Kelp forests in the Arctic Ocean could play a significantly larger role in the global carbon cycle than previously thought, as a study from the sea off Greenland shows. Accordingly, ocean currents and winter mixing processes help to permanently transport the carbon bound in seaweed into the deep sea. This means that it is removed from the atmospheric carbon cycle in the long term.
Although scientists have long known that seaweed grows very quickly and sequesters large amounts of carbon dioxide, it has been difficult to accurately quantify its contribution to carbon sequestration. Unlike seagrass, mangroves or salt marshes, the carbon in seaweed does not stay directly where it grows – i.e. in the soil or in the plants themselves. Seaweed grows on rocky coasts where organic material cannot be permanently deposited. Instead, broken parts of plants drift through the sea, often traveling long distances before they are eaten, decompose or ultimately end up in the depths of the ocean.
On the trail of the seaweed
An international research team led by Daniel Carlson from the Helmholtz Center Hereon in Geesthacht has now taken on exactly this question. Using a combination of observational data and modeling, the researchers were able to understand for the first time what actually happens to the carbon from the seaweed. To study the seaweed, the research team used southwest Greenland as a test area. A 2024 study estimated that this region exports nearly a million tons of carbon annually in the form of seaweed, ranking it 20th in the world for potential export of macroalgal carbon to the open ocean.
Using 305 surface drivers equipped with GPS trackers, Carlson and his colleagues tested previous assumptions about how long kelp stays in the study area. The result: While it was previously assumed that the seaweed drifts there for several months, the new data shows a significantly different picture: a particularly fast “express” current transports the seaweed into the open sea after just over twelve days on average. This happens at a time when the plants are still largely intact and have neither been eaten nor decomposed.
“Our research can now demonstrate for the first time that Greenland kelp carbon is effectively excluded from recycling through a chain of events,” reports Carlson. These include rapid, current-driven offshore transport, followed by an efficient transition from near-surface drift to sinking.
A natural transport mechanism into the depths
The study’s most exciting discovery is related to processes in the Labrador Sea. Strong winter storms there cause the water to mix significantly. This creates downward currents that act like a kind of natural pump. This is the so-called “alternating turbulence pump effect”. The currents can pull parts of the floating seaweed downwards – at speeds of up to nine meters per minute.
If the seaweed is pushed to depths of around 120 to 130 meters, its gas-filled bubbles lose their buoyancy due to the high water pressure. This effect cannot be reversed: the seaweed continues to sink and eventually reaches the deep sea floor, possibly at a depth of more than 2,000 meters. There the carbon-rich material decomposes very slowly or not at all.
“As climate change leads to the loss of Arctic sea ice, models predict that the distribution of kelp will expand not only in Greenland, but also throughout the Arctic. Our study provides a valuable blueprint for further investigations to accurately quantify the role of kelp in the ocean carbon cycle in other coastal regions,” says Carlson.
Source: Leibniz Institute for Baltic Sea Research Warnemünde; Specialist article: Science of the Total Environment, doi: 10.1016/j.scitotenv.2025.181247