Trophic structure and energy flow of continental slope of the northern South China Sea ecosystem

The continental slope is the transition area from the continental shelf to the deep sea. It has unique geographic ecological environment and active exchange of energy and matter fluxes. This study presents a mass-balanced trophic model for the continental slope of the northern South China Sea to explore the trophic structure, energy flow and system attributes of this ecosystem. The model consists of 22 functional groups, and biological and ecological data gathered from the surveys of fishery resources and marine ecological environment in this area during 2015–2016 using Ecopath with Ecosim 6.5 software. The fractional trophic levels of these ecological groups ranged from 1 to 4.47, with the highest TLs reached by dolphins in the food-web system. The Keystone Index (KSI) analysis indicated that the mesozooplankton and Sthenoteuthis oualaniensis are considered as key species in the system. The energy flow of this ecosystem was mainly dominated by detritus food chain, which accounts for 52% of the energy source. Using network analysis, the system network was mapped into a linear food chain and 6 integrated trophic levels were found with a mean transfer efficiency of 21.94%, of which 21.26% came from primary producer and 22.39% from detritus within the ecosystem．The results suggest that the continental slope of the northern South China Sea ecosystem has a high ecological conversion efficiency.