Playing Gaia

Phytoplankton doesn't need much to keep it happy. Just some sunlight and a few important macronutrients, such as nitrate, phosphate and silicic acid. However, in some areas of the ocean the abundance of phytoplankton is substantially lower than would be expected from the plentiful supply of nutrients. Such areas are known as "high-nutrient, low-cholorophyll" (HNLC) regions and make up 20% of the ocean surface.

Diatoms make up a large part of the phytoplankton (photo Wikipedia)

A feature often associated with HNLC regions is the low bioavailability of iron. Iron is critical for phytoplankton as it is required to make chlorophyll and important enzymes like nitrate reductase. It has, therefore, been suggested that iron bioavailability is the most important factor limiting phytoplankton population size in HNLC regions. But, zooplankton grazing and deep mixing of surface water that reduces the time that phytoplankton has access to light have also been hypothesised.

Progression of the largest recorded natural phytoplankton bloom in the Southern Ocean, which occurred earlier this year. It was thought to be caused by the addition of iron blown into the sea by strong offshore winds. At its peak, it measured 100 km north-south and 200 km east-west (images Jan Lieser & NASA Terra Modis).

A study, published in Nature, investigated the effect of adding iron to the water on phytoplankton abundance. It was not the first experiment to do so, there have been several. But, unlike the others, this experiment showed that much of the dead phytoplankton was transported into the deep sea. Which is important for our understanding of the role that the ocean plays in regulating the climate.

A map showing the location of iron fertilization experiments. The experiment published in Nature is the EIFEX study, which is the second from the right (image taken from Baar et al. 2004).

During photosynthesis, phytoplankton consumes carbonic acid, which is formed when carbon dioxide dissolves into water. If sufficient quantities of the carbon that the phytoplankton take up can be transported into the deep ocean, away from the atmosphere, it can effectively reduce the amount of carbon dioxide in the atmosphere, cooling the planet. Indeed, phytoplankton blooms may have played a role in triggering past ice ages.

The study recorded phytoplankton abundance and particles falling beneath the bloom at various depths for 37 days, capturing the rise and fall of the bloom. The bloom peaked after 24 days; chlorophyll concentrations was almost four times those at control sites and the bloom covered nearly 800 square kilometers. As the bloom collapsed, at least half of the biomass sank below 1,000 meters, most of which is likely to have reached the seafloor.

Such evidence indicates that at larger scales and over longer periods, algal blooms could transport climatically significant amounts of carbon into the deep sea. In surface waters, the carbon could be returned to the atmosphere in a few months. But, in deep sea sediments the carbon might not reach the atmosphere for centuries. However, more work needs to be done to confirm the effect of phytoplankton blooms on climate.

A surprising finding of the study was that light availability did not seem to effect the plankton bloom as much as might of been expected. Chlorophyll concentrations remained high down to 100 meters where light availability should limit phytoplankton abundance. Indeed, the results showed that chlorophyll concentrations were similar or higher than those recorded at shallower depths in previous iron fertilization experiments.

Reference:

Victor Smetacek, Christine Klaas, Volker H. Strass, Philipp Assmy, Marina Montresor, Boris Cisewski, Nicolas Savoye, Adrian Webb, Francesco d’Ovidio, Jesus M. Arrieta, Ulrich Bathmann, Richard Bellerby, Gry Mine Berg, Peter Croot, Santiago Gonzalez, Joachim Henjes, Gerhard J. Herndl, Linn J.Hoffmann, Harry Leach, Martin Losch, Matthew M. Mills, Craig Neill, Ilka Peeken, Rudiger Rottgers, Oliver Sachs, Eberhard Sauter, Maike M. Schmidt, Jill Schwarz, Anja Terbruggen, & Dieter Wolf-Gladrow (2012). Deep carbon export from a Southern Ocean iron-fertilized diatom bloom Nature, 487 (7407), 313-319 DOI: 10.1038/nature11229