| Abstract: Zooplankton, as essential secondary producers, are pivotal in maintaining the stability of marine ecosystems. The Bohai Sea, known for its rich biological resources, serves as a important spawning and feeding ground for numerous economic fish and shrimp in northern China. Studying the seasonal variations of zooplankton communities and their relationship with environmental factors is vital for assessing the current status of the Bohai Sea ecosystem and promoting the sustainable development of marine ecosystems. This study analyzed the community structure of zooplankton in the Bohai Sea using indices such as species turnover rate (A), dominance (Y), turnover rate of dominant species (R), Shannon-Wiener diversity index (H?), Margalef species richness index (d), and Pielou evenness index (J?), based on survey data collected during two cruises in June and October 2021.The results showed that a total of 43 phytoplankton species were collected in summer, with an average abundance was 34,722.6 ± 31,775.1 ind/m3. 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 distributed in Laizhou Bay and adjacent waters, whilethe high-value zones shifted to the Bohai Bay entrance and the central and southern Bohai Sea in autumn. A total of 41 zooplankton species (taxa) were identified, including 14 Copepod species, 13 pelagic larvae taxa, 8 Hydrozoa species, 2 Amphipoda species, and one species each of Ctenophora, Cladocera, Chaetognatha, and Tunicata. Due to the influence of high summer temperatures, the number of warm-water species significantly increased in autumn, resulting in higher species richness in autumn (36 species) compared to summer (30 species). Zooplankton abundance averaged higher in the summer (5,460.0 ind/m3) than in the autumn (4,116.9 ind/m3), with high-abundance zones occurring in warmer, lower-salinity areas of the sea. The spatial distribution of zooplankton abundance in summer was similar to that of phytoplankton, with primary concentrations in Laizhou Bay and Bohai Bay, while the distribution shifted to the central and southern Bohai Sea in autumn. The Shannon-Wiener diversity index (H?), Margalef richness index (d), and Pielou evenness index (J?) all exhibited higher values in autumn compared to summer. Dominant zooplankton species in summer included Acartia hongi, Oithona similis, Paracalanus parvus, Centropages abdominalis, Calanus sinicus, Acartia omorii, and Paracalanus crassirostris, while in autumn, the dominant species were A. hongi, P. parvus, P. crassirostris, O. similis, Bivalve larvae, Ditrichocorycaeus affinis, and Oikopleura dioica. Both P. parvus and A. hongi were significantly correlated with higher temperatures and higher phytoplankton abundance. Redundancy analysis revealed that P. parvus and A. omorii were significantly positively correlated with phytoplankton abundance in summer (p<0.05), while P. parvus and P. crassirostris showed significant positive correlations with temperature (p<0.01) and negative correlations 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 the zooplankton into three groups for both summer and autumn. In summer, Group A, found only at a few stations in the southern Bohai Sea, had 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 in Liaodong Bay and the central-northern Bohai Sea, had the lowest zooplankton abundance (1,373.5 ind/m3). Group E, in the central Bohai Sea, had relatively higher zooplankton abundance (3,818.4 ind/m3). Group F, mainly in Bohai Bay and Laizhou Bay, had the highest zooplankton abundance (6,074.7 ind/m3). SIMPER analysis indicated 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 include A. hongi, O. similis, P. parvus, and C. abdominalis. In Group B, four species contributed 85.3% to the similarity, namely A. hongi, P. parvus, P. crassirostris, and O. similis. In Group D, five species contributed 87.6%, including A. hongi, P. parvus, O. similis, Bivalve larvae, and Paracalanus crassirostris. In Group E, five species accounted for 83.2%, namely Acartia hongi, P. parvus, Bivalve larvae, P. crassirostris, and D. affinis. In Group F, four species contributed 81.4% to the similarity, including P. parvus, A. hongi, P. crassirostris, and O. similis. BIO-ENV analysis revealed that the best explanatory combination offactors for summer zooplankton community structure was surface temperature, surface salinity, bottom salinity, and phytoplankton abundance (ρ=0.303), while the best explanatory combination for autumn was surface salinity and surface dissolved oxygen (ρ=0.190). These findings are significance for understanding the seasonal variation characteristics and the 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, providing foundational data and scientific basis for seasonal variations of zooplankton in the Bohai Sea and adaptive management of coastal ecosystems. |
