Density‐driven facilitations increase ecological resilience under eutrophic stress

Abstract Eutrophication has been observed to decrease the ecological resilience of macrophyte‐dominated freshwater ecosystems, thereby resulting in more vulnerability to external perturbations and easily tipping into an algae‐dominated state. The stress‐gradient hypothesis (SGH) posits that plants exhibit positive plant–plant interaction (facilitation) in response to stress, potentially buffering the detrimental impacts of eutrophication. However, few studies attempted to link plant density with species‐species interactions and ecological resilience in the context of eutrophic stress. Here, we investigated how the density of neighbour plant species ( Potamogeton lucens ) affects the change rate of nutrients or Chl a (chlorophyll‐a) and target plant species ( Potamogeton maackianus ) along a gradient of nutrient levels (oligotrophic, mesotrophic, eutrophic) via a 42 days mesocosm experiment. Our objective was to corroborate: (1) species interactions may shift from competition to facilitation with the increased eutrophic stress. (2) High plant density is indispensable to generate facilitation and thereby augment ecological resilience under high eutrophic stress. Results showed that eutrophic scenarios significantly augmented chlorophyll‐a concentration and inhibited plant height, number of branches and leaves, showing that submerged macrophytes in eutrophic states are exposed to stressful conditions. However, the increasing density of neighbouring vegetation reduced nutrient and Chl a concentrations and enhanced the performance of Potamogeton maackianus in eutrophic conditions, but not in mesotrophic and oligotrophic conditions. Our results indicated that density‐dependent facilitation is more prevalent in eutrophic circumstances, and species interactions are likely transformed from competition to facilitation with increasing nutrient concentrations. Our research demonstrated that plant density could alter the relationship between facilitation and competition; high plant density is indispensable for the operation of the stress‐gradient hypothesis. Density‐dependent facilitation under severe conditions could mitigate the adverse influence of eutrophication. With freshwater ecosystems progressively subjected to eutrophication, harnessing the density‐dependent facilitation of submerged macrophytes in resilience‐driven management is crucial to inhibiting the transition to an algae‐dominated turbid state, which further broadens our understanding of the theory of alternative stable states in shallow lakes.