Net changes in nutrient concentrations below a point source input in two streams draining catchments with contrasting land uses

We examined net changes in ammonium-Nitrogen (NH4+-N), nitrate-Nitrogen (NO3−-N), dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP), and dissolved organic carbon (DOC) chloride-corrected ambient concentrations along a reach located below a wastewater treatment plant (WWTP) input in a non-agricultural (12 dates) and an agricultural (6 dates) stream. Based on those net changes, we estimated processing length (Snet) and mass transfer coefficient (Vf) of the cited nutrients. In the agricultural stream, results suggest that diffuse nutrient inputs from adjacent agricultural fields had a greater effect on water chemistry than the WWTP input, and probably overwhelmed the stream capacity to retain and transform nutrients. In the non-agricultural stream we observed consistent longitudinal trends below the WWTP input only for NH4+-N and NO3−-N. The tight coupling between longitudinal NH4+-N decreases and NO3−-N increases in the non-agricultural stream, and lack of longitudinal trends of DIN on most dates suggest that NH4+-N from the WWTP input was being nitrified along the reach. These results suggest that WWTP inputs favor conditions to support hot spots for chemoautotrophic activity.