We investigated the effect of temperature and light intensity on the inhibition of growth and photosynthetic activity of the cyanobacterium Microcystis aeruginosa mediated by eight allelochemicals (five polyphenols and three fatty acids) produced by the macrophyte Myriophyllum spicatum. Analysis of specific growth rate and maximum photosystem II quantum yield revealed the reduction in photosystem II activity to be the major mechanism responsible for growth inhibition of M. aeruginosa. The degree of growth inhibition varied with changes in temperature (20-30 degrees C) and light intensity (25-75 mu mol m(-2) s(-1)). The growth-inhibitory effect was greater at lower temperatures and light intensities; the decrease in M. aeruginosa growth at 20 degrees C was 1.9 times the rate observed at 30 degrees C, and the decrease at 25 mu mol m(-2) s(-1) was 1.5 times that at 75 mu mol m(-2) s(-1). These results therefore demonstrate that variation in water temperature and light intensity should be considered when estimating the potential effects of macrophytic anti-cyanobacterial allelochemicals in aquatic environments.