Seaweed forests, long overlooked in carbon storage discussions, play a far greater role in mitigating climate change than previously recognized. Recent studies by an international team of researchers at the University of Western Australia have found that millions of tons of seaweed-derived carbon are transported to the deep sea every year, where it can remain locked away for centuries.

Underwater rivers

While phytoplankton has traditionally been credited as the main source of carbon transported to ocean depths, seaweed has now emerged as a key player. According to the researchers, around 56 million tons of seaweed carbon reach deep-sea areas annually, some travelling over 5,000km from their coastal origins. This transfer occurs via currents of rapidly sinking dense water—“underwater rivers” formed by cold, dense coastal water sliding down the seafloor—which carry seaweed fragments far offshore.

This process is well documented off Western Australia, particularly during cooler months and storm events. Using ocean models, the researchers have now confirmed that it occurs globally. Kelp forests in Australia’s Great Southern Reef are major contributors, along with seaweed habitats in the United States, Chile, Indonesia and New Zealand.

Massive carbon sinks

Globally, seaweed forests cover an area twice the size of India and fix nearly one billion tons of carbon per year—comparable to Canada’s boreal forests. Of this, an estimated 10 to 170 million tons reach the deep ocean annually, the researchers stated.

Despite their importance, seaweed forests are under threat from warming seas, pollution and marine heatwaves. Some, like Tasmania’s giant kelp beds, are already in severe decline. Without action, their capacity to store carbon and support marine biodiversity may be lost.