Endemic macrophyte is more plastic than two cosmopolitan species in fluctuating water levels and nutrient-enriched conditions.
Plant communities within many of the world's waterways are losing diversity where flows are stabilised for security and enriched from land-use intensification. Understanding the phenotypic plasticity of plant species that protect and/or store their below-ground biomass during floods may help to promote them if problematic, overabundant species - like Phragmites australis and Typha domingensis - are less plastic. To investigate mechanisms underlying the plasticity of two cosmopolitan (Phragmites, Typha) and two endemic (Cycnogeton procerum, Cyperus gymnocaulos) macrophytes in response to disturbance (fluctuating hydrological regimes) across a nutrient gradient, we analysed historical data from pond experiments with single-species pots. Our analyses showed fluctuating water levels in nutrient-enriched conditions reduced total biomass in all species, with 65% and 46% reduction in Phragmites and Typha respectively. Contrary to expectations, only Cycnogeton allocated a higher proportion of biomass to protect (root) and store (rhizome) resources in fluctuating water levels and nutrient-enriched conditions (45% versus 23% in stable regime), and was the most plastic by allocating 75% more biomass to belowground in the least favourable conditions. Our results indicate that removing impediments to stochastic flooding disturbance could benefit some endemic species while reducing the productivity of overabundant Phragmites and Typha in environments enriched by human activity.