Structure And Energy Flow Of Dianshan Lake Ecosystem Based On Ecopath Model

Dianshan Lake, the biggest fresh water lake in Shanghai, covers an area of 62 km2 with mean depth of 2.0 m. It is regarded as the key water conservation district of Shanghai, providing drinking water for more than 10 million people. But with the rapid development of industry and agriculture in recent years, the water of Dianshan Lake has been drastically polluted by industry and living sewage from surrounding areas. What's worse is that blue-green algae bloom has emerged in some parts of the lake. The self-clean capability of Dianshan Lake has already degraded. The city government has provided several research projects to explore the reasons for ecological deterioration of this area. Based on a database report of bio-resources survey from September 2008 to September 2009, a mass-balance model of trophic interactions was constructed using Ecopath with Ecosim to represent the energy transfer through the trophic levels of the Dianshan Lake ecosystem, which represented the first Ecopath model of Dianshan Lake and consisted of 21 functional groups including birds, commercial fishes, phytoplanktons, zooplanktons, benthos, shrimps, mollusks, macrophytes and detritus. Each group represented the organisms with similar roles in the food web and covered the main trophic flow in Dianshan Lake.The results indicated that total throughput was 4098.50 t-km-2·year-1. The trophic levels of different groups ranged from 1 for detritus and phytoplanktons to 3.92 for large predators, and water bird group occupied the top level of ecosystem.According to the analysis of mixed trophic impact, the trawling and gillnet fishing were both considered as impacting components and the negative trophic impact of fishing on different groups of the system was relative highly. According to top-down control or bottom-up control in several predator-prey relationships, some species can be used to control blue-green algae bloom in some area, e.g. Hypophthalmichthys molitrix had negative trophic impact on the algae due to their predation relationship in the food web, while submerged macrophytes and other macrophytes had little influence on the algae due to their competitive relationship in the food web because of their low biomass to keep the niche. Through network analysis, the system network is mapped into a linear food chain and five main trophic levels are found with a mean transfer efficiency of 11.3% from detritus and 12.3% from the primary producers within the ecosystem, which was higher than the lakes nearby, but the transfer efficiency is just 7.0% from levelⅠto levelⅡand 3.3% from levelⅤtoⅥ, which means the transfer efficiency of Dianshan Lake ecosystem is too imbalanced. The importance of discard food and the system re-cycling rate in ecosystems was low, energy utilization rate could be increased. Most of the un-used detritus deposited on the bottom of lake and affected water transparency via being agitated by the storm.The indices reflecting ecosystem maturity and stability examined TPP/TR(total primary production/total respiration), CI(Connectance Index), FCI(Finn's cycling index) were 2.80,0.17,0.0189, respectively, showed that Dianshan Lake was in such a condition of environmental stress that it was out of the lake's self-clean capability. And that conducted top-down control or bottom-up control did not work in wholefood web, ecological theory lost the power in some parts of lake. The value of FCI,1.89%, indicated that a mass of energy could be further used in Dianshan Lake ecosystem. The value of CI,0.17, indicated that the relationships in the food web were loose, suggesting that fish diversity conservation and macrophytes restoration must be considered, while internal and external pollution sources were limited. Considering Odum's theory of ecosystem development, the ecosystem was placed on a low developmental stage.