Effects of nitrogen removal from wastewater on phytoplankton in eutrophic prairie streams

Biological nutrient removal (BNR) may be an effective strategy to reduce eutrophication; however, concerns remain about effects on receiving waters of removing both nitrogen (N) and phosphorus (P), rather than P alone. Phytoplankton abundance (as µg chlorophyll a/L) community composition nmol biomarker pigment/L) were quantified over 6 years in two connected eutrophic streams determine how algae cyanobacteria varied response a shift from tertiary (P removal) BNR (N wastewater treatment. sampled biweekly at nine stations May September analysed using generalised additive models (GAMs) quantify landscape patterns phototrophs identify potential causal relationships before (2010–2012) after (2017–2019) installation 2016. Analysis with GAMs showed that 69%–79% deviance phytoplankton could explained by date- site-specific variance stream flow, temperature, solute concentrations (mainly nutrients), whereas similar only effluent N content (δ15Nwater) as predictor c. 60% phototroph deviance. Prior BNR, levels chlorophytes) increased -rich effluent, their declined δ15N (diatoms, chlorophytes). Overall, declines total release (67%–97%) but not (c. 0%) due resulted 52 ± 7% decline relative upstream values, despite high inter-annual variation discharge baseline concentration. Nitrogen improved water quality N-limited ecosystems.