| Kiawah Island is a rapidly growing coastal community on a barrier island in the southeastern US. Best management practices mandate stormwater detention ponds to moderate the impacts of development on surrounding ecosystems. The stormwater detention pond network receives high levels of nitrogen (N) and phosphorus (P) loading from golf courses, residential areas, road runoff, atmospheric inputs, wildlife waste, and oceanic inputs. This system is characterized by eutrophic conditions and recurrent water quality degradation (hypoxia and anoxia, fish kills, and harmful algal blooms). Cyanobacterial blooms ( Microcystis aeruginosa, Anabaena sp., and Oscillatoria sp.) are common in these ponds during the summer and late fall and are attributed to nitrogen loading. The purpose of this study was to quantify planktonic cyanobacterial responses to manipulation of the form of N (NO3 -, NH4+, urea, or combination). Nutrient bioassays were performed in situ monthly (June to September 2009) in stormwater detention ponds K61 and K67 on Kiawah Island. The two study sites were similar in all physical parameters (size, temperature, pH) except salinity. Pond K61 was more brackish (average salinity 14 ppt) than pond K67 (average salinity 5 ppt). Urea additions resulted in significantly greater cyanobacterial biomass than the control or other N forms in pond K61 while NO3- additions promoted cyanobacterial biomass in pond K67. The molar DIN:DIP ratio averaged 0.15 for all treatments over the sampling period and had no significant affect on cyanobacterial abundance. The different cyanobacterial responses to N treatments may reflect differences in the cyanobacteria community composition and salinities at in the ponds. The importance of NO3- as a N source for cyanobacteria is well documented however, organic N (urea) may also stimulate cyanobacterial growth in mesohaline ponds. The use of urea-based fertilizers could possibly be a contributing factor to cyanobacterial blooms in this pond system. |
