Study On The Zooplankton Functional Groups In Taiwan Strait During Spring,Summer And Autumn Of 2015

Three oceanographic surveys were conducted in Taiwan Strait(TS)during spring(March to April),summer(June to July)and autumn(November)of 2015.The study area was geographically divided into two regions:the north and the south.Based on the data collected from those cruises,the seasonal and regional variations of species composition,dominant species,horizontal distribution and biodiversity of zooplankton in Taiwan Strait were studied in this paper.According to the size spectrum and feeding habits,the main zooplankton were classified into six functional groups:medusae(Med),giant crustacean(GC),herbivorous micro-crustacean(HMC),carnivorous micro-crustacean(CMC),chaetognath(CH)and tunicata(Tu).The horizontal distributions of the six groups in three seasons were illustrated respectively,and the main trophic structures of zooplankton in Taiwan Strait were outlined based on the results.Pearson correlation and redundancy analysis revealed correlations between functional groups and environmental variables,and between different groups.The investigated environmental variables included sea surface temperature(SST),sea surface salinity(SSS),chlorophyll-a(Chl.a)and dissolved oxygen(DO).The purpose of this study was to improve the understanding of the function of zooplankton to marine ecosystem and to accumulate reference data for further ecosystem modeling.The main results are as follows:1)The composition and distribution of zooplankton in TS274 zooplankton species and 33 groups of planktonic larvae were recorded from the samples collected on all the three cruises.Copepod and medusa were the two most dominant taxa in all seasons.Dominant species were seasonally and regionally varied:Calanus sinicus was most dominant species during spring in the north region,followed by Sugiura chengshanense;during summer,Flaccisagitta enflata had highest dominance in the north ragion while in the south region there was mainly Doliolum denliculatum during autumn,the number of Euchaeta concinna was the highest in the north region while Subeucalanus subcrassus dominated in the south region.The abundance of zooplankton was from 28.03 to 2468.25 ind./m3,and the biomass was from 79.44 to 1260.40 mg/m3.Both of abundance and biomass reached the highest in summer and were basically higher in coastal areas except that the abundance nearby the Taiwan Shoal was always high.The average abundance and biomass of zooplankton in the south region were higher than that in the north.2)Spatial and temporal distribution of zooplankton functional groups in TSMed were much more abundant during the spring and the summer,basically aggregating in coastal areas,the abundance of which could reach as high as 340.46 ind./m3.Med declined sharply in autumn,distributed as patch pattern.Compared with the other two seasons,the abundance of GC was remarkable higher during summer,at which time the aggregation areas of GC was located in the west side of Taiwan Shoal in the south region(around station A3)and in coastal area in the north region.The ratios of HMC were high in all the three seasons,and this group was more abundant in spring and summer,similar with Med.The horizontal distribution of HMC varied with the seasons switching:during spring,HMC was fairly similar to that of Med,while it aggregated in station A3 during summer and the trend of aggregation toward south was promoted during autumn,at which time another high-abundance zone of HMC showed up in station B1 nearshore in the west of studied area.CMC was the most stable functional group,especially in autumn when the total abundance of zooplankton decreased.CH was the third dominant functional group following HMC and Med,abundant during summer too.The distribution pattern of CH was similar to Med especially in the north region.Tu fluctuated widely:it was so rare during the spring that an blank area was observed near Haitan Island,but the abundance of Tu dramatically increased in summer,aggregating south forward,dominating all the other functional groups in the south region.In autumn,Tu drops again,aggregates in off-shore area.3)Influence factors to zooplankton functional groupsBase on the spatial and temporal variations of zooplankton functional groups,horizontal distribution of SSS and SST,and the results of statistic analysis,it was inferred that zooplankton functional groups were related to the shift of two main water masses,the Zhejiang-Fujian Coastal Water(ZFCW)and the Strait Warm Current Water(SWCW).Basically,Med and HMC preferred to aggregate in the region controlled by ZFCW while Tu was a good indicator of the strength of the warm current.In most cases,Euchaeta concinna was the typical species of CMC,tending to distribute in cold water.The exception occurred in the south area in summer where Pareuchaeta russelli aggregated in the Taiwan Shoal upwelling region.The proportion of Tu was high in summer,and the distribution of this fuctional group was mainly influenced by SSS.The amount of Ch1.a decreased dramatically in high-abundance zone of Tu,which indicated strong stress effect of that group on phytoplankton.The high synchronization of Med,CMC and DO in the north region in spring illustrated that DO was also an important factor to zooplankton functional groups.Additionally,positive correlation between different functional groups revealed the abundance of zooplankton functional groups was principally controlled by lower trophic level.