Wetland Plant Series: Common Duckweed

By Aaliyah Ross, Environmental Educator

Duckweed completely covers the water’s surface in one area of the swamp near the Boardwalk.

Duckweed completely covers the water’s surface in one area of the swamp near the Boardwalk.

If you’ve been to the park this summer, you’ve likely noticed the bright green floating mats covering the water’s surface in many areas of the swamp. Is it pollen? Is it algae? No, it’s duckweed!

What is Duckweed?

Duckweeds are a subfamily (Lemnoideae) of floating freshwater plants that are among the smallest flowering plants in the world. There are many duckweed species; the common duckweed (Lemna minor) is found in slow-moving freshwater throughout North America and is abundant in Phinizy Swamp Nature Park. The plant consists of 1-3 leaves with a single root extending into the water. Like many wetland plants, it contains air spaces (called aerenchyma) between its tissues that hold oxygen, allowing it to stay afloat.

Duckweed aerenchyma: A microscopic image of duckweed leaves showing the aerenchyma (air spaces) between its tissues that allow it to stay afloat. Source: www.waynesword.palomar.edu

Duckweed aerenchyma: A microscopic image of duckweed leaves showing the aerenchyma (air spaces) between its tissues that allow it to stay afloat. Source: www.waynesword.palomar.edu

Although duckweed can produce tiny flowers, the plant primarily reproduces asexually. As leaves grow, the plants divide and become separate (but genetically identical) individuals. Duckweed is often spread to other bodies of water on the feet of waterfowl.

A Bane to Some, a Boon to Others

Duckweed can be a nuisance in garden ponds, fishing ponds, and other calm bodies of water used primarily for recreation. It thrives in water with high levels of nitrogen and phosphorus, which can come from animal waste, lawn fertilizer runoff, leaking septic systems, and decomposing plant material. Although it is native, it can behave like an invasive species, reproducing quickly and forming large, thick colonies. This can block sunlight from other aquatic plants and deplete the water of oxygen.

A small handful of duckweed contains dozens of individual plants.

A small handful of duckweed contains dozens of individual plants.

Despite this, duckweed is extremely beneficial to many species. Its dense mats provide cover for small aquatic animals, including juvenile fish, insects and other macroinvertebrates. It also provides a nutritious food source for many waterfowl and fish. In fact, duckweed is cultivated for use in livestock and fish feed due to its high growth rate and protein content. It has also shown potential for use in biofuel production (Cui and Cheng, 2015).

A Natural Cleanser

Over 40 years of extensive research on duckweed has supported its potential for use in phytoremediation—the use of plants to remove pollutants from water or soil. Duckweed has been shown to remove nitrogen and phosphorus from swine wastewater (Chaipraprat et al., 2005), uranium and arsenic from mine drainage (Mkandawire et al., 2004; Mkandawire and Dudel, 2005), and even pharmaceuticals from wastewater (Brain et al., 2004, 2006)!

Pharmaceuticals (such as antibiotics, painkillers, birth control, and other hormone treatments) enter wastewater through the excretions of consumers, and also when expired medications are flushed down toilets. Most pharmaceuticals are not removed by current wastewater treatment methods, and as a result end up in streams. Some drugs can alter hormonal activity in aquatic organisms, causing detrimental effects such as sterility, developmental abnormalities, and death. Pharmaceuticals present a unique challenge for wastewater treatment because they persist in the environment and are active at extremely low concentrations. Additional treatment of wastewater with duckweed may prove to be an effective solution to this problem.