Sources of nitrogen to stream food webs in tributaries of the Red River Valley, Manitoba
Introduction
The rapid eutrophication of Lake Winnipeg has been linked to increased nutrients resulting from changes in land use, loss of wetlands and increases in runoff associated with climate change (Kling et al., 2011, Bunting et al., 2016, Schindler et al., 2012). Moreover, previous studies have identified that the majority of nutrients (approximately 68% of the total P load and 34% of the total N load) to Lake Winnipeg are delivered via the Red River (Environment Canada and Manitoba Water Stewardship, 2011). The mixed land use catchments of the Red River Valley (RRV) have thus become a priority area for management of nutrient loads to Lake Winnipeg (Schindler et al., 2012, Bunting et al., 2011). However, effective management strategies to protect or restore stream ecological conditions in mixed land use environments first requires the identification of key nutrient sources.
The main sources of nutrient loading to tributaries in the RRV are thought to be crop cultivation, livestock production and discharge of treated municipal wastewater effluent from rural communities (Yates et al., 2012, Painter et al., 2021). Stream nutrient loads associated with runoff events, such as snowmelt and large summer storms, have been linked to the amounts of agricultural activity in RRV catchments (Rattan et al., 2017, Rattan et al., 2021). In contrast, municipal wastewater effluent is discharged to RRV streams from treatment lagoons in short (i.e., two to three weeks) periods, once or twice between mid-May and late-October (Government of Manitoba, 2019). To date, the relative contribution of the different nutrient sources subsequently assimilated into food webs, and thus driving ecological conditions in RRV streams, is not known.
Isotopic composition of nitrogen, calculated as the ratio of heavy to light nitrogen isotopes (15N/14N, hereafter δ15N), is a useful tool for identifying anthropogenic nitrogen sources (Peterson and Fry, 1987, Kendall et al., 2007). In particular, synthetic fertilizer has a distinctive δ15N value (approx. −2 to +2‰) relative to human and animal wastes (approx. +7 to +20‰), making it possible to differentiate between aquatic food webs that utilize nitrogen originating from these different sources (Kendall et al., 2007). For example, Diebel and Vander Zanden (2009) used isotopic values of primary consumer invertebrates to identify that inorganic fertilizers were a more important source of nitrogen to food webs than livestock manure. Likewise, Loomer et al. (2015) traced the inputs of nitrogen from treated municipal wastewater using isotopic values of fish and invertebrates. Assessment of basal components of the food web, such as detrital organic matter and primary consumers (e.g., collector-gatherers), is an ideal approach to identify the importance of anthropogenic sources of δ15N to food webs. This is because the approach avoids potential confounding effects introduced by assessment of higher trophic levels that can incorporate material from multiple trophic levels (Kohzu et al., 2008, Peipoch et al., 2012).
Our study aimed to identify the primary anthropogenic sources of nitrogen to food webs in rural stream subcatchments of the RRV and how these sources vary seasonally. We achieved this goal by measuring δ15N of particulate organic matter (POM) and benthic macroinvertebrates collected in spring and summer from 20 subcatchments which exhibited a range of agricultural intensity and size of municipal wastewater lagoons. Anthropogenic nitrogen sources were identified by: 1) associating δ15N of POM and benthic macroinvertebrates with descriptions of the amount of nutrient producing activities (i.e., crop cultivation, livestock density and wastewater treatment lagoons), and 2) estimating the relative contribution of nitrogen sources (i.e., fertilizer and wastes) to POM and benthic macroinvertebrates. Our findings will enable managers to more effectively target mitigation strategies aimed at reducing nutrient loadings to RRV streams draining to Lake Winnipeg.
Section snippets
Study area and experimental design
Our study was conducted during spring (late May through early June) and summer 2014 (late July through August) in 20 subcatchments located in the agriculturally dominated landscape of the RRV, part of the Lake Winnipeg Basin located in southern Manitoba, Canada (Fig. 1). Study subcatchments were selected to exhibit gradients of nutrient-producing human activities. Descriptions of crop cultivation, livestock production and wastewater treatment (WWT) were generated following Yates et al. (2012).
Associations between δ15N and human activities
δ15N of FPOM in the spring had a mean (±standard deviation) of +6.3 (±2.9)‰ with a minimum value of +3.1‰ at LA03 and a maximum value of +13.2‰ at UR04 (Fig. 2, Electronic Supplementary Material (ESM) Table S1). In summer, the mean δ15N of FPOM was +6.8 (±2.7)‰, the maximum value remained at UR04 (+14.2‰), and the minimum was observed at BR04 (+4.1‰). Spring CPOM samples had a mean δ15N value of +6.0 (±3.2)‰ with a minimum of 2.9‰ at RO01 and a maximum of +14.5‰ at UR04. In summer, the mean
Discussion
Analysis of δ15N values in 20 subcatchments in the agriculturally-dominated landscape of the Red River Valley of southern Manitoba revealed that, when present, wastewater appears to be a more important source of nitrogen to stream food webs than agricultural sources. Moreover, variation in δ15N values of particulate organic matter and collector-gatherer invertebrates was best associated with indicators of the amount of wastewater and the greater contribution of waste sources to aquatic food
Conclusion
The past 70 years has seen a marked intensification in agricultural activities and associated growth in rural communities within the RRV. These land use changes have been linked to dramatic increases in nutrient loadings to streams within the RRV and eutrophication of Lake Winnipeg (Schindler et al., 2012). Our research provides critical information on how these land use activities are influencing ecological conditions within the tributaries of the RRV. In particular, we have identified that
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Funding was provided by the Lake Winnipeg Basin Initiative led by Environment Canada, the Tobacco Creek Model Watershed Research Consortium funded by the Canadian Water Network and separate Natural Sciences and Engineering Research Council Discovery Grants to A.G. Yates and J.M. Culp. Eric Luiker, Dave Hryn and Courtney Thompson provided assistance with sample collection. Kristie Heard and Kirk Roach assisted with sample processing. We thank two anonymous reviewers whose comments and
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2021, Journal of Great Lakes ResearchCitation Excerpt :For example, Carlson et al. (2013) found that stream nutrient concentrations often exceeded water quality guideline thresholds during lagoon discharge events in Deadhorse Creek, MB. Cormier et al. (2021) found that N associated with WWT sources was readily assimilated by biota in RRV streams and was in fact the dominant source of N to organisms in the streams most heavily impacted by WWT. The ecological consequents of WWT discharge is further exacerbated by the fact that streams in the RRV often experience low to no flow as the summer season progresses.
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2021, Science of the Total EnvironmentCitation Excerpt :Likewise, effluent from WWT systems, such as municipal lagoons, that lack advanced treatment is an important source of ammonium (Holeton et al., 2011) and it is likely that the ammonium discharged from municipal lagoons during the spring and summer months in the RRV is rapidly taken up by biota. Biotic uptake of waste derived ammonium is consistent with a study by Cormier et al. (2020), who reported 15N-enriched particulate organic matter and benthic invertebrates was associated with WWT at many of the same sites sampled for our study. Similarly, in their study of the La Tordera River in Spain, Ribot et al. (2012) observed rapid longitudinal depletion of WWT ammonium compared to nitrate during the summer season and hypothesized that preferential ammonium uptake by epilithic biofilm may play a key role in river DIN processing.
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