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Interspecifically induced phytoplankton colony formation, which is accompanied by increased particle size and sedimentation, plays important roles in enhancing anti-predation capability and alleviating competition among photosynthetic organisms. Induced morphological changes may indirectly affect the structure of food webs and thus influence ecosystem functions. In this study, the effects of microcystin-LR (MC-LR) on colony formation of Chlorella vulgaris under induction by the submerged macrophyte Potamogetom crispus were evaluated. The growth of C. vulgaris was significantly inhibited under P. crispus stress (10g FW L−1), and the adverse influences of extracts were considerably greater than those of exudates. In addition, the cell numbers per colony and colony proportion of C. vulgaris were significantly increased with the presence of P. crispus extracts. However, in contrast to our expectation, the addition of MC-LR exerted no significant effects on the growth and morphological changes of C. vulgaris under P. crispus induction. The present research results provide additional knowledge on interspecific interactions between submerged macrophyte and green algae in the eutrophic cyanotoxin-contaminated ecosystems. [ABSTRACT FROM AUTHOR]