Responses of Nutrients to the Precipitation Variation and Land Use in Subtropical Monsoonal Small Mountainous Rivers: A Case Study of Baixi Watershed

Small mountainous rivers are characterized by large instantaneous fluxes and susceptible to extreme weather events, which can rapidly transport materials into the sea and have a significant impact on the ecological environment of estuaries and bays. In order to investigate the seasonal characteristics of nutrients in small mountainous rivers in the subtropical monsoon region and the output pattern to the sea during heavy precipitation, surveys on the mountainous rivers were carried out in Baixi watershed in August 2020 (wet season), March 2021 (dry season) and June 2021 (Meiyu period). The results showed that the dissolved inorganic nitrogen (DIN) of the rivers has an average concentration of 752 µg L⁻¹ in the wet season and 1472 µg L⁻¹ in the dry season. The concentrations of dissolved inorganic phosphorus (DIP) in wet season and dry season were 63 µg L⁻¹ and 51 µg L⁻¹, respectively. Influenced by the changes of land use in sub-watersheds, DIN concentrations in the mainstream increased from 701 µg L⁻¹ in the upper reaches to 1284 µg L⁻¹ in the middle reaches. Two rainstorms during the Meiyu period in the watershed caused the pulse runoff in the river. The maximum daily runoff reached 70 times that before rains. The maximum daily fluxes of DIN and DIP were 109 and 247 times that before rains, respectively. In view that the watershed experienced several rainstorms in the wet season, the river, with pulse runoff, carries a large amount of nutrients into the sea in a short time, which will have a significant impact on the environment of Sanmen bay and its adjacent sea.