Aquatic community metabolism response to municipal effluent inputs in rivers quantified using diel δ18O values of dissolved oxygen
The spatial footprint over which municipal wastewater effluents cause changes to aquatic community structure and metabolism is key information required for the management of discharges into rivers. Longitudinal studies were undertaken on the Bow and South Saskatchewan rivers, Canada, to assess a new isotopic and modelling approach that combined O2 and δ18O-O2 diel (24 h) response curves to quantify changes in integrated community aquatic metabolism as a result of point-source wastewater inputs. Diel samplings were conducted over four seasons along 50 km transects at Calgary (Bow River) and Saskatoon (South Saskatchewan River). Diel O2 and δ18O-O2 cycles grew in magnitude downstream of effluent inputs in all seasons compared with upstream control sites. δ18O-O2 depletions clearly revealed the stimulating effect of effluent on aquatic photosynthesis. Diel isotopic mass balance modelling showed community metabolic responses to effluent inputs were most pronounced in the spring and summer when photosynthesis and respiration rates were about two- to three-fold higher than at upstream control sites. Our findings revealed that sewage treatment plant nutrient additions resulted in an enhanced metabolic footprint that extended beyond 50 km downstream.