The ocean's sequestration of anthropogenic carbon dioxide has major consequences for global change. Not only does it temper global warming by removing some of the greenhouse gas from the atmosphere, it also results in ocean acidification through the reaction of CO₂ and H₂O. While the environmental science community is well aware of these issues at a broad, global level, there are notable gaps in our understanding of changes occurring over smaller spaces and shorter times. This dissertation describes novel tools that are being developed and implemented in order to fill those gaps. Specifically, it focuses on autonomous, in situ systems designed to quantify total dissolved inorganic carbon (the sum of all chemical species formed in seawater directly through the addition of CO₂ to H₂O) and pH (a measure of the proton concentration in seawater. The first two chapters describe a "Micro-Rosette" for microfluidic quantification of total dissolved inorganic carbon (DIC) from profiling floats. Chapter 1 deals specifically with optimization of a gas diffusion cell, an integral component of any liquid-to-liquid (i.e., seawater to receiving solution) DIC extraction technique. 109 unique combinations of gas diffusion cell geometry, seawater volume, and diffusion time were tested to rigorously determine the optimal conditions needed in order to quantify DIC repeatably. Chapter 2 incorporates these findings and implements a sixteen-sample microfluidic collection manifold such that seawater can be captured and stored as a profiling float ascends from 2000 m to the surface and subsequently analyzed once the float returns to its "park depth" (̃1000 m), where it remains for over a week. Chapter 1 achieves better than 0.2% repeatability in DIC measurements; the multiplexed collection and analysis in Chapter 2 attains 1%. Chapters 3 and 4 pertain to in situ measurement of seawater pH with the Honeywell Durafet. Well over 100 packages implementing the Durafet have been deployed globally; Chapter 3 lays out best practices for utilizing those packages and validating the resulting data by comparing sensor results to discrete samples and thermodynamically and empirically- derived pH values. Chapter 4 illustrates the use of a Durafet packaged specifically for mobile platforms and surface pH mapping applications

We use autonomous nitrate (NO3-), oxygen (O 2), and dissolved inorganic carbon (DIC) observations to examine the relationship between ratios of C:N:O at an upwelling site in the Southern California Current System. Mean ratios and 95% confidence intervals observed by sensors over 8 months were NO3-:O2-=0.11- ±-0.002, NO3-:DIC-=-0.14-±-0.001, and DIC:O2-=0.83-±-0.01, in good agreement with Redfield ratios. Variability in the ratios on the weekly time scale is attributable to shifts in biological demand and nutrient availability and shown to exhibit a spectrum of values ranging from near 100% New Production to 100% Regenerated Production. Key Points Chemical sensors provide a means to observe the Redfield ratios Redfield ratios vary on the weekly timescale The f ratio ranges from 100% New to 100% Regenerated Production © 2014. American Geophysical Union. All Rights Reserved.

In April and May of 2020, a large phytoplankton bloom composed primarily of the dinoflagellate Lingulodinium polyedra reached historic levels in geographic expanse, duration, and density along the coast of southern California, United States, and Baja California Norte, Mexico. Here, we report the water quality parameters of dissolved oxygen and pH over the course of the red tide, as measured by multiple sensors deployed in various locations along San Diego County, and document the extent of mass organism mortality using field surveys and community science observations. We found that dissolved oxygen and pH corresponded with bloom dynamics, with extreme hypoxic and hyperoxic conditions occurring at multiple locations along the coast, most notably within select estuaries where dissolved oxygen reached 0 mg L−1 and hypoxia occurred for up to 254 consecutive hours, as well as along the inner shelf of the open coast where dissolved oxygen dropped as low as 0.05 mg L−1. Similarly, pH ranged widely (6.90–8.79) across the bloom over both space and time, largely corresponding with dissolved oxygen level. Extreme changes in dissolved oxygen and pH, in addition to changes to other water parameters that affect organismal health, ultimately led to documented mortalities of thousands of demersal and benthic fishes and invertebrates (primarily within estuarine and inner-shelf environments), and long-term surveys within one lagoon showed protracted changes to benthic infaunal density and species composition. In addition to field observations, we also quantified water quality parameters and organism mortalities from four local aquarium facilities, with varying levels of filtration and artificial oxygenation, and documented the morphological changes in the gills of captive-held Pacific sardine in response to the red tide. We show that multiple factors contributed to organismal stress, with hypoxia likely being the most widespread, but not the only, cause of mortality.