Seasonal characteristics in the water masses in Zhoushan fishing ground and adjacent region

The Zhoushan fishing ground is located in the shallow-water area of the northwest East China Sea.The formation and displacement of the fishing ground and variations in fishing season and fish crop are mainly affected by hydrologic conditions and nutrient contents there, whose variations, to agreat extent, depend on the occupancy range and vanishing-growing changes in water masses.Using the data observed before the year of 1990, the water masses in the northwest East China sea have been studied by some scientists at home and abroad, but the study on the water mass in this sea area after completion in the three gorges engineering has been not reported sofar.In this work, based on CTD and nutrient data obtained during the summer of 2001 and the winter of 2002, the water masses in Zhoushan fishing ground and adjacent region were divided by using the fuzzy clustering method, and the seasonal characteristics in distributions, thermohaline properties and nutrient contents in the water masses in this area were analyzed.The results showed that there are distinct seasonal characteristics in the distributions, ranges, therm ohaline property and nutrient contents of the water masses in this area.In the whole study area, there are three water masses, namely, Jiang-Zhe Coastal Water (JCW), Taiwan Warm Current Surface Water(TWCSW) and Yellow Sea(Huanghai Sea) Mixing Water (YSMW) in winter, while there are four water masses, namely, the JCW, the TWCSW, Taiwan Warm Current Deep Water (TWCDW) and the YSMW in summer.The JCW has a relative small occupancy range, low temperature, high salinity and high nutrients in winter; while it has a relative large occupancy range, high temperature, low salinity and low nutrients in summer.The TWCSW extends farther northwards in winter than in summer and its thickness is much larger in winter than in summer.In the TWCSW, temperature is low, salinity is high, SiO₃-Si and NO₃-N are relative high, and PO₄-P is low in winter, while the temperature high, salinitylow, SiO₃-Si and NO₃-N relative low, and PO₄-P high in summer.The TWCDW is a seasonal water mass, and there are relative rich nutrients in it.There are some seasonal charact eristics in the occupancy range and nutrient contents of the YSMW.