Using Stable Isotopes to Identify Sources of Organic and Inorganic Phosphorous and Nitrogen in the Twin Cities Watershed

EPA Grant Number: FP917432
Title: Using Stable Isotopes to Identify Sources of Organic and Inorganic Phosphorous and Nitrogen in the Twin Cities Watershed
Investigators: Bratt, Anika R
Institution: University of Minnesota
EPA Project Officer: Packard, Benjamin H
Project Period: September 1, 2012 through August 31, 2015
Project Amount: $126,000
RFA: STAR Graduate Fellowships (2012) RFA Text |  Recipients Lists
Research Category: Academic Fellowships , Fellowship - Aquatic Ecology


N and P are the most ubiquitous water pollutants across aquatic habitats. Urban contributions of these nutrients can be large, yet very little is known of urban sources because they are more diverse than any other system due to high densities of a variety of human activities. Stable isotope analyses often are applied to natural and agricultural systems. This research seeks to examine how they can be used in urban systems toward detailed and mechanistic understanding of N and P sources in human dominated areas. To address this broad question, the focus of the scope of this work will be as follows: (1) What are the sources of N and P in urban ecosystems? (2) What role does seasonal variation play, especially snowmelt, in these sources? (3) How do landscape features (e.g., tree species cover, management and soil characteristics) influence sources and transformations of N and P?


The study will address these questions using the Twin Cities metropolitan area watershed as a model site. Methods include collaboration with a local watershed management agency to collect both bi-monthly baseflow and event (immediately following precipitation) water samples in five urban watersheds in 2013 and 2014. Sources of dissolved inorganic and organic N in water samples using stable isotope analysis of d15N will be determined. Recent studies have demonstrated that 18OP (bound to P) can be an accurate and precise tracer of dissolved inorganic P in freshwater and estuarine ecosystems. These methods will be employed to elucidate urban sources of inorganic and organic P. The study will augment sampling regimes (i.e., long-term vs. intensive event sampling) in certain watersheds to investigate questions of seasonality and connections to landscape features.

Expected Results:

It is expected that human activity and terrestrial input (i.e., leaf litter, throughfall) will be the dominant sources of N and P, but that sources will vary greatly at small spatial scales. Preliminary work suggests that tree species cover, soil characteristics and snowmelt dynamics may play a large role in driving differences in sources and loads of N and P in baseflow and event runoff.

Potential to Further Environmental/Human Health Protection

N and P availability often limits primary production and, in excess, can fuel large algal blooms and subsequent decomposition and depletion of dissolved O2. Hypoxia and anoxia commonly are consequences of eutrophication in human-impacted aquatic ecosystems, and such impairment of freshwater and estuarine ecosystems has drastic implications for aquatic ecosystem health (i.e., gross disruptions of ecosystem processes and foodweb interactions). This reduced water quality ultimately can affect human wellbeing. Management and reduction of both N and P in these systems is necessary for ecosystem, and thereby human health.

Supplemental Keywords:

urban ecology, water quality, stable isotopes

Progress and Final Reports:

  • 2013
  • 2014
  • Final