A Study On The Zooplankton Community And Trophic Structure In Bohai Bay

Zooplankton is an important part of the marine ecosystem and plays a"pump"role in the ocean material cycle through the relationship between consumers and producers.In order to understand the role of zooplankton in the material cycle and energy flow of the marine ecosystem in Bohai Bay,this study was carried out in the Bohai Bay waters（117°72′-119°14′E and 37°98′-139°98′N）in the summer（July-August）and autumn（October-November）in 2020 for comprehensive marine surveys such as hydrology,chemistry and biology,17 and 19 sampling stations were set up in summer and autumn respectively.The characteristics of zooplankton community structure were analyzed and studied,and the trophic structure of zooplankton was studied by combining particle size classification and carbon and nitrogen stable isotope methods.In the community structure study,87 and 67 zooplankton species were identified in the summer and autumn in 2020,respectively.In summer,zooplankton larvae became the dominant group,accounting for27.59%of the total zooplankton species.Copepods were the second most dominant group,accounting for 26.44%of the species.In autumn,copepods were absolutely dominant and were the total zooplankton species.32.84%of the total,and 22.39%of zooplankton larvae.The abundance of zooplankton was higher in summer than in autumn.The range of zooplankton abundance in summer was 299.00～28219.47 ind./m³.Among them,the abundance of zooplankton larvae was the highest,followed by copepods.In summer,the abundance of cladocerans which in the bay and bay mouth waters was very high.The range of zooplankton abundance in autumn was 685.11～10807.00 ind./m³.The abundance of copepods was the largest,followed by pelagic larvae.The dominant group of chaetognatha in the third autumn was copepods,followed by pelagic larvae.There were 6 dominant species in both summer and autumn in Bohai Bay and the dominant species shared in summer and autumn were Paracalanus parvus,Oikopirura dioica,and Sagitta crassa.In addition,the dominant species in summer were Pseudevadne tergestina,Pseudevadne tergestina,and Pleurobrachia globosa,and the dominant species in autumn were Cotycaeus affinis,Cotycaeus affinis,and Oithona similis.The correlation analysis between zooplankton and environmental factors showed that temperature and salinity were the main factors affecting community structure.In the trophic structure study,zooplankton were divided into 5 sizes（200-500μm,500-1000μm,1000-2000μm,2000-4000μm,>4000μm）.Among the fractionated zooplankton,chaetognatha were the dominant group with>4000μm,and copepods were the dominant group for other fractions.In summer,the average biomass of zooplankton was 15.31±10.82 mg/m³,and in autumn,the average biomass of zooplankton was 13.89±9.06 mg/m³,and the biomass of 500-1000μm particle size was the highest.In summer,the total variation range of zooplanktonδ¹³C value in Bohai Bay is-28.61～-10.46‰,the total span is 18.15‰;the total variation range ofδ¹⁵N value is 3.00～14.95‰.In autumn,the total variation range ofδ¹³C value of zooplankton in Bohai Bay is-26.31～-12.51‰;the total variation range ofδ¹⁵N value were 5.54～14.27‰.The range ofδ¹³C was different in different regions,indicating that the carbon sources of zooplankton were different in different regions.Theδ¹³C span of plankton near the Haihe River was the largest,which may be because there was a large amount of terrestrial input at the estuary of the Haihe River,and the obtained carbon source was relatively abundant.Theδ¹⁵N of zooplankton was related to its trophic level.The range ofδ¹⁵N was different in different regions,indicating that the nitrogen sources of zooplankton were different in different regions.Theδ¹⁵N near the Haihe River was lower,which may be due to the restricted growth of zooplankton due to the high nutrient input from terrestrial sources in the nearshore area,which reduces the diversity and makes the species composition simpler,thus simplifying the food chain.Theδ¹³C andδ¹⁵N of zooplankton both increased with the grain size,indicating that there was a corresponding relationship between the size of the zooplankton and the level of its trophic level.Theδ¹⁵N values of the larger-sized zooplankton（>1000μm）were more dispersed than those of the smaller-sized zooplankton,indicating that the feeding habits of the larger-sized zooplankton were more complex and diverse.The average food chain length in summer was 3.48±0.95,and the average food chain length in autumn was 3.37±0.86.There was a food overlap between zooplankton size fractions in summer and autumn in Bohai Bay.This study provides scientific information for understanding the changes of marine ecosystems and biogeochemical cycles in the coastal waters of Bohai Bay.