Contrasting raz-rru stream metabolism and nutrient uptake downstream of urban wastewater effluent sites

Understanding how key stream ecosystem functions respond to wastewater treatment plant effluent is critical for assessing the ability of stream ecosystems to ameliorate anthropogenic nutrient loading and to effectively manage and restore impacted systems. We evaluated instream metabolism, reactive solute transport, and nutrient uptake along two 1.5 to 2-km-long reaches of a 2nd-order stream in the urbanized suburbs of Philadelphia, Pennsylvania, USA, each directly downstream of a wastewater treatment plant outfall. We compared metabolism based on resazurin-resorufin (Raz-Rru) with nutrient uptake and dissolved oxygen (DO) metabolism calculations. Plateau co-injections of the Raz-Rru metabolic tracer system and fluorescein provided integrated stream metabolism measurement. We sampled tracer concentrations hourly along longitudinal profiles and recorded them continuously, along with DO, at 2 discrete locations. The smaller reach 1, characterized by higher nutrient concentrations and canopy cover, had higher short-term transient storage and Raz uptake velocity. In contrast reach 2, with lower nutrient concentrations and less canopy cover, had higher nitrate and phosphorus uptake along with higher rates of gross primary productivity (GPP) and ecosystem respiration (ER). Temporal analysis indicated nitrate uptake increased over the afternoon at reach 2, whereas Raz uptake declined at both reaches. Our results suggest that nutrient uptake and GPP are sensitive to excessive nutrient concentrations and light in our system. In contrast, lower light and higher transient storage are likely driving larger, reach-scale spatial differences in Raz-based ER. Increasing nitrate uptake at reach 2, which lags behind diel DO concentrations, is likely the result of assimilatory N uptake coupled to GPP moderated by nitrification and denitrification, whereas decreasing rates of Raz transformation are likely related to diel variation in heterotrophic uptake. Our ability to resolve sub-daily changes in ER illustrates one of the key advantages of the Raz-Rru tracer system. However, our results also show the need for further investigation into the drivers of sub-daily ecosystem metabolism in streams as well as the mechanistic differences between DO- and Raz-based estimates of ER. Contrasting results from different measures of metabolic activity between reaches and over time highlight the complexity of metabolic processes in high-nutrient systems.