Assessing nitrogen sources in Lake Erhai : A spatially explicit modelling approach

Due to the vulnerable ecosystem environments, plateau lakes in the southwest of China face high eutrophication risks. Excessive nitrogen input is an important driver of eutrophication in plateau lakes. However, it is lacked of accurate analysis of spatial and temporal distribution characteristics of nitrogen sources for plateau lakes, which restricts local water environment management. Taking a typical plateau lake, Erhai Lake, as an example, this study applied the MARINA (Model to Assess River Inputs of Nutrients to seAs)-Lakes model to 26 sub-basins of Erhai Lake to analyze the temporal (1998–2016) characteristics of nitrogen inputs and identify the main pollution sources in different sub-basins. The results showed that the most obvious decreasing trends in river export of nitrogen occurred in 2016. It decreased from 2.9 in 2010–2.3 Mg in 2016, which is associated with the series of water quality protection measures implemented. By distinguish nitrogen forms, we found that dissolved inorganic nitrogen was mainly from diffuse source (synthetic fertilizer) whereas point source (directly discharge of animal manure) was the dominant source for dissolved organic nitrogen. The highest river export of nitrogen was from Zhonghe sub-basin in the western areas with high population density and intensive agricultural system, and the lowest was from Mijuhe sub-basin in the northern areas. Tourism was the dominant source of river export of nitrogen in several sub-basins, including Zhonghe, Meitao, Haidongqin, Boluo, and Jinli. Our study developed an explicit spatial analysis approach to quantify river export of nitrogen by sources at sub-basin scale. Based on the primary sources of nitrogen within each sub-basin, we could divide Erhai Lake basin into different management zones. These results can not only help decision-makers formulate accurate and effective lake protection policies for Erhai Lake, but also provide new insights for other plateau lakes.