Nature Note: Common Waterweed, Uncommon Adaptations

There is a surprisingly diverse array of plants that you may have interacted with only by feeling them brush against your toes or stick on your paddle. Submersed aquatic plants in Lake Champlain set the stage for a flurry of biological activity during the summer. In winter, most of these plants die back as ice forms over once sunny shallow bays, and the lake’s living beings slow down. However there is one species—common, plain, unremarkable, and often overlooked—that ignores this seasonal cue and weathers the cold: common waterweed (Elodea canadensis).

Common waterweed is an evergreen perennial that will photosynthesize at a reduced rate throughout the cold months. At this time of year, the limiting factor for photosynthesis under the waters of Lake Champlain is neither CO2, H2O, nor any nutrient, but sunlight. From December through February, when the days are short and the angle of the sun is low, less than half as much solar energy reaches our region as in the summer months. Ice on the surface of the lake further constrains available light – the ice is usually not clear but texturized, with bubbles that block the sun’s rays. Snow cover also affects light penetration—as little as half an inch of new snow can block two-thirds of the already reduced sunlight.

With less light, underwater photosynthesis is dramatically cut back. Most aquatic plant species “decide” that maintaining leaves in winter is not worthwhile. Instead, they have developed special adaptations that allow their greenery to die back in the fall while the plant is preserved as a tuber, turion, or rhizome (learn more about winter aquatic plant adaptions in our December 2023 Lake Look column). Common waterweed takes a different approach. As an evergreen, the plant will still use what little light breaks through to survive and grow, with photosynthesis taking place at a reduced rate.

What are the advantages of sticking out the winter? It takes substantial energy to produce new leaves every year, energy that common waterweed does not have to spare. Similar to pine trees, common waterweed will produce leaves that last more than one season. This investment means that the leaves are slow-growing, but it also makes them well-suited for harsh environments like the cold waters of Lake Champlain. Once the ice melts and the days grow longer, common waterweed has the advantage of soaking in the early spring sun before most other species have their leaves out.

When the winter season feels particularly dark or harsh, when you find yourself wondering when you last saw the sun, remember the resilience of common waterweed plants—if they can last under ice, you can last above it!