Phytoplankton, microscopic plant-like organisms that drift through the ocean, play a central role in the ocean's uptake of CO2. Through photosynthesis, plankton absorb carbon dioxide from the atmosphere, and some of this carbon dioxide is transported to the deep sea when the organisms die or are eaten.
In large parts of the world's oceans, however, growth is limited by a lack of iron, which is an essential nutrient. For this reason, researchers have for decades been investigating whether the ocean's CO2 uptake can be increased by adding dissolved iron to iron-deficient areas.
But how effective is this method, and what are its consequences for marine ecosystems? Professor Adam Martiny, DTU Aqua, has investigated this in a new study, which has been published in the prestigious scientific journal Nature.
Using an advanced ocean model, the researchers have simulated 60 years of iron addition in ten different ocean regions to investigate both the climate impact and the effects on marine ecosystems.
"There is a big difference depending on where you add iron to the sea. In some places, the iron has very severe consequences for the ecosystems, whilst in others the impacts are relatively limited. What is new about our study is that, for the first time, we can demonstrate the trade-off between CO2 uptake and ecological consequences across different ocean regions," says Adam Martiny.
He adds that once iron has been added to the ocean, it does not always remain in a single area but can move with the ocean currents, thereby affecting the chemistry and biology elsewhere in the world's oceans.
The Southern Ocean offers the best balance
The researchers find that the Southern Ocean around Antarctica offers the most favourable combination of CO2 removal and limited ecological consequences. Here, ocean currents transport both iron and nutrients to other areas, where they can continue to support biological production and carbon storage.
At the same time, the simulations show that the Southern Ocean is more resilient than the other areas studied. When the iron supply stops, the ecosystem returns relatively quickly to its original state, and the ecological impacts are less long-lasting than in, for example, the equatorial Pacific.
The equatorial Pacific Ocean also removes large quantities of CO2, but here the consequences are significantly greater. When plankton blooms following iron addition, it consumes large quantities of other nutrients. Ocean currents then transport this nutrient-poor water to other areas, which can reduce plankton production and affect food chains far away from where the iron was added.
