Ocean carbon capture project gets that sinking feeling

A geo-engineering project to soak up carbon dioxide in the Southern Ocean has reported disappointing results.

Scientists from the Lohafex project stimulated phytoplankton growth by fertilizing a 300sq km patch of ocean with six tonnes of dissolved iron.

Similar ocean fertilization experiments have shown that the resulting layer of algae absorbs CO2 before slowly sinking to the ocean floor, effectively removing the CO2 from the atmosphere.

As expected, the iron particles released in the Lohafex experiment stimulated the growth of phytoplankton. But according to the research team, the resultant algal bloom was different to that generated by other experiments and was mainly eaten by small crustacean zooplankton before it could sink.

"To our surprise, the iron-fertilized patch attracted large numbers of zooplankton predators belonging to the crustacean group known as amphipods," explained Victor Smetacek, co-chief scientist from the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association, who worked on the project.

Previous experiments in other parts of the ocean had stimulated the growth of diatoms, a type of algae that can not be consumed by crustacean zooplankton due to its hard silica shell.

Diatoms did not grow in the Lohafex experiment because natural blooms had previously been stimulated by iron from melting icebergs, streams or dust blown from Patagonia – meaning that much of the silicic acid in the area had already been extracted.

"A major finding was that other algal groups, although stimulated by iron fertilization, are unable to make blooms equivalent to those of diatoms," the scientists' report found.

They therefore concluded that due to low silicic acid content in the northern half of the Southern Ocean, iron fertilization would not lead to significant amounts of CO2 being removed from the atmosphere.

However, researchers remain confident that similar projects could still work in alternative ocean zones.

US geo-engineering firm Climos is planning a much larger experiment that could cover up to 40,000sq km of ocean, and hopes to fund future ocean fertilization projects by issuing carbon credits against the captured carbon emissions.