Marine phytoplankton play a very large role in the global carbon cycle, accounting for about 50% of the global biological (photosynthetic) uptake of CO2 . However, the efficiency of this process is sensitive to the availability of several essential macronutrients required by the various species comprising phytoplankton. Phosphorus, nitrogen, silicon, iron, and zinc are critical for the growth and reproduction of these organisms. In areas limited in one or more of these nutrients, fertilization of the sunlit euphotic zone with the limiting element can stimulate transient phytoplankton blooms to enhance CO2 uptake.
Iron (Fe) is the most practical of these nutrients, from a geoengineering perspective, for stimulating phytoplankton growth and photosynthesis. Three regions of the world ocean - the eastern equatorial Pacific, the subarctic Pacific, and the Southern (Antarctic) Oceans - are extremely limited in nutrient Fe, an element critical for phytoplankton photosynthesis (  and references therein). Areas suitable for fertilization have very specific characteristics: they are up-welling zones that are limited in an easily obtained and dispersed macronutrient.
Mitigation Option: Continuous broadcast spreading of phytoplankton macronutri-ents in nutrient-limited regions of the world's oceans, stimulating the biological carbon pump.
Feasibility: With careful screening of Candidate Ocean regions for their suitability vis-à-vis local ocean currents, biological composition and prevailing meteorology, fertilization can result in sequestration of atmospheric CO2.
The fraction of carbon bound by biological processes is dependent upon an array of factors. Phytoplankton are a heterogeneous lot, comprising diatoms, cyanobac-teria, dinoflagellates and other types of algae - all photoautotrophic organisms at the base of the ocean food web. Each species has preferred living conditions, responds differently to fertilization, and utilizes CO2 to differing degrees. The presence of predators (zooplankton) which can eat the phytoplankton increases the possibility that the carbon will be quickly recycled back into the atmosphere. Vertical advection of phytoplankton detritus is quite variable.
Calcification, the formation of insoluble forms of carbonate, is a necessary process for reproduction by coralline algae, an important species of phytoplankton present in coastal waters and the open ocean - and is highly pH dependent . Acidity drives the chemical equilibrium away from carbonate to favor the formation of bicarbonate (see Eq. 9.2). The current excess of atmospheric CO2 is driving the acidification of ocean surface layers. This suggests that the co-implementation of pH adjustment with Fe fertilization might be necessary for optimal CO2 drawdown.
Assuming the infrastructure existed to permit ready dispersion of the appropriate nutrient, identifying the best candidate areas appears to require a great deal of exploration and in situ experimentation.
Co-benefits and undesirable consequences: Effects on the ocean food web can be expected. Phytoplankton feed zooplankton (krill) which, in turn, feed a number of larger ocean species. The effects could favor the growth of currently depleted fish populations, or could alter the food web to favor other, less desirable species . Stimulating phytoplankton growth, sufficient to draw down a meaningful quantity of CO2, increases the risk of overgrowth of harmful algae and the creation of dead zones that could deplete fish stocks. Another unintended side effect could be the production of N2O - a gas with a high global warming potential - in the O2-depleted depths below the fertilized euphotic zone -.
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