Stormwater detention ponds are a common best management practice (BMP) designed to moderate the impacts of development on surrounding ecosystems. On Kiawah Island, South Carolina, stormwater detention ponds receive high levels of nitrogen (N) and phosphorus (P) loading from non-point sources, which results in eutrophic conditions, recurrent water quality degradation, and problematic cyanobacterial blooms during summer and fall. The purpose of this study was to quantify planktonic cyanobacterial responses to manipulations of the form of N (NO 3 -, NH 4 + , urea, or combinations) to determine their relative importance for cyanobacterial blooms. Nutrient addition bioassays were performed monthly (June to September 2009) in 2 brackish water ponds designated as K61 and K67. The 2 study sites were similar in all physical parameters except pond K61 was more brackish (salinity = 14 ± 2, mean ± SD) than pond K67 (6 ± 2). Urea additions resulted in a greater increase in cyanobacterial biomass than other N forms in pond K61 while urea and NH 4 + additions promoted growth in pond K67. The bioassays showed a relatively strong cyanobacteria response to the N additions for both ponds (except K67 in July). However, in all cases, the cyanobacterial contribution was always < 36% of total phytoplankton biomass. Our results suggest that both NH 4 + and urea may preferentially stimulate cyanobacterial growth in brackish stormwater detention ponds on Kiawah Island in the summer months. Although fertilizer application procedures and rates are not known for the areas drained by the stormwater detention ponds on Kiawah Island, our results suggest that the use of urea-based fertilizers, which produce both urea and NH 4 + in runoff, could foster the development and maintenance of cyanobacteria blooms.