Response of subtidal macrobenthic communities in Sansha Bay to different aquaculture modes

Sansha Bay is a typical complex mariculture bay in Fujian, China, where fish cage, macroalgae, and abalone farming coexist. To reveal the effects of different aquaculture modes on benthic ecosystems, we set 20 stations across four typical aquaculture zones in Sansha Bay in July 2025, including core, edge, and control areas, to investigate macrobenthic community structure, functional diversity, and their responses to environmental factors. A total of 44 macrobenthic species, belonging to 28 families, 18 orders, and 5 phyla, were recorded, including 23 annelids, 11 arthropods, 5 echinoderms, 4 mollusks, and 1 nemertean. The core stations of fish cage zones exhibited the lowest diversity, with a high proportion of individuals represented by dominant species relatively tolerant to organic enrichment, such as Sternaspis scutata. In contrast, the macroalgae and abalone culture areas showed relatively higher functional evenness, suggesting a more balanced distribution of community functional traits. The community characteristics of the abalone culture area were generally intermediate between those of the fish cage and macroalgae culture areas, which may be jointly influenced by sediment properties and specific feeding practices. Non-metric multidimensional scaling (NMDS) and analysis of similarities (ANOSIM) results revealed that the impact of the aquaculture model on community structure was significantly greater than that of geographic distance. Moving outward from the core zone, the fish cage area showed a certain tendency of decreasing disturbance, while the gradient in the macroalgae culture area was relatively weak. Functional trait ordination suggested partial differentiation of community functional composition among different aquaculture types. Redundancy analysis (RDA) indicated that sediment total phosphorus (STP) was an important environmental factor closely associated with community variation. Threshold analysis further identified the environmental tipping points for benthic community degradation as STP (602.7 mg·kg⁻¹) and sediment organic carbon (SOC) (1.018%). This study demonstrates that aquaculture impacts exhibit significant type-dependency and gradient effects, with fish cages leading to benthic functional simplification. These findings provide a scientific basis for ecosystem-based aquaculture carrying capacity assessment, sediment quality standard formulation, and differentiated aquaculture management in Sansha Bay.