Community structure of macrobenthos and ecological quality evaluation in the northern coastal areas of the Beibu Gulf

The Beibu Gulf is a typical semi-closed subtropical bay located in the northwest of the South China Sea. In recent years, there has been an increase in human activity in the Beibu Gulf, especially in the coastal waters, which may have a negative impact on macrobenthic organisms and reduce the benthic ecological quality. However, there were few related studies on this topic. The aim of this study is (1) to explore the current structure of macrobenthos communities in the northern Beibu Gulf, (2) to evaluate the benthic ecological quality status of the northern Beibu Gulf, and (3) to provide scientific insights for the ecological conservation and management of the Beibu Gulf. In this study, the macrobenthos was sampled at 12 sites in the northern Beibu Gulf, in the autumn (November) of 2022. The species composition, abundance, biomass, and diversity indices of macrobenthos were analyzed. To assess ecological quality comprehensively and reveal the disturbance in the northern Beibu Gulf, three methods were used, including AZTI’s Marine Biotic Index (AMBI) and multivariate AMBI (M-AMBI) index, as well as the Abundance-Biomass Comparison (ABC) curve analysis. A total of 30 species of macrobenthos were identified in the northern Beibu Gulf, primarily composed of annelids with 12 species, followed by mollusks (8 species) and arthropods (6 species), and fewer echinoderms and other species. The average abundance and biomass of macrobenthos were 148 ind./m² and 135.04 g/m², respectively. The dominant species were Timoclea scabra, Paraprionospio pinnata, and Aglaophamus dibranchis, which play significant roles in various ecological niches. The results of Cluster analysis and non-metric multidimensional scaling (nMDS) indicated that the macrobenthos communities could be divided into three sub-communities. The result of ABC curve analysis indicated that the macrobenthic community has not been disturbed. The results of the AMBI and M-AMBI indicated that almost all sites, except for one moderately disturbed site (S6), have been slightly disturbed or undisturbed, suggesting that the benthic habitat quality status is good or high. In summary, the preliminary results of this study indicate the macrobenthos communities in the northern Beibu Gulf received relatively lower disturbances, and the overall ecological quality status in this area was good.