Inputs from wastewater treatment plants (WWTP) increase nutrient loads in receiving streams, especially in Mediterranean regions due to low stream dilution. Further, these Mediterranean streams have a low capacity to reduce dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) excesses. We hypothesize that a low availability of labile carbon (C) can limit the heterotrophic activity of these streams; and thus, we expect that an increase in labile C will decrease DIN and SRP concentrations by promoting in-stream heterotrophic activity. We tested this hypothesis by adding a brewery by-product, as a source of labile C, to a WWTP-receiving stream and examining its effects on DIN (ammonium and nitrate) and SRP concentrations. We ran a 4-day C addition in a 180 meters reach of a forested headwater Mediterranean stream in July 2022, when 100% of the discharge was sourced by the WWTP effluent. We collected hourly water samples along the reach before and during the C addition. Diel oscillations of ammonium and nitrate concentrations together with longitudinal changes of both DIN and SRP forms during background conditions, suggest a high in-stream bioreactive capacity controlling nutrient concentrations. The in-stream capacity to process nutrients increased during the C addition, and the stream even shifted from being a source to a sink of DIN. Our results support the idea that the capacity of WWTP-receiving streams to mitigate nutrient loads is limited by the availability of labile C sources.

Primary Presenter: David Pineda Morante, CEAB-CSIC (dpineda@ceab.csic.es)

Authors:

David Pineda-Morante, CEAB-CSIC  (dpineda@ceab.csic.es)

Miquel Ribot, CEAB-CSIC  (mribot@ceab.csic.es)

Susana Bernal, CEAB-CSIC  (sbernal@ceab.csic.es)

Sara Castelar, CEAB-CSIC  (saracastelar@ceab.csic.es)

Esperança Gacia, CEAB-CSIC  (gacia@ceab.csic.es)

Anna Lupon, CEAB-CSIC  (alupon@ceab.csic.es)

Stephanie Merbt, CEAB-CSIC  (stephanie.merbt@ceab.csic.es)

Francesc Sabater, University of Barcelona  (fsabater@ub.edu)

Helena Guasch, CEAB-CSIC  (helena.guasch@ceab.csic.es)

Eugènia Martí, CEAB-CSIC  (eugenia@ceab.csic.es)