Nitrogen in urban stormwater caused as significant algal growth response as simulated soil erosion runoff despite differences in nitrogen forms.

Urban stormwater can accumulate significant nutrients and other pollutants from the catchment, leading to water quality degradation in receiving waters. However, the impact of nutrients in urban stormwater on the ecological processes of receiving environments, particularly compared with other point and diffuse sources of nutrients, has not been well studied. Understanding how different nutrient sources affect ecological responses can improve catchment-scale nutrient management and prioritization. This study investigated how nutrients in urban stormwater impact algal growth response using standardized bioassay, and compared the response with simulated soil erosion runoff (soil slurries), effluents from tertiary-treated sewage treatment plants (STPs) and aquaculture pond effluents. A total of 43 urban stormwater samples were examined from three local government areas in South East Queensland, Australia. The study showed that all urban stormwater treatments had significantly higher algal growth responses than a control treatment without nutrient addition. Total dissolved nitrogen (TDN) was the nutrient parameter most highly correlated with the algal response for urban stormwater samples. Urban stormwater showed a similar algal response to soil slurries and aquaculture pond effluent despite differences in nitrogen forms, and a significantly higher algal response and DOC:TDN ratio than STP effluent, at the same TDN concentration. Our findings indicate that urban stormwater is an important source of nitrogen and DOC with the potential to stimulate algal growth in nitrogen limited waterways. This study suggests that more effective control of urban stormwater nutrients and organic carbon is needed to improve water quality in receiving waters.