Michael Ellwood

Geo-engineering of the ocean could have big side-effects, scientist warns

Fertilisation of the ocean with iron, proposed as a "geo-engineering" technique to boost the amount of carbon dioxide locked up by marine organisms, would be "fraught with difficulties", according to an Australian National University scientist.

Michael Ellwood, a geochemist at the Research School of Earth Sciences, says methods proposed to use iron to stimulate the growth of phytoplankton, the ocean’s primary producers, might not work. And they could have drastic consequences.

The ocean is the biggest carbon dioxide sink, and photosynthesis by phytoplankton is the first stage of the “biological pump”, central to the oceanic component of the carbon cycle.

The tiny organisms absorb carbon dioxide dissolved in seawater, sequestering it when they, and the organisms that feed off them die and sink to the ocean floor.

Ellwood and colleagues have been studying the effect of essential trace elements on phytoplankton. They are filling big gaps in the knowledge of the ocean chemistry of the elements, which will be critical to the response of marine organisms to global warming.

In laboratory experiments, the scientists fertilised water samples from the Southern Ocean and Tasman Sea with various concentrations of iron and salts of nitrogen and phosphorus to see how the plankton responded.

The nutrients promoted growth of the organisms. However, addition of iron, which is limited in the Southern Ocean, would boost marine productivity and carbon sequestration only if the plankton had enough of the other elements.

The natural source of iron is dust blown off the land. But the source of nitrogen and phosphorus is deep ocean waters. The elements are delivered to the surface-hugging phytoplankton through the upwelling of colder water from deeper layers.

The warming of the oceans due to the greenhouse effect will strengthen the stratification of the oceans, reducing the phytoplankton’s supply of nitrogen and phosphorus, Ellwood says.

"There will be less communication between the deep water and surface waters" he says.

"It would be more difficult to fertilise the oceans with nitrogen and phosphorus than with iron because the concentrations required are three orders of magnitude higher."

And even if marine productivity were boosted, the increase in decaying organic matter on the ocean floor could deplete the water of oxygen, threatening the survival of organisms such as sponges that inhabit the deep.