Abstract
Urea can degrade water quality and stimulate toxic phytoplankton in P-rich lakes, yet little is known of its sources, abundance, or transportation in lotic systems, particularly within the Northern Great Plains. We measured physico-chemical parameters biweekly during May–September 2010–2012 at 16 stations along a 250 km lotic continuum to quantify spatial and temporal variation in urea concentrations and discharge, and to identify potential regulatory processes. Urea concentrations were similar to those in regional prairie lakes (range 5.2–792.1, median 78.6 μg N L−1) with variable seasonal mean (± SD) concentrations (96.6 ± 96.1 μg N L−1) and fluxes (4.22 × 105 ± 257.6 μg N s−1). Landscape analysis with generalized additive models explained 68.3% of deviance in urea concentrations, with high temporal variability predicted mainly by positive relationships with nutrient content and chlorophyte abundance, but not temperature, dissolved organic matter, bacterial abundance, or urban effluent. Seasonal analysis revealed that during spring, urea content was correlated negatively with leguminous forage cover (% area) and positively with stream discharge, oilseed and cereal crops, and shrubs or deciduous plants, while during summer, urea concentrations were correlated negatively with discharge and leguminous crop cover, as well as nutrient levels. Mean porewater urea concentrations (528.5 ± 229.8 μg N L−1) were over five-fold greater than stream concentrations, suggesting that hyporheic production may offset declining influx from terrestrial sources during summer. We conclude that urea may be ubiquitous in eutrophic prairie streams and that management of its export from land may reduce detrimental effects on downstream lakes.
Highlights
Dissolved organic nitrogen (DON) is increasingly recognized for its role in lotic nitrogen (N) cycles and its impacts on water quality (Berman and Bronk 2003; Wiegner et al 2006; Stanley and Maxted 2008) in lake (Berman 2001; Berman and Bronk 2003; Belisle et al 2016) and coastal ecosystems (Seitzinger and Sanders 1997; Anderson et al 2008; Korth et al 2011)
Instantaneous discharge rates (Figs. 2b, 4b) were significantly different among sampling months (Kruskal–Wallis test statistic = 30.61, p \ 0.01), with pairwise comparisons and post hoc Dwass-SteelChrichlow-Fligner tests indicating that both May and June were significantly greater than other months, but
Urea comprises * 70% of nitrogenous fertilizer application in the Canadian Prairies (Glibert et al 2006; Statistics Canada 2016), yet little is known of the patterns and controls of urea export to lotic ecosystems of the Northern Great Plains (NGP)
Summary
Dissolved organic nitrogen (DON) is increasingly recognized for its role in lotic nitrogen (N) cycles and its impacts on water quality (Berman and Bronk 2003; Wiegner et al 2006; Stanley and Maxted 2008) in lake (Berman 2001; Berman and Bronk 2003; Belisle et al 2016) and coastal ecosystems (Seitzinger and Sanders 1997; Anderson et al 2008; Korth et al 2011). While prior research on export of urea from watersheds to aquatic ecosystems has been limited mainly to temperate regions (Glibert et al 2005; Han et al 2015; Jackson 2016; Kibet et al 2016; King et al 2017), there are several reasons to expect that fluxes to freshwaters may be elevated in the grassland environments of the NGP These prairie watersheds include a variety of urea sources, including fertilizer applications (Davis et al 2016; Kibet et al 2016), intensive livestock operations (Peterson et al 2004; Kibet et al 2016), and urban effluents (Glibert et al 2006; Bogard et al 2012; Cozzi et al 2014). The timing of urea application in NGP (spring, fall) is concurrent with
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