Open ocean iron fertilization is a process by which phytoplankton primary productivity is increased in iron-deficient oceanic regions, potentially enhancing the carbon flux to the deep sea and drawing excess carbon dioxide (C02) from the atmosphere. This method has been proposed as a way to sequester carbon dioxide from the atmosphere and mitigate the effects of climate change. Six open ocean regions have been identified, where high dissolved concentrations of most nutrients occur year-round, and photosynthetic biomass is quite low. These “high-nutrient,low-chlorophyll a” (HNLC) zones are found in the Eastern Equatorial Pacific, the Northwest Pacific, the Northeast Pacific, the Northeastern Subarctic Pacific, the Western Subarctic Pacific and the Southern Ocean.
Several iron fertilization projects have been conducted so far that were designed to test the “iron hypothesis” that primary production in such HNLC regions is limited by low levels of the micronutrient iron, rather than by macronutrients such as nitrogen, which more typically limit primary production elsewhere. IRONEX I (in 1993) and IRONEX II (in 1995) were conducted in the Equatorial Pacific Ocean, SOIREE (in 1999) and EISENEX (in 2000) in the Southern Ocean, SEEDS I (in 2001) in the Northwest Pacific and SEEDS II (in 2004) in the Western Subarctic Pacific, SERIES (in 2002) in the Northeast Pacific, SoFEX (in 2002) in the Southern Ocean, EIFEX (in 2004) in the Southern Ocean, and CROZEX (in 2005) around the Crozet Islands south of South Africa. The key conclusions from the earlier experiments were that: (i) iron limits primary production in these regions; (ii) phytoplankton biomass can be increased over the short term (weeks) by the addition of iron; (iii) there is no evidence of increased carbon “export” to the dep sea within the time frame of the projects; and (iv) the composition of the phytoplankton community changes substantially upon the addition of iron.
In the context of efforts to curb greenhouse gases, several profit-driven entities have expressed interest in the process of ocean iron fertilization as a means of removing carbon from the atmopshere: some of the them are busy soliciting investors and interest in large-scale ocean fertilization projects. Michael Markels, founder of a company called Versar, for example, took out seven patents on iron fertilization strategies and set up a company called GreenSea Ventures. Coastal states as well have been identified as possible “hosts” for carbon credits by ocean fertilization projects carried out in waters under their jurisdiction.
Uncertainties linked to iron fertilization
Scientists are currently studying the many uncertainties associated with nutrient manipulation in the marine environment. Below is an overview of some the concerns and uncertainties associated with this method:
- Phytoplankton produce dimethylsulfide (CH3SCH3 or DMS), an important precursor for maritime sulfate aerosols and cloud condensation nuclei (CCN), which influence cloud properties and climate. During the SERIES project, in contrast with previous experiments, the iron fertilization resulted in a significant decrease in DMS concentrations. The decrease in DMS levels coincided with an increase in bacterial production and sulfur demand and a marked shift in the bacterial metabolism of dimethylsulfoniopropionate (DMSP), the algal precursor of DMS. Iron-enrichment resulted thus in both a low efficiency of carbon export to deeper waters and a reduction in DMS concentrations, leading to conditions that would not mitigate greenhouse warming;
- The demonstration that iron limits phytoplankton production does not preclude simultaneous limitation by other factors. Other possible limiting/co-limiting factors for phytoplankton production and growth may be low light conditions, vertical mixing, low temperature, low silicate concentrations and zooplankton grazing;
- It is not clear how efficient iron fertilization is as a means of removing carbon from the atmosphere: some scientists believe that it may be capabale of only an eight part per million (volume) drawdown in atmospheric pCO2 after 50 years;
- Iron fertilization may have to be done continuously to keep the additionally stored CO2 within the ocean;
- Increased productivity of diatoms (the most common type of phytoplankton) may boost nitrous oxide (N2O) and methane (CH4) production, which are both greenhouse gases, the first one responsible for ozone depletion; sinking of large phytoplankton blooms into the deep ocean may also reduce oxygen levels, potentially mobilizing existing iron (Fe) pools and triggering uncontrolled self-fertilization processes, thus extending anoxic regions with consequences for fisheries;
- According to Gervais et al the response in phytoplankton structure and productivity observed during an iron-induced algae bloom might also only be transient, and thus it is not clear whether results observed in these experiments would be sustained during a prolonged iron fertilization;
- Effects on animals and other macroscopic components of the food web are poorly known and extrapolation to larger, more relevant temporal and spatial scales is difficult. In fact, the proliferation of diatoms may inhibit zooplankton growth; and some pennate diatom strains (e.g., species of Pseudo-nitzschia) stimulate the production of a powerful biotoxin called domoic acid. The production of domoic acid seems to be related to availability of iron (among other nutrients such as nitrogen, phosphorus, and silicate), perhaps as a metal-binding ligand. Domoic acid can travel up the food chain, causing mortality in marine mammals and possibly even humans;
- Iron-fertilization in HNLC ocean regions would not consistently “zero out” global CO2 output under any realistic global CO2 emissions scenario. Iron fertilization in the Equatorial Pacific Ocean, for example, would hardly change the C02 levels in the atmosphere.
There is a legitimate concern that without an international regulatory regime in place setting some strict guidelines on iron fertilization projects (especially for climate change mitigation purposes), there could be a “carbon rush” with significant consequences to marine ecosystems.
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