New edition of the “OA-ICC Highlights”, May – September 2023

The new edition of the “OA-ICC Highlights” summarizes the project’s main activities and achievements over the period May – September 2023. This time our newsletter features the participation of the OA-ICC research staff in the 2023 ASLO Aquatic Sciences Meeting, the annual in-person meeting of the Executive Council of the Global ocean Acidification Observing Network (GOA-ON), a technical meeting on adaptation pathways for atoll islands and a training course on “blue carbon” for early-career scientists. Furthermore, a dedicated piece in this issue discusses Ocean Alkalinity Enhancement (OAE) in the context of potential ways to accelerate the ocean’s natural carbon sink.

Previous editions of the “OA-ICC Highlights”can be viewed here.

OA-ICC, 10 November 2023. Newsletter.

Challenging Greek primary students’ knowledge of ocean acidification using the carbon cycle context

The purpose of the present study was to investigate the impact of an intervention on primary school students’ construction of knowledge on ocean acidification and the development of their systems thinking. Eighty-five 11 to 12-year-old students from five different classes of two public primary schools in Greece participated in the 8-h intervention. The intervention included inquiry-based and knowledge-integration activities, and students worked in groups during all activities. Rich pictures, made by the groups at the beginning and the end of the intervention, were used to evaluate their progress in their knowledge concerning the carbon cycle, as well as in their systems thinking. Our findings showed that the intervention contributed to primary students’ conceptual knowledge of the carbon cycle and the inclusion of ocean acidification in the carbon cycle. It also helped them improve their systems thinking, indicating that students’ systems thinking at this age could be developed through formal instruction with interventions which emphasize content knowledge and use an earth systems approach. Moreover, our findings indicate that the systems thinking perspective can serve as an effective approach to help children better understand and critically engage with complex environmental issues, such as ocean acidification.

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Scientists publish 37-year record of ocean acidification off southern California

The longest ocean time series of dissolved carbon dioxide in the Pacific — part of the “Keeling Curve of the ocean” — is revealed.

For the first time, scientists at UC San Diego’s Scripps Institution of Oceanography have published nearly four decades’ worth of dissolved carbon dioxide measurements from waters off Southern California. The measurements reveal a slight but consistent trend of ocean acidification, a process characterized by a decrease in the ocean’s pH over time due to its absorption of carbon dioxide (CO2) from the atmosphere.

Since the early 1980s, samples of ocean carbonate chemistry have been collected by the California Cooperative Oceanic Fisheries Investigations (CalCOFI) program, which was established in 1949 to investigate the collapse of the sardine population off California. In a new study, Scripps Oceanography researchers present 37 years of measurements from CalCOFI Line 90 Station 90 (station 90.90), a measuring site located 450 kilometers (280 miles) off the coast of San Diego. The team’s findings were published on Nov. 3 in Communications Earth & Environment, a journal affiliated with Nature.

The measurements from station 90.90 establish the oldest time series of direct inorganic carbon observations in the Pacific Ocean. While measurements at the station carry on to the present day, the study details quarterly measurements collected from 1984 to 2021, with a gap from 2002 to 2008 due to a lack of funding. Notably, the data show that the seawater at the study site is getting more acidic, with a measured decrease in pH of 0.0015 per year.

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NOAA ocean acidification lecture, 1 December 2023, University of Delaware

Date and time: 1 December 2023, 11:30 a.m.

Location: Trabant Theater, University of Delaware

Dr Richard A. Feely, NOAA Senior Fellow at the NOAA Pacific Marine Environmental Laboratory, will give lecture on the chemical and ecological impacts of ocean acidification.

Please join us on Friday, Dec. 1, at 11:30 a.m. in Trabant Theater for a special School of Marine Science and Policy seminar on the chemical and ecological impacts of ocean acidification: “The Combined Effects of Ocean Acidification and Respiration on Habitat Suitability for Marine Calcifiers Along the West Coast of North America”.

The lecture will be followed by a reception in Trabant Multipurpose Rooms A/B.

Registration is required for this event. Register online.

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MOPGA 2024: Visiting fellowship program for early career researchers

Application deadline: 9 January 2024

Launched in 2018, the Make Our Planet Great Again (MOPGA) initiative has met with worldwide enthusiasm, generating expressions of interest from a large number of highly qualified students and researchers.  

France, as a major player in the fight against climate change and guarantor of the spirit of the Paris Agreement, is launching a new MOPGA visiting fellowship program geared towards strengthening scientific contributions to climate change issues raised by the COPs. 

This seventh Make Our Planet Great Again (MOPGA) call for applications is intended to welcome at least 40 early career researchers wishing to carry out their research in France. The program is funded by the French Ministry for Europe and Foreign Affairs, in collaboration with the French Ministry for Higher Education and Research, and implemented by Campus France. 

The MOPGA 2024 Visiting Fellowship Program for Early Career Researchers will support researchers working on topics listed in the “Research Themes” section.

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Porites’ coral calcifying fluid chemistry regulation under normal- and low-pH seawater conditions in Palau Archipelago: impacts on growth properties


  • •Palau’s reef has a long-term naturally acidified inshore seawater (pH ~ 7.85).
  • Porites corals up-regulate calcifying fluid pH (~8.41) at normal- and low-pH sites.
  • Porites corals adapt calcifying fluid chemistry to long-term low-pH conditions.
  • Porites shows 15 % lower skeletal density under low-pH (~7.85) vs. open-ocean (~8.03).


Ongoing ocean acidification is known to be a major threat to tropical coral reefs. To date, only few studies have evaluated the impacts of natural long-term exposure to low-pH seawater on the chemical regulation and growth of reef-building corals. This work investigated the different responses of the massive Porites coral living at normal (pHsw ~ 8.03) and naturally low-pH (pHsw ~ 7.85) seawater conditions at Palau over the last decades. Our results show that both Porites colonies maintained similar carbonate properties (pHcf, [CO32−]cf, DICcf, and Ωcf) within their calcifying fluid since 1972. However, the Porites skeleton of the more acidified conditions revealed a significantly lower density (~ 1.21 ± 0.09 g·cm−3) than the skeleton from the open-ocean site (~ 1.41 ± 0.07 g·cm−3). Overall, both Porites colonies exerted a strong biological control to maintain stable calcifying fluid carbonate chemistry that favored the calcification process, especially under low-pH conditions. However, the decline in skeletal density observed at low pH provides critical insights into Porites vulnerability to future global change.

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Effects of ocean acidification on growth and photophysiology of two tropical reef macroalgae

Macroalgae can modify coral reef community structure and ecosystem function through a variety of mechanisms, including mediation of biogeochemistry through photosynthesis and the associated production of dissolved organic carbon (DOC). Ocean acidification has the potential to fuel macroalgal growth and photosynthesis and alter DOC production, but responses across taxa and regions are widely varied and difficult to predict. Focusing on algal taxa from two different functional groups on Caribbean coral reefs, we exposed fleshy (Dictyota spp.) and calcifying (Halimeda tuna) macroalgae to ambient and low seawater pH for 25 days in an outdoor experimental system in the Florida Keys. We quantified algal growth, calcification, photophysiology, and DOC production across pH treatments. We observed no significant differences in the growth or photophysiology of either species between treatments, except for lower chlorophyll b concentrations in Dictyota spp. in response to low pH. We were unable to quantify changes in DOC production. The tolerance of Dictyota and Halimeda to near-future seawater carbonate chemistry and stability of photophysiology, suggests that acidification alone is unlikely to change biogeochemical processes associated with algal photosynthesis in these species. Additional research is needed to fully understand how taxa from these functional groups sourced from a wide range of environmental conditions regulate photosynthesis (via carbon uptake strategies) and how this impacts their DOC production. Understanding these species-specific responses to future acidification will allow us to more accurately model and predict the indirect impacts of macroalgae on coral health and reef ecosystem processes.

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California Ocean Acidification and Hypoxia Portal

The California OAH Portal is a centralized information center serving relevant, timely, and reliable OAH information to managers, researchers, industry, and other marine stakeholders. This new information hub is designed to support the automated generation of data-driven products to solve user needs within the region. The Portal integrates standardized, quality controlled data from diverse sources and platforms, incorporate existing data layers from models and satellites, and collaborate with state and West Coast partners to serve additional data streams and curated synthesis products. We serve automated and interoperable data and synthesis products that incorporate the most current data to generate indicators of status and trends. Data and information products will be downloadable and shareable for a variety of uses.

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Vacancy: Associate Research Scientist (Programme Support), IAEA Marine Environment Laboratories (open to U.S. citizens only)

Deadline for applications: 3 January 2024

Location: Radioecology Laboratory, IAEA Marine Environment Laboratories, Monaco

Organizational Setting
The Department of Nuclear Sciences and Applications implements the IAEA’s Major Programme 2, “Nuclear Techniques for Development and Environmental Protection”. This Major Programme comprises individual programmes on food and agriculture, human health, water resources, environment and radiation technologies. These programmes are supported by laboratories in Seibersdorf, Monaco and Vienna. The Major Programme’s objective is to enhance the capacity of Member States to meet basic human needs and to assess and manage the marine and terrestrial environments through the use of nuclear and isotopic techniques in sustainable development programmes.

The Radioecology Laboratory’s mission is to improve knowledge about the behaviour and fate of radionuclides and other contaminants in the environment, with a particular emphasis on the biosphere. It aims to assist and enhance Member States’ capabilities in the field of radioecology and its applications to ecotoxicology and biogeochemistry.

Main Purpose
Reporting to the Laboratory Head and Professional Staff, the Associate Research Scientist, (Programme Support) will facilitate and assure effective and efficient implementation of all GOA-ON (Global Ocean Acidification Observation Network) activities for the International Coordination Centre (OA-ICC), a PUI project, that promotes, facilitates and communicates global activities on ocean acidification. The Associate Research Scientist, (Programme Support )will enable the provision of the scientific and technical expertise and related support for the OA-ICC and GOA-ON and for ensuring the contribution of the GOA-ON to the achievement of SDG Target 14.3.

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SAROA webinar series: Coastal carbonate system dynamics in Southeast Asia, 29 November 2023

Date and time: 29 November 2023, 5 PM (IST)

Invited speaker: Dr. Patrick Martin, Associate Professor, Nanyang Technological University, Singapore
Venue: online (please register at:

The GOA-ON South Asia Regional Hub on Ocean Acidification (SAROA) will hold its fourth in a dedicated series webinar later this week. The invited guest speaker, Dr Patrick Martin, Associate Professor at the Asian School of the Environment, Nanyang Technological University, Singapore, will address the broader topic of changes in the coastal carbonate system in the region. The main focus of Dr Martin’s research is carbon cycling and understanding how it is processed biogeochemically at sea and what effects it may exert on marine communities and ecosystems.

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Warming, not acidification, favours survival of non-indigenous over native gammarid species

Anthropogenic disturbances, including non-indigenous species (NIS) and climate change, have considerably affected ecosystems and socio-economies globally. Despite the widely acknowledged individual roles of NIS and global warming in biodiversity change, predicting the connection between the two still remains a fundamental challenge and requires urgent attention due to a timely importance for proper conservation management. To improve our understanding of the interaction between climate change and NIS on biological communities, we conducted laboratory experiments to test the temperature and pCO2 tolerance of four gammarid species: two native Baltic Sea species (Gammarus locusta and G. salinus), one Ponto‐Caspian NIS (Pontogammarus maeoticus) and one North American NIS (Gammarus tigrinus). Our results demonstrated that an increase in pCO2 level was not a significant driver of mortality, neither by itself nor in combination with increased temperature, for any of the tested species. However, temperature was significant, and differentially affected the tested species. The most sensitive was the native G. locusta which experienced 100% mortality at 24 °C. The second native species, G. salinus, performed better than G. locusta, but was still significantly more sensitive to temperature increase than either of the NIS. In contrast, NIS performed better than native species with warming, whereby particularly the Ponto-Caspian P. maeoticus did not demonstrate any difference in its performance between the temperature treatments. With the predicted environmental changes in the Baltic Sea, we may expect shifts in distributions of native taxa towards colder areas, while their niches might be filled by NIS, particularly those from the Ponto-Caspian region. Although, northern colder areas may be constrained by lower salinity. Additional studies are needed to confirm our findings across other NIS, habitats and regions to make more general inferences.

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Autonomous high-frequency time-series observations of total alkalinity in dynamic estuarine waters

Total alkalinity (TA) is a variable that reflects the acid buffering capacity of seawater, and is key to studies of the global carbon cycle. Daily and seasonal TA variations are poorly constrained due to limitations in observational techniques, and this hampers our understanding of the carbonate system. High quality and high temporal resolution TA observations are required to constrain the controlling factors on TA. Estuarine and coastal waters usually have low TA values and may experience enhanced remineralization of organic matter in response to processes such as eutrophication and terrestrial organic matter input. Therefore, these waters are considered vulnerable to acidification as a consequence of ongoing atmospheric anthropogenic carbon dioxide uptake. An In Situ Analyzer for seawater Total Alkalinity (ISA-TA) was deployed for the first time in low salinitydynamic estuarine waters (Kiel Fjord, southwestern Baltic Sea). The ISA-TA and a range of additional sensors (for pH, pCO2, nitrate and temperature, salinity, dissolved oxygen) used to obtain ancillary data to interpret the TA variability, were deployed on a pontoon in the inner Kiel Fjord for approximately four months. Discrete samples (for TA, nutrients including NO3, soluble reactive phosphorus (SRP) and H4SiO4, chlorophyll a) were collected regularly to validate the ISA-TA and to interpret the TA data. The effects on TA in the study area of nitrate uptake and of other processes such as precipitation, run-off and mixing of different waters were observed. The difference between the TA values measured with the ISA-TA and TA of discretely collected samples measured with the Gran titration method was −2.6 ± 0.9 μmol kg−1 (n = 106), demonstrating that the ISA-TA provides stable and accurate TA measurements in dynamic, low salinity (13.2–20.8), estuarine waters. The TA and ancillary data recorded by the sensor suite revealed that physical mixing was the main factor determining the variability in TA in Kiel Fjord during the study period.

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Assessing impacts of coastal warming, acidification, and deoxygenation on Pacific oyster (Crassostrea gigas) farming: a case study in the Hinase area, Okayama Prefecture, and Shizugawa Bay, Miyagi Prefecture, Japan (Update)

Coastal warming, acidification, and deoxygenation are progressing primarily due to the increase in anthropogenic CO2. Coastal acidification has been reported to have effects that are anticipated to become more severe as acidification progresses, including inhibiting the formation of shells of calcifying organisms such as shellfish, which include Pacific oysters (Crassostrea gigas), one of the most important aquaculture resources in Japan. Moreover, there is concern regarding the combined impacts of coastal warming, acidification, and deoxygenation on Pacific oysters. However, spatiotemporal variations in acidification and deoxygenation indicators such as pH, the aragonite saturation state (Ωarag), and dissolved oxygen have not been observed and projected in oceanic Pacific oyster farms in Japan. To assess the present impacts and project future impacts of coastal warming, acidification, and deoxygenation on Pacific oysters, we performed continuous in situ monitoring, numerical modeling, and microscopic examination of Pacific oyster larvae in the Hinase area of Okayama Prefecture and Shizugawa Bay in Miyagi Prefecture, Japan, both of which are famous for their Pacific oyster farms. Our monitoring results first found Ωarag values lower than the critical level of acidification for Pacific oyster larvae in Hinase, although no impact of acidification on larvae was identified by microscopic examination. Our modeling results suggest that Pacific oyster larvae are anticipated to be affected more seriously by the combined impacts of coastal warming and acidification, with lower pH and Ωarag values and a prolonged spawning period, which may shorten the oyster shipping period and lower the quality of oysters.

Continue reading ‘Assessing impacts of coastal warming, acidification, and deoxygenation on Pacific oyster (Crassostrea gigas) farming: a case study in the Hinase area, Okayama Prefecture, and Shizugawa Bay, Miyagi Prefecture, Japan (Update)’

Chapter 30 – Sea cucumbers under ocean acidification and warming

Marine organisms are expected to be increasingly stressed by ocean acidification and ocean warming caused by the progressive anthropogenic increase in atmospheric CO2 levels and the absorption of approximately two-thirds of excess CO2 by the ocean. The responses of diverse ecological processes in economically and ecologically important holothuroids to the changing ocean have been of growing concern. Here we address some of them, including various aspects of gamete production, early life stages, biological function, and community interactions. In addition, future research needs and experimental considerations are highlighted.

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Ocean acidification impairs seagrass performance under thermal stress in shallow and deep water


  • Shallow and deep plants were exposed to ocean acidification and thermal stress;
  • Plants were unaffected by ocean acidification when not exposed to thermal stress;
  • Ocean acidification reduced plant performance under thermal stress;
  • Deep plants showed higher levels of heat stress at genetic and physiological levels;
  • Warming may play a key role in structuring future seagrass meadows.


Despite the effects of ocean acidification (OA) on seagrasses have been widely investigated, predictions of seagrass performance under future climates need to consider multiple environmental factors. Here, we performed a mesocosm study to assess the effects of OA on shallow and deep Posidonia oceanica plants. The experiment was run in 2021 and repeated in 2022, a year characterized by a prolonged warm water event, to test how the effects of OA on plants are modulated by thermal stress. The response of P. oceanica to experimental conditions was investigated at different levels of biological organization. Under average seawater temperature, there were no effects of OA in both shallow and deep plants, indicating that P. oceanica is not limited by current inorganic carbon concentration, regardless of light availability. In contrast, under thermal stress, exposure of plants to OA increased lipid peroxidation and decreased photosynthetic performance, with deep plants displaying higher levels of heat stress, as indicated by the over-expression of stress-related genes and the activation of antioxidant systems. In addition, warming reduced plant growth, regardless of seawater CO2 and light levels, suggesting that thermal stress may play a fundamental role in the future development of seagrass meadows. Our results suggest that OA may exacerbate the negative effects of future warming on seagrasses.

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Seasonal temperature variation in Zostera marina seedlings under ocean acidification

Objective: To investigate the responses of Zostera marina seedlings to the individual and combined stresses of seasonal temperature increase and ocean acidification (OA) caused by global climate change and anthropogenic factors. This data will help in efforts to protect and restore seagrass beds in temperate coastal zones of China.

Methods: A mesoscale experimental system was utilized to analyze stress response mechanisms at multiple levels – phenotype, transcriptome, and metabolome – during the seedling stage of Z. marina, a dominant temperate seagrass species in China. The study monitored the seedlings under varying conditions: increased seasonal temperature, OA, and a combination of both.

Results: Findings revealed that under high-temperature conditions, carotenoid biosynthesis was stimulated through the upregulation of specific metabolites and enzymes. Similarly, the biosynthesis of certain alkaloids was promoted alongside modifications in starch, sucrose, and nitrogen metabolism, which improved the plant’s adaptation to OA. Unique metabolic pathways were activated under OA, including the degradation of certain amino acids and modifications in the citric acid cycle and pyruvate metabolism. When subjected to both temperature and OA stresses, seedlings actively mobilized various biosynthetic pathways to enhance adaptability and resilience, with distinct metabolic pathways enhancing the plant’s response under diversified stress conditions. In terms of growth, all treatment groups exhibited significant leaf length increase (p < 0.05), but the weakest growth index was observed under combined stress, followed by the thermal treatment group. Conversely, growth under OA treatment was better, showing a significant increase in wet weight, leaf length, and leaf width (p < 0.05).

Conclusion: Seasonal temperature increase was found to inhibit the growth of Z. marina seedlings to some extent, while OA facilitated their growth. However, the positive effects of OA did not mitigate the damage caused by increased seasonal temperature under combined stress due to seedlings’ sensitivity at this stage. Our findings elucidate differing plant coping strategies under varied stress conditions, contingent on the initial environment. This research anticipates providing significant data support for the adaptation of Z. marina seedlings to seasonal temperature fluctuations and global oceanic events like OA, propelling the effective conservation of seagrass beds.

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North Cascades Audubon Society: ocean acidification and marine food webs in the Salish Sea

Date: 28 November 2023

Time: 7:00-9:00 pm

Location: Old City Hall, 121 Prospect Street, Bellingham, 98225 United States

Join Dr. Brooke Love, oceanographer, and WWU associate professor, for a discussion about how ocean acidification and climate change are unfolding in our local Washington waters. Ocean acidification is driven by the carbon dioxide being added to the atmosphere, which then changes the chemistry of the oceans. These changes can influence how hard it is to make a shell or how easy it is for plants and algae to grow. Ocean acidification can affect anything from the survival of tiny oysters to the sense of smell in fish, affecting marine food webs in varied and unpredictable ways. Brooke will teach us about some of the more common responses among different kinds of organisms in the Salish Sea, and she will also tell us how people and policymakers are addressing these oceanic changes.

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Ocean acidification will soon be irreversible, costing $400bn

Published by Back to Blue, a new report Ocean Acidification: Time for Action calls on international government action to step up in a bid to prevent the worst case scenario from unfolding. It also criticises the majority of countries for ‘ocean blindness’, and failing to factor this issue into climate change adaptation and mitigation plans. 

Currently, just 12 countries have in the world have ocean acidification action plans, yet if the problem is allowed to persist and become worse, some $400billion could be wiped off the global economy. 

As oceans are allowed to become more acidic, a direct result of absorbing increasing amounts of carbon dioxide, the effect on marine life is unforgiving, including the creation of so-called ‘dead zones’, and the destruction of finely balanced ecosystems. In turn, this is a major threat to the survival of coastal communities, many of which have developed due to the abundant riches found under water, not least fisheries, meaning the livelihoods of vast swathes of people now hangs in the balance.

According to data, policy advice and research institution the OECD, globally some three billion people rely on oceans for their income. In the U.S., for example, almost half the national GDP is tied to counties that are coastal adjacent, and more than three-million jobs, or one-in-45, are directly dependent on resources within the sea or Great Lakes. 

You can read the full report here.

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Code Blue: our oceans in crisis

In Australia, our love of the ocean is truly profound – most of us live near the coast, we surf it, camp by it, we marvel at its incredible beauty from its many pristine sandy shores and we are proud of the unique and wondrous sea life that inhabits it.

Our oceans are in trouble. As our climate changes, driven by the unchecked burning of fossil fuels, our seas are transforming before our eyes. Marine heatwaves are surging, coral reefs are on the brink, ice
sheets are melting at an alarming rate, currents are slowing and seas are rising. Put simply: the climate crisis is an ocean crisis.

The ocean is the beating heart of planet Earth, and the lifeblood for all humanity. It produces over half the oxygen we breathe. Its currents regulate our climate and weather. The marine life within it provides sustenance for billions. Our cultures, economies and very identity are tied to the sea.

We have pushed this wondrous, life-giving system to the brink by burning coal, oil and gas. More than 90 percent of the heat trapped by greenhouse gas emissions has been absorbed by the ocean. Parts of the ocean could reach a near-permanent heatwave state within decades.

Our iconic Great Barrier Reef may soon face annual mass coral bleaching. Entire island nations like Tuvalu and Kiribati could become uninhabitable this century as seas rise.

The ocean is a vital carbon sink, absorbing more than 30 percent of the carbon dioxide that humans emit by burning fossil fuels and clearing land. This has changed the chemical make-up of the entire ocean,
making it more acidic.

By absorbing excess heat, and carbon, the ocean has shielded us from the worst of climate change so far. But we are now seeing the consequences of its sacrifice. The climate crisis is no longer a far-off threat. The ocean is screaming a warning that cannot be ignored.

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Changes in the macrobenthic infaunal community of the Southern California continental margin over five decades in relation to oceanographic factors

Climate change has altered the physiochemical conditions of the coastal ocean but effects on infaunal communities have not been well assessed. Here, we used multivariate ordination to examine temporal patterns in benthic community composition from 4 southern California continental shelf monitoring programs that range in duration from 30 to 50 yr. Temporal changes were compared to variations in temperature, oxygen, and acidification using single-taxon random forest models. Species richness increased over time, coupled with a decline in overall abundance. Continental shelf macrobenthic communities from the 2010s comprised a broader array of feeding guilds and life history strategies than in the 1970s. Changing water temperature was associated with northward shifts in geographic distribution and increases in species abundance, while acidification was associated with southward shifts and declines in abundance of other species. Acidification was also associated with changes in depth distribution of benthic fauna, with shelled molluscs declining in abundance at depths most associated with increasing exposure to acidification. This broad-scale community-level analysis establishes causal hypotheses that set the stage for more targeted studies investigating shifts in abundance or distribution for taxa that appear to be responding to climate change-related disturbances.

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Editorial: the changing carbonate systems in coastal, estuarine, shelf areas and marginal seas

Editorial on the Research Topic
The changing carbonate systems in coastal, estuarine, shelf areas and marginal seas

Global atmospheric CO2 concentrations have increased from 320 ppm in the 1960s to the present-day value of 420 ppm, primarily due to anthropogenic activities. This increase influences the seawater carbonate system, impacting the marine ecosystem. There are still gaps that need to be resolved for predicting how these marine systems respond to current and future CO2 levels. Any actions to mitigate the change in pH will require adaptive management of multiple stressors across several spatial scales. Combined, these perspectives yield a more comprehensive picture of events during ocean acidification (OA).

This Research Topic brings together articles from different regions, including coastal, estuarine, and shelf areas and marginal seas, all susceptible to changing atmospheric conditions, riverine inputs, air-sea CO2 exchanges, and multiple acid-base reactions that can alter carbonate chemistry. Articles on the long-term trends of CO2 system descriptors and the interactions with calcifying organisms were also sought. The present Research Topic is primarily based on original articles devoted to carbonate systems in the marginal seas, but it is a pity that some interesting papers dealing with freshwater inflows, estuaries, and related coastal areas were not accepted.

Fransson et al. examined the effects of glacial and sea-ice meltwater on ocean acidification in the waters near the 79 North Glacier (79 NG) and the northeast Greenland shelf. The researchers investigated various ocean acidification factors and the influence of freshening, primary production, and air-sea CO2 exchange. One of the key findings was that the biological removal of CO2 through primary production played a crucial role in offsetting the negative impact of freshwater dilution on the aragonite saturation state (ΩAr), which is a measure of ocean acidification. This compensation effect was most pronounced in 2012, especially in the vicinity of the 79 NG front, where there was a significant presence of glacial meltwater and surface stratification. In 2016, a different scenario was observed, with a more homogenized water column due to sea-ice meltwater. In this case, the compensation effect of biological CO2 removal on ΩAr was weaker compared to 2012. The study also suggests that in the future, with ongoing climate and ocean chemistry changes, the increasing influence of meltwater may surpass the mitigating effects of biological CO2 removal. This could lead to unfavorable conditions for organisms that rely on calcium carbonate for their shells and skeletons. Thus, all the proposed factors need to be closely monitored as they could have significant implications for marine ecosystems and calcifying organisms in the face of ongoing environmental changes.

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