Unexpected shift from phytoplankton to periphyton in eutrophic streams due to wastewater influx
Nathanael T. Bergbusch, Nicole M. Hayes, Gavin L. Simpson, Peter R. Leavitt
Abstract
Abstract Pollution with nitrogen (N) and phosphorous (P) impairs streams by favoring suspended algae and cyanobacteria over diatom‐rich periphyton. Recently, wastewater treatment plants have been upgraded to biological nutrient removal to eliminate both P and N (mainly NH 4 + ), although little is known of the effects of this effluent on flowing waters. Here, we used high performance liquid chromatography to quantify how the abundance and composition of phytoplankton and periphyton varied in response to both influx of effluent produced by biological nutrient removal and physico‐chemical conditions in small, turbid, P‐rich streams of the northern Great Plains. At the catchment scale, analysis with generalized additive models (GAMs) explained 40.5–62.6% of deviance in total phototroph abundance (as Chl a ) and 72.5–82.5% of deviance in community composition (as biomarker carotenoids) in both planktonic and benthic habitats when date‐ and site‐specific physico‐chemical parameters were used as predictors. In contrast, GAMs using wastewater input (as aqueous δ 15 N) as a predictor explained up to 50% of deviance in Chl a , and ~60% of deviance in community composition, in both suspended (51.6% of Chl a , 67.1% of composition) and attached communities (21.5% of Chl a , 58.8% of composition). Phytoplankton was replaced by periphyton within a 60‐km wastewater‐impacted reach due to dilution of streams by transparent effluent and addition of urban NO 3 − , although predominance of phytoplankton was re‐established after confluence with higher‐order streams. Overall, influx of effluent shifted turbid, phytoplankton‐rich streams to clear ecosystems with abundant epilithon by improving water transparency and providing NO 3 − to favor benthic diatoms and chlorophytes.