Identification of nonpoint sources of nitrogen (N) and phosphorus (P) in urban systems is imperative to improving water quality and better managing eutrophication. Winter contributions and sources of annual N and P loads from urban watersheds are poorly characterized in northern cities because monitoring is often limited to warm-weather periods. To determine the winter export of N and P, we monitored stormwater outflow in a residential watershed in Saint Paul, Minnesota during 2012-2014. Our data demonstrate that winter melt events contribute a high percentage of annual N and P export (50%). We hypothesized that overwintering leaf litter that is not removed by fall street sweeping could be an important source to winter loads of N and P. We estimated contributions of this source by studying decomposition in lawns, street gutters, and catch basins during two winters. Rates of mass and N loss were negligible during both winters. However, P was quickly solubilized from decomposing leaves. Using mass balances and estimates of P leaching losses, we estimated that leaf litter could contribute 80% of winter total dissolved phosphorus (TDP) loading in this watershed (∼40% of annual TDP loading). Our work indicates that urban trees adjacent to streets likely represent a major source of P pollution in northern cities. Management that targets important winter sources such as tree leaves could be highly effective for reducing P loading and may mitigate eutrophication in urban lakes and streams in developed cities. (Figure Presented).
Bibliographical noteFunding Information:
We are grateful to Sandy Brovold, Michelle Rorer, and Christopher Buyarski for lab analysis of samples at UMN. This research was supported by grants from the Institute on the Environment and the Water Resources Center at the University of Minnesota and from Capital Region Watershed District. The authors are especially grateful to Capitol Region Watershed District for assistance with data collection and provision of data, and to Larry Baker and Paula Kalinosky for providing the model of municipal street sweeping in Prior Lake. We are also grateful to Elizabeth Beckman and Sharon Shinomiya for watching over our leaf litter incubations. Eric Lind contributed helpful advice and assistance with statistical analyses. A.B. was supported by an Environmental Protection Agency's STAR PhD fellowship. Feedback from three anonymous reviewers greatly improved this manuscript.
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