It’s not just the oceans, carbon dioxide is changing freshwater ecosystems too. Such is the key message behind a study by researchers at Ruhr-University Bochum in Germany.

Ocean acidification, often described as climate change’s ‘equally evil twin’, occurs when increased carbon dioxide in the atmosphere causes more carbon dioxide to be absorbed in seawater. The impacts of the acidification have been well studied, especially around organisms such as shellfish and corals. Meanwhile, surprisingly little is known about increases in carbon dioxide or the rate of acidification in freshwater. ‘Freshwater has been overlooked,’ says the team in Germany.

By analysing 35 years worth freshwater data from reservoirs in western Germany, the researchers found carbon dioxide levels in the water (or pCO2) was increasing annually. More worryingly, they saw that the reservoir water was becoming more acidic faster than seawater, at a pH change of 0.01 per year.

But how to detect the impacts of increased carbon dioxide on complex freshwater ecosystems? For this, they looked near the bottom of the food chain, focusing on the humble daphnia, a tiny crustacean common to many freshwater habitats. ‘The daphnia is a primary food source for higher levels of the food chain, and has long since been regarded as a key species of aquatic systems,’ says Linda Weiss, a biologist at Ruhr. Crucially, when a daphnia senses a predator, it responds by whipping out a helmet and spikes, making it harder to eat.

The study found increased carbon dioxide reduced these defences significantly, making the daphnia more vulnerable. The reduced ability, they found, was caused directly from the increase in pCO2, rather than from the indirect acidification of the water. With pCO2 increasing every year, scientists worry there will be similar impacts on other organisms and repercussions higher up the food chain. Weiss emphasises the need for further study on carbon dioxide and acidification in freshwater ecosystems, ‘we now want to know the global degree of this phenomenon.’ 

This was published in the March 2018 edition of Geographical magazine

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