Abstract: Supported by GIS and SPSS,the horizontal distribution of phytoplankton quantity and its correlation with water nutrients during highstand and lowstand seasons in culturing waters of Kaozhou Bay were studied with data interpolation,correlation analysis and regression analysis methods.The results showed that during the highstand,the distribution of phytoplankton quantity was even with the dense area locating in the estuary of Jilong River,while during the lowstand,it varied greatly with the dense area occurring in the waters from the mouth of the bay to the north of Yanzhou Island,which showed an increasing tendency from the northwest to the mouth of the bay.Comparing the two survey seasons,the phytoplankton quantity during highstand was obviously higher than that in the lowstand.The correlations between phytoplankton quantity and inorganic nitrate or phosphate were not significant,while there was a significant correlation between phytoplankton quantity and silicate which showed positive during the lowstand and negative during the highstand.

Key words: Reduction of N application, Wheat-maize cropping system, Residual nitrate N, N balance