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Wissenschaft
Why Coastal Ecosystems Must Be Strengthened – but Alone Cannot Offset the Impact of Fossil Fuel Emissions
Climate goals can only be met if carbon dioxide is removed from the atmosphere. Mangroves, kelp forests, and seagrass meadows take care of this naturally. How can these coastal ecosystems contribute to climate protection? And how can they be made more efficient and resilient in the face of change? The sea4soCiety research project, in which researchers from the U Bremen Research Alliance play a decisive role, is investigating these and similar questions.
In dispassionate scientific terms, mangroves are ecological service providers. These services are multifarious and their benefits extend well beyond those provided to local communities who depend on them most. Mangroves are used to make teas, medicines, building materials, and alcoholic beverages; their fruits supposedly taste like cheese. They serve as an aquatic nursery for fish, provide a habitat for mussels and other seafood, and their roots protect the coast from erosion and storm surges while purifying wastewater from the hinterland. At the same time, they also store organic molecules formed from carbon dioxide (CO2) in their sediment, their wood, and their leaves. Experts refer to this as “blue carbon.”
Dr. Martin Zimmer, however, is not only fascinated by the characteristics of the tidal plants, but the habitat itself. The almost mystical atmosphere in the morning haze before it gives way to heat and mosquitoes, the clacking sound in the crab caves as the water levels subside with a gulp, the calls of birds and monkeys, even the waist-deep mud, as well as the many, many mosquitoes and the humid heat all captivate him. “Mangrove forests are an extremely challenging wilderness,” Zimmer says. “It might sound melodramatic, but I find it soothing and inspiring to be there.”
Zimmer regularly spends time in mangrove forests. He is a professor for mangrove ecology at the University of Bremen and head of the Mangrove Ecology working group at the Leibniz Centre for Tropical Marine Research (ZMT), which are both member institutions of the U Bremen Research Alliance. Zimmer is also coordinator of the sea4soCiety research coalition, which he deems to be of great importance for tropical societies as well as for us. The acronym stands for “searching for solutions for carbon sequestration in coastal ecosystems,” that is, for the contribution that coastal ecosystems such as mangrove forests can make to mitigating climate change. How much CO2 do they sequester from the atmosphere? And how can they be protected and possibly strengthened so that they can continue to provide these ecological services in the future?
The initial goal was to measure and quantify – to scientifically collect clean data. During excursions in mangrove forests of Malaysia or Columbia, researchers took sediment samples by hand using foldable drills. These were then analyzed by local scientific partners and at the ZMT. Most of the CO2 has been stored in the sediment, often for decades, if not centuries. These analyses not only measure the amount, but also the long-term stability of the deposit area based on deforestation, coastal erosion, and increases in sea temperatures and levels. “This must be considered in our calculations and varies greatly depending on the region,” Zimmer has discovered. The stability of the organic material is analyzed as well.
People love to play fast and loose when speaking about the storage capacity of mangroves. “Most numbers are nonsense,” the researcher emphasizes, and warns of inflated expectations. “Mangrove forests are certainly very, very efficient carbon reservoirs, and on average sequester significantly more than terrestrial systems. However, they are not a panacea for combating climate change.” Together with seagrass meadows and salt marshes, they offset up to three percent of yearly global greenhouse emissions.
Martin Zimmer also clarifies a second widespread misconception, “Just planting mangroves is not the solution.” Seedlings do not store much CO2 and need many years to develop. They are too often planted in unfavorable locations with either too strong waves or suboptimal tidal currents. Experts estimate that of all plantings, 80 percent do not take hold.
In sea4soCiety, coastal ecosystems of the north such as salt marshes, seagrass meadows, and kelp forests are also studied. Around 40 scientists from nine universities and research institutions are participating in the project including the University of Bremen’s MARUM – Center for Marine Environmental Sciences. MARUM marine botanist, Professor Dr. Kai Bischof, is researching kelp forests off of Helgoland, which thrive on the island’s bedrock at a water depth of up to 13 meters. Like mangroves, kelp also sequester atmospheric CO2, albeit not as long term. After about ten years, they die and their biomass sinks to the bottom of the ocean or is washed ashore, releasing the greenhouse gas again. One approach is to determine whether this process can be minimized by recycling the remnants into biochar.
“Relying on nature-based solutions alone will not be sufficient to achieve our goal of net zero, let alone negative emissions,” says Professor Zimmer. The Cologne native came to Bremen in 2014 after having been stationed in Düsseldorf, Kiel, and Salzburg. “We need a host of measures.” Though he began his career as a foundational researcher, Zimmer is now more interested in applications. “At the ZMT, I quickly learned that our work also has a social dimension. I want to make a difference for our society and for those who live in the tropics.” The connection to the other research institutions within the U Bremen Research Alliance helps him with this. “Our areas of expertise complement each other well. This exchange is very beneficial.”
In the summer and in the spring of 2026, Zimmer will be working in mangrove forests in Malaysia and Colombia. This phase of sea4soCiety, which will be financed until 2027 by the Federal Ministry of Research, Technology and Space (BMFTR), will focus on the dynamics of carbon storage. How much is absorbed each year and how much is released?
The project is an eye-opener for him, Zimmer says, as it affirms the limited possibilities of natural coastal ecosystems in offsetting fossil fuel emissions. At the same time, it shows the need for behavioral changes. “Those who live close by are much more dependent on the ecosystem services than we are. We need to focus on making them more resilient now.”
Contact:
Merle El-Khatib
Communication und Marketing
Tel.: +49 421 218 60046
merle.el-khatib@vw.uni-bremen.de
About UBRA:
The University of Bremen and twelve federal and state financed non-university research institutes cooperate within the U Bremen Research Alliance. The Alliance includes research institutes of the four major German science organizations, i.e. Fraunhofer Society, Helmholtz Association, Leibniz Association and Max Planck Society, as well as the German Research Center for Artificial Intelligence.
Impact - The U Bremen Research Alliance science magazine Issue 12 (in German)
Since 2019 the Impact science magazine provides an exciting insight into the effects of cooperative research in Bremen. "The Power of Natural Climate Protectors" was published in issue 12 (September 2025) (in German).
https://www.bremen-research.de/fileadmin/user_upload/pdf/UNI_UBRA_Impact_2025_Au...
https://www.bremen-research.de/en/impressions/the-power-of-natural-climate-prote...
Prof. Dr. Martin Zimmer wants to ensure that mangroves continue to provide important ecosystem servi ...
Source: Jens Lehmkühler
Copyright: U Bremen Research Alliance
Researches large algae: Kai Bischof, professor at MARUM – Center for Marine Environmental Sciences a ...
Source: Jens Lehmkühler
Copyright: U Bremen Research Alliance
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