Finding a Cure Despite Uncertainties

Fishermen on Goree Island, Senegal- credit: Laurene Mainguy

By Laurene Mainguy, May 2009

Coral degradation would be dramatic for fish and invertebrates that find shelter and reproduce in coral reefs. It would also be a great threat to people who depend on tourism and coral trade to make a living.

Ocean acidification could have catastrophic consequences for those communities. In a recent study, a group of economists tried to evaluate the impact of ocean acidification on coral reefs in four climate scenarios, defined by the International Panel on Climate Change (IPCC). They found that 16% to 27% of corals will disappear by 2100, a loss that could be worth $870 billion.

The rest of the world population would be affected too. The ocean provides food, jobs and incomes to millions of people worldwide. In 2006, the total production of fish and mollusks was worth 170 billion dollars, according to the Food and Agriculture Organization.

Small changes are already happening. The lives of microorganims and pteropods already show signs of disruption. Those issues are likely to become more problematic affect more species through food web changes, although no one knows exactly how. �At this moment, it is very difficult, probably impossible to answer this question,�� said Atsushi Ishimatsu, a fish scientist at Nagasaki University.

Ecosystems with a great biodiversity might be better off than others, said Stephen Widdicombe, a benthic ecologist at the Plymouth Marine Laboratory. â€�When you have lots more species than "jobs" [breaking down plant material, eating other species, altering the environment in some ways] you will have each job being done by several species. Therefore, if you lose a few species, the jobs still get done and ecosystem function is not affected,â€� he said in an email. But the relationships between biodiversity and ecosystems are not yet clearly understood.

The scientific community has tried to come up with solutions to mitigate the effects of ocean acidification. One of them is iron fertilization. The idea is to spread iron dust into the sea to stimulate plankton growth, which in turn eat up carbon dioxide.

John Raven, a plankton biologist and former chair of the Royal Society working panel on ocean acidification, said iron fertilization could only be a minor contributor to atmospheric CO₂ mitigation.

''In terms of keeping carbon dioxide in the atmosphere below whatever threshold- we were talking about450 ppm- it migh have an effect. Maybe 20 ppm CO₂, that sort of range,'' he said.

But we don't know if iron fertilization would work on a large scale because only small-scale experiments have been done so far, Raven said. It can also be hard to track the effects of fertilization because the iron sometimes just disappear. ''It gets mixed and widely diluted. The iron gets stuck on mineral particles and precipitates, onto organsisms, so it gets lost in one way or another,'' Raven said.

Philip Boyd, a researcher at the New Zealand National Institute for Water and Atmospheric Research, agrees. He said iron fertilization expeditions can be unpredictable. ''The first experiment shut down after only five days when a mass of less dense water moved over the iron-fertilized water, pushing it far below the sunlit surface and ending the iron-induced bloom prematurely,'' he told the Woods Hole Oceanographic Institution.

Lohafex, the latest iron fertilization expedition, was carried out in Antarctica in February. The crew fertilized 186 miles square of the southern ocean with 4 tonnes of dissolved iron for 39 days. They found that phytoplankton doubled their biomass and ate up more CO₂ during the first two weeks, after which they did not grow any further and were grazed by more predators than usual.Contrary to previous expeditions, the CO₂ impact of Lohafex was quite small and scientists are still trying to understand why. ''The mechanisms await further analysis,'' said Uli Bathmann in an email.