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

Abstract 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) and community composition (as nmol biomarker pigment/L) were quantified over 6 years in two connected eutrophic streams to determine how algae and cyanobacteria varied in response to a shift from tertiary (P removal) to BNR (N and P removal) wastewater treatment. Phytoplankton were sampled biweekly at nine stations May to September and were analysed using generalised additive models (GAMs) to quantify landscape patterns of phototrophs and identify potential causal relationships both before (2010–2012) and after (2017–2019) BNR installation in 2016. Analysis with GAMs showed that 69%–79% of deviance in phytoplankton abundance and composition could be explained by date‐ and site‐specific variance in stream flow, temperature, and solute concentrations (mainly nutrients), whereas similar GAMs using only effluent N content (δ 15 N water ) as a predictor explained c . 60% of phototroph deviance. Prior to BNR, phytoplankton levels (mainly chlorophytes) increased with ‐rich effluent, whereas their abundance declined with δ 15 N after BNR (diatoms, chlorophytes). Overall, declines in total effluent release of N (67%–97%) but not P ( c . 0%) due to BNR resulted in a 52 ± 7% decline in phytoplankton abundance relative to upstream values, despite high inter‐annual variation in discharge and baseline chlorophyll a concentration. Nitrogen removal by BNR improved water quality in N‐limited ecosystems.