Influence des facteurs environnementaux et anthropiques sur la structure et le fonctionnement des peuplements benthiques du Golfe Normano-Breton.

Abstract : In the English Channel, the gulf of Saint-Malo is characterized by several environmental peculiarities including the occurrence of several residual gyres around the Channel Islands, one of the largest tidal flat in Europe in the Bay of Mont-Saint-Michel due to a high tidal range, and numerous human threats linked to tourism, shellfish farming and local fisheries. With regards to its great ecological and economical interest, the gulf is one of the main French coastal zone for which rapid enforcement of a coordinated conservation policy integrating the impact of human activities is necessary. In this context, to assess the impact of natural and anthropogenic environmental constraints on the macrobenthic community structure, two systems at two different scales were studied (i) the coarse sand Glycymeris glycymeris community in the gulf of Saint-Malo, and (ii) the intertidal flat of the bay of Mont-Saint-Michel. The main questions that we addressed were: (1) How do the hydrodynamic features of the Gulf of Saint Malo (i.e. permanent gyres) affect the structure and dynamics of benthic communities? (2) What is the relative role of environmental gradients and shellfish farming on benthic communities in the Bay of Mont-Saint-Michel? What is the ecological state of the bay? At the large scale of the gulf of Saint-Malo, a weak relationship was reported between the five principal gyres (i.e. Flamanville, Guernsey, Jersey, Saint-Brieuc bay and the Mont-Saint-Michel bay) and the structure of the coarse sand G. glycymeris community. The homogenization of the benthic structure was favored by the permeability of the local gyres and species distribution is mainly governed by thermal fronts. However, differences were obtained on the demographic structure of the three main bivalves (i.e. Paphia rhomboides, G. glycymeris and Timoclea ovata) in relation to these gyres suggesting that connectivity between gyres is reduced. In spite of a clear elevation of temperature estimated to 1.5 °C over the last three decades, a comparative study on the distribution of some benthic species between our data and earlier sampling (i.e. 70's) did not reveal strong modifications, excepted for few cases. At this scale, the temperature rising effect may be associated with fishing effort to explain decrease or proliferation of some species. At the scale of the bay of Mont-Saint-Michel, the distributions of the benthic assemblages and the four main bivalves (i.e. Abra alba, Cerastoderma edule, Macoma balthica and Spisula ovalis) followed a typical vertical gradient. They were strongly influenced by environmental factors (i.e. tidal height, sediment grain size) while the shellfish activity played only a minor role: locally, it may favour the development of opportunistic species including mainly oligochaetes and cirratulid polychaetes. The main environmental consequence of shellfish farming is probably the introduction of the alien gastropod Crepidula fornicata which greatly controls primary production within the bay. The analysis of the demographic structure of the bivalves suggests that sediment instability may affect the spatial pattern of recruitment of some species. The ecological quality status of the Mont-Saint-Michel Bay was determined using six biotic indices defined under European Water Framework Directive (H', AMBI, BOPA, M-AMBI, UK index, DK index). The bay is classified as non-polluted and in good or high state although the ecological status values calculated with the various indices were different. For example, some indices accuracy was down weighted by the influence of environmental constraints typical of tidal flat which strongly affect species richness or total densities. Finally, an additional map was done based on the Natura 2000 habitat criteria.
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Submitted on : Tuesday, March 23, 2010 - 12:57:48 PM
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Rima Jihane Trigui. Influence des facteurs environnementaux et anthropiques sur la structure et le fonctionnement des peuplements benthiques du Golfe Normano-Breton.. Océan, Atmosphère. Muséum national d'histoire naturelle - MNHN PARIS, 2009. Français. ⟨tel-00466280⟩

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