Popes Creek in 2021: An Over-Enriched Estuary at George Washington Birthplace

Estuaries — where saltwater meets freshwater — are among the most dynamic places in coastal parks, and among the most vulnerable to nutrient enrichment, mostly from human activity. The Northeast Coastal and Barrier Network (NCBN) monitors water quality in its parks' estuaries. 2021 status at George Washington Birthplace National Monument: over-enriched.

The monitored estuarine area. Image from the National Park Service

The site

The monument lies on the tidal Potomac River in Westmoreland County, Virginia; the Potomac shore is its northern boundary, and three small sub-basins drain into the river. Its most important estuary is Popes Creek, which exchanges water with the Potomac only through a restricted mouth. Monitoring covers all tidal parts of Popes Creek — about 137.7 ha outside the park and 4.7 ha inside.

Nutrient enrichment is a primary management concern. NCBN's monitoring protocol tracks how estuarine resources respond to changing nutrient inputs, using three indicators, each rated against thresholds consistent with the U.S. Environmental Protection Agency's national coastal condition reporting and the National Oceanic and Atmospheric Administration's National Estuarine Eutrophication Assessment:

The light attenuation cutoffs come from EPA turbidity criteria for areas with significant submerged aquatic vegetation (SAV).

Bottom-water dissolved oxygen: good

Why it matters: when nutrients feed an overgrowth of nuisance algae, more organic matter settles into the sediments, and microbes decomposing it use up oxygen in the bottom water. That can bring longer, wider anoxia and toxic sulfide, and in the end change which animals live there and how many.

2021: good across 95.2% of the sampled area and fair across 4.8% — like the five previous index periods, and the third in which more than 90% was good.

Bottom-water dissolved oxygen condition by percent of estuarine area. Image from the National Park Service

Surface chlorophyll-a: poor

Why it matters: extra nutrients feed phytoplankton, whose growth is often nutrient-limited, so rising chlorophyll-a points to rising nutrient loads and poorer water quality.

2021: poor across 82.2% of the area and fair across 17.8% — slightly better than earlier years, with less poor and more fair area. Only one year, 2011, has had any area in good condition.

Surface chlorophyll-a condition by percent of estuarine area. Image from the National Park Service

Light attenuation: poor

Why it matters: submerged aquatic vegetation provides important ecosystem services and depends on light. How far light reaches depends on phytoplankton — linked to nutrients — and on suspended and dissolved matter. NCBN tracks it whether or not SAV is present, as a guide to overall eutrophication.

2021: poor across 100% of the area, as in every previous index period.

Water-column light attenuation condition by percent of estuarine area. Image from the National Park Service

Conclusion

Dissolved oxygen has been good across most of Popes Creek, but chlorophyll-a and light attenuation show it remains eutrophic. The flood-tide delta at its mouth restricts flushing and traps eroded sediment and nutrient-rich runoff from homes, farms and the Potomac — and trapped nutrients drive eutrophication.

Park staff collecting water quality data in a monitored estuary. NPS photo / Michaela Compo

Sources

Based on "2021 Estuarine Nutrient Enrichment Status Report: George Washington Birthplace NM," Northeast Coastal and Barrier Network, National Park Service; a work of the United States government in the public domain. The source's "National Oceanic and Atmospheric Association" is corrected to Administration.