Denitrification in Hangzhou Bay: The role of nutrient dynamics in a tidal and human-impacted estuarine system.

Denitrification is a key pathway of nitrogen removal in estuarine ecosystems, yet its regulation is shaped by complex interactions between environmental conditions and human activities. In semi-enclosed estuaries like Hangzhou Bay, long hydraulic retention times and sustained nitrogen inputs from upstream sources have intensified eutrophication, posing significant ecological pressures. Field surveys revealed clear spatial and seasonal variability in denitrification rates, ranging from less than 2.5 ng N g^-1 h^-1 in March to more than 5 ng N g^-1 h^-1 in June. To identify the drivers of this variability, we combined field observations with statistical modeling. Nitrate (NO₃^-) emerged as the primary predictor of denitrification, underscoring its pivotal role as a substrate for denitrifying microorganisms, while temperature emerged as a secondary driver, controlling microbial activity through seasonal fluctuations. Sensitivity analyses confirmed the dominant influence of NO₃^- and temperature, but a negative correlation between NO₃^- and denitrification rate suggests that nitrogen excess in Hangzhou Bay results from limited removal efficiency rather than the substrate scarcity. These findings illuminate the complex interplay between anthropogenic nutrient inputs and natural hydrodynamic processes, providing new insights into nitrogen cycling in estuarine environments and supporting the development of management strategies to mitigate eutrophication and strengthen ecosystem resilience.