Water blooms sequester a large amount of CO2 from the surface layers
Palma. 19th July 2012. An international study, with participation of the Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB) researcher Jesús M. Arrieta, shows that water blooms –massive accumulations of plankton–, can store carbon in the ocean floor and sequester a large amount of carbon dioxide (CO2) from the surface layers. This is one of the results from research conducted in 2004 in the Southern Ocean as part of the European Iron Fertilization Experiment (EIFEX), now published by the journal Nature in its latest issue.
«Controlled fertilization experiments allow us to test hypotheses and quantify processes that cannot be studied in the laboratory. The general objective of the study is to expand knowledge and better understand natural ocean processes that mitigate climate change», explains the researcher Jesús M. Arrieta.
Diatoms that sequester CO2
During the development of EIFEX, researchers onboard the German icebreaker Polarstern fertilized a circular area of approximately 14 km in diameter within a swirl. They used 7 tons of iron sulphate, equivalent to one hundredth of a gram per square meter. The addition of iron resulted in a massive growth of diatoms (a type of seaweed with a siliceous shell) in a layer of about 100 meters deep. At the end of the experiment, more than the 50% of these seaweeds sank quickly reaching depths of over 1,000 meters deep, carrying some of the carbon fixed by photosynthesis in the surface layers.
«The results of this study contrast with the 12 previous experiments of iron fertilization carried out by other projects, because the biomass generated by fertilization during EIFEX was higher. In addition, the bloom developed in an oceanic layer of about 100 meters thick, much deeper than previously we thought that was the lower limit for the formation of these massive accumulations of plankton», added Arrieta.
Iron as a nutrient
Iron plays an important role in many biochemical processes such as photosynthesis and is essential for ocean life and absorption of CO2 from the atmosphere. In the case of phytoplankton, it increases the efficiency of the photosynthetic machinery because enzymes that bear iron participate actively in the transfer of energy in the photosynthetic apparatus. In some ocean areas the nutrient concentrations and the conditions for the growth of phytoplankton are optimal but there is an iron deficiency that limits photosynthesis and biological productivity in general.
«All previous experiments have shown that iron fertilization produces a stimulation of photosynthesis in these ocean regions, stimulating absorption of atmospheric CO2. However, these experiments failed to elucidate which was the final destination of the carbon fixed by phytoplankton. The duration of the EIFEX campaign was longer than the previous ones and, in addition, the intensity of the bloom added to the great depth at which growth was detected resulted in a large quantity of biomass generated. The union of all these factors allowed us to detect the collapse of the bloom and its fast sink to a great depth», says the researcher of the IMEDEA.
Iron fertilization has been proposed as a possible strategy to reduce CO2 atmospheric concentrations: encouraging the uptake of CO2 on the surface so that when the bloom collapses, it sinks to great depths. «Although our results show that the general principle of this idea works, to extend the experiment to the scale that would be required to produce a significant decrease in the concentration of CO2 in the atmosphere would have unpredictable effects on the ecosystem. It is also highly likely to cause damage much more important than the problem to be solved», added Arrieta.
Citation: Victor Smetacek, Christine Klaas et al. (2012). Deep carbon export from a Southern Ocean iron fertilized diatom bloom. Nature DOI:10.1038/nature11229