Abstract: Liuxi River, across the Tropic of Cancer, is a typical river in transition between tropics and subtropics. To understand the seasonal and spatial variation of nutrient concentrations in the river, eight investigations on water quality were carried out at 20 sites covering the whole river from February 2015 to February 2017. We analyzed the seasonal dynamics and spatial distribution of N and P concentrations and the potential affecting factors by multivariate statistical methods. The concentrations of TN and TP ranged from 0.29 to 11.88 mg·L^-1 and 0.01 to 0.59 mg·L^-1, respectively. TN concentration in the dry season was higher than that in the flood season, regulated by rainfall and affected by river dilution. TP concentration in the flood season was higher than that in the dry season, which was regulated by rainfall and significantly affected by exogenous input. Seasonal variations of TN and TP were similar in the dry season, but different in the flood season. Spatial variations of NO₃^--N and TN concentrations were similar, which could be divided into three distinct sections: the mid-value section of upstream of Liuxihe Reservoir, the low-value section from downstream of the Liuxihe Reservoir to the Jiekou Bridge, and the high-value section of downstream of Jiekou Bridge. Nutrient retention in two large reservoirs located in the upstream was one of the main factors accounting for the spatial difference. The concentrations of NH₄⁺-N and TP increased from upstream to downstream. The concentrations of N and P were significantly higher in the third-order stream than in the first and second-order streams, which were significantly correlated with the land use types. The high proportion of building land, open land and cultivated land increased the concentrations of TP, NH₄⁺-N and NO₃^--N, while the increased proportion in forest land contributed to soil and water conservation and reduced the concentrations of N and P in the river. Therefore, the regulation of upstream reservoirs and the increase of forest land in downstream urban areas are helpful to improve the water quality of Liuxi River.

Key words: Manas River Basin, land use change, geo-informatic spectrum, geo-spectrum unit