A (very) Brief Introduction to Aquatic Plants

Three of the four types of aquatic plants

The Lakes of Missouri Volunteer Program devotes a lot of time to discussing, controlling and measuring algae in lakes. In our zeal for all things algae, we tend to overlook the other green nutrient-consumers in lakes, namely the "higher order" aquatic plants, or macrophytes. This article is a brief look at some types of aquatic plants we find in Missouri.

First up are the free-floating plants. Thanks to their rapid growth rate, free-floating plants can quickly cover the entire surface of a lake or pond. In this group, watermeal, duckweed, and water fern are common in Missouri. These plants are typically viewed as a nuisance by lake users, and managers work hard to eradicate them. Watermeal is similar in shape to duckweed, though individual plants are much smaller and have no hanging roots. Water fern has hanging roots, can be green or red in appearance, and has multiple small overlapping leaves. Both water fern and duckweed are capable of taking nitrogen from the atmosphere and converting it into usable nutrient form (nitrate). Like soybeans and clover on land, water fern and duckweed "fix nitrogen" thanks to a symbiotic relationship with blue-green algae (cyanobacteria). Because of this feature, rice farmers will often introduce these small aquatic plants to their paddies as a natural fertilizer source.

Above left: Watermeal (free-floating plant). Above right: Water lilies (floating leaf plant) and duckweed (free-floating plant)

Unlike the free-floating plants that pull other nutrients from the air and the water column, rooted aquatic plants primarily draw nutrients from the lake substrate via their roots. The location of the stems and leaves relative to the water further segregates the rooted aquatic plants into 3 subgroups: submersed, floating leaf, and emergent.

Submersed plants generally live under the water's surface. In some cases, sexual parts may break the surface for pollination, but the plants largely remain underwater. Examples of submersed plants include milfoil, naiad, coontail and pondweed.

Submersed Plants. Above, from left: Coontail, Pondweed, Milfoil

Floating-leaf plants, such as water lily and lotus, have waxy, floating leaves that rest on the surface of the water. All of the photosynthesis takes place on the upward-facing surface of the leaf, so the chloroplasts (part of a plant cell that contains chlorophyll) are concentrated here.

While rooted under water, emergent plants have leaves and/or stems that break through the surface. Pollination occurs above the water's surface for all emergent plants. Cattails and arrow-heads are familiar examples of emergent plants, as are many grass-like species (wild rice, rushes, sedges).

Emergent Plants. Above, from left: Arrow-head, cattail

Aquatic plants can be annoying to people who are fishing or boating, but they are important to a healthy aquatic ecosystem. Small fish use aquatic plants as habitat to hide from predators. Naturally, predators like to hunt among aquatic plants, thus making "weed beds" prime fishing structures. Aquatic plants can consume nutrients and shade lake surfaces, thereby inhibiting algae growth. They can also be beautiful to look at. You can have too much of a good thing, though, and many aquatic plant species are capable of taking over a lake, making fishing, boating and swimming nearly impossible. In some situations, weed control becomes necessary. Then it's time to call in the professionals.