Aquatic plants

connecting aquatic and terrestrial worlds

Prompted by a discussion with another naturalist, I’ve been thinking about plants that grow in fresh or brackish waters and their unsung importance to animals. So this essay is about aquatic plants (collectively called macrophytes) such as pond lilies (Nuphar), milfoil (Myriophyllum), burreed (Sparganium) , buckbean (Menyanthes), pondweed (Potomogeton), water crowfoot (Ranunculus), ditch grass (Ruppia), arrowhead (Sagittaria), and some sedges (Carex) that play many ecological roles relative to animals. Therefore they also have numerous ramifying effects on many aspects of local ecosystems. Here are some examples.

Northern-Milfoil-underwater-by-Bob-Armstrong
Northern Milfoil. Photo by Bob Armstrong

These aquatic plants are eaten by animals. For example, Canada geese nibble the shoots of Lyngbye sedge out on the wetlands; later in the season they grub up the root, leaving characteristic divots. Sedge stands closer to the forest edge are grazed by bears, deer, and sometimes moose. Moose forage on buckbean and other aquatics (see this video by Bob Armstrong), which are reportedly very digestible forage plants and a good source of minerals. Geese, swans, and ducks graze on the leaves of milfoil, ditchgrass, burreed, ditch grass and other species too. Geese and ducks eat the seeds of sedge, milfoil, burreed, ditchgrass , and other species, in some cases passing undigested seeds through the digestive tract and thus dispersing the seeds. All of those animals are, at some point in their lives, potential prey for various predators.

Trumpeter-Swans-feeding-on-milfoil-in-Twin-Lakes-by-Bob-Armstrong
Swans feeding on milfoil. Photo by Bob Armstrong

Beavers (and humans and muskrats) dig up and eat the nutritious tubers of arrowhead; beavers also eat the yellow pond lilies, buckbean, and soft leaves of several species. Beavers are habitat engineers, creating pond habitat for nesting birds and juvenile salmon. They are prey for carnivores such as wolves; a recent report concludes that wolves are able to plan ahead to set up ambushes for beavers, as well as just running them down on land.

Some of these macrophytes (e.g., water crowfoot, buckbean, arrowhead, pond lilies) produce flowers that are pollinated by insects. The visiting insects may obtain nectar or pollen as food, and they are prey for several kinds of birds.

Damselflies have evolved an unusual use for these plants: female damselflies insert their eggs in the leaves and stems of various aquatic plants, sometimes submerging themselves for several minutes. The emerging larvae are predators on other insects and are themselves (as both larvae and adults) prey for other insects, fish, birds, and frogs.

Macrophytes provide protective cover for small fish, such as sticklebacks and salmon fry, which in turn are prey for larger fish, birds (such as kingfishers and mergansers), otter, and mink. Similarly, toad tadpoles and some aquatic insects hang out in the watery ‘forests’ of pondweed or milfoil, temporarily hiding from predatory insects, fish, or birds.

In addition to providing food, cover, and egg-laying sites, the standing ‘forests’ of aquatic plants provide a handy substrate for dense coatings of algae. Photosynthesis of the algae produces oxygen that improves the breathability of the water. The algae are eaten by toad tadpoles and by herbivorous invertebrates such as snails, which in turn are prey for fish and birds.

These ecological connections are relevant to local ponds (such as Twin Lakes) that are sometimes managed to reduce the density of milfoil and other macrophytes. The species of milfoil in those ponds has been identified as a native species (northern milfoil, Myriophyllum sibiricum). A study of this species (and the invasive Eurasian milfoil, M. spicatum) in eastern North America showed that the native species generally supported more snails and other invertebrates than the invasive species. Those rich communities of invertebrates provide food for fish and waterfowl. Some of the waterfowl also graze directly on milfoil. Thus it becomes important to understand the ecological effects of reducing milfoil density in the lakes. How is the foraging of fish and birds changed? Also, perhaps reducing the density of the native milfoil facilitates invasion by the Eurasian species (widespread in North America and it might be in our area too), which supports poorer invertebrate communities. Furthermore, the invader can hybridize with the native species, changing its palatability or digestibility along with the associated composition of the algal community, with resultant effects on the animals that use milfoil. Hmmm, a potential research project awaiting attention…