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What is all that Green Stuff on Glade Run Lake?

Updated: Feb 4, 2023

Written by Dave Fowler, Board Member with the Glade Run Lake Conservancy

Many visitors to Glade Run Lake often comment on the unsightly green "scum" they see on the water's surface, asking what can be done to get rid of it. To some, it may come as a surprise to know that all that green material is actually a combination of different plants (depending on where in the lake you're looking), and may not be as bad as you think, nor will it last into the wintertime.

Dave Fowler, blog author, paddles on Glade Run Lake in October 2021. The green "scum" visible on the lake is actually a combination of two plants; duck weed and water meal. The plants seen to the right of Dave's boat are Eurasian water-milfoil, an aquatic invasive species.

The green "scum" on the surface on Glade Run Lake is not primarily algae and not really scummy if you touch it. It is comprised of two plant species, water meal (Wolffia sp.) and duckweed (Lemna sp.), both of which are among the smallest flowering plants in the world.

Water meal can be identified by its texture; it feels like corn meal if you get it on your fingers. It's also very small; about the size of a poppy seed, and has no roots. Duckweed is larger than water meal, much softer, and has roots. It is, however, also a very tiny plant - smaller than a pencil eraser.

Water meal and duckweed (above) are both present on the surface of Glade Run Lake. In this image, Dave is pointing to several duckweed plants.

Both water meal and duckweed are great food sources for beavers, muskrats, and other wildlife that use the lake, but they can cause issues after dying off in the winter, causing a potential drop in the lake's oxygen levels. Spread of these plants generally occurs via waterfowl, and management can be difficult in areas where their presence may hinder recreational activities.

All species of Wolffia found in Pennsylvania are native to our state, according to the Biota of North America Program. There is at least one non-native species of duckweed found in Pennsylvania; dotted duckweed (Landoltia punctata); however, other species of Lemna occurring in the Commonwealth are native to our state.

Note: Some of the native duckweed species found in Pennsylvania are rare and thus important to protect because of their ecological value. For reference, any plant species native to Pennsylvania are beneficial to our insect and wildlife populations, providing basic needs such as food and shelter. Native plants also serve a key role in the healthy functioning of our local ecosystems.

The exact type of Wolffia sp. and Lemna sp. found in Glade Run Lake have not yet been determined to a species level, so we do not have this information at present. However, both are non-toxic and will disappear with the onset of colder temperatures.

Other areas of Glade Run Lake have a thick layer of hair algae (i.e., filamentous algae) on top of a growth of Eurasian water-milfoil (Myriophyllum spicatum). Filamentous algae is non-toxic and a normal habitant of Pennsylvania lakes. Unfortunately, Eurasian water-milfoil is a different story as it is a known invasive aquatic plant in Pennsylvania and in the surrounding region. It can spread to new, un-infested locations on boats, trailers, and other recreational equipment and provides a great example of why we all need to thoroughly clean our boats and gear prior to traveling to other waterbodies to enjoy our favorite outdoor pastimes.

(Left and middle): Filamentous algae on the surface of Glade Run Lake; note the water meal scattered on top of the algae. (Right): Eurasian water-milfoil, photographed in Glade Run Lake.

Control for all these species is best achieved through lowering the amount of nitrogen and phosphorus in the lake waters. The Glade Run Lake Conservancy is working to monitor the water coming into Glade Run Lake in order to best protect the watershed that includes the lake.

For further questions regarding Glade Run Lake and the plants or animals that you observe there, feel free to contact the Glade Run Lake Conservancy at any time via our website.


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