Structuring effects of long-term eutrophication stress on macrobenthic assemblages in Xiangshan Bay, the East China Sea

Eutrophication has been identified as a global environmental issue to marine ecology. To investigate the effects of long-term eutrophication on the composition and dominant species of macrobenthos in Xiangshan Bay, macrobenthos and aquatic environmental factors were sampled and measured in August (wet season) every year from 2012 to 2022 in Xiangshan Bay. The results showed that the average content of dissolved inorganic nitrogen (DIN) in the surface seawater of Xiangshan Bay was (0.71 ± 0.17) mg/L, and the average content of dissolved inorganic phosphorus (DIP) was (0.037 ± 0.011) mg/L. Among all five stations collected, three of them indicated severe eutrophication, while other two stations displayed moderate organic pollution in their water quality. A total of 44 macrobenthic species were identified, including 7 phyla, 9 classes, and 35 families. Annelids and Mollusca emerged as the dominant macrobenthic taxa (31.82 % and 25.00 %, respectively). The annual average abundance was 218 ind.m⁻², with Annelida Polychaeta displaying the highest abundance, accounting for 46.33 % of the total. Within the macrobenthic assemblages, 9 dominant species were determined, with Ophiura kinbergi and Sternaspis chinensis consistently dominating over the five-year period. Notably, the abundance of O. kinbergi exhibited a strong positive correlation with DIP and water quality index (A) (P < 0.01), as well as a significant positive correlation with eutrophication index (EI) and temperature (T) (P < 0.05). The comparative analysis with historical data revealed that pollution-tolerant macrobenthos had become prevalent, featuring smaller body sizes and greater adaptability to long-term eutrophication, particularly in the innermost bay area.