Ocean fertilisation

After exploring terrestrial geoengineering techniques, I thought it would be interesting to gain a marine perspective.

Ocean fertilisation is a CDR technique that causes shifts in marine biological processes; termed the “biological carbon pump” (Figure 1). This is done by ‘fertilising’ the upper-oceans with limiting nutrients, such as iron (Fe), nitrates (N) and phosphorus (P). The intention is to cause a phytoplankton bloom large enough to sequester substantial atmospheric CO₂ (IPCC, 2011).

Figure 1: 'Biological pump' - Marine carbon cycle processes in upper layer and downward flux (Source: NASA Earth Observatory)

Nutrient limitations

Marine phytoplankton require light, CO₂ and nutrients such as N, P and Fe to photosynthesise. Whilst such nutrients are readily available, the slow-recycling of ¼ of total nutrients means <1% reaches the deep benthic (bottom-dwelling) zone (Williamson et al, 2012) (Figure 1). Shortages of Fe, in particular, have been found to limit biological production of phytoplankton in many oceans (Moore et al, 2013).

Fertilisation process

• Fe is added into the upper ocean
• An increase in photosynthetic activity enhances phytoplankton biomass
• Carbon is sequestered via gas exchange through the air-sea interface
• Carbon moves further down into the ocean column via processes such as decomposition, excretion and water mixing.

Carbon sequestration potential

Below is a map of chlorophyll concentration in the world’s oceans; an indicator of phytoplankton biomass via photosynthetic rates and thus carbon intake. Although this shows the large extent of carbon sequestration potential, it is not spatially uniform. Whilst blooms are evident in the Northern Hemisphere Oceans (Arctic, North Atlantic and parts of the North Pacific Oceans), it is not so in tropical and mid-latitudes.

Figure 2: Chlorophyll concentration in March-June 2006, using NASA's Aqua Satellite (Source: NASA)

It would seem that equatorial and mid-latitude regions are severely nutrient-limited due to low productivity, and therefore would benefit from ocean fertilisation. In theory, this could cause a phytoplankton bloom: however it is not so straightforward. A palaeoclimatological study suggests that although iron dust-fertilisation was 2-3 times greater in the Last Glacial Maximum than in the Holocene, phytoplankton productivity in the equatorial Pacific Ocean remained the same (Costa et al, 2016).

There is considerable scientific interest in fertilising the Southern Ocean. Modelling suggests it has the greatest carbon sequestration potential. In the Late Pleistocene, periods of high-dust influx in the Southern Ocean have been associated with the onset of cold glacial periods, with a possible reduction of 30 ppmv of CO₂. This has also been found to correlate with extensive diatom mats: indicating intensive phytoplankton productivity (Martinez-Garcia,2011).

Many experiments suggest iron-fertilisation in the Fe-limited Southern Ocean could be a potential geoengineering strategy (Salter, 2015), however some regions associated with the South Georgia Islands are naturally Fe-rich (Robinson et al, 2016). It is evident then that some parts of ocean systems may not benefit from iron-fertilisation, as was once suggested.

Unintended consequences

Ocean acidification: It has been suggested that Fe-fertilisation can enhance deep-sea acidification (Cao and Caldeira, 2010). This is due to the downward flux of carbon in oceans (Figure 1), such as decomposing phytoplankton sinking to the ocean floor and releasing carbon (Miquel et al, 2015). Benthic species which have calcium carbonate shells, such as molluscs, can be subject to dissolution. In turn, this can disrupt the functioning of the marine food web (Gazeau et al, 2013). 

Toxic phytoplankton blooms: There is a potential for toxic phytoplankton to capitalise on nutrients and cause HAB's (harmful algal blooms) (Trick et al, 2010), which could alter phytoplankton community composition by favouring larger sized phytoplankton (Subramaniam et al, 2016). HAB's can kill fish, birds and mammals and cause anoxic conditions by depleting oxygen.