Seasonal and regional differences in long-term changes in large mesozooplankton (>505 µm) biomass and abundance in a semi-enclosed subtropical bay

Obvious spatiotemporal heterogeneity is a distinct characteristic of ecosystems in subtropical bays. To aid targeted management and ecological restoration in long and narrow semi-enclosed subtropical bays, we analyzed seasonal and regional differences in long-term changes (1980–2019) in the biomass and abundance of large mesozooplankton (LMZ; >505 µm) in Xiangshan Bay, Zhejiang, China. We found spatiotemporal heterogeneity in the historical changes of LMZ. Significant negative trends in LMZ biomass were found in the inner and middle bay during the warm season (summer and autumn), when the nutrient concentration (especially dissolved inorganic nitrogen) and temperature increased simultaneously. Nutrient changes in Xiangshan Bay began in the late 1980s or early 1990s, coinciding with large-scale fish cage development. A rapid decline in LMZ biomass occurred after 2005 when power plants commenced operation, accelerating the warming trend. Therefore, the joint stress of eutrophication and warming likely precipitated the decline in LMZ biomass. Conversely, a significant increase in LMZ biomass was found in the outer bay in spring. This trend was consistent with the trend of LMZ biomass near the Changjiang (Yangtze) River estuary, which indicates that the pelagic ecosystem in the outer bay was affected by water from the Changjiang River estuary during spring. Based on our results, ecosystem management and restoration in semi-enclosed subtropical bays should focus on internal waters, which have a poor capacity for water exchange. For Xiangshan Bay, the changes in the Changjiang River estuary ecosystem during the cold season (winter and spring) should also be considered.