洱海入湖河流弥苴河下游氮磷季节性变化特征及主要影响因素

入湖河流携带污染物对洱海水环境的影响日益明显,对洱海入湖水量最大的河流——弥苴河下游水体氮磷进行了连续采样分析,以期为河口湿地建设和水质改善提供基础数据.结果表明:1)弥苴河水质介于地表水Ⅲ-Ⅴ类之间,主要污染物为氮和磷,其中总氮平均浓度为1.17 mg/L,最高浓度达到2.00 mg/L;总磷平均浓度为0.06 mg/L;2)弥苴河下游总氮、总磷浓度丰水期高于枯水期,并呈现出季节性变化规律；3)弥苴河下游水体总氮、总磷年均浓度远高于洱海水体总氮、总磷年均浓度,其中总氮高出2.10倍,总磷高出2.90倍；4)弥苴河下游河段非点源污染占据主导地位.

Seasonal variation of nitrogen and phosphorus in Miju River and Lake Erhai and influencing factors

Over previous years, increasing amounts of pollutants carried by rivers entering Lake Erhai are suspected to have affected its water quality.The long-term effects of human life and industry on the rivers include deterioration of upstream and river vegetation and serious soil erosion, which are main contributory sources of sediment and nutrients flowing into Lake Erhai. Industrial and human wastes delivered directly into streams obviously affect the water quality of Lake Erhai. Consequently, the study of the water quality of the main lowland river entering Lake Erhai and analysis of the current pollution conditions of Lake Erhai will allow appropriate countermeasures to be taken and, correspondingly allow the nitrogen and phosphorus pollution load to be reduced. This has practical significance in managing the water environment of Lake Erhai and mitigating environmental stress arising from unreasonable human development and utilization of natural resources.A survey and analysis were undertaken of the major nutrient distribution in Miju River, the largest river flowing into Lake Erhai, and the results contrasted with those of the water of the Lake. The purpose was to provide data for the planned construction of a wetland in the estuarine region of the Miju River, to mitigate the pollutant loading. Thirteen sampling points were located in the lower reaches of the Miju River, an area seriously impacted by human activities. The water quality of the Miju River is between III and V, based on The People's Republic of China surface water environment quality standards. The two major pollution sources are total nitrogen (TN) and total phosphorus (TP), with mean concentrations of 1.17 mg/L and 0.06 mg/L, respectively. The maximum TN concentration exceeded 2.00 mg/L, and concentrations varied monthly. NO^-₃-N concentrations varied from 0.21 to 1.42 mg/L, and NH₃-N concentrations ranged from 0.08 to 1.23 mg/L. Mean concentrations of NO^-₃-N and NH₃-N were 0.56 mg/L and 0.39 mg/L, respectively. Concentrations of both N and P in the Miju River varied seasonally, being much higher in the wet season than in the dry season. The maximum concentration of NO^-₃-N was in the dry season, while for NH₃-N it was in the early wet season. Contents of N and P in the Miju River were 2.10 and 2.90 times higher, respectively, than in Lake Erhai. Mean annual concentrations of TN and TP in Lake Erhai between 1990 and 2000 were 0.286 mg/L and 0.018 mg/L, respectively, and were 0.55 mg/L and 0.024 mg/L between 2004 and 2008. Annual concentrations of TN and TP in the dock of Erhai were 0.91 mg/L and 0.03 mg/L, respectively, which far exceeded the recorded water quality data. With an increase of rainfall, N concentrations were first positively correlated and then negatively related later, TP concentrations were more affected by the rainfall which is a principal characteristic of non-point source polluted rivers. Nitrogen and phosphorus, derived mainly from irrigated farming and cultivated land fertilization, sewage, trash, livestock manures and plant waste from cultivated land, were carried by rainfall runoff. In a broad sense, non-point pollution played a leading role in the pollutant concentrations observed in the Miju River.