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ID: 287253 • Letter: W
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Impact of iron fertilization on global climate
An alternative to point source carbon capture and storage is the proposed use of ocean phytoplankton communities as natural carbon capturing and removal machinery. In this model, oceanic carbon uptake is stimulated by enhancing phytoplankton productivity (usually through iron fertilization, see below), drawing CO2 into the oceans and ultimately sequestering it in the deep ocean. Nearly two decades ago, Martin and Fitzwater demonstrated that phytoplankton productivity in high-nitrate low-chlorophyll (HNLC) regions is limited by the availability of iron. Iron is an essential element for photosynthesis and many other biological processes, yet in many open ocean environments, surface iron concentrations are extremely low. Since this discovery, several large-scale iron fertilization experiments have confirmed the original hypothesis. In nearly every experiment, diatoms were among the dominant phytoplankton species to respond to iron enrichment. Recent assessments suggest diatom-mediated export production can influence climate change through uptake and sequestration of atmospheric CO2 . Thus, iron fertilization of the oceans has the potential to draw-down atmospheric CO2 levels by stimulating large blooms of diatoms.
Given the success of Pseudo-nitzschia species in iron enrichment experiments, it is important to investigate further their potential to produce toxin to better evaluate the consequences of proposed large-scale ocean fertilization to mitigate increasing atmospheric carbon
Impact of iron fertilization on local fish population
Iron is the limiting micronutrient in the Southern Ocean and experiments have demonstrated that addition of soluble iron to surface waters results in phytoplankton blooms, particularly by large diatoms. Antarctic krill (Euphausia superba) eat diatoms and recycle iron in surface waters when feeding. Baleen whales eat krill, and, historically, defecation by baleen whales could have been a major mechanism for recycling iron, if whale faeces contain significant quantities of iron. We analysed the iron content in 27 samples of faeces from four species of baleen whale. Faecal iron content (145.9 ± 133.7 mg kg1) is approximately ten million times that of Antarctic seawater, suggesting that it could act as a fertilizer. Furthermore, we analysed the iron content of seven krill species and of muscle tissue of two species of baleen whales; all samples had high iron levels. Using these figures, together with recent estimates of the range and biomass of krill, we calculate that the Antarctic krill population contains 24% of the total iron in the surface waters in its range. Thus, krill can act as a long-term reservoir of iron in Antarctic surface waters, by storing the iron in their body tissue. Pre-exploitation populations of whales and krill must have stored larger quantities of iron and would have also recycled more iron in surface waters, enhancing overall ocean productivity through a positive feedback loop. Thus, allowing the great whales to recover might actually increase Southern Ocean productivity through enhancing iron levels in the surface layer.
Evaluation of Ocean Iron Fertilization using three elements of sustainability
Fertilizing the ocean with iron (or other nutrients) has been proposed as a mechanism for mitigating climate change, by accelerating the uptake of CO2 by the ocean.In order for such strategies to work, three criteria must be met:
(a) ocean fertilization must lead to increased growth of phytoplankton, packaging carbon and nutrients together into organic material;
(b) this organic material must be transferred into the deep ocean so that it does not simply get recycled near the surface releasing its carbon back to the atmosphere; and
(c) this transfer of carbon from the surface ocean to the deep ocean must result in a compensating transfer of carbon from the atmosphere into the surface ocean.
Research performed by NOAA, its academic partners and the wider research community has shown that while the first part of this chain of events is likely to occur (phytoplankton growth increases following ocean fertilization), the inevitability of the second two criteria is far from certain. Reliably quantifying any net uptake of carbon by the ocean following ocean fertilization is not currently possible, especially in the face of negative feedbacks that would tend to release carbon back to the atmosphere.Research using ocean models to simulate the effects of ocean fertilization suggests that the maximum impact of ocean fertilization on ocean carbon uptake is likely to be a small fraction of what is required to stabilize atmospheric CO2 concentrations at twice the preindustrial concentration.
The maximum impact achieved in modeling studies requires continually fertilizing millions of square kilometers of ocean over hundreds of years, a feat that is not technologically feasible.
Ocean fertilization efforts that require manipulating ecosystems at a large scale could also potentially interfere with other legitimate uses of the sea. Potential negative consequences include undesirable changes in the structure and function of marine ecosystems, including ecosystems that support economically important fisheries; reductions in ocean productivity in regions affected by but remote from the fertilization site; increases in mid-ocean and deep ocean oxygen depletion; changes in ocean biogeochemistry that enhance the production of nitrous oxide and methane, greenhouse gases that have a higher heat-trapping potential than CO2 on a molecule per molecule basis; stimulation of Harmful Algal Bloom forming species, and; net increases in ocean acidification
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