Structural and functional responses of macrobenthic communities to seasonal hypoxia and implications for ecological...

This study examined the structural and functional responses of macrobenthic communities in Bohai Bay to seasonal low-oxygen conditions. Based on field observations conducted in June 2024 (normoxic conditions) and August 2024 (a seasonal low-oxygen period), we examined changes in community structure, functional diversity, and ecological quality. Functional diversity was assessed using Functional Richness (FRic), Functional Evenness (FEve), Functional Divergence (FDiv), and Functional Dispersion (FDis), while ecological quality was evaluated using the AZTI Marine Biotic Index (AMBI) and the multivariate AMBI (M-AMBI). Relationships among environmental conditions, species composition, and functional traits were explored using RLQ analysis. During the low-oxygen period, total macrobenthic abundance increased whereas biomass declined, accompanied by increased dominance of opportunistic taxa, particularly small-bodied polychaetes. Oxygen depletion acted as a strong environmental filter on macrobenthic functional traits, favoring tolerant trait modalities while suppressing oxygen-sensitive functional groups. Although AMBI and M-AMBI generally classified ecological conditions as good, FEve declined markedly under low-oxygen conditions, indicating pronounced functional convergence despite relatively stable taxonomic composition. These findings suggest that seasonal oxygen depletion primarily restructures macrobenthic communities through trait-based filtering rather than immediate species loss, and that integrating functional diversity metrics-especially FEve-with traditional biotic indices can provide earlier and more comprehensive diagnosis of ecosystem degradation in coastal systems.