Abstract: Based on the investigation data during the high-water (July,2003) and low-water (January,2004) periods,a causal analysis was made on the variation of zooplankton distribution in the north branch waters of the Changjiang Estuary.The results showed that in high-water period,the average of zooplankton biomass was 234.38 mg·m^-3,being 141.35 mg·m^-3 in flood tide and 327.40 mg·m^-3 in ebb tide,while in low-water period,it was 188.81 mg·m^-3,being 184.69 mg·m^-3 in flood tide and 192.93 mg·m^-3 in ebb tide.The biomass increased from the east to the west in flood tide,but a contrary trend was observed in ebb tide.The species number did not change obviously both in flood tide and in ebb tide.The value of diversity index (H') was higher in flood tide than in ebb tide.In high-water period,the biomass near the north shore was higher than that near the south shore,but it was contrary in the ebb tide.The difference between the waters of two shores was not obvious in low-water period as in high-water period,though the trend of biomass variation was similar.The variation of zooplankton distribution in the north branch waters of the Changjiang Estuary had a close relation with the seasonal changes of zooplankton biomass outside the Changjiang Estuary and the tide,but not significantly related with the Changjiang runoff water.Coriolis force accounted for the difference of zooplankton biomass in the waters of two shores via tide movement.

Key words: Landscape, Isolation, Type divergence, Nearest neighbor index, Dispersion index