Nutrient Budgets and Recent Decadal Variations in a Highly Eutrophic Estuary: Hangzhou Bay, China

Wu, B.; Jin, H.; Gao, S.; Xu, J., and Chen, J., 2020. Nutrient budgets and recent decadal variations in a highly eutrophic estuary: Hangzhou Bay, China. Journal of Coastal Research, 36(1), 63–71. Coconut Creek (Florida), ISSN 0749-0208.Hangzhou Bay is one of China's most eutrophic waterbodies, yet water quality observations within the bay are few, and the estuary's nutrient dynamics are poorly understood. Five cruises (summer, autumn, winter, spring) were conducted in Hangzhou Bay and adjacent waters between July 2006 and November 2007 and in August 2018. A steady-state box model was also constructed to estimate summer and winter nutrient fluxes. The results show that the spatial distributions and temporal variations of nutrients within the bay are significantly influenced by not only direct inputs from rivers around the bay but also the Changjiang River plume to the north. Bay concentrations of nitrate (), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi) were quite high throughout the year, especially in the head of the bay. Relative to concentrations measured in 1981–1982, estuarine concentrations of and DIP had increased, whereas DSi had been stable. The model results indicate that riverine discharge is the major source of nutrients to Hangzhou Bay. In both summer and winter, nutrients were exported from the bay to the East China Sea. The fluxes of , dissolved inorganic nitrogen (DIN), DIP, and DSi were higher in summer than winter, but the flux of ammonium was higher in winter. Nutrient fluxes to Hangzhou Bay observed here during 2006–2007 are much higher than those observed in the 1980s. Additionally, nutrient abundances in Hangzhou Bay are significantly higher than those reported for other coastal embayments in China. The difference in algal blooms inside and outside the bay is possibly due to the difference in turbidity or the hydrological conditions.