Phytoplankton: 3 papers and a photo

Oceanic phytoplankton are microscopic plant-like single cell organisms (either bacteria or algae) which photosynthesize and thus form the basis of the marine food chain. (The plankton that are not autotrophic, which instead consume the phytoplankton (i.e., they are heterotrophic) are called zooplankton — single-celled or metazoic animals.) Phytoplankton are responsible for anywhere from 20% to 50% of all photosynthetic activity on earth, and therefore are the major organic factor in the earth’s carbon cycle and the principle source of organically produced oxygen. The viability of these organisms is critically important for animal (including human) life on earth and plays an essential role in the earth’s climate (because they represent a major carbon sink). The life cycle of these organisms are characterized by great pulses. The explosive population growth that makes visible vats of green on the ocean in spring-time are known as blooms. Blooms are reduced by zooplankton and other grazers (including such large animals as the baleen whale). Why these blooms appear and then disappear is the question.

In 1953 Norwegian oceanographer Harald Ulrik Sverdrup published a paper entitled “On Conditions for the Vernal Blooming of Phytoplankton” (subscription required) in ICES Journal of Marine Science, in which he developed the so-called “critical depth hypothesis.” The question addressed was: What initiates an algal bloom? The paper developed a model which explains that blooms begin when the depth of the surface mixed layer becomes sufficiently shallow in spring for phytoplankton to receive adequate light for net positive growth. Ten years of satellite data from the Sea-viewing Wide Field of view Sensor, however, shows that the blooms begin in mid-winter (actually, the fractional net rate of change in phytoplankton biomass becomes positive in winter) rather than the spring, as Sverdrup’s model predicts. In place of Sverdrup’s hypothesis and based on the satellite data, Michael Behrenfeld, professor of botany at Oregon Statue University, proposes what he calls a “Dilution Recoupling Hypothesis.” The study is published as “Abandoning Sverdrup’s Critical Depth Hypothesis on phytoplankton blooms” in 91 Ecology 977-89 (April 2010). The hypothesis “decouples” the rate of growth /destruction in Sverdrup’s hypothesis and posits that: destruction of phytoplankton by grazers is not constant; the ocean is mixed by winter storms bringing nutrients and dormant algae to the surface where population growth is then subjected to maximal conditions; grazing only catches up by the end of the bloom with the delayed population increase of zooplankton.

The question of how mid-oceanic algae are able to survive in areas with little nitrate is addressed in another paper, K.S. Johnson, S.C. Riser, D.M. Karl, “Nitrate supply from deep to near-surface waters of the North Pacific subtropical gyre,” 465 Nature (June 24, 2010). Monterey Bay Aquarium Research Institute (MBARI) chemical oceanographer Ken Johnson and coauthors Stephen Riser of the University of Washington and David Karl of the University of Hawaii show that mid-ocean algae obtain nitrate from deep water, up to 250 meters below the surface. According to a press release by the MBARI:

“Johnson … used a robotic drifter called an Apex float, which automatically moves from the sea surface down to 1,000 meters and then back again, collecting data as it goes. Researchers at the University of Washington outfitted this drifter with an oxygen sensor and a custom version of Johnson’s In Situ Ultraviolet Spectrophotometer, which measures nitrate concentrations in seawater.

“From January through October of each year [2007-09], the instruments on the drifter showed a gradual increase in oxygen concentrations in the upper 100 meters of the ocean. At the same time, the float detected a gradual decrease in concentrations of nitrate in deeper waters, from 100 to 250 meters below the surface.”

The oxygen production in the upper ocean layer correlated with the nitrate depletion in the lower layers (in the proportion expected by photosynthesis). They also found that the amount of algal growth predicted by this amount of photosynthetic activity confirmed by the actual algal growth rates measured during the University of Hawaii’s oceanographic cruises in that part of the Pacific.

Johnson, et al., speculate that ocean eddies (or short-term transport events) are responsible for bringing the nitrate from the depths (at which photosynthesis is impossible) to the upper waters.

Finally, while nitrate may be the limiting macronutrient for algae, in the Southern Ocean iron is the limiting micronutrient. In Stephen Nichol, et al., “Southern Ocean iron fertilization by baleen whales and Antarctic krill,” 11 Fish and Fisheries 203-09 (March 30, 2010), Stephen Nicol of the Australian Antarctic Division, based in Kingston, Tasmania has found large amounts of iron in the feces of baleen whale.  He posits that the iron is recycled from phytoplankton eaten by zooplankton which are in turn eaten by the whales. He believes that before commercial whaling, baleen whale faeces may have accounted for some 12% of the iron in the upper layers of the Southern Ocean. Compare also the paper entitled, Trish J. Lavery, et al., “Iron defecation by sperm whales stimulates carbon export in the Southern Ocean,” published on-line in the Proceedings of the Royal Society B (June 16, 2010), which shows that Southern sperm whale are responsible for fertilizing the ocean with 50 tons of iron. That results (by its contribution to phytoplankton growth) in the net removal of 200,000 tons of carbon from the atmosphere.

All of this discussion about phytoplankton, however, was simply an introduction to the beautiful image of a phytoplankton bloom off the coast of Iceland, taken by the Moderate Resolution Imaging Spectroradiometer on NASA’s Aqua satellite on June 24, 2010. A collection of other NASA images of phytoplankton bloom (with commentary by NASA) is hosted by Geology.com.

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