Thematic Issue: Water of the Taihu Lake
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Quality of surface waters is a severe problem worldwide (Dimberg and Bryhn 2014; Harmon et al. 2014; Christia et al. 2014; Renjith et al. 2013; Sedlacek et al. 2013; Antunes et al. 2013; Rinke et al. 2013; Watchorn et al. 2013; George et al. 2013) but particularly important to China because of the water resources use from the huge rivers originating in the Tibetan plateau flowing through the North China Plain into the Pacific Ocean. The water quality issue includes nutrients (Ji et al. 2014; Wu et al. 2013), in particular phosphorus (Chen and Zhao 2014; Chen et al. 2014; Yuan et al. 2014; Zhu et al. 2013; Liu and Chang 2013) and many other pollutants. To develop solutions for a sustainable water resources management, in addition to technological solutions also socio-economic studies are important to conduct for governance, regulation and acceptance issues (Antonellini et al. 2014; Kerschbaumer et al. 2015; Subagadis et al. 2014; Wang et al. 2014) as well as organic pollutants (Tilman et al. 2013) and microbiological hazards (Stoll et al. 2012; Xueping et al. 2014).
Recent Thematic Issues in EES were dealing with Integrated Water Resources Management under different hydrological, climatic and socio-economic conditions (Kalbus et al. 2012; Grathwohl et al. 2013; Seegert et al. 2014) including topics like progress in monitoring and modeling concepts (Beinhorn et al. 2005; Beyer et al. 2006; Kalbacher et al. 2012; Rink et al. 2012). The protection of aquatic ecosystems and the security of drinking water provision are becoming an increasing economic challenge in water management. The Chinese government recognizes the importance and complexity of the situation and has initiated a program titled “Major Water Program of Science and Technology for Water Pollution and Governance” (2006–2020). A series of EES Thematic Issues are dedicated to water problems in China and approaches for their solution (Song et al. 2015; Wang et al. 2015). Bergmann et al. (2012) provided a comprehensive Special Issue on results of the joint research work of German and Chinese groups dealing with the Yangtze Three Gorges Dam in China—The Yangtze-Hydro Project. In the years 2009–2012, SWISSWATER worked on the project “Lake Dianchi Kunming, Project Clear Lake”. Using mathematical modeling, scenarios were made on the effects of various measures on the state of the Dianchi Lake (EAWAG 2012).
Lakes in China—Taihu
Lakes play an important role for the economic and social development in China—as they are one of the most important natural water resources. In recent decades, as a result of rapid economic and social development, fast urbanization and lack of pollution prevention or treatment, many lakes in China suffer from several sources of pollution (e.g. nitrogen, phosphorus, heavy metals). The problems of water pollution and eutrophication are, therefore, increasingly severe. A large number of lakes remain in a state of high ecological risk, with frequent algal blooms (Wang et al. 2015).
Lake Taihu is a large freshwater lake in the Yangtze Delta plain near Shanghai, China. The lake belongs to Jiangsu province and the southern shore forms its border with Zhejiang province. With an area of 2250 square kilometers (869 sq mi) and an average depth of 2 meters (6.6 ft), it is the third-largest freshwater lake in China, after Poyang and Dongting. The lake houses about 90 islands, ranging in size from a few square meters to several square kilometers. The lake is connected to more than 220 rivers. In recent years, Lake Tai has been plagued by pollution as a result of rapid economic growth in the surrounding region (source: https://en.wikipedia.org/wiki/Lake_Tai). This region is one of the most developed economic areas in China; with a 0.4 % land area and 3 % population, it developed 11.6 % of the GDP in China. From late 1980s, Lake Taihu has suffered from algal blooms due to its high nutrient level and shallow water.
Lake Taihu has been investigated in numerous studies (more than 1000 articles in the Web-of-Science, and more than 20 works in EES). We briefly discuss very recent EES works related to the Lake Taihu dealing with remote sensing approaches for large-scale and long-term lake observation, on-site lake monitoring (Rinke et al. 2013), water quality assessment, and basic research to provide a context for the present Thematic Issue.
Chen et al. (2013) developed a new method for estimation of chlorophyll-a concentration retrievals in the highly turbid coastal waters such as the Yellow and Yangtze River estuaries as well as large shallow lakes. In a subsequent work, Chen and Zhao (2014) improved their approach further for estimating MODIS signals when water bodies are highly turbid and applied the model to the Lake Taihu. Huang et al. (2014b) used a time series MODIS data from 2000 to 2011 to extract and characterize algal blooms from the Taihu Lake study area. The use of remote sensing data becomes more and more important to provide meaningful data for hydrological analysis. Hochmuth et al. (2015) studied actual evapotranspiration rates (ETa) computed on the basis of Landsat TM/ETM + and MODIS data in the Heihe River watershed, NW China. Thevs et al. (2015) conducted a similar study in the Tarim basin.
Chen et al. (2014) used stable isotopes (N-15) as a robust indicator of nitrogen (N) source. The joint use stable isotopes (nitrogen N-15 and oxygen O-18-NO3) values can provide useful information about nitrate source discrimination and N cycle process. Data from the Lake Taihu were investigated to identify the primary nitrate sources and assess nitrate biogeochemical processes. Han et al. (2014) incubated collected sediment samples from Lake Taihu in aerobic/anaerobic conditions (with and without air extraction) to estimate the potential nitrate reductase activities. He et al. (2013) analyzed the distribution and sources of DDTs (dichlorodiphenyltrichloroethane) in the Poyang Lake (upstream of the Yangzte River from the Lake Taihu). DDT is a typical organic compound characterized by high bioaccumulability, toxicity, and persistence aquatic environments and is, therefore, important to many lakes. The microbiological transformation of chlorinated pollutants in Yangtze River sediments has been demonstrated recently (Kranzioch et al. 2013, 2015). Huang et al. (2014a) modeled the effects of environmental variables on short-term spatial changes in phytoplankton biomass in the Lake Taihu. Better understanding the short-term response of phytoplankton biomass on environmental variables is needed for issuing early warnings of harmful algal blooms in aquatic ecosystems. Predicting harmful algal blooms are particularly challenging in large shallow lakes such as the Lake Taihu due to their complex mixing patterns (Huang et al. 2014a). Ji et al. (2014) developed an approach for determining lake nutrient standards in different regions of China (Yungui Plateau ecoregion and Eastern Plain ecoregion lakes, Taihu). The most predictive indicator was found to be total phosphorus (TP). Yuan et al. (2014) determined total organic carbon (TOC), total nitrogen (TN) and the phosphorus species concentrations of sediment cores taken from Zhushan Bay, Meiliang Bay, and East Lake regions in Lake Taihu. The experimental results showed an exposed eutrophication trend in the northern area of Lake Taihu. Alam et al. (2014) studied the transport of pesticides in the underground rivers connected to urban lakes. To better understand the eutrophication and pollution processes of lakes it is important to recognize their link to subsurface aquatic systems and reactive transport processes therein (Beinhorn et al. 2005; Beyer et al. 2006). This is particularly important in karst areas which are widely spread in China (Yang et al. 2011).
In the present Taihu Thematic Issue, Li et al. (2015a) proposed a method for quantitative groundwater pollution assessment based on grey relational analysis and established a multiple linear regression equation to predict the groundwater pollution intensity in 18 contaminated sites. Li et al. (2015b) collected and analyzed field data from routine monitoring sites to examine the effects of water level fluctuation on the nitrate concentration in groundwater. Ma et al. (2015) studied the effects of coexistent ions on nitrate reduction by microbial NZVI in a laboratory simulated groundwater environment and implied microbial NZVI as a potential approach for in situ groundwater remediation.
Zhang et al. (2015) presented the dataset of dissolved organic nitrogen (DON) in 17 lakes in Eastern China, and analyzed the different DON concentration patterns in non-river-connected lakes and in river-isolated lakes, as well as in summer and in winter. Huang et al. (2015a, b) studied the eutrophication mechanisms in Lake Taihu through a sediment–water–algae laboratory experiment, and linked hydrodynamic disturbance with sediment resuspension, phosphorus release, and algal growth in Lake Taihu. Wang et al. (2015) presented a statistical approach which considered the nonstationarity caused by step change in time series, and used this approach to analyze the data from the past decades to examine temporal trends of TN and TP in four Chinese lakes, including Lake Dongting, Lake Poyang, Lake Chaohu and Lake Taihu.
Chinese lakes—here the Taihu Lake became an important research object worldwide. Within the German research initiative CLIENT (International Partnership for sustainable climate protection as well as environmental technologies and services) funded by German Federal Ministry of Education and Research (BMBF) several projects started recently to support China in solving the water problems, e.g. SinoWater (Grant Number 02WCL1335A, coordinated by Forschungsinstitut für Wasser- und Abfallwirtschaft an der RWTH Aachen (FiW) e.V.), SIGN (Grant Number 02WCL1336A, coordinated by TZW: DVGW-Technologiezentrum Wasser Karlsruhe) and Urban Catchments (Grant Number 02WCL1337A, coordinated by Helmholtz Centre for Environmental Research UFZ and TU Dresden). The funding of the German–Chinese cooperation projects by BMBF and Chinese Ministry of Science and Technology (MoST) based on the joint declaration of 07.05.2015 is very much appreciated. We are thankful to Prof. Dohmann for his comments to this editorial.