Date Published: February 6, 2017
Publisher: Public Library of Science
Author(s): Zebin Tian, Binghui Zheng, Lijing Wang, Liqiang Li, Xing Wang, Hong Li, Stefan Norra, Yiguo Hong.
In order to protect the water quality of Dongting Lake, it is significant to find out its nitrogen pollution characteristics. Using long-term monthly to seasonally data (1997–2014), we investigated the spatial and temporal variations in nitrogen in Dongting Lake, the second largest freshwater lake in China. The average concentrations of total nitrogen (TN) in the eastern, southern, and western parts of the lake were 1.77, 1.56, and 1.35 mg/L, respectively, in 2014. TN pollution was generally worse in the southern area than in the western area. Concentrations showed temporal variation, and were significantly higher during the dry season than during the wet season. Based on the concentration and growth rate of TN, three different stages were identified in the long term lake data, from 1997 to 2002, from 2003 to 2008, and from 2009 to 2014, during which the concentrations and the growth rate ranged from 1.09–1.51 mg/L and 22.09%-40.03%, 1.05–1.57 mg/L and -9.05%-7.74%, and 1.68–2.02 mg/L and 57.99%-60.41%, respectively. The main controls on the lake water TN concentrations were the quality and quantity of the lake inflows, spatial and temporal variations in hydrodynamic conditions within the lake (flow velocity, flow direction), and point and nonpoint inputs from human activities. Diffuse nutrient losses from agricultural land are a significant contributor. As a priority, the local government should aim to control the pollutant inputs from upstream and non-point nutrient losses from land.
Significant disturbances from anthropogenic activities over the past decades have accelerated the process of eutrophication and threatened the health of aquatic ecosystems [1, 2]. Excessive nutrient inputs are the main reason for lake eutrophication , and surplus nitrogen (N) is one of the main driving factors . In particular, more than 80% of eutrophic waters in China suffer from serious N pollution [5, 6]. N concentrations distributions are related to the hydrodynamic conditions and anthropogenic input intensity in a river basin . Over the past decade, most studies have identified that different industrial and agricultural activities under different hydrodynamic conditions are the main sources for nitrogen loads [8–12]. However, there is no significant reduction in nitrogen concentration, after the government implemented effective abatement of point sources pollution . The contribution from fertilizer N applications has become a major cause of TN accumulation . Studies showed that China has become the largest consumers of nitrogen fertilizers in the world, which accounted for 35.1% of the world’s nitrogen fertilizer application . While the linkage between anthropogenic activities N input and nitrogen concentrations have been poorly documented . Thus, it is important to understand and characterize the sources of human-induced N inputs and their contributions to the N concentrations. Riverine transport is the principal pathway by which point and non-point sources move from land to lake . The upper rivers not only bring in the N loads, but also affect the hydrodynamic conditions of the lake , eventually influence the distribution and transportation of nitrogen . However, studies to date rarely provide an integrated analysis of the upper reaches and the lake body [19, 20] and the driving mechanisms for N concentrations distributions associated with upper rivers. Therefore, it is important to reveal the spatial and temporal variations in nitrogen response to the changes of hydrodynamic conditions caused by upper rivers and the N inputs from different sources.
Results from our study have demonstrated that TN concentrations decreased from the river inflows to the central lake area, and that TN pollution was more serious in the southern area than in the western area. Concentrations of TN varied seasonally, and were significantly higher during the dry season than during the wet season. The spatial and temporal variations in the TN concentrations were attributed to the positions of the upstream inflows to the lake and seasonal variations in the quantity and quality of their inputs. Spatio-temporal variations in hydrodynamic conditions (flow velocity, flow direction, etc.) in the lake water body were the second most important factor.