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Dense mats of floating vegetation form complex structures that support high biodiversity in wetlands. Since the 1960s, however, high-density stands of Stratiotes aloides have shown strong declines throughout Europe and restoration efforts have often failed, both of which have been linked to high nitrogen (N) input rates. The low success rate of reintroduction is striking, since seemingly healthy, high-density stands are observed to grow under similar environmental conditions. We therefore hypothesise that N tolerance of S. aloides increases with density due to joint N uptake and detoxification. To test our hypothesis, we set up a controlled full factorial mesocosm experiment in which we manipulated N loading and S. aloides density, and investigated interacting effects on habitat biogeochemistry and plant performance. High-density S. aloides stands strongly lowered ammonium availability through shared uptake …