Oxygen Depletion Threatens Life in the World’s Mangrove Forests

As the oceans warm, it becomes harder for fish in mangrove forests to breathe. This is shown by a unique new study from researchers at the University of Gothenburg.

‘In some systems, conditions will become so extreme that only the most adapted species will survive,’ says the study’s lead author, marine chemist Gloria Reithmaier.

^{Text: Amanda Saveland}

The world’s mangrove forests act as nurseries for many fish species. Sheltered by the roots of the trees, they can search for food undisturbed without being eaten by larger predatory fish. As a result, the number of fish living there is estimated to be around 20,000 more per hectare per year compared to unvegetated seabeds. This also affects nearby ecosystems, such as coral reefs, where the fish migrate once they have grown.

Previous estimates have valued these ecosystem services at 92.3 million kronor. However, the vital role played by mangroves is now under threat from global warming. As the oceans warm, carbon dioxide levels also rise, causing oxygen levels in the mangrove forests to reach critically low levels whilst carbon dioxide levels become dangerously high.

‘What surprised me most was how many mangrove forests are already exceeding the threshold at which it becomes difficult for fish to breathe,’ says Gloria Reithmaier.

How this affects the fish

In the premises where she works at the University of Gothenburg, ventilation is regulated based on the carbon dioxide levels in the rooms. If they exceed a certain limit, the fans start spinning faster to bring in more oxygen.

‘If there is a lot of carbon dioxide and not enough oxygen in a room, we notice it straight away. We get tired and drowsy, and it’s exactly the same for the fish. They become disoriented and move more slowly, and if the situation becomes too serious, they die,’ she explains.

Together with her research colleagues, she has measured the levels of oxygen and carbon dioxide in the water across a total of 23 mangrove forests worldwide. Collecting the data has required advanced technology and intensive monitoring. The equipment has needed to be maintained every few hours, in difficult terrain featuring both crocodiles and metre-high tidal surges.

‘The fact that we have managed to collect this high-resolution data over extended periods of time is what makes the study unique,’ says the marine chemist.

A critical window of opportunity is closing

It is the tides that cause mangroves to be more affected by increased carbon dioxide levels than other marine environments.

There is already a natural oxygen deficiency that occurs when the tide is low, which only the most highly adapted species can cope with. The other, more sensitive fish must instead wait to enter the mangroves until the tide rises and oxygen-rich water flows in again.

This is a crucial window of opportunity for much of the life in the forests. But as the oceans get warmer and the amount of carbon dioxide in the atmosphere increases, the periods of extreme oxygen depletion and elevated carbon dioxide levels are becoming longer and longer.

‘In some systems, this window of opportunity will disappear completely,’ says Gloria Reithmaier.

The most species-rich forests are hit hardest

The study was based on various climate scenarios and developed models of how mangrove forests will be affected by different levels of warming. Even today, they observed that several areas, particularly those closest to the equator, were already exceeding the threshold where it became difficult for fish to breathe during parts of the day.

‘Those most affected are also the ecosystems that are most valuable in terms of biodiversity. In the Amazon, for example, our models showed that this harmful condition will become constant in the future,’ says Gloria Reithmaier.

Economic consequences

Today, around 4 million people depend on mangrove forests for their livelihoods. Most of them live in developing countries and engage in small-scale fishing. An income that risks disappearing if fish stocks in the mangroves decline.

‘It is mainly the fish we are most interested in eating that will be hardest hit, as they are the most sensitive to this change in water quality. So this will have major consequences for many people,’ says Gloria Reithmaier.

Reduced emissions make a difference

Above all, she believes that more fish will be eaten when they are forced away from their protected nursery habitats. The lack of oxygen does not kill them, as they can swim away when the oxygen disappears. But there are also other species, such as oysters, which do not have the same ability to move.

‘So climate change will most likely lead to reduced biodiversity in the mangroves,’ she says.

At the same time, the study showed that the outlook for life in the mangroves varies greatly depending on which climate scenario is used as a basis.

– ‘In scenarios where we actually do something about climate change now, where everyone contributes and tries to reduce emissions, the effects are actually not as severe,’ says Gloria Reithmaier.

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