Tropical mangroves already push fish to their breathing limits, and warming will make it worse

Mangrove forests are suffocating. Not in the dramatic, visible way of a forest fire or a bleached coral reef, but in a slow tightening of the chemical conditions that fish need to breathe. Researchers at the University of Gothenburg have now measured exactly how tight those conditions are, across 23 mangrove systems on multiple continents, and the picture is worse than many expected.

The study, the first to simultaneously measure both oxygen and carbon dioxide across mangroves worldwide, reveals that many tropical systems already operate at the edge of what marine life can tolerate. Under climate warming scenarios, those edges get pushed further, reducing the windows of time when fish can safely use mangroves as nurseries, feeding grounds, and shelter.

The tidal breathing cycle

Mangrove waters follow a daily rhythm dictated by tides. At low tide, decomposition of organic matter consumes oxygen while producing carbon dioxide. Oxygen drops, CO2 rises, and conditions become hostile to all but the most tolerant species. When the tide comes in, fresh seawater brings oxygen and flushes out accumulated CO2. These high-tide windows are when more sensitive species, including many commercially important fish, enter the mangroves.

Gloria Reithmaier, a marine chemist at the University of Gothenburg and lead author, found that many mangrove systems, particularly in warm tropical regions, already experience prolonged periods of extreme low oxygen and high CO2. The recovery windows at high tide are shorter than might be assumed, leaving less time for sensitive species to safely enter and use the habitat.

The Amazon and India are already at the limit

The global survey revealed a clear geographic pattern. Mangroves closer to the equator, where water temperatures are highest and decomposition rates fastest, experience the most extreme conditions. Systems in the Amazon and India showed particularly elevated CO2 levels even at baseline.

Compared with mangroves at higher latitudes, these tropical systems already operate close to the physiological limits of the species that inhabit them. The margin between current conditions and conditions that would exclude sensitive fish is thin.

Every warming scenario makes it worse

Using multiple climate projections, the researchers modeled how mangrove water chemistry would change as ocean temperatures rise and atmospheric CO2 increases. Across all scenarios, stressful conditions became more severe and lasted longer. The time available for fish to enter and use mangroves shrank.

In some projections, conditions in tropical mangroves reach levels where many fish species would struggle to breathe at all. The species most affected are likely to be the ones that are least tolerant of low oxygen and high CO2, which tend to be the commercially valuable species that coastal communities depend on for food and income.

Who stands to lose the most

The consequences are not distributed equally. Tropical developing countries, where fisheries and coastal livelihoods depend most heavily on mangrove ecosystems, face the sharpest declines. These are also the countries with the least capacity to adapt through alternative protein sources or economic diversification.

Increasing environmental stress may gradually reduce biodiversity in mangrove ecosystems, favoring only the most tolerant species. Over time, this could transform mangroves from diverse nurseries supporting complex food webs into simplified systems dominated by a handful of hardy species with limited commercial or nutritional value.

What the data cannot predict

The study models chemical changes based on temperature and CO2 projections, but biological responses to those changes are harder to forecast. Some fish species may adapt. Others may shift their timing, entering mangroves only during the briefest high-tide windows. Interactions between species, the arrival of new heat-tolerant competitors, and local management decisions will all shape outcomes in ways that a chemical model cannot capture.

The measurements also represent snapshots in time, not continuous monitoring. Tidal cycles, seasonal variation, and year-to-year fluctuations all affect the conditions fish actually experience. Permanent sensor arrays in key mangrove systems would provide a more detailed and dynamic picture.

Still, the core finding is difficult to dismiss: the world's tropical mangroves are already breathing hard, and rising temperatures will only tighten the constraints on the species that depend on them.