Community structure and seasonal variation of zooplankton in coastal Beibu Gulf

A comprehensive investigation on the population of zooplankton was conducted at 44 stations in the Beibu Gulf coastal area in the dry,wet and normal seasons in 2017. In total,251 zooplankton species including 24 larvae were identified during the investigation,among which 138,134,and 191 species occurred in the dry,wet and normal seasons,respectively. All zooplankton could be grouped into four ecotypes: estuary,offshore warm-water, offshore warm-temperature and eurytopic. There were 9 dominant species during the sampling period,with protozoa dominating the community in dry season,Cladocera,Copepoda and pelagic larvae dominating in the wet season, Decapoda and pelagic larvae in the normal season.The annual average abundance of zooplankton was 789.95 ind./m~3. Dry season had the largest abundance(1540.19 ind./m~3), followed by normal(457.58 ind./m~3) and wet(372.08 ind./m~3) seasons. The annual average biomass of zooplankton was 252.40 mg/m~3, with the highest biomass during normal season(385.01 mg/m~3),followed by dry(221.41 mg/m~3) and wet(150.78 mg/m~3) seasons.The highest diversity index(3.16) was found in normal season, followed by wet season(2.35), while the least diversity(2.22) presented in the dry season. Both the horizontal distribution of biomass and abundance of zooplankton increased gradually moving offshore in dry and wet seasons, while remaining relatively evenly distributed in normal season. Seasonal variation of biomass and abundance was determined by composition and dominant species. Correlation analysis shows that the distribution of biomass and abundance of zooplankton in the coastal waters of Beibu gulf are influenced by suspended matter and nutrients. Compared with the other bays, the population structure of zooplankton in the coast of the Beibu Gulf exhibited a clear tendency toward miniaturization that require additional attention.