Role de la diversite fonctionnelle de la communaute a Macoma balthica (estuaire du Saint-Laurent, Quebec, Canada) sur les flux biogeochimiques a l'interface eau-sediment et sur le melange particulaire

The objective of this Ph.D. thesis is to characterise and quantify the effects of the dominant functional groups of the Macoma balthica community, maintained separately and in various combinations, on: (i) biogeochemical fluxes at the sediment-water interface (oxygen, dissolved organic carbon, ammonium, nitrate, phosphate), (ii) sediment mixing, and (iii) bacterial communities. The functional groups of the M. balthica community of the St Lawrence estuary (Quebec, Canada) were defined according their sediment mixing mode: two biodiffusers (M. balthica and Mya arenaria) and one gallery-diffuser (Nereis virens) were studied. Experiments were performed in replicated microcosms with the functional groups being either isolated (allopatry) and combined (sympatry); control microcosms contained sediment but no macrofauna. All treatments were subjected to identical experimental conditions over the duration of the experiment (40 days). Microcosms contained similar biovolumes of organisms, these volumes being determined from natural densities. The space occupied by macro-invertebrates was thus similar in all treatments. The results clearly show that the biodiffusers and the gallery-diffuser, at similar biovolumes, have different effects on the stimulation of biogeochemical fluxes, on particle transport towards deeper layers, and on bacterial activity. The effects of the gallery-diffuser are highest and vary over time, due to the spatio-temporal dynamic of biogenic structures and irrigation activities, and to different maximal burrowing depths. The different burrowing depths of the two biodiffusers also explain the observed intra-functional variation in nitrate and ammonium fluxes, and in bacterial activity. The combination of both functional groups (gallery-diffuser+biodiffusers) has important effects on measured parameters, the latter often being higher than would be predicted based on additive effects. Positive spatial interactions between the two functional groups explain these effects. The interactions between functional groups, which essentially act on nitrate, phosphate, ammonium and oxygen fluxes, suggest that, at similar biovolumes, the burrowing depth of each functional group and the spatial organisation of these groups are key factors in the stimulation of sedimentary biogeochemical processes. The results obtained are directly comparable due to the use of similar biovolumes, and thus validate the functional approach for the study of bioturbation and its effects on biogeochemical fluxes, sediment mixing and bacterial communities. Further research is proposed to (i) develop the functional approach for studies based only on solute exchange at the sediment-water interface, (ii) refine the notion of biovolume, and (iii) examine in more detail the effects of relationships between functional groups within a benthic community on coastal sediment functioning.