Ocean iron fertilisation, one of the most discussed CDR geoengineering proposals, deliberately tries to stimulate biological activity in the upper ocean. New research shows this in turn affects ecology at the ocean floor too. Let’s just hope sea cucumbers don’t eat all of the carbonate we had intended to store for eternity.

Just two days ago we’ve discussed the importance of Antarctic krill for ocean iron fertilisation, as the crustacean helps recycle iron from the ocean floor to allow phytoplankton growth in the upper layers of the ocean, where photosynthesis allows CO2 absorption.

In the Southern Ocean [and an estimated 40 percent of oceans worldwide] we learned, iron is a growth-limiting nutrient – add iron, and you’ll likely stimulate overall biological activity and biomass production, hopefully – but far from certainly – also increasing carbon sedimentation at the ocean floor, thus removing CO2 and creating a route to actually lowering atmospheric CO2 concentrations.

CDR geoengineering

Deliberate attempts at such are defined as ‘carbon dioxide removal’ geoengineering, or CDR geoengineering, a fundamental difference to [SRM] geoengineering proposals that try to directly influence the energy balance of the planet, but do not try to restore carbon concentrations.

Now a research group with scientists from the UK universities of Liverpool, Southampton and Aberdeen has studied the ecological effects of ocean iron fertilisation around the Crozet Islands, somewhere in between the Indian Ocean and the Southern Ocean – and recently published in PLoS ONE.

Natural iron fertilisation

The researchers played safe. They did not add iron themselves but, just like the krill we reported on earlier, let nature do it herself. The Crozet Islands are volcanic and locally deposit iron-rich sediments in the water. The British compared one ocean site under influence of this natural iron fertilisation, to another one, deprived of extra iron, some 460 kilometres away. On both sites the ocean was around 4200 meters deep. Both sites were climatologically comparable and ecologically connected.

More iron, more biomass

They note several differences: the site with natural iron fertilisation had an algae bloom starting in the southern hemisphere spring. Also the amount of organic matter that sank to the ocean floor was higher in the iron site [this is good news to CDR geoengineers]. Because this organic matter also had a higher nutrient content it stimulated biological activity at the ocean floor too, with a larger abundance of sea cucumbers and brittle stars. The biodiversity implications were however unclear, with some subspecies of both animals preferring either the iron-rich or the iron-deprived site.

Overall living biomass on the ocean floor was larger at the iron fertilisation site.

CO2 perspective

Although living biomass is a carbon reservoir in itself – like forests on land – this is not directly the thing that we should hope for if we want to see lots of CO2 sequestrated. From a geoengineering perspective ecology would be handier if it state to the basic: eat (& absorb CO2), die (& sink to the ocean floor), pile up – and become limestone. Gravediggers are welcome to turn up and recycle some nutrients like the iron, but they’d better not redistribute the calcium carbonates, re-releasing the CO2.

Ecology perspective

The researchers themselves look at their findings not from a climatological, but from an ecological perspective. From this they consider the discovered high biological activity good news – as it shows the increase of dead organic matter does not appear to create anoxia – through rotting processes – at the ocean floor, a feared ‘dead zone’ [although anoxic zones can actually be a refuge for marine life] under iron fertilisation sites.

© Rolf Schuttenhelm | www.bitsofscience.org