Depleting oxygen levels due to rising CO2 in mangrove waters threatening fish nurseries

Mangrove-lined estuaries are experiencing high levels of environmental stress due to rising CO₂ levels caused by climate change, leading to a condition called hypercapnic hypoxia.

Hypercapnic hypoxia, which is a high CO₂ and low oxygen state, pushes estuaries, where the tide meets the stream, into a stressful chemical state. It mostly occurs during low tide, at low-salinity sites and in warm tropical regions.

A new study published in AGU | Advancing Earth and space science assessed 23 mangrove sites across the globe and revealed that most sites already experience mild (34-43 per cent of the time) or severe (6 per cent–32 per cent) hypercapnic hypoxia.

“Climate change will decrease oxygen concentrations by 5-35 per cent and increase CO₂ concentrations by 8-60 per cent in mangrove waters by 2100,” the researchers said.

It added that these events will become more intense and 15 times more frequent under extreme climate scenarios.

The unfavourable chemical conditions reduce the window during which fish can safely enter mangrove nurseries, which begins to shrink.

“Overall, hypercapnic hypoxia events will occur more frequently, last longer, and become more severe. These shifts will reduce mangrove biodiversity and deteriorate habitat quality for commercially valuable fish. The strongest impact is expected in tropical developing countries,” the study noted.

Temperature acts as a secondary but powerful driver. A 10°C increase in water temperature (from 20°C to 30°C) correlates with a 30 per cent decrease in dissolved oxygen and a 50 per cent increase in CO₂.

The study warned that current short-term events (<6 hours) will transition into prolonged exposures. Under heatwave scenarios, 78 per cent of sites will experience mild hypoxia lasting 12 to 24 consecutive hours. Notably, in the Amazon, hypercapnia is projected to persist 100 per cent of the time during extreme heatwaves.

Mangrove forests provide habitats for numerous marine species, enhancing biodiversity and supporting fisheries.

The paper said that typical mangrove fish species are found across a wide dissolved oxygen range of 30–110 per cent saturation. Low-tolerance fish are often reef-associated species, with increased size and diversity occurring above 70–80 per cent air saturation.

“Several of the medium- to low-tolerance species are highly relevant for fisheries in developing countries, for example, common silver-biddy (Gerres oyena), silver grunt (Pomadasys argenteus), pink ear emperor (Lethrinus lentjan), and Indian goatfish (Parupeneus indicus). Increased hypercapnic hypoxia will thus shift mangrove species composition away from large reef-associated species favoured by fishers,” the study observed, noting these are likely to be impacted.

Providing food, shelter, and protection from predators, mangroves create attractive nursery grounds for commercially and ecologically valuable crab, shrimp, and fish species, increasing populations in connected ecosystems such as coral reefs and seagrass meadows, the study underlined.

It is likely to threaten the livelihoods of millions of people.

“Mangroves support ∼20,000 additional fish per hectare per year compared to unvegetated areas, an ecological service worth $10 million. Globally, around 4 million fishers rely on mangrove ecosystems, with the majority located in developing countries such as Brazil, Indonesia, and Tanzania. To preserve socioeconomic benefits and guide coastal management, it is crucial to understand what drives mangrove habitat function,” the authors noted.

The researchers noted that these predictions may be conservative, as the data were collected in well-flushed tidal channels; conditions deeper inside mangrove forests are likely to be even more extreme.