In order to monitor the health of ecosystems over time, scientists need to use bioindicators that are both reliable and very widespread. It is even more important that these bioindicators should be used prudently in order to avoid unreliable interpretations. Recent work at the Laboratory of Oceanology of the University of Liege at the STARESO oceanographic station (Corsica) have made it possible to optimize the information provided by two major bioindicators in the Mediterranean Sea: the marine plant posidonia and mussels. If the theories are confirmed, these two “sentinel” species point to a marked improvement in the environment of Southern Europe. This is potentially great news that should not be underestimated.

Few tourists are aware of the fact that the beautiful turquoise reflections of the Mediterranean are partly due to posidonia seagrasses. These flowering plants, which belong to the group of marine magnoliophytes, established themselves in the area surrounding the Mediterranean around 8,000 years ago during the last rise in water levels in the enormous intercontinental zone. This aesthetic quality of the seagrasses (in particular, the species Posidonia oceanica, which is abundant in the Mediterranean) is not what interests the scientists most but rather its morphological particularities. Like all plants, posidonia seagrasses have a large surface area available for photosynthesis and the exchange of nutrients. The leaves are flat and linear (60 centimeters long by 1 centimeter wide) and these are densely criss-crossed by a creeping rhizome whose roots are anchored in the marine sediments. Thanks to this particular configuration there can be anything up to 3,000 posidonia leaves per square meter! Posidonia oceanica only covers 1% to 2% of the sea bed but, with such characteristics, we can clearly see that it plays a fundamental role in the oxygen, carbon, nitrogen and phosphorous cycles even beyond the fringe of coastline that it colonizes (from a depth ranging from 0 to 45 m).

The particular advantage of the Marine and Oceanographic Research Station (STARESO) of the University of Liege, situated in the Bay of Calvi in Upper-Corsica is the fact that it has an available and extensive database of information relating to posidonia. Because it is able to accumulate pollutants that are dissolved in seawater and trapped by the sediments, this plant is traditionally considered as an excellent bioindicator of the state of the marine environment. As a photosynthetic sessile organism, it is capable of accumulating trace elements such as the most well-known pollutants (the “traditional” trace elements such as lead, cadmium, mercury...) as well as so-called “rare” trace elements (such as antimony, vanadium, bismuth...). This quality of “witness” or “memory” of the quality of the ecosystem is shared by posidonia with other living organisms that are sometimes very different. This is the case with mussels and, in particular, Mytilus galloprovincialis, the Mediterranean mussel. Like all mussels, Mytilus galloprovincialis feeds on suspended particles and its environment provides precious information with regard to the presence of pollution.

Another advantage of these bioindicators is the fact that they are complementary: mussels, which are artificially kept in pockets of open water, essentially reflect the quality of the water column; posidonia reveals the quality of the seabed and supplies precious information on the accumulation of pollutants in coastal sediments year after year. The first of the above bioindicators gives a general idea of the current state of a very large area and the second gives a much more accurate idea by taking account of lighter spatial variations (integrated in time) in pollution levels. The identification of reliable bioindicators is one thing but putting them to good use is another! A few years ago, Jonathan Richir, who was then a PhD student at the Oceanology Laboratory of the University of Liege, had already demonstrated that the presence of zinc in in the purple sea urchin (Paracentrotus lividus, another bioindicator) need not necessarily be considered as an indicator of pollution. In fact, during reproduction, the female urchin concentrates this essential element for the imminent release of her gametes. Ignoring facts such as the sex of the harvested sea urchins during sample-taking operations would expose scientists to serious biases in the interpretation of data.