Stream Water Quality: How Much Phosphorus is Too Much Phosphorus?

By Emma Brentjens, NCRN I&M Science Communications Intern

Takeaways:

• Too much phosphorus in streams can harm aquatic animals, shift stream communities, and contribute to algal blooms and fish kills in the Chesapeake Bay.
• Total phosphorus concentrations vary between and within National Capital Region (NCR) national parks. Prince William Forest Park has the highest overall percentage of phosphorus measurements passing EPA standards since 2016, while Antietam National Battlefield, Harpers Ferry National Historical Park, and Rock Creek Park have a lower percentage of passing measurements.
• More sensitive lab analysis used since 2016 reveals that phosphorus levels in NCR streams are in better condition than previously reported.

Water quality monitoring in Henson Creek at National Capital Parks-East.

NPS/Kelsey Graczyk

Streams are an important part of National Capital Region (NCR) parks. They provide habitat for diverse aquatic animals and recreational opportunities for park visitors. Most NCR park streams drain into the Potomac River, which in turn drains to the Chesapeake Bay, the largest estuary in the United States.

The National Capital Region Inventory & Monitoring Network (NCRN) measures several water quality parameters, including total phosphorus concentration, to understand stream health. The phosphorus concentration in a stream can tell us a lot about the health of the stream and surrounding land.

Where Does Phosphorus Come From?

Hard surfaces like concrete and pavement accelerate the movement of stormwater, which carries pollutants into streams and rivers.

U.S. Geological Survey

Phosphorus is a nutrient that occurs naturally in low amounts in soil and water. In water, phosphorus levels can surge when rainwater carries pollutants from farmland and urban development into streams. Farm fertilizers often contain inorganic phosphorus, and hard surfaces like concrete or pavement quickly channel polluted water into nearby streams.
Some forms of phosphorus can stay dormant in soil and sediment for a long time. This “legacy phosphorus,” is accumulated in sediment often over years of fertilizer use and can reenter streams when sediments are disturbed after a storm or other event.

Young’s Branch at Manassas National Battlefield Park. Suspended sediment makes the water appear cloudy.

U.S. National Park Service

Phosphorus Often Tied to Sediment, Affects Oxygen

Phosphorus is linked to other stream characteristics (sediment and oxygen) that impact aquatic animals. Stream sediments not only release phosphorus into the water but also determine habitat availability for many fish and macroinvertebrates that cannot tolerate a high amount of sediment on the stream bed.

Phosphorus also impacts the amount of dissolved oxygen in the water, which plants and animals need to survive. When too much phosphorus or other nutrients enter the water, they can cause an overgrowth of bacteria and algae. This process is called eutrophication. Eutrophication often depletes water of dissolved oxygen, placing stress on fish, amphibians, and smaller aquatic organisms. Long-term nutrient pollution can cause species shifts by favoring organisms that are tolerant of poor environmental conditions.

Moving stream water tends to have more dissolved oxygen because it has more contact with the air. The effects of eutrophication are more pronounced in slow-moving waters like the Chesapeake Bay.

How Do We Measure Phosphorus?

NCRN I&M scientists track total phosphorus in streams, which accounts for both dissolved and solid phosphorus, including organic and inorganic forms. Measurements are compared to nutrient standards set by the EPA to protect waterways as habitat for aquatic organisms and recreational spaces for people.

EPA nutrient standards vary by region based on soil and weather patterns. NCRN parks fall within two different nutrient ecoregions and are evaluated on two different standards.

Ecoregion IX, Southeastern Temperate Forested Plains and Hills, which has a total phosphorus threshold of below 0.037 milligrams per liter, includes:

• George Washington Memorial Parkway
• Manassas National Battlefield Park
• Monocacy National Battlefield
• National Capital Parks-East
• Prince William Forest Park
• Rock Creek Park
• Wolf Trap National Park for the Performing Arts

Ecoregion XI, The Central and Eastern Forested Uplands, which has a total phosphorus threshold of below 0.01 milligrams per liter, includes:

• Antietam National Battlefield
• Catoctin Mountain Park
• Harpers Ferry National Historical Park

(NCRN does not monitor streams in Chesapeake & Ohio Canal National Historical Park.)

Status of Phosphorus in the NCR

Previous NCRN reporting on levels of phosphorus in streams based on 2005–2015 data reported higher phosphorus concentrations. Since 2016, water samples are analyzed for total phosphorus by external, EPA-certified labs. Water samples collected from 2005–2015 were analyzed for different forms of phosphorus in-house, with less sensitive tests. The newer analysis is more reliable, so NCRN no longer reports on phosphorus data from the 2005–2015 period.

Total phosphorus trends based on 2016–2024 data vary between and within parks. Streams at Manassas, Monocacy, Prince William, and Wolf Trap generally have more acceptable phosphorus levels according to EPA standards. Some parks that have fewer measurements meeting EPA standards include Antietam, Harpers Ferry, and Rock Creek Park (Figure 1).

There are many environmental factors that influence water quality. It is unclear why some parks have more acceptable total phosphorus measurements than others, particularly those with similar landscapes like Monocacy and Antietam. The disparity may be due to land cover in the stream’s watershed, different land management practices, or another factor.

Figure 1. The percentage of total phosphorus measurements that passed EPA standards in individual streams (listed along the y-axis) between 2016 and 2024. Streams are grouped by park.

Narrative figure description:

This figure shows the percentage of total phosphorus measurements collected between 2016 and 2024 that meet EPA standards. Percent of passing measurements ranges from 0 to 100 on the x-axis while park and stream names are listed along the y-axis. Passing total phosphorus measurements in the National Capital Region vary, with Prince William Forest Park generally having the highest percentage of passing scores. Antietam National Battlefield, Harpers Ferry National Historical Park, and Rock Creek Park generally have lower percentages of passing scores.  

Phosphorus often peaks in late summer or autumn

The factors driving this pattern are unclear. Lower water levels or greater fertilizer application during the growing season may play a role. Occasional spikes in phosphorus concentrations, like those seen at many parks in January 2022, are possibly due to run off from storms.

Variation from year to year is more difficult to discern. Many streams show no clear trend in phosphorus concentrations over time, while others like Flowing Springs Run at Harpers Ferry and Sharpsburg Creek at Antietam, show increasing phosphorus over the last five years. The NCRN Water Quality Visualizer shows trends in phosphorus and other water quality parameters. Example plots from the visualizer displaying seasonal and annual variation in total phosphorus are shown below in Figure 2.

Figure 2. Total phosphorus concentrations from 2017–2024 at Sharpsburg Creek and Broad Branch.

Narrative figure description:

This graph shows total phosphorus measurements between 2017 and 2024 at Sharpsburg Creek in Antietam National Battlefield (top) and Broad Branch in Rock Creek Park (bottom). Total phosphorus as milligrams per liter is on the y-axis, while year is listed along the x-axis. Each point represents one total phosphorus measurement. Antietam is in the Central and Eastern Forested Uplands EPA nutrient ecoregion, which has a total phosphorus threshold of 0.01 milligrams per liter. Rock Creek Park is in the Southeastern Temperate Forested Plains and Hills nutrient ecoregion, which has a total phosphorus threshold of 0.037 milligrams per liter. A black dotted line indicates these thresholds for each stream. 

Parks Taking Action & Land Management Practices

National parks in the NCR are taking actions to protect water quality and reduce total phosphorus (as well as other pollutants) in streams. These include land management practices that:

• minimize the effects of fertilizers,
• prevent soil erosion, or
• slow down stormwater.

Parks with land in agricultural leases including Antietam, Manassas, and Monocacy battlefields give preference to farming practices that limit fertilizer applications and minimize runoff.

At Antietam, the park is also planting large areas of trees and shrubs along streams and restoring woodlands to create riparian buffers that can absorb pollution.

Wetland protection work at Catoctin, C&O Canal, Harpers Ferry, and Monocacy has included removing invasive species and restoring native plants. Wetlands act as giant water filters, storing stormwater and capturing sediment and nutrients before they enter streams.

Wolf Trap National Park for the Performing Arts utilizes no-mow zones to control erosion and allow for the reestablishment of native plants. Harpers Ferry has reduced grassland mowing to protect ground nesting birds. These no- and low-mow practices, while implemented for different purposes, benefit water quality by slowing runoff and increasing the absorption of water into the soil.

The Native Meadow, a no-mow zone at Wolf Trap National Park for the Performing Arts

NPS Photo

At Rock Creek Park and National Capital Parks-East (NACE) parks in Washington, DC, there are several completed, ongoing, and future stream restoration projects. These large-scale projects work to reduce excessive sediment loads and restore stream habitat in highly eroded urban streams. Many of these projects are carried out in partnership with the DC Department of Energy and Environment.

In 2026, NACE plans to restore portions of Fort Dupont Creek through re-engineering and vegetation planting to stabilize sediment. At Rock Creek Park, they have restored Broad Branch, Milkhouse Run, and Bingham Run, and are planning 11 more stream restoration projects to reduce erosion and improve downstream water quality.

For some NCR parks, the watersheds of monitored streams sit largely outside of park boundaries and are beyond direct park influence. However, by protecting natural landscapes and using best management practices within park boundaries, these parks still benefit the larger Chesapeake Bay watershed.

Harmful algal blooms (HABs) in the Chesapeake Bay are often caused by dinoflagellates, a type of algae. These organisms give the water a reddish-brown color.

Alicia Pimental/Chesapeake Bay Program

Downstream in the Chesapeake Bay

The Bay has long suffered from algal blooms (sudden and excessive growth of algae) due to nutrient pollution. Phosphorus loading and warming waters have led to more intense algal blooms in the Chesapeake Bay and across the country. Mahogany tides, which are caused by a type of algae called dinoflagellates, are common in the Bay and often associated with foul odors and fish kills. Cyanobacteria, also called blue-green algae, produce toxins that can harm humans and animals.

Learn More About the National Park Service's Inventory & Monitoring Efforts

To help protect natural resources ranging from bird populations to forest health and water quality, National Park Service scientists perform ecological Inventory & Monitoring (I&M) work in parks across the country. The National Capital Region, Inventory & Monitoring Network (NCRN I&M) serves national parks in the greater Washington, DC area (DC, MD, VA, and a bit of WV). To learn more about NCRN water quality monitoring, you can visit the NCRN Stream Water webpage, Water Quality Visualizer, and our water monitoring products on the NPS DataStore. 

Further Reading

• Chen, Crystal. 2024. What We’re Learning and Why it Matters: Long-Term Monitoring in the National Capital Region.
• Chesapeake Bay Foundation. 2013. Nitrogen & Phosphorus.
• Chesapeake Bay Program. Nutrient Runoff.
• DC Department of Energy & Environment. Stream and Habitat Restoration.
• Harpers Ferry National Historical Park. 2018. Forty-two wetland areas identified for restoration in four Western Maryland parks. https://www.nps.gov/hafe/learn/news/wetlandsfonsi.htm
• McAdory, Myra. 2020. A harmful algal bloom caught red handed. Chesapeake Bay Program.
• Nortrup, Megan. 2014. National Capital Region Network Resource Brief: Water Monitoring.
• University of Michigan. Legacy Phosphorus.
• Wolf Trap National Park for the Performing Arts. No-Mow Zones at Wolf Trap.