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Context and objectives

The earth’s oceans are becoming more and more acidic owing to CO2 emissions into the atmosphere (Fig. 1). In an experimental environment, the growth of reef-building organisms (such as corals and calciferous algae), i.e. the quantity of calciferous skeleton deposited by these organisms (“calcification”) decreases with a drop in the pH of the surrounding environment (Figs. 2 and 3). Diminished growth of coral reefs is therefore expected in the course of the 21st century, reaching perhaps 40% according to certain projections, possibly even more, owing to the additional adverse effects of global warming.

This being the context, the proposal is : 1) to add to the monitoring networks of the quality of the marine environment currently implemented by the Regional Marine Institute (ARVAM-IFREMER), by setting up a station for measuring CO2 in the marine environment. There is currently no monitoring station of the impact of CO2 emissions in the western Indian Ocean. 2) to research exactly how important CO2 is in controlling on-site reef calcification, as the latter is also affected by light, temperature and nutritional content (which in turn are impacted by human activity on the catchment areas). For the time being, the scientific literature has only published data resulting from laboratory work on isolated organisms, with very few dealing with the interactions between the various parameters controlling calcification. Field data is required in order to refine these projections and contribute to thinking on emission reduction.

Figure 1

The CO2 emitted into the atmosphere dissolves in the ocean’s surface, where it releases an acid (H+). The latter reacts with the “carbonates” already present in seawater (CO32-), resulting in a reduction in the quantity of carbonates. But carbonates are indispensable for the synthesis of the skeleton of reef-building organisms – corals, but also the calciferous algae that stabilize the reef’s framework. The growth of these organisms, but also that of sea-urchins, shells, and so on, is significantly perturbed by the decreasing quantities of carbonates, to such a degree that some authors consider that the survival of coral ecosystems in their entirety is jeopardized.

Figure 2

Both photographs above show the same species, but on the left, it has been cultivated in a “normal” environment, while on the right, the environment has been acidified by adding CO2. On the right, the polyps (living coral tissue) are uncovered. In experiments using the quantities of CO2 in the range of what is expected towards the end of the 21st century, the coral skeletons are not dissolved, but their growth rate is slowed down considerably.

Figure 3 On the left, in a “normal” environment, (red) calciferous algae can be observed developing normally on the experimental substratum (plexiglas cylinders). On the right, in a CO2 enriched environment, the calciferous algae have been replaced by filamentous seaweed, diatoms, etc., which play no role in reef-building.


Environmental monitoring will be carried out on a continuous basis, using sensors placed off the developed reef, in order to calculate calcification using the obtained measurements. Calcification will then be related to the environmental parameters (carbonate content, light, temperature, and nutritional content.)


ECOMAR and ARVAM. The external partners are : GéoSciences Réunion, the University of Western Australia, and the University of Hawaii.