Macroalgae are Taking Over – and Could Change How the Ocean Works

Over the past 20 years, macroalgae blooms have increased at an unprecedented rate. This is shown by a new study, which warns that the spread could lead to major changes in the ecology of the sea.

^{Text: Amanda Saveland}              

Using over a million satellite images, the researchers behind the study have mapped how macro- and microalgae blooms have changed globally between 2003 and 2022. Among other things, the images show that macroalgae mats in the tropical Atlantic and western Pacific have increased by 13.4 per cent per year. These blooms have increased particularly over the last decade, which researchers link to global warming.

‘What is remarkable is that the main increase in both floating macroalgae and surface accumulations of microalgae has occurred over the past decade, in line with the accelerated global ocean warming since 2010,’ the report’s authors write.

More blooms to come

According to the study, macroalgae in particular, but also certain types of microalgae, benefit when the oceans warm up. The blooms are also exacerbated by nutrients released from agriculture, for example, especially phosphorus, which causes eutrophication. This is something that researchers believe will lead to even greater blooms in the future.

Regime shift as macroalgae take over

At the same time, the researchers see that the impact differs between different types of algae. Even though microalgae are also increasing, it is not at all at the same rapid rate as macroalgae. The study also shows that the presence of phytoplankton, which produces about half of all oxygen on Earth, has not increased in places where macroalgae have bloomed heavily.

“If this is the case, we believe that a regime shift in oceanographic conditions has already occurred, favouring macroalgae. This will have profound consequences for the atmospheric radiation balance and light conditions in the ocean, as well as for carbon storage, ocean biogeochemistry and the stability of the upper ocean layer,” the researchers warn.

Impact on the sea and on land

This is therefore a shift in which large algae are favoured at the expense of microscopic algae, changing the way the sea functions. The dense algae mats make it more difficult for light to penetrate the sea surface, impairing the sea’s ability to store carbon. In addition, the study points to economic effects when algae wash up in places where many people make their living from tourism and who risk losing their income when the chalk-white beaches are covered with foul-smelling masses of rotting algae.

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