A changing urban phosphorus fingerprint: Internal P dynamics alter phosphorus speciation in urban aquatic systems

Background/Question/Methods

Urban environments are known for their large phosphorus (P) footprint that negatively affects water quality in downstream ecosystems. The reduction of the urban derived P is an important management objective that requires knowledge of the source and biogeochemistry of this nutrient. Very few studies have examined the biogeochemistry of P and the variation in the different P forms (i.e. inorganic, dissolved organic, or particulate) in urban environment. The aim of this study was to examine the dynamics of different types of P in urban ponds, which are generally constructed to ameliorate water quality from urban runoff. We analyzed different P forms (TP, PP and dissolved organic P; DOP) in surface water samples, collected from 12 urban ponds located in Southern Ontario, Canada in the spring and late summer of 2012. We focused our study especially on the concentrations of DOP and that of two separate forms with different bioavailability; phosphomonoesters and phosphodiesters, measured by enzymatic hydrolysis. 

Results/Conclusions

Urban derived-suspended particulates and particulate P (PP) were significantly related with Chl-a contents in urban ponds with slight increase of concentrations from surface water along the water flow while DOP concentrations remained relatively stable from inflow to outflow. During the late summer, we found a clear a pattern in the prevalence of two different organic P forms within ponds from inflow to outflow water. This pattern was one where phosphomonoester was 60 % of DOP in urban runoff entering the pond, but was reduced to <10 % of DOP at the outflow locations. In contrast, phosphodiester concentration increased through urban ponds, accounted for over 90 % of DOP in the outflow waters. Since phosphodiester is more labile P form to algae and microbes than phosphomonoester, this increased phosphodiesters in outflow waters can be a potential threat to downstream ecosystems. Our results indicate that different P forms behave differently in urban ponds. Compared to particulates which were removed efficiently in urban ponds, DOP stayed at considerable quantities in standing water and only forms of DOP pool was modified by internal processes in urban ponds.