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Nitrogen Fixation And Phytoplankton Blooms In The Southwest Indian Ocean

Date:
August 19, 2009
Source:
National Oceanography Centre, Southampton (UK)
Summary:
New observations help understand the massive blooms of microscopic marine algae -- phytoplankton -- in the seas around Madagascar and its effect on the biogeochemistry of the southwest Indian Ocean.

Researchers based at the National Oceanography Centre, Southampton report on a 2005 hydrographic survey south and east of Madagascar while aboard the royal research ship RRS Discovery
Credit: Image courtesy of National Oceanography Centre, Southampton

Observations made by Southampton scientists help understand the massive blooms of microscopic marine algae – phytoplankton – in the seas around Madagascar and its effect on the biogeochemistry of the southwest Indian Ocean.

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The observations were made by researchers based at the National Oceanography Centre, Southampton (NOCS) during a 2005 hydrographic survey south and east of Madagascar while aboard the royal research ship RRS Discovery. The fully analysed results are published in the journal Geophysical Research Letters.

Nitrogen-fixing bacteria convert atmospheric nitrogen into nitrogen compounds that organisms can then use as food. This process is thought to be important in areas of the ocean where nitrogen-based nutrients are otherwise in short supply, and the researchers confirm that this is indeed the case in the region south of Madagascar.

But there were some surprises. Previously, it has been thought that the large-scale autumn bloom that develops in this region is driven by nitrogen-fixing blue-green algae, or cyanobacteria, called Trichodesmium, colonies of which the researchers found to be abundant. However, the 2005 bloom was dominated by a diatom – a type of phytoplankton – the cells of which play host to another nitrogen-fixing cyanobacterium called Richella intracellularis, with Trichodesmium apparently playing second fiddle.

Diatoms have relatively large cells, and when they die they sink down the water column, carrying with them carbon that is ultimately derived from carbon dioxide drawn from the atmosphere though the process of photosynthesis. "Carbon dioxide is a greenhouse gas, and enhanced export of carbon to the deep ocean in the bodies of diatoms is an important natural mechanism by which the ocean regulates atmospheric carbon dioxide and our climate," says team member Dr Alex Poulton of NOCS.

The researchers believe that their findings will have an impact on modelling and satellite studies of the Madagascar bloom. "Future research will also need to account for the magnitude of carbon export associated with diatoms and their nitrogen-fixing guests in the southwest Indian Ocean, and indeed other subtropical oceanic settings," says Dr Poulton.


Story Source:

The above story is based on materials provided by National Oceanography Centre, Southampton (UK). Note: Materials may be edited for content and length.


Journal Reference:

  1. Poulton, A.J., Stinchcombe, M.C. & Quartly, G.D. High numbers of Trichodesmium and diazotrophic diatoms in the southwest Indian Ocean. Geophysical Research Letters, 2009; 36 (15): L15610 DOI: 10.1029/2009GL039719

Cite This Page:

National Oceanography Centre, Southampton (UK). "Nitrogen Fixation And Phytoplankton Blooms In The Southwest Indian Ocean." ScienceDaily. ScienceDaily, 19 August 2009. <www.sciencedaily.com/releases/2009/08/090814103233.htm>.
National Oceanography Centre, Southampton (UK). (2009, August 19). Nitrogen Fixation And Phytoplankton Blooms In The Southwest Indian Ocean. ScienceDaily. Retrieved October 31, 2014 from www.sciencedaily.com/releases/2009/08/090814103233.htm
National Oceanography Centre, Southampton (UK). "Nitrogen Fixation And Phytoplankton Blooms In The Southwest Indian Ocean." ScienceDaily. www.sciencedaily.com/releases/2009/08/090814103233.htm (accessed October 31, 2014).

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