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Deep-Sea Bioluminiscence Blooms in the Mediterranean Sea

Por: | 30 de octubre de 2013

By Rosa Martínez, University of Barcelona Press

Image: The North-West Mediterranean Sea is the best region to study dense water formation. (Photo: José Luis Casamor, UB Research Group on Marine Geosciences)

Permanent deep-sea’s darkness is sometimes lightened by biogenic light blooms, a phenomenon so-called ‘deep-sea bioluminescence’. It is the ability of numerous marine organisms to emit light by chemical processes. But deep-sea bioluminescence blooms are connected with dense water formation, a process originated by the cooling of surface waters, which provides nutrients and oxygen to marine communities.

The discovery is based on an inter-disciplinary research carried out with ANTARES telescope, the first underwater equipment to detect high-energy neutrinos. From March to July in 2009 and 2010, the undersea telescope ANTARES, located close to Nice coast, detected a great increase of abyssal pelagic organism bioluminescence in the Gulf of Lion; it constitutes a unique data set, never recorded before.

Bioluminescence: An Evidence of Biological Activity in Deep Waters

Marine organism bioluminescence is a well-known phenomenon. About 90% of abyssal organisms are able to emit light; this ability responds to defence, food or reproduction needs, and some behaviour guidelines. However, the origins of this process remain unknown.

Miquel Canals, head of the Research Group on Marine Geosciences of the University of Barcelona and first author of an article, published in Nature (2006), which described the phenomenon of dense water cascading in North-Western Mediterranean Sea, explains that “the process is produced by dense water formation. The research proves quick connections among the atmosphere, the ocean — even in deep waters — and marine biological communities. Data show a quick response of deep water pelagic ecosystem to external stimuli. In short, all these processes are correlated”.

Extreme Values up to 9,000 kHz in Deep Waters

In 2009 and 2010, the values detected by ANTARES light sensors — between 40 and 100 kHz — went up to 9,000 kHz, an extreme value which enabled to relate bioluminescence to salinity and temperature changes in Gulf of Lion waters. This extreme value is due to the vertical mixing of deep water and the sinking of surface water.

“The observed area”, adds Canals, “located at a depth of about 2.4 kilometres, has suffered for some years the phenomenon so-called ‘open-sea convection’, a process similar to dense water cascading”. Convection results in the formation of deep water owing to the combination of atmospheric forcing and regional circulation that lead the water column to overturn. Dense deep water formation occurs during late winter and early spring due to cold, strong and persistent northern winds, Mistral and Tramontane.

Canals, who was associated research director of the Training and Research Centre on Marine Environment (CEFREM) of the University of Perpignan, points out that “these periods, which match the end of winter, spring and the beginning of summer, show surface waters’ cooling. The phenomenon responds to the cooling and subsequent sinking of saltier water currents; and, consequently, salinity is increased. 

The research group suggests that the measurement of bioluminescence should be considered a priority method to continually assess deep water biological activity. The objective is to better understand how marine ecosystems work and their relation to atmospheric and oceanic circulation and, in the last resort, to global climate change.

Finally, it is important to remember that, within the Mediterranean Sea, the group led by Miquel Canals has recently published an article about the trawling effect on the seabed (Nature, 2012), a publication honoured with an editorial in The New York Times.


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