Seawater pCO2 Research Articles

Elevated temperature and carbon dioxide levels alter growth rates and shell composition in the fluted giant clam, Tridacna squamosa

Giant clams produce massive calcified shells with important biological (e.g., defensive) and ecological (e.g., habitat-forming) properties. Whereas elevated seawater temperature is known to alter giant clam shell structure, no study has examined the effects of a simultaneous increase in seawater temperature and partial pressure of carbon dioxide (pCO2) on shell mineralogical composition in these species. We investigated the effects of 60-days exposure to end-of-the-century projections for seawater temperature (+ 3 °C) and pCO2 (+ 500 µatm) on growth, mineralogy, and organic content of shells and scutes in juvenile Tridacna squamosa giant clams. Elevated temperature had no effect on growth rates or organic content, but did increase shell [24Mg]/[40Ca] as well as [40Ca] in newly-formed scutes. Elevated pCO2 increased shell growth and whole animal mass gain. In addition, we report the first evidence of an effect of elevated pCO2 on element/Ca ratios in giant clam shells, with significantly increased [137Ba]/[40Ca] in newly-formed shells. Simultaneous exposure to both drivers greatly increased inter-individual variation in mineral concentrations and resulted in reduced shell N-content which may signal the onset of physiological stress. Overall, our results indicate a greater influence of pCO2 on shell mineralogy in giant clams than previously recognized.

Persistent Continental Shelf Carbon Sink at the Ieodo Ocean Research Station in the Northern East China Sea

Hourly (2017–2021) to seasonal (2015–2021) inorganic C data were collected at the Ieodo Ocean Research Station (32.07°N and 125.10°E) in the northern East China Sea (ECS), located under the influence of the nutrient-rich Changjiang Diluted Water (CDW). An increase in phytoplankton biomass from April to mid-August (the warming period) equalized much of the temperature-driven increase in the surface pCO2 and thus, made the northern ECS a moderate sink of atmospheric CO2. From November to March (the cooling period), a large pCO2 reduction, driven by a temperature reduction, and a high air–sea CO2 exchange rate, because of high windspeeds, transformed the basin into a substantial CO2 sink, yielding an annual net C uptake of 61.7 g C m–2 yr–1. The effects of biological production and temperature change on seawater pCO2 (and thus, the net air–sea CO2 flux) were decoupled each season and acted in concert to increase the net annual CO2 sink by the region. The present study provided the observational and mechanistic lines of evidence for confirming “continental shelf C pump”—a mechanism in the shallow waters of the continental shelves that accumulate a significant amount of C (via reinforced cooling and promoted biological C uptake) that is transported from the basin surface waters to the interior of the adjacent deep ocean. In the future, an increasing input of anthropogenic nutrients into the northern ECS is likely to make the region a stronger CO2 sink.

Monthly and seasonal variations in the surface carbonate system and air–sea CO2 flux of the Yellow Sea

Surface carbonate chemistry in the Yellow Sea was investigated based on discrete seawater samples collected from 2017 to 2020 at the Socheongcho Ocean Research Station (S-ORS; 37.423°N, 124.738°E). Records of carbon parameters, including seawater CO2 partial pressure (pCO2), revealed considerable seasonal variations, with amplitudes comparable to those observed across the western part of the Yellow Sea. The study site acted as a modest sink (−0.13 mol C m−2 yr−1) for atmospheric CO2. Biological processes (primary production and respiration) and physical conditions (temperature and degree of stratification) determined seawater pCO2, which fluctuated on an intraseasonal timescale between oversaturated and undersaturated with respect to atmospheric pCO2. Variation in pCO2 was significant in summer, depending on the biological carbon drawdown and tidal mixing-induced upwelling (increased pCO2 up to ~1000 μatm). The intraseasonal variability in seawater pCO2 may bias estimated air–sea CO2 fluxes, if measurements with a coarser (seasonal) time resolution are used.

Meta-analysis suggests negative, but pCO2-specific, effects of ocean acidification on the structural and functional properties of crustacean biomaterials.

Crustaceans comprise an ecologically and morphologically diverse taxonomic group. They are typically considered resilient to many environmental perturbations found in marine and coastal environments, due to effective physiological regulation of ions and hemolymph pH, and a robust exoskeleton. Ocean acidification can affect the ability of marine calcifying organisms to build and maintain mineralized tissue and poses a threat for all marine calcifying taxa. Currently, there is no consensus on how ocean acidification will alter the ecologically relevant exoskeletal properties of crustaceans. Here, we present a systematic review and meta‐analysis on the effects of ocean acidification on the crustacean exoskeleton, assessing both exoskeletal ion content (calcium and magnesium) and functional properties (biomechanical resistance and cuticle thickness). Our results suggest that the effect of ocean acidification on crustacean exoskeletal properties varies based upon seawater pCO2 and species identity, with significant levels of heterogeneity for all analyses. Calcium and magnesium content was significantly lower in animals held at pCO2 levels of 1500–1999 µatm as compared with those under ambient pCO2. At lower pCO2 levels, however, statistically significant relationships between changes in calcium and magnesium content within the same experiment were observed as follows: a negative relationship between calcium and magnesium content at pCO2 of 500–999 µatm and a positive relationship at 1000–1499 µatm. Exoskeleton biomechanics, such as resistance to deformation (microhardness) and shell strength, also significantly decreased under pCO2 regimes of 500–999 µatm and 1500–1999 µatm, indicating functional exoskeletal change coincident with decreases in calcification. Overall, these results suggest that the crustacean exoskeleton can be susceptible to ocean acidification at the biomechanical level, potentially predicated by changes in ion content, when exposed to high influxes of CO2. Future studies need to accommodate the high variability of crustacean responses to ocean acidification, and ecologically relevant ranges of pCO2 conditions, when designing experiments with conservation‐level endpoints.

Influence of ocean warming and acidification on habitat-forming coralline algae and their associated molluscan assemblages

When ocean warming and acidification impact habitat-forming species, substantial alterations to the supported ecological communities and associated ecosystems are likely to follow. Here, we used experimental manipulations to test the hypotheses that ocean warming and acidification would negatively affect habitat-forming coralline algal turfs and the diverse molluscan assemblages they support. Boulders covered in a turf of Amphiroa anceps with intact faunal assemblages were subjected to an orthogonal combination of current (~ 23 °C) and future (~ 26 °C) ocean temperatures, and current (~ 430 µatm) and future (~ 880 µatm) seawater pCO 2 . Ocean warming negatively impacted the cover and photosynthetic efficiency of Amphiroa fronds, whereas ocean acidification caused the biomass per unit area and the frond density of Amphiroa turf to be 56% and 59% less than current ocean conditions, respectively. Ocean acidification also caused a significant change in the structure of molluscan assemblages associated with Amphiroa turf, which included a 43% and a 61% reduction in the species richness and overall abundance of molluscs, respectively. The results demonstrate that coralline algal turfs are particularly vulnerable to ocean climate change, which has implications for the biodiversity and ecosystem functions supported by these globally distributed foundation species. • We assessed whether ocean warming and acidification impacts habitat-forming coralline algal turfs and their associated molluscan assemblages. • Ocean warming negatively impacted the cover and photosynthetic efficiency of coralline fronds. • Ocean acidification caused a 56% and a 59% reduction in the biomass and frond density of coralline turfs, respectively. • Ocean acidification caused a decrease in the richness and abundance of molluscs in coralline turfs by 43% and 61%, respectively.

The Potential of Kelp Saccharina japonica in Shielding Pacific Oyster Crassostrea gigas From Elevated Seawater pCO2 Stress

Ocean acidification (OA) caused by elevated atmospheric CO2 concentration is predicted to have negative impacts on marine bivalves in aquaculture. However, to date, most of our knowledge is derived from short-term laboratory-based experiments, which are difficult to scale to real-world production. Therefore, field experiments, such as this study, are critical for improving ecological relevance. Due to the ability of seaweed to absorb dissolved carbon dioxide from the surrounding seawater through photosynthesis, seaweed has gained theoretical attention as a potential partner of bivalves in integrated aquaculture to help mitigate the adverse effects of OA. Consequently, this study investigates the impact of elevated pCO2 on the physiological responses of the Pacific oyster Crassostrea gigas in the presence and absence of kelp (Saccharina japonica) using in situ mesocosms. For 30 days, mesocosms were exposed to six treatments, consisting of two pCO2 treatments (500 and 900 μatm) combined with three biotic treatments (oyster alone, kelp alone, and integrated kelp and oyster aquaculture). Results showed that the clearance rate (CR) and scope for growth (SfG) of C. gigas were significantly reduced by elevated pCO2, whereas respiration rates (MO2) and ammonium excretion rates (ER) were significantly increased. However, food absorption efficiency (AE) was not significantly affected by elevated pCO2. The presence of S. japonica changed the daytime pHNBS of experimental units by ~0.16 units in the elevated pCO2 treatment. As a consequence, CR and SfG significantly increased and MO2 and ER decreased compared to C. gigas exposed to elevated pCO2 without S. japonica. These findings indicate that the presence of S. japonica in integrated aquaculture may help shield C. gigas from the negative effects of elevated seawater pCO2.

Effects of seawater pCO2 on the skeletal morphology of massive Porites spp. corals

Ocean acidification alters the dissolved inorganic carbon chemistry of seawater and can reduce the calcification rates of tropical corals. Here we explore the effect of altering seawater pCO2 on the skeletal morphology of 4 genotypes of massive Porites spp. which display widely different calcification rates. Increasing seawater pCO2 causes significant changes in in the skeletal morphology of all Porites spp. studied regardless of whether or not calcification was significantly affected by seawater pCO2. Both the median calyx size and the proportion of skeletal surface occupied by the calices decreased significantly at 750 µatm compared to 400 µatm indicating that polyp size shrinks in this genus in response to ocean acidification. The coenosteum, connecting calices, expands to occupy a larger proportion of the coral surface to compensate for this decrease in calyx area. At high seawater pCO2 the spines deposited at the skeletal surface became more numerous and the trabeculae (vertical skeletal pillars) became significantly thinner in 2 of the 4 genotypes. The effect of high seawater pCO2 is most pronounced in the fastest growing coral and the regular placement of trabeculae and synapticulae is disturbed in this genotype resulting in a skeleton that is more randomly organised. The study demonstrates that ocean acidification decreases the polyp size and fundamentally alters the architecture of the skeleton in this major reef building species from the Indo-Pacific Ocean.

A monthly surface <i>p</i>CO<sub>2</sub> product for the California Current Large Marine Ecosystem

Abstract. A common strategy for calculating the direction and rate of carbon dioxide gas (CO2) exchange between the ocean and atmosphere relies on knowledge of the partial pressure of CO2 in surface seawater (pCO2(sw)), a quantity that is frequently observed by autonomous sensors on ships and moored buoys, albeit with significant spatial and temporal gaps. Here we present a monthly gridded data product of pCO2(sw) at 0.25∘ latitude by 0.25∘ longitude resolution in the northeastern Pacific Ocean, centered on the California Current System (CCS) and spanning all months from January 1998 to December 2020. The data product (RFR-CCS; Sharp et al., 2022; https://doi.org/10.5281/zenodo.5523389) was created using observations from the most recent (2021) version of the Surface Ocean CO2 Atlas (Bakker et al., 2016). These observations were fit against a variety of collocated and contemporaneous satellite- and model-derived surface variables using a random forest regression (RFR) model. We validate RFR-CCS in multiple ways, including direct comparisons with observations from sensors on moored buoys, and find that the data product effectively captures seasonal pCO2(sw) cycles at nearshore sites. This result is notable because global gridded pCO2(sw) products do not capture local variability effectively in this region, suggesting that RFR-CCS is a better option than regional extractions from global products to represent pCO2(sw) in the CCS over the last 2 decades. Lessons learned from the construction of RFR-CCS provide insight into how global pCO2(sw) products could effectively characterize seasonal variability in nearshore coastal environments. We briefly review the physical and biological processes – acting across a variety of spatial and temporal scales – that are responsible for the latitudinal and nearshore-to-offshore pCO2(sw) gradients seen in the RFR-CCS reconstruction of pCO2(sw). RFR-CCS will be valuable for the validation of high-resolution models, the attribution of spatiotemporal carbonate system variability to physical and biological drivers, and the quantification of multiyear trends and interannual variability of ocean acidification.

Surface Seawater pCO2 Variation after a Typhoon Passage in the Kuroshio off Eastern Taiwan

In this study, two cruises were conducted across the mainstream of the Kuroshio off eastern Taiwan before and after the passage of Typhoon Saola in summer 2012. The continuous underway pCO2 (i.e., partial pressure of CO2) measurements revealed that surface seawater pCO2 (SS pCO2) displayed spatial variations in response to typhoon passage. The simulated results showed that the mixed-layer deepening after typhoon passage had a minor effect on SS pCO2 variation because pCO2 decrease driven by temperature dropdown and enhanced biological production fueled by nutrients input was largely compensated by pCO2 increase caused by salinity increase and dissolved inorganic carbon input from the subsurface layer. By contrast, the advection pattern showed significant change before and after the typhoon, which could play a major role in controlling the variation of SS pCO2. In the exit area of the cyclonic eddy, SS pCO2 decreased, while in the area of its arrival, SS pCO2 increased. Besides, the discharge of freshwater and the intrusion of the South China Sea subsurface could result in SS pCO2 increase in the nearshore area. The present study highlights that more advection changes need to be considered to better understand the impact of the typhoon on SS pCO2, especially in the strong current area, such as the Kuroshio.

Mesoscale Advective and Biological Processes Alter Carbon Uptake Capacity in a Shelf Sea

Marine uptake of carbon dioxide reduces the accumulation of carbon dioxide in the atmosphere. Continental shelf seas are essential for carbon uptake from the atmosphere, but are also highly variable environments, for which uncertainties of carbon budget estimates are large. Recent studies indicate that their carbon sink capacity is weakening. A way to reduce the uncertainty of carbon budgets is to increase our observational capacity, for example through FerryBox installations on Ships-of-Opportunity. Here, we compare FerryBox observations in the North Sea for the fall seasons of 2019 and 2020. We show that short-lived mesoscale events can be characterized when the sampling resolution is adequately high, and that these events cause changes in essential environmental variables on the same magnitude as seasonal cycles. Whether advective or biological in origin, these events rapidly lowered seawater pCO2 by 8–10% and influenced the carbon uptake capacity. We demonstrate the importance of resolving and integrating the variability of these smaller features in regional carbon budget assessments and advocate for the tuning of models in order to capture this small-scale variability.

The combined effects of ocean acidification and copper on the physiological responses of the tropical coral Stylophora pistillata

A decrease in ocean pH of 0.3 units will likely double the proportion of dissolved copper (Cu) present as the free metal ion, Cu2+, the most bioavailable form of Cu, and one of the most common marine pollutants. We assess the impact of ocean acidification and Cu, separately and in combination, on calcification, photosynthesis and respiration of sub-colonies of a single tropical Stylophora pistillata colony. After 15 days of treatment, total calcification rates were significantly decreased in corals exposed to high seawater pCO2 (∼1000-μatm, 2100 scenario) and at both ambient (1.6–1.9 nmols) and high (2.5–3.6 nmols) dissolved Cu concentrations compared to controls. The effect was increased when both stressors were combined. Coral respiration rates were significantly reduced by the combined stressors after 2 weeks of exposure, indicating the importance of experiment duration. It is therefore likely rising atmospheric CO2 will exacerbate the negative effects of Cu pollution to S. pistillata.

Abrupt upwelling and CO2 outgassing episodes in the north-eastern Arabian Sea since mid-Holocene

Identifying the causes and consequences of natural variations in ocean acidification and atmospheric CO2 due to complex earth processes has been a major challenge for climate scientists in the past few decades. Recent developments in the boron isotope (δ11B) based seawater pH and pCO2 (or pCO2sw) proxy have been pivotal in understanding the various oceanic processes involved in air-sea CO2 exchange. Here we present the first foraminifera-based δ11B record from the north-eastern Arabian Sea (NEAS) covering the mid-late Holocene (~ 8–1 ka). Our record suggests that the region was overall a moderate to strong CO2 sink during the last 7.7 kyr. The region behaved as a significant CO2 source during two short intervals around 5.5–4 ka and 2.8–2.5 ka. The decreased pH and increased CO2 outgassing during those abrupt episodes are associated with the increased upwelling in the area. The upwelled waters may have increased the nutrient content of the surface water through either increased supply or weaker export production. This new dataset from the coastal NEAS suggests that, as a potential result of changes in the strength of the El-Nino Southern Oscillation, the region experienced short episodes of high CO2 outgassing and pre-industrial ocean acidification comparable to or even greater than that experienced during the last ~ 200 years.

Marine CO<sub>2</sub> system variability along the northeast Pacific Inside Passage determined from an Alaskan ferry

Abstract. Information on marine CO2 system variability has been limited along the northeast Pacific Inside Passage despite the region's rich biodiversity, abundant fisheries, and developing aquaculture industry. Beginning in 2017, the Alaska Marine Highway System M/V Columbia has served as a platform for surface underway data collection while conducting twice weekly ∼1600 km transits between Bellingham, Washington, and Skagway, Alaska. Marine CO2 system patterns were evaluated using measurements made over a 2-year period, which revealed the seasonal cycle as the dominant mode of temporal variability. The amplitude of this signal varied spatially and was modulated by the relative influences of tidal mixing, net community production, and the magnitude and character of freshwater input. Surface water pHT (total hydrogen ion scale) and aragonite saturation state (Ωarag) were determined using carbon dioxide partial pressure (pCO2) data with alkalinity derived from a regional salinity-based relationship, which was evaluated using intervals of discrete seawater samples and underway pH measurements. High-pCO2, low-pHT, and corrosive Ωarag conditions (Ωarag<1) were seen during winter and within persistent tidal mixing zones, and corrosive Ωarag values were also seen in areas that receive significant glacial melt in summer. Biophysical drivers are shown to dominate pCO2 variability over most of the Inside Passage except in areas highly impacted by glacial melt. pHT and Ωarag extremes were also characterized based on degrees of variability and severity, and regional differences were evident. Computations of the time of detection identified tidal mixing zones as strategic observing sites with relatively short time spans required to capture secular trends in seawater pCO2 equivalent to the contemporary rise in atmospheric CO2. Finally, estimates of anthropogenic CO2 showed notable spatiotemporal variability. Changes in total hydrogen ion content ([H+]T), pHT, and Ωarag over the industrial era and to an atmospheric pCO2 level consistent with a 1.5 ∘C warmer climate were theoretically evaluated. These calculations revealed greater absolute changes in [H+]T and pHT in winter as opposed to larger Ωarag change in summer. The contemporary acidification signal everywhere along the Inside Passage exceeded the global average, with specific areas, namely Johnstone Strait and the Salish Sea, standing out as potential bellwethers for the emergence of biological ocean acidification (OA) impacts. Nearly half of the contemporary acidification signal is expected over the coming 15 years, with an atmospheric CO2 trajectory that continues to be shaped by fossil–fuel development.

Decreased Diversity and Abundance of Marine Invertebrates at CO2 Seeps in Warm-Temperate Japan.

Japan has many coastal carbon dioxide seeps as it is one of the most volcanically active parts of the world. These shallow seeps do not have the spectacular aggregations of specialist fauna seen in deep-sea vent systems but they do have gradients in seawater carbonate chemistry that are useful as natural analogues of the effects of ocean acidification on marine biodiversity, ecosystem function and fisheries. Here, we compare macroinvertebrate diversity and abundance on rocky habitats at ambient (mean ≤ 410 µatm) and high (mean 971-1484 µatm) levels of seawater pCO2 in the warm-temperate region of Japan, avoiding areas with toxic sulphur or warm-water conditions. We show that although 70% of intertidal taxa and 40% of shallow subtidal taxa were able to tolerate the high CO2 conditions, there was a marked reduction in the abundance of corals, bivalves and gastropods in acidified conditions. A narrower range of filter feeders, grazers, detritivores, scavengers and carnivores were present at high CO2 resulting in a simplified coastal system that was unable to retain the high standing stocks of marine carbon biomass found in ambient conditions. It is clear that cuts in CO2 emissions would reduce the risks of climate change and ocean acidification impacts on marine biodiversity, shellfish production and biomass in the rocky coastal shores of this region.

Derivation of seawater <i>p</i>CO<sub>2</sub> from net community production identifies the South Atlantic Ocean as a CO<sub>2</sub> source

Abstract. A key step in assessing the global carbon budget is the determination of the partial pressure of CO2 in seawater (pCO2 (sw)). Spatially complete observational fields of pCO2 (sw) are routinely produced for regional and global ocean carbon budget assessments by extrapolating sparse in situ measurements of pCO2 (sw) using satellite observations. As part of this process, satellite chlorophyll a (Chl a) is often used as a proxy for the biological drawdown or release of CO2. Chl a does not, however, quantify carbon fixed through photosynthesis and then respired, which is determined by net community production (NCP). In this study, pCO2 (sw) over the South Atlantic Ocean is estimated using a feed forward neural network (FNN) scheme and either satellite-derived NCP, net primary production (NPP) or Chl a to compare which biological proxy produces the most accurate fields of pCO2 (sw). Estimates of pCO2 (sw) using NCP, NPP or Chl a were similar, but NCP was more accurate for the Amazon Plume and upwelling regions, which were not fully reproduced when using Chl a or NPP. A perturbation analysis assessed the potential maximum reduction in pCO2 (sw) uncertainties that could be achieved by reducing the uncertainties in the satellite biological parameters. This illustrated further improvement using NCP compared to NPP or Chl a. Using NCP to estimate pCO2 (sw) showed that the South Atlantic Ocean is a CO2 source, whereas if no biological parameters are used in the FNN (following existing annual carbon assessments), this region appears to be a sink for CO2. These results highlight that using NCP improved the accuracy of estimating pCO2 (sw) and changes the South Atlantic Ocean from a CO2 sink to a source. Reducing the uncertainties in NCP derived from satellite parameters will ultimately improve our understanding and confidence in quantification of the global ocean as a CO2 sink.

High-latitude calcified coralline algae exhibit seasonal vulnerability to acidification despite physical proximity to a non-calcified alga

The emergent responses of vulnerable species to global change can vary depending on the relative quality of resources available to support their productivity under increased stress, as well as the biotic interactions with other species that may alter their access to these resources. This research tested how seawater pCO2 may interact with seasonal light availability to affect the photosynthesis and calcification of high-latitude coralline algae, and whether the responses of these calcified macroalgae are modified by physical association with a non-calcified seaweed. Through an in situ approach, our study first investigated how current seasonal environmental variation affects the growth of the understory coralline algae Crusticorallina spp. and Bossiella orbigniana in Southeast Alaska's kelp forests. We then experimentally manipulated pH to simulate end-of-century acidification scenarios, light regime to simulate seasonal light availability at the benthos, and pairings of coralline algal species with and without a fleshy red alga to examine the interactive effects of these variables on coralline productivity and calcification. Our results indicate that: 1) coralline species may face net dissolution under projected future winter pH and carbonate saturation state conditions, 2) differences in seasonal light availability in productive, high-latitude waters may not be distinct enough to modify coralline algal net calcification, and 3) association with a non-calcified red alga does not alter the response of these coralline algal species to ocean acidification scenarios. This research highlights the necessity of incorporating locally informed scenarios of environmental variability and community interactions when predicting species’ vulnerability to global change.

Effects of ocean acidification on the performance and interaction of fleshy macroalgae and a grazing sea urchin

When predicting the response of marine ecosystems to climate change, it is increasingly recognized that understanding the indirect effects of ocean acidification on trophic interactions is as important as studying direct effects on organism physiology. Furthermore, comprehensive studies that examine these effects simultaneously are needed to identify and link the underlying mechanisms driving changes in species interactions. Using an onshore ocean acidification simulator system, we investigated the direct and indirect effects of elevated seawater pCO2 on the physiology and trophic interaction of fleshy macroalgae and the grazing sea urchin Lytechinus variegatus. Macroalgal (Dictyota spp.) biomass increased despite decreased photosynthetic rates after two-week exposure to elevated pCO2. Algal tissue carbon content remained constant, suggesting the use of alternative carbon acquisition pathways beneficial to growth under acidification. Higher C:N ratios driven by a slight reduction in N content in algae exposed to elevated pCO2 suggest a decrease in nutritional content under acidification. Urchin (L. variegatus) respiration, biomass, and righting time did not change significantly after six-week exposure to elevated pCO2, indicating that physiological stress and changes in metabolism are not mechanisms through which the trophic interaction was impacted. Correspondingly, urchin consumption rates of untreated macroalgae (Caulerpa racemosa) were not significantly affected by pCO2. In contrast, exposure of urchins to elevated pCO2 significantly reduced the number of correct foraging choices for ambient macroalgae (Dictyota spp.), indicating impairment of urchin chemical sensing under acidification. However, exposure of algae to elevated pCO2 returned the number of correct foraging choices in similarly exposed urchins to ambient levels, suggesting alongside higher C:N ratios that algal nutritional content was altered in a way detectable by the urchins under acidification. These results highlight the importance of studying the indirect effects of acidification on trophic interactions simultaneously with direct effects on physiology. Together, these results suggest that changes to urchin chemical sensing and algal nutritional quality are the driving mechanisms behind surprisingly unaltered urchin foraging behavior for fleshy macroalgae under joint exposure to ocean acidification. Consistent foraging behavior and consumption rates suggest that the trophic interaction between L. variegatus and fleshy macroalgae may be sustained under future acidification. However, increases in fleshy macroalgal biomass driven by opportunistic carbon acquisition strategies have the potential to cause ecological change, depending on how grazer populations respond. Additional field research is needed to determine the outcome of these results over time and under a wider range of environmental conditions.

Experimental Studies on the Impact of the Projected Ocean Acidification on Fish Survival, Health, Growth, and Meat Quality; Black Sea Bream (Acanthopagrus schlegelii), Physiological and Histological Studies.

Simple SummaryThis study’s data suggest that under the projected scenarios of ocean acidification by 2100 and beyond, significant negative impacts on growth, health, and meat quality are expected, particularly on black sea bream, and will be susceptible to the scientifically approved fish having a weaker resistance to diseases and environmental changes if CO2 emissions in the atmosphere are not curbed. Knowing the expected consequences, mitigation measures are urgently needed.Acidification (OA), a global threat to the world’s oceans, is projected to significantly grow if CO2 continues to be emitted into the atmosphere at high levels. This will result in a slight decrease in pH. Since the latter is a logarithmic scale of acidity, the higher acidic seawater is expected to have a tremendous impact on marine living resources in the long-term. An 8-week laboratory experiment was designed to assess the impact of the projected pH in 2100 and beyond on fish survival, health, growth, and fish meat quality. Two projected scenarios were simulated with the control treatment, in triplicates. The control treatment had a pH of 8.10, corresponding to a pCO2 of 321.37 ± 11.48 µatm. The two projected scenarios, named Predict_A and Predict_B, had pH values of 7.80-pCO2 = 749.12 ± 27.03 and 7.40-pCO2 = 321.37 ± 11.48 µatm, respectively. The experiment was preceded by 2 weeks of acclimation. After the acclimation, 20 juvenile black sea breams (Acanthopagrus schlegelii) of 2.72 ± 0.01 g were used per tank. This species has been selected mainly due to its very high resistance to diseases and environmental changes, assuming that a weaker fish resistance will also be susceptibly affected. In all tanks, the fish were fed with the same commercial diet. The seawater’s physicochemical parameters were measured daily. Fish samples were subjected to physiological, histological, and biochemical analyses. Fish growth, feeding efficiency, protein efficiency ratio, and crude protein content were significantly decreased with a lower pH. Scanning electron microscopy revealed multiple atrophies of microvilli throughout the small intestine’s brush border in samples from Predict_A and Predict_B. This significantly reduced nutrient absorption, resulting in significantly lower feed efficiency, lower fish growth, and lower meat quality. As a result of an elevated pCO2 in seawater, the fish eat more than normal but grow less than normal. Liver observation showed blood congestion, hemorrhage, necrosis, vacuolation of hepatocytes, and an increased number of Kupffer cells, which characterize liver damage. Transmission electron microscopy revealed an elongated and angular shape of the mitochondrion in the liver cell, with an abundance of peroxisomes, symptomatic of metabolic acidosis.

Red Tide Events and Seasonal Variations in the Partial Pressure of CO2 and Related Parameters in Shellfish-Farming Bays, Southeastern Coast of Korea

Mixed results have been reported on the evaluation of the coastal carbon cycle and its contribution to the global carbon cycle, mainly due to the shortage of observational data and the considerable spatiotemporal variability arising from complex biogeochemical factors. In this study, the partial pressure of carbon dioxide (pCO2) and related environmental factors were measured in the Jinhae–Geoje–Tongyeong bay region of the southeastern Korean Peninsula in February 2014, August 2014, April 2015, and October 2015. The mean pCO2 of surface seawater ranged from 215 to 471 μatm and exhibited a high correlation with the surface seawater temperature when data for August were excluded (R2 = 0.69), indicating that the seasonal variation in CO2 could be largely attributed to the variation in seawater temperature. However, a severe red tide event occurred in August 2014, when the lowest pCO2 value was observed despite a relatively high seawater temperature. It is considered that the active biological production of phytoplankton related to red tides counteracted the summer increase in pCO2. Based on the correlation between pCO2 and temperature, the estimated decrease in pCO2 caused by non-thermal factors was approximately 200 μatm. During the entire study period, the air–sea CO2 flux ranged from −14.2 to 3.7 mmol m–2 d–1, indicating that the study area served as an overall sink for atmospheric CO2, and only functioned as a weak source during October. The mean annual CO2 flux estimated from the correlation with temperature was −5.1 mmol m–2 d–1. However, because this estimate did not include reductions caused by sporadic events of biological production, such as red tides and phytoplankton blooms, the actual uptake flux is considered to be higher. The mean saturation state (ΩAr) value of carbonate aragonite was 2.61 for surface water and 2.04 for bottom water. However, the mean ΩAr of bottom water was <2 in August and October, and the ΩAr values measured at some of the bottom water stations in August were <1. Considering that the period from August to October corresponds to the reproduction and growth stages of shellfish, such low ΩAr values could be very damaging to shellfish production and the aquaculture industry.

Seawater chemistry of a modern subtropical ‘epeiric’ sea: Spatial variability and effects of organic decomposition

Direct studies of recent epeiric seawater chemistry and their geological significance are relatively few compared to the far more abundant studies from ancient epeiric marine carbonates. Acknowledging the limitations of analogue studies in the modern glaciated world, we propose that the recent Abu Dhabi offshore and lagoonal areas share important attributes with the shallow-marine coastal zones of a subtropical epeiric carbonate sea. The study area in Abu Dhabi is dwarfed by the vast epeiric seas of the geological past, but its seawater properties and related geochemistry have relevance for paleoenvironmental interpretations for similar ancient systems. This study documents the geochemical properties of seawater along the Abu Dhabi coast, and discusses their trends within a spatial context. Results show a positive correlation between salinity, temperature, elemental concentrations, and oxygen isotopic composition of seawater, indicating a significant influence of evaporation on seawater chemistry. A distal-to-proximal negative correlation between seawater Mg/Ca and Sr/Ca ratios may be related to a landward increase in abundance of high-Mg calcite foraminifera. After normalization to chloride, seawater elemental (Ca2+, Mg2+, Sr2+) concentrations and alkalinity show decreasing trends in a landward direction, consistent with seawater aging from the open marine to restricted lagoonal settings. Lower ΔAlk (alkalinity change) : ΔCa2+ (Ca2+ change) ratios in seawater compared with the theoretical value, and more negative δ13CDIC values relative to typical (sub)tropical oceans in the study area indicate an influence of factors apart from CaCO3 precipitation on properties of aged seawater. An input of alkalinity and 12C-enriched carbon by organic carbon decomposition (via sulfate reduction) during early diagenesis is proposed to account for these deviations from typical values. The input of alkalinity and 12C-enriched carbon by organic carbon decomposition may be further related to reduced seawater pCO2 and early-marine cementation. Findings discussed here suggest that organic carbon decomposition during early-marine diagenesis may influence the geochemical properties of the coastal areas of shallow epeiric seas, and thus assumedly produce significant differences in proxy data compared with those from carbonates deposited in open oceans.