People in and downstream of cities rely on lakes and rivers for a variety of services. Despite progress made in reducing point sources of pollution to urban surface waters, these waters continue to be impaired by so called “non-point” pollution, particularly excessive inputs of nitrogen (N) and phosphorus (P). We’ve been taking a watershed approach to understanding urban water pollution, quantifying inputs and outputs of N and P in urban subwatersheds of the Mississippi River, in Saint Paul, Minnesota, USA towards improving management of urban nutrient pollution. Household actions of lawn fertilization and pet ownership contributed the majority of watershed N and P inputs, respectively. N and P exhibited contrasting dynamics within watersheds. In contrast to many non-urban watersheds that exhibit high P retention, these urban watersheds have high street density that enhanced transport of P-rich materials like tree leaves from landscapes to stormwater, likely contributing to surface water degradation. High apparent N retention likely resulted from unmeasured watershed N losses to the atmosphere and groundwater. These contrasting dynamics suggest that N management should emphasize reducing watershed inputs from residential fertilizer, while P management should focus on reducing watershed P inputs and transport from vegetated landscapes to streets and storm drains through leaves and lawn runoff.
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