Time-varying Characteristics Of Zooplankton Biomass In The Northern South China Sea Based On Acoustic Scattering

In this dissertation,a set of submerged buoy-seabed-based observation systems deployed in the northern waters of the South China Sea are used to invert the acoustic backscatter data of ADCP to obtain the time series of plankton biomass.Combined with the ocean current data and temperature data from synchronous observations,as well as the cloud cover and shortwave radiation data from the ERA5 dataset,the time-varying characteristics of zooplankton biomass in this sea area are analyzed,and the possible mechanisms affecting the changes in zooplankton biomass are discussed.The dissertation mainly discusses the following four time-scale changes:(1)Diurnal characteristics.Affected by sunlight,the zooplankton in the observation sea area has a typical vertical migration of day and night,that is,during the day,the zooplankton gathers in the waters shallower than 130 m at night,and gathers in the water layer at the depth of 180-236 m during the day.(2)Half-moon variation characteristics.The zooplankton biomass in the whole water body has a semi-monthly change,which is shown as follows: during the neap tide,the zooplankton biomass is more;during the spring tide,the zooplankton biomass is less.Two possible mechanisms were believed to responsible to the variation of the zooplankton biomass.On one hand,the strong tidal current during the spring tide led to the deterioration of the living environment of zooplankton and reduced its biomass.On the other hand,the strong tidal current during the spring tide changed the vertical migration characteristics of zooplankton,which migrated to the sea bottom,making it difficult to be observed..(3)Intra-seasonal variation characteristics.Among the three months of August,September and October,the zooplankton biomass was the least in September.Combined with the cloud cover and shortwave radiation data in this sea area,this dissertation believes that light is the main factor causing the intra-seasonal variation,and the light change is caused by the least cloud cover controlled by high pressure in the sea area in September.(4)Seasonal variation characteristics.By extracting the overall trend of zooplankton biomass during the observation period,it can be seen that the zooplankton biomass is larger in autumn than in summer.Combined with previous studies,this dissertation suggests that in addition to light,seasonal variation in water temperature may be an important factor in the seasonal changes in zooplankton biomass.