Zooplankton are important food organisms in the marine ecosystem, and their community structure and distribution reflect the productivity of the waters. To investigate the zooplankton structure and the environmental factors affecting ecological niche differentiation in the waters of Tahe Bay, Lushun, a survey was conducted in September 2021 and March, April, and November 2022 in the waters of Tahe Bay. The results showed that there are 31 species representing four phyla, with an annual mean abundance of 12.42 × 10³ ind/m³, dominated by Copepoda (13 species, 41.94%), with zooplankton richness indices ranging from 0.83 to 2.44, diversity indices ranging from 0.84 to 2.42, and evenness indices ranging from 0.14 to 0.84. Pearson’s correlation of zooplankton abundance and community diversity with environmental factors, such as water temperature, salinity, dissolved oxygen, NH₃-N, and NO₃-N, was significant in the waters of Tahe Bay. There were 13 dominant species, mainly consisting of broad-niche species, among which Oithona similis was the dominant species in all four seasons; the degree of niche overlap of the dominant species was related to the seasons, with a serious niche overlap accounting for 81.0% in September 2021, and there was no niche overlap in March 2022; 33.3% of the species had a severe ecological niche overlap in April 2022, with a serious niche overlap accounting for 86.7% in November 2022. The results of redundancy analysis (RDA) indicated that water temperature, salinity, DO, and DIP are the main environmental factors affecting the ecological niche differentiation of the dominant species of zooplankton.

Phytoplankton serve as crucial producers in marine ecosystems, and their community composition and populations’ dispersion directly or indirectly influence the productivity of marine waters via the trophic cascade effect within the food chain. A survey was undertaken in September 2021 and March, April, and November 2022 to examine the phytoplankton structure and the environmental variables influencing ecological niche differentiation in the waters of Tahe Bay in Lushun, Dalian city, China. The findings indicated that there are 83 species representing forty-one genera and six phyla, with an annual mean abundance of 22.13 × 10⁴ ind·m⁻³, predominantly represented by Bacillariophyta (65 species, constituting 78.31% of the total species). The phytoplankton richness indices varied from 0.83 to 4.99, diversity indices ranged from 2.03 to 2.80, and evenness indices spanned from 0.28 to 0.84. Pearson’s correlation between phytoplankton abundance and community diversity with environmental parameters, including water temperature, salinity, pH, DO, NH₃-N, and NO₃-N, was substantial in Tahe Bay’s waters. The dominant species, comprising 20 species across three phyla, primarily include broad-niche species, with Paralia sulcata being the most prevalent species, except during summer. The extent of ecological niche overlap among the dominant species varied by season, exhibiting 40.0% severe overlap in September 2021, 100.0% severe overlap in March 2022, 93.0% severe overlap in April 2022, and 58.0% severe overlap in November 2022. The findings of redundancy analysis (RDA) and canonical correspondence analysis (CCA) revealed that COD, water temperature, NO₃-N, DIP, NO₂-N, and NH₃-N are the primary environmental variables influencing the ecological niche differentiation of dominant species of phytoplankton. The results of the study elucidate the alteration rules of dominant species and the stability of the community structure of the phytoplankton community in this sea area, thereby offering a theoretical foundation for the scientific assessment of the ecological health of the area and the sustainable utilization of marine biological resources.