Abstract: Investigations on macrobenthic fauna were conducted at 29 sampling stations between the Sanmen Bay and Jiaojiang Estuary during August and November 2019, and April and December 2020. Their seasonal distribution and associations with environmental factors were analyzed. The results showed that a total of 68 macrobenthic species were collected. In terms of taxonomic composition, the specimen comprised 33 annelid species, 13 mollusk species, 12 crustacean species, 3 cnidarian species, and 7 species from other animal groups. Annelids and mollusks together accounted for 67.6% of the total number of species. Across the four cruises, nine dominant species were identified: Turritella bacillum, Siliqua minima, Notomastus latericeus, Aglaophamus dibranchis, Mediomastus chinensis, Sternaspis sculata, Cossurella dimorpha, Virgularia gustaviana, and Eocylichna braunsi. The seasonal average Shannon diversity index ranged from 0.78 to 2.07, average Margalef species richness index from 0.20 to 0.65, average Pielou evenness index from 0.51 to 0.92, and average Simpson dominance index from 0.29 to 0.54. All the four indices showed significant seasonal differences. Beta diversity was primarily driven by the turnover component. Generalized additive model (GAM) analysis (adjusted R²= 0.513) revealed significant nonlinear effects of phytoplankton abundance, chemical oxygen demand (COD), and water color on inhabiting density, highlighting the interconnected influence of the biological, chemical, and optical environment on its variability. Canonical correspondence analysis (CCA) identified two primary environmental gradients governing community differentiation: a synergistic salinity-silicate gradient (Axis 1) and a synergistic water temperature-chlorophyll-a gradient (Axis 2), which collectively explained 39.69% of the variation of species-environment relationship. Cluster analysis divided the 29 stations into three groups. Group I was dominated by pollution-tolerant species (A. dibranchis, S. sculata), indicating terrigenous organic pollution. Group II was characterized by the coexistence of A. dibranchis and V. gustaviana, reflecting a tidal-regulated sand-mud mixed habitat. Group III, with V. gustaviana as the indicator species, represented a low-disturbance sandy environment.

Key words: macrobenthos, biodiversity, canonical correspondence analysis (CCA), environmental factor, community structure