Abstract: Algal blooms caused by eutrophication in fresh water are one of the major environmental problems in the world. Using
biological methods to control algal growth, especially based on allelopathic inhibitory effects of aquatic macrophytes on
phytoplankton growth, have been received world-wide attention. In this study, the allelopathic activity of the invasive macrophyte,
Eichhornia crassipes (water hyacinth), on blue-green algae, Microcystis aeruginosa (PCC7806) was investigated using coexistence
assay. Our results showed that water hyacinth had disparate effects on the growth of M. aeruginosa (PCC7806) under different initial
algal densities. Under lower initial algal density (OD650 = 0.10 and OD650 = 0.05), the algal growth was significantly inhibited by
water hyacinth (inhibition ratio was 95.6% and 97.3%, respectively). While it was stimulated at higher initial algal densities (OD650
= 0.20). Water hyacinth inhibited the growth of algae mainly through its root system. Culture water from water hyacinth and aqueous
methanol extracts from dry roots samples also showed inhibition effects on algal growth. The inhibition effects increased as the
increase of crude extract concentration, suggest that water hyacinth may excrete inhibitory substances from root system and show
allelopathic inhibitory potential to the growth of M. aeruginosa.