Denitrification is the only process by which nitrogen can be permanently lost from ecosystems; the removal of excess nitrogen by stream bacteria, therefore, is a critical ecosystem service. Left unchecked, excess nitrogen in waterways leads to water quality problems, such as harmful algal blooms and regional hypoxia. Few studies, however, have addressed impacts of land-use change on the communities of bacteria performing denitrification. To address this research gap, our study focuses on a) characterizing differences between denitrifying bacteria (denitrifier) communities in urban versus forested streams and b) linking denitrifier community structure to denitrification rates. We expect denitrifier community structure to shift in response to selection pressures imposed by the environmental changes that follow watershed urbanization, including increased hydrologic disturbances, simplified channel structure, and elevated nutrient and chemical pollutant concentrations. Furthermore, we postulate a strong link between community composition and denitrification rates because denitrifier taxa differ in the way they respond to the environment and the efficiency with which they perform denitrification. To test these predictions, we conducted a field survey of 4 paired urban and reference forested streams (n=8), collecting data on denitrification rates, denitrifier community structure (i.e., abundance, taxa richness, and composition), and a suite of abiotic environmental factors.
Results/Conclusions
Standard biogeochemical models based on abiotic environmental factors predict high denitrification rates in ecosystems with high nitrate, high dissolved organic carbon, and low dissolved oxygen, conditions typical of urban streams, including those in our survey. Our survey, however, found lower denitrification rates in urban versus forested streams, suggesting that some as yet unidentified factor is inhibiting denitrification in urban streams; denitrifier community structure may be responsible. The denitrifier communities in our urban stream sediments appear to be compositionally distinct from their forested stream counterparts, based on culture-independent terminal restriction fragment length polymorphism (tRFLP) analyses of nirK genes. nirK genes code for a key enzyme in the denitrification pathway. Denitrifier abundance (based on quantitative PCR results) and species richness (based on tRFLP data), on the other hand, were comparable between urban and forested stream communities. We speculate that pollutant resistant taxa dominate the urban denitrifier communities and that pollutant resistance comes at a cost (or tradeoff) of lowered enzyme activity. A better understanding of the impacts of watershed urbanization on denitrifying bacteria community structure and function is a pre-requisite to effective management and restoration of urban streams.
