Estimation of anthropogenic CO2 inventories in the ocean

The source and accumulation of anthropogenic carbon in the U.S. East Coast

The ocean has absorbed anthropogenic carbon dioxide (Canthro) from the atmosphere and played an important role in mitigating global warming. However, how much Canthro is accumulated in coastal oceans and where it comes from have rarely been addressed with observational data. Here, we use a high-quality carbonate dataset (1996-2018) in the U.S. East Coast to address these questions. Our work shows that the offshore slope waters have the highest Canthro accumulation changes (ΔCanthro) consistent with water mass age and properties. From offshore to nearshore, ΔCanthro decreases with salinity to near zero in the subsurface, indicating no net increase in the export of Canthro from estuaries and wetlands. Excesses over the conservative mixing baseline also reveal an uptake of Canthro from the atmosphere within the shelf. Our analysis suggests that the continental shelf exports most of its absorbed Canthro from the atmosphere to the open ocean and acts as an essential pathway for global ocean Canthro storage and acidification.

Time series (2003-15) analysis of selected physicochemical parameters in Indian Ocean: Cumulative impacts prediction on coral bleaching using machine learning

Coral bleaching is an important ecological threat worldwide, as the coral ecosystem supports a rich marine biodiversity to survive. Sea surface temperature was considered a major culprit; however, later it was observed that other water parameters like pH, tCO2, fCO2, salinity, dissolved oxygen, etc. also play a significant role in bleaching. In the present study, all these parameters of the Indian Ocean area for 15 years (2003-2017) were collected and analysed using machine learning language. The main aim is to see the cumulative impacts of various ocean parameters on coral bleaching. Introducing machine learning in environmental impact assessment studies is a new approach, and the prediction of coral bleaching using simulation of physico-chemical parameters interactions shows 94.4 % accuracy for the prediction of the future bleaching event. This study can be probably the first step in the application of the machine learning language for the prediction of coral bleaching in the field of marine science.

Development of SPEEK-Coated IrOx Sensor for High-Biomass Aquatic pH Monitoring

Instability is a key challenge for current pH sensors in practical applications, especially in aquatic environments with high biomass and redox substances. Herein, we present a novel approach that uses a highly stable IrOx sensing layer enveloped in a composite film of SPEEK doped with a silicon-stabilized ionic liquid (SP-IrOx). This design mitigates drift due to sensitive layer variations and minimizes interference from complex external conditions. After exhibiting robustness under moderately reducing conditions caused by S2-, I-, and ascorbic acid, the SP-IrOx sensor's efficacy was validated through real-time pH measurements in demanding aquatic settings. These included laboratory algal culture medium, sediment substrates, and mussel aquaculture areas. The sensor sustained accuracy and stability over extended periods of 6-8 days when compared to calibrated commercial electrodes. The deviations from reference samples were minimal, with a variance of no more than 0.03 pH units in mussel aquaculture areas (n = 17) and 0.07 pH units in an algal culture medium (n = 37). As a potentiometric, this solid-state electrode features a compact structure and low energy consumption, making it an economical and low-maintenance solution for precise pH monitoring in diverse challenging environments with high biomass and turbidity.

Storage and redistribution of anthropogenic CO2 in the western North Pacific: The role of subtropical mode water transportation

Oceanic uptake and storage of anthropogenic CO2 (CANT) are regulated by ocean circulation and ventilation. To decipher the storage and redistribution of CANT in the western North Pacific, where a major CANT sink develops, we investigated the water column carbonate system, dissolved inorganic radiocarbon and ancillary parameters in May and August 2018, spanning the Kuroshio Extension (KE, 35-39 °N), Kuroshio Recirculation (KR, 27-35 °N) and subtropical (21-27 °N) zones. Water column CANT inventories were estimated to be 40.5 ± 1.1 mol m-2 in the KR zone and 37.2 ± 0.9 mol m-2 in the subtropical zone. In comparison with historical data obtained in 2005, relatively high rates of increase of the CANT inventory of 1.05 ± 0.20 and 1.03 ± 0.12 mol m-2 yr-1 in the recent decade were obtained in the KR and subtropical zones, respectively. Our water-mass-based analyses suggest that formation and transport of subtropical mode water dominate the deep penetration, storage, and redistribution of CANT in those two regions. In the KE zone, however, both the water column CANT inventory and the decadal CANT accumulation rate were small and uncertain owing to the dynamic hydrology, where the naturally uplifting isopycnal surfaces make CANT penetration relatively shallow. The findings of this study improve the understanding of the spatiotemporal variations of CANT distribution, storage, and transport in the western North Pacific.

Genome Study of α-, β-, and γ-Carbonic Anhydrases from the Thermophilic Microbiome of Marine Hydrothermal Vent Ecosystems

Carbonic anhydrases (CAs) are metalloenzymes that can help organisms survive in hydrothermal vents by hydrating carbon dioxide (CO₂). In this study, we focus on alpha (α), beta (β), and gamma (γ) CAs, which are present in the thermophilic microbiome of marine hydrothermal vents. The coding genes of these enzymes can be transferred between hydrothermal-vent organisms via horizontal gene transfer (HGT), which is an important tool in natural biodiversity. We performed big data mining and bioinformatics studies on α-, β-, and γ-CA coding genes from the thermophilic microbiome of marine hydrothermal vents. The results showed a reasonable association between thermostable α-, β-, and γ-CAs in the microbial population of the hydrothermal vents. This relationship could be due to HGT. We found evidence of HGT of α- and β-CAs between Cycloclasticus sp., a symbiont of Bathymodiolus heckerae, and an endosymbiont of Riftia pachyptila via Integrons. Conversely, HGT of β-CA genes from the endosymbiont Tevnia jerichonana to the endosymbiont Riftia pachyptila was detected. In addition, Hydrogenovibrio crunogenus SP-41 contains a β-CA gene on genomic islands (GIs). This gene can be transferred by HGT to Hydrogenovibrio sp. MA2-6, a methanotrophic endosymbiont of Bathymodiolus azoricus, and a methanotrophic endosymbiont of Bathymodiolus puteoserpentis. The endosymbiont of R. pachyptila has a γ-CA gene in the genome. If α- and β-CA coding genes have been derived from other microorganisms, such as endosymbionts of T. jerichonana and Cycloclasticus sp. as the endosymbiont of B. heckerae, through HGT, the theory of the necessity of thermostable CA enzymes for survival in the extreme ecosystem of hydrothermal vents is suggested and helps the conservation of microbiome natural diversity in hydrothermal vents. These harsh ecosystems, with their integral players, such as HGT and endosymbionts, significantly impact the enrichment of life on Earth and the carbon cycle in the ocean.

Nuclear Reprocessing Tracers Illuminate Flow Features and Connectivity Between the Arctic and Subpolar North Atlantic Oceans

Releases of anthropogenic radionuclides from European nuclear fuel reprocessing plants enter the surface circulation of the high-latitude North Atlantic and are transported northward into the Arctic Ocean and southward from the Nordic Seas into the deep North Atlantic, thereby providing tracers of water circulation, mixing, ventilation, and deep-water formation. Early tracer studies focused on ¹³⁷Cs, which revealed some of the first significant insights into the Arctic Ocean circulation, while more recent work has benefited from advances in accelerator mass spectrometry to enable the measurement of the conservative, long-lived radionuclide tracers ¹²⁹I and ²³⁶U. The latest studies of these tracers, supported by simulations using the North Atlantic-Arctic Ocean-Sea Ice Model (NAOSIM) and enhanced by the use of transit time distributions to more precisely accommodate mixing, have provided a rich inventory of transport data for circulation in the Arctic and North Atlantic Oceans that are of great importance to global thermohaline circulation and climate.

Direct Air Capture of CO2 through Carbonate Alkalinity Generated by Phytoplankton Nitrate Assimilation

Despite the consensus that keeping global temperature rise within 1.5 °C above pre-industrial level by 2100 reduces the chance for climate change to reach the point of no return, the newest Intergovernmental Panel on Climate Change (IPCC) report warns that the existing commitment of greenhouse gas emission reduction is only enough to contain the warming to 3-4 °C by 2100. The harsh reality not only calls for speedier deployment of existing CO₂ reduction technologies but demands development of more cost-efficient carbon removal strategies. Here we report an ocean alkalinity-based CO₂ sequestration scheme, taking advantage of proton consumption during nitrate assimilation by marine photosynthetic microbes, and the ensuing enhancement of seawater CO₂ absorption. Benchtop experiments using a native marine phytoplankton community confirmed pH elevation from ~8.2 to ~10.2 in seawater, within 3-5 days of microbial culture in nitrate-containing media. The alkaline condition was able to sustain at continued nutrient supply but reverted to normalcy (pH ~8.2-8.4) once the biomass was removed. Measurements of δ¹³C in the dissolved inorganic carbon revealed a significant atmospheric CO₂ contribution to the carbonate alkalinity in the experimental seawater, confirming the occurrence of direct carbon dioxide capture from the air. Thermodynamic calculation shows a theoretical carbon removal rate of ~0.13 mol CO₂/L seawater, if the seawater pH is allowed to decrease from 10.2 to 8.2. A cost analysis (using a standard bioreactor wastewater treatment plant as a template for CO₂ trapping, and a modified moving-bed biofilm reactor for nitrate recycling) indicated that a 1 Mt CO₂/year operation is able to perform at a cost of ~$40/tCO₂, 2.5-5.5 times cheaper than that offered by any of the currently available direct air capture technologies, and more in line with the price of $25-30/tCO₂ suggested for rapid deployment of large-scale CCS systems.

Role of oceanic abiotic carbonate precipitation in future atmospheric CO2 regulation

The oceans play a major role in the earth’s climate by regulating atmospheric CO₂. While oceanic primary productivity and organic carbon burial sequesters CO₂ from the atmosphere, precipitation of CaCO₃ in the sea returns CO₂ to the atmosphere. Abiotic CaCO₃ precipitation in the form of aragonite is potentially an important feedback mechanism for the global carbon cycle, but this process has not been fully quantified. In a sediment-trap study conducted in the southeastern Mediterranean Sea, one of the fastest warming and most oligotrophic regions in the ocean, we quantify for the first time the flux of inorganic aragonite in the water column. We show that this process is kinetically induced by the warming of surface water and prolonged stratification resulting in a high aragonite saturation state (Ω_Ar ≥ 4). Based on these relations, we estimate that abiotic aragonite calcification may account for 15 ± 3% of the previously reported CO₂ efflux from the sea surface to the atmosphere in the southeastern Mediterranean. Modelled predictions of sea surface temperature and Ω_Ar suggest that this process may weaken in the future ocean, resulting in increased alkalinity and buffering capacity of atmospheric CO₂.

Spatial and temporal trends of anthropogenic carbon storage in typical marginal seas along the Asia continent in the northern hemisphere

Global climate change is an indisputable fact, and anthropogenic disturbances are the likely driving mechanisms; moreover, marginal seas tend to respond faster than the global ocean. In this study, the transit time distribution method was used to estimate the anthropogenic carbon (C_ant) in the typical marginal seas along the west side of North Pacific Ocean and the Arctic Ocean. From the South China Sea (SCS) to the Arctic Ocean (AO), the range of C_ant storage gradually increased with latitude. The maximum and minimum rates of ~0.6 mol C·m^-2·yr^-1, and ~0.2 mol C·m^-2·yr^-1 were seen in the AO and SCS, respectively. In the short term, warming and decline of ice cover may promote the transfer of excess CO₂ from the atmosphere to the water interior; but on a longer time scale, a positive feedback (i.e., reduced CO2 absorption) may occur due to warming. Accordingly, the AO will likely no longer be a CO₂ sink in the future when the sea ice disappears completely.

The impact of mobile demersal fishing on carbon storage in seabed sediments

Subtidal marine sediments are one of the planet's primary carbon stores and strongly influence the oceanic sink for atmospheric CO₂ . By far the most widespread human activity occurring on the seabed is bottom trawling/dredging for fish and shellfish. A global first-order estimate suggested mobile demersal fishing activities may cause 0.16-0.4 Gt of organic carbon (OC) to be remineralized annually from seabed sediment carbon stores (Sala et al., 2021). There are, however, many uncertainties in this calculation. Here, we discuss the potential drivers of change in seabed sediment OC stores due to mobile demersal fishing activities and conduct a literature review, synthesizing studies where this interaction has been directly investigated. Under certain environmental settings, we hypothesize that mobile demersal fishing would reduce OC in seabed stores due to lower production of flora and fauna, the loss of fine flocculent material, increased sediment resuspension, mixing and transport and increased oxygen exposure. Reductions would be offset to varying extents by reduced faunal bioturbation and community respiration, increased off-shelf transport and increases in primary production from the resuspension of nutrients. Studies which directly investigated the impact of demersal fishing on OC stocks had mixed results. A finding of no significant effect was reported in 61% of 49 investigations; 29% reported lower OC due to fishing activities, with 10% reporting higher OC. In relation to remineralization rates within the seabed, four investigations reported that demersal fishing activities decreased remineralization, with three reporting higher remineralization rates. Patterns in the environmental and experimental characteristics between different outcomes were largely indistinct. More evidence is urgently needed to accurately quantify the impact of anthropogenic physical disturbance on seabed carbon in different environmental settings and to incorporate full evidence-based carbon considerations into global seabed management.

Enhanced mercury reduction in the South Atlantic Ocean during carbon remineralization

Mercury (Hg) in seawater is subject to interconversions via (photo)chemical and (micro)biological processes that determine the extent of dissolved gaseous mercury (DGM) (re)emission and the production of monomethylmercury. We investigated Hg speciation in the South Atlantic Ocean on a GEOTRACES cruise along a 40°S section between December 2011 and January 2012 (354 samples collected at 24 stations from surface to 5250 m maximum depth). Using statistical analysis, concentrations of methylated mercury (MeHg, geometric mean 35.4 fmol L^-1) were related to seawater temperature, salinity, and fluorescence. DGM concentrations (geometric mean 0.17 pmol L^-1) were related to water column depth, concentrations of macronutrients and dissolved inorganic carbon (DIC). The first-ever observed linear correlation between DGM and DIC obtained from high-resolution data indicates possible DGM production by organic matter remineralization via biological or dark abiotic reactions. DGM concentrations projected from literature DIC data using the newly discovered DGM-DIC relationship agreed with published DGM observations.

The role of soils in the regulation of ocean acidification

Soils play an important role in mediating chemical weathering reactions and carbon transfer from the land to the ocean. Proposals to increase the contribution of alkalinity to the oceans through 'enhanced weathering' as a means to help prevent climate change are gaining increasing attention. This would augment the existing connection between the biogeochemical function of soils and alkalinity levels in the ocean. The feasibility of enhanced weathering depends on the combined influence of what minerals are added to soils, the formation of secondary minerals in soils and the drainage regime, and the partial pressure of respired CO2 around the dissolving mineral. Increasing the alkalinity levels in the ocean through enhanced weathering could help to ameliorate the effects of ocean acidification in two ways. First, enhanced weathering would slightly elevate the pH of drainage waters, and the receiving coastal waters. The elevated pH would result in an increase in carbonate mineral saturation states, and a partial reversal in the effects of elevated CO2. Second, the increase in alkalinity would help to replenish the ocean's buffering capacity by maintaining the 'Revelle Factor', making the oceans more resilient to further CO2 emissions. However, there is limited research on the downstream and oceanic impacts of enhanced weathering on which to base deployment decisions. This article is part of the theme issue 'The role of soils in delivering Nature's Contributions to People'.

Perspective: Increasing blue carbon around Antarctica is an ecosystem service of considerable societal and economic value worth protecting

Precautionary conservation and cooperative global governance are needed to protect Antarctic blue carbon: the world's largest increasing natural form of carbon storage with high sequestration potential. As patterns of ice loss around Antarctica become more uniform, there is an underlying increase in carbon capture-to-storage-to-sequestration on the seafloor. The amount of carbon captured per unit area is increasing and the area available to blue carbon is also increasing. Carbon sequestration could further increase under moderate (+1°C) ocean warming, contrary to decreasing global blue carbon stocks elsewhere. For example, in warmer waters, mangroves and seagrasses are in decline and benthic organisms are close to their physiological limits, so a 1°C increase in water temperature could push them above their thermal tolerance (e.g. bleaching of coral reefs). In contrast, on the basis of past change and current research, we expect that Antarctic blue carbon could increase by orders of magnitude. The Antarctic seafloor is biophysically unique and the site of carbon sequestration, the benthos, faces less anthropogenic disturbance than any other ocean continental shelf environment. This isolation imparts both vulnerability to change, and an avenue to conserve one of the world's last biodiversity refuges. In economic terms, the value of Antarctic blue carbon is estimated at between £0.65 and £1.76 billion (~2.27 billion USD) for sequestered carbon in the benthos around the continental shelf. To balance biodiversity protection against society's economic objectives, this paper builds on a proposal incentivising protection by building a 'non-market framework' via the 2015 Paris Agreement to the United Nations Framework Convention on Climate Change. This could be connected and coordinated through the Antarctic Treaty System to promote and motivate member states to value Antarctic blue carbon and maintain scientific integrity and conservation for the positive societal values ingrained in the Antarctic Treaty System.

Global Ocean Spectrophotometric pH Assessment: Consistent Inconsistencies

Ocean acidification (OA)-or the decrease in seawater pH resulting from ocean uptake of CO₂ released by human activities-stresses ocean ecosystems and is recognized as a Climate and Sustainable Development Goal Indicator that needs to be evaluated and monitored. Monitoring OA-related pH changes requires a high level of precision and accuracy. The two most common ways to quantify seawater pH are to measure it spectrophotometrically or to calculate it from total alkalinity (TA) and dissolved inorganic carbon (DIC). However, despite decades of research, small but important inconsistencies remain between measured and calculated pH. To date, this issue has been circumvented by examining changes only in consistently measured properties. Currently, the oceanographic community is defining new observational strategies for OA and other key aspects of the ocean carbon cycle based on novel sensors and technologies that rely on validation against data records and/or synthesis products. Comparison of measured spectrophotometric pH to calculated pH from TA and DIC measured during the 2000s and 2010s eras reveals that (1) there is an evolution toward a better agreement between measured and calculated pH over time from 0.02 pH units in the 2000s to 0.01 pH units in the 2010s at pH > 7.6; (2) a disagreement greater than 0.01 pH units persists in waters with pH < 7.6, and (3) inconsistencies likely stem from variations in the spectrophotometric pH standard operating procedure (SOP). A reassessment of pH measurement and calculation SOPs and metrology is urgently needed.

Lead seasonality in humans, animals, and the natural environment

Lead adversely impacts the health of humans, animals, and the natural environment. Higher lead burdens in warm weather occur in humans, domesticated and wild animals; land and water species; urban and rural, developed and pristine environments. The array of evidence suggests that lead seasonality is multifactorial within the natural world, including humans. Seasonally higher temperatures, solar radiation, humidity and anthropogenic pollution result in lower pH (acidification) in air, water and soil. Environmental acidification increases lead's bioavailability and mobility thus intensifying human, animal and plant exposures. In addition, lead seasonality in the biosphere is influenced by higher growth rates, slightly increased exposures, and more Vitamin D metabolism. Methodologically, we applied a One Health perspective to EPA's Integrated Science Assessments of Lead to review the published literature, supplemented with subsequent and related publications to assess data on the seasonality of lead exposure across species and through the earth's systems. Our integrated assessment suggests that: 1) 'Seasonality' is a multifactorial, terrestrial phenomenon affecting the natural world; human activities have exacerbated natural cyclicities that impact lead exposures across species. 2) To be sustainable, human lead remediation strategies must consider the total environment. 3) Global warming and climate change events may increase lead exposures and toxicity to all species throughout the natural environment.

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

PURPOSE OF REVIEW

We summarize recent progress on autonomous observations of ocean carbonate chemistry and the development of a network of sensors capable of observing carbonate processes at multiple temporal and spatial scales.

RECENT FINDINGS

The development of versatile pH sensors suitable for both deployment on autonomous vehicles and in compact, fixed ecosystem observatories has been a major development in the field. The initial large-scale deployment of profiling floats equipped with these new pH sensors in the Southern Ocean has demonstrated the feasibility of a global autonomous open-ocean carbonate observing system.

SUMMARY

Our developing network of autonomous carbonate observations is currently targeted at surface ocean CO₂ fluxes and compact ecosystem observatories. New integration of developed sensors on gliders and surface vehicles will increase our coastal and regional observational capability. Most autonomous platforms observe a single carbonate parameter, which leaves us reliant on the use of empirical relationships to constrain the rest of the carbonate system. Sensors now in development promise the ability to observe multiple carbonate system parameters from a range of vehicles in the near future.

The Variable Southern Ocean Carbon Sink

The CO₂ uptake by the Southern Ocean (<35°S) varies substantially on all timescales and is a major determinant of the variations of the global ocean carbon sink. Particularly strong are the decadal changes characterized by a weakening period of the Southern Ocean carbon sink in the 1990s and a rebound after 2000. The weakening in the 1990s resulted primarily from a southward shift of the westerlies that enhanced the upwelling and outgassing of respired (i.e., natural) CO₂. The concurrent reduction in the storage rate of anthropogenic CO₂ in the mode and intermediate waters south of 35°S suggests that this shift also decreased the uptake of anthropogenic CO₂. The rebound and the subsequent strong, decade-long reinvigoration of the carbon sink appear to have been driven by cooling in the Pacific Ocean, enhanced stratification in the Atlantic and Indian Ocean sectors, and a reduced overturning. Current-generation ocean models generally do not reproduce these variations and are poorly skilled at making decadal predictions in this region.

Caprellid amphipods (Caprella spp.) are vulnerable to both physiological and habitat-mediated effects of ocean acidification

Ocean acidification (OA) is one of the most significant threats to marine life, and is predicted to drive important changes in marine communities. Although OA impacts will be the sum of direct effects mediated by alterations of physiological rates and indirect effects mediated by shifts in species interactions and biogenic habitat provision, direct and indirect effects are rarely considered together for any given species. Here, we assess the potential direct and indirect effects of OA on a ubiquitous group of crustaceans: caprellid amphipods (Caprella laeviuscula and Caprella mutica). Direct physiological effects were assessed by measuring caprellid heart rate in response to acidification in the laboratory. Indirect effects were explored by quantifying caprellid habitat dependence on the hydroid Obelia dichotoma, which has been shown to be less abundant under experimental acidification. We found that OA resulted in elevated caprellid heart rates, suggestive of increased metabolic demand. We also found a strong, positive association between caprellid population size and the availability of OA-vulnerable O. dichotoma, suggesting that future losses of biogenic habitat may be an important indirect effect of OA on caprellids. For species such as caprellid amphipods, which have strong associations with biogenic habitat, a consideration of only direct or indirect effects could potentially misestimate the full impact of ocean acidification.

Spatial and temporal variations of particulate organic carbon in the Yellow-Bohai Sea over 2002-2016

The Yellow-Bohai Sea (YBS) is a typical marginal sea in the Northwest Pacific Ocean; however, little is known about the dynamics of particulate organic carbon (POC) and underlying mechanisms. Here, we analyze the spatial and temporal variations of surface POC derived from MODIS-Aqua during 2002–2016. Overall, POC is higher in the Bohai Sea (315–588 mg m⁻³) than in the Yellow Sea (181–492 mg m⁻³), and higher in the nearshore than in the offshore. Surface POC is highest in spring in the YBS, and lowest in winter (summer) in the Bohai Sea (the Yellow Sea). The spatial and seasonal patterns of POC are due to combined influences of primary productivity, water exchange, sediment resuspension and terrestrial inputs. Surface POC shows an overall decreasing trend prior to 2012 followed by an upward trend until 2015 in the YBS, which is almost opposite to chlorophyll; the decrease (increase) may result from strengthened (weakened) water exchange with the East China Sea through the Yellow Sea Warm Current. Declined terrestrial runoff is also partly responsible for the decrease prior to 2012. Our study suggests that water exchange and sediment resuspension are dominant factors regulating the spatial and temporal variability of POC in the YBS.

Structural and physiological responses of Halodule wrightii to ocean acidification

Coastal areas face high variability of seawater pH. Ocean acidification (OA) and local stressors are enhancing this variability, which poses a threat to marine life. However, these organisms present potential phenotypic plasticity that can offer physiological and structural tools to survive in these extreme conditions. In this study, we evaluated the effects of elevated CO₂ levels and consequent pH reduction on the physiology, anatomy and ultrastructure of the seagrass Halodule wrightii. A mesocosm study was conducted in an open system during a 30-day experiment, where different concentrations of CO₂ were simulated following the natural variability observed in coastal reef systems. This resulted in four experimental conditions simulating the (i) environmental pH (control condition, without CO₂ addition) and (ii) reduced pH by - 0.3 units, (iii) - 0.6 units and (iv) - 0.9 units, in relation to the field condition. The evaluated population only suffered reduced optimum quantum yield (Y(II)), leaf width and cross-section area under the lowest CO₂ addition (- 0.3 pH units) after 30 days of experiment. This fitness commitment should be related to carbon concentration mechanisms present in the evaluated species. For the highest CO₂ level, H. wrightii demonstrated a capacity to compensate any negative effect of the lowest pH. Our results suggest that the physiological behaviour of this primary producer is driven by the interactions among OA and environmental factors, like irradiance and nutrient availability. The observed behaviour highlights that high-frequency pH variability and multifactorial approaches should be applied, and when investigating the impact of OA, factors like irradiance, nutrient availability and temperature must be considered as well.

Marine Microbial Gene Abundance and Community Composition in Response to Ocean Acidification and Elevated Temperature in Two Contrasting Coastal Marine Sediments

Marine ecosystems are exposed to a range of human-induced climate stressors, in particular changing carbonate chemistry and elevated sea surface temperatures as a consequence of climate change. More research effort is needed to reduce uncertainties about the effects of global-scale warming and acidification for benthic microbial communities, which drive sedimentary biogeochemical cycles. In this research, mesocosm experiments were set up using muddy and sandy coastal sediments to investigate the independent and interactive effects of elevated carbon dioxide concentrations (750 ppm CO2) and elevated temperature (ambient +4°C) on the abundance of taxonomic and functional microbial genes. Specific quantitative PCR primers were used to target archaeal, bacterial, and cyanobacterial/chloroplast 16S rRNA in both sediment types. Nitrogen cycling genes archaeal and bacterial ammonia monooxygenase (amoA) and bacterial nitrite reductase (nirS) were specifically targeted to identify changes in microbial gene abundance and potential impacts on nitrogen cycling. In muddy sediment, microbial gene abundance, including amoA and nirS genes, increased under elevated temperature and reduced under elevated CO2 after 28 days, accompanied by shifts in community composition. In contrast, the combined stressor treatment showed a non-additive effect with lower microbial gene abundance throughout the experiment. The response of microbial communities in the sandy sediment was less pronounced, with the most noticeable response seen in the archaeal gene abundances in response to environmental stressors over time. 16S rRNA genes (amoA and nirS) were lower in abundance in the combined stressor treatments in sandy sediments. Our results indicated that marine benthic microorganisms, especially in muddy sediments, are susceptible to changes in ocean carbonate chemistry and seawater temperature, which ultimately may have an impact upon key benthic biogeochemical cycles.

Natural Variability and Anthropogenic Trends in the Ocean Carbon Sink

Since preindustrial times, the ocean has removed from the atmosphere 41% of the carbon emitted by human industrial activities. Despite significant uncertainties, the balance of evidence indicates that the globally integrated rate of ocean carbon uptake is increasing in response to increasing atmospheric CO₂ concentrations. The El Niño-Southern Oscillation in the equatorial Pacific dominates interannual variability of the globally integrated sink. Modes of climate variability in high latitudes are correlated with variability in regional carbon sinks, but mechanistic understanding is incomplete. Regional sink variability, combined with sparse sampling, means that the growing oceanic sink cannot yet be directly detected from available surface data. Accurate and precise shipboard observations need to be continued and increasingly complemented with autonomous observations. These data, together with a variety of mechanistic and diagnostic models, are needed for better understanding, long-term monitoring, and future projections of this critical climate regulation service.

Changes in Ocean Heat, Carbon Content, and Ventilation: A Review of the First Decade of GO-SHIP Global Repeat Hydrography

Global ship-based programs, with highly accurate, full water column physical and biogeochemical observations repeated decadally since the 1970s, provide a crucial resource for documenting ocean change. The ocean, a central component of Earth's climate system, is taking up most of Earth's excess anthropogenic heat, with about 19% of this excess in the abyssal ocean beneath 2,000 m, dominated by Southern Ocean warming. The ocean also has taken up about 27% of anthropogenic carbon, resulting in acidification of the upper ocean. Increased stratification has resulted in a decline in oxygen and increase in nutrients in the Northern Hemisphere thermocline and an expansion of tropical oxygen minimum zones. Southern Hemisphere thermocline oxygen increased in the 2000s owing to stronger wind forcing and ventilation. The most recent decade of global hydrography has mapped dissolved organic carbon, a large, bioactive reservoir, for the first time and quantified its contribution to export production (∼20%) and deep-ocean oxygen utilization. Ship-based measurements also show that vertical diffusivity increases from a minimum in the thermocline to a maximum within the bottom 1,500 m, shifting our physical paradigm of the ocean's overturning circulation.

In situ sensor technology for simultaneous spectrophotometric measurements of seawater total dissolved inorganic carbon and pH

A new, in situ sensing system, Channelized Optical System (CHANOS), was recently developed to make high-resolution, simultaneous measurements of total dissolved inorganic carbon (DIC) and pH in seawater. Measurements made by this single, compact sensor can fully characterize the marine carbonate system. The system has a modular design to accommodate two independent, but similar measurement channels for DIC and pH. Both are based on spectrophotometric detection of hydrogen ion concentrations. The pH channel uses a flow-through, sample-indicator mixing design to achieve near instantaneous measurements. The DIC channel adapts a recently developed spectrophotometric method to achieve flow-through CO2 equilibration between an acidified sample and an indicator solution with a response time of only ∼ 90 s. During laboratory and in situ testing, CHANOS achieved a precision of ±0.0010 and ± 2.5 μmol kg(-1) for pH and DIC, respectively. In situ comparison tests indicated that the accuracies of the pH and DIC channels over a three-week time-series deployment were ± 0.0024 and ± 4.1 μmol kg(-1), respectively. This study demonstrates that CHANOS can make in situ, climatology-quality measurements by measuring two desirable CO2 parameters, and is capable of resolving the CO2 system in dynamic marine environments.

Marine bacterial communities are resistant to elevated carbon dioxide levels

It is well established that the release of anthropogenic-derived CO2 into the atmosphere will be mainly absorbed by the oceans, with a concomitant drop in pH, a process termed ocean acidification. As such, there is considerable interest in how changes in increased CO2 and lower pH will affect marine biota, such as bacteria, which play central roles in oceanic biogeochemical processes. Set within an ecological framework, we investigated the direct effects of elevated CO2, contrasted with ambient conditions on the resistance and resilience of marine bacterial communities in a replicated temporal seawater mesocosm experiment. The results of the study strongly indicate that marine bacterial communities are highly resistant to the elevated CO2 and lower pH conditions imposed, as demonstrated from measures of turnover using taxa–time relationships and distance–decay relationships. In addition, no significant differences in community abundance, structure or composition were observed. Our results suggest that there are no direct effects on marine bacterial communities and that the bacterial fraction of microbial plankton holds enough flexibility and evolutionary capacity to withstand predicted future changes from elevated CO2 and subsequent ocean acidification.

Aquatic primary production in a high-CO2 world

Here, we provide a review of the direct effect of increasing CO2 on aquatic primary producers through its function as a source of carbon, focusing our analysis on the interpretation of this increase as an increase in the availability of a resource. This provides an interesting context to evaluate ecological and evolutionary theories relating to nutrient availability and leads us to: the assessment of theories about limitation of productivity and the integration of CO2 into the co-limitation paradigm; the prediction of community composition and of change in communities from known changes in the environment; and evaluation of the potential for evolutionary adaptation in conditions that increase growth.

High-frequency spectrophotometric measurements of total dissolved inorganic carbon in seawater

A new spectrophotometric method was developed to achieve continuous measurements of total dissolved inorganic carbon (DIC) in seawater. It uses a countercurrent flow design and a highly CO2-permeable membrane (Teflon AF 2400) to achieve flow-through CO2 equilibration between an acidified sample and an indicator solution with a fast response time of ~22 s. This method improves the spatiotemporal resolution by more than 1 order of magnitude compared to the existing spectrophotometric method. The flow-through equilibration allows for continuous (~1 Hz) detection and real-time data smoothing. The method had a short-term precision of ± 2.0 μmol kg(-1) for a given flow-through sample. It achieved a field precision of ± 3.6 μmol kg(-1) and successfully captured high DIC variability down to minute scales. Measurements by the new method over the typical range of oceanic DIC showed good agreement with measurements made by an established method (mean differences -1.6 to 0.3 μmol kg(-1) with 1σ ± 6.0-6.7 μmol kg(-1)). This level of precision and accuracy is comparable to that of the existing spectrophotometric method. The characteristics of the new method make it particularly suitable for high-frequency, submerged measurements required for mobile observing platforms in the ocean. It can also be adapted for high-frequency, spectrophotometric measurements of seawater CO2 fugacity.

Biological impacts of enhanced alkalinity in Carcinus maenas

Further steps are needed to establish feasible alleviation strategies that are able to reduce the impacts of ocean acidification, whilst ensuring minimal biological side-effects in the process. Whilst there is a growing body of literature on the biological impacts of many other carbon dioxide reduction techniques, seemingly little is known about enhanced alkalinity. For this reason, we investigated the potential physiological impacts of using chemical sequestration as an alleviation strategy. In a controlled experiment, Carcinus maenas were acutely exposed to concentrations of Ca(OH)2 that would be required to reverse the decline in ocean surface pH and return it to pre-industrial levels. Acute exposure significantly affected all individuals' acid-base balance resulting in slight respiratory alkalosis and hyperkalemia, which was strongest in mature females. Although the trigger for both of these responses is currently unclear, this study has shown that alkalinity addition does alter acid-base balance in this comparatively robust crustacean species.

Possible effects of global environmental changes on Antarctic benthos: a synthesis across five major taxa

Because of the unique conditions that exist around the Antarctic continent, Southern Ocean (SO) ecosystems are very susceptible to the growing impact of global climate change and other anthropogenic influences. Consequently, there is an urgent need to understand how SO marine life will cope with expected future changes in the environment. Studies of Antarctic organisms have shown that individual species and higher taxa display different degrees of sensitivity to environmental shifts, making it difficult to predict overall community or ecosystem responses. This emphasizes the need for an improved understanding of the Antarctic benthic ecosystem response to global climate change using a multitaxon approach with consideration of different levels of biological organization. Here, we provide a synthesis of the ability of five important Antarctic benthic taxa (Foraminifera, Nematoda, Amphipoda, Isopoda, and Echinoidea) to cope with changes in the environment (temperature, pH, ice cover, ice scouring, food quantity, and quality) that are linked to climatic changes. Responses from individual to the taxon-specific community level to these drivers will vary with taxon but will include local species extinctions, invasions of warmer-water species, shifts in diversity, dominance, and trophic group composition, all with likely consequences for ecosystem functioning. Limitations in our current knowledge and understanding of climate change effects on the different levels are discussed.

Observations of CFCs and SF6 as ocean tracers

An advantage of using chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6) as tracers of ocean circulation is that the time-dependent source functions permit calculation of rates for ocean processes. These compounds are also sensitive indicators highlighting interior ocean regions where surface-derived anomalies can be transported on timescales of decades. Significant applications for CFCs have been for the deep limb of the Atlantic meridional overturning circulation, upper ocean ventilation, and biogeochemical rates, including apparent oxygen utilization rates and anthropogenic CO2 inventories. Although CFCs have started to decrease in the atmosphere, SF6 continues to increase. There are benefits to measuring both CFCs and SF6: A large global CFC data set exists; CFCs are still increasing in older waters; SF6 expands estimates of age; and calculations of anthropogenic CO2 inventory are enhanced. Thus, the outlook for using CFCs as tracers for oceanic processes, and in particular in concert with SF6, remains very positive.