Co-existing growth relationships of a lipid-producing alga Chlorella sp.HQ were investigated with Microcystis aeruginosa under different inoculation ratio(1:1,1:5 and 5:1).Chlorella outgrew Microcystis in all co-existing medium(SE,1/2 SE + 1/2 BG11 and BG11)under 1:1 inoculation ratio,and exerted a significant inhibition on Microcystis in 1/2 SE + 1/2 BG11 medium,which might be allelopathy.Under 5:1 inoculation ratio cell aggregation was enhanced on co-existing Chlorella compared with mono-cultures,which was a possible adaption to Microcystis toxin.Nutrient concentration in co-existing environments affected algal allelopathic interaction,including property(inhibition/stimulation)and extent.Monod model was used to predict possible biomass of Chlorella sp.HQ in open pond systems and N,P requirements.Analysis of excitation-emission matrix(EEM)from three-dimensional fluorescence spectra showed a good distinction between Chlorella sp.HQ and M.aeruginosa in different co-existing environments.A new quantitative method on algal density was developed by three-dimensional fluorescence spectra to provide reference for algal growth monitoring in open pond systems,which showed good linear correlation with haemacytometer measurement(r ≥ 0.92).