By Dr. David Wartinbee, for the Redoubt Reporter
Photo courtesy of Dr. David Wartinbee. Lemna minor plants, also called duckweeds, float on the water’s surface. Their thalli are only about one-eighth of an inch to one-quarter of an inch long. The single root of each plant can be seen dangling in the water below.
While learning to take off and land on floats this summer, I visited a large number of lakes on the Kenai Peninsula. I developed a fondness for one particular lake because of the variety of different organisms I saw there.
Along the edge of this (to be unnamed) lake, I spotted one of the largest bull moose I have ever seen. There were all kinds of waterfowl in the area, including grebes, loons and a family of swans. I carefully watched for this swan family and if they were anywhere near my intended landing spot, I went to another lake to practice. On a couple occasions, I stopped to wander around the shoreline and was rewarded with a number of ecologically interesting discoveries.
Like many peninsula lakes, this one was home to a large population of freshwater mussels that live in the bottom substrates. Along the shoreline, I found a couple places where river otters had frequently dined on mussels and left their shells in large, scattered piles. Their short slides and spraint areas were also well-used.
In other areas of this lake were signs of beavers harvesting aspen saplings and some early construction work on a bank lodge. Here and there were signs of bears visiting the shoreline, as well.
While the animal fauna around this lake was compelling, I was actually most impressed with the plants in and around the lake. The diversity of rooted aquatic vascular plants was as great as any lake I have visited in Alaska. There were towering bulrushes in some areas that stood 6 feet above the water, and lots of floating Nuphar leaves with their yellow flowers. There were bladderworts, several Potamogetan species, and the list could go on. However, while all the larger plants were truly interesting, I was most excited to find some of the very tiniest flowering plants called duckweeds.
Duckweeds are very small, simple plants that can be found all over North and South America, Europe and along the Pacific Rim. Because of their tiny size and hardy nature, they can easily be carried from one place to another by traveling waterfowl. So their presence here should not come as a major surprise.
I found two distinctly different species in the lake — Lemna minor and Lemna trisulca. I then collected a number of each species and placed them in an indoor aquarium. They have thrived and I have far more individual plants than when I started.
Lemna minor have flattened, oval-shaped, leaflike structures that are usually referred to as thalli. These thalli are no more than a quarter inch at their longest. When one thallus reaches a particular size, another thallus begins to bud out along the outer edge of the parent thallus.
These connections usually remain intact and the plant may consist of half a dozen related thalli that are attached and floating together. When the water is warm and nutrients are abundant, the tiny plant can reproduce itself in as few as three days. Because of its rapid growth potential, many southern ponds with high levels of nutrients become completely covered with these tiny plants.
It is believed that Shakespeare was referring to duckweeds covering a pond when, in “King Lear” (Act III, Scene 4), he writes, “… drinks of the green mantle of the stagnant pool.” That very situation rarely happens in Alaska because our lake waters don’t warm up all that much, and there are only limited amounts of nutrients dissolved in the water.
The vegetative tissue in the thallus performs photosynthesis while drawing nutrients out of the water through its single, 2-inch-long root. Some tissues within the thallus are modified to capture air, so the plant floats on the surface of the water. Other cells within the thallus store starch late in the season that cause the thallus to sink. Once submerged on the bottom sediments, the thallus overwinters with the stored starch and re-floats in the spring.
Duckweeds are flowering plants that produce a tiny flower. The flowers are so small and simple that they are usually only seen under a microscope with 10 to 20 magnifications. When I was a graduate student at Pitt’s Pymatuning Laboratory of Ecology, we found a number of flowering duckweeds. The word got out and pretty soon everyone in the lab, including all the faculty members, had come to see these rare flowers. Each flower produces only one seed.
With a name like duckweed, there must be an association with ducks. Besides helping with the plant’s worldwide distribution, some ducks do feed on these plants. However, even though duckweeds are rich in proteins and lipids, ducks must augment their diet with other foods to survive.
Because duckweeds like Lemna minor grow quite rapidly in nutrient-rich waters, they are used for bioremediation. Many sewage treatment plants have outflow lagoons where treated water resides for many days. Duckweeds in the lagoons can remove a substantial portion of the dissolved nutrients, like nitrogen and phosphorous.
While these are some of the smallest flowering plants in existence, Duckweeds can play a big role in their habitat. In Alaska, they are an abundant plant that is most often overlooked or maybe never even seen.
In a future column, I’ll take a look at the duckweeds’ larger species, Lemna trisulca.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.
