Ecosystem engineering by tubificid worms stimulates macrophyte growth in poorly oxygenated wetland sediments

P>1. Positive interactions driven by ecosystem engineers have been determined to be important community forces in stressed environments. By ameliorating habitat conditions, ecosystem engineering can create available ecological niches for other species. In poorly oxygenated sediments of freshwater wetlands, small invertebrates such as tubificid worms and chironomid larvae are known to function as active bioturbators; however, their effects on the growth and physiology of organisms which are constrained by low oxygenation of sediments have never been studied. 2. We examined whether the common bioturbator, Tubifex tubifex, significantly influences the growth and the physiological state of two plant species, Elodea canadensis and Myriophyllum spicatum, in experimental systems simulating a water-sediment interface of wetlands. We also quantified the influence of plant-animal interactions on biogeochemical processes (fluxes of oxygen and nitrogen at the water-sediment interface) and microbial compartment in sediments. 3. Tubificid worms stimulated growth of aboveground and belowground biomasses of the two plants through reduction in the anoxic stress in sediments. Myriophyllum spicatum, which was the best adapted to sedimentary anaerobic conditions, essentially increased its biomass whereas E. canadensis, less adapted to anaerobic conditions, shifted its root metabolism from anaerobic to aerobic. 4. Biogeochemical processes were not significantly influenced by plant-animal interactions: (i) oxygen flux from overlying water to sediments probably reached a threshold that could not be raised by the increased plant biomass induced by worms and (ii) nitrogen fluxes were essentially linked to bioturbation activities of worms. 5. Our study confirmed that the reduction in constraining variables by physical habitat modifications (ecosystem engineering) may play a crucial role in community and ecosystem processes. The fact that positive interactions measured between ecosystem engineers and plant species in anoxic wetland sediments were highly dependent on the ecophysiology of plant species suggests an extension of this first study to a wide range of macrophytes in order to determine the main plant functional traits driving plant-animal interactions in wetland sediments.