Dissolved organic matter addition alters diel nitrogen and phosphorus uptake downstream of a wastewater treatment plant effluent

Abstract Wastewater treatment plant (WWTP) effluents deliver high loads of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) to receiving streams and can generate dissolved organic carbon (DOC): nutrient imbalances, potentially constraining microbial DIN and SRP processing and increasing downstream nutrient export. We hypothesized that increasing bioavailable DOC supply and altering DOC: nutrient stoichiometry would stimulate heterotrophic activity and enhance in-stream DIN and SRP uptake downstream of a WWTP effluent. We conducted a four-day reach-scale enrichment with a brewery-derived dissolved organic matter (DOM) solution rich in bioavailable DOC in a Mediterranean headwater stream fed exclusively by WWTP effluent. Hourly samples were collected upstream and at two downstream stations relative to the DOM injection point to quantify spatiotemporal variation in net nutrient uptake. Stoichiometric analysis suggested that heterotrophic nutrient uptake was constrained by DOC and DIN relative to high SRP availability. DOM enrichment partly alleviated DOC limitation, and rapid net DOC removal occurred within the first meters downstream of the injection point, coinciding with strong oxygen depletion and benthic mat development. During enrichment, the reach shifted from net DIN source to sink, mainly through enhanced nitrate uptake. Ammonium uptake showed a weaker, more localized, and diel-dependent response, whereas no clear effect was detected for SRP uptake. Despite enhanced local uptake, whole-reach reductions in DIN and SRP export remained < 15% relative to WWTP nutrient loads. Overall, our results support using system-specific C:N:P stoichiometry to guide DOC-based nutrient attenuation strategies by identifying C, N, and P limitation or excess in WWTP-receiving streams.