Abstract: The recent years have wit nessed the continual expansion of red tides in the coastal waters of the world and its serious damage to marine fishery industries and threats to human health.For mitigation and prevention of red tide disasters,resear chers have at tached great attention to the studies on the prewarning and forecasting of red tide events.Based on the microsm experiments this paper discusses the correlation between phytoplankton diversity and apparent oxygen increment(AOI)and its application to the prewarning of red tide events.The water samples for the experiment were collected at surface waters respectively from the stations near Baozhu Islet at the inner bay and Qingyu Islet at the outer bay of Xiamen.Each water sample was subsampled into five boron-silicate glass bottles each of 500 cm³ in the filed and shipped back to land.Two groups of subsam ples were then put into a open pond of 1 m deep and let the phytoplankt on in the subsamples to grow naturally under the natural sunlight and water temperature.During the experimental period,one subsample was taken from each group for the ident ification and counting of phytoplankt on and the determination of physical and chemical parameters.The experiment results indicate that in the fo rmation process of a red tide event,phytoplankt on diversity index Hdecreased greatly while dominancy D₂ increased remarkably.The papertakes Apparent Oxygen Increment(AOI,the differences between field concentration of dissolved oxygen and the saturated oxygen concentration under field temperature and salinity conditions)as the measurem entindex for phytoplankt on biomass during the red tide event in the study on the correlation between H' and D₂ and phytoplankt on biomass.The results indicated that H' and AOI were highly negatively correlated and D₂ and AOI highly exponentially.The correlation implies that during the red tide formation process,there is a cause and effectrelation ship between phytoplankt ondiversity and biomass.Based on the correlation modes between H' and D₂ and AOI and by taking AOI index in red tide assessment as the reference,the paper proposes the index values for both H' and D₂ for red tide assessment.The test by field monitoring data indicated that these two indexes can be used as the supplementary indexes in the assessment of red tide strength.