Flow regime and nutrient input control invasive alien aquatic plant distribution and species composition in small closed estuaries.

Nutrient pollution is facilitating the encroachment of invasive aquatic plants in various water bodies globally. This study investigated seasonal aquatic macrophyte responses in two temporarily closed estuaries with different nutrient inputs. Consistent effluent discharge from the upstream wastewater treatment works (WWTW) facilitated the establishment of numerous freshwater invasive alien aquatic plants (IAAPs) in the uThongathi Estuary. IAAPs (Myriophyllum aquaticum, Pistia stratiotes and Pontederia crassipes) were only displaced from the estuary after high flow events (>5 m s). In the less polluted uMdlotane Estuary nutrient pulses (>1 mg/L DIN) associated with high rainfall changed the aquatic macrophyte composition. The nutrient tolerant non-rooted Ceratophyllum demersum outcompeted the rooted submerged macrophyte Stuckenia pectinata. Species composition changed in response to flow and nutrient inputs, with the exception of emergent macrophytes, such as the grass Echinochloa pyramidalis, that remained consistent in cover and distribution in the uMdlotane Estuary. This study demonstrated that aquatic macrophytes are more responsive to nutrient inputs in unimpacted estuaries compared to consistently nutrient-rich systems where flow is an important driver of IAAPs community dynamics. Many temporarily closed estuaries are subjected to nutrient pollution from WWTWs and restoration efforts such as diversion of discharges to constructed wetlands needs urgent implementation.