Impact of nutrient loading on phytoplankton: a mesocosm experiment in the eutrophic Lake Taihu, China

An increased nutrient loading drives eutrophication of lake ecosystems. Nutrient loading has two different origins: (1) internal loading due to nutrients release from sediments and (2) external loading due to nutrients entering the lake ecosystem from catchment and atmospheric deposition. A 10-day mesocosm experiment was conducted to assess the effects of two nutrient origins on algal photosynthetic capacity, biomass, and phytoplankton community composition of Lake Taihu in the Yangtze River zone. The experiment was specifically designed to mimic (1) internal loading including sediment release in both static and turbulent situations, and (2) external loading including addition of P only, N only, and co-addition of both N and P. Sediment resuspension significantly increased nutrient loading and decreased algal photosynthetic capacity. Resuspension brought benthic algae into water column, resulting in an increase of algal biomass. Nutrient addition significantly changed N:P ratios and phytoplankton community composition. Nutrient addition optimized the dominant species. N Addition favored cyanobacteria reproduction, while P addition did not benefit algal growth and chlorophytes was the dominant in summer. With N and P co-addition, both cyanobacteria and chlorophytes were dominant. Hence, a long-term nutrient management strategy for Taihu watershed should reduce the inputs of both N and P.