LONDON – Plankton, tiny marine organisms, are a good way of cleansing the atmosphere of one of the main greenhouse gases, carbon dioxide. To do this they need dissolved iron to help them to grow, and if they lack iron then they cannot do much to reduce CO2 levels.

So the eruption in 2010 of an Icelandic volcano gave scientists a perfect opportunity to see how much the cataclysm helped the plankton by showering them with unexpected clouds of iron.

Their verdict, published in the journal Geophysical Research Letters – the volcano certainly helped, but not for long enough to make much difference.

This is a blow to some supporters of geo-engineering, who have suggested that one way to tackle climate change is large-scale seeding of the oceans with iron to stimulate plankton to absorb more carbon dioxide.

The volcano's impact was assessed by a team led by scientists from the UK's National Oceanography Centre, Southampton, who were on a shipboard research expedition in the area at the time.

Air travel to a standstill

The April 2010 eruption of the volcano Eyjafjallajökull hurled clouds of ash several kilometers into the atmosphere, bringing air travel to a standstill across Europe and, in a less noticeable effect, seeding the seas south of Iceland with ash.

In many parts of the ocean the productivity of phytoplankton – microscopic plants at the base of the marine food chain – is limited by the availability of dissolved iron.

In 2007 the team had shown that, after a large spring bloom, phytoplankton in the Iceland Basin failed to grow much because it lacked iron. The scientists wanted to see whether the ash from Eyjafjallajökull supplied enough iron to sustain the spring blooms for longer than usual.

The team found that the five-week eruption supplied enough dissolved iron to increase the number of phytoplankton cells within a region of the North Atlantic stretching across 570,000 square kilometers – or 220,000 square miles, about the size of France.

Short-lived effect

But the effect was short-lived as the extra iron resulted in the rapid removal of biological nitrate, depriving the phytoplankton of nitrogen they also needed.

The team – from Southampton, the University of Cape Town and the Norwegian Institute for Air Research – conducted three research voyages in 2010 investigating ocean productivity in the area affected by ash from Eyjafjallajökull.

They took samples of ash and dust in the atmosphere, and of nutrients in the ocean, and also measured the activity of the phytoplankton.

The north Atlantic Ocean is globally important, as it is a sink for atmospheric carbon dioxide, said Eric Achterberg, chief scientist for the research cruise and lead author of the study.

"A limit to the availability of iron in this region means that the ocean is less efficient in its uptake of atmospheric carbon dioxide," he said.

"The additional removal of carbon by the ash-stimulated phytoplankton was therefore only 15 percent to 20 percent higher than in other years, making for a significant but short-lived change to the biogeochemistry of the Iceland Basin."

The National Oceanography Centre develops technology for coastal and deep ocean research. It is based in Southampton and Liverpool.

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.