Letter: Caution: the kelping hand may not be that helpful

From Paul Broady, Christchurch, New Zealand

Future Chronicles imagines a time when we use seaweed to remove atmospheric carbon dioxide. But there would be impacts from growing and then sinking many million tonnes of seaweed to the ocean floor (9 November, p 22).

At the surface, the seaweed would take up nutrients otherwise used by phytoplankton. The result would be less CO₂ removed by sinking phytoplankton and a cut in food for fish production.

The seaweed would decompose at depth, causing deoxygenation and releasing CO₂. The latter would acidify the water. The release of nutrients during decomposition would cause eutrophication. These effects would all affect deep-sea ecology.

Maybe the use of seaweed will be part of the solution to climate change. But we should proceed with great caution before applying it on a grand scale.