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The Ulva prolifera bloom is considered one of the most serious ecological disasters in the Yellow Sea in the past decade, forming a carbon sink in its source area within a short period but becoming a carbon source at its destination. To explore the effects of different environmental changes on seawater dissolved carbon pools faced by living U. prolifera in its originating area, U. prolifera were cultured in three sets with different light intensity (54, 108, and 162 μmol m -2  s -1 ), temperature (12, 20, and 28 °C) and nitrate concentration gradients (25, 50, and 100 μmol L -1 ). The results showed that moderate light (108 μmol m -2  s -1 ), temperature (20 °C), and continuous addition of exogenous nitrate significantly enhanced the absorption of dissolved inorganic carbon (DIC) in seawater by U. prolifera and most promoted its growth. Under the most suitable environment, the changes in the seawater carbonate system were mainly dominated by biological production and denitrification, with less influence from aerobic respiration. Facing different environmental changes, U. prolifera continuously changed its carbon fixation mode according to tissue δ 13 C results, with the changes in the concentrations of various components of DIC in seawater, especially the fluctuation of HCO 3 - and CO 2 concentrations. Enhanced light intensity of 108 μmol m -2  s -1 could shift the carbon fixation pathway of U. prolifera towards the C 4 pathway compared to temperature and nitrate stimulation. Environmental conditions at the origin determined the amount of dissolved carbon fixed by U. prolifera. Therefore, more attention should be paid to the changes in marine environmental conditions at the origin of U. prolifera, providing a basis for scientific management of U. prolifera., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

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 CO 2 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 CO 2 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 CO 2 excretion rate than those in the control group (pH 8.1). In comparison, at the same pH level, the CO 2 excretion rate increased when seawater TA level was elevated. No significant correlation between the CO 2 excretion rate and seawater pCO 2 levels (P > 0.05) was found; however, a significant correlation (P < 0.05) between CO 2 excretion rate and TA-DIC (the difference between total alkalinity and dissolved inorganic carbon) was observed. Blue mussel has a significantly higher CO 2 excretion rate than the other three species in the CO 2 excretions per unit mass of soft parts, with no significant difference observed among these three species. However, in terms of CO 2 excretion rate per unit mass of gills, abalone has the highest CO 2 excretion rate, while significant differences were found between each species. Our studies indicate that the CO 2 buffering capacity impacts the CO 2 excretion rate of four shellfish species largely independent of pCO 2 . Since CO 2 excretion is related to acid-base balancing, the results imply that the effects of other carbonate parameters, particularly the CO 2 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., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Globally, oyster reef restoration is one of the most widely applied coastal restoration interventions. While reefs are focal points of processes tightly linked to the carbonate system such as shell formation and respiration, how these processes alter reef carbonate chemistry relative to the surrounding seawater is unclear. Moreover, coastal systems are increasingly impacted by coastal acidification, which may affect reef carbonate chemistry. Here, we characterized the growth of multiple constructed reefs as well as summer variations in pH and carbonate chemistry of reef-influenced seawater (in the middle of reefs) and ambient seawater (at locations ~50 m outside of reefs) to determine how reef chemistry was altered by the reef community and, in turn, impacts resident oysters. High frequency monitoring across three subtidal constructed reefs revealed reductions of daily mean and minimum pH (by 0.05-0.07 and 0.07-0.12 units, respectively) in seawater overlying reefs relative to ambient seawater (p < .0001). The proportion of pH measurements below 7.5, a threshold shown to negatively impact post-larval oysters, were 1.8×-5.2× higher in reef seawater relative to ambient seawater. Most reef seawater samples (83%) were reduced in total alkalinity relative to ambient seawater samples, suggesting community calcification was a key driver of modified carbonate chemistry. The net metabolic influence of the reef community resulted in reductions of CaCO 3 saturation state in 78% of discrete samples, and juvenile oysters placed on reefs exhibited slower shell growth (p < .05) compared to oysters placed outside of reefs. While differences in survival were not detected, reef oysters may benefit from enhanced survival or recruitment at the cost of slowed growth rates. Nevertheless, subtidal restored reef communities modified seawater carbonate chemistry in ways that likely increased oyster vulnerability to acidification, suggesting that carbonate chemistry dynamics warrant consideration when determining site suitability for oyster restoration, particularly under continued climate change., (© 2023 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

A 45-day trial was conducted to study the effect of seawater total alkalinity (TA) level up- and downregulation on the growth performance and calcification of Haliotis discus hannai Ino, while seawater pH was maintained at pH NBS  = 8.1. Although seawater was not acidified, the results showed that TA downregulation caused a significant reduction (P < 0.05) in the somatic tissue growth of juvenile abalone, while TA upregulation significantly increased growth performance (P < 0.05). Similar to the impacts of pH reduction, TA downregulation also induces a decline in CO 2 buffering capacity, which may be the reason why somatic tissue growth was reduced, as lowered CO 2 buffering capacity was reported to shift the acid-base balancing of abalone. Parts of the periostracum layer weremissing and exposed the inner shell layers of the individuals from the TA-downregulated group. Scanning electron microscopy (SEM) results showed calcium carbonate densely deposited onto the inner shell in the control and TA-upregulated groups, while sparsely deposited calcium carbonate was observed in the TA-downregulated group. The C: N ratio in the shell of individuals from the TA-downregulated group was significantly lower than that of the other two groups, indicating that less inorganic carbon was added to the shell. As a result, abalone grew lighter and thinner shells in TA-downregulated seawater. Although seawater was not acidified, TA downregulation also caused a reduction in the calcium carbonate saturation state (Ω), which induced the erosion of the surface shell and the interruption of calcium carbonate generation. In conclusion, although seawater pH remained at ambient levels, the lowered CO 2 buffering capacity and Ω induced by seawater TA downregulation also showed a detrimental effect on the growth and calcification of Pacific abalone. The impact of ocean acidification on the growth of abalone should not be assessed using only seawater pH and/or pCO 2 but rather taking into account all of carbonate chemistry, particularly the CO 2 buffering capacity. Abalone cultivation is suggested to be carried out in seawater with a higher level of CO 2 buffering capacity and Ω, which can be achieved through integrated culture with seaweed or increasing the seawater TA level., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Dissolved inorganic carbon (DIC) is a key component of the global carbon cycle and plays a critical role in ocean acidification and proliferation of phototrophs. Its quantification at a high spatial resolution is essential for understanding various biogeochemical processes. We present an analytical method for 2D chemical imaging of DIC by combining a conventional CO 2 optode with localized electrochemical acidification from a polyaniline (PANI)-coated stainless-steel mesh electrode. Initially, the optode response is governed by local concentrations of free CO 2 in the sample, corresponding to the established carbonate equilibrium at the (unmodified) sample pH. Upon applying a mild potential-based polarization to the PANI mesh, protons are released into the sample, shifting the carbonate equilibrium toward CO 2 conversion (>99%), which corresponds to the sample DIC. It is herein demonstrated that the CO 2 optode-PANI tandem enables the mapping of free CO 2 (before PANI activation) and DIC (after PANI activation) in complex samples, providing high 2D spatial resolution (approx. 400 μm). The significance of this method was proven by inspecting the carbonate chemistry of complex environmental systems, including the freshwater plant Vallisneria spiralis and lime-amended waterlogged soil. This work is expected to pave the way for new analytical strategies that combine chemical imaging with electrochemical actuators, aiming to enhance classical sensing approaches via in situ (and reagentless) sample treatment. Such tools may provide a better understanding of environmentally relevant pH-dependent analytes related to the carbon, nitrogen, and sulfur cycles.

This research characterizes the temporal and spatial variability of the seawater carbonate chemistry on the near-shore waters of the northern Persian Gulf and Makran Sea. In general, normalized total alkalinity (nA T ) showed a westward decrease along the coasts of Makran Sea and the Persian Gulf. Intertidal seawater was always supersaturated in terms of calcium carbonate minerals during the daytime. Rocky shore waters in the Persian Gulf were sinks for CO 2 in the winter during the daytime. The nA T decreased from Larak to Khargu Island by 81 μmol/kg. As expected, the two hypothetical drivers of bio-calcification, i.e., Ω and the [HCO 3 - ]/[H + ] ratio, were significantly related at a narrow range of ambient temperature. However, as data were pooled over seasons and study sites, in contrast to Ω Ar , the [HCO 3 - ]/[H + ] ratio showed a slight dependence on temperature, suggesting that the ratio should be investigated as a more reliable factor in future biocalcification researches., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The processes that occur at the micro-scale site of calcification are fundamental to understanding the response of coral growth in a changing world. However, our mechanistic understanding of chemical processes driving calcification is still evolving. Here, we report the results of a long-term in situ study of coral calcification rates, photo-physiology, and calcifying fluid (cf) carbonate chemistry (using boron isotopes, elemental systematics, and Raman spectroscopy) for seven species (four genera) of symbiotic corals growing in their natural environments at tropical, subtropical, and temperate locations in Western Australia (latitudinal range of ~11°). We find that changes in net coral calcification rates are primarily driven by pH cf and carbonate ion concentration [ CO 3 2 - ] cf in conjunction with temperature and DIC cf . Coral pH cf varies with latitudinal and seasonal changes in temperature and works together with the seasonally varying DIC cf to optimize [ CO 3 2 - ] cf at species-dependent levels. Our results indicate that corals shift their pH cf to adapt and/or acclimatize to their localized thermal regimes. This biological response is likely to have critical implications for predicting the future of coral reefs under CO 2 -driven warming and acidification., (© 2018 John Wiley & Sons Ltd.)

Ocean acidification threatens many marine organisms, especially marine calcifiers. The only global-scale solution to ocean acidification remains rapid reduction in CO 2 emissions. Nevertheless, interest in localized mitigation strategies has grown rapidly because of the recognized threat ocean acidification imposes on natural communities, including ones important to humans. Protection of seagrass meadows has been considered as a possible approach for localized mitigation of ocean acidification due to their large standing stocks of organic carbon and high productivity. Yet much work remains to constrain the magnitudes and timescales of potential buffering effects from seagrasses. We developed a biogeochemical box model to better understand the potential for a temperate seagrass meadow to locally mitigate the effects of ocean acidification. Then we parameterized the model using data from Tomales Bay, an inlet on the coast of California, USA which supports a major oyster farming industry. We conducted a series of month-long model simulations to characterize processes that occur during summer and winter. We found that average pH in the seagrass meadows was typically within 0.04 units of the pH of the primary source waters into the meadow, although we did find occasional periods (hours) when seagrass metabolism may modify the pH by up to ±0.2 units. Tidal phasing relative to the diel cycle modulates localized pH buffering within the seagrass meadow such that maximum buffering occurs during periods of the year with midday low tides. Our model results suggest that seagrass metabolism in Tomales Bay would not provide long-term ocean acidification mitigation. However, we emphasize that our model results may not hold in meadows where assumptions about depth-averaged net production and seawater residence time within the seagrass meadow differ from our model assumptions. Our modeling approach provides a framework that is easily adaptable to other seagrass meadows in order to evaluate the extent of their individual buffering capacities. Regardless of their ability to buffer ocean acidification, seagrass meadows maintain many critically important ecosystem goods and services that will be increasingly important as humans increasingly affect coastal ecosystems., (© 2018 The Authors. Ecological Applications published by Wiley Periodicals, Inc. on behalf of Ecological Society of America.)

Accumulation of anthropogenic CO 2 is significantly altering ocean chemistry. A range of biological impacts resulting from this oceanic CO 2 accumulation are emerging, however, the mechanisms responsible for observed differential susceptibility between organisms and across environmental settings remain obscure. A primary consequence of increased oceanic CO 2 uptake is a decrease in the carbonate system buffer capacity, which characterizes the system's chemical resilience to changes in CO 2 , generating the potential for enhanced variability in pCO 2 and the concentration of carbonate [ CO 3 2 - ], bicarbonate [ HCO 3 - ], and protons [H + ] in the future ocean. We conducted a meta-analysis of 17 shipboard manipulation experiments performed across three distinct geographical regions that encompassed a wide range of environmental conditions from European temperate seas to Arctic and Southern oceans. These data demonstrated a correlation between the magnitude of natural phytoplankton community biological responses to short-term CO 2 changes and variability in the local buffer capacity across ocean basin scales. Specifically, short-term suppression of small phytoplankton (<10 μm) net growth rates were consistently observed under enhanced pCO 2 within experiments performed in regions with higher ambient buffer capacity. The results further highlight the relevance of phytoplankton cell size for the impacts of enhanced pCO 2 in both the modern and future ocean. Specifically, cell size-related acclimation and adaptation to regional environmental variability, as characterized by buffer capacity, likely influences interactions between primary producers and carbonate chemistry over a range of spatio-temporal scales., (© 2018 John Wiley & Sons Ltd.)

The role of rising atmospheric CO 2 in modulating estuarine carbonate system dynamics remains poorly characterized, likely due to myriad processes driving the complex chemistry in these habitats. We reconstructed the full carbonate system of an estuarine seagrass habitat for a summer period of 2.5 months utilizing a combination of time-series observations and mechanistic modeling, and quantified the roles of aerobic metabolism, mixing, and gas exchange in the observed dynamics. The anthropogenic CO 2 burden in the habitat was estimated for the years 1765-2100 to quantify changes in observed high-frequency carbonate chemistry dynamics. The addition of anthropogenic CO 2 alters the thermodynamic buffer factors (e.g., the Revelle factor) of the carbonate system, decreasing the seagrass habitat's ability to buffer natural carbonate system fluctuations. As a result, the most harmful carbonate system indices for many estuarine organisms [minimum pH T , minimum Ω arag , and maximum pCO 2(s.w.) ] change up to 1.8×, 2.3×, and 1.5× more rapidly than the medians for each parameter, respectively. In this system, the relative benefits of the seagrass habitat in locally mitigating ocean acidification increase with the higher atmospheric CO 2 levels predicted toward 2100. Presently, however, these mitigating effects are mixed due to intense diel cycling of CO 2 driven by aerobic metabolism. This study provides estimates of how high-frequency pH T , Ω arag , and pCO 2(s.w.) dynamics are altered by rising atmospheric CO 2 in an estuarine habitat, and highlights nonlinear responses of coastal carbonate parameters to ocean acidification relevant for water quality management., Competing Interests: The authors declare no conflict of interest.

Ocean alkalinity enhancement (OAE) via the enhanced weathering of alkaline minerals is a promising carbon dioxide removal (CDR) technology. Theoretically, these includes iron and steel slags, although their dissolution kinetics in seawater are unknown. Here, we conducted lab-scale experiments to assess the alkalinity generation potential and dissolution kinetics of various slags in seawater. We show that the alkalinity generated per mass of iron slag was logarithmic, i.e., higher amounts of iron slag added had diminishing alkalinity returns. In contrast, the relatively quick dissolution of steel slags and their linear generation of alkalinity per mass of feedstock dissolved in seawater makes them better OAE candidates. Furthermore, despite the presence of potentially toxic metals in these feedstocks, their low to non-existent presence as dissolution products suggests that harmful concentrations should not be reached, at least for the slag tested here. Finally, if all steel slag produced annually was used for OAE, between 10 and 22 gigatonnes of CO2 could be captured cumulatively by 2,100, highlighting significant CDR potential by slags. [ABSTRACT FROM AUTHOR]

Global-scale ocean acidification has spurred interest in the capacity of seagrass ecosystems to increase seawater pH within crucial shoreline habitats through photosynthetic activity. However, the dynamic variability of the coastal carbonate system has impeded generalization into whether seagrass aerobic metabolism ameliorates low pH on physiologically and ecologically relevant timescales. Here we present results of the most extensive study to date of pH modulation by seagrasses, spanning seven meadows (Zostera marina) and 1000 km of U.S. west coast over 6 years. Amelioration by seagrass ecosystems compared to non-vegetated areas occurred 65% of the time (mean increase 0.07 ± 0.008 SE). Events of continuous elevation in pH within seagrass ecosystems, indicating amelioration of low pH, were longer and of greater magnitude than opposing cases of reduced pH or exacerbation. Sustained elevations in pH of >0.1, comparable to a 30% decrease in [H+ ], were not restricted only to daylight hours but instead persisted for up to 21 days. Maximal pH elevations occurred in spring and summer during the seagrass growth season, with a tendency for stronger effects in higher latitude meadows. These results indicate that seagrass meadows can locally alleviate low pH conditions for extended periods of time with important implications for the conservation and management of coastal ecosystems.

Compared with the open ocean, knowledge of pH variability in coastal waters is rudimentary, especially in Africa. This is concerning as quantifying local pH conditions is critical when assessing the response of coastal species to future ocean acidification scenarios. The objective of this study was to capture some of the variability in pH at scales and sites relevant to coastal marine organisms in a South African temperate embayment (Algoa Bay, Indian Ocean). We used a sampling approach that captured spatial (at a resolution of ∼10 km), monthly and diel (24-hour) variability in pH and associated physical and biological parameters at offshore and shallow inshore sites in Algoa Bay. We found that pH and associated parameters (temperature, calculated pCO2, chlorophyll a) varied over space and time in Algoa Bay. The range in pH was 0.30 units at offshore sites and 0.46 at inshore sites, and the average pH was 8.10 (SD 0.06) and 8.10 (SD 0.13) at these sites, respectively, which is typical for coastal environments. Our results showed that both biological factors (at the offshore sites) and salinity (at the inshore sites) may influence temporal and spatial variability in pH. We also identified a shallow inshore site with high levels of macroalgal growth that had consistently higher average daytime pH levels (8.33 [SD 0.07]), which may serve as an ocean acidification refuge for coastal marine species. This is the first comprehensive pH-monitoring study to be implemented in a nearshore coastal area in Africa and provides recommendations for monitoring in other understudied regions. [ABSTRACT FROM AUTHOR]

Seawater carbonate chemistry links Earth's climate and carbon cycle through the production and preservation of carbonate sediments. Models and carbonate facies abundance records have generated hypotheses about trajectories of seawater carbonate chemistry, including responses to key events in the evolutionary history of carbonate biomineralizers. However, tests of these hypotheses have remained elusive. We applied a novel proxy for the carbonate mineral saturation state (Ω) of seawater based on the diameters of ooids—concentrically‐coated carbonate sand grains—to estimate Ω, dissolved inorganic carbon, alkalinity, and pH of seawater spanning Phanerozoic time. Reconstructed Ω values decreased sharply around ∼120 Ma, which we interpret as the fingerprint of the Mid‐Mesozoic Revolution of planktic calcifiers. Shifts in Ω across Ordovician time also suggest a possible causal relationship with the Great Ordovician Biodiversification Event. Our results demonstrate that ooid sizes are a useful tool for reconstructing Earth's ancient carbon cycle. Plain Language Summary: Earth's oceans play an important role in removing carbon from Earth's surface environments. One of the ways this happens is through the production and burial of calcium carbonate sediments, which include shells and calcium carbonate minerals formed in other ways. The chemistry of the oceans, including pH and the concentrations of carbonate ions, affect how easy it is for carbonate minerals to form, and whether they can survive long enough to be permanently buried on the seafloor. Tracking these aspects of the chemistry of ancient oceans can enable scientists to better understand the balance of carbon entering and exiting Earth's surface environments in the past. Until recently, we have not had the right tools to extract this information from sedimentary rocks. Here, we used a recently developed method that uses the sizes of ooids—sand grains that grow by accumulating concentric layers of calcium carbonate—to track ancient ocean carbonate chemistry. We compared the results of our new approach with previous efforts and found that our approach works well. Our work demonstrates that ooid size measurements can improve our understanding of ancient oceans. Key Points: We estimated seawater calcite saturation states spanning Phanerozoic time using a proxy based on ooid sizeWe combined our calcite saturation state values with pCO2 and seawater chemistry proxy data to calculate seawater carbonate chemistryDissolved inorganic carbon, alkalinity, and pH estimates based on the ooid size proxy agree with predictions from carbon cycle models and carbonate facies records [ABSTRACT FROM AUTHOR]

Sharp increases in atmospheric CO2 are resulting in ocean warming, acidification and deoxygenation that threaten marine organisms on continental margins and their ecological functions and resulting ecosystem services. The relative influence of these stressors on biodiversity remains unclear, as well as the threshold levels for change and when secondary stressors become important. One strategy to interpret adaptation potential and predict future faunal change is to examine ecological shifts along natural gradients in the modern ocean. Here, we assess the explanatory power of temperature, oxygen and the carbonate system for macrofaunal diversity and evenness along continental upwelling margins using variance partitioning techniques. Oxygen levels have the strongest explanatory capacity for variation in species diversity. Sharp drops in diversity are seen as O2 levels decline through the 0.5-0.15 ml l(-1) (approx. 22-6 µM; approx. 21-5 matm) range, and as temperature increases through the 7-10°C range. pCO2 is the best explanatory variable in the Arabian Sea, but explains little of the variance in diversity in the eastern Pacific Ocean. By contrast, very little variation in evenness is explained by these three global change variables. The identification of sharp thresholds in ecological response are used here to predict areas of the seafloor where diversity is most at risk to future marine global change, noting that the existence of clear regional differences cautions against applying global thresholds.

To quantitatively assess the inorganic carbon cycle in the eastern Indian sector of the Southern Ocean (80–150°E, south of 60°S), we measured ocean surface temperature, salinity, total alkalinity (TA), the partial pressure of carbon dioxide (pCO2), and concentrations of chlorophyll‐a (chl a), dissolved inorganic carbon (DIC), and nutrients during the KY18 survey (December 2018–January 2019). The sea–air CO2 flux in this region was −8.3 ± 12.7 mmol m−2 day−1 (−92.1 to +10.6 mmol m−2 day−1). The ocean was therefore a weak CO2 sink. Based on the DIC and TA in the temperature minimum layer, we estimated the change of pCO2 from winter to summer (δpCO2) due to changes in water temperature, salinity, and biological activity (photosynthesis). The spatial distribution of pCO2 in the western part (80–110°E) of the study area was mainly driven by biological activity, which decreased pCO2 from December to early January, and in the eastern part (110–150°E) by temperature, which increased pCO2 from January to February. We also examined the changes in the CO2 concentrations (xCO2) over time by comparing data from 1996 with our data (2018–2019). The oceanic and atmospheric xCO2 increased by 23 and 45 ppm in 23 years, respectively. These changes of ocean xCO2 were mainly driven by an increase in CO2 uptake from the atmosphere as a result of the rise in atmospheric xCO2 and increase in biological activity associated with the change in the water‐mass distribution. Plain Language Summary: Determining the drivers of seasonal changes in ocean CO2 is important for estimating the global carbon cycle. Seawater samples were collected in the eastern Indian sector of the Southern Ocean to investigate the carbonate chemistry of the ocean surface water. During winter and summer, the partial pressure of carbon dioxide (pCO2) at the ocean surface decreased and increased due to photosynthesis and temperature changes, respectively. Surface water pCO2 was lower than the pCO2 of the atmosphere in summer: the result was CO2 uptake by the ocean. A comparison of data from 1996 with our data (2018–2019) indicated that the CO2 concentration (xCO2) of the seawater had increased by 23 ppm. This increase was due primarily to an increase in uptake of CO2 from the atmosphere as a result of the rise of atmospheric xCO2. Key Points: The eastern Indian sector of the Southern Ocean absorbs atmospheric CO2 weakly in summerBiological activity and increases in water temperature change the pCO2 from winter to summerChanges in atmospheric pCO2 and water masses explained the interannual variations of dissolved CO2 in the ocean [ABSTRACT FROM AUTHOR]

The aragonite saturation state (Ωarag) was determined for the surface waters of the western Arctic Ocean over 3 years, from 2016 to 2018, in an investigation of the present state of acidification of its waters and the main factors controlling the spatial and temporal variations in the surface Ωarag. The study area was divided into the Chukchi marginal area (CMA) and the East Siberian marginal area (ESMA) along a longitude of 180°E. In the CMA, the surface Ωarag during the study period ranged from 0.86 to 1.77, with an average of 1.16, indicating near saturation with respect to aragonite. In the ESMA, the surface Ωarag during the study period ranged from 1.01 to 2.21, with a higher average (1.59) than the CMA. Aragonite undersaturation in the ESMA was not observed during any of the measurement periods, so ocean acidification was less serious there than in the CMA. The surface Ωarag of the CMA was mainly determined by the mixing of seawater and freshwater introduced from rivers and/or sea ice, whereas in the ESMA it was influenced by the mixing of seawater and freshwater but also biological production and lateral mixing. Plain Language Summary: The study was conducted in the western Arctic Ocean and included the Northwind Ridge, Chukchi Plateau, Chukchi Abyssal Plain, Chukchi Sea Slope, East Siberian Sea Slope, and Mendeleyev Ridge. The waters encompassed by these sites are highly vulnerable to acidification because of the inflow of lower‐pH water from the Pacific Ocean through the Bering Sea and the current rapid reduction in the amount of sea ice. The study area was divided into the Chukchi marginal area (CMA) and the East Siberian marginal area (ESMA) along a longitude of 180°E. In the CMA, the surface waters were almost saturated with respect to aragonite but in the ESMA they were undersaturated, indicating that oceanic acidification was more serious in the CMA than in the ESMA. In the near future, the aragonite undersaturation in the most of the surface waters of the CMA will prohibit the survival of calcareous organisms and may lead to their extinction from this area. However, a similar short‐term scenario is not expected in the ESMA, due to its relatively high biological production, which favors aragonite saturation and will thus delay aragonite undersaturation of the surface water. Key Points: In the Chukchi marginal area (CMA), aragonite saturation state (Ωarag) at the surface was determined mainly by the mixing of seawater and fresh waterIn the East Siberian marginal area, Ωarag was affected by freshwater mixing, biological production, and lateral mixingIn the near future, most of surface waters in the CMA will be undersaturated with respect to aragonite [ABSTRACT FROM AUTHOR]

To examine the ocean acidification of coastal water as the result of the oceanic uptake of anthropogenic atmospheric CO2, we initiated acidification monitoring in the eastern part of the Tsugaru Strait, through which the Tsugaru Warm Current flows eastward from the Sea of Japan to the North Pacific. Annual mean pH and CaCO3 saturation state during 2012–2019 decreased considerably throughout all depths at rates of 0.0030–0.0051 and 0.017–0.036 years−1, respectively. These rates of decrease are faster than those caused by increasing atmospheric CO2 and faster than those observed during previous research. These fast rates are attributed to an enhanced increase in dissolved inorganic carbon concurrently with increases in salinity and density caused by elevated mixing of the upper and deeper waters from the Sea of Japan at the western part of the strait. The elevated mixing is attributable to the strengthening of the Tsugaru Warm Current. Plain Language Summary: Approximately 30% of the total amount of CO2 released to the atmosphere by human activities has accumulated in the global ocean. This oceanic uptake of CO2 has resulted in ocean acidification. In coastal waters, the acidification affects marine organisms, thus coastal ecosystems may be more vulnerable to acidification than the open ocean. To examine the extent to which acidification has advanced in the eastern part of the Tsugaru Strait, through which the Tsugaru Warm Current passes from the Sea of Japan to the North Pacific, we initiated a time‐series observation of acidification. The pH reduction is found to have enhanced considerably across the whole depth during 2012–2019 at a rate faster than that caused by increasing atmospheric CO2 and at the highest rates observed during previous research. The rapid pH reduction is found to be attributable to the enhanced rate of increase of dissolved inorganic carbon concurrently with increases in salinity and density caused by elevated mixing of the upper and deeper waters from the Sea of Japan at the western strait due to the strengthening of the Tsugaru Warm Current. In other straits that are connected to the open ocean, the strengthening of their throughflow may also accelerate acidification. Key Points: The eastern part of the Tsugaru Strait has become significantly more acidic throughout its depth during 2012–2019The pH reduction is occurring at faster rates than reported elsewhere and faster than expected for oceanic uptake of anthropogenic CO2Rapid pH reduction is caused by enhanced increase of dissolved inorganic carbon, owing to a strengthened Tsugaru Warm Current [ABSTRACT FROM AUTHOR]

Abstract Oceanic carbonate chemistry during the Cenozoic has affected the climatology, ecology, and marine geology of our planet; yet, we have limited means to know the evolution of that chemistry, due to a lack of preserved and unaltered seawater samples and a continuing paucity of proxies. Modeling is often used to address this problem; here, we offer a simple, data-driven, secular timescale, inverse model for the mean, Cenozoic, carbonate chemistry of the oceans. Inputs for the model include carbonate compensation depth (CCD), CaCO 3 burial, seawater temperature, atmospheric CO 2 and carbonate ion records, as well as a simple set of original, but justified, assumptions. The model retrodicts the total dissolved inorganic carbon (DIC), carbonate alkalinity (CAlk), and pH of the surface and deep waters of the ocean. The retrodicted DIC and CAlk records do not indicate any unusually elevated values in the early Cenozoic, as found in some past studies. If the CCD record from Lyle et al. (2008) is employed, the changes in DIC and CAlk appear entirely related to changes in the alkalinity input to the pelagic oceans and atmospheric CO 2 ; however, with the CCD from Pälike et al. (2012) , the increases in DIC and CAlk during the last 15 Ma reflect the effects of ocean cooling. Using either CCD-record, our model provides consistent retrodictions of the available pH record. Our results are not consistent with many past modeling assumptions, such as constancy of alkalinity in surface waters, or the ratio of shallow and deep carbonate ion concentrations. Finally, we use our results to provide new estimates of atmospheric CO 2 based on Boron isotopes and find significantly lower CO 2 values in the early Cenozoic than previous values. Highlights • Inverse model of the secular carbonate chemistry of the pelagic ocean during the Cenozoic. • Retrodicted carbonate alkalinity does not exhibit large changes suggested by others. • Retrodicted pH agrees with observations from proxies. • Increase in alkalinity in the past 15 Ma may be due to the effects of cooling. • Recalculated Boron-based p CO 2 is comparatively much lower in the early Cenozoic. Graphical abstract [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]

It has been hypothesized that highly productive coastal ecosystems, such as seagrass meadows, could lead to the establishment of ocean acidification (OA) refugia, or areas of elevated pH and aragonite saturation state (Ωa) compared to source seawater. However, seagrass ecosystems experience extreme variability in carbonate chemistry across short temporal and small spatial scales, which could impact the pH buffering capacity of these potential refugia. Herein, short-term (hourly to diel) and small-scale (across 0.01-0.14 km2) spatiotemporal carbonate chemistry variability was assessed within two seagrass meadows in order to determine their short-term potential to elevate seawater pH relative to source seawater. Two locations at similar latitudes were chosen in order to compare systems dominated by coarse calcium carbonate (Bailey’s Bay, Bermuda) and muddy silicate (Mission Bay, CA, USA) sediments. In both systems, spatial variability of pH across the seagrass meadow at any given time was often greater than diel variability (e.g., the average range over 24 h) at any one site, with greater spatial variability occurring at low tide in Mission Bay. Mission Bay (spatial ΔpH = 0.08 ± 0.08; diel ΔpH = 0.12 ± 0.01; mean ± SD) had a greater average range in both temporal and spatial seawater chemistry than Bailey’s Bay (spatial ΔpH = 0.02 ± 0.01; diel ΔpH = 0.03 ± 0.00; mean ± SD). These differences were most likely due to a combination of slower currents, a larger tidal range, and more favorable weather conditions for photosynthesis (e.g., sunny with no rain) in Mission Bay. In both systems, there was a substantial amount of time (usually at night) when seawater pH within the seagrass beds was lower relative to the source seawater. Future studies aimed at assessing the potential of seagrass ecosystems to act as OA refugia for marine organisms need to account for the small-scale, high-frequency carbonate chemistry variability in both space and time, as this variability will impact where and when OA will be buffered or intensified. [ABSTRACT FROM AUTHOR]

Total alkalinity (TA) is a popularly measured carbonate system parameter and is widely used in calculations of key carbonate system descriptors such as the calcium carbonate saturation state, an important indicator of ocean acidification. Organic alkalinity (OrgAlk) is recognised as a contributor to TA in coastal waters, with this having implications on the use of TA to calculate key carbonate chemistry descriptors. As titratable charge groups of OrgAlk can act as unknown acid-base species, the inclusion of the total concentration and apparent dissociation constants of OrgAlk in carbonate calculations involving TA is required to minimise uncertainty in computed speciation. Here we present an investigation of the prevalence and properties of OrgAlk as well as the impact of OrgAlk on carbonate chemistry calculations in a transitional waterbody. Water samples were collected during low and high tide over a 5-week period in Rogerstown Estuary, Dublin Ireland. TA and OrgAlk were measured using modified Global Ocean Acidification Observing Network (GOA-ON) titration apparatus in conjunction with OrgAlkCalc , an open-source Python based computational programme. pH was measured on the total scale using meta-cresol purple (mCP) as the indicator dye. Dissolved inorganic carbon (DIC), the partial pressure of CO 2 (pCO 2), in situ pH on the total scale (pH T) and the saturation state of aragonite (∆Ω A) were calculated using pH and both OrgAlk adjusted TA and measured TA as the input parameters. Optical analysis of DOM was conducted to compliment OrgAlk characterisations and to further elucidate OrgAlk sources and dynamics. OrgAlk charge groups concentrations ranged from 35,198 μ mol · kg−1, with the highest concentrations observed in more marine waters. Two apparent charge groups were associated with OrgAlk, with pK values of 4.38 ± 0.27 and 6.95 ± 0.43. Differences between calculated carbonate system parameters when using OrgAlk adjusted TA and non-OrgAlk adjusted TA ranged from 88 to 254 μ mol · kg−1 DIC, −98–67 μ atm pCO 2 , −0.02–0.12 pH T and 0.02–0.64 ∆Ω A. Variability in the differences in calculated carbonate systems was largely a factor of OrgAlk charge group concentration and pK. This work highlights the importance of considering OrgAlk if using TA as an input parameter in carbonate system investigations of coastal waters. • OrgAlk can be an important fraction of TA in coastal waters. • OrgAlk acid-base properties are suggestive of carboxyl and phenolic-like signals. • Complementary fluorescence analysis of DOM can elucidate origins and transformations of OrgAlk. • OrgAlk can lead to the propagation of uncertainty on calculated carbonate parameters. [ABSTRACT FROM AUTHOR]

Evidence of ocean acidification (OA) throughout the global ocean has galvanized some coastal communities to evaluate carbonate chemistry variations closer to home. An impediment to doing this effectively is that, often, only one carbonate system parameter is measured at a time, while two are required to fully constrain the inorganic carbon chemistry of seawater. In order to leverage the abundant single-carbonate-parameter datasets in Washington State for more rigorous OA research, we have characterized an empirical relationship between total alkalinity (TA) and salinity (TA = 47.7 × S + 647; 1 σ = ±17 μmol kg) for regional surface waters (≤25 m) that is robust in the salinity range from 20 to 35 for all seasons. The relationship was evaluated using 5 years of 3-h contemporaneous observations of salinity, carbon dioxide partial pressure ( pCO), and pH from a surface mooring on the outer coast of Washington. In situ pCO observations and salinity-based estimates of TA were used to calculate pH for comparison with in situ pH measurements. On average, the calculated pH values were 0.02 units lower than the measured pH values across multiple pH sensor deployments and showed extremely high fidelity in tracking the measured high-frequency pH variations. Our results indicate that the TA-salinity relationship will be a useful tool for expanding single-carbonate-parameter datasets in Washington State and quality controlling dual pCO-pH time series. [ABSTRACT FROM AUTHOR]

• Hatching duration and hatching rate in Haliotis discus hannai is susceptible to increasing pCO2 level. • CaCO3 deposition in larval abalone shell is susceptible to calcium carbonate saturation state (Ω) rather than pCO2 or pH. • Abalone shell formation relies heavily on inorganic carbon supply from seawater. • Different seawater carbonate parameters play different roles in affecting early embryonic development and shell formation of the Pacific abalone. pH or p CO 2 are usually taken to study the impact of ocean acidification on molluscs. Here we studied the different impact of seawater carbonate parameters on embryonic development and calcification of the Pacific abalone (Haliotis discus hannai). Early embryonic development was susceptible to elevated p CO 2 level. Larvae hatching duration was positively and hatching rate was negatively correlated with the p CO 2 level, respectively. Calcium carbonate (CaCO 3) deposition of larval shell was found to be susceptible to calcium carbonate saturation state (Ω) rather than p CO 2 or pH. Most larvae incubated in seawater with Ω arag = 1.5 succeeded in shell formation, even when seawater p CO 2 level was higher than 3700 μatm and pH T was close to 7.4. Nevertheless, larvae failed to generate CaCO 3 in seawater with Ω arag ≤ 0.52 and control level of p CO 2 , while seawater DIC level was lowered (≤ 852 μmol/kg). Surprisingly, some larvae completed CaCO 3 deposition in seawater with Ω arag = 0.6 and slightly elevated DIC (2266 μmol/kg), while seawater p CO 2 level was higher than 2700 μatm and pH T was lower than 7.3. This indicates that abalone may be capable of regulating carbonate chemistry to support shell formation, however, the capability was limited as surging p CO 2 level lowered growth rate and jeopardized the integrity of larval shells. Larvae generated thicker shell in seawater with Ω arag = 5.6, while adult abalone could not deposit CaCO 3 in seawater with Ω arag = 0.29 and DIC = 321 μmol/kg. This indicates that abalone may lack the ability to directly remove or add inorganic carbon at the calcifying sites. In conclusion, different seawater carbonate parameters play different roles in affecting early embryonic development and shell formation of the Pacific abalone, which may exhibit limited capacity to regulate carbonate chemistry. [ABSTRACT FROM AUTHOR]

Understanding the ocean carbon sink and its future acidification-derived changes requires accurate and precise measurements with good spatiotemporal coverage. In addition, a deep knowledge of the thermodynamics of the seawater carbonate system is key to interconverting between measured and calculated variables. To gain insights into the remaining inconsistencies in the seawater carbonate system, we assess discrete water column measurements of carbon dioxide fugacity (f CO 2), dissolved inorganic carbon (DIC), total alkalinity (TA), and pH measured with unpurified indicators, from hydrographic cruises in the Atlantic, Pacific, and Southern Oceans included in GLODAPv2.2020 (19,013 samples). An agreement of better than ± 3% between f CO 2 measured and calculated from DIC and pH is obtained for 94% of the compiled dataset, while when considering f CO 2 measured and calculated from DIC and TA, the agreement is better than ± 4% for 88% of the compiled dataset, with a poorer internal consistency for high-CO 2 waters. Inspecting all likely sources of uncertainty from measured and calculated variables, we conclude that the seawater carbonate system community needs to (i) further refine the thermodynamic model of the seawater carbonate system, especially K 2 , including the impact of organic compounds and other acid-base systems on TA; (ii) update the standard operating procedures for the seawater carbonate system measurements following current technological and analytical advances, paying particular attention to the pH methodology that is the one that evolved the most; (iii) encourage measuring discrete water column f CO 2 to further check the internal consistency of the seawater carbonate system, especially given the new era of sensor-based seawater measurements; and (iv) develop seawater Certified Reference Materials (CRMs) for f CO 2 and pH together with seawater CRMs for TA and DIC over the range of values encountered in the global ocean. Our conclusions also suggest the need for a re-evaluation of the adjustments applied by GLODAPv2 to pH, which were based on DIC and TA consistency checks but not supported by f CO 2 and DIC consistency. • Internal consistency assessment of discrete measurements of f CO 2 , DIC, TA, and pH. • Current uncertainties in calculated f CO 2 are larger than climate-quality goal requirements. • High-CO 2 waters have poorer internal consistency. • A more comprehensive understanding of the thermodynamics, especially K 2 , is needed. • Need for updated standard operating procedures and wide-range reference materials. [ABSTRACT FROM AUTHOR]

Abstract: Equilibrium oxygen isotope fractionation factors (, and ) are fundamental geochemical parameters that characterize 18O partitioning in the CO2–H2O system. These constants were established in laboratory experiments using deionized H2O (e.g., Beck et al. (2005) and references therein). The applicability of these constants in environmental waters, including natural seawater, appears questionable due to potentially strong ionic interactions in such aqueous media. For instance, considerable portions of carbonate ions in seawater exist as cation–CO3 2− ion complexes such as MgCO3 0. In this study, quantitative BaCO3 precipitation experiments were performed to examine the effect of Mg2+ concentrations on the oxygen isotope equilibrium between dissolved inorganic carbon (DIC) species and H2O. Our results from Mg2+-free control experiments in which BaCO3 samples were precipitated from simple NaHCO3 solutions were in good agreement with empirical results from three independent studies and with theoretical calculations. BaCO3 precipitations from solutions with Mg2+ concentrations higher than 2.5mM caused intolerable quantities of Mg(OH)2 co-precipitation, which interfered with δ18O measurements. Within the limit of 2.5mM of [Mg2+], the MgCO3 0 abundance in the total carbonate ions ([CO3 2−]T) and [DIC] was varied over approximately 0 to 40% and 0 to 36%, respectively, by manipulating solution chemistry. Despite such chemical treatment, there was no effect of Mg2+-addition on . These results suggest that the presence of Mg2+ in solutions has a negligible effect on the oxygen isotope equilibrium in the CO2–H2O system. In seawater, Mg2+ is the most important cation that forms complexes with CO3 2−. Hence, if our results also hold at higher [Mg2+] and higher ionic strength, they imply that the applicability of freshwater-based equilibrium fractionation factors is not compromised by ionic interactions in seawater. [Copyright &y& Elsevier]

• We show spatial and seasonal patterns of the carbonate system in the northern basin of the South China Sea (SCS) based, on a thus far, the largest dataset in a major world marginal sea. • Exchange fluxes of the carbonate variables between the northern SCS and West Philippine Sea (WPS) via the Luzon Strait are constrained in the upper layer by Kuroshio intrusion and over the full depth range. • We reveal significant spatial and seasonal changes in the carbonate chemistry of the deep SCS below 2000 db, attributable likely to spatial and seasonal variations in organic matter decomposition. Using a most comprehensive, high quality, high-resolution dataset for any marginal sea up to depths >2000 db, we examined the seasonality of the carbonate system in the northern South China Sea (nSCS) and exchange with the West Philippine Sea (WPS) during 2009–2011. The carbonate system dynamics demonstrated evident spatial and seasonal variations. Winter exhibited the highest average surface dissolved inorganic carbon (DIC) concentrations (1936 ± 15 μmol kg−1), and summer had the lowest (1882 ± 12 μmol kg−1), primarily associated with more abundant freshwater inputs in summer. At 100 db depth, decreased DIC and total alkalinity (TA) values were observed within the Luzon Strait vicinity due to the influence of WPS waters. Higher DIC and TA concentrations were found within the central nSCS basin. The average Kuroshio contribution to the DIC inventory in the upper 150 db was seasonally significant, ranging between 11 and 32%, with the highest contributions during spring and winter. Below 2000 db, nSCS basin-averaged DIC was significantly higher than WPS-averaged DIC (∼23 µmol kg−1 difference) due to more organic matter decomposition in the nSCS basin. Within the basin, average deep water DIC values were highest in autumn, and averaged concentrations at >18.5°N were lower than at ≤18.5°N. Our datasets and analysis imply that (i) the significant seasonal and spatial patterns of carbonate chemistry in the nSCS are controlled by a combination of large-scale and smaller mesoscale physical processes; (ii) extrinsically from Asian monsoons via seasonal freshwater discharge and dynamic exchanges with open ocean waters; and (iii) intrinsically, through seasonal vertical mixing as well as mesoscale processes and their subsequent new productions. The seasonal and spatial variability in carbonate parameters established here serves as an essential baseline to monitor future changes to the nSCS and to compare with other marginal sea systems. [ABSTRACT FROM AUTHOR]

Prior studies of surface seawater CO 2 partial pressure (p CO 2) and air-sea CO 2 fluxes have primarily been conducted on the eastern Bering Sea shelf area, with a paucity of data in the Bering Sea basin. In order to assess the surface variability and the air-sea CO 2 fluxes for a more complete set of different regions, underway surface seawater p CO 2 and related parameters were investigated across the Bering Sea basin, slope and shelf in July 2010 during the 4th Chinese National Arctic Research Expedition (CHINARE). The surface p CO 2 exhibited large spatial variability and was observed to vary from 137 μatm in the central Bering Strait to 481 μatm in the western Bering Strait. In the central Bering Strait, the high supersaturation with respect to the atmospheric p CO 2 (378 ± 2 μatm) was driven by the upwelling event. The neutral or weak CO 2 sink in the Bering Sea basin and eastern nearshore region were related to high nutrient low chlorophyll status and riverine input, respectively. Biological process maintained the most shelf and slope areas as a strong CO 2 sink. Overall, despite a small CO 2 outgassing area the whole Bering Sea still acted as a net ocean CO 2 sink of −6.4 ± 0.9 mmol m−2 d−1 in summer. • Air-sea CO 2 fluxes of a more complete set of different Bering Sea regions than most prior studies were reported. • Despite a small CO 2 outgassing area the Bering Sea still acted as a net ocean CO 2 sink of -6.4±0.9 mmol m-2 d-1 in summer. • Large spatial variations of p CO 2 and air-sea CO 2 fluxes were observed corresponding to diverse controlling processes. [ABSTRACT FROM AUTHOR]

Kelp forests are known as key habitats for species diversity and macroalgal productivity; however, we know little about how these biogenic habitats interact with seawater chemistry and phototroph productivity in the water column. We examined kelp forest functions at three locales along the Olympic Peninsula of Washington state by quantifying carbonate chemistry, nutrient concentrations, phytoplankton productivity, and seawater microbial communities inside and outside of kelp beds dominated by the canopy kelp species Nereocystis luetkeana and Macrocystis pyrifera. Kelp beds locally increased the pH, oxygen, and aragonite saturation state of the seawater, but lowered seawater inorganic carbon content and total alkalinity. Although kelp beds depleted nitrate and phosphorus concentrations, ammonium and dissolved organic carbon (DOC) concentrations were enhanced. Kelp beds also decreased chlorophyll concentrations and carbon fixed by phytoplankton, although kelp carbon fixation more than compensated for any difference in phytoplankton production. Kelp beds entrained distinct microbial communities, with higher taxonomic and phylogenetic diversity compared to seawater outside of the kelp bed. Kelp forests thus had significant effects on seawater chemistry, productivity and the microbial assemblages in their proximity. Thereby, the diversity of pathways for carbon and nitrogen cycling was also enhanced. Overall, these observations suggest that the contribution of kelp forests to nearshore carbon and nitrogen cycling is greater than previously documented. [ABSTRACT FROM AUTHOR]

Microorganisms confined to annual sea ice in the Southern Ocean are exposed to highly variable oxygen and carbonate chemistry dynamics because of the seasonal increase in biomass and limited exchange with the underlying water column. For sea-ice algae, physiological stress is likely to be exacerbated when the ice melts; however, variation in carbonate speciation has rarely been monitored during this important state-transition. Using pulse amplitude modulated fluorometry (Imaging-PAM, Walz), we documented in situ changes in the maximum quantum yield of photosystem II (Fv/Fm) of sea-ice algae melting out into seawater with initial pH values ranging from 7.66 to 6.39. Although the process of ice-melt elevated seawater pH by 0.2-0.55 units, we observed a decrease in Fv/Fm between 0.02 and 0.06 for each unit drop in pH during real-time fluorescence imaging. These results are considered preliminary but provide context for including carbonate chemistry monitoring in the design of future sea ice state-transition experiments. Imaging-PAM is a reliable technology for determining Fv/Fm, but is of limited use for obtaining additional photosynthetic parameters when imaging melting ice. [ABSTRACT FROM AUTHOR]