Hydropower development is the key strategy in many developing countries for energy supply,climate-change mitigation and economic development.However,it is commonly assumed that river dams retain nutrients and therefore reduce downstream primary productivity and fishery catches,compromising food security and causing trans-boundary disputes.Contrary to expectation,here we found that a cascade of reservoirs along the upper Mekong River increased downstream bioavailability of nitrogen and phosphorus.The dams caused phytoplankton density to increase with hydraulic residence time and stratification of the stagnant reservoirs caused hypoxia at depth.This allowed the release ofbioavailable phosphorus from the sediment and an increase in dissolved inorganic nitrogen as well as a shift in nitrogen species from nitrate to ammonium,which were transported downstream by the discharge of water from the base of the dam.Our findings provide a new perspective on the environmental impacts of river dams on nutrient cycling and ecosystem functioning,with potential implications for sustainable development ofhydropower worldwide.