Seasonal changes of ecological characteristics of pelagic Mollusc in Yangtze River estuary

Based on the data of four seasonal investigations in Yangtze River estuary(28°00′-32°00′N,122°00′-123°30′E)from the year 2002 to 2003,the distribution pattern of pelagic mollusc,the seasonal changes of total abundance and the species composition were analyzed.Furthermore,environmental adaptation of dominant species was deduced through scatter plot of abundance and temperature-salinity.The results showed that the seasonal abundance were 64.61 ind/m3 in spring,191.84 ind/m3 in summer,59.11 ind/m3 in autumn,and 1.4 ind/ m3 in winter.There were twenty-four species in four seasons.The mollusc species number peaked in summer (21 species),followed by autumn (10 species),spring (8 species) and winter (2 species).Numerically,Limacina trochiformis was the main dominant specis in spring,attributed to its low temperature adaptation.Atlanta peroni was the dominant species in summer and Creseis acicula was the dominant species in autumn due to their adaptation to relatively high temperature.Agadina stimpsoniji was the dominant species in winter,adapted to low temperature.In spring,pelagic mollusc aggregated in the waters where Yellow Sea Coastal Current meets Changjiang Diluted Water.While in autumn,the pelagic monllusc were abundant at the warm water side of the mixed water mass where the Taiwan Warm Current meets Yangtze River diluted water.The pelagic mollusc species number was few in the low temperature of winter,because most pelagic mollusc is warm water species.Abundance and species number were low in the nearshore than in the offshore of the East China Sea,because most of the pelagic mollusc are offshore water species.