Spatial and temporal variation of benthic ecological quality evaluation in the Bohai Bay (China) using benthic indices

Bohai Bay, a semi-enclosed bay in northern China, is an important spawning and nursery ground for marine organisms but has experienced long-term anthropogenic pressures associated with land-based pollution, coastal engineering, and aquaculture activities. Assessing the ecological status of benthic habitats is therefore essential for evaluating the effectiveness of recent management and restoration measures. Based on summer surveys conducted from 2019 to 2023, benthic macrofauna and environmental data were used to evaluate benthic ecological quality in Bohai Bay using three commonly applied indices: the Shannon-Wiener index (H’), AZTI’s Marine Biotic Index (AMBI), and multivariate AMBI (M-AMBI). The three indices showed generally consistent spatial patterns and interannual trends, although differences in sensitivity were observed. H’ and M-AMBI exhibited clearer spatial gradients and higher concordance with habitat conditions, whereas AMBI produced more clustered classifications. Overall, benthic ecological quality in Bohai Bay displayed a significant improving trend during the study period. The proportion of stations classified as slightly disturbed or undisturbed increased from 81% in 2019 to 96% in 2023, while stations in good or high ecological status increased from 56% to 76%. Spatial heterogeneity remained evident, with stations showing moderate disturbance or poor status mainly associated with localized human activities such as aquaculture, anchorage, and marine engineering. Correlation analysis and redundancy analysis further indicated that benthic communities were jointly influenced by water-column and sediment conditions. Active phosphate in the water column was closely associated with broader regional gradients, whereas sediment sulfide played a key role in shaping local benthic ecological quality and community composition. These results indicate that pollution reduction and coastal management efforts have contributed to measurable improvements in benthic ecological quality in Bohai Bay, while localized human pressures continue to influence benthic habitats, and this study provides a multi-index and multivariate framework for assessing benthic ecosystem responses to human activities that supports adaptive management of Bohai Bay and other semi-enclosed coastal systems.