting the physiological processes of marine organisms, altering biogeochemical cycles, and changing chemical equilibria throughout the world’s oceans. It is difficult to measure pH broadly, in large part because accurate pH measurement technology is expensive, bulky, and requires technical training. Here, we present the development and evaluation of a hand-held, affordable, field-durable, and easy-to-use pH instrument, named the pHyter, which is controlled through a smartphone app. We determine the accuracy of pH measurements using the pHyter by comparison with benchtop spectrophotometric seawater pH measurements, measurement of a certified pH standard, and comparison with a proven in situ instrument, the iSAMI-pH. These results show a pHyter pH measurement accuracy of ±0.046 pH or better, which is on par with interlaboratory seawater pH measurement comparison experiments. We also demonstrate the pHyter’s ability to conduct both temporal and spatial studies of coastal ecosystems by presenting data from a coral reef and a bay, in which the pHyter was used from a kayak. These studies showcase the instrument’s portability, applicability, and potential to be used for community science, STEM education, and outreach, with the goal of empowering people around the world to measure pH in their own backyards. 

Abstract In 2021, the Ocean Thematic Centre of the European Research Infrastructure “Integrated Carbon Observation System” conducted an international partial pressure of carbon dioxide (pCO₂) instrument intercomparison. The goal was to understand how different types of instrumentation for the measurement of oceanpCO₂compare to each other. During the two‐week long experiment, we installed various instruments in a tank facility using natural sea water (North Sea). These included direct air–water equilibration systems and membrane‐based flow‐through instruments along with submersible sensors and instruments that are normally installed on buoys and autonomous surface vehicles. In situ instruments were installed inside the tank and the flow‐through instruments were fed the same water using a pumping system. We changed the temperature (between 10°C and 28°C) and the seawaterpCO₂(between 250 and 800μatm) to observe instrument responses over a wide range. Since there is no reference for surface oceanpCO₂measurements, we agreed on a set of instruments serving as intercomparison reference. All data from the different instruments were then compared against the intercomparison reference during periods of stable temperature andpCO₂. The study provides important information to enhance future ocean carbon monitoring networks, but makes no direct recommendation for the use of any specific sensor. A major finding is that equilibration through direct air–water contact appears to be more consistent and independent of external factors than equilibration through a membrane or photometric detection. We found several instruments with no temperature measurements at the location of equilibration or with uncalibrated temperature sensors introducing significant uncertainty in the results. 

Abstract Total alkalinity (A_T) is an important parameter in the study of aquatic biogeochemical cycles, chemical speciation modeling, and many other important fundamental and anthropogenic (e.g., industrial) processes. We know little about its short‐term variability, however, because studies are based on traditional bottle sampling typically with coarse temporal resolution. In this work, an autonomous A_Tsensor, named the Submersible Autonomous Moored Instrument for Alkalinity (SAMI‐alk), was tested for freshwater applications. A comprehensive evaluation was conducted in the laboratory using freshwater standards. The results demonstrated excellent precision and accuracy (± 0.1%–0.4%) over the A_Trange from 800 to 3000 μmol L⁻¹. The system had no drift over an 8 d test and also demonstrated limited sensitivity to variations in temperature and ionic strength. Three SAMI‐alks were deployed for 23 d in the Clark Fork River, Montana, with a suite of other sensors. Compared to discrete samples, in situ accuracy for the three instruments were within 10–20 μmol L⁻¹(0.3–0.6%), indicating good performance considering the challenges of in situ measurements in a high sediment, high biofouling riverine environment with large and rapid changes in temperature. These data reveal the complex A_Tdynamics that are typically missed by coarse sampling. We observed A_Tdiel cycles as large as 60–80 μmol L⁻¹, as well as a rapid change caused by a runoff event. Significant errors in inorganic carbon system modeling result if these short‐term variations are not considered. This study demonstrates both the feasibility of the technology and importance of high‐resolution A_Tmeasurements.