Aragonite Saturation State Research Articles

Rapid Acidification of the Arctic Chukchi Sea Waters Driven by Anthropogenic Forcing and Biological Carbon Recycling

AbstractThe acidification of coastal waters is distinguished from the open ocean because of much stronger synergistic effects between anthropogenic forcing and local biogeochemical processes. However, ocean acidification research is still rather limited in polar coastal oceans. Here, we present a 16 year (2002–2018) observational dataset in the Chukchi Sea during the rapid sea‐ice melting season to determine the long‐term changes in pH and aragonite saturation state (Ωarag). We found that pH and Ωarag significantly declined in the water column with average rates of −0.0095 ± 0.0027 years−1 and −0.0333 ± 0.0098 years−1, respectively, and are 4–6 times faster than those solely due to increasing atmospheric CO2. We attributed the rapid acidification to the increased dissolved inorganic carbon owing to a combination of ice melt‐induced increased atmospheric CO2 invasion and subsurface remineralization induced by a stronger surface biological production as a result of the increased inflow of the nutrient‐rich Pacific water.

Implementing the iCORAL (version 1.0) coral reef CaCO3 production module in the iLOVECLIM climate model

Abstract. Coral reef development is intricately linked to both climate and the concentration of atmospheric CO2, specifically through temperature and carbonate chemistry in the upper ocean. In turn, the calcification of corals modifies the concentration of dissolved inorganic carbon (DIC) and total alkalinity in the ocean, impacting air–sea gas exchange, atmospheric CO2 concentration, and ultimately the climate. This feedback between atmospheric conditions and coral biogeochemistry can only be accounted for with a coupled coral–carbon–climate model. Here we present the implementation of a coral reef calcification module into an Earth system model. Simulated coral reef production of the calcium carbonate mineral aragonite depends on photosynthetically active radiation, nutrient concentrations, salinity, temperature, and the aragonite saturation state. An ensemble of 210 parameter perturbation simulations was performed to identify carbonate production parameter values that optimize the simulated distribution of coral reefs and associated carbonate production. The tuned model simulates the presence of coral reefs and regional-to-global carbonate production values in good agreement with data-based estimates, despite some limitations due to the imperfect simulation of climatic and biogeochemical fields driving the simulation of coral reef development. When used in association with methods accounting for bathymetry changes resulting from different sea levels, the model enables assessment of past and future coral–climate coupling on seasonal to millennial timescales, highlighting how climatic trends and variability may affect reef development and the resulting climate–carbon feedback.

Surface carbonate dynamics in a temperate coastal system in the Northern Yellow Sea, China

The carbon sink/source in estuaries and coastal seas may have significant implications for regional climate change. This study investigated seawater carbon dioxide partial pressure (pCO2), aragonite saturation state (Ωarag), and related carbonate parameters in the Yalu River estuary and the adjacent Northern Yellow Sea, China, in September 2023. The results indicate that seawater pCO2 ranged from 261 to 1121 μatm, while Ωarag ranged from 0.49 to 3.34 in the study area. High pCO2 were observed in the estuarine and northwestern areas, whereas low pCO2 were found in the southern offshore area. In the estuarine area, high pCO2 levels were attributed to the input of diluted water from the Yalu River. In the southern areas, low pCO2 were a result of primary production, while community respiration played an important role in increasing seawater pCO2 in the northwestern area. On the other hand, seawater Ωarag was extremely low in the estuarine area, and greater than 1.5 in the southern offshore area. Ωarag in the study area was mainly controlled by water mixing and temperature. The study area acted as a source of atmospheric CO2 with an average air-sea CO2 flux of 6.2 mmol m−2d−1 in autumn, which was relatively high compared with other marginal seas. The high pCO2 and low Ωarag associated with diluted water inputs and community respiration could weaken the ability of coastal oceans to uptake CO2. Therefore, it is crucial to explore ways to reduce CO2 levels in riverine and adjacent coastal seas.

Underestimation of calcium carbonate saturation state in marginal seas due to the disregard of calcium ion addition: A case study of the Bohai sea, China

Seawater calcium ion (Ca2+) concentration was investigated based on the potentiometric titration method during the summer of 2018 in the Bohai Sea, China. The measured Ca2+ concentration ranged from 7760 to 9739 μmol kg−1 and deviated from the theoretical Ca2+ values, which were estimated from the calcium/salinity ratio. The excess calcium (Ca2+excess) ranged from 186 to 1229 μmol kg−1, showing a decreasing trend from the estuary to the nearshore, and then the offshore areas. Riverine input was an important source of seawater Ca2+excess in the Bohai Sea. Biological activity was another factor in regulating seawater Ca2+excess by precipitation in the Yellow River estuary and dissolution in other area of the Bohai Sea. Furthermore, the aragonite saturation state (Ωarag) values calculated from the measured Ca2+ concentrations showed a significant deviation from the values calculated from the theoretical Ca2+ concentrations, especially in the estuarine area with a maximum difference of 18.5%. Therefore, the disregard of the calcium addition would lead to an underestimation of the calcium carbonate saturation state and a deviation in the assessment of ocean acidification in marginal seas.

Climatological distribution of ocean acidification variables along the North American ocean margins

Abstract. Climatologies, which depict mean fields of oceanographic variables on a regular geographic grid, and atlases, which provide graphical depictions of specific areas, play pivotal roles in comprehending the societal vulnerabilities linked to ocean acidification (OA). This significance is particularly pronounced in coastal regions where most economic activities, such as commercial and recreational fisheries and aquaculture industries, occur. In this paper, we unveil a comprehensive data product featuring coastal ocean acidification climatologies and atlases, encompassing the fugacity of carbon dioxide, pH on the total scale, total hydrogen ion content, free hydrogen ion content, carbonate ion content, aragonite saturation state, calcite saturation state, Revelle factor, total dissolved inorganic carbon content, and total alkalinity content. These variables are provided on 1° × 1° spatial grids at 14 standardized depth levels, ranging from the surface to a depth of 500 m, along the North American ocean margins, defined as the region between the coastline and a distance of 200 nautical miles (∼370 km) offshore. The climatologies and atlases were developed using the World Ocean Atlas (WOA) gridding methods of the NOAA National Centers for Environmental Information (NCEI) based on the recently released Coastal Ocean Data Analysis Product in North America (CODAP-NA), along with the 2021 update to the Global Ocean Data Analysis Project version 2 (GLODAPv2.2021) data product. The relevant variables were adjusted to the index year of 2010. The data product is available in NetCDF (https://doi.org/10.25921/g8pb-zy76, Jiang et al., 2022b) on the NOAA Ocean Carbon and Acidification Data System: https://www.ncei.noaa.gov/data/oceans/ncei/ocads/metadata/0270962.html (last access: 15 July 2024). It is recommended to use the objectively analyzed mean fields (with “_an” suffix) for each variable. The atlases can be accessed at https://www.ncei.noaa.gov/access/ocean-carbon-acidification-data-system/synthesis/nacoastal.html (last access: 15 July 2024).

Spectrophotometrically derived seawater CO2‐system assessments: Parameter calculations using pH do not require measurements at standard temperatures

AbstractThe temperature range of Earth's open‐ocean waters is roughly 0–30°C, yet our understanding of the seawater carbon dioxide (CO2) system is largely derived from analyses conducted within a narrow temperature range (e.g., laboratory temperature of 20°C or 25°C). Herein, we address two aspects of open‐ocean CO2‐system measurements and modeling: (1) a highly precise spectrophotometric technique is used to determine bicarbonate dissociation constants (K2) in seawater at temperatures as low as 3°C and (2) a cruise dataset uniquely including total scale pH measurements at two temperatures is used for CO2‐system internal consistency comparisons at 12°C and 25°C. Our pK2 parameterization (where pK = −log K) is applicable for broad ranges of salinity (20 ≤ SP ≤ 40) and temperature (3°C ≤ t ≤ 35°C). Our CO2‐system internal consistency evaluation (comparison of measured and calculated CO2‐system parameters) utilized data obtained during NOAA's 2021 West Coast Ocean Acidification Cruise: total alkalinity (TA), total dissolved inorganic carbon (DIC), pH measured at 25°C, and pH measured at 12°C (n = 265). Results demonstrate that, relative to calculations utilizing the TA, DIC pair, agreement between measured and calculated parameters is improved when either TA or DIC is paired with pH measurements at either temperature. Calculations of CO2 fugacity (fCO2) and aragonite saturation state (Ωar) using pH measurements made at 25°C or 12°C (paired with either TA or DIC) are statistically indistinguishable. Results also suggest that the temperature dependence of current CO2‐system dissociation constants need further refinement.

Anthropogenic CO2, air–sea CO2 fluxes, and acidification in the Southern Ocean: results from a time-series analysis at station OISO-KERFIX (51° S–68° E)

Abstract. The temporal variation of the carbonate system, air–sea CO2 fluxes, and pH is analyzed in the southern Indian Ocean, south of the polar front, based on in situ data obtained from 1985 to 2021 at a fixed station (50°40′ S–68°25′ E) and results from a neural network model that reconstructs the fugacity of CO2 (fCO2) and fluxes at monthly scale. Anthropogenic CO2 (Cant) is estimated in the water column and is detected down to the bottom (1600 m) in 1985, resulting in an aragonite saturation horizon at 600 m that migrated up to 400 m in 2021 due to the accumulation of Cant. At the subsurface, the trend of Cant is estimated at +0.53±0.01 µmol kg−1 yr−1 with a detectable increase in the trend in recent years. At the surface during austral winter the oceanic fCO2 increased at a rate close to or slightly lower than in the atmosphere. To the contrary, in summer, we observed contrasting fCO2 and dissolved inorganic carbon (CT) trends depending on the decade and emphasizing the role of biological drivers on air–sea CO2 fluxes and pH inter-annual variability. The regional air–sea CO2 fluxes evolved from an annual source to the atmosphere of 0.8 molC m−2 yr−1 in 1985 to a sink of −0.5 molC m−2 yr−1 in 2020. Over 1985–2020, the annual pH trend in surface waters of -0.0165±0.0040 per decade was mainly controlled by the accumulation of anthropogenic CO2, but the summer pH trends were modulated by natural processes that reduced the acidification rate in the last decade. Using historical data from November 1962, we estimated the long-term trend for fCO2, CT, and pH, confirming that the progressive acidification was driven by the atmospheric CO2 increase. In 59 years this led to a diminution of 11 % for both aragonite and calcite saturation state. As atmospheric CO2 is expected to increase in the future, the pH and carbonate saturation state will decrease at a faster rate than observed in recent years. A projection of future CT concentrations for a high emission scenario (SSP5-8.5) indicates that the surface pH in 2100 would decrease to 7.32 in winter. This is up to −0.86 lower than pre-industrial pH and −0.71 lower than pH observed in 2020. The aragonite undersaturation in surface waters would be reached as soon as 2050 (scenario SSP5-8.5) and 20 years later for a stabilization scenario (SSP2-4.5) with potential impacts on phytoplankton species and higher trophic levels in the rich ecosystems of the Kerguelen Islands area.

How Much Can Riverine Biogeochemical Fluxes Affect the Arctic Ocean Acidification?

AbstractArctic rivers carry not only large amounts of freshwater but also biogeochemical materials into the ocean and play important roles in Arctic ocean acidification (OA). This study quantitatively evaluated the effects of riverine biogeochemical fluxes (R‐BGC; carbon and nutrients) on the Arctic marine carbonate system and OA using multi‐decadal experiments (1979–2018) with a pan‐Arctic sea ice–ocean model. Improved initial and lateral boundary conditions of carbonate properties, observation‐based riverine biogeochemical data, and land model‐based interannually varying riverine freshwater discharge were adopted to enable more realistic experiments. The model simulated negative trends in aragonite saturation state (Ω) and pH in most regions of Arctic Ocean regardless of R‐BGC. The increased riverine freshwater promoted more OA through the higher dilution effect. Compared to the experiment with riverine discharge of only freshwater, the inclusion of R‐BGC caused positive anomalies in Ω and pH (by ∼0.14 and ∼0.03 in central basins, and by ∼0.15 and ∼0.06 in shelf seas, respectively). In the central basins, these anomalies were caused mostly by carbon (total alkalinity and dissolved inorganic carbon) of the R‐BGC. In the shelf seas, nutrient (nitrate and silicate) fluxes also contributed ∼14% and ∼32% of the anomalies owing to the enhanced primary production and a corresponding reduction in seawater pCO2. R‐BGC mitigated OA (ΔΩ = −1.53 × 10−3 year−1 and ΔpH = −0.56 × 10−3 year−1) in regions where riverine freshwater was accumulated (i.e., the Canada Basin, Chukchi Cap, Eurasian Basin, and East Siberian Sea). This study stressed the importance of including R‐BGC for OA model projection.

The impact of aragonite saturation variability on shelled pteropods: An attribution study in the California Current System.

Observations from the California Current System (CalCS) indicate that the long-term trend in ocean acidification (OA) and the naturally occurring corrosive conditions for the CaCO3 mineral aragonite (saturation state Ω < 1) have a damaging effect on shelled pteropods, a keystone group of calcifying organisms in the CalCS. Concern is heightened by recent findings suggesting that shell formation and developmental progress are already impacted when Ω falls below 1.5. Here, we quantify the impact of low Ω conditions on pteropods using an individual-based model (IBM) with life-stage-specific mortality, growth, and behavior in a high-resolution regional hindcast simulation of the CalCS between 1984 and 2019. Special attention is paid to attributing this impact to different processes that lead to such low Ω conditions, namely natural variability, long-term trend, and extreme events. We find that much of the observed damage in the CalCS, and specifically >70% of the shell CaCO3 loss, is due to the pteropods' exposure to naturally occurring low Ω conditions as a result of their diel vertical migration (DVM). Over the hindcast period, their exposure to damaging waters (Ω < 1.5) increases from 9% to 49%, doubling their shell CaCO3 loss, and increasing their mortality by ~40%. Most of this increased exposure is due to the shoaling of low Ω waters driven by the long-term trend in OA. Extreme OA events amplify this increase by ~40%. Our approach can quantify the health of pteropod populations under shifting environmental conditions, and attribute changes in fitness or population structure to changes in the stressor landscape across hierarchical time scales.

Seasonality and response of ocean acidification and hypoxia to major environmental anomalies in the southern Salish Sea, North America (2014–2018)

Abstract. Coastal and estuarine ecosystems fringing the North Pacific Ocean are particularly vulnerable to ocean acidification, hypoxia, and intense marine heatwaves as a result of interactions among natural and anthropogenic processes. Here, we characterize variability during a seasonally resolved cruise time series (2014–2018) in the southern Salish Sea (Puget Sound, Strait of Juan de Fuca) and nearby coastal waters for select physical (temperature, T; salinity, S) and biogeochemical (oxygen, O2; carbon dioxide fugacity, fCO2; aragonite saturation state, Ωarag) parameters. Medians for some parameters peaked (T, Ωarag) in surface waters in summer, whereas others (S, O2, fCO2) changed progressively across spring–fall, and all parameters changed monotonically or were relatively stable at depth. Ranges varied considerably for all parameters across basins within the study region, with stratified basins consistently the most variable. Strong environmental anomalies occurred during the time series, allowing us to also qualitatively assess how these anomalies affected seasonal patterns and interannual variability. The peak temperature anomaly associated with the 2013–2016 northeast Pacific marine heatwave–El Niño event was observed in boundary waters during the October 2014 cruise, but Puget Sound cruises revealed the largest temperature increases during the 2015–2016 timeframe. The most extreme hypoxia and acidification measurements to date were recorded in Hood Canal (which consistently had the most extreme conditions) during the same period; however, they were shifted earlier in the year relative to previous events. During autumn 2017, after the heat anomaly, a distinct carbonate system anomaly with unprecedentedly low Ωarag values and high fCO2 values occurred in parts of the southern Salish Sea that are not normally so acidified. This novel “CO2 storm” appears to have been driven by anomalously high river discharge earlier in 2017, which resulted in enhanced stratification and inferred primary productivity anomalies, indicated by persistently and anomalously high O2, low fCO2, and high chlorophyll. Unusually, this CO2 anomaly was decoupled from O2 dynamics compared with past Salish Sea hypoxia and acidification events. The complex interplay of weather, hydrological, and circulation anomalies revealed distinct multi-stressor scenarios that will potentially affect regional ecosystems under a changing climate. Further, the frequencies at which Salish cruise observations crossed known or preliminary species' sensitivity thresholds illustrates the relative risk landscape of temperature, hypoxia, and acidification anomalies in the southern Salish Sea in the present day, with implications for how multiple stressors may combine to present potential migration, survival, or physiological challenges to key regional species. The Salish cruise data product used in this publication is available at https://doi.org/10.25921/zgk5-ep63 (Alin et al., 2022), with an additional data product including all calculated CO2 system parameters available at https://doi.org/10.25921/5g29-q841 (Alin et al., 2023).

The Combined Effects of Ocean Acidification and Respiration on Habitat Suitability for Marine Calcifiers Along the West Coast of North America

AbstractThe California Current Ecosystem (CCE) is a natural laboratory for studying the chemical and ecological impacts of ocean acidification. Biogeochemical variability in the region is due primarily to wind‐driven near‐shore upwelling of cold waters that are rich in re‐mineralized carbon and poor in oxygen. The coastal regions are exposed to surface waters with increasing concentrations of anthropogenic CO2 (Canth) from exchanges with the atmosphere and the shoreward transport and mixing of upwelled water. The upwelling drives intense cycling of organic matter that is created through photosynthesis in the surface ocean and degraded through biological respiration in subsurface habitats. We used an extended multiple linear‐regression approach to determine the spatial and temporal concentrations of Canth and respired carbon (Cbio) in the CCE based on cruise data from 2007, 2011, 2012, 2013, 2016, and 2021. Over the region, the Canth accumulation rate increased from 0.8 ± 0.1 μmol kg−1 yr−1 in the northern latitudes to 1.1 ± 0.1 μmol kg−1 yr−1 further south. The rates decreased to values of about ∼0.3 μmol kg−1 yr−1 at depths near 300 m. These accumulation rates at the surface correspond to total pH decreases that averaged about 0.002 yr‐1; whereas, decreases in aragonite saturation state ranged from 0.006 to 0.011 yr‐1. The impact of the Canth uptake was to decrease the amount of oxygen consumption required to cross critical biological thresholds (i.e., calcification, dissolution) for marine calcifiers and are significantly lower in the recent cruises than in the pre‐industrial period because of the addition of Canth.

Insights into carbonate environmental conditions in the Chukchi Sea

Abstract. Healthy Arctic marine ecosystems are essential to the food security and sovereignty, culture, and wellbeing of Indigenous Peoples in the Arctic. At the same time, Arctic marine ecosystems are highly susceptible to impacts of climate change and ocean acidification. While increasing ocean and air temperatures and melting sea ice act as direct stressors on the ecosystem, they also indirectly enhance ocean acidification, accelerating the associated changes in the inorganic carbon system. Yet, much is to be learned about the current state and variability of the inorganic carbon system in remote, high-latitude oceans. Here, we present time series (2016–2020) of pH and the partial pressure of carbon dioxide (pCO2) from the northeast Chukchi Sea continental shelf. The Chukchi Ecosystem Observatory includes a suite of subsurface year-round moorings sited amid a biological hotspot that is characterized by high primary productivity and a rich benthic food web that in turn supports coastal Iñupiat, whales, ice seals, walrus (Odobenus rosmarus), and Arctic cod (Boreogadus saida). Our observations suggest that near-bottom waters (33 m depth, 13 m above the seafloor) are a high carbon dioxide and low pH and aragonite saturation state (Ωarag) environment in summer and fall, when organic material from the highly productive summer remineralizes. During this time, Ωarag can be as low as 0.4. In winter, when the site was covered by sea ice, pH was &lt;8 and Ωarag remained undersaturated under the sea ice. There were only two short seasonal periods with relatively higher pH and Ωarag, which we term ocean acidification relaxation events. In spring, high primary production from sea ice algae and phytoplankton blooms led to spikes in pH (pH &gt; 8) and aragonite oversaturation. In late fall, strong wind-driven mixing events that delivered low-CO2 surface water to the shelf also led to events with elevated pH and Ωarag. Given the recent observations of high rates of ocean acidification and a sudden and dramatic shift of the physical, biogeochemical, and ecosystem conditions in the Chukchi Sea, it is possible that the observed extreme conditions at the Chukchi Ecosystem Observatory are deviating from the carbonate conditions to which many species are adapted.

Drivers of CO2-carbonate system variability in the coastal ocean south of Honolulu, Hawai’i

This study examines carbonate chemistry variability from 2008 to 2021 in subtropical coastal waters adjacent to Honolulu, Hawai’i. We use surface seawater carbon dioxide partial pressure (pCO2sw) measurements obtained every three hours from two buoys located along the south shore of O’ahu near anthropogenically impacted fringing reefs. The Ala Wai buoy was located 200 m offshore of a canal draining most of Honolulu, while the Kilo Nalu buoy was 1.3 miles (2 km) to the northwest, at a similar distance from shore with fewer terrestrial inputs. We compare pCO2sw variability from diurnal to interannual time scales. A trend analysis reveals a statistically significant increase in pCO2sw of +1.84 ± 0.27 µatm per year over the 11-year period. This rate is slightly lower than the average atmospheric growth rate observed during the same timeframe. In contrast to a nearby open-ocean site, the coastal sites experience amplified shorter-term variability, while seasonal to inter-annual variability is comparable to the open ocean. Ala Wai exhibits greater ranges than Kilo Nalu in all carbonate system variables due to its proximity to the Ala Wai Canal outflow. We examine the drivers that may explain both the similarities and contrasts in carbon dynamics observed between the two locations. Drivers of aragonite saturation state (ΩAr), an important variable for quantifying ocean acidification, are isolated from the in-situ time-series. Interannual salinity variations both due to freshwater pulses and large-scale regional salinity changes have a larger impact on ΩAr than temperature changes, which mostly have an effect seasonally. A large biological contribution to ΩAr is suspected, and further investigated using TA/DIC ratios normalized to median salinity and their slopes. Observed ratios at the south shore sites are evaluated relative to expected ratios derived from an open-ocean reference. Results suggest that dissolution and respiration are the primary biogeochemical processes occurring at these coastal sites. This highlights the significance of carbonate dissolution in anthropogenically impacted coastal waters, which is likely buffering acidification due to anthropogenic CO2 and freshwater inputs at these sites.

Remotely sensed retrieval of air-sea carbon flux and acidification risk in Chinese Bohai Sea based on a semi-analytical mechanism model with hour-level GOCI image and ERA5 reanalysis data

Marine carbon sinks act as a buffer against global warming, but raise the risk of acidification, especially in the marginal shelf seas which are rich in terrigenous carbon input. The Chinese Bohai Sea performs generally as a weak carbon source while carbon fluxes (fCO2), pH, and aragonite saturation states (Ωarag) vary in time and space under intensive land-sea interaction. However, there are still 1) insufficient spatiotemporal resolution in existing remotely sensed retrieval of carbon flux, 2) inadequate analytic mechanism for existing empirical models, which are not suitable for case II waters in Bohai Sea, and 3) limited research on remotely sensed retrieval of acidification risk expressed as pH and Ωarag. Thus, we proposed a semi-analytical mechanism algorithm (MeSAA) to gain seawater partial pressure of CO2 (pCO2sw) with hour-level GOCI imagery and seawater carbonate equilibrium equation (CO2SYS). With the assistance of ERA5 reanalysis data, the gained pCO2sw was then used to obtain fCO2 by using the sea-air CO2 partial pressure difference (ΔpCO2) method. Similarly, with the assistance of remotely sensed retrieval, two indices, pH and Ωarag were also gained from CO2SYS to identify the acidification risk. The results showed that Bohai Sea was a weak carbon source with the positive values of fCO2 (0.5–6 mmol C·m−2·day−1) and total emission (0.956 Tg C·yr−1). It suffered from a high acidification risk, especially in three bays with low values of Ωarag (1.15–1.3) from May to September 2011. Although photosynthetic carbon sequestration was intensive near shore, it could not consume the large amount of the rich carbon input, and resulting a monthly increase for pCO2sw and fCO2 and a monthly decrease for pH and Ωarag from May to September. The distribution of pCO2sw was in accord with a former study, but the values were not. The local parameter adjustment of MeSAA in the Bohai Sea was analyzed for this issue, so did the effect of uncertainty analysis of mapping at different time scales (hourly and daily). Moreover, the contrast of absolute deviation and relative deviation on different time scales verified the advantage of hour-level GOCI imagery in improving assessment accuracies. This study gained a more precise change trend of pCO2sw, fCO2, pH, and Ωarag in the Bohai Sea from May to September 2011, which would be beneficial to the study of the carbon cycle in the marginal sea of the shelf under the conditions of climate change.

Short-terms variability of water properties and phytoplankton blooms along the central eastern Florida shelf edge

The central eastern Florida shelf edge is a highly dynamic area that supports an important yet vulnerable deep-sea coral reefs ecosystem, the Oculina Coral Habitat Area of Particular Concern. Rapid and large short-terms (days to weeks) changes of bottom temperature (up to 9 °C) and pCO2 (up to 180 μatm) were observed at the shelfbreak during a two-month (May 13-July 10, 2017) deployment of a lander package. An analysis suggests that these changes are the combined results of tides, the Gulf Stream meandering, and submesoscale eddies and filaments. The processes responsible for sub-tidal variability may include 1) the Gulf Stream frontal movements, 2) upwelling/downwelling of slope waters in association with the Gulf Stream variability, and 3) submesoscale processes and associated vertical movements. Satellite images also frequently show a narrow plume of elevated chlorophyll concentration that stretches from the coast northward up to >200 km along the Gulf Stream front during late spring and early summer. Our analysis indicates that these phytoplankton blooms in the plume are likely supported by the nutrient supply from the nutrients-rich slope waters to the shelf edge and subsequent local vertical mixing. Carbon export associated with these blooms can be an important food source to the Oculina corals. Upwelling of slope waters, on the other hand, will lead to increased CO2 and reduced pH and aragonite saturation state along the shelf edge. Therefore, these dynamic processes may have strong impacts on the health and sustainability of the Oculina coral ecosystem.

Mississippi River Chemistry Impacts on the Interannual Variability of Aragonite Saturation State in the Northern Gulf of Mexico

AbstractIn the northern Gulf of Mexico shelf, the Mississippi‐Atchafalaya River System (MARS) impacts the carbonate system by delivering freshwater with a distinct seasonal pattern in both total alkalinity (Alk) and dissolved inorganic carbon (DIC), and promoting biologically driven changes in DIC through nutrient inputs. However, how and to what degree these processes modulate the interannual variability in calcium carbonate solubility have been poorly documented. Here, we use an ocean‐biogeochemical model to investigate the impact of MARS's discharge and chemistry on interannual anomalies of aragonite saturation state (ΩAr). Based on model results, we show that the enhanced mixing of riverine waters with a low buffer capacity (low Alk‐to‐DIC ratio) during high‐discharge winters promotes a significant ΩAr decline over the inner‐shelf. We also show that increased nutrient runoff and vertical stratification during high‐discharge summers promotes strong negative anomalies in bottom ΩAr, and less intense but significant positive anomalies in surface ΩAr. Therefore, increased MARS discharge promotes an increased frequency of suboptimal ΩAr levels for nearshore coastal calcifying species. Additional sensitivity experiments further show that reductions in the Alk‐to‐DIC ratio and nitrate concentration from the MARS significantly modify the simulated interannual ΩAr patterns, weakening the positive surface ΩAr anomalies during high‐discharge summers or even producing negative surface ΩAr anomalies. Our findings suggest that riverine water carbonate chemistry is a main driver of interannual variability in ΩAr over river dominated ocean margins.

Spatial variability of aragonite saturation state (Ωarag) in Indonesian coastal waters

The effects of Ocean acidification (OA) on the coastal waters of small islands in Indonesia have yet to be extensively studied. This research aims to investigate the process of OA in the coastal waters of small Indonesian islands and examine how land-sea interactions impact carbonate mineral saturation. We collected seawater samples from seven locations on small islands in Indonesia between 2015 and 2021 to analyze the aragonite saturation state. The result shows that most of Indonesia's coastal waters are accompanied by supersaturation of aragonite saturation state (Ωarag>1). Selayar Island's waters had the highest aragonite saturation, averaging 4.96 ± 0.48, while Pari Island's coastal waters had the lowest, averaging 2.49 ± 0.50. Salinity had the greatest effect on Ωarag in all of the sampling sites, ranging from 24.13% to 52.92%, except Aceh Island, where temperature had a greater impact (34.35%) than salinity (26.99%). By the end of this century, Ωarag is predicted to decline based on projections related to climate change. Small island coastal waters are expected to experience a more substantial decline compared to those near the mainland, ranging from 4.71% to 79.58%. The coastal waters of Weh and Selayar Island are probably going to decline the greatest, while the coastal waters of Sorong (mainland) are probably going to decline the least. All seven sampling locations are expected to observe the decrease. This decline will be observed at all seven sampling locations, with Ωarag values ranging from 1.91 to 3.35.

The role of marine fish-produced carbonates in the oceanic carbon cycle is determined by size, specific gravity, and dissolution rate

Rising CO2 emissions have heightened the necessity for increased understanding of Earth's carbon cycle to predict future climates. The involvement of marine planktonic species in the global carbon cycle has been extensively studied, but contributions by marine fish remain poorly characterized. Marine teleost fishes produce carbonate minerals (‘ichthyocarbonates’) within the lumen of their intestines which are excreted at significant rates on a global scale. However, we have limited understanding of the fate of excreted ichthyocarbonate. We analyzed ichthyocarbonate produced by three different marine teleosts for mol%MgCO3 content, size, specific gravity, and dissolution rate to gain a better understanding of ichthyocarbonate fate. Based on the species examined here, we report that 75 % of ichthyocarbonates are ≤0.91 mm in diameter. Analyses indicate high Mg2+ content across species (22.3 to 32.3 % mol%MgCO3), consistent with previous findings. Furthermore, ichthyocarbonate specific gravity ranged from 1.23 to 1.33 g/cm3, and ichthyocarbonate dissolution rates varied among species as a function of aragonite saturation state. Ichthyocarbonate sinking rates and dissolution depth were estimated for the Atlantic, Pacific, and Indian ocean basins for the three species examined. In the North Atlantic, for example, ~33 % of examined ichthyocarbonates are expected to reach depths exceeding 200 m prior to complete dissolution. The remaining ~66 % of ichthyocarbonate is estimated to dissolve and contribute to shallow water alkalinity budgets. Considering fish biomass and ichthyocarbonate production rates, our results support that marine fishes are critical to the global carbon cycle, contributing to oceanic alkalinity budgets and thereby influencing the ability of the oceans to neutralize atmospheric CO2.

More Than Marine Heatwaves: A New Regime of Heat, Acidity, and Low Oxygen Compound Extreme Events in the Gulf of Alaska

AbstractRecent marine heatwaves in the Gulf of Alaska have had devastating impacts on species from various trophic levels. Due to climate change, total heat exposure in the upper ocean has become longer, more intense, more frequent, and more likely to happen at the same time as other environmental extremes. The combination of multiple environmental extremes can exacerbate the response of sensitive marine organisms. Our hindcast simulation provides the first indication that more than 20% of the bottom water of the Gulf of Alaska continental shelf was exposed to quadruple heat, positive hydrogen ion concentration [H+], negative aragonite saturation state (Ωarag), and negative oxygen concentration [O2] compound extreme events during the 2018–2020 marine heat wave. Natural intrusion of deep and acidified water combined with the marine heat wave triggered the first occurrence of these events in 2019. During the 2013–2016 marine heat wave, surface waters were already exposed to widespread marine heat and positive [H+] compound extreme events due to the temperature effect on the [H+]. We introduce a new Gulf of Alaska Downwelling Index (GOADI) with short‐term predictive skill, which can serve as indicator of past and near‐future positive [H+], negative Ωarag, and negative [O2] compound extreme events near the shelf seafloor. Our results suggest that the marine heat waves may have not been the sole environmental stressor that led to the observed ecosystem impacts and warrant a closer look at existing in situ inorganic carbon and other environmental data in combination with biological observations and model output.

Spatial symmetry and contrasting controls of surface pH and aragonite saturation state in the western North Pacific

Oceanic uptake of anthropogenic CO2 causes a decrease in seawater pH and aragonite saturation state (Ωarag), a process known as ocean acidification (OA). The western North Pacific is a hotspot for anthropogenic CO2 sinks; however, the spatiotemporal variability of pH and Ωarag and their controlling mechanisms remain unexplored. In this study, we provide high-frequency and high-precision underway measurements of sea surface pCO2 and pH to investigate the distribution and drivers of OA metrics across different hydrochemical gradients in the western North Pacific in late spring 2018, a season with the highest primary production in the year. Our results show that the surface pH reached near air-sea equilibrium in the subtropical zone but gradually increased northward across the Kuroshio Recirculation (KR) zone and peaked in the Kuroshio Extension (KE) zone. We found that sea surface temperature played the most prominent role in regulating pH, which was also counteracted by the effects of air–sea gas exchange and vertical mixing. In contrast, the distribution of Ωarag largely mirrored the pH and was governed by air–sea gas exchange and vertical mixing, the effects of which on Ωarag were enhanced by temperature. Biological activity thrived in the KE zone to increase both pH and Ωarag, which further reinforced the latitudinal pattern of pH, but weakened that of Ωarag. These findings are based on direct in situ measurements of pH and improve our understanding of the spatiotemporal variability of OA metrics in the western North Pacific region.