Application of mass-balance modelling to assess the effects of ecological restoration on energy flows in a subtropical reservoir, China

Highlights: • Hydrophytic establishment limited the production/biomass of the plankton community. • Biomanipulation enhanced the energy flows along herbivorous/molluscivorous food chains. • The varying keystone groups indicated a shift in trophic control from top-down to wasp-waist. • The restored system was more mature, cycled, organized, and web-like than before. • A fishery policy was proposed to harvest the stocked fish at 0.27–6.53 g/m²/year. -- Abstract: Eutrophication is a leading cause of impairment of lentic water bodies throughout the world. To inhibit algal blooms and remove excess nutrients, a 10,000 m² restoration project consisting of vegetation establishment and fish manipulation was conducted in the eutrophic bay of the Yantian Reservoir, southern China. Three Ecopath models were constructed to assess the recovery effects at an ecosystem level, and time series data were simulated to propose a fishery policy. During the restoration, 1) the redundant primary production flowing back to detritus decreased due to the increased predation of four stocked fish with different feeding habits; 2) the transfer efficiencies (TEs) through trophic levels increased due to the reinforced energy flows along the planktivorous, herbivorous, and molluscivorous food chains; 3) the groups that had the highest keystoneness shifted from carnivorous fish to invertivorous fish and omnivorous shrimp, indicating the shift of mixed trophic impacts from top-down to wasp-waist control; and 4) the changing indices of path length, flow fluxes, matter cycling, and network information showed that the restored system was more mature, developed, and organized than before. To sustain the long-term energy balance and functioning of the ecosystem, the maximum fishing yields (0.37–8.53 g/m²/year) were determined to maintain the relative biomass (close to 1) of stocked fish and wild tilapia by harvesting their annual production.