Directive Cadre sur l’Eau - Masses d’Eau Fortement Modifiées (MEFM) - Prise en compte de l’impact des activités conchylicoles sur l’élément de qualité biologique « faune benthique invertébrée » dans l’évaluation du potentiel écologique. Cas des masses d’eau côtière FRFC02 (Pertuis Charentais) et FRGC01 (baie du Mont Saint Michel) - Convention 2010 - Action 7

Shellfish farming is a major economic activity developed in the French coastal areas and causes more or less extended changes ontohydromorphological characteristics of water bodies. These changes may induce their designation as heavily modified water bodies (HMWB) under the Water Framework Directive (WFD 2000/60/EC). This is, for example, the case of the coastal water body FRFC02 "Pertuis Charentais", which houses a major part of the Marennes-Oléron shellfish production area. Scientific literature describing the effects of shellfish aquaculture on benthic communities, as well as downgrading assessments obtained in 2007 regarding the parameter "benthic invertebrate fauna", raise the question about the opportunity of considering the water body FRGC01 “Mont Saint Michel Bay” as a HMWB at the next 2013 deadline. In the case of the heavily modified water body FRFC02, the WFD monitoring network as currently defined, does not allow any impact on the biological quality element "benthic invertebrate fauna” to be consistently detected during the period 2007 to 2009, as a consequence of the intense oyster and mussel farming in the Bay of Marennes-Oléron. Consequently, and given the current state of knowledge, it seems inappropriate to propose different biological conditions from the reference conditions involved in the assessment of “natural” water bodies. In contrast, in the water body FRGC01, it seems that the intense shellfish farming activity partly affects the results of evaluation given by the indicator “benthic invertebrate fauna” used in the context of the WFD. The hydro-sedimentary environment of monitoring points incriminated consists in intertidal muddy-sands and, assuming that it would be decided to classify water body FRGC01 as a HMWB for the 2013’s WFD inventory, biological conditions corresponding to the maximum ecological potential (MEP) could be proposed for this type of habitat. Following a methodological approach inspired by the suitable European guidance document, an AMBI index value of 2 (instead of 1 in "natural" condition) could be proposed to characterize the biological conditions of this water body at the maximum ecological potential. Other conditions of reference for the calculation of the M-AMBI (35 for species richness and 4 for H') in the water body would remain unchanged. In any case, the designation as a HMWB does not absolve from the implementation of measures to mitigate the impact of shellfish farming. Moreover, the impact on benthic field generated by other pressures, also identified in water bodies covered by this study, should also be evaluated (invasive species introduced by the shellfish farming practices, professional and recreational shellfish fishing).