Ocean acidification

Is it possible to simultaneously address the increase of the concentration of carbon dioxide (CO₂) in the atmosphere and the resulting acidification of the oceans? The research of the project DESARC-MARESANUS, a collaboration between the Politecnico di Milano and the CMCC Euro-Mediterranean Center on Climate Change Foundation, explores the feasibility of this process, its chemical and environmental balance, and the benefits for the marine sector, focusing on the Mediterranean basin.

It is now widely recognized that in order to reach the target of limiting global warming to well below 2°C above pre-industrial levels (as the objective of the Paris agreement), cutting the carbon emissions even at an unprecedented pace will not be sufficient, but there is the need for development and implementation of active Carbon Dioxide Removal (CDR) strategies. Among the CDR strategies that currently exist, relatively few studies have assessed the mitigation capacity of ocean-based Negative Emission Technologies (NET) and the feasibility of their implementation on a larger scale to support efficient implementation strategies of CDR. The ocean plays a particular role in the climate system acting as significant sink of atmospheric heat and CO₂; this has caused the additional hazard of ocean acidification, that is the pH reduction of ocean seawater since the pre-industrial period, that is unprecedented in the last 65 million years and has significant implications for marine organisms affecting their metabolic regulation and capability to form calcium carbonate, destabilizing the ecosystem and ultimately threatening vital ecosystem services. Among the ocean-based NETs, artificial ocean alkalinization via the dissolution of Ca(OH)₂, known in short as ocean liming, has attracted attention due to its capability of contemporarily addressing two issues: global warming via increased levels of CO₂ and ocean acidification.

A new study recently published in Frontiers in Climate exploresthe case of ocean alkalinization in detail. The research, realized by the Euro-Mediterranean Center on Climate Change Foundation (CMCC) and the Politecnico di Milano within the Desarc-Maresanus project, with the financial support of Amundi and the collaboration of CO2APPS, presents an analysis of marine alkalinization applied to the Mediterranean Sea taking into consideration the regional characteristics of the basin. Researchers used a set of simulations of alkalinization based on current shipping routes to quantitatively assess the alkalinization efficiency via a coupled physical-biogeochemical high-resolution model (NEMO-BFM) for the Mediterranean Sea (1/16° horizontal resolution that is ~6 km) under an RCP4.5 scenario over the next decades. The alkalinization strategies applied in this study to the Mediterranean Sea illustrate the potential of ocean alkalinization to mitigate climate change by increasing the air-sea flux of CO₂ across the basin and counteracting acidification. In contrast to previous studies, the analyzed scenarios offer a clear pathway into practical implementation being based on realistic levels of lime discharge using the current network of cargo and tanker shipping routes across the Mediterranean Sea.

Two different approaches of alkalinization scenarios have been explored: one with a constant annual discharge of lime over the entire scenario period and another with gradually increasing alkalinization levels proportional to the pH decreases in the baseline scenario RCP4.5. The simulations used in the study suggest the potential of nearly doubling the carbon-dioxide uptake rate of the Mediterranean Sea after 30 years of alkalinization, and of neutralizing the mean surface acidification trend of the baseline scenario without alkalinization over the same time span.

A more recent paper carried out within the project and just published, realizes an estimate of the potential of maritime transport for ocean liming and atmospheric CO₂ removal, highlighting a very high potential discharge of slaked lime in the sea by using the existing global commercial fleet of bulk carrier and container ships. For some closed basins, such as the Mediterranean Sea where traffic density is relatively high, the potential of ocean alkalinization, also with low discharge rates, is far higher than what is needed for counteracting ocean acidification. Therefore, the results of this study highlight from one hand the need for further research for a more precise assessment of the technical aspects of this approach and potential criticalities, from another hand indicates the potential of a regional implementation of ocean liming to the Mediterranean Sea based on the existing network of tankers and cargo ships.

“These two publications provide a key contribution to the international and national scientific and technical communities working to find solutions to these two issues – atmospheric CO₂ removal and counteracting ocean acidification – which we will have to tackle in the future. Even if further investigations are needed, these results are encouraging”, states Stefano Caserini, Professor of Mitigation of Climate Change at Politecnico di Milano and Project leader of the project Desarc-Maresanus.

“In these works the idea of ocean alkalinisation as a mitigation strategy for climate change is for the first time assessed on the base of a technically feasible pathway of implementation providing a first step towards a real-world application. In addition, even if the full ecological consequences of this strategy still require additional research, a solution is indicated that may stabilise the acidity of the seawater counteracting acidification without risking dramatic alterations of the seawater chemistry in the opposite direction, which as of today would have largely unknown consequences.” states the main author of the first article, Momme Butenschön, Lead Scientist of the Research Unit on Earth System Modelling at the CMCC Foundation Euro-Mediterranean Center on Climate Change (CMCC).

Read the full papers published in Frontiers in Climate:

Butenschön M., Lovato T., Masina S., Caserini S., Grosso M. (2021), Alkalinization Scenarios in the Mediterranean Sea for Efficient Removal of Atmospheric CO2 and the Mitigation of Ocean Acidification. Frontiers in Climate – Negative Emission Technologies, volume 3, 11 pages, DOI: 10.3389/fclim.2021.614537

Caserini S., Pagano D., Campo F., Abbá A., De Marco S., Righi D., Renforth P., Grosso M. (2021) Potential of maritime transport for ocean liming and atmospheric CO₂ removal. Frontiers in Climate – Negative Emission Technologies. 3:575900. https://doi.org/10.3389/fclim.2021.575900

Website: www.desarc-maresanus.net

The 2020 Prime Minister’s MacDiarmid Emerging Scientist Prize Winner is Dr Christopher Cornwall from Te Herenga Waka Victoria University of Wellington, whose research focuses on the impacts of ocean acidification on marine organisms, including seaweeds and various calcifying algae and corals.

Ocean acidification is the result of changes in seawater pH levels due to increasing levels of atmospheric carbon dioxide. It is one of the biggest threats to the world’s oceans and has the potential to cause the collapse of ecosystems.

Chris’s work investigating ocean acidification, and career in research, has centred largely around three major questions:

1) How is ocean acidification leading to a decline in the growth of organisms with calcium carbonate skeletons?

2) Can reef-building organisms acclimatise or adapt to ocean acidification?

3) How will ocean acidification reduce the growth of coral reefs globally?

Expansive study shows seagrass meadows can buffer ocean acidification

Spanning six years and seven seagrass meadows along the California coast, a paper from the University of California, Davis, is the most extensive study yet of how seagrasses can buffer ocean acidification.

The study, published today in the journal Global Change Biology, found that these unsung ecosystems can alleviate low pH, or more acidic, conditions for extended periods of time, even at night in the absence of photosynthesis. It found the grasses can reduce local acidity by up to 30 percent.

“This buffering temporarily brings seagrass environments back to preindustrial pH conditions, like what the ocean might have experienced around the year 1750,” said co-author Tessa Hill, a UC Davis professor in the Department of Earth and Planetary Sciences and Bodega Marine Laboratory.

Marine forests

When picturing seagrasses, you might think of slimy grasses that touch your feet as you walk along the shoreline. But a closer look into these underwater meadows reveals an active, vibrant ecosystem full of surprises.

California’s seagrass meadows are home to a wide diversity of organisms, as seen in this video of research conducted by UC Davis’ Aurora Ricart, Melissa Ward, Tessa Hill and colleagues. The team deployed high-precision sensors into the meadows and found seagrass can make water less acidic. (Melissa Ward/UC Davis)

Sea turtles, bat rays, leopard sharks, fishes, harbor seals, seahorses and colorful sea slugs are just some of the creatures that visit seagrass ecosystems for the food and habitat they provide. They are nursery grounds for species like Dungeness crab and spiny lobster, and many birds visit seagrass meadows specifically to dine on what’s beneath their swaying blades of grass.

“It’s a marine forest without trees,” said lead author Aurora M. Ricart, who conducted the study as a postdoctoral scholar at UC Davis Bodega Marine Laboratory and is currently with Bigelow Laboratory for Ocean Sciences. “The scale of the forest is smaller, but all of the biodiversity and life that is in that forest is comparable to what we have in terrestrial forests.”

Night and day

For the study, the scientists deployed sensors between 2014 and 2019, collecting millions of data points from seven seagrass meadows of eelgrass stretching from Northern to Southern California. These include Bodega Harbor, three locations in Tomales Bay, plus Elkhorn Slough, Newport Bay and Mission Bay.

Aurora Ricart, left, and Melissa Ward stand aboard a research vessel while conducting UC Davis field work aimed at understanding seagrass’ capacity to buffer ocean acidification. (Courtesy Melissa Ward/UC Davis)

Buffering occurred on average 65 percent of the time across these locations, which ranged from nearly pristine reserves to working ports, marinas and urban areas.

Despite being the same species, eelgrass behavior and patterns changed from north to south, with some sites increasing pH better than others. Time of year was also an important factor, with more buffering occurring during the springtime when grasses were highly productive.

Seagrasses naturally absorb carbon as they photosynthesize when the sun is out, which drives this buffering ability. Yet the researchers wondered, would seagrasses just re-release this carbon when the sun went down, canceling out that day’s buffering? They tested that question and found a welcome and unique finding:

“What is shocking to everyone that has seen this result is that we see effects of amelioration during the night as well as during the day, even when there’s no photosynthesis,” Ricart said. “We also see periods of high pH lasting longer than 24 hours and sometimes longer than weeks, which is very exciting.”

Northern California’s Bodega Harbor and Tom’s Point within Tomales Bay stood out as being particularly good at buffering ocean acidification. Pinpointing why and under what conditions that happens across varied seascapes remains among the questions for further study.

Climate change, shellfish and ocean acidification

The study carries implications for aquaculture management, as well as for climate change mitigation and conservation and restoration efforts.

Globally, ocean acidification is on the rise while seagrass ecosystems are in decline. As more carbon dioxide is emitted on the planet, about a third is absorbed by the ocean. This changes the pH balance of the water and can directly impede the shell formation of species like oysters, abalone and crab.

“We already knew that seagrasses are valuable for so many reasons — from climate mitigation to erosion control and wildlife habitat,” said co-author Melissa Ward, a UC Davis graduate student researcher at the time of the study and currently a postdoctoral researcher at San Diego State University. “This study shows yet another reason why their conservation is so important. We now have a piece of evidence to say the state’s directive to explore these ideas for ameliorating ocean acidification is a valuable thread to follow and merits more work.”

Researchers at the UC Davis Bodega Marine Laboratory and its interdisciplinary Bodega Ocean Acidification Research Group are working with coastal communities, shellfish growers, policymakers and other scientists on a variety of research projects aimed at understanding how changing seawater chemistry impacts ecologically and economically important coastal species in California.

Additional co-authors on the study include Eric Sanford, Sarah Merolla, Priya Shukla, Aaron T. Ninokawa, Kristen Elsmore and Brian Gaylord of UC Davis Bodega Marine Laboratory; Kristy J. Kroeker of UC Santa Cruz; and Yuichiro Takeshita of Monterey Bay Aquarium Research Institute.

The study was funded by California Sea Grant and the California Ocean Protection Council.

Kat Kerlin, University of California Davis, 31 March 2021. Press release.

New challenges to a once tried-and-true method for assessing reef health

Carnegie’s Manoela Romanó de Orte and Ken Caldeira led a research team that deployed a cutting-edge incubator to monitor the metabolic activity of coral and algae in an area of Australia’s Great Barrier Reef that had been damaged by tropical cyclones. The CISME, or Coral In Situ Metabolism and Energetics, instrument is a small chamber that can be placed directly on the coral surface and allow scientists to monitor coral growth by measuring changes in seawater chemistry. Credit: image courstesy of Ken Caldeira.

Washington, DC– Algae colonizing dead coral are upending scientists’ ability to accurately assess the health of a coral reef community, according to new work from a team of marine science experts led by Carnegie’s Manoela Romanó de Orte and Ken Caldeira. Their findings are published in Limnology and Oceanography.

Corals are marine invertebrates that build tiny exoskeletons, which accumulate to form giant coral reefs. Widely appreciated for their beauty, these reefs are havens for biodiversity and crucial for the economies of many coastal communities. But they are endangered by ocean warming, seawater acidification, extreme storms, pollution, and overfishing.

Shelled organisms helped buffer ocean acidification by consuming less alkalinity from seawater.

Microscopic fossilized shells are helping geologists reconstruct Earth’s climate during the Paleocene-Eocene Thermal Maximum (PETM), a period of abrupt global warming and ocean acidification that occurred 56 million years ago. Clues from these ancient shells can help scientists better predict future warming and ocean acidification driven by human-caused carbon dioxide emissions.

Led by Northwestern University, the researchers analyzed shells from foraminifera, an ocean-dwelling unicellular organism with an external shell made of calcium carbonate. After analyzing the calcium isotope composition of the fossils, the researchers concluded that massive volcanic activity injected large amounts of carbon dioxide into the Earth system, causing global warming and ocean acidification.

They also found that global warming and ocean acidification did not just passively affect foraminifera. The organisms also actively responded by reducing calcification rates when building their shells. As calcification slowed, the foraminifera consumed less alkalinity from seawater, which helped buffer increasing ocean acidity.

The oceans are becoming more acidic because of the rapid release of carbon dioxide (CO₂) caused by anthropogenic (human) activities, such as burning of fossil fuels. So far, the oceans have taken up around 30% of all anthropogenic CO₂ released to the atmosphere. The continuous increase of CO₂ has a substantial effect on ocean chemistry because CO₂ reacts with water and carbonate molecules. This process, called ‘ocean acidification,’ lowers pH, and calcium carbonate becomes less available. This is a problem for calcifying organisms, such as corals and molluscs, that use calcium carbonate as the main building blocks of their exoskeleton.

In particular, organisms that build their shells from a type of calcium carbonate known as ‘aragonite’ are in trouble because aragonite is extremely soluble in sea water. Sea butterflies, tiny, swimming sea snails, build their shells of aragonite. Therefore, they are also known as ‘the canaries of the coalmine’ because they are expected to be amongst the first organisms to be affected by ocean acidification.

Team will assess vulnerability of aquaculture and restoration efforts in Chesapeake Bay

The excess carbon dioxide responsible for global warming also increases the acidity of seawater, challenging the growth and survival of oysters and other shellfish. A team led by researchers at William & Mary’s Virginia Institute of Marine Science is now helping oyster growers and restoration specialists better manage their future responses to acidification in the Chesapeake Bay.

The team, funded by the NOAA Ocean Acidification Program, is led by VIMS researchers Marjy Friedrichs and Emily Rivest, along with David Wrathall of Oregon State University. Other team members include Mark Brush, Pierre St-Laurent, and Karen Hudson of VIMS, Aaron Bever of Anchor QEA, and Bruce Vogt of NOAA’s Chesapeake Bay Office. The team calls their project STAR, for Shellfish Thresholds and Aquaculture Resilience.

“Coastal acidification and its associated co-stressors present a serious and credible threat to the success of both oyster aquaculture and oyster restoration in the Bay,” says Friedrichs. The co-stressors include nutrient pollution, warmer Bay waters, and pulses of freshwater from rainstorms made more intense by global atmospheric changes. Previous research has shown these factors can intensify the negative impacts caused by ocean acidification alone.

Rising ocean temperatures and ocean acidification could have negative cumulative effects that slow the growth of tropical coral reefs, according to a UCLA-led study published in early January.

The study examined the growth of two types of tropical corals – commonly known as cauliflower coral and hood coral – at different water temperatures and acidities. At a lower temperature, the corals were able to compensate for the acidification and continue building their skeletons. But at a higher temperature, the corals grew much slower.

In order for coral reefs to survive despite constant degradation by waves and human activity, individual reef-building corals must be able to efficiently build their skeletons through a process called calcification, said Maxence Guillermic, a UCLA postdoctoral researcher and lead author of the study. The coral must maintain the correct internal carbonate conditions to allow the skeletal building material, calcium carbonate, to precipitate, he added.

Researchers from the University of Tsukuba find that in the presence of ocean acidification, feedback loops keep degraded turf algal states stabilized, inhibiting the recruitment of coral and other algae.

Tsukuba, Japan—It’s tough out there in the sea, as the widespread loss of complex marine communities is testament to. Researchers from Japan have discovered that ocean acidification favors degraded turf algal systems over corals and other algae, thanks to the help of feedback loops.

In a study published this month in Communications Biology, researchers from the University of Tsukuba have revealed that ocean acidification and feedback loops stabilize degraded turf algal systems, limiting the recruitment of coral and other algae.

A NCCOS and NOAA Ocean Acidification Program sponsored study investigated how physical properties such as winds, tides, and currents impact estuarine acidification and carbonate chemistry in the Chesapeake Bay estuary, a complex and little studied undertaking. A coupled hydrodynamic-carbonate chemistry model was used to understand the wind-driven variability in the estuarine carbonate system. Large temporal pH fluctuations and low pH events were documented that could negatively impact acidification-sensitive species such as oysters.

A major challenge in the study of estuarine acidification is the strong temporal and spatial variability of carbonate chemistry resulting from a wide array of physical forces such as winds, tides and river flows. Most studies of estuarine carbonate system dynamics have been limited to the along-channel direction, while lateral pH dynamics (e.g., channel to shore) has received less attention.