DIEL VERTICAL MIGRATION OF ZOOPLANKTON IN SANYA BAY,HAINAN PROVINCE, CHINA

Zooplankton was collected by diel vertically-stratified sampling in November of 1998 and January, April and August of 1999 at a station in the Sanya Bay which is located in 109°28′6″E, 18°13′36″N and has a water depth of 15m. The results showed that the characteristics of diel vertical migration of total zooplankton abundance had an obvious seasonal variation. In spring, zooplankton abundance centralized in upper and middle layers (0-10m); while in summer, zooplankton migrated vertically, sinking by day and rising by night. The zooplankton in the Sanya Bay could be grouped into two types, i.e. species with and without evident diel vertical migration. The species with evident diel vertical migration could be further divided into three types: 1) species sinking by day and rising by night; 2) species rising by day and sinking by night; 3) species rising at dusk and dawn, sinking by day and night. The species without evident diel vertical migration could be also divided into two types, i.e. species mainly distributed in upper and middle layers and species equably distributed in each layer. Besides these, the diel vertical migration of some species was irregular. Light was an important factor affecting the diel vertical migration of zooplankton. The diel vertical migration of zooplankton was irrelevant to the phytoplankton density. In summer, macrozooplankton Sagitta enflata and Subeucalanus subcrassus could exceed the top of thermocline into surface layer. The extension of diel vertical migration of zooplankton appeared to be related to individual size, water depth and natural behavior of species.