Abstract: Maowei Sea is the inner sea of Qinzhou Bay in Guangxi, a typical semi-enclosed estuarine bay, which is both a high-productivity area and an important ecological function zone, and has high ecological value. However, the water exchange capacity of Maowei Sea is limited, its environmental carrying capacity is weak, and land-based pollution is prone to accumulate. Human activities have a significant impact on it. As the estuary of the national major project Pinglu Canal, the construction and operation of the canal will bring about a sharp increase in ship traffic, an increase in land-based pollution input, drastic changes in hydrological conditions, and an increased risk of invasive alien species. All these factors are highly likely to cause severe disturbances to the benthic habitat of Maowei Sea, leading to sediment damage, water quality deterioration, loss of key species' habitats, and degradation of community structure. Macrobenthos are key indicator organisms reflecting the health of the ecosystem. Therefore, conducting seasonal background surveys of the macrobenthos community in Maowei Sea is of vital importance. Given the insufficient ecological surveys of macrobenthos in this sea area in recent years, this study aims to comprehensively assess the community structure, species composition, dominant species, abundance, biomass, and biodiversity of macrobenthos through comparative surveys in winter (dry season) and summer (wet season) (December 2023 and June 2024), reveal the seasonal dynamic change patterns, and identify key environmental drivers (such as heavy metals), providing baseline data and scientific basis for the study of ecological changes after the opening of the Pinglu Canal. A total of 131 species belonging to 8 phyla and 84 families were recorded, with fewer species observed in winter (65 species from 7 phyla and 43 families) than in summer (100 species from 8 phyla and 79 families). Polychaete was the dominant group in both seasons (36 species each), followed by crustacean (14 species in winter and 30 species in summer). The dominant species in winter were Branchiostoma belcheri, Micronephthys oligobranchia, and Heteromastus filiformis, with Branchiostoma belcheri showing the highest dominance index (Y=0.042). In summer, the dominant species were Xenophthalmus pinnotheroides, Micronephthys oligobranchia, and Heteromastus filiformis, among which Xenophthalmus pinnotheroides had the highest dominance index (Y=0.045). Winter mean abundance and biomass (97.69ind/m² and 11.594g/m²) were lower than those in summer (442.06ind/m² and 40.573g/m²). Winter exhibited higher Shannon-Wiener diversity (H′=1.696) and Pielou evenness (J′=0.841) than summer (H′=1.656, J′=0.715), whereas the Margalef richness index (D) was slightly lower in winter (1.746) than that in summer (1.809). Cluster analysis and nMDS ordination divided the winter survey stations into 5 groups and the summer ones into 7 groups, which profoundly reveals the seasonal dynamics of environmental heterogeneity in the Maowei Sea ecosystem. Analyses of abundance, biomass, and environmental factors indicated that winter abundance was significantly negatively correlated with sediment cadmium (Cd) content (P<0.05), while biomass showed a significant positive correlation with abundance (P<0.05). In summer, abundance was significantly positively correlated with water depth (P<0.01). Based on historical survey data, this study found that the species number and diversity of macrobenthos in Maowei Sea, Qinzhou Bay, are on a downward trend, with polychaetes gradually becoming the dominant group. There is a potential risk of the community evolving towards smaller-sized species. It is suggested that on the basis of deepening the research on community structure, long-term observational data on macrobenthos and environmental factors should be systematically accumulated, and the control of nitrogen and phosphorus input and land-based pollution (such as agricultural runoff) should be strengthened. In winter, the changes in sediment organic matter should be closely monitored, and in summer, pollution prevention and control should be intensified to provide scientific support for the ecological protection and management of this sea area.

Key words: Macrozoobenthos, Community Structure, Biodiversity, Environmental Factors, Qinzhou Bay