What’s at the Bottom of the Potomac River?

Laura Birsa pulls the ponar onto the boat
Laura Birsa pulls the ponar onto the boat

Written by: Chelsea Gray

Each year, researchers and graduate students from PEREC gather data on Gunston Cove, located just downriver from the Norman M. Cole Jr. Pollution Control Plant. This study has been used to determine the health of the Potomac River for over three decades.

How is a river’s health determined?

The health of an ecosystem can’t be measured directly, however, you can make an educated guess by gathering information on the flora and fauna found in an ecosystem. For a river, that often involves looking at benthic macro-invertebrates, or the small animals that live on the bottom of a river.

The bottom of the Potomac is not just mud, but is a complex ecosystem of organisms that include benthic macro-invertebrates, fish, and submerged aquatic vegetation (SAV). Many “bugs” found in the Potomac begin their lives as larvae that live under the water’s surface. For example, midge larvae (Chironomids) are quite commonly found at Gunston Cove, where they provide food for the fish that swim in the river. Freshwater clams (Corbicula) and snails (Gastropods) are also very common, and their shells are commonly found in the shallow waters near docks.

A good ponar sample includes lots of bottom sediment! Hidden in all that mud are the organisms that will give us a clue to the health of the river.

Certain species are more sensitive to pollution then others. By looking at the number and type of benthic macro-invertebrates found in an area, scientists can make an educated guess on how polluted a river might be. The more “sensitive to pollution” organisms that are found, the less likely there are high levels of pollutants in the water.

How can you sample the bottom of a river?

In order to gather samples of the organisms that reside on the river bottom, a ponar, sometimes called a grabber, is used. This is a metal “scoop” that is dropped over the side of the boat. When the ponar hits the bottom, the two scoops close, digging into the mud. Once the arms are securely closed around a chunk of sediment, the ponar is pulled back onto the boat.

Samples are rinsed using river water.

Since it is living animals that help determine the health of an ecosystem, the mud is rinsed off, and a sieve is used to catch any living organisms.

While they are fun to look at, a pile clams isn’t particularly informative for the purpose of the study. Instead, all the organisms captured by the ponar are preserved and taken back to the lab. There, each individual organism is counted and identified, giving us a good picture of the health of the river bottom.

Since benthic samples have been gathered over several decades, with the same equipment, all the results can be reliably compared over time.

Has the Potomac changed over the years?

Clams are a common sight in the Potomac River.

The Potomac River has seen a slow, but consistent increase in the over biodiversity of benthic macro-invertebrates. While they still have a small population size, organisms are that are semi-sensitive to pollution, such as amphipods, have been found in recent years.

This means that each year, the river is just a little bit cleaner.



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