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In Mauritania, seagrass beds emerge at low tide but are doing better underwater

Eelgrass are flowering plants living in the sea, where they form seagress beds in shallow areas. Some species grow in the intertidal zone and are subject to severe environmental stresses during low tide, such as extreme temperatures, exposition to ultra-violet rays or desiccation. What are the consequences on the biological production of these seagrass beds, which are one of the world's most productive ecosystems?

This study dealt with the aerial and underwater carbon metabolism in Zostera noltii, a species distributed from Europe to West Africa; it was conducted on the Banc d’Arguin in Mauritania, where half of the extensive area (500 km²) emerging at low tide is covered by dense beds of this species, present there at the southern limit of its distribution area.



The study site, near Zira island; seagrass beds are shown in light grey

Small seagrass areas, either emerged (at low tide) or immerged (at high tide), were enclosed within opaque or transparent chambers where carbon concentration was continuously measured, in air or in water according to the case. This made possible the computation of net carbon fluxes between the plants and their environment; these fluxes are the result of two antagonistic processes: production (which fixes carbon into vegetal matter through photosynthesis) and respiration (which extracts energy from organic matter and rejects carbon into the air or the water).


Data collection under the transparent dome, with computer recording of CO2 concentration. At low tide, walking on the seagrass beds of tha Banc d'Arguin sometimes requires the use of snowshoes to cross mud flats (photo CNRS / E. Amice)

These measures showed that the metabolism of plants is much slower during low-tide emersion: respiration is reduced by half and photosynthesis by a factor of 7. This large difference in primary production is mainly related to the pH and oxygen content of the thin layer of water remaining at low tide, as well as the mutual shading of leaves. Respiration is limited by the oxygen deficit at night.

The analyses of carbon isotopes in the leaves allowed to determine the origin of carbon fixed within vegetal tissues, as the isotopic "signature" of carbon differs when it is provided by the air (as gaseous CO2) or by water (as carbonate ions and dissolved CO2). They confirm that it is during immersion periods that Z. noltii produces the largest part of its living matter.


The paper

Clavier J., Chauvaud L., Carlier A., Amice E., Van der Geest M., Labrosse P., Diagne A., Hily C., 2011. Aerial and underwater carbon metabolism of a Zostera noltii seagrass bed in the Banc d'Arguin, Mauritania. Aquatic Botany 95(1) : 24-30.


The journal

Aquatic Botany is an international scientific journal dealing with submerged, floating and emergent plants in marine and freshwater ecosystems. It publishes fundamental studies on plants (molecular biology, biochemistry, physiology,…) and their communities (structure, function, dynamics,…), but also applied research on plant-dominated aquatic systems, including the consequences of disturbance (e.g. transplantation, influence of herbicides and other chemicals, thermal pollution, biological control, grazing and disease), the use of aquatic plants, conservation of resources, and all aspects of aquatic plant production and decomposition.


The authors

This paper results from the collaboration of scientists from France (Lemar, IUEM-Brest), Netherlands (NIOZ-Texel) and Mauritania (IMROP-Nouadhibou). Works were financed by the franco-mauritanian project PACOBA.



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