Determining Critical Nutrient Thresholds Needed to Control Harmful Cyanobacterial Blooms in Eutrophic Lake Taihu, China

Nutrient overenrichment has led to dramatic increases in harmful cyanobacterial blooms, creating serious threats to drinking water supplies, ecological and economic sustainability of freshwater ecosystems. Nutrient-cyanobacterial bloom interactions were examined in eutrophic Lake Taihu, China. In situ microcosm nutrient dilution bioassays and mesocosm nutrient addition experiments were conducted to determine nitrogen (N) and phosphorus (P) concentration and load thresholds needed to control cyanobacterial bloom formation. Blooms were dominated by toxic, non N-2 fixing Microcystis spp, from May to December. Dilution bioassays showed seasonality in nutrient limitation, with P-availability controlling prebloom spring conditions and N-availability controlling summer-fall blooms. Nutrient dilution and enrichment bioassays indicated that total nitrogen (TN) and total phosphorus (TP) concentration thresholds should be targeted at below 0.80 mg L-1 and 0.05 mg L-1, respectively, to limit intrinsic growth rates of Microcystis dominated blooms. Based on estimates of nutrient loading and observed stoichiometry of phytoplankton biomass, 61-71% TN and 20-46% TP reduction are necessary to bring Taihus phytoplankton biomass to acceptable sub-bloom conditions of less than 20 mu g L-1 chlorophyll a.