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Another challenge to confront with geoengineering: Ocean plankton toxins

Wednesday, November 10th, 2010

There have been several critiques of geoengineering as a climate mitigation tool.  Two of the most incisive, in my opinion, come from science and ethics.

The first is a 2007 paper in PNAS by Matthews and Caldeira showing that if we establish aerosol clouds or space reflectors while doing nothing to reduce carbon emissions, we run the risk of catastrophic rates of warming (2-4 degrees C per decade) if these systems were to fail.

The second is a recent piece in Slate by my colleague, Dale Jamieson, who argued that there is no moral and legal authority to know how and when to deploy geoengineering or by how much.

One proposed geoengineering tool is fertilizing the world’s oceans with iron.  The premise behind this idea was developed by John Martin in 1990, who is often quoted as saying something like, “Give me a tanker of iron, and I’ll give you an ice age.” Micronutrients like iron and zinc are extremely limiting to phytoplankton growth in the open ocean—orders of magnitude moreso than nutrients we typically think of in common fertilizers, like nitrogen and phosphorus.  Dumping iron into the oceans has been shown to stimulate algal blooms, and the creation of this biomass consumes CO2 from the surface waters and atmosphere, thereby helping to mitigate rising CO2 from fossil fuels.  In theory, some of this biomass should sink to the deep ocean where it is sequestered for centuries, but this has yet to be shown definitively on a wide scale.

In a forthcoming paper in the Proceedings of the National Academy of Sciences, Mary Silver and colleagues show that there is another potential risk of geoengineering resulting from ocean iron fertilization…

It turns out that some of the phytoplankton stimulated by these iron additions secrete toxins in the water at concentrations that are potentially harmful to marine life:

Near-surface waters ranging from the Pacific subarctic (58°N) to the Southern Ocean (66°S) contain the neurotoxin domoic acid (DA), associated with the diatom Pseudo-nitzschia. Of the 35 stations sampled, including ones from historic iron fertilization experiments (SOFeX, IronEx II), we found Pseudo-nitzschia at 34 stations and DA measurable at 14 of the 26 stations analyzed for DA. Toxin ranged from 0.3 fg·cell−1 to 2 pg·cell−1, comparable with levels found in similar-sized cells from coastal waters. In the western subarctic, descent of intact Pseudo-nitzschia likely delivered significant amounts of toxin (up to 4 μg of DA·m−2·d−1) to underlying mesopelagic waters (150–500 m). By reexamining phytoplankton samples from SOFeX and IronEx II, we found substantial amounts of DA associated with Pseudo-nitzschia. Indeed, at SOFeX in the Antarctic Pacific, DA reached 220 ng·L−1, levels at which animal mortalities
have occurred on continental shelves. Iron ocean fertilization also occurs naturally and may have promoted blooms of these ubiquitous algae over previous glacial cycles during deposition of iron-rich aerosols. Thus, the neurotoxin DA occurs both in coastal and oceanic waters, and its concentration, associated with changes in Pseudo-nitzschia abundance, likely varies naturally with climate cycles, as well as with artificial iron fertilization. Given that iron fertilization in iron-depleted regions of the sea has been proposed to enhance phytoplankton growth and, thereby, both reduce atmospheric CO2 and moderate ocean acidification in surface waters, consideration of the potentially serious ecosystem impacts associated with DA is prudent.

Mary W. Silvera, Sibel Bargu, Susan L. Coale, Claudia R. Benitez-Nelson, Ana C. Garcia, Kathryn J. Roberts, Emily Sekula-Wood, Kenneth W. Bruland, and Kenneth H. Coale (2010). Toxic diatoms and domoic acid in natural and iron enriched waters of the oceanic Pacific Proceedings of the National Academy of Sciences : 10.1073/pnas.1006968107

Photo credit: emdot

One Response to “Another challenge to confront with geoengineering: Ocean plankton toxins”

  1. charlie says:

    nice catch! There is another PNAS paper that came out this year documenting the same concern. (“Iron enrichment stimulates toxic diatom production in high-nitrate, low-chlorophyll areas”, Trick et al 2010)

    geoengineering… well, we’re ALREADY geoengineers; we started on that course when we started digging up the carboniferous and dumping it into the atmosphere, if not earlier. Thusfar, humans have been pretty terrible geoengineers. But the proposed schemes, like ocean fertilization and aerosols, all have this unsubtle character to them that makes them susceptible to the complexity of real-world systems. Someday we may know enough to come up with a successful geoengineering scheme, but right now the best solution seems to me to be the obvious one: Cut carbon emissions.


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