Influence de l'aquaculture sur les flux biogeochimiques dans le couplage pelagos-benthos Iles-de-la-Madeleine (Quebec, Canada)

Aquaculture is an ancestral practice which is now in expansion to respond to the need of increasing world population. Aquaculture increases nutrient fluxes and oxygen consumption in the environment and leads to eutrophication and anoxic events which could damage the environment and the aquaculture production. Although aquaculture is a new activity in Canada, the federal government aims to sustainable aquaculture. Since 2003, scientist team work to create a carrying capacity model of the Iles-de-la-Madeleine lagoons, newly exploited by bivalve farming. This PhD study is part of this project. The three objectives of this study are: (i) to test the summer influence of mussel and scallop cultures on the benthic environment in Grande-Entree and Havre-aux-Maisons lagoons, (ii) to show that suspended cultures, in their integrity, could increase biogeochemical fluxes in adjacent water, and (iii) to compare the benthic and pelagic influence of suspended aquaculture structures on the main biogeochemical cycles to highlight that the pelagic influence of suspended bivalve cultures on ecosystem functioning should not be neglected. This comparative approach is one of the originalities of this PhD thesis. Two in situ and one tank experiments were carried out in the Iles-de-la-Madeleine archipelago during the summers 2003 and 2004. Biogeochemical fluxes were measured at the water-sediment interface and at the interface of aquaculture structures using dark chambers (benthic, pelagic and metabolic). Experimental design of each study was balanced and adapted to analyse environmental disturbances. The results of theses studies show that in contrast to scallop cultures, mussel cultures induce organic enrichment in the sediment and decrease benthic macrofauna abundance. Mussel and scallop cultures increase benthic nutrient fluxes but not oxygen consumption. Aquaculture structures act as settlement and growth substrates for cultivated bivalves, as well as for biofouling organisms. As bivalve metabolism, the metabolism of the associated fauna and the degradation of organic matter trapped within the structure contribute to increase biogeochemical fluxes in adjacent water. Suspended mussel and scallop cultures increase biogeochemical fluxes in the water column in culture zones and contribute to the main biogeochemical cycles. Their contribution should not be neglected in comparison to that of the benthic interface. Suspended aquaculture structures play an important role in nitrogen cycle (especially one of nitrate and nitrite) whereas the benthic interface plays a great role in silicate cycle. The pelagic influence of suspended aquaculture needs to be included, in integrity, in carrying capacity models. Acquisition of seasonal and spatial data should be the next step after this PhD study. Aquaculture is known to favour attraction of megafaunal predators and proliferation of algae and plants, the effect of the latter on biogeochemical cycles could be studied in the future to obtain an over-all knowledge of the influence of aquaculture on lagoon ecosystem functioning in the Iles-de-la-Madeleine.