Plankton (from the Greek planktos, or wanderer) comprise a vast assortment of life-forms with limited or no swimming power that largely drift in the ocean. They are divided into the plantlike phytoplankton and the animal-like zooplankton.

Phytoplankton comprise at least four thousand species of plants that, as on land, use sunlight in the process of photosynthesis to generate sugars and other high-energy organic compounds. They live in the ocean's euphotic zone, the sunlit upper layer, which is only tens of meters deep. Phytoplankton range in size from microscopic to a millimeter in length and include single-celled organisms and tiny clumps of algae.

Because the ocean covers 71 percent of the Earth's surface, phytoplankton are a major driver of Earth's carbon cycle. Their photosynthesis extracts carbon dioxide (CO2) from surrounding water and replaces it with oxygen. The resulting changes in oceanic gas levels change atmospheric gas levels as well. Some phytoplankton (such as coccolithophores) accrete calcium carbonate (CaCO3) for shielding. If the carbon contained in their bodies reaches the ocean depths, it may be sequestered there for years, or even for millions of years.

Oceanic nutrient levels are major determinants of phytoplankton productivity. The most fertile areas are river estuaries, shallow waters, and upwelling areas in the deep ocean. Phytoplankton are eaten by zooplankton (Adhiya, 2001). Zooplankton range mostly from microscopic to the size of small snails, although jellyfish, some more than 2 meters in diameter, also swim weakly and can be classed as zooplankton. Zooplankton slow the carbon cycle, because they consume phytoplankton and emit CO2. Phytoplankton "blooms" of increased productivity are quickly followed by surging zooplankton populations.

Conversely, zooplankton contribute to carbon cycling by accreting carbonate shells, dropping fecal pellets, and falling to the sea bottom upon death. Together, phytoplankton and zooplankton directly or indirectly feed all the rest of the animals in the ocean, including human fisheries. Oceanic acidification may affect both planktonic food production and planktonic capture of CO2. Increasing CO2 levels in the atmosphere translate into higher levels in oceanic surface waters and favor increased photosynthesis. However, increased CO2 levels also cause greater ocean acidity. Increased acidity in the waters may strongly hinder carbonate shell-building among plankton and other marine life-forms. If so, it would slow the marine carbon-capture process and contribute to even greater atmospheric CO2 levels.

In 1988, oceanographer John Martin declared, "Give me half a tanker of iron, and I can start a new ice age." Martin was referring to phytoplankton growth in the "bluewater desert" areas of the deep ocean. The creatures' growth rate is often limited by the availability of trace amounts of iron. It has been hypothesized that ice ages may result from large amounts of wind-blown dust enriching the oceans with iron. Such enrichment could have caused phytoplankton blooms that reduced atmospheric CO2 levels, thus reducing the greenhouse effect and cooling the planet (Field, 2004).

Similarly, Martin suggested that artificial iron fertilization in the oceans might reduce global warming. (A major campaign of oceanic fertilization would be a species of geoengineering.) Several limited experiments of a few hundred square kilometers and a few days duration have confirmed major iron fertilization in the Pacific Ocean and in the southern ocean around Antarctica.

Phytoplankton emit the sulfur-bearing gases dimethyl sulfide (DMS, CH3SCH3) and carbonyl sulfide (COS). Their breakdown product, sulfur dioxide (SO2), produces airborne particles (aerosols) that reflect visible light but allow infrared light (heat waves) to pass through, thus causing more cooling. Ocean fertilization could be self-funding, because a part of the increased planktonic production could be harvested via increased fisheries production. The major objections to oceanic fertilization are as follows: First, as noted, zooplankton populations grow to feed on phytoplankton blooms, and they quickly return much CO2 to the atmosphere. Second, even if it is not eaten, much planktonic biomass decays and gives up captured carbon before it can sink to the bottom, so it is not an efficient carbon sink. Third, if fertilization were widely implemented, organic material reaching the deep ocean would vastly increase. In decaying, it could harm marine life by lowering oxygen levels in the deep waters. Fourth, plankton emit some greenhouse gases (GHGs), such as oxides of nitrogen, that might cancel some aerosol cooling. Finally, aerosols released by fertilization in sufficient amounts to cause noticeable cooling would also significantly decrease the sunlight available for photosynthesis. For these reasons, opponents of oceanic fertilization argue that it might result in some mitigation of global warming, but it could never represent a comprehensive solution to the problem.

References

1. Adhiya, Jagat, and Sallie W. Chisholm. "Is Ocean Fertilization a Good Carbon Sequestration Option?" Cambridge, Mass.: MIT Press, 2001.

2. Field, Christopher B., and Michael R. Rapauch, eds. The Global Carbon Cycle: Integrating Humans, Climate, and the Natural World. Washington, D.C.: Island Press, 2004.

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