In aquatic ecosystems, invertebrate bioturbation signifi cantly infl uences microbial activities and biogeochemical processes in sediments by modifying water and sediment fl uxes at the water-sediment interface. We apply the concept of ecosystem engineering to develop a qualitative general understanding of the role of bioturbation on microbial processes in different benthic environments. We hypothesized that the effects of the bioturbation mode (sediment reworking, biogenic structure building, bioirrigation) on microbial processes vary between diffusion-and advection-dominated benthic environments because bioturbation does not have the same infl uences on hydrological exchanges (and the fl ux of resources for micro-organisms living in sediments) at the water-sediment interface of the two systems. To test this hypothesis, we experimentally compared the infl uence of three bioturbation modes (fi ne-sediment reworking, U-shaped structure burrowing, and gallery-network burrowing) in a diffusion-dominated system (fi ne sediments/low interstitial fl ow rates) and an advectiondominated system (coarse sediments/advection of water in sediments). Our analysis demonstrated that bioturbation modes in the two systems had different impacts on microbial activities. For instance, U-shaped tube burrowing by animals increased O 2 consumption in the diffusion-dominated system but produced the opposite effect in the advection-dominated system. The infl uence of bioturbation was also negatively related to interstitial fl ow rate, the bioturbation having a higher infl uence on O 2 consumption in the diffusion-dominated system than in the advection-dominated system. According to our hypothesis, bioturbation modifi ed microbial processes in sediments depend on the hydrological characteristics of the system. In the diffusion-dominated system, invertebrate bioturbation can produce water fl uxes at the watersediment interface that may strongly infl uence microbial processes in sediments. In contrast, in the advectiondominated system, invertebrate bioturbation can only modify the water circulation patterns in sediments, moderately affecting microbial processes. Consequently, it is necessary to use a conceptual framework which takes into account the features of sediment habitats in order to allow a better prediction of bioturbation effects on sediment biogeochemistry. With this aim, the conceptual scheme of ecosystem engineers can be an organizing principle to integrate the complex relationships among physical habitat, bioturbation mode and microbial activity.

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