It is another nail in the coffin of using ocean fertilisation to cool the planet. Early results from the latest field experiment suggest the technique will fail.
"I think we are seeing the last gasps of ocean iron fertilisation as a carbon storage strategy," says Ken Caldeira of the Carnegie Institution at Stanford University.
Earlier this month, the controversial Indian-German Lohafex expedition fertilised 300 square kilometres of the Southern Atlantic with six tonnes of dissolved iron. The iron triggered a bloom of phytoplankton, which doubled their biomass within two weeks by taking in carbon dioxide from the seawater. Dead bloom particles were then expected to sink to the ocean bed, dragging carbon along with them.
Instead, the bloom attracted a swarm of hungry copepods. The tiny crustaceans graze on phytoplankton, which keeps the carbon in the food chain and prevents it from being stored in the ocean sink. Researchers from the Alfred Wegener Institute for Polar and Marine Research reported that the copepods were in turn eaten by larger crustaceans called amphipods, which serve as food for squid and fin whales.
The grazing effect had not been seen in previous fertilisation experiments. These had caused blooms of diatoms, a type of phytoplankton that is protected against grazers by a hard shell of silica. But the Lohafex experiment did not trigger a diatom bloom because there was little silicic acid available in the water for diatoms to build their shells from.
Lohafex researchers say the results suggest that using iron fertilisation to increase the ocean carbon sink would rely on a complex chain of events, making it difficult to control. The Southern Ocean is thought to be the planet's largest ocean carbon sink. But most of the northern half of the region is low on silicic acid, ruling it out as an option for carbon fertilisation.
The researchers tried to provoke a second bloom by fertilising the same patch of ocean three weeks later, with no success â€“ most probably because the water was already saturated in iron.
"It seems that if it is possible to fertilise enough ocean to make a difference to climate, we would need to turn vast ocean ecosystems into giant plankton farms," says Caldeira.