Effects of tide and water masses on the distribution of zooplankton in different parts of Hangzhou Bay

Based on the data collected from three oceanographic surveys in the east area of north (NE) (121.67°E–121.87°E, 30.68°N–30.83°N), the east area of south (SE) (121.60°E–121.85°E, 29.95°N–30.24°N), and the west area of north (NW) (121.31°E–121.56°E, 30.58°N–30.77°N) Hangzhou Bay during the autumn of 2009, 2011 and 2012, we analyzed the distribution patterns and composition of dominant zooplankton species in the three areas. The effects of the tide, water masses, and environmental factors on the distribution patterns of zooplankton were explored as well. The results indicated that the total biomass increased gradually from nearshore to offshore in the NW, while the total abundance increased from offshore to nearshore. In the NE, the total biomass increased from offshore to nearshore, however the peak abundance occurred in the central survey waters. In the SE, both total biomass and total abundance increased from offshore to nearshore. The average biomass of zooplankton in the NW, NE and SE was 70.78 mg/m3, 84.10 mg/m3, and 580.58 mg/m3, respectively. The average abundance in the three areas was 47.98 ind/m3, 98.89 ind/m3, and 578.88 ind/m3, respectively. The analysis of variance shows that there was a significant difference in the zooplankton abundance in the different parts of Hangzhou Bay; and there was a significant difference in dominant species in the three areas as well. The primary dominant species in the NW were Tortanus vermiculus, Labidocera euchaeta and Paracalanus aculeatus in the NE; and Labidocera sinilobata in the SE. The difference in zooplankton distribution in the three areas may be majorly influenced by the runoff strength of the Yangtze River and Qiantang River, seawater invasion from the East China Sea, and the difference of tidal currents between the north and south parts. The association between the distribution of zooplankton and environmental parameters, which was calculated by the stepwise regression analysis, indicates that temperature and salinity are the two major factors that influence the distribution pattern of zooplankton in the three different parts of Hangzhou Bay.