太湖流域河流水质状况对景观背景的响应

为探索流域水质对景观背景的响应,以太湖流域为研究对象,在0.5—24 km共9个尺度上运用冗余分析研究了土地利用、河网密度、降水量、地形等景观背景因子与河流水化学指标的关系。结果表明:2006—2010年太湖流域河流水质状况总体较差,但整体有逐渐改善的趋势,超标水质指标主要包括总磷(TP)、氨氮(AN)、生化需氧量(BOD)、化学需氧量(COD)和溶解氧(DO),上游地区主要表现为林区和平原水网区的差异,下游地区主要表现为河段上下游间的差异。河流水质受到多种景观背景因子的综合影响,并表现出尺度依赖性和区位差异性。AN、TP、DO在流域上游与聚落用地正相关,在下游则与耕地、河网密度正相关。COD、BOD在流域上游主要与自然湿地负相关,与人工湿地正相关,在下游则与坡度负相关,与河网密度正相关。总方差贡献率在上下游表现出一致的尺度依赖特征,均在较小(0.5—1 km)和较大(16 km)两个尺度上具有较高的解释能力。自然湿地和坡度,河网密度和耕地分别为上游、下游地区在较小和较大尺度上解释能力最高的景观因子。

Response of river water quality to background characteristics of landscapes in Taihu Lake basin

Taihu Lake basin is one of the most populous regions in China, with high urbanization and economic development. This area has faced serious water environmental problems since the 1980's as the amount of pollutants produced and discharged into the rivers and lakes have increased. Non-point source pollution-mainly caused by land use and land cover change-is considered the most important cause of this contamination. Most earlier work researching the relationship between rivers and the landscapes through which they flow in this area simply focused on the impact of land use on water quality at a single spatial scale, and lacked a consideration of natural factors such as geography and climate conditions. To explore the response of water quality to background characteristics of landscapes at a watershed scale, we examined the correlation of 9 landscape factors characterizing land use, stream density, precipitation, and topography with 5 water chemistry indicators of the rivers in Taihu Lake basin. Using redundancy analysis, correlations were conducted at 9 spatial scales from 0.5 km to 24 km, respectively. The results show an overall poor status for the water quality of the rivers, but with an improving trend. The indicators with concentrations exceeding the national standard include total phosphorus (TP), ammonia nitrogen (AN), biochemical oxygen demand (BOD), chemical oxygen demand (COD), and dissolved oxygen (DO). Water quality also varies significantly by location: in the upstream basin, apparent differences are observed between the plains areas with dense stream networks and the forested areas, while in the downstream basin the rivers flowing out of Taihu Lake show a significant difference between their upper and lower reaches. The investigated landscape factors have a significant effect on water quality of the rivers; the water chemistry indicators show scale- and location-dependent responses to the landscape factors. Moreover, different indicators have distinct responses. Specifically, AN, TP, and DO are positively correlated with settlement cover in the upstream basin, but with farmland cover and stream density in the downstream basin. BOD and COD exhibit a negative correlation with natural wetlands and a positive correlation with artificial wetlands in the upstream basin, while being negatively correlated with slope and positively correlated with stream density in the downstream basin. The explanatory power on the variance in water quality of each landscape factor shows apparent scale-dependent features. With increasing spatial scale, the curves of total explanatory power show a consistent pattern in the upstream and downstream basins, presenting double peaks near the scales of 0.5-1 km and 16 km. In the upstream basin, natural wetlands and slope present the highest exploratory power at the smaller and larger scales, respectively. In the downstream basin, stream density has the largest effect at small scales and farmland has the largest effect at a large scales.