Abstract
External nitrogen (N) inputs originating from human activities act as essential nutrients accumulation in aquatic ecosystems or it is exported elsewhere, where the assimilation capacity is surpassed. This research presents a multi-annual case study of the dissolved inorganic nitrogen (DIN) in an urban river in Ontario (Canada), assessed changes in N downstream of the largest wastewater treatment plant (WTP) in the watershed. Changes in the DIN effluent discharge, in-river concentrations and loads were observed comparing the intra- and inter-annual variability (2010–2013) before, during and after WTP upgrades. These upgrades reduced the ammonium concentration in the river from 0.44 to 0.11 mg N-NH4+/L (year average), but the N load in the effluent increased. In the river, nitrate and ammonium concentrations responded to seasonal variability, being higher during the low temperature (>10 °C) and high flow seasons (spring and spring melt). Among years, changes in the DIN concentration are likely controlled by the effluent to river dilution ratio, which variability resides on the differences in river discharge between years. This suggest that the increasing trend in the DIN concentration and loads are the result of agricultural and urban additions, together with reduced N assimilation, in addition to N loads responding to variable river discharge. Finally, we propose monitoring both concentrations and loads, as they provide answers to different questions for regulatory agencies and water managers, allowing tailored strategies for different purposes, objectives and users.
Highlights
Nitrogen (N) is an essential nutrient that is sometimes limiting in aquatic ecosystems [1,2,3,4]
Ammonium concentration consistently bedischarged by the KWTP and the magnitude of the upgrades, a large part of the results low 5 mg N-NH4+/L has been observed in the effluent since May 2013
The Impact of the Upgrades at the Kitchener Wastewater Treatment Plant grades are seen in the DO and the Dissolved inorganic nitrogen (DIN) concentrations at Blair (5700 m downstream of the KWTP)
Summary
Nitrogen (N) is an essential nutrient that is sometimes limiting in aquatic ecosystems [1,2,3,4]. Primary producers meet their N requirements by a combination of N fixation, mineralization, atmospheric deposition and surface runoff [5]. NO3 − + NH4 + ) entering into rivers and streams originates from human activities [9,10,11]. Surplus N inputs can be from point sources (wastewater effluent, septic systems) and non-point sources (agricultural runoff, atmospheric deposition). The capacity of a catchment to process, retain or export reactive nitrogen depends primarily on its in-stream productivity, nutrient availability, channel morphology and river discharge [12,13,14]
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