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Gloria Reithmaier, marine chemist at the University of Gothenburg.

Credit: Michael Reithmaier

The tidal zones of mangrove forests function as vital nursery grounds for numerous fish species. Scientists at the University of Gothenburg have analyzed carbon dioxide and oxygen concentrations in 23 mangrove regions around the globe. Their findings warn that these ecosystems are facing growing pressure as ocean temperatures continue to climb.

Water conditions in mangroves shift continuously with the tides. During low tide, oxygen concentrations decline while carbon dioxide accumulates, creating challenging conditions for fish and other marine organisms. Only species specially adapted to mangrove habitats can withstand such extremes. When the tide comes in, oxygen-rich seawater flushes the area and reduces carbon dioxide levels. These higher-tide periods enable more sensitive species, including commercially valuable fish, to enter the mangroves to feed or take refuge.

Extreme conditions

To determine how frequently these tidal cycles expose marine life to stressful conditions, researchers measured oxygen and carbon dioxide levels in 23 mangrove ecosystems worldwide. This marks the first global assessment in which both gases were examined together across multiple mangrove sites, uncovering widespread patterns of environmental strain.

“I was surprised to find that many mangrove systems are already experiencing very harsh conditions,” says Gloria Reithmaier, marine chemist at the University of Gothenburg and lead author of the study. “Particularly in warm tropical regions, extended periods of low oxygen and elevated carbon dioxide leave only short windows of time for sensitive fish species to access these habitats.”

As global warming drives up ocean temperatures, the researchers used various climate scenarios to project how mangrove waters may evolve under increasing heat and rising carbon dioxide levels. In every scenario examined, stressful conditions intensified and persisted longer. This trend reduces the time available for fish and other marine organisms to use mangroves as protective habitats. In some projections, environmental conditions approach thresholds at which many fish species would struggle to survive.

Highly diverse habitats

“In regions such as the Amazon and India, carbon dioxide levels are already extremely high. Compared to mangroves located farther from the equator, tropical systems are functioning close to their environmental limits,” says Gloria Reithmaier. “These biologically rich ecosystems may be the first to lose their more sensitive species as ocean temperatures and carbon dioxide concentrations continue to rise.”

Growing environmental stress could progressively diminish biodiversity within mangrove ecosystems, favoring only the most resilient species. The consequences are expected to be most severe in tropical developing nations, where fisheries and coastal communities depend heavily on healthy mangrove systems.

“The species people value most for fishing are likely among those most vulnerable to these changes,” says Gloria Reithmaier.

Journal

Geophysical Research Letters

DOI

10.1029/2025GL119355

Method of Research

Observational study

Article Title

Climate Change Will Enhance Hypercapnic Hypoxia Threatening Mangrove Habitats

Article Publication Date

13-Feb-2026