Abstract:Zooplankton are essential secondary producers that are pivotal in maintaining the stability of marine ecosystems. The Bohai Sea, which is known for its rich biological resources, serves as an important spawning and feeding ground for numerous economically important fish and shrimp species in northern China Sea. Studying seasonal variations in zooplankton communities and their relationships with environmental factors is vital for assessing the current status of the Bohai Sea ecosystem and promoting the sustainable development of marine ecosystems. In this study, we analyzed the community structure of zooplankton in the Bohai Sea using various indices, such as species turnover rate (A), dominance (Y), turnover rate of dominant species (R), Shannon-Wiener diversity index (Hʹ), Margalef richness index (d), and Pielou evenness index (Jʹ), based on survey data collected during two cruises in June (summer) and October (autumn) 2021. The results showed that 43 phytoplankton species were collected during summer, with an average abundance of (34,722.6±31,775.1) ind./m³. A total of 67 phytoplankton species were identified in autumn, with an average abundance of (464,376.6±422,191.0) ind./m³. The high-value zones for phytoplankton abundance in summer were mainly located in Laizhou Bay and adjacent waters, whereas the high-value zones shifted to the Bohai Bay entrance, and central and southern Bohai Sea in autumn. A total of 41 zooplankton species (taxa) were identified, including 14 copepod species, 13 pelagic larval taxa, 8 Hydrozoa species, 2 Amphipoda species, and one species each of Ctenophora, Cladocera, Chaetognatha, and Tunicata. Owing to the influence of high summer temperatures, the number of warm-water species significantly increased in autumn, yielding higher species richness during autumn (36 taxa) than during summer (30 taxa). Zooplankton abundance averaged higher in summer (5,460.0 ind./m³) than in autumn (4,116.9 ind./m³), with high-abundance zones occurring in warmer, lower-salinity marine areas. The spatial distribution of zooplankton abundance in summer was similar to that of phytoplankton, with primary concentrations in Laizhou Bay and Bohai Bay, whereas the distribution shifted to the central and southern Bohai Sea in autumn. The Hʹ, d, and Jʹ all exhibited higher values in autumn than in summer. The dominant zooplankton species in summer included the following: Acartia hongi, Oithona similis, Paracalanus parvus, Centropages abdominalis, Calanus sinicus, Acartia omorii, and Paracalanus crassirostris. Conversely, the dominant species in autumn comprised the following: A. hongi, P. parvus, P. crassirostris, O. similis, bivalvia larvae, Ditrichocorycaeus affinis, and Oikopleura dioica. Both P. parvus and A. hongi exhibited significant correlations with elevated temperatures and increased phytoplankton abundance. Redundancy analysis revealed that P. parvus and A. omorii were significantly positively correlated with phytoplankton abundance in summer (P＜0.05). Moreover, P. parvus and P. crassirostris were significantly positively correlated with temperature (P＜0.01) and negatively correlated with salinity and dissolved oxygen (P＜0.05). In autumn, D. affinis was positively correlated with temperature (P＜0.05), and O. dioica exhibited a highly significant negative correlation with salinity (P＜0.01). Cluster analysis classified zooplankton into three groups in both summer and autumn. During summer, Group A, located at select stations in the southern Bohai Sea, exhibited the lowest zooplankton abundance (371.0 ind./m³). Group B, located in the Bohai Bay and Laizhou Bay regions, had the highest zooplankton abundance (12,937.5 ind./m³). Group C encompassed the central and northern Bohai Sea regions. In autumn, Group D, primarily located in Liaodong Bay and the central–northern Bohai Sea, had the lowest zooplankton abundance (1,373.5 ind./m³). Group E, situated in the central Bohai Sea, had a relatively high zooplankton abundance (3,818.4 ind./m). Group F, mainly located in Bohai Bay and Laizhou Bay, had the highest zooplankton abundance (6,074.7 ind./m³). SIMPER analysis revealed that four zooplankton species, each contributing over 5%, accounted for cumulative contribution rates of 87.7% and 82.9% in Groups A and C, respectively. These species included A. hongi, O. similis, P. parvus, and C. abdominalis. In Group B, four species contributed 85.3% to the similarity: A. hongi, P. parvus, P. crassirostris, and O. similis. In Group D, five species accounted for 87.6% of the similarity: A. hongi, P. parvus, O. similis, bivalvia larvae, and P. crassirostris. In Group E, five species accounted for 83.2% of the similarity: A. hongi, P. parvus, bivalvia larvae, P. crassirostris, and D. affinis. In Group F, four species contributed 81.4% to the similarity: P. parvus, A. hongi, P. crassirostris, and O. similis. BIO-ENV analysis demonstrated that the best explanatory combination of factors for summer zooplankton community structure was surface temperature, surface salinity, bottom salinity, and phytoplankton abundance (ρ=0.303). Conversely, the best explanatory combination for autumn was surface salinity and surface dissolved oxygen (ρ=0.190). These findings are important for understanding the seasonal variation characteristics and dynamic changes in the marine ecological environment. They are crucial for assessing the current status of the Bohai Sea ecosystem and promoting the sustainable development of marine ecosystems. The findings also provide foundational data and a scientific basis for the seasonal variations in zooplankton in the Bohai Sea and the adaptive management of coastal ecosystems.
