Inhibitory effects of Cynodon dactylon from a reservoir drawdown zone on Microcystis aeruginosa and Scenedesmus quadricauda

Abstract The dominant plant Cynodon dactylon in the drawdown zone of Danjiangkou Reservoir may release allelochemicals affecting adjacent algal communities under submerged conditions, but its impact on the growth and interactions of competing algal species remains unclear. This study examined the effects of Cynodon dactylon aqueous extracts on Microcystis aeruginosa and Scenedesmus quadricauda through monoculture and three co-culture treatments with initial ratios of 75%:25%, 50%:50%, and 25%:75%. Extracts (0, 0.5, 1.5, 3.0 g L −1 ) were applied over 16 days, monitoring cell density, specific growth rate, and modeling growth/competition dynamics using Logistic and Lotka-Volterra models. Key findings: (1) In monoculture, Microcystis aeruginosa outperformed Scenedesmus quadricauda ; without extracts, but extract additions suppressed both species dose-dependently, with stronger inhibition on Microcystis aeruginos a. (2) In co-culture without extract, Microcystis aeruginos a dominated; extract addition reversed this, with higher concentrations progressively suppressing Microcystis aeruginos a while inhibiting Scenedesmus quadricauda less due to its greater tolerance. (3) Lotka-Volterra modeling suggested the extract reduced Microcystis aeruginosa ’s carrying capacity and intrinsic growth rate, with competition coefficients changing quantitatively. (4) ANOVA revealed species, extract concentration, and initial proportion all critically influenced growth rates. Initial proportion had strongest explanatory power ( R 2 = 0.319), followed by concentration ( R 2 = 0.228) and species ( R 2 = 0.150). Significant interactions highlighted concentration and proportion dependent inhibition without altering core competition dynamics. These findings provide new insights into how riparian vegetation affects algal community structure in hydro-fluctuation zones, yet the ecological safety of these extracts for algal bloom control requires further evaluation with integrated toxin monitoring.