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The water quality of Erhai Lake is good as a whole, but its decreasing trend is not optimistic; the water ecosystem of Erhai Lake has changed dramatically, showing an obvious degenerating trend. The main reasons for the ecosystem degeneration are as follows: inflow pollution load exceeding the water environmental capacity, not favorable water habitat for the propagation of submerged plants, and the weak management for the lake basin. To prevent the degeneration of Erhai ecosystem, two key problems should be solved: one is to reduce the inflow pollution load to prevent the increase of nitrogen and phosphorus concentrations in the overlying water; and the other is to implement the ecosystem regulation, namely improve the stability and biodiversity of Erhai ecosystem by habitat improvement, ecological restoration and ecological regulation. In terms of the preventive measures against the Erhai ecosystem degeneration, the pollution source control in combination with ecosystem regulation should be taken as the focus. Pollution source control, habitat improvement, ecological restoration, ecological regulation and watershed management should be emphasized simultaneously. To be specific, the key tasks for preventing Erhai ecosystem degeneration include five points namely, the reduction of pollution loading into lake, adjustment of ecological fishery, restoration of submerged vegetation, watershed integrated management, and emergency treatment of water blooms. [ABSTRACT FROM AUTHOR]

Soluble organic nitrogen (SON) has been hypothesized to play a central role in biogeochemical nitrogen cycle in freshwater lake resulting in deterioration in water quality. In this study, we investigated the composition and structure of DON in sediments from a typical eutrophic lake, Erhai Lake in China, to understand organic nitrogen molecular transformation involved in nitrogen release in the lake, and to assess DON potential effect on the lake water quality. Our results indicated that total soluble nitrogen (TSN) and SON in sediments had similar spatial distribution trend. The dominant component of SON fractions in the lake region had a significant difference that the north region was HON, the south region was hydrophobic acidic (HOA), but the middle region was hydrophilic matter (HIM). Three-dimensional fluorescence spectroscopy analysis of SON fractions indicated that HOA was dominated by humic-like substances, whereas HIM, HON, and hydrophobic basic were dominated by protein-like substances, which suggested that SON of the south region in the lake mostly originates from human activities. The higher content (31.7 %) and fluorescence characteristics of HIM in SON indicated that the SON of the lake sediments had a higher bioavailability and might remarkably impact the nitrogen cycle in water. Thus, the control of SON and HIM fraction in lake sediments was important to protect the lake water and to prevent the lake eutrophication. [ABSTRACT FROM AUTHOR]

In view of the problems of high P concentration in the sediments and low P concentration in the overlying waters of Erhai Lake (EH), an indoor static incubation experiment was performed to determine the P release characteristics of sediments at different depths. The differences in the P forms of the sediments before and after the incubation experiment were compared, and the impact of such forms on the characteristics of the P release of sediments at different depths and that of P release on the water quality of EH were also investigated. Results were as follows: (1) Dissolved organic P (DOP) is the main form of P released at the sediment-water interface of EH, which is approximately six times that of soluble reactive P (SRP), which indicates that the DOP release of sediments may have a more important impact on P composition and the content of the overlying water of EH than SRP. The release flux of the DOP of sediments at the surface layer is higher than that at the bottom layer. The vertical distribution of the release flux of SRP is characterized by an increase at the surface layer and a decline at the lower layer. (2) Different P release potentials exist for different forms of P in the sediments of EH. Among the different P forms, the loosely absorbed inorganic P (NHCl-rp) is the main source of SRP that is released from the sediments into the overlying waters. (3) The release flux of total P (TP) at the sediment-water interface of EH amounts to 114.2 t/a, which accounts for 87.2 % of the total external P input in the lake. Although the amount of the P release of sediments under indoor static simulation conditions is higher than that under real conditions, the trend of the P release of the sediments of EH is accurate. According to this result, the contribution of the sediment P release to the TP concentration of the overlying water is up to 31 %, reaching a concentration of 9.4 µg/L. The annual P release of the sediments of EH influences the P concentration in the overlying water. The contribution of the sediment P release to the TP concentration of overlying water in the northern part of EH is obviously higher than to that of the other parts. The water quality of this kind of lake can be improved by controlling the P release of sediments, particularly the release of OP. [ABSTRACT FROM AUTHOR]

Purpose: The content and composition of dissolved organic matter (DOM) in sediment directly affect nutrient cycling and material exchange in lake ecosystems. This study investigated the content and composition of DOM and its fractions in sediments, as well as the relationship between the different parameters and nitrogen (N) forms in DOM. The main aim of this study was to evaluate the compositional characteristics of DOM, hydrophobic bases (HOB), hydrophobic acids (HOA), hydrophobic neutral fractions (HON), and hydrophilic matter (HIM) in sediments from Erhai Lake, China. Materials and methods: Seven surface sediment samples with different environmental characteristics were collected. The DOM in the sediment was fractionated into HOB, HOA, HON, and HIM using XAD-8 resin based on compound hydrophobicity. The contents of DOM and its fractions were measured using a TOC analyzer. The structural characteristics of DOM and its fractions were investigated using fluorescence spectroscopy and UV-Vis absorbance. Correlation analyses were carried out to better understand the relationships between the parameters of the spectral characteristics and the contents of the different N forms in DOM and its fractions. Results and discussion: The content, spatial distribution and structure of DOM and its fractions in Erhai Lake sediment were affected by water depth and aquatic plants. The DOM content in sediment ranged from 0.2 to 0.5 g kg. HON accounted for 41.3 to 85.7 % of DOM, whereas HIM constituted 15.0 to 58.7 % and was significantly negatively correlated with HON ( R = 0.856, P < 0.01). HIM was more dominant than the hydrophobic matter in the southern site, but was less abundant in the north and central sites. Sediment DOM and its fractions comprised protein- and humic-like substances. The HOA and HIM comprised humic-like substances, which were the most abundant in the southern site. Conclusions: (1) Hydrophobic fractions are the major components of DOM in the sediments from the seven sites in Erhai Lake. (2) DOM and its fractions mainly originated from microbial sources. (3) The A/ A ratio is useful for evaluating the contents of N forms. The structure of DOM and its fractions are important in affecting the contents of DON. Nitrate (NO-N) contributes to eutrophication, and thus cannot be ignored from studies of Erhai Lake sediment. [ABSTRACT FROM AUTHOR]