Comparison of capture efficiency for zooplankton in the northern South China Sea,using two plankton mesh sizes

Zooplankton is a categorization spanning a wide range of organism sizes. Different mesh sizes of the plankton net used in zooplankton research can lead to different results. In order to understand whether the zooplankton community structure has been altered by different mesh sizes of the plankton net in the northern South China Sea, we investigated the catching efficiency in August 2007 using two plankton nets of different mesh sizes. The mesh sizes of large and medium nets are 505 and 160 μm, respectively. The results show that the capture efficiencies of the two nets are not significantly different for large size zooplankton, such as medusae, euphausiids, chaetognathes, and so on. Species of larger zooplankton caught by the large net are higher than those by the medium net. The difference between the two plankton nets was mainly due to the loss of small-size copepods in the samples collected by the 505 μm mesh net. The abundance of 13 dominant small copepods(Temora turbinate, Paracalanus gracilis, Oncaea venusta, and so on) caught by the medium net was 11 times of that by the large net. It suggested that two plankton nets can compensate each other. In addition, although species composition of zooplankton differed in samples collected by nets of different mesh sizes, the zooplankton communities from the two nets are similar under cluster analysis. Zooplankton communities from both nets can be divided into onshore and offshore groups, either with large or medium size plankton net.