We must avoid hasty interpretations!

After several years of research, the work of Jonathan Richir (currently doing post-doctoral work as the scientific head of the project "STARECAPMED") and his colleagues has resulted in similar conclusions, but this time concerning the Mediterranean mussel and posidonia. “It is during the period when she gathers food in order to produce her gametes that the Mediterranean mussel accumulates the highest quantity of trace elements”, explains Sylvie Gobert, professor of Oceanography and head of the Oceanology Laboratory of the faculty of sciences of ULg. “Then when reproduction occurs, the sudden release of gametes into the environment causes a peak in the concentration of trace elements in the mussels. In contrast with the gametes, trace elements are not re-released into the environment; this peak in the concentration of trace elements can be explained by the significant loss of body mass (around 20 %) caused by the liberation of gametes. But pollution in the water column does not vary nonetheless! In other words, the variations in trace elements that can be found in mussels do not necessarily always reveal the degree of pollution of the environment, but rather the ecophysiological activities of the organism”. The same type of observation was made for the posidonia observed on the French coast (in the PACA region of Corsica), in the Mediterranean. “We carried our sampling operations very close to each other and spread out over a period of several years in both polluted and non-polluted areas. Thanks to these sampling operations, we noticed that the cycles of trace elements occur exactly in parallel with the growth of the plant. Therefore during their growth, the young shoots accumulate trace elements in their tissues. But in autumn, the oldest and most contaminated leaves become detached and, because of this, the concentrations of trace elements in the biomass of the leaves decrease dramatically. We are therefore in the presence of an artefact because the concentration in the area does not vary”! This leads to the conclusion once again that; the bioindicator reflects more its own physiology in this type of situation rather than the state of the environment.

Does all this mean that the bioindicator qualities of these organisms should be re-examined? Sylvie Gobert’s answer to this is a definitive no. “The values for the concentrations are there and need not be questioned for any reason. On the other hand, these kinds of results should encourage researchers to be more vigilant in their sample-taking activities and the way they interpret their results. In the light of our observations, it will no longer be acceptable to take several disparate studies and compare their results without focussing on the part of the plant analysed or the period of the year during which the samples were taken. It becomes imperative to harmonize the sampling practices”.