Mytiliculture et ecosysteme cotier: Modelisation integree de leurs interactions dans la Lagune de Grande-Entree (Iles-de-la-Madeleine, Quebec)

In the context of over-exploited marine resources, aquaculture should provide an increasing part of the food demand worldwide. Thanks to the simple technical level and the low investments it requires, shellfish culture is developing rapidly. This development takes place mainly in coastal areas where it already raises the question of the carrying capacity of the receiving ecosystems. The complexity of such a problem, mixing aspects of the hydrodynamics of the system, the biogeochemical processes influencing the production and consumption of bivalve food and the ecophysiology of these molluscs, requires the set up of adequate tools. Numerical modelling provides the ideal framework for this kind of study.;In the first part of the work, finite element numerical modelling based on field data is used to study the tidal and tidally induced residual circulation dynamics of the coupled "restricted" and "leaky" coastal lagoon system of the Magdalen Islands. Havre-aux-Maisons Lagoon (HML) is of a "restricted" nature with a neutral inlet in terms of tidal asymmetry. Grande-Entree Lagoon (GEL) is of a "leaky" nature with a marked ebb dominance at the inlet due to direct interactions between the main astronomical tidal constituents. The imbalance caused by the different tidal filtering characteristics of both inlets combines with the internal morphological asymmetries of the system to produce a residual throughflow from HML to GEL. The residual circulation is also characterized by strongest values at both inlets, very weak residual currents in HML deep basin and a dipole of residual eddies over the deeper areas of GEL.;In the second step of the study, a biogeochemical model coupled to the hydrodynamic model of the previous section is developed to study the dynamics of GEL's ecosystem sheltering a commercial mussel aquaculture farm. The model calibration is based on the objective comparison of model results and observations from a field sampling program both in terms of state variable concentrations and process rate magnitudes. A sensitivity analysis is then carried out to test the predictive ability of the model. The results of both the calibration process and the sensitivity analysis strongly establish the capacity of the model to accurately reproduce the lagoon's ecosystem dynamics, including the effects of cultured mussels. These dynamics are characterized in summer by the dominance of recycling processes among inorganic nitrogen input mechanisms and the dominant role of the microbial food web, especially the microzooplankton, in setting the productivity level of the system. The mussel culture activity in its present state does not seem to exert any major influence on the system dynamics at the global scale.;Finally, the calibrated fine resolution physical-biogeochemical model is improved with the addition of a dynamic energy budget (DEB) and used to investigate the local and system scale interactions between the mussel farm and the receiving coastal ecosystem. Using a set of published parameters for the DEB, the coupled model reproduces both the local mussel growth and its spatial repartition over the farm area quite accurately. Mussel related process rates are also well reproduced, allowing the study of mussel/environment interactions.;The main goal of the present thesis is to develop a numerical tool integrating the three aspects of the carrying capacity problem and to consider the diverse spatial scales involved, which is lacking from previous studies. The model will then be used to investigate the interactions between the mussel farm and the ecosystem of Grande-Entree lagoon, Magdalen Islands, Gulf of Saint-Lawrence.;Results show the local importance of cultured mussels in the cycling of nitrogen within the cultivation area. Despite the strongly reduced influence exerted by the mussel farm at the scale of the entire system, the culture activity still has the ability to alter the structure of Grande-Entree lagoon's ecosystem.;The coupled model results show that the mussel stock could be greatly increased before reaching the maximum production capacity of GEL. However, when the ecological aspect is accounted for, using model results along with objective criteria such as the depletion footprint curve, the overall carrying capacity of GEL must be significantly reduced.;The coupled fine scale numerical model developed for this study gives the opportunity to assess the ecological carrying capacity of a coastal region for shellfish culture accounting for both local and system scale processes.