Mangroves are widely recognised for storing carbon, earning them a prominent place in discussions of climate change and “blue carbon”. New research by Professor Benoit Thibodeau, Assistant Professor in the Department of Earth and Environmental Sciences and the School of Life Sciences at The Chinese University of Hong Kong (CUHK), and PhD candidate Wang Ziyan has revealed another important function of these coastal forests: their capacity to remove nitrogen pollution from coastal waters.
Published in Earth’s Future, the study estimates that mangroves remove approximately 870,000 tonnes of nitrogen pollutants from coastal waters each year, representing an ecosystem service worth more than US$8.5 billion annually. The researchers refer to this overlooked function as “blue nitrogen”.
While carbon has dominated international discussions of climate mitigation, excess nitrogen presents a parallel environmental challenge. Nitrogen from agriculture, wastewater and other human activities can enter coastal waters, stimulating algal growth. As the algae decompose, oxygen levels fall, creating “dead zones” in which marine life struggles to survive.
“Planetary boundaries represent the Earth’s safety limits, and we have already crossed seven of the nine,” Professor Thibodeau explains. “Among them, biogeochemical flows, and nitrogen pollution in particular, rank as one of the most severely transgressed.”
From blue carbon to blue nitrogen
Professor Thibodeau’s research into nitrogen removal began with blue carbon, i.e. carbon stored in marine and coastal environments. Rather than simply measuring how much carbon mangroves store, he wanted to understand the processes that enable them to do so. Hong Kong’s mangroves provided a natural laboratory. As Professor Thibodeau and his colleagues studied the interactions between carbon, nitrogen and sulfur in mangrove sediments, they measured rates of denitrification, through which microorganisms convert reactive nitrogen into harmless nitrogen gas.
This work belongs to biogeochemistry, an interdisciplinary field that examines how elements such as carbon, nitrogen and sulfur move through living organisms, soils, sediments, water and the atmosphere. In coastal ecosystems, these processes help determine how nutrients are stored, recycled or removed, influencing water quality and the services provided by wetlands.
“As we began measuring nitrogen removal processes like denitrification in Hong Kong, we realised the rates were surprisingly significant,” Professor Thibodeau says. The team then compiled data from mangrove ecosystems worldwide to assess the global importance of nitrogen removal.
The analysis showed that mangroves remove nitrogen pollution at a rate nearly twice that of terrestrial soils. Although they occupy less than 0.1% of the world’s land area, their contribution to coastal water quality is substantial. The estimated value of this natural purification service is more than 12 times the value of mangrove carbon sequestration credits, according to the study’s comparison.
The research direction was not obvious at the outset. “In the early stages, it was challenging to convince funding agencies to support work in this field,” Professor Thibodeau recalls. “So yes, it was a risk.” In this case, a question about carbon storage opened a new line of inquiry into nitrogen cycling and the wider functions of mangrove ecosystems.
A natural laboratory for a resilient city
Hong Kong’s coastal wetlands have played a central role in this research. Sites such as Mai Po enable researchers to study nutrient movement, microbial activity and the relationship between coastal habitats and water quality.
For Professor Thibodeau, mangroves are important to Hong Kong not only because they store carbon, but also because they provide coastal protection and help improve water quality. Their roots can stabilise shorelines and dissipate wave energy, while their sediments support nitrogen removal.
“These ecosystems are crucial for Hong Kong, though perhaps not for the reasons most people would expect,” he says. “As a low-lying coastal city increasingly exposed to typhoons and storm surges, Hong Kong benefits enormously from the natural buffering that mangroves provide.”
He stresses that mangroves cannot solve nitrogen pollution on their own. Reducing pollution at its source, through better food production, wastewater treatment and nutrient management, must remain the priority.
Mangroves can nevertheless provide an important buffer by intercepting and removing part of the nitrogen that would otherwise reach coastal waters. They also support biodiversity, store carbon and help protect shorelines.
“Too often, wetlands are seen as empty land awaiting development, with their services taken for granted precisely because they are invisible and unpriced,” Professor Thibodeau says.
By quantifying nitrogen removal and estimating its economic value, the research provides evidence to inform conservation and coastal management by comparing the benefits of protecting natural systems with the costs of relying solely on engineered infrastructure.
“These ecosystems deliver a combination of benefits – coastal defence, water purification, biodiversity and carbon storage,” he says. “If we can learn to see them as valuable infrastructure in their own right, I am confident they can play a defining role in building a more resilient and sustainable city.”
An unexpected journey
Professor Thibodeau’s interest in mangroves began unexpectedly. Raised in Canada, where mangroves do not exist, he first encountered them during his postdoctoral fellowship at the University of Tokyo, while conducting research on coral reefs in Okinawa.
During a field trip to Iriomote Island, he wandered into a mangrove forest. The trees’ tangled prop roots, the richness of life in the mud and the crabs emerging as the tide receded left a lasting impression.
“I was struck by the extraordinary architecture of the trees, with their tangled prop roots rising out of the mud, by the sheer richness of life around me, and by the countless crabs that emerged from their burrows as the tide receded,” he recalls. “It was a world unlike anything I had known, and it left a lasting mark.”
His work later brought him to Hong Kong, where collaborations with experts in marine biology and biodiversity led him to study mangrove food webs and sediment processes. These exchanges gradually expanded his research from biodiversity to the wider ecological functions of coastal wetlands.
Research without borders
Professor Thibodeau describes Hong Kong as a distinctive base for environmental research, providing access to diverse coastal environments while attracting researchers with different scientific backgrounds and perspectives. He has found the same spirit of dialogue and exchange at CUHK, where interdisciplinary collaboration is central to his work. The University’s academic freedom has given his research group the space to pursue questions that were not yet fully established.
For blue nitrogen, that freedom was particularly important. The move from blue carbon towards nitrogen removal was neither obvious nor well-funded at the beginning. It required the researchers to follow evidence as it emerged and remain open to unexpected findings. “An institution that understands this, and that trusts its researchers to take intellectual risks, is one that creates the conditions for genuine breakthroughs,” Professor Thibodeau says.
The research has transformed an observation in Hong Kong’s mangrove sediments into a global perspective on coastal wetlands not simply as habitats to be protected, but as living systems that help societies build healthier and more resilient coasts. “CUHK allows scholars to pursue the questions that genuinely matter to us and to follow our instincts, even when they lead us down unconventional paths,” Professor Thibodeau concludes.
