Eutrophication drives regime shift via changes in stoichiometric homeostasis-based submerged macrophyte assemblages

Homeostasis for phosphorus (HP) in submerged macrophytes may influence the susceptibility of lakes to regime shifts; however, the mechanisms linking submerged macrophyte HP to regime shifts remain unclear. We conducted an in situ mesocosm experiment to compare the dynamic responses of a high-HP species, Potamogeton maackianus , and a low-HP species, Hydrilla verticillata , to different phosphorus (P) level gradients, as well as their effects on phytoplankton inhibition. The biomass of P. maackianus under mesotrophic P (MP; P concentration 0.05 mg L −1 ) and eutrophic P (EP; P concentration 0.10 mg L −1 ) conditions was either non-significantly different from, or lower than that under oligotrophic P conditions (OP; P concentration under detection limit of 0.01 mg L −1 ). Conversely, H. verticillata biomass under EP was significantly higher than that under MP on day 90, whereas it died under OP. This variable response of submerged macrophyte species to P level gradients increased the relative growth advantage of H. verticillata compared to P. maackianus during eutrophication. The inhibition ratio of phytoplankton (IRP) for P. maackianus was ~15 times higher than that for H. verticillata under EP. Our study demonstrated a trend that submerged macrophyte assemblage IRP increased along with its assemblage HP. Thus, the changes in submerged macrophyte assemblages from high-HP species-dominance to low-HP species-dominance would erode its phytoplankton inhibition capacity, and further promote the regime shift from a clear-water state to a turbid state. Our results advance the regime shift theory from an ecological stoichiometry perspective and highlight the importance of high-HP submerged macrophyte species in the restoration of eutrophic lakes.