Models Show Seaweed Cultivation Could Deplete Nutrients but Iron Fertilization Might Still Remove Tens of Billions of Tonnes of CO2

Two modeling studies published this month conclude that widespread seaweed cultivation intended to remove carbon dioxide from the atmosphere would rapidly deplete ocean nutrients and reduce phytoplankton populations. Manon Berger at the University of Bern and colleagues modeled annual production of 20 billion tonnes of seaweed within 200 nautical miles of coastlines.

After 25 years, growth fell 95 percent because of shortages of nitrogen, phosphorus and iron.

The same scenario cut global phytoplankton growth by as much as 8 percent. Only patches off Senegal and southern Australia, covering 0.05 percent of the ocean, allowed seaweed to grow without substantially lowering phytoplankton, the model indicated. Berger said the restricted area means gigatonne-scale removal is not feasible.

Andrew Yool at the UK National Oceanography Centre led a second study that added iron fertilization. That approach removed up to 40 billion tonnes of CO2 each year but halved ocean plankton biomass. The infrastructure would cover 14 percent of the ocean surface, mainly in the Southern Ocean and northern Pacific and Atlantic.

Without iron fertilization, the modeled seaweed operations failed to offset plankton losses and raised atmospheric CO2 by as much as 700 million tonnes per year, according to the study published in Biogeosciences. Earlier commercial efforts have already encountered limits. The U.S.

Start-up Running Tide raised $70 million to sink seaweed on wooden pucks but closed last year after running out of financing. Dutch company Kelp Blue has raised at least $2 million to expand cultivation in Namibia for fertilizer and claims its operations could eventually sequester 500 million tonnes of CO2 annually.