Indirect facilitation promotes macrophyte survival and growth in freshwater ecosystems threatened by eutrophication

1. Eutrophication is a major threat for freshwater ecosystems. Submerged aquatic plants (macrophytes) can maintain clear water conditions in eutrophic lakes by competing with phytoplankton for light and nutrients. The interactions between macrophytes and phytoplankton may lead to indirect facilitation among plants and the maintenance of high macrophyte diversities in eutrophic conditions. Nonetheless, the role of indirect facilitation in promoting macrophyte-dominated clear water states under eutrophication has never been demonstrated experimentally despite important implications for these ecosystems. 2. We predicted that (i) submerged aquatic plants buffer negative consequences of eutrophication by strongly affecting biotic (phytoplankton) and environmental conditions (light, nutrients) in the water column, (ii) competition with phytoplankton results in indirect facilitation among submerged aquatic plants, and (iii) the response to indirect facilitation depends on the tolerance of submerged aquatic plants to light attenuation by phytoplankton. 3. We experimentally simulated eutrophication through fertilization and manipulated the presence of neighbouring plants in a mesocosm. By manipulating the presence of neighbours with and without fertilization, we were able to test whether competitive or facilitative interactions occur during the eutrophication of lakes. 4. Fertilization caused turbid water states by increasing phytoplankton content and light attenuation. The presence of neighbouring plants reduced phytoplankton growth and promoted survival and biomass production of macrophytes under eutrophication. 5. Synthesis. Indirect facilitation among plants can buffer the direct negative interactions between aquatic plants and phytoplankton. Indirect facilitation may prevent the decline in aquatic plant diversity of freshwater ecosystems threatened by eutrophication. Because the experimental design used here is comparable to the manipulations frequently carried out across a wide range of terrestrial ecosystems, this study may contribute to the comparison of patterns and processes in aquatic and terrestrial environments.