What’s Green and Growing in the River?

by Jon Markovich

Wissahickon Creek, PA

Wissahickon Creek, Pennsylvania

The Mid-Atlantic Region has many great walking, biking, and hiking trails that meander through the woods and provide us with the chance to escape into the natural environment.  One of my favorite activities on a hike is to stop along the trail to check out a nearby river or stream.  It’s nice to relax and admire the view, listen as the water flows, and to see the different types of plants growing in and around the water.

Before becoming an environmental scientist, I wouldn’t have known that the extent and type of aquatic plants can indicate the health of a waterbody.  In our region there are many beneficial species of submerged aquatic vegetation (SAV).  SAV are rooted underwater plants that provide wildlife with food and habitat, and add oxygen to the water.  In fact, a positive sign in the Chesapeake Bay restoration efforts has been an increase in SAV. The more SAV, the better for the Bay.

Not so with another common type of aquatic growth – algae.  These are a large and diverse group of organisms that lack many typical characteristics of true plants.  Algae can grow on the bottom of a stream or float freely in the water.  While algae can be important to an aquatic ecosystem, too much can cause problems.

Excessive algal growth can negatively alter habitat and create low oxygen problems for aquatic life.  In addition, it can decrease water clarity for SAV, making it hard for them to get the sunlight they need to grow.  Some types of large algal blooms even pose a human health risk by producing toxics compounds.  Also in recent years, excess filamentous algae – long hair-like strands of algae growing on streambeds – has been a concern for potentially affecting recreation, such as fishing, boating, and swimming.  Specific effects could include tangled fishing lures, slippery rocks, and an overall unsightly appearance.

With several thousand different species of algae and SAV, it can be confusing to figure out what you see growing in a river or stream.  The Interstate Commission on the Potomac River Basin Commission (ICPRB) recently presented several tips to identify algae and SAV.  I spent some time hunched over a microscope to test these out, but with this handout they’ve created, you won’t need any scientific tools!

ICPRB is asking citizens in the Potomac basin to help by reporting areas where the water always seems green with algae.  You can share your observations using ICPRB’s new Water Reporter smartphone app which helps target local research efforts to study how excess algal growth affects aquatic life and the activities we like to do in the water.

Next time you’re out hiking, check out a local stream and see what types of aquatic plants are growing.  Can you answer the question “What’s green and growing in the river?”


About the Author: Jon Markovich joined EPA’s Water Protection Division in 2014 and works in the impaired waters and Total Maximum Daily Load programs. In his spare time, Jon enjoys hiking, kayaking and camping in the Mid-Atlantic Region’s many great state parks.


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