Using the δ15N of submerged biomass for assessing changes in the nitrogen cycling in a river receiving wastewater treated effluent

We measured the changes in concentration and isotopic composition of the two major dissolved inorganic nitrogen species (ammonium and nitrate) and the isotopic composition of submerged biomass, in order describe the changes in the nitrogen cycling at a river receiving treated effluent before and after upgrades at a wastewater treatment plant in southwestern Ontario (Canada). We observed that changes in the δ15N of macrophytes and periphyton are correlated to changes in the quality of treated effluent. We explored two scenarios, differential assimilation of dissolved nitrogen species without isotopic fractionation using a two end-member mixing model, and isotopic fractionation due to assimilation (αassimilation) in macrophytes. The two end-member mixing model suggested different contribution per N specie, and αassimilation was estimated to be around 1.005 and 1.012 before and after upgrades, respectively. Thus, we provided evidence that the δ15N of submerged biomass can be used as a proxy for in-stream nitrogen cycling and complementary to water quality monitoring for evaluating changes in the nitrogen cycling in urban catchments.