Food web structure and ecosystem properties of the largest impounded lake along the eastern route of China's South-to-North Water Diversion Project
Introduction
To alleviate water shortage problems in arid regions and promote development, numerous inter-basin water transfer projects have been launched in both developed and developing countries and regions (Ghassemi and White, 2007, Zhuang, 2016). However, there are ample evidences that inter-basin water diversion would result in significant changes in related aquatic ecosystems, such as introduction of aquatic organisms, changes in hydrologic condition, alteration of habitat, and changes in water quality which will finally affect the related aquatic ecosystem structure and functions (Davies et al., 1992, Meador, 1992, Grant et al., 2012). Scientists globally pay increasing attention on the ecological understanding of aquatic ecosystems in order to make effective decisions on ecosystem management and conservation (Zhang, 2009).
China's South-to-North Water Diversion Project (SNWDP), includes eastern, central, and western routes, is a large-scale strategic project to relieve water shortages and sustain economic and social development in northern China (http://www.nsbd.gov.cn/). The Eastern Route will span four largest river basins in China, as Yangtze River basin, Yellow River basin, Huai River basin and Hai River basin and the diverted water will pass through and impound a chain of lakes and link-up with the northern section of the Beijing-Hangzhou Grand Canal (Zhang, 2009). The Eastern Route will eventually transfer 14.8 billion m3 of water annually from the Yangtze River to northern China by a complex hydraulic system of interconnected lakes, rivers and canals, water level would be improved step by step (Tian et al., 2013). The interconnected lakes, such as Hongze Lake (HZL), Gaoyou Lake (GYL), Luoma Lake (LML), Nansi Lake (NSL) and Dongping Lake (DPL) were thus used as the impounded reservoirs and water channels. The first-stage project have been completed at the end of 2013, therewith, water from the Yangtze River would impound the five lakes and water level improved step by step in the lakes.
Hongze Lake, the fourth largest freshwater lake in China, is also the largest impounded reservoir on the eastern route of the SNWDP (Zhang, 2010). HZL ecosystems historically provide multiple benefits that are fundamental to human wellbeing, like navigation junction, flood-and-drought resisting, fishing and aquaculture, biodiversity conservation and tourism in eastern China (Wang and Dou, 1998). However, along with the water diversion project, there should be a set of predictable threats to the lake ecosystems and foodweb, for instance, introduction of aquatic organisms, changes in flow regime, alteration in habitat and changes in water quality (Meador, 1992, Zhang, 2009). Therefore, a systematic understanding and assessment of the structure and function of lake ecosystem should be urgently needed to make effective strategies to maintain the aquatic ecosystem health. More important, numerous national or local projects were intensively funded recently to assess the potential effects of the SNWDP on aquatic ecosystems, while the current status of ecosystem structure and functions should be even more important for the subsequent and comparable studies. In contrast to its importance, global understanding of the HZL ecosystem is still poor. To adequately evaluate potential ecological impacts as the result of the water transfer, a full ecological understanding of structure and function of the lake ecosystem is essential. Among the top five largest freshwater lakes in China, there is least studies focused on HZL with only 50 SCI papers, compared with Taihu Lake (2152), Dongting Lake (443), Poyang Lake (699) and Chaohu Lake (356) (search on 2016/5/13 in “Web of Science-Core Collection”). Moreover, most of the previous studies only conducted regular monitoring on aquatic communities (Ren et al., 2014, Tian et al., 2013), water quality (Zhang et al., 2015, Zhang et al., 2015), fishes and fisheries (Lin et al., 2013) without exception. However, up until now, little is known about the ecosystem status and food web structure of HZL as the interactions of biodiversity within an ecosystem are very complex. Moreover, an ecosystem based analysis is always a data intensive research which a mass of fundamental ecological data should be required (Guo et al., 2013).
During the past thirty years, Ecopath with Ecosim (EwE) models have been widely used all over the world to describe the trophic relationship and the quantitative ecosystem properties, due to its convenience to use compared to most other ecosystem modelling techniques (Christensen et al., 2005, Villasante et al., 2016). This approach was among the first ecosystem-level simulation model to be freely accessible and user friendly, which contributed to its worldwide uptake and popularity as a key tool for the ecosystem-approach to fisheries and aquatic resources. Up to now, > 500 ecosystem models using the EwE approach have been published which are referenced by > 700 citations per year, on average over the last decade (Colléter et al., 2015, Villasante et al., 2016); EwE has also been recognized as one of the top ten scientific breakthroughs in the US National Oceanographic and Atmospheric Administration (NOAA)’s 200-year history (http://celebrating200years.noaa.gov/breakthroughs/ecopath/) (Coll et al., 2015).
This study aims to construct a global ecological understanding of the trophic interactions and ecosystem properties for the important, yet poorly studied HZL ecosystem. Our main objectives were to 1) quantify the food web structure and trophic interactions; and 2) evaluate the ecosystem properties and health status of HZL before the water was routinely transferred. And finally, propose ecosystem-based management strategies for the large freshwater ecosystem based on the features of HZL ecosystem in the context of inter-basin water diversion. Our findings are critically important for effective decision making and policy development in terms of sustainability of economic development and conservation of aquatic ecosystem health along the SNWDP. The results will also contribute to guiding monitoring and assessing the ecological and environmental consequences of the inter-basin water diversion in the future.
Section snippets
Study area and sampling procedure
Hongze Lake (33° 06′–33° 40′ N, 118° 10′–118° 52′ E), which lies in the middle reach of the Huai River, is the fourth largest freshwater lake in China with a surface area around 1597 km2, and a mean water depth of 1.7 m (Wang and Dou, 1998). Since the end of 2013, along with the water transfer at the first stage of the SNWDP (eastern route), Hongze Lake was used as the largest impounded reservoir and water channel (Zhang, 2010).
In the current study, seasonal field samplings were conducted in the
Basic input and output variables
The balanced Ecopath model with the pedigree index (0.462) and the measure of fit (2.384) indicated that the input parameters of the model were based on reliable data sources and the model was also robust with a high level of confidence (Table 4).
A set of basic input variables and the estimated parameters from the model are listed in Table 4. Generally, the EE values of all functional groups are < 1, and all of the P/Q values are between 0.05 and 0.3, meeting the requirements of a balanced
Food web structure in HZL
The current study contributed to constructing the first mass-balance model for HZL ecosystem where a globally ecological understanding was urgently needed. The model is useful for assessing the ecosystem health and making ecosystem-based management decisions for the large freshwater system in the context of large scale inter-basin water diversion. EwE has become the most widely adopted modelling framework to facilitate a more holistic ecosystem-based approach to fisheries management. However,
Acknowledgements
We are grateful to the Hongze Lake Fishery Management Commission for providing the fishery statistic data and kindly assistance in field sampling. We also highly appreciated our colleagues from Fishery Ecology Lab and Aquatic Ecosystems Health Lab for the assistance of field sampling and fruitful comments in these studies. This work was funded by National Natural Science Foundation of China (No. 31602158) and Chinese Academy of Sciences (Y62302, Y45Z04, Y55Z06, and QYZDB-SSW-SMC041).
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