2004 Progress Report: Attenuation of Non-Point Source Nitrogen Pollution in a Coastal Watershed: Implications for Nutrient Management

EPA Grant Number: R830652
Title: Attenuation of Non-Point Source Nitrogen Pollution in a Coastal Watershed: Implications for Nutrient Management
Investigators: Piehler, Michael F. , Whalen, Stephen C. , Band, L. R. , Paerl, Hans
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Packard, Benjamin H
Project Period: January 20, 2003 through January 19, 2006 (Extended to January 19, 2007)
Project Period Covered by this Report: January 20, 2004 through January 19, 2005
Project Amount: $737,555
RFA: Nutrient Science for Improved Watershed Management (2002) RFA Text |  Recipients Lists
Research Category: Water , Water and Watersheds


The objective of this research project is to provide detailed field and experimental data on transformations of nitrogen (N) in agricultural and forested catchments. These data will be organized into a process-based spatial model to predict the quantity and form of nitrogen exported.

Progress Summary:

Water Quality Monitoring

A primary component of this project is development of a nitrogen mass balance for representative forest and agricultural ecosystems in the lower Neuse River watershed. The study sites are located within the South River Basin, which discharges to the Neuse River Estuary. Mass balances were developed by weekly streamwater sampling and more frequent automated flow-paced sampling during storm events. Data compiled from August-December 2003 showed that although only one-half of the total flow volume occurred during storm events, the majority of the dissolved inorganic nitrogen export occurred during this time. These results suggest that storm-derived nutrients are a major component of the nutrient export from this watershed but also that instream retention during baseflow is significant.

Higher nutrient concentrations, particularly nitrate, were observed in the agricultural watershed compared to the forest. In the forest, ammonia was the dominant form of inorganic nitrogen, whereas nitrate dominated at the farm. Both streams exhibited a rapid rise in streamflow in response to increased precipitation suggesting low storage capacity in both catchments. In the agricultural watershed, the concentrations were higher on the falling limb compared to the rising limb at equivalent flow rates. A potential mechanism for this response is the release of low nitrogen groundwater followed by a delayed flushing of terrestrial nitrogen. In contrast, in the forested catchment, nitrate and ammonium concentrations were higher on the rising limb of the hydrograph than on the lower limb at equivalent discharges. An explanation for this observation is that a store of nitrogen (from mineralization and nitrification) in the stream and shallow terrestrial soils was flushed downstream in response to the storm event.

Denitrification in Headwater Stream Sediments

The spatial and temporal variability of denitrification (the biological transformation of NO3- to N2) was assessed in both watersheds on a seasonal basis. Results showed a distinct spring maximum and higher rates in the farm than the forest. In addition, denitrification rates in the first order drainage ditches were generally higher than larger canals and the natural stream channel in both watersheds. However, these results were highly variable, and differences were not statistically significant making definitive conclusions on controlling factors impossible. As a result, experimental manipulations of several potential variables were conducted based on those that were suggested by seasonal data patterns.

The effects of temperature, nitrate, and carbon were investigated in the first order ditches in both watersheds. At the farm, nitrate stimulated rates above unamended controls but were lower when nitrate and carbon were added together. The effect was more pronounced when combined with increased temperatures. These data suggested that denitrifiers were nitrate limited but that competition for nitrate by aerobes may have controlled nutrient fluxes when a labile carbon source was available. Similar manipulations at the forest showed no effect of increased nitrate during the spring but a stimulatory effect during the fall. Carbon additions did not affect denitrification rates but did increase respiration rates. Nitrate fluxes measured in the nitrogen-amended cores during both spring and fall indicated significant uptake by sediment communities above requirements for denitrifiers. Low benthic chlorophyll α, high sediment carbon, and high respiration suggested that uptake by aerobic bacteria outcompeted denitrifiers for nitrate.

The importance of denitrification to instream retention was estimated for the agricultural watershed by extrapolating rates measured in sediment cores to total stream bed area of first order ditches and second order canals. During the 5-month period from August-December 2003, 1800 kg of nitrogen were exported based on data collected at the watershed outlet during baseflow and storm events. Denitrification accounted for approximately 30 percent of total nitrogen exported.

Nitrogen and Phosphorus Attenuation Within the Stream Network

Prediction of the watershed-derived nutrient load delivered downstream is confounded by instream processes acting during transport. The effect of instream nutrient uptake within a stream network on the total watershed nutrient export was investigated at multiple spatial and temporal scales from August to December 2003 in the agricultural watershed. Whole-ecosystem uptake rates of NH4 and PO4 were calculated for first and second order streams using short-term additions of inorganic nutrients: mass transfer velocities ranged from 0.1–25 mm min-1. Instream processes were found to attenuate 65-98 percent of daily NH4 flux and 78-98 percent of daily PO4 flux in first order drainage ditches. A numerical relationship was developed to calculate the gross retention efficiency (percent of upstream nutrient load retained) based on discharge and was applied to the daily nutrient load at the watershed outlet to estimate retention in canals. Instream nutrient uptake was estimated to retain all NH4 and PO4 load during baseflow, and 76 percent and 49 percent of the NH4 and PO4 loading during stormflows. The stream network in this watershed during this time period temporarily disconnected the terrestrial watershed from the estuary during baseflow, and substantially reduced inorganic nutrient exports to the estuary during storm events.

Future Activities:

The magnitude of nitrogen nutrient pools, rates of transformation among pools, and ecosystem imports and exports will be continued in both forested and agricultural watersheds as described in the initial proposal. A watershed budget is currently being developed for both watersheds, starting with flow and nutrient data collected in July 2003. Budgetary expenditures for the reporting period were in line with level of work that has been completed.

Journal Articles on this Report : 4 Displayed | Download in RIS Format

Other project views: All 23 publications 8 publications in selected types All 8 journal articles
Type Citation Project Document Sources
Journal Article Ensign SH, Doyle MW. In-channel transient storage and associated nutrient retention: evidence from experimental manipulations. Limnology and Oceanography 2005;50(6):1740-1751. R830652 (2004)
R830652 (2005)
  • Full-text: ASLO PDF
  • Abstract: ASLO Abstract
  • Journal Article Paerl HW, Valdes LM, Piehler MF, Stow CA. Assessing the effects of nutrient management in an estuary experiencing climatic change: the Neuse River Estuary, North Carolina. Environmental Management 2006;37(3):422-436. R830652 (2004)
    R830652 (2005)
    R828677C001 (Final)
  • Abstract from PubMed
  • Full-text: SpringerLink Full Text
  • Abstract: SpringerLink
  • Other: SpringerLink Full Text PDF
  • Journal Article Piehler MF, Twomey LJ, Hall NS, Paerl HW. Impacts of inorganic nutrient enrichment on phytoplankton community structure and function in Pamlico Sound, NC, USA. Estuarine, Coastal and Shelf Science 2004;61(2):197-209. R830652 (2004)
    R828677C001 (2003)
    R828677C001 (2004)
  • Full-text: EPA(only)
  • Abstract:
  • Other: EPA(only)
  • Journal Article Twomey LJ, Piehler MF, Paerl HW. Phytoplankton uptake of ammonium, nitrate and urea in the Neuse River Estuary, NC, USA. Hydrobiologia 2005;533(1-3):123-134. R830652 (2004)
  • Abstract: Springerlink
  • Supplemental Keywords:

    chemical transport, estuary, ecological effects, ecosystem, Mid-Atlantic, agriculture,, RFA, Scientific Discipline, Air, INTERNATIONAL COOPERATION, Water, ECOSYSTEMS, Ecosystem Protection/Environmental Exposure & Risk, Aquatic Ecosystems & Estuarine Research, Water & Watershed, Environmental Chemistry, climate change, Air Pollution Effects, Aquatic Ecosystem, Water Quality Monitoring, Environmental Monitoring, Terrestrial Ecosystems, Ecological Risk Assessment, Atmosphere, Watersheds, anthropogenic stress, bioassessment, anthropogenic processes, watershed classification, nutrient transport, ecosystem monitoring, watershed management, biodiversity, nutrient flux, conservation, diagnostic indicators, ecosystem indicators, biota diversity, Mystic Lake, aquatic ecosystems, bioindicators, watershed sustainablility, water quality, biological indicators, ecosystem stress, watershed assessment, conservation planning, nitrogen uptake, ecosystem response, aquatic biota, land use, restoration planning

    Relevant Websites:

    http://www.marine.unc.edu/Paerllab/ Exit

    Progress and Final Reports:

    Original Abstract
  • 2003 Progress Report
  • 2005 Progress Report
  • Final