In early March, the Indian-German Lohafex expedition dropped six tonnes of dissolved iron across 300 square kilometres of sea. The triggered phytoplankton bloom doubled in biomass within two weeks by absorbing carbon dioxide from the water. The researchers expected that dead bloom particles would sink to the ocean floor, taking carbon with them.
Instead, the bloom attracted a swarm of copepods, tiny animals that graze on phytoplankton. In doing so, they keep carbon in the food chain. The copepods themselves were eaten by larger crustaceans called amphipods, which are eaten by squid and fin whales.
The copepod grazing effect had not been seen in earlier iron fertilisation tests. Those experiments had caused blooms of diatoms, a type of phytoplankton that is protected against grazers by a hard shell of silica. With little silicic acid in the water for diatoms to build their shells from, the Lohafex experiment did not trigger a diatom bloom.
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. “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,” said Caldeira.
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