Climate change is expected to reduce the ability of kelp forests to sequester and store carbon

Kelp forms vast algal forests along temperate coasts, which sequester large amounts of atmospheric carbon. But according to a study by Karen Filbee-Dexter of the University of Western Australia and the Institute of Marine Research and colleagues, published in the open-access journal PLOS Biologywarming oceans could reduce the ability of kelp forests to sequester carbon for long periods in deep ocean reserves, exacerbating the effects of climate change.

Carbon from kelp can feed other organisms as it decays, or it can be trapped in the deep ocean, where it can remain for hundreds or thousands of years. To understand how the ability of kelp forests to build long-term carbon stores will change as the climate warms, researchers studied kelp decomposition rates at 35 locations in the Northern Hemisphere. They collected fragments of fresh kelp and placed them in a mesh bag in a plastic cage – allowing water and microbes to drain but excluding large herbivores – and tethered them to the seabed for 4 at 18 weeks.

They found that sea temperature had a strong influence on the rate of decomposition, with kelp fragments in cooler waters degrading more slowly. Modeling has shown that kelp decomposition is slow enough that a significant proportion can reach deep ocean carbon sinks. But the continued rise in sea temperature due to climate change could accelerate decomposition, reducing the amount of carbon stored in the long term. A projected increase in sea temperature of 0.4 degrees Celsius by 2050 could reduce the carbon sequestration potential of decomposing kelp by 9%.

However, kelp forests are expected to expand into higher latitude waters as the climate warms, and slower decay rates here could make these forests a major contributor to long-term ocean carbon stocks. according to the authors.

Filbee-Dexter says their “experiment measured kelp carbon turnover in twelve regions of the northern hemisphere and found that carbon degradation was strongly related to ocean temperature. This suggests that carbon storage kelp could be reduced as the ocean warms and kelp forests in cold polar environments have the potential to be important blue carbon ecosystems.”