Summary of Nitrogen, Phosphorus, and Suspended-Sediment Loads and Trends Measured at the Chesapeake Bay Nontidal Network Stations for Water Years 2009–2018

Prepared by Douglas L. Moyer and Joel D. Blomquist, U.S. Geological Survey

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The Chesapeake Bay nontidal network (NTN) currently consists of 123 stations throughout the Chesapeake Bay watershed. Stations are located near U.S. Geological Survey (USGS) stream-flow gages to permit estimates of nutrient and sediment loadings and trends in the amount of loadings delivered downstream. Routine samples are collected monthly, and 8 additional storm-event samples are also collected to obtain a total of 20 samples per year, representing a range of discharge and loading conditions (Chesapeake Bay Program, 2020). The Chesapeake Bay partnership uses results from this monitoring network to focus restoration strategies and track progress in restoring the Chesapeake Bay.

Methods

Changes in nitrogen, phosphorus, and suspended-sediment loads in rivers across the Chesapeake Bay watershed have been calculated using monitoring data from 123 NTN stations (Moyer and Langland, 2020). Constituent loads are calculated with at least 5 years of monitoring data, and trends are reported after at least 10 years of data collection. Additional information for each monitoring station is available through the USGS website “Water-Quality Loads and Trends at Nontidal Monitoring Stations in the Chesapeake Bay Watershed” (https://cbrim.er.usgs.gov). This website provides State, Federal, and local partners as well as the general public ready access to a wide range of data for nutrient and sediment conditions across the Chesapeake Bay watershed. In this summary, results are reported for the 10-year period from 2009 through 2018. All annual results are based on a water year, which extends from October 1 through September 30.

The USGS computes load and trend results from the NTN to display (1) the range in loads of nitrogen, phosphorus, and suspended sediment; and (2) the trends in these loads. Most of the NTN sites whose data were used for the analysis had data collected since 2009 (Figure 1; Table 1). Load results from each NTN station are normalized by the respective drainage area to present the results as per-acre loads (also known as yields) to facilitate the comparison of loads and trends between sites. The total number of NTN stations analyzed for their total nitrogen, total phosphorus, and suspended-sediment loads and trends varies because of the length of the data-collection record and because of the presence or absence of targeted water-quality samples collected during stormflow conditions (Chanat and others, 2015). Trends in loads at the NTN stations are flow-normalized to remove the year-to-year variability in river flow; by doing so, changes in nitrogen, phosphorus, and suspended-sediment loads resulting from changing sources, delays associated with storage and transport of historical inputs, and (or) implemented management actions are identified.

Patterns in Loads and Trends Across the Chesapeake Bay Watershed (2009–18)

Changes in loads for nitrogen, phosphorus, and suspended sediment are provided for 2009 through 2018. The monitoring station locations where loads are lower in the end year than in the start year are classified as having improving conditions, whereas those where the loads are higher in the end year than in the start year are classified as having degrading conditions. A location is classified as having no trend if there is no discernable difference between the loads in the start year and those in the end year.

Patterns in Total Nitrogen Loads

  • Average annual total nitrogen loads for 2009 through 2018 range from 1.23 pounds per acre (lb/acre) to 29.10 lb/acre (Figure 2) with a combined average load for this period of 7.00 lb/acre. The average 4 annual loads have been divided into three groups representing the following three categories: low (blue), medium (yellow), and high (red).
  • Forty-one percent of the NTN stations are improving, whereas 40 percent are degrading, and the remaining 19 percent are showing no trend.
    • 37 of 90 stations (41 percent) have improving trends, with load reductions ranging from 0.09 to 5.49 lb/acre (Figure 3, green bars)
    • 36 of 90 stations (40 percent) have degrading trends, with load increases ranging from 0.09 to 1.80 lb/acre (Figure 3, orange bars)
    • 17 of 90 stations (19 percent) show no statistical change (Figure 3, gray bars)

Patterns in Total Phosphorus Loads

  • Average annual total phosphorus loads for 2009 through 2018 range from 0.13 lb/acre to 2.01 lb/acre (Figure 4) with a combined average load for this period of 0.52 lb/acre. The average annual loads for phosphorus also are divided into three categories: low (blue), medium (yellow), and high (red).
  • Forty-four percent of the NTN stations are improving, whereas one-third are degrading, and the remainder are showing no trend.
    • 29 of 66 stations (44 percent) have improving trends, with load reductions ranging from 0.0001 to 0.36 lb/acre (Figure 5, green bars)
    • 21 of 66 stations (32 percent) have degrading trends, with load increases ranging from 0.01 to 0.63 lb/acre (Figure 5, orange bars)
    • 16 of 66 stations (24 percent) show no statistical change (Figure 5, gray bars)

Patterns in Suspended-Sediment Loads

  • Average annual suspended-sediment loads for 2009 through 2018 range from 18 to 1,920 lb/acre (Figure 6) with a combined average load for this period of 427 lb/acre. The average annual loads for suspended-sediments also are divided into three categories: low (blue), medium (yellow), and high (red).
  • Twenty percent of the NTN stations are improving, whereas nearly 42 percent are degrading, and the remainder are showing no trend.
    • 13 of 66 stations (20 percent) have improving trends, with load reductions ranging from 1.01 to 480 lb/acre (Figure 7, green bars)
    • 28 of 66 stations (42 percent) have degrading trends, with load increases ranging from 7.83 to 861 lb/acre (Figure 7, orange bars)
    • 25 of 66 stations (38 percent) show no statistical change (Figure 7, gray bars)

The Chesapeake Nontidal Monitoring Network and the Role of the USGS

The NTN is a partnership implemented among the States in the watershed, the U.S. Environmental Protection Agency, the USGS, and the Susquehanna River Basin Commission. A network of monitoring stations has been established and is sampled using standardized protocols and quality-assurance procedures designed to measure nitrogen, phosphorus, and suspended-sediment loads and changes in these loads over time. The initial network formed in about 1985 with coordinated monitoring at the nine River Input Monitoring (RIM) stations (Table 1). The RIM information is reported every year (Moyer and Blomquist, 2019), and the summary of the latest results are available by way of the “Water-Quality Loads and Trends at Nontidal Monitoring Stations in the Chesapeake Bay Watershed” website at https://cbrim.er.usgs.gov/data/RIM_Load_Trend_Summary_1985-2018_Combined.pdf . In 2004, the Chesapeake Bay Program formalized the network, and a period of expansion followed. In 2010 and 2011, the network was further. expanded to address the Total Maximum Daily Load (TMDL) requirements. The network currently has 123 sites designed to measure changes in nitrogen, phosphorus, and suspended sediment in the Chesapeake Bay watershed. Through this partnership, nitrogen, phosphorus, and suspended-sediment loads and trends are determined based on (1) continuous streamflow monitoring, (2) extensive water-quality sampling, and (3) advanced statistical analysis. The USGS performs the analysis for computing loads and trends.

Additional Information and USGS Contacts

For more information on this topic, visit the “Water-Quality Loads and Trends at Nontidal Monitoring Stations in the Chesapeake Bay Watershed” website at https://cbrim.er.usgs.gov/, or contact:

Doug Moyer, Richmond, VA, Joel Blomquist, Baltimore MD,

For more information on USGS Chesapeake Bay Studies

Scott Phillips, or visit http://chesapeake.usgs.gov/