It is the first time that the amount of organic carbon converted into carbon dioxide by ultraviolet light has been determined in the open sea. The researchers reached the conclusion that each year, ultraviolet radiation converts two to three percent of organic carbon into carbon dioxide. This does not seem a lot, but it is still eight times the amount converted by plankton in the top ten metres of water. It is also twice as much as the amount of carbon supplied by rivers. The researchers made these statements on the basis of measurements they had carried out on the ship Tydeman in the Atlantic Ocean.
Temperature measurements of the uppermost hundred metres of the ocean revealed that the top 20 to 40 metres consists of layers which do not mix with each other during the day. This stratification ensures that both the plankton and the organic carbon are exposed to sunlight throughout the day.
According to the biologists the plankton can withstand this large dose of sunlight. This can be compared to sunburn. When the sunbather seeks shelter the skin can recover to a large extent. However, if the plankton receive too much ultraviolet radiation too often, they can no longer cope with this and die. They are probably replaced by new species but the effect of these species shifts on the food chains is not yet clear.
After organic carbon in the soil, the ocean forms the largest source of organic carbon in the biosphere. Heterotrophic plankton remove this carbon from the ocean and use it partly for growth and partly for metabolism. This conversion of organic carbon into carbon dioxide provides the plankton with energy. The carbon dioxide leaves the ocean and ends up in the atmosphere, where it contributes to the greenhouse effect.
For further information please contact Prof. Gerhard Herndl (Netherlands Institute for Sea Research), tel. +31 (0)222 369507 (home) or +31 (0)6 51260745 (mobile), fax +31 (0)222 319674, e-mail herndl@nioz.nl. The findings have just been published: Herndl, G.J., I. Obernosterer, 2002: UV radiation and pelagic bacteria. In: Ecological Studies. Vol 153. UV-radiation and arctic ecosystems. [Ed] D.O. Hessen, Springer Verlag, Berlin, Heidelberg, 245-259. This spring a chapter will be published in a book about plankton and their role in food chains.
The research was funded by the Netherlands Organisation for Scientific Research (NWO).
