Nutrient changes from Zhoushan coastal upwelling fluctuations since 1900 and its implications for fisheries

This study aims to elucidate the environmental changes signified by biogenic components, assess fluctuations in upwelling over the past century, and evaluate the potential risks associated with variations in coastal upwelling intensity on the degradation of fisheries within the Zhoushan Fishing Ground. High-resolution sediment records were established to reveal long-term variations in Zhoushan coastal upwelling. Results showed a significant reduction in nutrient components between 1900s and 1950s, suggesting a decline in upwelling strength. Since the 1960s, lightened δ¹⁵N and the incline of nutrients suggests an increase in anthropogenic nutrients influx, and increased OP and CUI_SST (SST-based coastal upwelling index) indicates strengthening of upwelling during the 1950s to 1980s. A positive correlation between organic phosphorus (OP) and δ¹³C has been observed since the 1960s, implying that P is a limiting nutrient due to increasing anthropogenic N influx, and the change in CUI_SST coincides well with our OP records. OP is a potential indicator of upwelling strength because upwelling contributes to over 90 % of the P on the East China Sea shelf. A prominent decline in the nutrient composition occurred from the 1980s to 2010s, despite substantially increased anthropogenic nutrients influx and eutrophication in adjoining Changjiang delta regions in the past decades. Weakening upwelling and the consequent decreasing nutrients influx has presumably hindered recovery and thus contributed to the degradation of fishery resources in recent decades.