Posts Tagged 'chemistry'

An automated microfluidic analyzer for in situ monitoring of total alkalinity

We have designed, built, tested, and deployed an autonomous in situ analyzer for seawater total alkalinity. Such analyzers are required to understand the ocean carbon cycle, including anthropogenic carbon dioxide (CO2) uptake and for mitigation efforts via monitoring, reporting, and verification of carbon dioxide removal through ocean alkalinity enhancement. The microfluidic nature of our instrument makes it relatively lightweight, reagent efficient, and amenable for use on platforms that would carry it on long-term deployments. Our analyzer performs a series of onboard closed-cell titrations with three independent stepper-motor driven syringe pumps, providing highly accurate mixing ratios that can be systematically swept through a range of pH values. Temperature effects are characterized over the range 5–25 °C allowing for field use in most ocean environments. Each titration point requires approximately 170 μL of titrant, 830 μL of sample, 460 J of energy, and a total of 105 s for pumping and optical measurement. The analyzer performance is demonstrated through field data acquired at two sites, representing a cumulative 25 days of operation, and is evaluated against laboratory measurements of discrete water samples. Once calibrated against onboard certified reference material, the analyzer showed an accuracy of −0.17 ± 24 μmol kg–1. We further report a precision of 16 μmol kg–1, evaluated on repeated in situ measurements of the aforementioned certified reference material. The total alkalinity analyzer presented here will allow measurements to take place in remote areas over extended periods of time, facilitating affordable observations of a key parameter of the ocean carbon system with high spatial and temporal resolution.

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A biogeochemical alkalinity sink in a shallow, semiarid estuary of the Northwestern Gulf of Mexico

Estuarine total alkalinity (TA), which buffers against acidification, is temporally and spatially variable and regulated by complex, interacting hydrologic and biogeochemical processes. During periods of net evaporation (drought), the Mission-Aransas Estuary (MAE) of the northwestern Gulf of Mexico experienced TA losses beyond what can be attributed to calcification. The contribution of sedimentary oxidation of reduced sulfur to the TA loss was examined in this study. Water column samples were collected from five stations within MAE and analyzed for salinity, TA, and calcium ion concentrations. Sediment samples from four of these monitoring stations and one additional station within MAE were collected and incubated between 2018 and 2021. TA, calcium, magnesium, and sulfate ion concentrations were analyzed for these incubations. Production of sulfate along with TA consumption (or production) beyond what can be attributed to calcification (or carbonate dissolution) was observed. These results suggest that oxidation of reduced sulfur consumed TA in MAE during droughts. We estimate that the upper limit of TA consumption due to reduced sulfur oxidation can be as much as 4.60 × 108 mol day−1 in MAE. This biogeochemical TA sink may be present in other similar subtropical, freshwater-starved estuaries around the world.

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Recent trends and variability in the oceanic storage of dissolved inorganic carbon

Several methods have been developed to quantify the oceanic accumulation of anthropogenic carbon dioxide (CO2) in response to rising atmospheric CO2. Yet, we still lack a corresponding estimate of the changes in the total oceanic dissolved inorganic carbon (DIC). In addition to the increase in anthropogenic CO2, changes in DIC also include alterations of natural CO2. Once integrated globally, changes in DIC reflect the net oceanic sink for atmospheric CO2, complementary to estimates of the air-sea CO2 exchange based on surface measurements. Here, we extend the MOBO-DIC machine learning approach by Keppler et al. (2020a) to estimate global monthly fields of DIC at 1° resolution over the top 1500 m from 2004 through 2019. We find that over these 16 years and extrapolated to cover the whole global ocean down to 4000 m, the oceanic DIC pool increased close to linearly at an average rate of 3.2+/-0.7 Pg C yr^-1. This trend is statistically indistinguishable from current estimates of the oceanic uptake of anthropogenic CO2 over the same period. Thus, our study implies no detectable net loss or gain of natural CO2 by the ocean, albeit the large uncertainties could be masking it. Our reconstructions suggest substantial internal redistributions of natural oceanic CO2, with a shift from the mid-latitudes to the tropics and from the surface to below 200 m. Such redistributions correspond with the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. The interannual variability of DIC is strongest in the tropical Western Pacific, consistent with the El Nino Southern Oscillation.

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Climate-driven changes of global marine mercury cycles in 2100

Significance

One concern caused by the changes in the ocean due to climate change is the potential increase of neurotoxic methylmercury content in seafood. This work quantifies the impact of global change factors on marine mercury cycles. The air–sea exchange is influenced by wind speed weakening and solubility drop of mercury due to seawater warming. The decreased biological pump shrinks the methylation substrate and causes weaker methylation. The advantageous light environment resulting from less attenuation by sea ice and phytoplankton increases the photodegradation potential for seawater methylmercury. Responses of seawater methylmercury can propagate to biota, which is also modulated by the changes in anthropogenic emissions and ocean ecology. Our results offer insight into interactions among different climate change stressors.

Abstract

Human exposure to monomethylmercury (CH3Hg), a potent neurotoxin, is principally through the consumption of seafood. The formation of CH3Hg and its bioaccumulation in marine food webs experience ongoing impacts of global climate warming and ocean biogeochemistry alterations. Employing a series of sensitivity experiments, here we explicitly consider the effects of climate change on marine mercury (Hg) cycling within a global ocean model in the hypothesized twenty-first century under the business-as-usual scenario. Even though the overall prediction is subjected to significant uncertainty, we identify several important climate change impact pathways. Elevated seawater temperature exacerbates elemental Hg (Hg0) evasion, while decreased surface wind speed reduces air–sea exchange rates. The reduced export of particulate organic carbon shrinks the pool of potentially bioavailable divalent Hg (HgII) that can be methylated in the subsurface ocean, where shallower remineralization depth associated with lower productivity causes impairment of methylation activity. We also simulate an increase in CH3Hg photodemethylation potential caused by increased incident shortwave radiation and less attenuation by decreased sea ice and chlorophyll. The model suggests that these impacts can also be propagated to the CH3Hg concentration in the base of the marine food web. Our results offer insight into synergisms/antagonisms in the marine Hg cycling among different climate change stressors.

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Ocean acidification in emission-driven temperature stabilization scenarios: the role of TCRE and non-CO2 greenhouse gases

Future ocean acidification mainly depends on the continuous ocean uptake of CO2 from the atmosphere. The trajectory of future atmospheric CO2 is prescribed in traditional climate projections with Earth System Models, leading to a small model spread and apparently low uncertainties for projected acidification, but a large spread in global warming. However, climate policies such as the Paris Agreement define climate targets in terms of global warming levels and as traditional simulations do not converge to a given warming level, they cannot be used to assess uncertainties in projected acidification for these warming levels. Here, we perform climate simulations that converge to given temperature levels using the Adaptive Emission Reduction Algorithm (AERA) with the Earth System Model Bern3D-LPX at different setups with different transient climate response to cumulative carbon emissions (TCRE) and choices between reductions in CO2 and non-CO2 forcing agents. With these simulations, we demonstrate that uncertainties in surface ocean acidification are an order of magnitude larger than the usually reported inter-model uncertainties from simulations with prescribed atmospheric CO2. Uncertainties in acidification at a given stabilized temperature are dominated by TCRE and the choice of emission reductions of non-CO2 greenhouse gases. High TCRE and relatively low reductions of non-CO2 greenhouse gases, for example, necessitate relatively strong reductions in CO2 emissions and lead to relatively little ocean acidification at a given temperature level. The results suggest that choices between reducing emissions of CO2 versus non-CO2 agents should consider the economic costs and ecosystem damage of ocean acidification.

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Limits and CO2 equilibration of near-coast alkalinity enhancement

Ocean alkalinity enhancement (OAE) has recently gained attention as a potential method for carbon dioxide removal (CDR) at gigatonne (Gt) scale, with near-coast OAE operations being economically favorable due to proximity to mineral and energy sources. In this paper we study critical questions which determine the scale and viability of OAE. Which coastal locations are able to sustain a large flux of alkalinity at minimal pH and ΩArag (aragonite saturation) changes? What is the interference distance between adjacent OAE projects? How much CO2 is absorbed per unit of alkalinity added? How quickly does the induced CO2 deficiency equilibrate with the atmosphere? Choosing relatively conservative constraints on ΔpH or ΔOmega, we examine the limits of OAE using the ECCO LLC270 (0.3) global circulation model. We find that the sustainable OAE rate varies over 1–2 orders of magnitude between different coasts and exhibits complex patterns and non-local dependencies which vary from region to region. In general, OAE in areas of strong coastal currents enables the largest fluxes and depending on the direction of these currents, neighboring OAE sites can exhibit dependencies as far as 400 km or more. At these steady state fluxes most regional stretches of coastline are able to accommodate on the order of 10s to 100s of megatonnes of negative emissions within 300 km of the coast. We conclude that near-coastal OAE has the potential to scale globally to several Gt CO2 yr−1 of drawdown with conservative pH constraints, if the effort is spread over the majority of available coastlines. Depending on the location, we find a diverse set of equilibration kinetics, determined by the interplay of gas exchange and surface residence time. Most locations reach an uptake efficiency plateau of 0.6–0.8 mol CO2 per mol of alkalinity after 3–4 years, after which there is only slow additional CO2 uptake. Regions of significant downwelling (e.g., around Iceland) should be avoided by OAE deployments, as in such locations up to half of the CDR potential of OAE can be lost to bottom waters. The most ideal locations, reaching a molar uptake ratio of around 0.8, include North Madagascar, California, Brazil, Peru and locations close to the Southern Ocean such as Tasmania, Kerguelen and Patagonia, where the gas exchange appears to occur faster than the surface residence time. However, some locations (e.g., Hawaii) take significantly longer to equilibrate (up to 8–10 years) but can still eventually achieve high uptake ratios.

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Climate change: stressor on marine buffer system

Oceans are natural carbonate buffer systems and work as a carbon sink in the environment which is much larger than the atmospheric and terrestrial carbon content. The global carbon cycle is maintained by the continuous gaseous exchange during photosynthesis and respiration. The atmospheric CO2 also gets dissolved into the ocean water and forms weak carbonic acid. Thus, ocean water is a mixture of various numerous weak acids and bases and stays in contact with the atmosphere and other minerals as sediments. All of them together make the ocean an excellent buffer for neutralizing small changes in its composition. But the recent increase in industrialization and anthropogenic activities are causing the increase in atmospheric CO2 and climate change. More atmospheric CO2 is being dissolved in ocean water and carbon is being released from oceanic carbon sink making the ocean more acidic. Since industrialization, ocean water pH has dropped by 0.1 unit which indicated approximately a 30% increase in hydrogen ion concentration and 16% decrease in carbonate ion concentration relative to the preindustrial value. As a result of ocean acidification, there are devastating effects on ocean biota. An increase in sea surface temperature and deoxygenation are other climate change-related stressors on the ocean system.

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Variability of pCO2 and FCO2 in the Mexican Pacific during 25 years

Oceanographic features acting on different spatial-temporal scales influence the variation in the partial pressure of CO2 (pCO2) and ocean-atmosphere CO2 flux (FCO2). In this work, we regionally characterize regions of variability in the Mexican Pacific (MP) based on these chemical properties. We also evaluate the seasonal and interannual changes of each region: in the California Current System (CCS), Cabo Corrientes (CC), and Gulf of Tehuantepec (GT) regions. Sea surface temperature (SST), salinity, wind, pCO2, and FCO2 data from 1993 to 2018 were analyzed. Bayesian t-tests (95% credibility intervals) determined showed that the three regions had high probabilities of being different. Typical FCO2 values in the CCS were higher (−27.6–29.8 mmol C m−2 d−1) than those of the CC and GT regions (−19.9–25.8 and − 11.8–12.5 mmol C m−2 d−1, respectively). The highest positive seasonal variation of FCO2 (mean ± standard deviation) was found in the CCS and CC (∼4.6 ± 4.2 mmol C m−2 d−1) regions during spring, and in the GT region (1.2 ± 2 mmol C m−2 d−1) in autumn due to the strong northerly winds. It was found that during ENSO conditions the MP was a source (4.0 and 3.9 mol C m−2 y−1 for El Niño and La Niña, respectively), although on average over the last 25 years included in the study the MP acted as a slight-CO2 sink (∼10.9 ± 0.005 mol C m−2).

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Advancing best practices for assessing trends of ocean acidification time series

Assessing the status of ocean acidification across ocean and coastal waters requires standardized procedures at all levels of data collection, dissemination, and analysis. Standardized procedures for assuring quality and accessibility of ocean carbonate chemistry data are largely established, but a common set of best practices for ocean acidification trend analysis is needed to enable global time series comparisons, establish accurate records of change, and communicate the current status of ocean acidification within and outside the scientific community. Here we expand upon several published trend analysis techniques and package them into a set of best practices for assessing trends of ocean acidification time series. These best practices are best suited for time series capable of characterizing seasonal variability, typically those with sub-seasonal (ideally monthly or more frequent) data collection. Given ocean carbonate chemistry time series tend to be sparse and discontinuous, additional research is necessary to further advance these best practices to better address uncharacterized variability that can result from data discontinuities. This package of best practices and the associated open-source software for computing and reporting trends is aimed at helping expand the community of practice in ocean acidification trend analysis. A broad community of practice testing these and new techniques across different data sets will result in improvements and expansion of these best practices in the future.

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The DIC carbon isotope evolutions during CO2 bubbling: implications for ocean acidification laboratory culture

Ocean acidification increases pCO2 and decreases pH of seawater and its impact on marine organisms has emerged as a key research focus. In addition to directly measured variables such as growth or calcification rate, stable isotopic tracers such as carbon isotopes have also been used to more completely understand the physiological processes contributing to the response of organisms to ocean acidification. To simulate ocean acidification in laboratory cultures, direct bubbling of seawater with CO2 has been a preferred method because it adjusts pCO2 and pH without altering total alkalinity. Unfortunately, the carbon isotope equilibrium between seawater and CO2 gas has been largely ignored so far. Frequently, the dissolved inorganic carbon (DIC) in the initial seawater culture has a distinct 13C/12C ratio which is far from the equilibrium expected with the isotopic composition of the bubbled CO2. To evaluate the consequences of this type of experiment for isotopic work, we measured the carbon isotope evolutions in two chemostats during CO2 bubbling and composed a numerical model to simulate this process. The isotopic model can predict well the carbon isotope ratio of dissolved inorganic carbon evolutions during bubbling. With help of this model, the carbon isotope evolution during a batch and continuous culture can be traced dynamically improving the accuracy of fractionation results from laboratory culture. Our simulations show that, if not properly accounted for in experimental or sampling design, many typical culture configurations involving CO2 bubbling can lead to large errors in estimated carbon isotope fractionation between seawater and biomass or biominerals, consequently affecting interpretations and hampering comparisons among different experiments. Therefore, we describe the best practices on future studies working with isotope fingerprinting in the ocean acidification background.

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Rapid increase of pCO2 and seawater acidification along Kuroshio at the east edge of the East China Sea

Graphical abstract

Highlights

  • Fast rates of pCO2 increase and seawater acidification were found along the Kuroshio mainstream near the East China Sea
  • Under sustained DIC increase, faster seawater warming led to rapid rates of pCO2 increase and acidification
  • It potentially implies a gradual loss of oceanic CO2 uptake in ocean margins under climate change.

Abstract

Rates of seawater acidification and rise of partial pressure of CO2 (pCO2) at ocean margins are highly uncertain. In this study, nine years of time-series data sampled during 2010–2018 along Kuroshio Current near the East China Sea (ECS) were investigated. We found trends of surface seawater pCO2 at 3.70 ± 0.57 μatm year−1 and pH at −0.0033 ± 0.0009 unityear−1, both of which were significantly greater than those reported from other oceanic time series. Mechanistic analysis showed that seawater warming caused rapid rates of pCO2 increase and acidification under sustained DIC increase. The faster pCO2 growth relative to the atmosphere resulted in the CO2 uptake through the air-sea exchange declining by ~50 % (~−0.8 to −0.4 mol C m−2 year−1) over the study period. Our results imply that rapidly warming boundary currents could potentially present an elevated pCO2 trend, leading to a gradual reduction and eventual loss of oceanic CO2 uptake under climate change.

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A space-time mosaic of seawater carbonate chemistry conditions in the north-shore Moorea coral reef system

The interplay between ocean circulation and coral metabolism creates highly variable biogeochemical conditions in space and time across tropical coral reefs. Yet, relatively little is known quantitatively about the spatiotemporal structure of these variations. To address this gap, we use the Coupled Ocean Atmosphere Wave and Sediment Transport (COAWST) model, to which we added the Biogeochemical Elemental Cycling (BEC) model computing the biogeochemical processes in the water column, and a coral polyp physiology module that interactively simulates coral photosynthesis, respiration and calcification. The coupled model, configured for the north-shore of Moorea Island, successfully simulates the observed (i) circulation across the wave regimes, (ii) magnitude of the metabolic rates, and (iii) large gradients in biogeochemical conditions across the reef. Owing to the interaction between coral net community production (NCP) and coral calcification, the model simulates distinct day versus night gradients, especially for pH and the saturation state of seawater with respect to aragonite (Ωα). The strength of the gradients depends non-linearly on the wave regime and the resulting residence time of water over the reef with the low wave regime creating conditions that are considered as “extremely marginal” for corals. With the average water parcel passing more than twice over the reef, recirculation contributes further to the accumulation of these metabolic signals. We find diverging temporal and spatial relationships between total alkalinity (TA) and dissolved inorganic carbon (DIC) (≈ 0.16 for the temporal vs. ≈ 1.8 for the spatial relationship), indicating the importance of scale of analysis for this metric. Distinct biogeochemical niches emerge from the simulated variability, i.e., regions where the mean and variance of the conditions are considerably different from each other. Such biogeochemical niches might cause large differences in the exposure of individual corals to the stresses associated with e.g., ocean acidification. At the same time, corals living in the different biogeochemical niches might have adapted to the differing conditions, making the reef, perhaps, more resilient to change. Thus, a better understanding of the mosaic of conditions in a coral reef might be useful to assess the health of a coral reef and to develop improved management strategies.

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Understanding the current use and future needs of CO2 in seawater certified reference materials

Certified reference materials (CRMs) are fundamental for accurate and precise measurements of seawater CO2 system parameters and research related to ocean acidification and oceanic carbon cycles. Currently, there is a single global source of reference materials for total alkalinity, dissolved inorganic carbon, and pH in seawater and a calibrated HCl titrant for seawater alkalinity analysis (Andrew Dickson Laboratory, Scripps Institution of Oceanography, University of California San Diego). When production of these materials was halted during lab closures due to the Covid-19 pandemic, a shortage of CRMs ensued and highlighted the risks associated with having a single producer of CRMs. Distribution of CRMs was halted from for a year starting in March 2020. The U.S. Interagency Working Group on Ocean Acidification, which is responsible for coordinating U.S. federal activities related to ocean acidification, is engaging in efforts to increase resilience in the production and distribution of reference materials for the quality control of measurements of seawater CO2 system parameters. Increasing resilience of CRM production includes exploring multiple nodes of production inside the U.S. and whether a country outside of the United States could develop a production site. In parallel with U.S. efforts, the Global Ocean Acidification Observing Network (GOA-ON) is also working to advance efforts to improve international CRM resilience through its program for the UN Decade of Ocean Science for Sustainable Development: OARS, Ocean Acidification Research for Sustainability.

A new model for CRM production and certification, both within the US and internationally, must be informed by an understanding of the current and future use of CRMs. Specifically, it is vital to understand who uses CRMs, how and where CRMs are used, how many CRMs are currently used, and how many CRMs are expected to be used in the future. To better understand these aspects of CRM use, the GOA-ON executive secretariat created a questionnaire on CRM usage in collaboration with the Interagency Working Group on Ocean Acidification. The questionnaire was shared with the carbonate chemistry research community in April 2021. It was released approximately one month after Dr. Andrew Dickson presented a webinar in which he discussed his current reference material production system at Scripps Institution of Oceanography and options for the future of CRM production. The questionnaire was made available on social media platforms, including the Ocean Acidification Information Exchange, and was shared with webinar attendees. Additionally, the Dickson laboratory, GOA-ON, and the Ocean Carbon and Biogeochemistry Program (OCB) shared the questionnaire with their contacts. The questionnaire was made available along with a link to a recording of Dr. Dickson’s webinar and a link to an Ocean Acidification Information Exchange post about the webinar which contains questions, discussions, and a pdf copy of the presentation slides. Members of the OA and carbonate chemistry research communities voluntarily elected to participate in the questionnaire. It was encouraged that only one representative from each laboratory or research group provide answers.

A total of 247 individuals voluntarily responded to the questionnaire, although not every participant responded to every question. This document describes the responses that were received. All responses are presented in aggregate form as all individual responses are confidential and will not be released publicly.

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Climate change amelioration by marine producers: does dominance predict impact?

Climate change threatens biodiversity worldwide, and assessing how those changes will impact communities will be critical for conservation. Dominant primary producers can alter local-scale environmental conditions, reducing temperature via shading and mitigating ocean acidification via photosynthesis, which could buffer communities from the impacts of climate change. We conducted two experiments on the coast of southeastern Alaska to assess the effects of a common seaweed species, Neorhodomela oregona, on temperature and pH in field tide pools and tide pool mesocosms. We found that N. oregona was numerically dominant in this system, covering >60% of habitable space in the pools and accounting for >40% of live cover. However, while N. oregona had a density-dependent effect on pH in isolated mesocosms, we did not find a consistent effect of N. oregona on either pH or water temperature in tide pools in the field. These results suggest that the amelioration of climate change impacts in immersed marine ecosystems by primary producers is not universal and likely depends on species’ functional attributes, including photosynthetic rate and physical structure, in addition to abundance or dominance.

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Chapter 10 – Carbonate chemistry, carbon cycle, and its sequestration in aquatic system

Carbon is the universal currency used by biota to store and expend energy. Oceans act as a reservoir for almost 30% of the atmospheric carbon dioxide. The oceans store carbon in three forms: dissolved inorganic carbon (CO2 , HCO3, and CO32−), dissolved organic carbon (both small and large organic molecules), and particulate organic carbon (live organisms or fragments of dead plants and animals). They also store it in the form of black carbon (BC). Carbon keeps on exchanging between the aquatic and terrestrial ecosystems via atmosphere. Inorganic carbon is absorbed and released at the interface of the ocean’s surface and surrounding air, through the process of diffusion. This exchange of inorganic carbon takes place only in the form of CO 2, which forms carbonate when dissolved in seawater. The formation of carbonate allows oceans to take up and store a much larger amount of carbon than would be possible if dissolved CO2 remained in that form. Carbon is also cycled through the ocean by the biological processes of photosynthesis, respiration, and decomposition of aquatic plants. The changes in the chemistry of the ocean due to acidification have a great impact on marine life as well as corals and foraminifera. Since the concentration of carbon dioxide has increased rapidly in the last few decades, it becomes crucial for us to fully understand the carbonate processes and the various source and sink of carbon in the aquatic system in order to mitigate the negative effects of global warming and climate change.

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Water motion and pH jointly impact the availability of dissolved inorganic carbon to macroalgae

The supply of dissolved inorganic carbon to seaweeds is a key factor regulating photosynthesis. Thinner diffusive boundary layers at the seaweed surface or greater seawater carbon dioxide (CO2) concentrations increase CO2 supply to the seaweed surface. This may benefit seaweeds by alleviating carbon limitation either via an increased supply of CO2 that is taken up by passive diffusion, or via the down-regulation of active carbon concentrating mechanisms (CCMs) that enable the utilization of the abundant ion bicarbonate (HCO3). Laboratory experiments showed that a 5 times increase in water motion increases DIC uptake efficiency in both a non-CCM (Hymenena palmata, Rhodophyta) and CCM (Xiphophora gladiata, Phaeophyceae) seaweed. In a field survey, brown and green seaweeds with active-CCMs maintained their CCM activity under diverse conditions of water motion. Whereas red seaweeds exhibited flexible photosynthetic rates depending on CO2 availability, and species switched from a non-CCM strategy in wave-exposed sites to an active-CCM strategy in sheltered sites where mass transfer of CO2 would be reduced. 97–99% of the seaweed assemblages at both wave-sheltered and exposed sites consisted of active-CCM species. Variable sensitivities to external CO2 would drive different responses to increasing CO2 availability, although dominance of the CCM-strategy suggests this will have minimal impact within shallow seaweed assemblages.

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Report on ocean acidification monitoring in the Western Indian Ocean

Carbon dioxide (CO2) emissions from human activities are largely absorbed by the ocean, accounting for about one-third of the total emissions over the past 200 years from the combustion of fossil fuels, the production of cement, and changes in land use (Sabine et al., 2004). The uptake of CO2 by the ocean benefits society by moderating the rate of climate change but also causes unprecedented changes to ocean chemistry, decreasing the pH of the water and leading to a suite of chemical changes collectively known as ocean acidification. Like climate change, ocean acidification is a growing global problem that will intensify with continued CO2 emissions and has the potential to change marine ecosystems and affect benefits to society.

The chemistry of the ocean is changing at an unprecedented rate and magnitude due to anthropogenic carbon dioxide emissions; the rate of change exceeds that which has occurred for at least the past hundreds of thousands of years. Unless anthropogenic CO2 emissions are substantially curbed, or atmospheric CO2 is controlled by some other means, the average pH of the ocean will continue to fall. Ocean acidification has demonstrated impacts on many marine organisms. While the ultimate consequences are still unknown, there is a risk of ecosystem change that may threaten coral reefs, fisheries, protected species, and other natural resources of value to society.

Since the start of the Industrial Revolution, the average pH of ocean surface waters has decreased by about 0.1 units, from about 8.2 to 8.1. Model predictions show an additional 0.2–0.3 drop in pH by the end of the century, even under optimistic scenarios. Perhaps more important is that the rate of this change exceeds any known change in ocean chemistry for at least 800,000 years. The major changes in ocean chemistry caused by increasing atmospheric CO2 are well understood and can be precisely calculated, despite some uncertainty resulting from biological feedback processes.

However, the direct biological effects of ocean acidification are less certain and will vary among organisms, with some coping well and others not at all. The long-term consequences of ocean acidification for marine biota are unknown, but changes in many ecosystems and the services they provide to society appear likely based on current understanding.

In response to these concerns, WIOMSA launched ocean acidification projects in six countries: Kenya, Mauritius, Mozambique, Seychelles, South Africa and Tanzania, with the support of the Swedish International Development Cooperation Agency and institutional partners in the WIO region. The research provides a baseline that will foster the development of an integrated science strategy for ocean acidification monitoring, research and impact assessment. It presents a review of the current state of knowledge on ocean acidification in the WIO region and identifies the gaps in information required to improve understanding and address the consequences of ocean acidification.

The report consists of seven chapters. Chapter 1 introduces the subject of ocean acidification and chapters 2 to 7 summarize the esults of ocean acidification monitoring in the six countries that participated in the four-year monitoring project. Lessons learned and recommendations are presented for each country.

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Simulated impact of ocean alkalinity enhancement on atmospheric CO2 removal in the Bering Sea

Abstract

Ocean alkalinity enhancement (OAE) has the potential to mitigate ocean acidification (OA) and induce atmospheric carbon dioxide (CO2) removal (CDR). We evaluate the CDR and OA mitigation impacts of a sustained point-source OAE of 1.67 × 1010 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 pCO2 and increasing aragonite saturation state and pH. Then, enhanced air-to-sea CO2 exchange follows with an approximate e-folding time scale of 5 weeks. Meaningful modeled OA mitigation with reductions of >10 μatm pCO2 (or just under 0.02 pH units) extends 100–100,000 km2 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 CO2 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.

Key Points

  • We used regional ocean model to simulate single point-source ocean alkalinity enhancement in the Bering Sea
  • The steady state carbon dioxide removal efficiency was near one in years 3+ of the simulation
  • The meaningful modeled ocean acidification mitigation is confined to the region near the alkalinity addition

Plain Language Summary

The Intergovernmental Panel on Climate Change suggests that carbon dioxide (CO2) removal (CDR) approaches will be required to stabilize the global temperature increase at 1.5–2°C. In this study, we simulated the climate mitigation impacts of adding alkalinity (equivalent to 667,950 metric ton NaOH yr−1) in Unimak Pass on the southern boundary of the Bering Sea. We found that adding alkalinity can accelerate the ocean CO2 uptake and storage and mitigate ocean acidification near the alkalinity addition. It takes about 3.6 months for the Ocean alkalinity enhancement impacted area to take up the extra CO2. The naturally cold and carbon rich water in the Bering Sea and the tendency of Bering Sea surface waters to linger near the ocean surface without mixing into the subsurface ocean both lead to high CDR efficiencies (>96%) from alkalinity additions in the Bering Sea. However, even with high efficiency, it would take >8,000 alkalinity additions of the kind we simulated to be operating by the year 2100 to meet the target to stabilize global temperatures within the targeted range.

Continue reading ‘Simulated impact of ocean alkalinity enhancement on atmospheric CO2 removal in the Bering Sea’

Alkalinity and nitrate dynamics reveal dominance of anammox in a hyper-turbid estuary

Total alkalinity (TA) regulates the oceanic storage capacity of atmospheric CO2. TA is produced along two general pathways, weathering reactions and anaerobic respiration of organic matter, e.g., by denitrification, the anaerobic reduction of nitrate (NO3-) to elemental nitrogen (N2). Anammox, is another anaerobic pathway, yields N2 as its terminal product via comproportionation of ammonium (NH4+) and nitrite (NO2-); this is, however, without release of alkalinity as a byproduct. In order to investigate these two nitrate / nitrite respiration pathways and their resulting impact on TA generation, we sampled the highly turbid estuary of the Ems River, discharging into the North Sea in June 2020. We sampled a transect from the Wadden Sea to the upper tidal estuary, five vertical profiles during ebb tide, and fluid mud for incubation experiments in the hyper-turbid tidal river. The data reveal a strong increase of TA and DIC in the tidal river, where stable nitrate isotopes indicate water column denitrification as the dominant pathway. In the fluid mud of the tidal river, the TA data imply only low denitrification rates, with the majority of the N2 being produced by anammox (> 90 %). The relative abundances of anammox and denitrification, respectively, thus exert a major control on the CO2 storage capacity of adjacent coastal waters.

Continue reading ‘Alkalinity and nitrate dynamics reveal dominance of anammox in a hyper-turbid estuary’

Chapter 11 – Mediterranean Sea general biogeochemistry

This chapter gives an overview of the general biogeochemistry in the Mediterranean Sea explaining the particularities of the main biogeochemical variables and the physical, biological, and geochemical processes driving their distribution in the main basins of this marginal sea. Each subsection focuses on one essential variable, starting from dissolved oxygen and following inorganic nutrients, dissolved organic carbon and the CO2 system. A brief overview on the utility of those biogeochemical variables to identify water masses is also given. The chapter concludes with a summary of the projections and threats on biogeochemistry in the Mediterranean Sea under different future climate change scenarios.

Continue reading ‘Chapter 11 – Mediterranean Sea general biogeochemistry’

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