The oceanic uptake of anthropogenic CO2 has resulted in ocean acidification (OA). Macroalgae farming has the potential to mitigate OA by removing CO2 from the surface water via photosynthesis. However, continuous in-situ observations of marine carbonate chemistry related to macroalgae farming remain limited, leaving its effectiveness in addressing OA uncertain. To address these knowledge gaps, this study examined a 2-acre Saccharina latissima, sugar kelp, farm located at Point Judith, Rhode Island, as a case study to assess the potential of sugar kelp aquaculture in mitigating local OA. Over the full growing season from December 2022 to May 2023, high-temporal-resolution (every 30–60 minutes) measurements of surface temperature, salinity, dissolved oxygen and pH were taken inside and outside the kelp farm. The results demonstrate that sugar kelp farming does not significantly impact the carbonate system, thus providing negligible OA mitigation locally. Specifically, a temporary, local-scale CO2 reduction and higher pH occurred during very early kelp growth in early February, but was reversed by a higher surface CO2, exaggerating OA, starting in mid-February. Over the entire observation period, kelp growth resulted in a 5.1 ± 11.6 μatm increase of pCO2 per week compared to the control site in the surface, a signal which is small compared to the substantial natural variability. However, the minimal pCO2 difference at the kelp farm may be reflective of the relatively small cultivation area (2 acres) or depressed growth of phytoplankton, resulting from nutrient competition between the kelp and in-situ phytoplankton. This study underscores the need for future sustained observations to evaluate the impact of seaweed cultivation on OA mitigation and the carbon cycle at the ecosystem scale.

The increase in atmospheric carbon dioxide (CO2) over the last 200 years has largely been mitigated by the ocean’s function as a carbon sink. However, this continuous absorption of CO2 by seawater triggers ocean acidification (OA), a process in which water becomes more acidic and more depleted in carbonate ions that are essential for calcifiers. OA is well-studied in open ocean environments; however, understanding the unique manifestation of OA in coastal ecosystems presents myriad challenges due to considerable natural variability resulting from concurrent and sometimes opposing coastal processes—e.g. eutrophication, changing hydrological conditions, heterogeneous biological activity, and complex water mass mixing. Developing a mechanistic understanding of carbonate chemistry variability and its drivers across different time scales is a critical first step in identifying the anthropogenic OA signal against background variability and predicting future OA in coastal systems. This study analyzed high temporal resolution pH data collected during 2022 and 2023 from Narragansett Bay, RI—a mid-sized, urban estuary that since 2005 has undergone a 50% reduction in nitrogen loading—with weekly, discrete bottle samples to verify sensor data. Over a year’s worth of data revealed a distinct diurnal cycle of pH, with pH increasing during the day and decreasing during the night, with an average daily range between 0.05 and 0.1 pH units. Further, we observed a strong seasonal cycles with higher mean pH in winter (8.07 ± 0.15) and lower mean pH in summer (7.72 ± 0.07). By separating the drivers of pH variability into effects from temperature, salinity, water mass mixing, biological activity, and air-sea gas flux, we determined that biological production has the most significant influence on pH from daily to annual timescales and in episodic pH changes. To a lesser extent, the seasonal air-sea CO2 exchange and temperature cycle further modified pH on monthly to seasonal timescales. The dominant influence of biological activity in modulating pH has allowed Narragansett Bay’s nutrient reductions, which have been successful in increasing bottom water DO and pH conditions, to modestly reduce summertime surface pH through reduced primary production. This study offers an in-depth understanding of Narragansett Bay’s natural carbonate variability and highlights the sensitivity of an estuary to water management policy. These findings will benefit future OA prediction and will ultimately assist in making environmental management decisions in coastal estuaries with implications for multiple coastal stakeholders.

This study presents the first regional‐scale analysis to quantify decadal trends and drivers of surface ocean acidification (OA) across the highly sensitive Pacific‐Arctic Region (PAR) using a consistent trend methodology. From 1993 to 2021, the Southern PAR acidified at rates comparable to the global average, with declining by 0.018 units and aragonite saturation state decreasing by 0.063 units , primarily driven by anthropogenic uptake. In contrast, the Bering Strait exhibited slower acidification, with declining by 0.011 units and decreasing by 0.020 units —substantially lower than previously reported—likely due to increased primary productivity. The Northern PAR experienced the most rapid acidification: decreased by 0.028 units and by 0.078 units , with the Beaufort Gyre acidifying 2–4 times faster than the global mean. This rapid change was driven by rising atmospheric and significant freshening linked to sea ice melt and increased riverine input, which reduced the ocean's buffering capacity. Continued warming will likely exacerbate acidification in regions transitioning from multi‐year to seasonal ice. While local processes such as primary productivity can temporarily counteract OA, whether they can offset rising anthropogenic levels remains unclear. This underscores the importance of biogeochemical models that integrate climatic and biological feedbacks, enabling accurate forecasts of OA changes and their impacts on marine ecosystems. These findings highlight the urgent need for sustained monitoring in the PAR, where accelerating changes threaten critical ecosystems.

Bivalve shells, a natural alkaline material, play a crucial role in coastal carbon cycles by influencing total alkalinity (TA) and dissolved inorganic carbon (DIC). This study investigated oyster shell dissolution in Narragansett Bay, Rhode Island, under varying pCO(2) conditions, revealing TA regeneration rates of 4-56 mu mol L-1 d(-1), which could mitigate localized ocean acidification (OA). Notably, significant dissolution occurred even in oversaturated waters (Omega calcite > 1) due to corrosive microenvironments created by microbial respiration. Although shell formation (calcification) emits CO2, TA regeneration (shell dissolution) buffers OA when the carbonate chemistry of the water is corrosive, offsetting the initial CO2 emissions. Therefore, recycling shells enhances ecosystem resilience by buffering acidification stress for OA-sensitive organisms. This research highlights the need to revisit shell management policies to promote sustainable aquaculture and sheds light on the potential of incorporating this nature-based alkaline material into ocean alkalinity enhancement strategies for improved coastal carbon management.

To assess the consequences of nutrient reduction strategies on water quality under climate change, we investigated the long-term dynamics of dissolved oxygen (DO) and pH in Narragansett Bay (NB), a warming urbanized estuary in Rhode Island, where nitrogen loads have declined due to extensive wastewater treatment plant upgrades. We use 15 years (January 2005-December 2019) of measurements from the Narragansett Bay Fixed Site Monitoring network. Nutrient-enhanced phytoplankton growth can increase DO in the upper water column while subsequent respiration can reduce water column DO and enhance bottom water acidification, and vice-versa. We observed significant decreases in surface DO levels, concurrent with a significant increase in bottom DO, associated with the nitrogen load reduction. Surface DO decline was primarily attributed to reduced intensity of primary productivity, supported by a concurrent decrease in surface chlorophyll concentrations. Meanwhile, the influence of reduced organic matter respiration led to the increase of bottom DO levels by 9 µmol kg-1 (approximately 0.2 mg L-1 for typical summer temperature and salinity) over a 15-year period, which overcame the opposite influence of oxygen reduction from solubility decreases due to warming temperatures. In contrast, long-term changes in surface pH have not exhibited discernible trends beyond natural variability, likely due to the complex and sometimes opposing influences of biological activity and changing river flow conditions. We observed a slight increase in bottom pH, associated with the increase in DO in bottom water. Notably, future variations in freshwater discharge, particularly linked to extreme precipitation events, may further influence water carbonate chemistry and thereby impact pH dynamics. This study highlights the necessity of long-term time series measurements in helping understand the impacts of environmental management practices in improving water quality in coastal regions during a changing climate.

As elsewhere in the global ocean, the Gulf of Alaska is experiencing the rapid onset of ocean acidification (OA) driven by oceanic absorption of anthropogenic emissions of carbon dioxide from the atmosphere. In support of OA research and monitoring, we present here a data product of marine inorganic carbon chemistry parameters measured from seawater samples taken during biannual cruises between 2008 and 2017 in the northern Gulf of Alaska. Samples were collected each May and September over the 10 year period using a conductivity, temperature, depth (CTD) profiler coupled with a Niskin bottle rosette at stations including a long-term hydrographic survey transect known as the Gulf of Alaska (GAK) Line. This dataset includes discrete seawater measurements such as dissolved inorganic carbon and total alkalinity, which allows the calculation of other marine carbon parameters, including carbonate mineral saturation states, carbon dioxide (CO2), and pH. Cumulative daily Bakun upwelling indices illustrate the pattern of downwelling in the northern Gulf of Alaska, with a period of relaxation spanning between the May and September cruises. The observed time and space variability impart challenges for disentangling the OA signal despite this dataset spanning a decade. However, this data product greatly enhances our understanding of seasonal and interannual variability in the marine inorganic carbon system parameters. The product can also aid in the ground truthing of biogeochemical models, refining estimates of sea–air CO2 exchange, and determining appropriate CO2 parameter ranges for experiments targeting potentially vulnerable species. Data are available at https://doi.org/10.25921/x9sg-9b08 (Monacci et al., 2023).

Quantifying air-sea carbon dioxide (CO2) flux from observations is subject to uncertainties due to missing data, uneven data distribution, and a relatively short observation period in the Gulf of Mexico (GOM). Despite the publication of multiple seawater partial pressure of CO2 (pCO(2sw)) products, their reliabilities in the GOM have been relatively understudied. We compare the Surface Ocean CO2 Atlas (SOCAT) observation-based synthesis with eight regional and global machine-learning pCO(2sw) data products in the GOM. SOCAT reveals significant spatial and seasonal variations in pCO(2sw) in the GOM owing to complex local nonthermal (physical and biological) dynamics, particularly in the Louisiana Shelf (LAS) and Western Florida Shelf (WFS). The regional pCO(2sw) data product outperforms the global products in capturing small-scale pCO(2sw) variations. When averaging climatology across the entire northern GOM, the spatial heterogeneity of pCO(2sw) and CO2 flux resulting from local nonthermal processes tends to counterbalance across the entire GOM in all pCO(2sw) data products. Consequently, the regional data product and the ensemble mean of seven global products yield pCO(2sw) climatology that closely aligns with the SOCAT observations with a small difference (< +/- 3 mu atm). During the overlapping period from 2003 to 2017 (15 years), the average flux from the eight products indicates that the entire GOM is CO2-neutral, with an ocean uptake flux of 0.08 +/- 0.12 mol C/m(2)/yr or 1.50 +/- 2.25 TgC/yr, which is about 0.6 % of the global coastal ocean CO2 sink. Observations show that the pCO(2sw) trend also exhibits notable spatial differences, with the river plume area acting as an increasing CO2 sink and the WFS acting as an increasing CO2 source. Due to limited observations and large spatiotemporal variations, the true values of the decadal trend still have large uncertainties in the highly dynamic river plume area. In most other subregions, pCO(2sw) increases following atmospheric CO2. Uncertainties persist across all pCO(2sw) data products in simulating the decadal trend, given that the regional product displays essentially no trend (<0.5 mu atm/yr), while the ensemble average of global products exhibits a trend that follows atmospheric pCO(2) (similar to+2.0 mu atm/yr). Our findings demonstrate that existing pCO(2sw) data products effectively simulate the climatology of pCO(2sw) in the GOM, providing valuable information for CO2 flux quantification in the GOM. Future research should emphasize the development of pCO(2sw) data products designed to accurately predict small-scale variations and temporal shifts, while also delving into the underlying dynamics responsible for these changes.

Abstract. The Amazon River plume plays a critical role in shaping the carbonate chemistry over a vast area in the western tropical North Atlantic. We conduct a sensitivity analysis of hypothetical ocean alkalinity enhancement (OAE) via quicklime addition in the Amazon River watershed, examining the response of carbonate chemistry and air–sea carbon dioxide flux to the alkalinity addition. Through a series of sensitivity tests, we show that the detectability of the OAE-induced alkalinity increment depends on the perturbation strength (or size of the alkalinity addition, ΔTA) and the number of samples: there is a 90 % chance to meet a minimum detectability requirement with ΔTA>15 µmol kg−1 and sample size >40, given background variability of 15–30 µmol kg−1. OAE-induced pCO2 reduction at the Amazon plume surface would range between 0–25 µatm when ΔTA=20 µmol kg−1, decreasing with increasing salinity (S). Adding 20 µmol kg−1 of alkalinity at the river mouth could elevate the total carbon uptake in the Amazon River plume (1532) due to its large size, comprising approximately 80 % of the S>15 plume area. However, the lowest-salinity region (S<15) has a greater drop in surface ocean partial pressure of CO2 (pCO2sw) due to its low buffer capacity, potentially allowing for observational detectability of pCO2sw reduction in this region. Reduced outgassing in this part of the plume, while more uncertain, may also be important for total additional CO2 uptake. Such sensitivity tests are useful in designing minimalistic field trials and setting achievable goals for monitoring, reporting, and verification purposes.

Ocean alkalinity enhancement (OAE) has the potential to mitigate ocean acidification (OA) and induce atmospheric carbon dioxide (CO 2 ) removal (CDR). We evaluate the CDR and OA mitigation impacts of a sustained point‐source OAE of 1.67 × 10 10 mol total alkalinity (TA) yr −1 (equivalent to 667,950 metric tons NaOH yr −1 ) in Unimak Pass, Alaska. We find the alkalinity elevation initially mitigates OA by decreasing p CO 2 and increasing aragonite saturation state and pH. Then, enhanced air‐to‐sea CO 2 exchange follows with an approximate e ‐folding time scale of 5 weeks. Meaningful modeled OA mitigation with reductions of >10 μatm p CO 2 (or just under 0.02 pH units) extends 100–100,000 km 2 around the TA addition site. The CDR efficiency (i.e., the experimental seawater dissolved inorganic carbon (DIC) increase divided by the maximum DIC increase expected from the added TA) after the first 3 years is 0.96 ± 0.01, reflecting essentially complete air‐sea CO 2 adjustment to the additional TA. This high efficiency is potentially a unique feature of the Bering Sea related to the shallow depths and mixed layer depths. The ratio of DIC increase to the TA added is also high (≥0.85) due to the high dissolved carbon content of seawater in the Bering Sea. The air‐sea gas exchange adjustment requires 3.6 months to become (>95%) complete, so the signal in dissolved carbon concentrations will likely be undetectable amid natural variability after dilution by ocean mixing. We therefore argue that modeling, on a range of scales, will need to play a major role in assessing the impacts of OAE interventions.

By compiling boreal summer (June to October) CO2 measurements from 1989 to 2019 on the Bering and eastern Chukchi Sea shelves, we find that the study areas act as a CO2 sink except when impacted by river runoff and wind-driven upwelling. The CO2 system in this area is seasonally dominated by the biological pump especially in the northern Bering Sea and near Hanna Shoal, while wind-driven upwelling of CO2-rich bottom water can cause episodic outgassing. Seasonal surface Delta fCO(2) (oceanic fCO(2) - air fCO(2)) is dominantly driven by temperature only during periods of weak CO2 outgassing in shallow nearshore areas. However, after comparing the mean summer Delta fCO(2) during the periods of 1989-2013 and 2014-2019, we suggest that temperature does drive long-term, multi-decadal patterns in Delta fCO(2). In the northern Chukchi Sea, rapid warming concurrent with reduced seasonal sea-ice persistence caused the regional summer CO2 sink to decrease. By contrast, increasing primary productivity caused the regional summer CO2 sink on the Bering Sea shelf to increase over time. While additional time series are needed to confirm the seasonal and annual trajectory of CO2 changes and ocean acidification in these dynamic and spatially complex ecosystems, this study provides a meaningful mechanistic analysis of recent changes in inorganic carbonate chemistry. As high-resolution time series of inorganic carbonate parameters lengthen and short-term variations are better constrained in the coming decades, we will have stronger confidence in assessing the mechanisms contributing to long-term changes in the source/sink status of regional sub-Arctic seas.

1 State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China, National Institute of Biology, Marine Biology Station Piran, Piran, Slovenia, Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, United States, National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory, Seattle, WA, United States, Department of Oceanography, Dalhousie University,

Information on the sources and transformations of particulate organic N (PN) and dissolved organic N (DON) at the coastal interface remains insufficient due to technological difficulties and complicated features of intensive physical mixing and rapid biological activities. Here, we investigated the spatial distribution of concentrations and isotopic compositions of PN and DON in the Changjiang plume during the summer flood period. In average, DON and PN accounted for 25.6 +/- 12.1% and 8.1 +/- 9.1% (n = 55), respectively, of the total N pool, with the remaining N primarily in the form of nitrate (NO3-). Mean delta N-15 values were the lowest for DON (-0.1 +/- 2.7 parts per thousand, n = 58) and slightly higher for PN (2.0 +/- 1.6 parts per thousand, n = 101), and the highest for NO3- (6.5 +/- 2.2 parts per thousand, n = 67), suggesting multiple transformations had occurred to differentiate isotopic characteristics among the three N pools. By applying a conservative mixing model, we found DON deficits (-3.5 +/- 3.7 mu mol L-1, n = 43) and negative shift in delta N-15(DON) (-3.6 +/- 2.2 parts per thousand, n = 43) in the Changjiang plume, revealing nonconservative DON behaviors. In the offshore surface plume where Chlorophyll a was high, the most likely cause is the DON uptake by phytoplankton with a strong inverse isotope effect (around -40 parts per thousand). This DON assimilation by phytoplankton contributed to similar to 16 +/- 12% of the PN production, with the remaining supported by NO3- assimilation, producing an overall isotope effect of 4-9 parts per thousand. However, in waters near the river mouth and at the bottom of the offshore plume where total suspended matter concentrations were high (> 5 mg L-1), the DON deficit was most likely induced by the selective adsorption of N-15 enriched moieties of DON onto particulate surfaces (with an isotope effect of -20 parts per thousand to -5 parts per thousand). Unlike dissolved organic carbon to behave conservatively in most estuaries, our results show that active transformations had occurred between the DON and PN pools in the Changjiang plume. (c) 2021 Published by Elsevier B.V.

OPINION article Front. Mar. Sci., 01 November 2021Sec. Marine Biogeochemistry https://doi.org/10.3389/fmars.2021.729992

Abstract The effect of photomineralization on the carbon cycle in a eutrophic, semiarid estuary (Baffin Bay, Texas) was investigated using closed‐system incubations. Photochemical production rate of dissolved inorganic carbon ranged from 0.16 to 0.68 μM hr−1, with a daily removal of 0.3∼1.5% of the standing stock of dissolved organic carbon (DOC). The photomineralization rate was negatively correlated with chlorophyll a concentration, suggesting that plankton‐derived DOC was less photoreactive to solar radiation. The stable carbon isotope composition (δ13C∼ −18.6‰) of degraded DOC, as calculated using the DIC “Keeling” plot, further indicated high photochemical lability of 13C‐enriched DOC in this semiarid environment. Our finding showed that photomineralization of 13C‐enriched DOC is an important component of carbon cycle in this system, and this process does not necessarily remove 13C‐depleted organic carbon as observed in other coastal systems.

It is known that surface water eutrophication enhances bottom water ocean acidification via respiration in coastal oceans. However, the role of benthic processes in influencing bottom water acidification has not been sufficiently explored. We examined this issue by analyzing a 10-year summer carbonate chemistry dataset in bottom water together with recent benthic flux measurements and literature benthic flux data in the northern Gulf of Mexico. The difference between the observed and estimated pH (Omega) values calculated from anthropogenic CO2 increase and water column aerobic respiration were defined as Delta pH (Delta Omega). We found that Delta pH and Delta Omega values in hypoxic condition were -0.03 0.04 (mean standard deviation) and -0.15 0.39, respectively. Both Delta pH and Delta Omega values in hypoxic conditions were significantly lower than zero (p < 0.05). The net results of anaerobic respiration, oxidation of reduced chemcials, burial of iron sulfide minerals, and possible CaCO3 dissolution may have led to an alkalinity to DIC production ratio of less than 1 in porewater. This caused the ratio of alkalinity to dissolved inorganic carbon fluxes from sediment to bottom water to be less than 1, which led to additional bottom water acidification. Our analysis and model simulations demonstrate that severe hypoxic and anoxic conditions, which correspond to less water movement, favor the accumulation of benthic respiration products, leading to additional pH and reductions. The findings on sediment processes contributing to acidification in bottom waters provide new insights into the sensitivity of coastal ocean acidification to low-oxygen conditions under current and future climates and anthropogenic nutrient loading scenarios. Plain Language Summary The ongoing decrease in seawater pH as a result of uptake of anthropogenic carbon dioxide (CO2) from the atmosphere is known as ocean acidification, which can be enhanced by oxygen-consuming respiration in the water column. Meanwhile, regions of coastal hypoxia (dissolved oxygen <2 mg L-1 or 63 mol L-1) have increased in size and number during the last several decades because of water column eutrophication. Previous studies take only the anthropogenic CO2 intrusion and aerobic respiration (respiration that consumes oxygen) into consideration when predicting the water column pH and carbonate mineral saturation. However, we found that anaerobic respiration (respiration that does not consumes dissolved oxygen) and the subsequent alkalinity removal via metal-sulfide burials in sediments can further decrease pH and carbonate mineral saturation. Therefore, the bottom water acidification states are more aggravated in hypoxic conditions than previous estimations in the northern Gulf of Mexico. To our knowledge, this is the first study that uses data from multiple years and systematically examines the role of benthic fluxes on ocean acidification in eutrophic coastal bottom waters. The finding has profound implications for similar coastal systems where eutrophication-induced bottom ocean acidification is likely more severe than we used to think. Key Points Observed pH and Omega in hypoxic water in the nGoM are lower than those estimated from anthropogenic CO2 increase and organic carbon respiration The lower pH and Omega values are caused by neglecting benthic anaerobic respiration Benthic anaerobic respiration and subsequent alkalinity removal further increase the susceptibility of coastal waters to ocean acidification

Year of emergence (YoE) is the year when an environment and the organisms within begin to experience significant different conditions (two times of natural variability) from the pre-industrial conditions (~1770 C.E.). This study calculates the global surface ocean YoEs for pH, partial pressure of CO(CO) and aragonite saturation (Ω) from a recent calculated surface ocean carbonate chemistry data product. The data product is calculated from the Surface Ocean CO Atlas version 6 (SOCATv6) with modeled CO changes in the global surface ocean from the ESM2M model. We find that CO, pH and Ω generally emerged from preindustrial conditions in the open ocean by the year 1950, while these properties have still not yet emerged along many ocean margins. We also find that Ω had a significantly delayed YoE compared to pH and CO. The delayed YoE for Ω is caused by its lasting sensitivity to temperature variability, which increases the natural variability experienced by organisms, and a partial cancellation of the long term acidification trend by the global warming. Together, YoEs presented here highlight that there are hotspots (open ocean) and coldspots (ocean margins that were impacted by boundary currents) for the emergence of anthropogenic signals. Continuous data collection and synthesis are needed to further examine the impact of ocean acidification on ecosystem health.

Decadal-scale pH trends for the open ocean are largely monotonic and controlled by anthropogenic CO2 invasion. In estuaries, though, such long-term pH trends are often obscured by a variety of other factors, including changes in net metabolism, temperature, estuarine mixing, and riverine hydrogeochemistry. In this study, we mine an extensive biogeochemical database in two North Carolina estuaries, the Neuse River estuary (NeuseRE) and New River estuary (NewRE), in an effort to deconvolute decadal-scale trends in pH and associated processes. By applying a Generalized Additive Mixed Model (GAMM), we show that temporal changes in NewRE pH were insignificant, while pH decreased significantly throughout much of the NeuseRE. In both estuaries, variations in pH were accompanied by increasing river discharge, and were independent of rising temperature. Decreases in bottom-water pH in the NeuseRE coincided with elevated primary production in surface waters, highlighting the importance of eutrophication on long-term acidification trends. Next, we used a simple mixing model to illustrate the impact of changing river discharge on estuarine carbonate chemistry. We found that increased riverine alkalinity loads to the NewRE likely buffered the impact of CO2-intrusion-induced acidification. In the NeuseRE, however, elevated dissolved inorganic carbon loads further decreased the buffering capacity, exacerbating the effects of CO2-intrusion-driven acidification. Taken together, the findings of this study show that future trajectories in estuarine pH will be shaped by complex interactions among global-scale changes in climate, regional-scale changes in precipitation patterns, and local-scale changes in estuarine biogeochemistry.