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Rising sea surface temperatures and ocean acidification (OA) may threaten the ability of calcified organisms to build carbonate reefs, but it is unclear if particular reefs have the capacity to tolerate global change. Current understanding of the effects of OA on coral reefs originates from single-species laboratory studies largely focused on scleractinian corals. Traditionally, these experiments attempt to mimic static future conditions under the assumption that coastal regimes are as constant as -- and will acidify at the same rate as -- open ocean surface waters. Predictions based on these oversimplified scenarios are unrealistic because numerous benthic organisms, including calcifiers and primary producers, significantly alter the bulk seawater carbonate chemistry over a diurnal cycle. Further, the prevalence of recently appreciated extreme diel fluctuations in pH across some reefs suggests that benthic species may be acclimated to future carbonate conditions. To look for potential OA refugia on reefs, a research team from the Scripps Institute of Oceanography (University of California at San Diego) and the Lamont Dougherty Earth Observatory (Columbia University) will undertake a unique mechanistic study on Palmyra Atoll, a remote uninhabited island in the central Pacific that lacks degradation from local human influence. They will explore the strengths and controls of biogeochemical feedbacks from coral reef benthic community assemblages to the seawater chemistry above and experimentally determine how this natural fluctuation affects physiological responses of key taxa to OA. Specifically they will: (1) tightly integrate a novel benthic flux technique in situ that allows continuous, high-temporal resolution measurements of net ecosystem metabolic rates (production and calcification) with an ongoing high spatial resolution benthic community dynamics study to quantify feedbacks of known species assemblages to observed natural spatiotemporal variability in seawater carbonate chemistry; and (2) use small scale common garden CO2 enrichment experiments and productivity/respiration assays in the lab paired with reciprocal transplant experiments in situ to empirically quantify the effects of elevated and/or fluctuating pCO2 on growth, calcification and photophysiology of common framework building organisms and their benthic competitors. This should allow them to examine the coupled interactions between OA and diverse benthic coral reef organisms in their natural environment in the absence of other confounding human impacts. BROADER IMPACTS. Ocean acidification is expected to affect a number of ecosystem goods and services that human societies have come to depend on. Specifically, the physical protection that coral reefs provide from coastal erosion will be reduced, potentially exacerbating the effects of sea level rise, and the effects of OA on fisheries and tourism are yet to be determined. One central goal is to work with local communities and organizations to better educate the public about how OA will affect people, societies and the natural resources they use and to give individuals tools that can allow them to be part of the solution. To that end, the research team has established strong partnerships with a local non-profit public broadcasting service KPBS, San Diego to develop novel products for OA communication and education. Further, they will work with the Birch Aquarium at SIO to develop an exhibit that communicates the results of the project. Finally, the project will directly train interns, students and researchers and will promote diversity through a variety of outreach activities including public lectures, training of aquarium docents and the development of web-based educational materials.

Information on marine CO2 system variability has been limited along the Inside Passage of the Pacific Northwest coast of North America despite the region’s rich biodiversity, abundant fisheries, and developing aquaculture industry. Beginning in 2017, the Alaska Marine Highway System M/V Columbia has served as a platform for surface underway data collection while conducting twice weekly ~1600-km transits between Bellingham, Washington and Skagway, Alaska. This dataset allowed for the assessment of marine CO2 system patterns along the Inside Passage, including quantification of the relative importance of key drivers in shaping pCO2 variability. Surface water pH and aragonite saturation state (Ωarag) were determined using the pCO2 data with alkalinity from a regional salinity-based relationship, which was evaluated with discrete seawater samples and underway pH measurements. Low pH and corrosive (Ωarag < 1) Ωarag conditions were seen during winter and in persistent tidal mixing zones, and corrosive Ωarag values were also seen in areas that receive significant glacial melt in summer. The time-of-detection was computed and revealed that tidal mixing zones may be sentinel observing sites with relatively short time spans of observation needed to capture secular trends in seawater pCO2 equivalent to the contemporary atmospheric CO2 increase. Finally, anthropogenic CO2 was estimated and showed notable time and space variability. We theoretically considered the change in hydrogen ion concentration ([H+]), pH, and Ωarag over the industrial era and to an atmospheric pCO2 level consistent with a 1.5 °C warmer climate and revealed greater changes in [H+] and pH in winter as opposed to larger Ωarag change in summer. In addition, the contemporary acidification signal everywhere along the Inside Passage exceeded the global average, with Johnstone Strait and the Salish Sea standing out as potential bellwethers for biological OA impacts. In theory, roughly half the acidification signal experienced thus far over the industrial era may be expected over the coming 15 years with an atmospheric CO2 trajectory that continues to be shaped by fossil-fuel development.

Global-scale ocean acidification has spurred interest in the capacity of seagrass ecosystems to increase seawater pH within crucial shoreline habitats through photosynthetic activity. However, the dynamic variability of the coastal carbonate system has impeded generalization into whether seagrass aerobic metabolism ameliorates low pH on physiologically and ecologically relevant timescales. Here we present results of the most extensive study to date of pH modulation by seagrasses, spanning seven meadows (Zostera marina) and 1000km of U.S. west coast over 6years. Amelioration by seagrass ecosystems compared to non-vegetated areas occurred 65% of the time (mean increase 0.07±0.008SE). Events of continuous elevation in pH within seagrass ecosystems, indicating amelioration of low pH, were longer and of greater magnitude than opposing cases of reduced pH or exacerbation. Sustained elevations in pH of >0.1, comparable to a 30% decrease in [H+ ], were not restricted only to daylight hours but instead persisted for up to 21days. Maximal pH elevations occurred in spring and summer during the seagrass growth season, with a tendency for stronger effects in higher latitude meadows. These results indicate that seagrass meadows can locally alleviate low pH conditions for extended periods of time with important implications for the conservation and management of coastal ecosystems.

In situ autonomous chemical sensors, combined with the right deployment platforms provide novel, powerful tools for oceanographers to observe biogeochemical processes on unprecedented spatial and temporal scales. However, many aspects of chemical sensor technology have not yet reached full maturity, preventing routine use by the community at large. This dissertation aims to fill this critical need in ocean observing technology, with a focus on Ion Sensitive Field Effect Transistor (ISFET) pH sensors for profiling float applications. Following a brief introduction to the current status of marine chemical sensor technology, the four chapters address the various steps involved in sensor development: sensor characterization, calibration, data quality control (QC), and a modeling effort using sensor data. Chapter 2 introduces a simple QC protocol for profiling float oxygen data by comparison to a monthly climatology. This protocol can constrain O₂ at the surface to better than 3%, and detect sensor drift with high confidence. Similar approaches can be taken to QC other chemical sensors data from profiling floats. Chapter 3 characterizes the response of the ISFET pH sensor and the Chloride-Ion Selective Electrode by comparison to the hydrogen electrode and the silver-silver chloride electrode, respectively. Both electrodes showed near-Nernstian response, thus the error in pH due to non-theoretical behavior of the electrodes is negligible over the oceanic range of pH and salinity. Chapter 4 quantifies the effect of pressure on the pH of certified tris buffer prepared in synthetic seawater. Assignment of pH values to certified buffer solutions is essential for sensor calibration. As the number of pH sensors deployed under high pressures is expected to increase, this chapter will fill a critical need in sensor validation and traceability. Chapter 5 presents habitat-specific ocean acidification projections between 2012 and 2100 for 4 habitats in the upper 100 m of the Southern California Bight. The projections were generated by combining high frequency pH sensor data, a regional empirical relationship of the CO₂ system, and hydrographic data to characterize the properties of upwelled waters. Habitat specific acidification signals were predicted, and implications for future ocean acidification research are discussed