Sources of nitrogen to stream food webs in tributaries of the Red River Valley, Manitoba

Lake Winnipeg has undergone rapid eutrophication over the last several decades with a large portion of the associated nutrient loads delivered via the Red River. Consequently, subcatchments of the Red River Valley (RRV) are priorities for nutrient management, an aim that requires identification of key nutrient sources to stream food webs. We identified the primary sources of nitrogen to food webs of rural streams of the RRV, as well as variation in nitrogen input between spring and summer. We measured 815N of particulate organic matter (POM) and collector-gatherer invertebrates collected in spring and summer from 20 subcatchments, which exhibited a range of agricultural intensity and size of municipal wastewater lagoons. 815N values of POM and collector-gatherer invertebrates were best predicted by the presence of wastewater treatment lagoons, with 815N values increasing with the number of people served by lagoons in spring and summer. When present, wastewater contributed a greater proportion of nitrogen to stream food webs than agricultural sources. Waste sources also had a greater relative contribution to food webs in summer than spring. Despite wastewater lagoons releasing nitrogen in short-term, pulsed discharges, the influence of wastewater on food web nitrogen persisted from the summer release into the following spring. Based on the observed importance of wastewater as a source of nitrogen to stream food webs, we recommend management agencies consider additional actions to reduce nutrient losses from wastewater treatment facilities as well as agricultural lands to more effectively protect aquatic ecosystems. (c) 2020 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research.

Automated summary

The study found that wastewater treatment facilities are a major source of nitrogen in rural stream food webs, with a greater contribution in summer compared to spring. The impact of wastewater on nitrogen persists from summer releases into the following spring, indicating a high degree of persistence.