Tiny plant-like organisms called phytoplankton dominate biological production in the sunlit surface waters of the world's oceans and, through the process of photosynthesis, sequester large amounts of atmospheric carbon dioxide.
A proportion of the carbon is exported to the deep ocean, and because carbon dioxide is a greenhouse gas, this so-called 'biological carbon pump' helps prevent runaway global climate warming.
Iron is an essential micro nutrient for phytoplankton growth. In high-nutrient, low-chlorophyll (HNLC) oceanic regions, phytoplankton growth is limited by low iron availability. Classical HNLC regions, which account for about a third of the world's oceans, include the Southern Ocean and the subpolar North Pacific.
In contrast, it has been widely assumed that iron supply does not limit biological production of the high-latitude (>50 degrees N) North Atlantic Ocean. Here, winter cooling causes the sinking of nutrient-depleted surface waters and their replacement by deep nutrient-rich water. This winter 'overturning' replenishes surface water nutrients, and in the spring, when light intensities increase, a large phytoplankton bloom develops, leading to high rates of carbon export.
However, in many regions of the open North Atlantic, including the Iceland and Irminger Basins, residual amounts of nitrate persist into the summer period, after the spring bloom has ceased. This represents an inefficiency of the biological carbon pump that is potentially of global significance to the partitioning of carbon between the atmosphere and ocean.
Phytoplankton are grazed upon and some of the larger phytoplankton species such as diatoms with low grazing mortality are susceptible to silicate shortage. Traditionally, it has been believed that these factors, acting in concert, might be sufficient to terminate the spring bloom leaving some nitrate unused. However, there have been indications that phytoplankton might simply run out of iron before they are able to exploit any remaining other nutrients.
Now a team of scientists from the National Oceanography Centre, Southampton, have tested this 'iron limitation hypothesis' for the high-latitude North Atlantic Ocean. Their measurements were performed on a cruise aboard the RRS Discovery within the central Iceland Basin during the summer of 2007, and the findings are published this month in the scientific journal Global Biogeochemical Cycles.
The researchers found that the concentration of dissolved iron in surface waters was very low, as was biological production, despite the presence of residual nitrate. Experimental addition of iron to bottles containing seawater samples increased photosynthetic efficiency, chlorophyll concentrations, and growth of several types of phytoplankton, including the ubiquitous Emiliania huxleyi, a coccolithophore.
"These results, backed up by additional experiments, are extremely exciting," said team member Maria Nielsdottir: "They provide strong evidence that low iron availability limits summer biological production in the high-latitude North Atlantic. This has only previously been suspected, but helps explain why the spring phytoplankton bloom does not continue well into the summer and why residual amounts of nitrate remain unused."
The central Iceland Basin receives little iron input from continental sources or from atmospheric dust, and some iron is also lost through detrital sinking. Moreover, the new findings suggest that iron brought to the surface during winter overturning is insufficient to support maintenance of the phytoplankton bloom into the summer.
Nielsdottir, a research student at the University of Southampton's School of Ocean and Earth Science based at the National Oceanography Centre, said: "In effect, the high-latitude North Atlantic is a seasonal HNLC region, whereas classic, HNLC regions such as the Southern Ocean remain in this condition throughout the year."
The failure of the phytoplankton community to exploit residual nitrate remaining in the summer reduces the effectiveness of the biological carbon pump. "This is important," says Nielsdottir, "because the high-latitude North Atlantic is second only to the Southern Ocean in its potential to lower atmospheric carbon dioxide and un used nitrate in the surface highlight the potential for even higher CO2 drawdown, high levels of which are an important cause of global climate warming."
For more information contact the NOCS Press Officer Dr Rory Howlett on +44 (0)23 8059 8490 Email: email@example.com
Images are available from the NOCS Press Office (Tel. 023 8059 6100).
Maria Nielsdottir: email firstname.lastname@example.org; telephone +44 (0) 23 8059 6237
The work was supported by the National Oceanography Centre with a PhD to MCN, Natural Environment Research Council, Oceans2025 and the Faroese Ministry of Interior and Law. The authors are Maria Nielsdottir, C. Mark Moore, Richard Sanders, Daria Hinz and Eric Achterberg, University of Southampton's School of Ocean and Earth Science based at the National Oceanography Centre, Southampton.
Citation: Nielsdóttir, M. C., C. M. Moore, R. Sanders, D. J. Hinz, and E. P. Achterberg (2009), Iron limitation of the postbloom phytoplankton communities in the Iceland Basin, Global Biogeochem. Cycles, 23, GB3001 , doi:10.1029/2008GB003410.
Rory Howlett | EurekAlert!
Scientists shed light on carbon's descent into the deep Earth
19.07.2017 | European Synchrotron Radiation Facility
Thawing permafrost releases old greenhouse gas
19.07.2017 | GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre
Physicists have developed a new technique that uses electrical voltages to control the electron spin on a chip. The newly-developed method provides protection from spin decay, meaning that the contained information can be maintained and transmitted over comparatively large distances, as has been demonstrated by a team from the University of Basel’s Department of Physics and the Swiss Nanoscience Institute. The results have been published in Physical Review X.
For several years, researchers have been trying to use the spin of an electron to store and transmit information. The spin of each electron is always coupled...
What is the mass of a proton? Scientists from Germany and Japan successfully did an important step towards the most exact knowledge of this fundamental constant. By means of precision measurements on a single proton, they could improve the precision by a factor of three and also correct the existing value.
To determine the mass of a single proton still more accurate – a group of physicists led by Klaus Blaum and Sven Sturm of the Max Planck Institute for Nuclear...
The research team of Prof. Dr. Oliver Einsle at the University of Freiburg's Institute of Biochemistry has long been exploring the functioning of nitrogenase....
A one trillion tonne iceberg - one of the biggest ever recorded -- has calved away from the Larsen C Ice Shelf in Antarctica, after a rift in the ice,...
Physics supports biology: Researchers from PTB have developed a model system to investigate friction phenomena with atomic precision
Friction: what you want from car brakes, otherwise rather a nuisance. In any case, it is useful to know as precisely as possible how friction phenomena arise –...
21.07.2017 | Event News
19.07.2017 | Event News
12.07.2017 | Event News
21.07.2017 | Physics and Astronomy
21.07.2017 | Life Sciences
21.07.2017 | Physics and Astronomy