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Four species of shellfish, blue mussel (Mytilus galloprovincialis), Pacific abalone (Haliotis discus hannai), zhikong scallops (Chlamys farreri), and Pacific oyster (Crassostrea gigas), were exposed to decoupled carbonate system variables to investigate the impacts of different seawater carbonate parameters on the CO2 excretion process of mariculture shellfish. Six experimental groups with two levels of seawater pH (pH 8.1 and pH 7.7) and three levels of total alkalinity (TA = 1000, 2300, and 3600 μmol/kg, respectively) were established, while pH 8.1 and TA = 2300 μmol/kg was taken as control. Results showed that the CO2 excretion rates of these tested shellfish were significantly affected by the change in carbonate chemistry (P < 0.05). At the same TA level, animals incubated in the acidified group (pH 7.7) had a lower CO2 excretion rate than those in the control group (pH 8.1). In comparison, at the same pH level, the CO2 excretion rate increased when seawater TA level was elevated. No significant correlation between the CO2 excretion rate and seawater pCO2 levels (P > 0.05) was found; however, a significant correlation (P < 0.05) between CO2 excretion rate and TA-DIC (the difference between total alkalinity and dissolved inorganic carbon) was observed. Blue mussel has a significantly higher CO2 excretion rate than the other three species in the CO2 excretions per unit mass of soft parts, with no significant difference observed among these three species. However, in terms of CO2 excretion rate per unit mass of gills, abalone has the highest CO2 excretion rate, while significant differences were found between each species. Our studies indicate that the CO2 buffering capacity impacts the CO2 excretion rate of four shellfish species largely independent of pCO2. Since CO2 excretion is related to acid–base balancing, the results imply that the effects of other carbonate parameters, particularly the CO2 buffering capacity, should be studied to fully understand the mechanism of how acidification affects shellfish. Besides, the species difference in gill to soft parts proportion may contribute to the species difference in responding to ocean acidification. [ABSTRACT FROM AUTHOR]

The Western Tropical Atlantic Ocean (WTAO) is crucial for understanding CO2 dynamics due to inputs from major rivers (Amazon and Orinoco), substantial rainfall from the Intertropical Convergence Zone (ITCZ), and CO2-rich waters from equatorial upwelling. This study, spanning 1998 to 2018, utilized sea surface temperature (SST) and sea surface salinity (SSS) data from the PIRATA buoy at 8°N 38°W to reconstruct the surface marine carbonate system. Empirical models derived total alkalinity (TA) and dissolved inorganic carbon (DIC) from SSS, with subsequent estimation of pH and fCO2 from TA, DIC, SSS, and SST data. Linear trend analysis showed statistically significant temporal trends: DIC and fCO2 increased at a rate of 0.7 μmol kg-1 year-1 and 1.539 μatm year-1, respectively, and pH decreased at a rate of -0.001 pH units year-1, although DIC did not show any trend after data was de-seasoned. Rainfall analysis revealed distinct dry (July to December) and wet (January to June) seasons, aligning with lower and higher freshwater influence on the ocean surface, respectively. TA, DIC, and pH correlated positively with SSS, exhibiting higher values during the dry season and lower values during the wet season. Conversely, fCO2 correlated positively with SST, showcasing higher values during the wet season and lower values during the dry season. This emphasizes the influential roles of SSS and SST variability in CO2 solubility within the region. Finally, we have analysed the difference between TA and DIC (TA-DIC) as an indicator for ocean acidification and found a decreasing trend of -0.93 ± 0.02 mmol kg-1 year-1, reinforcing the reduction in the surface ocean buffering capacity in this area. All trends found for the region agree with data from other stations in the tropical and subtropical Atlantic Ocean. In conclusion, the use of empirical models proposed in this study has proven to help filling the gaps in marine carbonate system data in the Western Tropical Atlantic. [ABSTRACT FROM AUTHOR]

In the oligotrophic Southeastern Mediterranean Sea (SEMS), it has been shown that dissolved inorganic nutrient (DIN) from fresh submarine groundwater discharge (FSGD) enhance primary production in coastal waters. In this study pH, Total Alkalinity (TA) and DIN of seawater and fresh water in a sea‐cave in the northern part of the Israeli Mediterranean coast and a nearby contact spring, respectively, were measured during October 2018–March 2020. The results show gradients of measured salinity, TA, pH and DIN along the cave axis year‐round, suggesting that they are influenced by FSGD. The seawater near the back of the cave was supersaturated with respect to atmospheric CO2 nearly year‐round and there is a strong positive divergence from its regional open‐water thermal dependence, which suggests that FSGD is also a source of atmospheric CO2 in this region. Comparison of TA, salinity and pCO2 from the back of the sea cave to their corresponding values from an abrasion platform monitoring site, ca. 3 km south of the cave, suggests that FSGD is occurring along the entire shoreline in this region. Thus, despite the increased productivity due to FSGD mediated nutrient enrichment of adjacent coastal waters of the oligotrophic SEMS, they are still a source of atmospheric CO2 nearly year‐round. Finally, the apparent trends of seawater acidification (ΔpH/Δt = −0.006 yrs−1) and pCO2 increase (+8 ppmV yr−1) observed at the nearby monitoring site since 2013 are explained by increased groundwater recharge and resulting FSGD total alkalinity compared to dissolved inorganic carbon inputs (ΔTA/ΔDIC = 1:1.2). Plain Language Summary: Submarine groundwater discharge is an important source of dissolved inorganic nutrients that support organic matter production in the Mediterranean Sea. Furthermore, the contribution of fresh groundwater discharge to the alkalinity budget of the Mediterranean Sea compared to riverine inputs is also significant. Approximately half of the groundwater discharged into the Mediterranean originates from karst lithologies. In this study we followed the annual variations in seawater chemical properties in the Rosh‐Hanikra sea‐cave, located along the northern Mediterranean coast of Israel. The results suggest that fresh groundwater discharge supports the observed gradients of measured seawater chemistry along the cave axis year‐round with a stronger influence during the wintertime compared to summertime and that it helps maintain coastal waters as a source of atmospheric CO2 year‐round. Furthermore, the data suggest that submarine groundwater discharge, which is enriched in dissolved inorganic carbon increases the rate of ocean acidification associated with increasing atmospheric CO2 in the coastal waters of this region. Key Points: Fresh submarine groundwater discharge (FSGD) is apparent in seawater properties of the Rosh‐Hanikra Mediterranean Sea sea‐cave year‐roundFSGD into coastal waters in the Rosh‐Hanikra region causes them to be a source of atmospheric CO2 year‐roundFSGD augments seawater acidification along the northern Mediterranean coast of Israel despite nutrient enrichment and increased productivity [ABSTRACT FROM AUTHOR]

The Western Tropical Atlantic Ocean (WTAO) is crucial for understanding CO2 dynamics due to inputs from major rivers (Amazon and Orinoco), substantial rainfall from the Intertropical Convergence Zone (ITCZ), and CO2-rich waters from equatorial upwelling. This study, spanning 1998 to 2018, utilized sea surface temperature (SST) and sea surface salinity (SSS) data from the PIRATA buoy at 8°N 38°W to reconstruct the surface marine carbonate system. Empirical models derived total alkalinity (TA) and dissolved inorganic carbon (DIC) from SSS, with subsequent estimation of pH and fCO2 from TA, DIC, SSS, and SST data. Linear trend analysis showed statistically significant temporal trends: DIC and fCO2 increased at a rate of 0.7 µmol kg-1 year-1 and 1.539 µatm year-1, respectively, and pH decreased at a rate of -0.001 pH units year-1, although DIC did not show any trend after data was de-seasoned. Rainfall analysis revealed distinct dry (July to December) and wet (January to June) seasons, aligning with lower and higher freshwater influence on the ocean surface, respectively. TA, DIC, and pH correlated positively with SSS, exhibiting higher values during the dry season and lower values during the wet season. Conversely, fCO2 correlated positively with SST, showcasing higher values during the wet season and lower values during the dry season. This emphasizes the influential roles of SSS and SST variability in CO2 solubility within the region. Finally, we have analysed the difference between TA and DIC (TA-DIC) as an indicator for ocean acidification and found a decreasing trend of -0.93 ± 0.02 μmol kg-1 year-1, reinforcing the reduction in the surface ocean buffering capacity in this area. All trends found for the region agree with data from other stations in the tropical and subtropical Atlantic Ocean. In conclusion, the use of empirical models proposed in this study has proven to help filling the gaps in marine carbonate system data in the Western Tropical Atlantic.

Caulerpa is a widely distributed genus of chlorophytes (green macroalgae) which are important for their dietary, social and coastal ecosystem value. Ocean acidification (OA) threatens the future of marine ecosystems, favouring macroalgal species that could benefit from increased seawater carbon dioxide (CO2) concentrations. Most macroalgae species possess CO2 concentrating mechanisms (CCMs) that allow active uptake of bicarbonate (HCO3−). Those species without CCMs are restricted to using CO2, which is currently the least abundant species of dissolved inorganic carbon (DIC) in seawater. Thus, macroalgae without CCMs are predicted to be likely benefit from OA. Caulerpa is one of the rare few genera that have species both with and without CCMs. The two most common Caulerpa species in New Zealand are C. geminata (possesses a CCM) and C. brownii (non-CCM). We investigated the responses of growth, photo-physiology and DIC utilisation of C. geminata and C. brownii to four mean seawater pH treatments (8.03, 7.93, 7.83 and 7.63) that correspond to changes in pH driven by increases in pCO2 simulating future OA. There was a tendency for the mean growth rates for C. brownii (non-CCM) to increase under lower pH, and the growth rates of C. geminata (CCM) to decline with lower pH, although this was not statistically significant. However, this is likely because variability in growth rates also increased as seawater pH declined. There were few other differences in physiology of both species with pH, although there was tendency for greater preference for CO2 over HCO3− uptake in the CCM species with declining seawater pH. This study demonstrates that DIC-use alone does not predict macroalgal responses to OA. [ABSTRACT FROM AUTHOR]

Dissolved inorganic carbon (DIC) represents an important but poorly constrained form of lateral carbon flux to the oceans. With high precipitation rates, large glaciers, and dense temperate rainforest, Southeast Alaska plays a critical role in the transport of carbon to the Gulf of Alaska (GOA). Previous estimates of DIC flux across the Southeast Alaska Drainage Basin (SEAKDB) are poorly constrained in space and time. Our goal was to incorporate recent measurements of DIC concentrations with previous measurements from the U.S. Geological Survey in order to model the spatial and temporal patterns of riverine DIC transport from SEAK to the GOA. We aggregated DIC concentration measurements from 1957 to 2020 and associated measurements of mean daily discharge. We then constructed load estimation models to generate concentration predictions across 24 watersheds. By spatially matching measurements of DIC with SEAKDB watersheds, we extrapolated concentration predictions across 2,455 watersheds encompassing approximately 190,000 km2. Models were aggregated according to two factors, the presence of karst and the discharge regime. Finally, monthly flux predictions were generated for each watershed using predicted concentrations and runoff estimates from the Distributed Climate Water Balance Model. Mean annual DIC flux from the SEAKDB was 2.36 Tg C with an average yield of 12.52 g C m−2. Both karst presence and flow regimes modified DIC flux and speciation across coastal marine areas. The high resolution of DIC flux estimates will provide useful inputs for describing seasonal C dynamics, and further refines our understanding of C budgets in the Pacific temperate rainforest and the surrounding marine environment. Plain Language Summary: Understanding how carbon moves through ecosystems is critical in a changing climate. Dissolved carbon in aquatic environments plays a critical role in driving large‐scale processes such as ocean acidification, which represents a threat to many marine ecosystems. Despite the importance of understanding and accounting for carbon as it moves through the environment, the transfer of dissolved inorganic carbon (DIC) (such as carbon dioxide) from the terrestrial environment to the marine environment is often overlooked. Streams and rivers transfer carbon from land to ocean and represent a significant source of carbon to the marine environment, especially in areas that have large amounts of freshwater discharge such as Southeast Alaska. In this study, we created a model which generates predictions for how much DIC is entering the marine environment of Southeast Alaska. For each of 2,455 watersheds identified in this region we calculated monthly flux estimates which we grouped into large marine zones. Our overall flux estimate agrees well with previous estimates, but here our model provides more highly resolved spatial and temporal flux values which reveals seasonal and geographic patterns of DIC transfer from rivers to the marine environment. Key Points: Lateral flux of dissolved inorganic carbon (DIC) is not well resolved in time and space in Southeast AlaskaWe present robust models for DIC flux on a watershed basis that are spatially and temporally explicitCalculated DIC flux from Southeast Alaska watersheds is heavily influenced by streamflow regime and karst presence [ABSTRACT FROM AUTHOR]

Quantifying the flux of dissolved inorganic carbon (DIC) from the Chukchi shelf to the deep ocean (i.e., the continental shelf pump) is of great significance for understanding the carbon cycle and ocean acidification in the changing Arctic Ocean. Using a coupled ocean‐sea ice‐biogeochemical model, this study shows that the Pacific inflow drives the highly efficient continental shelf pump in the Chukchi Sea, with a multi‐year average of 828 Tg C‐DIC exported to deep basins during 1998–2015. The Pacific annual inflow carries 385 Tg C‐DIC to the Chukchi Sea in summertime (May–September) and 436 Tg C in wintertime (October–April). Downstream, DIC is mainly exported via Barrow Canyon (BC) (266 Tg C, 64%) in summer and along the shelf under the 100 m isobath (217 Tg C, 53%) in winter. This variability corresponds to the seasonal variation of local winds. The stronger winter northeasterly winds hinder the outflow through BC. In both seasons, interannual variability exists in the proportion of DIC exports through different pathways. The fraction of DIC export via BC has a significant correlation (r > 0.8) with the along‐canyon wind speed. This is further related to the sea level pressure (SLP) gradient between the Arctic and North Pacific. A weaker SLP gradient leads to weaker local northeasterly wind, and thus favors a higher proportion of DIC export via BC and lower DIC export through other pathways. This process plays a key role in the redistribution of shelf‐sourced DIC in deep basins of the Arctic Ocean. Plain Language Summary: The Chukchi Sea exports acidified water to the western Arctic Ocean Basin, where there is undergoing unprecedented ocean acidification in recent decades. Quantifying the dissolved inorganic carbon (DIC) flux from the Chukchi shelf to deep basins is of great significance for understanding the carbon cycle and ocean acidification in the changing Arctic Ocean. This study reveals that the Pacific inflow determines the annual DIC flux from the Chukchi Sea to deep basins by analyzing a hindcast simulation of a three‐dimensional ocean–sea ice–biogeochemical model. DIC is mainly exported via Barrow Canyon (BC) in summer and through the 100 m isobath pathway in winter. The year‐to‐year variations of proportions of DIC export through different pathways are regulated by the along‐canyon wind speeds in both summer and winter. The local wind is associated with the sea level pressure (SLP) gradient between the Arctic and North Pacific, in which the weaker SLP gradient leads to weaker northeasterly wind and subsequently favors a higher proportion of DIC export via BC. Thus, the change in atmospheric conditions will adjust the redistribution of DIC in the western Arctic Ocean Basin by affecting the ocean circulation structure. Key Points: Pacific inflow drives the continental shelf pump for dissolved inorganic carbon (DIC) (828 Tg C yr−1) exported from the Chukchi Sea to deep basinsIn summer, DIC is mainly exported via Barrow Canyon (BC) (266 Tg C) while through 100 m isobath pathway (217 Tg C) in winterInterannually, local winds regulate the proportion of DIC exports between BC and other pathways [ABSTRACT FROM AUTHOR]

This study combines measurements of dissolved inorganic carbon (DIC), total alkalinity (TA), pH, earth observation (EO), and ocean model products with deep learning to provide a good step forward in detecting changes in the ocean carbonate system parameters at a high spatial and temporal resolution in the North Atlantic region (Long. −61.00° to −50.04° W; Lat. 24.99° to 34.96° N). The in situ reference dataset that was used for this study provided discrete underway measurements of DIC, TA, and pH collected by M/V Equinox in the North Atlantic Ocean. A unique list of co-temporal and co-located global daily environmental drivers derived from independent sources (using satellite remote sensing, model reanalyses, empirical algorithms, and depth soundings) were collected for this study at the highest possible spatial resolution (0.04° × 0.04°). The resulting ANN-estimated DIC, TA, and pH obtained by deep learning shows a high correspondence when verified against observations. This study demonstrates how a select number of geophysical information derived from EO and model reanalysis data can be used to estimate and understand the spatiotemporal variability of the oceanic carbonate system at a high spatiotemporal resolution. Further methodological improvements are being suggested. [ABSTRACT FROM AUTHOR]

In a future ocean, dissolved organic carbon (DOC) release by seaweed has been considered a pathway for organic carbon that is not incorporated into growth under carbon dioxide (CO2) enrichment/ocean acidification (OA). To understand the influence of OA on seaweed DOC release, a 21- day experiment compared the physiological responses of three seaweed species, two which operate CO2 concentrating mechanisms (CCMs), Ecklonia radiata (C. Agardh) J. Agardh and Lenormandia marginata (Hooker F. and Harvey) and one that only uses CO2 (non-CCM), Plocamium cirrhosum (Turner) M.J. Wynne. These two groups (CCM and non-CCM) are predicted to respond differently to OA dependent on their affinities for Ci (defined as CO2 + bicarbonate, HCO3 −). Future ocean CO2 treatment did not drive changes to seaweed physiology—growth, Ci uptake, DOC production, photosynthesis, respiration, pigments, % tissue carbon, nitrogen, and C:N ratios—for any species, regardless of Ci uptake method. Our results further showed that Ci uptake method did not influence DOC release rates under OA. Our results show no benefit of elevated CO2 concentrations on the physiologies of the three species under OA and suggest that in a future ocean, photosynthetic CO2 fixation rates of these seaweeds will not increase with Ci concentration. [ABSTRACT FROM AUTHOR]

Multi‐decadal trends of the total dissolved inorganic carbon (DIC) concentrations and consequent ocean acidification in surface seawater were investigated on the basis of data from shipboard measurements conducted since 1996 along the 165°E repeat line in the western North Pacific. The observed trends exhibited clear meridional variabilities, with higher rates in the subtropical and tropical zones and lower rates in the subarctic zone, with a DIC range from +0.09 ± 0.14 to +1.64 ± 0.16 μmol kg−1 yr−1 and pH range from −0.0023 ± 0.0034 to −0.0281 ± 0.0059 decade−1. DIC and acidification trends were consistent with those expected from the atmospheric CO2 concentrations at nearly all latitudinal zones, but were significantly different at some latitudes. We attribute the significantly lower rates observed in the central western Subarctic Gyre and southern Subtropical Gyres primarily to the variabilities in upward DIC supply from the subsurface associated with the variability in ocean circulation. However, the higher rate observed to the south of the Kuroshio Extension appears to have been caused by the change in winter vertical mixing related to the change in its stable/unstable paths. Plain Language Summary: CO2 uptake by the ocean leads to a change in the state of CO2 chemistry in seawater, commonly referred to as "ocean acidification," and also plays an important role in mitigating global warming by reducing the atmospheric CO2 concentration. To project future ocean acidification under different socio‐economic scenarios, we need to understand the current trends in oceanic CO2 concentrations and acidification and the impact of climate change on these trends. Based on the data obtained from shipboard observations extending from the subarctic to equatorial zones in the western North Pacific over the last 25 years, we observed a clear meridional variability in ocean acidification trends, which was generally consistent with the trends expected from the increase in the atmospheric CO2 concentration. However, in some regions, the trends in oceanic CO2 concentration and acidification in surface seawater are likely influenced by the temporal variability in ocean circulation, in addition to the atmospheric CO2. These results suggest that the sensitivity of ocean acidification trends to climate change varies greatly between oceanic regions within the western North Pacific. Key Points: Meridional variability was found in the trends of dissolved inorganic carbon in the surface layer along the 165°E repeat lineExtremely slow rates of increase observed in the subarctic and tropics are attributed to ocean circulation variabilityA fast increase in the south of the Kuroshio Extension is likely associated with the variability in its path [ABSTRACT FROM AUTHOR]

The acidification of the marginal seawater was a more intricate process than the ocean. Although some studies have been done on seasonal acidification in the bottom water of Chinese marginal seas, research on surface water acidification has still been insufficient. We analyzed the acidification properties and controlling factors in the Bohai Sea (BS) and Yellow Sea (YS) surface water during April 2023. The observation showed that the average surface water pH of the BS, North Yellow Sea (NYS), and South Yellow Sea (SYS) were 8.09 ± 0.06, 8.13 ± 0.05, and 8.15 ± 0.05. Phytoplankton significantly impacted pH and Ω arag , while riverine inputs and biological activity played a vital role in controlling DIC and TA. The Yellow River significantly impacted the BS. The North Yellow Sea Cold Water Mass had a limited impact on acidification, while the South Yellow Sea Cold Water Mass significantly affected the SYS. Regarding seasonal fluctuations, Ω arag was significantly higher in summer than in other seasons. DIC and TA showed different patterns in both the BS and YS, with a minimal fluctuation in pH. Over the last two decades, the pH in the BS showed a slight annual decline, and the rate of change was (−1.45 ± 2.19) × 10−5 yr−1. In contrast, the NYS and SYS have slightly risen, with rates of change of (2.39 ± 1.24) × 10−5 and (1.23 ± 0.76) × 10−5 yr−1. We believed that surface water acidification in the BS and YS did not follow the expected trend of significant acidification observed in open oceanic regions. Instead, the acidification process in these marginal seas was dominated by local factors such as riverine inputs, biological activity, and cold water masses, resulting in minimal pH changes over the last two decades. [Display omitted] • River inputs and phytoplankton influenced BS and YS surface seawater acidification. • Summarizing data of past 20 years on surface water acidification in BS and YS. • The BS and YS surface water may not be acidified. [ABSTRACT FROM AUTHOR]

This study combines measurements of dissolved inorganic carbon (DIC), total alkalinity (TA), pH, earth observation (EO), and ocean model products with deep learning to provide a good step forward in detecting changes in the ocean carbonate system parameters at a high spatial and temporal resolution in the North Atlantic region (Long. −61.00° to −50.04° W; Lat. 24.99° to 34.96° N). The in situ reference dataset that was used for this study provided discrete underway measurements of DIC, TA, and pH collected by M/V Equinox in the North Atlantic Ocean. A unique list of co-temporal and co-located global daily environmental drivers derived from independent sources (using satellite remote sensing, model reanalyses, empirical algorithms, and depth soundings) were collected for this study at the highest possible spatial resolution (0.04° × 0.04°). The resulting ANN-estimated DIC, TA, and pH obtained by deep learning shows a high correspondence when verified against observations. This study demonstrates how a select number of geophysical information derived from EO and model reanalysis data can be used to estimate and understand the spatiotemporal variability of the oceanic carbonate system at a high spatiotemporal resolution. Further methodological improvements are being suggested.

Coral reefs are iconic ecosystems with immense ecological, economic and cultural value, but globally their carbonate‐based skeletal construction is threatened by ocean acidification (OA). Identifying coral species that have specialised mechanisms to maintain high rates of calcification in the face of declining seawater pH is of paramount importance in predicting future species composition, and growth of coral reefs. Here, we studied multiple coral species from two distinct volcanic CO2 seeps in Papua New Guinea to assess their capacity to control their calcifying fluid (CF) chemistry. Several coral species living under conditions of low mean seawater pH, but with either low or high variability in seawater pH, were examined and compared with those living in 'normal' (non‐seep) ambient seawater pH. We show that when mean seawater pH is low but highly variable, corals have a greater ability to maintain constant pHcf in their CF, but this characteristic was not linked with changes in abundance. Within less variable low pH seawater, corals with limited reductions in pHcf at the seep sites compared with controls tended to be more abundant at the seep site than at the control site. However, this finding was strongly influenced by a single species (Montipora foliosa), which was able to maintain complete pHcf homeostasis. Overall, although our findings indicate that there might be an association between ecological success and greater pHcf homeostasis, further research with additional species and at more sites with differing seawater pH regimes is required to solidify inferences regarding coral ecological success under future OA. [ABSTRACT FROM AUTHOR]

We measured the carbonate system (between 2015 and 2018) in an isolated and a well‐connected temperate estuary, both known for shellfish growth. We evaluated end‐member model estimates of inorganic carbon, alkalinity, pH, mineral saturation states (Ωa), and pH sensitivity (βDIC). We find winter conditions are estimated within observational uncertainty. Spring‐summer primary productivity elevates observed pH and Ωa above theoretical lines, beyond uncertainty. Both estuaries are sensitive in winter and likely to experience rapid pH changes with increased inorganic carbon inputs. Summer pH sensitivity is reduced by productivity and is least sensitive in the midsalinity region. We estimate carbon increased by up to 49 μmol kg−1, since the pre‐industrial period resulting in significant decreases in pH (0.2) and Ωa (0.5). The largest pH decrease occurred outside the minimum buffer zone, at higher salinities where carbon increase was greatest. The largest pH decrease occurred in winter, but the largest Ωa decrease occured in summer. Plain Language Summary: The ocean has absorbed about a third of human carbon dioxide emissions, increasing its acidity. Increasing acidity negatively affects marine organisms such as shellfish and fish that build calcium carbonate structures because they must spend more energy building and maintaining these structures. Most farmed and wild shellfish live in estuarine environments where acidification has not been widely studied. We made complete carbon system measurements over several years and across all seasons, in two distinct estuaries with significant aquaculture, where no previous carbon data existed. We estimated key acidification parameters and evaluated our ability to predict these parameters using only salinity, once properties in the freshest and saltiest water in the systems were well defined in all seasons. Our simple "mixing model" estimates winter conditions well, but during summer, phytoplankton blooms take up carbon and make conditions more favorable than predicted. We found that both estuaries are sensitive to future increases in carbon and are likely to experience rapid changes in chemistry. We estimate that human activity has already caused significant increases in inorganic carbon and associated acidification. The largest change critical to shell‐building organisms has occurred during the summer, and at higher salinities that are typical of grow sites. Key Points: End‐member mixing estimates winter pH and mineral saturation states in temperate estuaries within measurement uncertaintySeasonal productivity keeps pH and mineral saturation states above theoretical mixing curves by up to ∼1 pH and ∼2.5 Ωa, respectivelySince the preindustrial period nearshore estuarine pH declined by 0.05–0.2 with largest decline outside the minimum buffer zone [ABSTRACT FROM AUTHOR]

Ocean acidification typically reduces the calcification rates of massive Porites spp. corals, but increasing seawater temperatures (below the stress and bleaching threshold) can offset this effect. Here, we use δ11B to reconstruct the pH of the calcification media (pHECM) used to precipitate the skeleton in poritid corals cultured over a range of seawater pCO2 and at 25 °C and 28 °C. Increasing temperature had no significant effect on pHECM at high pCO2 although corals increased their calcification rates. pHECM was reduced at 28 °C compared to 25 °C at low seawater pCO2, although calcification rates remained constant. Increasing calcification rates could reflect the positive influence of temperature on aragonite precipitation rate, an increase in calcification media saturation state or a change in the concentration/behaviour of the skeletal organic matrix. The two temperatures utilized in this study were within the seasonal range at the coral collection site and do not represent a heat stress scenario. Increasing seawater temperatures may promote calcification in some corals in the future but are unlikely to benefit the majority of corals, which are already living close to their maximum thermal tolerance limits. [ABSTRACT FROM AUTHOR]

The atmospheric partial pressure of CO2 (pCO2) has been increasing dramatically since the beginning of the industrial revolution and about 30% of the CO2 produced by anthropogenic activities was absorbed by the ocean. This led to a perturbation of the seawater carbonate chemistry resulting in a decrease of the average surface ocean pH by 0.1 and termed ocean acidification (OA). Projections suggest that pCO2 may reach 900 μatm by the end of the twenty-first century lowering the average pH of the surface ocean by 0.4 units. The negative impacts of OA on many species of marine invertebrates such as mollusks, echinoderms, and crustaceans are well documented. However, less attention has been paid to the impacts of low pH on fitness and immune system in crustaceans. Here, we exposed Pacific white shrimps to 3 different pHs (nominal pH 8.0, 7.9, and 7.6) over a 100-days experiment. We found that, even though there were no significant effects on fitness parameters (survival, growth and allometries between length and weight), some immune markers were modified under low pH. A significant decrease in total hemocyte count and phenoloxidase activity was observed in shrimps exposed to pH 7.6 as compared to pH 8.0; and phagocytosis rate significantly decreased with decreasing pH. A significant increase in superoxide production was also observed at pH 7.6 as compared to pH 8.0. All these results suggest that a 100-days exposure to pH 7.6 did not have a direct effect on fitness but lead to a modulation of the immune response.

Ocean acidification (OA) is a global environmental concern, and submarine groundwater discharge (SGD) is a potentially process that enhances OA. This review summarizes the relationship between two types of constituents carried by SGD into China's seawater and OA. 1) Current research predominantly concentrates on constituent fluxes from SGD, neglecting its ecological impacts on carbon and nutrients budgets, as well as the mechanisms between carbon and nutrients. 2) Uncertainties persist in SGD research methods and acidification characterization. 3) There's a need to enhance quantitative research methods of SGD-OA, particularly in areas with intricate biogeochemical processes. Effective identification methods are crucial to quantify SGD's contribution to OA. Investigating core scientific questions, including SGD's impact on OA rates and scales, is paramount. While the primary focus is on SGD-OA research in China, insights gained from novel perspectives could have broader value for coastal management globally. [Display omitted] • Uncertainties exist in SGD and coastal acidification characterization. • Relative paucity of studies on SGD input carbon at different scales in China. • Consider coupling of SGD-carried nutrients and carbon to coastal acidification. • Consider conducting qualitative and quantitative studies on SGD and acidification. [ABSTRACT FROM AUTHOR]

The Paleocene–Eocene Thermal Maximum (PETM) (55.6 Mya) was a geologically rapid carbon-release event that is considered the closest natural analog to anthropogenic CO2 emissions. Recent work has used boron-based proxies in planktic foraminifera to characterize the extent of surface-ocean acidification that occurred during the event. However, seawater acidity alone provides an incomplete constraint on the nature and source of carbon release. Here, we apply previously undescribed culture calibrations for the B/Ca proxy in planktic foraminifera and use them to calculate relative changes in seawater-dissolved inorganic carbon (DIC) concentration, surmising that Pacific surface-ocean DIC increased by +1,010+1,415−646 µmol/kg during the peak-PETM. Making reasonable assumptions for the pre-PETM oceanic DIC inventory, we provide a fully data-driven estimate of the PETM carbon source. Our reconstruction yields a mean source carbon δ13C of −10‰ and a mean increase in the oceanic C inventory of +14,900 petagrams of carbon (PgC), pointing to volcanic CO2 emissions as the main carbon source responsible for PETM warming. [ABSTRACT FROM AUTHOR]

The Arctic is undergoing unprecedented environmental change. Rapid warming, decline in sea ice extent, increase in riverine input, ocean acidification and changes in primary productivity are creating a crucible for multiple concurrent environmental stressors, with unknown consequences for the entire arctic ecosystem. Here, we synthesized 30 years of data on the stable carbon isotope (δ13C) signatures in dissolved inorganic carbon (δ13C‐DIC; 1977–2014), marine and riverine particulate organic carbon (δ13C‐POC; 1986–2013) and tissues of marine mammals in the Arctic. δ13C values in consumers can change as a result of environmentally driven variation in the δ13C values at the base of the food web or alteration in the trophic structure, thus providing a method to assess the sensitivity of food webs to environmental change. Our synthesis reveals a spatially heterogeneous and temporally evolving δ13C baseline, with spatial gradients in the δ13C‐POC values between arctic shelves and arctic basins likely driven by differences in productivity and riverine and coastal influence. We report a decline in δ13C‐DIC values (−0.011‰ per year) in the Arctic, reflecting increasing anthropogenic carbon dioxide (CO2) in the Arctic Ocean (i.e. Suess effect), which is larger than predicted. The larger decline in δ13C‐POC values and δ13C in arctic marine mammals reflects the anthropogenic CO2 signal as well as the influence of a changing arctic environment. Combining the influence of changing sea ice conditions and isotopic fractionation by phytoplankton, we explain the decadal decline in δ13C‐POC values in the Arctic Ocean and partially explain the δ13C values in marine mammals with consideration of time‐varying integration of δ13C values. The response of the arctic ecosystem to ongoing environmental change is stronger than we would predict theoretically, which has tremendous implications for the study of food webs in the rapidly changing Arctic Ocean. [ABSTRACT FROM AUTHOR]

Laboratory studies that test the responses of coastal organisms to ocean acidification (OA) typically use constant pH regimes which do not reflect coastal systems, such as seaweed beds, where pH fluctuates on diel cycles. Seaweeds that use CO2 as their sole inorganic carbon source (non-carbon dioxide concentrating mechanism species) are predicted to benefit from OA as concentrations of dissolved CO2 increase, yet this prediction has rarely been tested, and no studies have tested the effect of pH fluctuations on non-CCM seaweeds. We conducted a laboratory experiment in which two ecologically dominant non-CCM red seaweeds (Callophyllis lambertii and Plocamium dilatatum) were exposed to four pH treatments: two static, pHT 8.0 and 7.7 and two fluctuating, pHT 8.0 ± 0.3 and 7.7 ± 0.3. Fluctuating pH reduced growth and net photosynthesis in C. lambertii , while P. dilatatum was unaffected. OA did not benefit P. dilatatum , while C. lambertii displayed elevated net photosynthetic rates. We provide evidence that carbon uptake strategy alone cannot be used as a predictor of seaweed responses to OA and highlight the importance of species-specific sensitivity to [H+]. We also emphasize the importance of including realistic pH fluctuations in experimental studies on coastal organisms. [ABSTRACT FROM AUTHOR]

Abstract Pairs of marine carbonate system variables are often used to calculate others, but those results are seldom reported with estimates of uncertainties. Although the procedure to propagate these uncertainties is well known, it has not been offered in public packages that compute marine carbonate chemistry, fundamental tools that are relied on by the community. To remedy this shortcoming, four of these packages were expanded to calculate sensitivities of computed variables with respect to each input variable and to use those sensitivities along with user-specified estimates of input uncertainties (standard uncertainties) to propagate uncertainties of calculated variables (combined standard uncertainties). Sensitivities from these packages agree with one another and with analytical solutions to within 0.01%; similar agreement among packages was found for the combined standard uncertainties. One package was used to quantify how propagated uncertainties vary among computed variables, seawater conditions, and the chosen pair of carbonate system variables that is used as input. The relative contributions to propagated uncertainties from the standard uncertainties of the input pair of measurements and various other input data (equilibrium constants etc) were explored with a new type of diagram. These error-space diagrams illustrate that further improvement beyond today's state-of-the-art measurement uncertainties for the input pair would generally be ineffective at reducing the combined standard uncertainties because the contribution from the constants is larger. Likewise, using much more uncertain measurements of the input pair does not always substantially worsen combined standard uncertainty. The constants that contribute most to combined standard uncertainties are generally K 1 and K 2 , as expected. Yet more of the propagated uncertainty in the computed saturation states of aragonite and calcite comes from their solubility products. Thus percent relative combined standard uncertainties for the saturation states are larger than for the carbonate ion concentration. Routine propagation of these uncertainties should become standard practice. Highlights • Uncertainty propagation added to four public software packages that make CO 2 system calculations • New type of diagram helps to assess how propagated uncertainty changes with different input uncertainties • Uncertainties from the constants often dominate propagated uncertainty, so that measurement uncertainty plays little role • Relative uncertainties are larger for CaCO 3 saturation states than for CO 3 2- because of uncertainties in solubility products [ABSTRACT FROM AUTHOR]

Abstract Ocean acidification typically reduces calcification in tropical marine corals but the mechanism for this process is not understood. We use skeletal boron geochemistry (B/Ca and δ11B) to reconstruct the calcification fluid DIC of corals cultured over both high and low seawater pCO 2 (180, 400 and 750 μatm). We observe strong positive correlations between calcification fluid pH and concentrations of the DIC species potentially implicated in aragonite precipitation (be they CO 3 2−, HCO 3 − or HCO 3 − + CO 3 2−). Similarly, with the exception of one outlier, the fluid concentrations of precipitating DIC species are strongly positively correlated with coral calcification rate. Corals cultured at high seawater pCO 2 usually have low calcification fluid pH and low concentrations of precipitating DIC, suggesting that a reduction in DIC substrate at the calcification site is responsible for decreased calcification. The outlier coral maintained high pH CF and DIC CF at high seawater pCO 2 but exhibited a reduced calcification rate indicating that the coral has a limited energy budget to support proton extrusion from the calcification fluid and meet other calcification demands. We find no evidence that increasing seawater pCO 2 enhances diffusion of CO 2 into the calcification site. Instead the overlying [CO 2 ] available to diffuse into the calcification site appears broadly comparable between seawater pCO 2 treatments, implying that metabolic activity (respiration and photosynthesis) generates a similar [CO 2 ] in the vicinity of the calcification site regardless of seawater pCO 2. Highlights • Calcification fluid pH and [precipitating DIC] are positively correlated in all corals. • [Precipitating DIC] and coral calcification rate are positively correlated in all but one outlier coral. • Corals cultured at high seawater pCO 2 usually have low fluid pH and [precipitating DIC]. • Maintaining high [precipitating DIC] at high seawater pCO 2 is at the expense of other calcification processes. [ABSTRACT FROM AUTHOR]

Abstract: Ocean acidification (OA) is a major threat to marine ecosystems, particularly coral reefs which are heavily reliant on calcareous species. OA decreases seawater pH and calcium carbonate saturation state (Ω), and increases the concentration of dissolved inorganic carbon (DIC). Intense scientific effort has attempted to determine the mechanisms via which ocean acidification (OA) influences calcification, led by early hypotheses that calcium carbonate saturation state (Ω) is the main driver. We grew corals and coralline algae for 8–21 weeks, under treatments where the seawater parameters Ω, pH, and DIC were manipulated to examine their differential effects on calcification rates and calcifying fluid chemistry (Ωcf, pHcf, and DICcf). Here, using long duration experiments, we provide geochemical evidence that differing physiological controls on carbonate chemistry at the site of calcification, rather than seawater Ω, are the main determinants of calcification. We found that changes in seawater pH and DIC rather than Ω had the greatest effects on calcification and calcifying fluid chemistry, though the effects of seawater carbonate chemistry were limited. Our results demonstrate the capacity of organisms from taxa with vastly different calcification mechanisms to regulate their internal chemistry under extreme chemical conditions. These findings provide an explanation for the resistance of some species to OA, while also demonstrating how changes in seawater DIC and pH under OA influence calcification of key coral reef taxa. [ABSTRACT FROM AUTHOR]

We conducted a comprehensive analysis of a variety of geochemical data including total alkalinity (TA), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), major ions, stable isotopes, and submarine groundwater discharge, to understand biogeochemical and hydrologic processes driving the seasonal to annual estuarine buffering capacity in Nueces Bay, Texas. These measurements, together with statistical analysis and geochemical modeling, show large variability of freshwater influence. TA consumption, common to spring seasons, was mainly driven by CaCO 3 precipitation and, to some extent, by aerobic respiration. TA production occurred in some parts of the bay during summer, fall and winter, likely driven by denitrification. CaCO 3 dissolution is stimulated by input of undersaturated river waters following significant flooding events. Since consumption and production of TA was not necessarily associated with different salinity zones, SGD, identified to be significant year-round, likely offsets the effects of salinity changes. Net DIC and TA fluxes exceeded dissolved organic carbon flux by an order of magnitude, except for winter 2014 when it was in the same order of magnitude. In addition to generally larger SGD rates when compared to other studies, production of TA (DIC and DOC) in the bottom sediments, as observed in this study, leads to larger fluxes, especially for the driest season (winter 2014), in the mid-bay area (6.27 · 10 6 μM m −2 d −1 ). Consistently larger inputs occur along the shoreline stations (6.14 · 10 6 μM m −2 d −1 ) following the flood recession, when compared to mid-bay (1.26 · 10 6 μM m −2 d −1 ) and are associated with lower SGD following the summer 2015 flooding. This study demonstrates that the carbonate chemistry of estuaries in semiarid areas is affected by non-conservative processes because of seasonal variability of hydroclimatic conditions. [ABSTRACT FROM AUTHOR]

Quantifying carbon fluxes associated with submarine groundwater discharge (SGD) remains challenging due to the complex biogeochemistry of the carbonate system in the subterranean estuary (STE). Here we conducted time series measurements of total alkalinity (TAlk) and dissolved inorganic carbon (DIC) in a well-studied coastal aquifer (Waquoit Bay, Massachusetts, USA). Groundwater samples were collected monthly from May 2009 to June 2010 across the freshwater-saltwater mixing zone of the Waquoit Bay (WB) STE. The concentrations of both TAlk and DIC in zero-salinity groundwater were variable, but were lower than those in the bay water ( S ∼ 28). DIC underwent slightly non-conservative mixing between low and intermediate salinities while there was an apparent additional DIC source at high salinity (>20) in all seasons. TAlk concentrations exhibited even stronger variations, with evidence of both production and consumption in high salinity zones, and consistent TAlk consumption at intermediate salinity in summer and fall (June–December, 2009). The increases in DIC and TAlk at high salinity were attributed to aerobic respiration and denitrification in WB sediments during bay water recharge of the STE. We infer that the loss of TAlk at intermediate salinity reflects H + production as reduced compounds (e.g. Fe 2+ ) are oxidized within the STE. In terms of impacts on surface water inorganic carbon budgets, the SGD-derived DIC flux was mainly controlled by seasonal changes in SGD while a combination of TAlk concentration variability and SGD drove the TAlk flux. SGD-derived DIC, aqueous CO 2 , and H + fluxes to the bay were ∼40–50% higher in summer vs. in winter, a result of enhanced marine groundwater flux and significant TAlk removal (proton addition) during periods of high seawater intrusion. Furthermore, the SGD-derived DIC flux was consistently greater than TAlk flux regardless of season, indicating that SGD serves to reduce the CO 2 buffering capacity of surface water. Our results highlight the importance of seasonality and subsurface biogeochemical processes on the subterranean estuary carbonate system and the resulting impact on SGD-derived TAlk, DIC, aqueous CO 2 , and H + fluxes to the coastal ocean. [ABSTRACT FROM AUTHOR]

Reef coral calcification depends on regulation of pH in the internal calcifying fluid (CF) in which the coral skeleton forms. However, little is known about calcifying fluid pH (pHCF) regulation, despite its importance in determining the response of corals to ocean acidification. Here, we investigate pHCF in the coral Stylophora pistillata in seawater maintained at constant pH with manipulated carbonate chemistry to alter dissolved inorganic carbon (DIC) concentration, and therefore total alkalinity (AT).We also investigate the intracellular pH of calcifying cells, photosynthesis, respiration and calcification rates under the same conditions. Our results show that despite constant pH in the surrounding seawater, pHCF is sensitive to shifts in carbonate chemistry associated with changes in [DIC] and [AT], revealing that seawater pH is not the sole driver of pHCF. Notably, when we synthesize our results with published data, we identify linear relationships of pHCF with the seawater [DIC]/[H+] ratio, [AT]/ [H+] ratio and [CO32- ]. Our findings contribute new insights into the mechanisms determining the sensitivity of coral calcification to changes in seawater carbonate chemistry, which are needed for predicting effects of environmental change on coral reefs and for robust interpretations of isotopic palaeoenvironmental records in coral skeletons. [ABSTRACT FROM AUTHOR]

The Moorea Coral Reef Long Term Ecological Research program affords a unique opportunity to study the implications of ocean acidification (OA) for coral reefs, as ongoing ecological and physical monitoring there provides a context in which the patterns of community change attributed to OA can be detected. We used mesocosms to study the impacts of OA on corals and calcified algae, and conducted experiments to compare multiple species and evaluate how they are affected by changing concentrations of the different forms of dissolved inorganic carbon. Our results reveal taxonomic variation in the response to OA, with some corals and algae showing signs of resistance to OA conditions. This discovery is informing an urgent debate over the form in which coral reefs will survive (if at all) in the future.

This research assesses the thermodynamic consistency of recent marine CO 2 system measurements in United States coastal waters. As one means of assessment, we compared aragonite saturation states calculated using various combinations of measured parameters. We also compared directly measured and calculated values of total alkalinity and CO 2 fugacity. The primary data set consists of state-of-the-art measurements of the keystone parameters of the marine CO 2 system: dissolved inorganic carbon (DIC), total alkalinity (TA), CO 2 fugacity ( f CO 2 ), and pH. This study is the first thermodynamic CO 2 system intercomparison based on measurements obtained using purified meta cresol purple as a pH indicator. The data are from 1890 water samples collected during NOAA's West Coast Ocean Acidification Cruise of 2011 (WCOA2011) and NOAA's Gulf of Mexico and East Coast Carbon Cruise of 2012 (GOMECC-2). Calculations of in situ aragonite saturation states (Ω A ) near the saturation horizon exhibited differences on the order of ± 10% between predictions based on the (DIC, TA) pair of measurements vs. the (pH, DIC), ( f CO 2 , DIC), or ( f CO 2 , pH) pairs. Differences of this magnitude, which are largely attributable to the imprecision of Ω A calculated from the (DIC, TA) pair, are roughly equivalent to the magnitude of Ω A change projected to occur over the next several decades due to ocean acidification. These observations highlight the importance of including either pH or f CO 2 in saturation state calculations. Calculations of TA from (pH, DIC) and ( f CO 2 , DIC) showed that internal consistency could be achieved if minor subtractions of TA (≤ 4 μmol kg − 1 ) were applied to samples of salinity < 35. The extent of thermodynamic consistency is also exemplified by the small offset between TA calculated from (DIC, pH) and that calculated from (DIC, f CO 2 ): ~ 3 μmol kg − 1 , which is similar to the accuracy of the TA measurements. Systematic trends can be detected in the offsets between measured and calculated parameters, but for this high-quality data set the magnitude of methodological improvements required to achieve exact thermodynamic consistency is quite small. [ABSTRACT FROM AUTHOR]

The Southern Ocean plays a major role in mediating the uptake, transport, and long-term storage of anthropogenic carbon dioxide (CO 2 ) into the deep ocean. Examining the magnitude and spatial distribution of this oceanic carbon uptake is critical to understanding how the earth's carbon system will react to continued increases in this greenhouse gas. Here, we use the extended multiple linear regression technique to quantify the total and anthropogenic change in dissolved inorganic carbon (DIC) along the S04P and P16S CLIVAR/U.S. Global Ocean Carbon and Repeat Hydrography Program lines south of 67°S in the Pacific sector of the Southern Ocean between 1992 and 2011 using discrete bottle measurements from repeat occupations. Along the S04P section, which is located in the seasonal sea ice zone south of the Antarctic Circumpolar Current in the Pacific, the anthropogenic component of the DIC increase from 1992 to 2011 is mostly found in the Antarctic Surface Water (AASW, upper 100 m), while the increase in DIC below the mixed layer in the Circumpolar Deep Water can be primarily attributed to either a slowdown in circulation or decreased ventilation of deeper, high CO 2 waters. In the AASW we calculate an anthropogenic increase in DIC of 12–18 μmol kg − 1 and an average storage rate of anthropogenic CO 2 of 0.10 ± 0.02 mol m − 2 yr − 1 for this region compared to a global average of 0.5 ± 0.2 mol m − 2 yr − 1 . In surface waters this anthropogenic CO 2 uptake results in an average pH decrease of 0.0022 ± 0.0004 pH units yr − 1 , a 0.47 ± 0.10% yr − 1 decrease in the saturation state of aragonite (Ω Aragonite ) and a 2.0 ± 0.7 m yr − 1 shoaling of the aragonite saturation horizons (calculated for the Ω Aragonite = 1.3 contour). [ABSTRACT FROM AUTHOR]

Seawater CO 2 measurements are being made with increasing frequency as interest grows in the ocean's response to changing atmospheric CO 2 levels and to climate change. The ultimate usefulness of these measurements depends on the data quality and consistency. An inter-laboratory comparison was undertaken to help evaluate and understand the current reliability of seawater CO 2 measurements. Two seawater test samples of different CO 2 content were prepared according to the usual method for the creation of seawater reference materials in the Dickson Laboratory at Scripps Institution of Oceanography. These two test samples were distributed in duplicate to more than 60 laboratories around the world. The laboratories returned their measurement results for one or more of the following parameters: total alkalinity ( A T ), total dissolved inorganic carbon ( C T ), and pH, together with information about the methods used and the expected uncertainty of the measurements. The majority of laboratories reported A T and C T values for all their measurements that were within 10 μmol kg −1 of the assigned values (i.e. within ± 0.5%), however few achieved results within 2 μmol kg −1 (i.e. within ± 0.1%), especially for C T . Results for the analysis of pH were quite scattered, with little suggestion of a consensus value. The high-CO 2 test sample produced results for both C T and pH that suggested in many cases that CO 2 was lost during analysis of these parameters. This study thus documents the current quality of seawater CO 2 measurements in the various participating laboratories, and helps provide a better understanding of the likely magnitude of uncertainties in these measurements within the marine science community at the present time. Further improvements will necessarily hinge on adoption of an improved level of training in both measurement technique and of suitable quality control procedures for these measurements. [ABSTRACT FROM AUTHOR]

Respiration and calcification rate were estimated to quantify the effect of Zhikong scallop Chlamys farreri on marine CO system in Sanggou Bay, China. The C. farreri population in Sanggou Bay sequestered 78.06 ± 5.76 g C m y for shell formation, while the CO fluxes due to calcification and respiration were 53.95 ± 3.98 and 71.69 ± 6.51 g C m y, respectively. In order to eliminate the additional CO released from calcification and respiration process of C. farreri, Gracilaria lemaneiformis were introduced into the integrated system and its role was validated by in situ mesocosm methods. Eight mesocosms (1,000 L) were deployed over 42-h period and consisted of four treatments: seaweed-only, scallop-only (SP), seaweed integrated with scallop (SS), and control (C). The aqueous CO concentration and partial pressure of CO in SP treatments were significantly higher than the other three treatments ( p < 0.01), while there were no difference between SS treatments and C treatments ( p > 0.05). Furthermore, compared with the SP treatments, the presence of the G. lemaneiformis can keep the seawater pH stable. These findings suggest that seaweed and shellfish integrated aquaculture practice cannot only reduce dissolved inorganic carbon but also can alleviate ocean acidification. [ABSTRACT FROM AUTHOR]

Rising atmospheric CO2-concentrations will have severe consequences for a variety of biological processes. We investigated the responses of the green alga Ulva lactuca ( Linnaeus) to rising CO2-concentrations in a rockpool scenario. U. lactuca was cultured under aeration with air containing either preindustrial pCO2 (280 μatm) or the pCO2 predicted by the end of the 21st century (700 μatm) for 31 days. We addressed the following question: Will elevated CO2-concentrations affect photosynthesis (net photosynthesis, maximum relative electron transport rate ( rETR(max)), maximum quantum yield ( Fv/Fm), pigment composition) and growth of U. lactuca in rockpools with limited water exchange? Two phases of the experiment were distinguished: In the initial phase (day 1-4) the Seawater Carbonate System ( SWCS) of the culture medium could be adjusted to the selected atmospheric pCO2 condition by continuous aeration with target pCO2 values. In the second phase (day 4-31) the SWCS was largely determined by the metabolism of the growing U. lactuca biomass. In the initial phase, Fv/Fm and rETR(max) were only slightly elevated at high CO2-concentrations, whereas growth was significantly enhanced. After 31 days the Chl a content of the thalli was significantly lower under future conditions and the photosynthesis of thalli grown under preindustrial conditions was not dependent on external carbonic anhydrase. Biomass increased significantly at high CO2-concentrations. At low CO2-concentrations most adult thalli disintegrated between day 14 and 21, whereas at high CO2-concentrations most thalli remained integer until day 31. Thallus disintegration at low CO2-concentrations was mirrored by a drastic decline in seawater dissolved inorganic carbon and HCO3− . Accordingly, the SWCS differed significantly between the treatments. Our results indicated a slight enhancement of photosynthetic performance and significantly elevated growth of U. lactuca at future CO2-concentrations. The accelerated thallus disintegration at high CO2-concentrations under conditions of limited water exchange indicates additional CO2 effects on the life cycle of U. lactuca when living in rockpools. [ABSTRACT FROM AUTHOR]

Abstract: Acidification of the oceans, as a consequence of anthropogenic CO2 emissions has reduced the pH at the surface of oceans by 0.1 unit compared to pre-industrial values. The reduction of seawater pH changes the relative proportion of the inorganic carbon (Ci) species which could potentially affect the modes of Ci assimilation by marine microalgae. In this study the effects of changes in external pH on the modes of Ci uptake over the pH range 5.0 to 7.5 were determined mass spectrometrically in Stichococcus minor Naegeli and S. cylindricus Butcher et Umbauk. Both species were found to tolerate a broad range of pH from pH 5.0 to 9.5 but the optimum for growth of both species was 8.2. Both species were also found to grow over a range of salinities and are best described as brackish water species rather than marine species, since they grow over wide ranges of salinity and pH. Neither species expresses external carbonic anhydrase (CA) activity. In both species, cells grown at pH 5.0, where the bulk of dissolved inorganic carbon (DIC) is in the form of CO2, active HCO3 − and CO2 uptake were absent and cells appear to take up CO2 by diffusion. However, active HCO3 − uptake was present in cells of both species grown at pH 6.0, 7.0 and 7.5 but active CO2 uptake was not detectable. Cells of both species, when grown at pH 8.2, display both active CO2 uptake and active HCO3 − uptake. [Copyright &y& Elsevier]

Dissolved inorganic carbon (DIC) assimilation is essential to the reef-building capacity of crustose coralline algae (CCA). Little is known, however, about the DIC uptake strategies and their potential plasticity under ongoing ocean acidification (OA) and warming. The persistence of CCA lineages throughout historical oscillations of p CO 2 and temperature suggests that evolutionary history may play a role in selecting for adaptive traits. We evaluated the effects of p CO 2 and temperature on the plasticity of DIC uptake strategies and associated energetic consequences in reef-building CCA from different evolutionary lineages. We simulated past, present, moderate (IPCC RCP 6.0) and high p CO 2 (RCP 8.5) and present and high (RCP 8.5) temperature conditions and quantified stable carbon isotope fractionation (13ε), organic carbon content, growth and photochemical efficiency. All investigated CCA species possess CO 2 -concentrating mechanisms (CCMs) and assimilate CO 2 via diffusion to varying degrees. Under OA and warming, CCA either increased or maintained CCM capacity, which was associated with overall neutral effects on metabolic performance. More basal taxa, Sporolithales and Hapalidiales, had greater capacity for diffusive CO 2 use than Corallinales. We suggest that CCMs are an adaptation that supports a robust carbon physiology and are likely responsible for the endurance of CCA in historically changing oceans. • Ambient diffusive CO 2 use of reef-building crustose coralline algae ranges from 35 to 65%. • Algae largely maintain or increase bicarbonate use under ocean acidification and warming. • Maintained or increased bicarbonate use is associated with sustained metabolic performance. • Lineage predicts inorganic carbon uptake strategy. • Proposed initial framework for inorganic carbon uptake strategies in crustose coralline algae. [ABSTRACT FROM AUTHOR]

The Cenozoic Era has been characterized by large perturbations to the oceanic carbon cycle and global climatic changes, but quantifying the magnitude and cause of these shifts is still subject to considerable uncertainty. The boron/calcium (B/Ca) ratio of fossil planktic foraminifera shells is a promising tool for reconstructing surface ocean carbonate chemistry during such events. Previous studies indicate that symbiont-bearing, planktic foraminiferal B/Ca depends on the [B(OH) 4 − /DIC] ratio of seawater and potentially, when combined with foraminiferal δ 11 B proxy reconstructions of B(OH) 4 − , an opportunity to reconstruct surface ocean DIC in the geologic past. There are, however, two barriers towards interpreting records from the pre-Pleistocene era: (1) changes in seawater major ion chemistry in the past might have affected foraminiferal B/Ca; and (2) modern foraminifera species show variable B/Ca calibration sensitivities that cannot be constrained in now-extinct species. Here we address these challenges with new experiments in which we have cultured modern, symbiont-bearing foraminifera Globigerinoides ruber (pink) and Trilobatus sacculifer in seawater with simulated early Cenozoic seawater chemistry (high [Ca], low [Mg], and low [B] T). We explore mechanisms that can account for the inter-species trends that are observed in foraminiferal B/Ca, and propose a framework that can be used to apply B/Ca calibrations to now-extinct species for reconstructing climate perturbations under varying seawater chemistries. • Paleocene seawater chemistry affects planktic foraminifer boron/calcium proxy sensitivity. • T. sacculifer and O. universa shell boron content is similar to that of Paleogene species. • We present a new framework for applying B/Ca calibrations to the early Cenozoic. • Our new approach allows application of calibrations from modern species to extinct ones. [ABSTRACT FROM AUTHOR]