Due to the large-scale application of Eichhornia crassipes (Mart.) Solms on the bioremediation of eutrophic lake in China, the influence of growth, physiological characteristics, microcystins production and release of M. aeruginosa by E. crassipes was investigated. Meanwhile, the release risk of nutrients from M. aeruginosa and the accumulation risk of microcystins in E. crassipe were explored through semi-continuous co-existence experiments. Our results indicated that M. aeruginosa was promoted by E. crassipes to undergo the cell death. Under the stress of E. crassipes, direct damage of phycocyanin and phycocyanin/allophycocyanin ratio in M. aeruginosa occurred, while the photosystem II-Hill reaction in M. aeruginosa was not interrupted. The PC/APC levels in the treatment of 10% and 20% water exchange rate were respectively decreased to 54.93% +/- 7.07% and 55.81% +/- 1.97% of the level in their relative controls after 8 days. Then, the final significant decrease of specific superoxide dismutase activity and the striking elevation of malondialdehyde content in M. aeruginosa could be the results of oxidative damage by E. crassipes. Algal malondialdehyde content in the treatment of 10% and 20% water exchange rate were respectively 2.95 +/- 0.074 and 2.22 +/- 0.086 times of the level in their relative controls on day 8. The release of nutrients from M. aeruginosa was accelerated because the decay and lysis of algal cells were promoted by E. crassipes. After 12-day co-existence experiments, the concentration of total dissolved nitrogen in water was brought back to the initial level and the release of total dissolved phosphorus was faster than nitrogen nutrients under the stress of E. crassipes. In addition, the microcystins production in M. aeruginosa was not stimulated and the extracellular microcystins were significantly eliminated by the influence of E. crassipes. The extracellular microcystins contents in the treatment of 10% and 20% water exchange rate were respectively decreased to 12.07 microg x L(-1) +/- 0.63 microg x L(-1) and 11.36 microg x L(-1) +/- 0.04 microg x L(-1) after 12 days. But the microcystins level in the whole plants of E. crassipes co-cultured with M. aeruginosa for 12 days was only about 5.95 ng x g(-1) +/- 0.76 ng x g(-1) FW. The increase of water exchange rate could reduce the damage of M. aeruginosa by E. crassipes, but had no significant effect on the microcystins elimination.