Spatiotemporal Evolution of Nitrogen Species and Physico-chemical Characteristics in a Gate-controlled Canal Impacted by Mariculture

Centralized discharge of mariculture tailwater significantly impacts coastal river quality. This study investigates the Fuzhuang Drainage Canal in Tianjin to analyze the evolution of physicochemical indicators and nitrogen speciation during the mariculture cycle (May–September). Results show the main channel is heavily influenced by high salinity, with a mean EC of 30900 μS/cm. Nitrogen speciation exhibits significant spatial heterogeneity driven by hydraulic conditions and aquaculture discharge. Notably, during the September harvest, gate regulation coupled with high-load tailwater causes a rapid transition from oxidative to reductive states, leading to marked ammonia and nitrite accumulation. This study identifies the harvest period as a critical management window, providing a scientific basis for refined water quality scheduling in coastal aquaculture zones.