The “blue forest”* in figures: first global inventory of carbon stored by seagrass meadows
CEAB-CSIC participates in international work demonstrating the capacity of living parts of marine plants to retain up to 40 million tonnes of carbon worldwide
Spanish National Research Council (CSIC)
image:
Seagrass meadows and the researchers working in them
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An international study led by CEAB-CSIC and published in Nature Communications presents the first global assessment of blue carbon accumulated in the living parts of seagrass plants. According to the results, their leaves, rhizomes and roots store up to 40 million tonnes of carbon worldwide. To this figure must be added the carbon stored in the seabed, which can remain sequestered for thousands of years, as long as the meadow persists. The data confirm that, despite covering a very small area, these ecosystems play a key role in absorbing atmospheric CO₂, transforming it, and retaining it.
The research was led by the Centre for Advanced Studies of Blanes (CEAB-CSIC) and BIOSFERA Research & Conservation, with participation from Edith Cowan University (Australia), the University of Western Australia, James Cook University, the Institute of Marine Sciences (ICM-CSIC), King Abdullah University of Science and Technology (KAUST), and the Institute of Marine and Coastal Research (CONICET, Argentina).
This is the first global “inventory” of seagrass meadows as blue carbon sinks. It includes calculations of the CO₂ they capture, their production —the transformation of carbon dioxide into new plant biomass—, and the carbon they store. It also provides figures on the emissions resulting from their loss.
In addition to presenting a global overview, the article offers detailed data by regions, countries, and meadow types. This makes it possible to quantify the contribution of each area, sea, and ocean to the carbon cycle, enabling territories to understand the importance of their own “blue forests”.
Seagrass meadows: a hidden treasure
Seagrass meadows, such as those formed by Posidonia, cover an area of between 160,000 and 266,000 km² worldwide. Despite their small size, they function as true “blue forests”: they capture CO₂, one of the main greenhouse gases, transform it into organic carbon through photosynthesis, and store it in their leaves and roots. Part of this carbon becomes incorporated into the seabed, where it can remain sequestered for millennia, for as long as the meadow endures.
This process makes them such efficient natural carbon sinks that, per unit area, they are comparable to or even exceed tropical forests. On average, they accumulate around 1.5 tonnes of carbon per hectare in the living parts of seagrass plants and fix nearly 7 tonnes each year.
Differences by genera and regions
Capture capacity varies by genus. Meadows formed by persistent genera, such as Posidonia in the Mediterranean, store more carbon in their structure, while opportunistic and colonising genera stand out for their rapid growth and high annual capacity to capture CO₂.
There are also clear differences between seas. In the Mediterranean, for instance, meadows retain significant amounts of carbon in the seabed, but their annual capture rate is moderate. By contrast, in regions such as the North Pacific or the temperate Atlantic, the opposite occurs: meadows are composed of smaller, shorter-lived plants, but with very rapid growth, enabling them to capture more CO₂ than Mediterranean meadows. In other words: some accumulate more carbon in the long term, while others excel in the speed with which they fix this gas.
Threats and avoidable emissions
Despite their crucial role, seagrass meadows are in constant decline due to urban development, pollution, and global warming. Their loss generates between 154 and 256 gigagrams of CO₂ equivalent each year, solely from their living biomass. Australia, Spain, Mexico, Italy, and the United States account for over 80% of these emissions linked to seagrass loss.
Blue carbon markets and nature restoration
Researchers point out that these data pave the way for the inclusion of seagrass meadows in carbon credit markets, alongside forests, mangroves, and saltmarshes. This could boost their conservation and restoration.
“Seagrass meadows are a cornerstone in the fight against climate change. Conserving them not only preserves biodiversity, but also avoids emissions and contributes to capturing carbon naturally,” says Enric Gomis, PhD student at CEAB-CSIC and BIOSFERA, and first author of the study.
“What is new about this work is that, for the first time, we have a global balance of blue carbon in seagrass meadows. This enables us to better understand their role on the planet and to open the door to global conservation policies and carbon credit markets, as well as other initiatives to restore nature and benefit from their ecosystem services,” adds Òscar Serrano, CEAB-CSIC researcher and coordinator of the study. “It has been proven that seagrass meadows are a key element in the fight against climate change. Conserving them —in addition to safeguarding the diversity of marine life forms, improving water quality, or protecting the coastline— removes CO₂ from the atmosphere,” concludes the researcher.
The authors stress that protecting these ecosystems is a natural, highly efficient, and also cost-effective tool to address the global climate challenge. At a time when it is urgent to reduce greenhouse gas emissions and enhance all measures that mitigate them, the protection of these “underwater forests” emerges as a feasible and powerful solution.
Gomis, E., Strydom, S., Foster, N.R. et al. Global estimates of seagrass blue carbon stocks in biomass and net primary production. Nat Commun 16, 9530 (2025). https://doi.org/10.1038/s41467-025-64667-6
CSIC Comunicación
comunicacion@csic.es
Journal
Nature Communications
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
Global estimates of seagrass blue carbon stocks in biomass and net primary production
Article Publication Date
3-Nov-2025
The driver of Sargassum blooms in the Atlantic Ocean
Upwelling of phosphorus-rich deep water promotes an N-fixing symbiont of the Sargassum algae giving it a competitive advantage
Max Planck Institute for Chemistry
By the beginning of June this year, approximately 38 million tons of Sargassum drifted towards the coasts of the Caribbean islands, the Gulf of Mexico, and northern South America, marking a negative record. Especially during the summer months, the brown algae accumulate on beaches, decomposing and emitting a foul odor. This not only repels tourists but also threatens coastal ecosystems. In the open ocean, Sargassum seaweed floating on the surface serves as nourishment and habitat for numerous marine species.
The algae originally come from the Sargasso Sea, located east of Florida. However, since 2011, researchers have repeatedly observed the so-called Great Atlantic Sargassum Belt, a gigantic carpet of gulfweed that drifts from the equator towards the Caribbean when easterly winds prevail. Until now, the sources of nutrients phosphorus (P) and nitrogen (N) fueling their rapid growth was unclear. It was hypothesized that nutrient runoff from overfertilization and rainforest deforestation might be responsible. However, these processes cannot explain the increase in Sargassum biomass observed during the past years.
An international research team led by the Max Planck Institute for Chemistry has now uncovered the main mechanism behind these algae blooms. The researchers have also identified the climatic conditions that facilitate this phenomenon, enabling them to develop a predictive system for future stranding events of Sargassum.
Extra nitrogen provided by cyanobacteria growing on the algae
In the latest issue of the journal Nature Geoscience, the researchers from Mainz explain how strong wind-driven upwelling near the equator transports phosphorus to the ocean's surface and moves it northward into the Caribbean. This increase in phosphorus availability benefits cyanobacteria that grow on the brown algae. These bacteria can capture atmospheric gaseous nitrogen (N2) and convert it into a form usable by the algae, a process called nitrogen fixation. Cyanobacteria are known to colonize Sargassum algae, forming a symbiotic relationship where Sargassum benefits from an additional nitrogen source. According to the new study this symbiotic relationship offers a competitive advantage with respect to other algae in the Equatorial Atlantic and can explain past changes in Sargassum biomass.
Nitrogen isotopes bound in coral cores have unveiled nitrogen fixation rates over the past 120 years
The researchers have identified the connection between algal blooms, increased nitrogen fixation, and the upwelling of cool, nutrient-rich deep water by analyzing coral cores from diverse Caribbean locations. Corals are vital archives to reconstruct past changes in the ocean because during their growth they incorporate chemical signatures from the water in their calcareous skeletons. By analyzing coral annual growth layers, which are akin to tree rings, researchers can reveal changes in the chemical composition of the ocean over the past centuries.
In this study the Max Planck researchers have analyzed the nitrogen isotopic composition of corals to reconstruct the amount of nitrogen fixed in the ocean by microorganisms over the last 120 years. During nitrogen fixation bacteria lower the ratio of stable nitrogen isotopes 15N to 14N in the ocean. Thus, periods of low 15N to 14N analyzed in the coral layers indicate times of high nitrogen fixation rates. Seawater samples collected by the research vessel Eugen Seibold were used to calibrate the nitrogen isotopic composition of modern corals demonstrating that they record nitrogen fixation.
Since 2011, algae growth and nitrogen fixation have remained coupled
Jonathan Jung, a PhD student at the Max Planck Institute for Chemistry and first author of the study, explains, "In the first set of measurements we noticed two significant increases in nitrogen fixation in 2015 and 2018, two years of record Sargassum blooms. So we compared our coral reconstruction with annual Sargassum biomass data, and the two records aligned perfectly! At that time, however, it was not at all clear whether there was a causal link.”
The researchers identified a connection after examining both sets of measurements. It turned out that not only the maximum values but the entire data series for algae growth and nitrogen fixation, including minimum values, have been coupled since 2011. This timing is important because, in 2010, strong winds displaced brown algae for the first time from the Sargasso Sea to the tropical Atlantic.
The research team concluded that the excess of phosphorus is the key factor in Sargassum blooms by eliminating other possibilities. One previous theory suggested that iron-rich Saharan dust, which frequently blows from Africa to the Atlantic, promotes the growth of the algae. However, the dust input did not correlate with biomass. Similarly, nutrient inputs from the Amazon or Orinoco rivers did not correlate with observations of Sargassum blooms.
The new mechanism can be used to improve predictions of future Sargassum blooms
In their publication, the researchers therefore describe a mechanism in which phosphorus from upwelling deep water and nitrogen from nitrogen fixation drive algal blooms observed during the past decades. Geochemist Jung adds: “Our mechanism explains the variability of Sargassum growth better than any previous approaches. However, there is still uncertainty as to whether and to what extent other factors also play a role.”
The supply of phosphorus occurs by cooler sea surface temperatures in the tropical North Atlantic and warmer temperatures in the southern Atlantic. These temperature variations cause changes in air pressure, leading to wind anomalies that displace surface water and allow phosphorus-rich water from the deep sea to flow in.
According to Mainz researchers, observing winds, sea temperatures, and the resulting upwelling changes in the equatorial Atlantic can improve predictions of Sargassum growth. Alfredo Martínez-García, group leader at the Max Planck Institute for Chemistry and senior author of the study, explains: “Ultimately, the future of Sargassum in the tropical Atlantic will depend upon how global warming affects the processes that drive the supply of excess phosphorous to the equatorial Atlantic”. His team plans to provide a more detailed view of these processes by measuring new coral records from different locations across the Caribbean. The researchers expect that these new findings can guide efforts to mitigate the impacts of the blooms on Caribbean reef ecosystems and coastal communities.
Journal
Nature Geoscience
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Equatorial upwelling of phosphorus drives Atlantic N2 fixation and Sargassum blooms
Article Publication Date
5-Nov-2025
