Changes and Influencing Factors of the Zooplankton Community in the Eutrophic Waters of Jinghai Bay

Abstract

Coastal ecosystems are critical to biodiversity owing to the strong interactions between terrestrial and marine environments. Zooplankton are important secondary producers and major drivers of biological pumps in marine ecosystems and changes in their communities may affect coastal ecosystems. Therefore, it is important to comprehensively evaluate the effects of various environmental factors on the changes in zooplankton communities in coastal ecosystems. Previous research has been limited to a single study on the environmental quality or biological communities in Jinghai Bay, and there have been no relevant reports on the analysis and evaluation of the relationship between changes in zooplankton communities and environmental factors in Jinghai Bay. To study the relationship between zooplankton communities and environmental factors in Jinghai Bay, four survey voyages of environmental factors and zooplankton were conducted in May (spring) and August (summer) from 2021 to 2022. Redundancy analysis (RDA) and Pearson correlation analysis were used to explore the effects of environmental factors on dominant zooplankton species and communities. The results showed that the degree of eutrophication in Jinghai Bay increased significantly from 2021 to 2022, particularly during the summer of 2022 (average eutrophication index EI > 200). The distribution of EI showed that the EI was higher in estuaries in the surveyed sea area, and the source of nutrients may be river-diluted water. In addition, there were significant seasonal changes in salinity, especially in the summer of 2022 (the average salinity dropped to 12.06). The average zooplankton abundance in 2021 was higher than that in 2022, and in the summer of 2022, when eutrophication was most severe, the average zooplankton abundance dropped to 54.07 ind./m3. However, during the summer of 2022, which had the lowest abundance, the biomass did not show the same proportion of decline, which requires further study. In addition, Principal Co-ordinates Analysis (PCoA) revealed significant differences between the 2022 summer zooplankton communities and the previous three voyages. A total of 47 species of 10 groups of zooplankton were identified during the four voyages, among which copepods were the most dominant species in spring and pelagic larvae in summer. Acartia pacifica and Zoea larva (Brachyura) were the dominant species on all four voyages. From 2021 to 2022, the dominance of Acartia pacifica showed a continuously decreasing trend. In the spring and summer of 2022, Zoea larva (Brachyura) became the dominant species in the sea area, but the relative abundance and dominance of Zoea larva (Brachyura) decreased in summer. The number of dominant species, across both years, reached an apex (7 species) in the summer of 2022. The interannual and seasonal variation trends of the zooplankton communities, as shown by the Shannon-Wiener diversity index (H′) and Pielou's evenness index (J), in Jinghai Bay were similar. In terms of seasonal changes, H′ and J in spring 2021 were higher than those in summer. In 2022, the opposite trend was observed, with H′ and J higher in summer than in spring. From the perspective of inter-annual change, both the H′ and J in 2022 were higher than those in 2021. Generally, from 2021 to 2022, the trend of change of zooplankton biodiversity in the Jinghai Bay area gradually increased. RDA showed that the dominant species, Acartia pacifica and Zoea larva (Brachyura), which appeared in all four surveys, were negatively correlated with EI during spring and summer in both years. Eurytemora pacifica, which only appeared in spring, was positively correlated with salinity during the spring of the two years, whereas in the summer of 2022, the newly emerged dominant species Penilia avirostris, Gammaridae, and Schmackeria poplesia were positively correlated with EI, dissolved inorganic phosphate (DIP), and chemical oxygen demand (COD), and negatively correlated with salinity. Pearson correlation analysis showed that EI was negatively correlated with the abundance of zooplankton in all four voyages, positively correlated with abundance and biomass in spring, and negatively correlated with abundance and biomass in summer. In addition, Pearson correlation analysis showed that salinity had an impact on the zooplankton diversity index. Based on the correlation analysis of the RDA and Pearson correlation coefficient, eutrophication was found to be the main environmental factor affecting the community structure and abundance of zooplankton in Jinghai Bay, and salinity affected the seasonal variation in zooplankton. In addition, through PCoA analysis and comparison of data from 2021 and 2022, it was found that occasional events, such as typhoons, may also be an important reason for changes in the zooplankton community. The results of this study will be conducive to a follow-up study of the ecosystem of Jinghai Bay and understanding the factors controlling the dynamics of its zooplankton community, which in turn will contribute to the scientific management of the ecological environment of Jinghai Bay and an in-depth understanding of the mechanisms of zooplankton community change in the bay.

Keywords

• eutrophication
• salinity
• typhoon
• zooplankton
• community structure
• jinghai bay