Based on the 9-year monthly monitoring data,the response of water quality to the economic development activities and the protection countermeasures in catchment and water body of Tianmuhu Reservoir,a meso-eutrophic reservoir with an area of 12 km² located in southeastern of China,was studied.The result showed that the high-density big-head carp (Hypophthalmichthys nobilis) cultivation benefited the algal bloom in the growth season of phytoplankton.Fishery adjustment,with decrease the bighead carp biomass and increase the silver carp (Hypophthalmichthys molitrix) to big-head carp ratio,significantly decreased the peak value of summer phytoplankton biomass.While when the biomass decreased to relative low value,such as 15 mg/L,the control ability of fishery to phytoplankton became weak.The influence of nutrient condition and climate factor became more important to the growth of phytoplankton.Agricultural and tourism development dramatically enhanced the nutrients runoff to the reservoir,especially the tea-farming in the steep slopes strongly increase the spring nitrogen concentration in the reservoir.Low rainfall could significantly decrease the nitrogen concentration in the water on the seasonal scale.This study suggests fishery adjustment,strictly controlling agriculture and tourism development,decreasing the fertilizer application,and expanding the wetland buffer area in the catchment,are the fundamental countermeasures for the water protection in the reservoirs located at southern China like Tianmuhu Reservoir.