Spatial-temporal distribution of carbon, nitrogen and phosphorus concentrations and eutrophication evaluation of mountainous small river in a cascaded damming

梯级筑坝对河流水环境演化的影响是国内外关注的热点.小型山区河流高密度梯级开发对水体生源要素的空间格局以及水环境演化的累积影响特征尚不清楚.以重庆市五布河为研究对象,对流域内8个"河流-水库-下泄水"交替系统中表层水体理化因子及碳(C)、氮(N)、磷(P)形态组成进行季节性监测,探讨了梯级筑坝对小型河流生源要素空间格局及水体富营养化风险影响的累积特征及驱动机制.结果表明:梯级水电开发对五布河流域水生生境和生源要素空间分配的影响具有潜在的累积效应,各库区水体碳氮磷浓度均呈逐级增加的空间规律;水库段的有机碳及不同形态的氮、磷浓度均高于入库河流,因此水体养分浓度呈现出河段尺度(即单个河流-水库-下泄水系统)和流域尺度(即上游至下游)耦合的空间变异模式.上游水库中溶解性氮、磷的再释放及下泄输移能够补给下游库区,加之下游水库泥沙对氮、磷的吸附-沉积作用的减弱,导致水体氮、磷总量及溶解性氮、磷的占比沿程增加,呈现梯级筑坝对水环境演化的累积影响.梯级筑坝影响下河流碳氮磷总量的相关性减弱,而溶解性养分间的相关性增强,形成了特殊的养分协同演化;水库群之间水力滞留时间的差异与水体碳氮磷浓度具有较好的线性关系,是影响流域养分分配的关键因素.五布河流域水体均为高富营养化风险,由于梯级筑坝下水体溶解性养分的逐级增加,下游水库水华风险更大;水力滞留时间与水体富营养化指数及叶绿素a浓度呈显著正向关,结合氮磷比特征的分析,本研究认为外源磷输入控制及水力滞留时间的调节是五布河梯级水库富营养化防控的有效途径.;Rivers, as the channel connecting terrestrial and marine ecosystems and transporting a large amount of nutrients to ocean, are important for maintaining the global material cycle. The construction of cascade dams leads to river fragmentation and changes the distribution of carbon, nitrogen and phosphorus in the water. At present, a lot of studies have been conducted on the ecological effects of cascade development in large rivers, but few studies focus on small mountainous rivers. The spatial-temporal patterns and controls of the distribution of carbon, nitrogen and phosphorus from cascade river-reservoir systems in the small mountainous rivers remain unclear. In this study, we investigated the spatial-temporal variations of carbon(C), nitrogen(N) and phosphorus(P) concentrations along a cascade damming river (Wubu River) in Southwest China. The results showed that carbon, nitrogen and phosphorus showed an obvious cumulative effect, that was, the contents of C, N and P gradually increased from the first reservoir upstream to the last reservoir downstream underlying cascade dams. And, the organic carbon, as well as nitrogen and phosphorus in the reservoir section, were higher than in those inflow rivers. Thus, different forms of carbon, nitrogen, and phosphorus concentration in the reach (river-reservoir-release water system) scale and river basin scale (the upstream to downstream) displayed the double coupling variation pattern. Primary production and plankton proliferation were the main mechanisms of organic carbon accumulation in the river. At the same time, cascade reservoirs may form a series of nitrogen and phosphorus storage tanks and reactors to promote the release and transport of dissolved nitrogen and phosphorus. The re-release and transport of dissolved N and P in the upstream reservoir can replenish the downstream reservoir, increasing the content of N and P in the downstream reservoir and the proportion of dissolved N and P, thus leading to the cumulative effect of water deterioration. The decrease in the adsorption-deposition effect of sediment on nitrogen and phosphorus nutrients in downstream reservoirs also leads to the accumulation of soluble nitrogen and phosphorus concentrations. The correlation of C, N and P in the river was weakened, but there was a significant relationship between dissolved nutrients, forming a special nutrient co-evolution. Furthermore, the hydraulic retention time of the reservoir groups had a good linear relationship with water C, N and P contents, which was the key factor affecting nutrient allocation in the watershed. Seasonal variations of total organic carbon and dissolved organic carbon were highest in spring and lowest in winter in Wubu River Basin. Total nitrogen and nitrate nitrogen in the dry season (winter and spring) were significantly higher than those in the wet season (summer and autumn), and other nutrient forms had no obvious seasonal pattern. Rainfall dilution, algal reproduction, microbial decomposition and re-release were the main processes leading to the seasonal variation of nutrients. The eutrophication risk also presented an increasing trend from upstream to downstream because of the dam. There was also a good linear relationship between hydraulic retention time and eutrophication risk. Combining the analysis of N:P ratio characteristics, it was suggested that exogenous P control and optimal allocation of hydraulic retention time are effective approaches for eutrophication control in the Wubu River Basin.