Fertilising the ocean with iron to draw down carbon dioxide does far less ecological damage in the Southern Ocean than near the equator. That is the central result of a modelling study published in the journal Nature.
The idea behind ocean iron fertilisation is simple. Across large stretches of open water, phytoplankton have everything they need to grow except iron. Add it, and they bloom and take up carbon dioxide, some of which sinks into the deep sea. What has never been settled is how much carbon stays down there, and what continuous fertilisation does to everything else in the water.
Researchers from the University of California, Irvine, the National Center for Atmospheric Research and the Technical University of Denmark tested this with an ocean biogeochemical model. It links about a dozen plankton types with the nitrogen cycle, the oxygen budget and ocean currents. The team ran ten ocean regions through 60 years of continuous fertilisation, followed by 60 years of recovery.
Where the harm lands
The equatorial Pacific performed well on carbon and badly on everything else. Blooms there sustained themselves by stripping nutrients from surrounding water, an effect the authors call nutrient robbing. Fertilising 0.35 percent of the ocean surface cut populations of large zooplankton by at least a tenth across 14 percent of the global ocean, according to the account in Anthropocene Magazine. Oxygen-starved zones expanded, and plankton communities shifted towards smaller, less nutritious species. Large zooplankton are a main food source for fish, including the tuna stocks the Pacific islands depend on.
The Southern Ocean behaved differently. Fertilising 0.2 percent of the global surface increased the area in which large zooplankton grew by 13 percent, as Earth.com reported. Dead zones barely changed, and the region recovered quickly once the iron stopped.
"The Central Pacific should be avoided for OIF due to the severe ecological risks, and the Southern Ocean may offer the best balance between carbon sequestration and ecological risk over decadal timescales," said Jun Yu of UC Irvine's department of earth system science.
The climate payoff stays modest. Sixty years of fertilisation removed a net 1.1 to 5.3 parts per million of carbon dioxide, and more than half of that leaked back into the atmosphere within decades. What is easy to observe from satellites, the brightness of a bloom, turns out to be a poor guide to how much carbon actually sinks. Without a reliable measure of that, selling the result as carbon credits remains hard to justify.