Plant Identification and Management

What Aquatic Plants Need

Aquatic plants require sunlight, nutrients (especially nitrogen and phosphorus), carbon dioxide, oxygen, and water. Sufficient sunlight must penetrate through the water column and reach the bottom substrate for submersed and emergent plants to establish. Aquatic plants often cannot establish and grow in deeper waters when there is not enough sunlight. Therefore, most aquatic plants grow in shallow waters near the shoreline, and turbid or muddy waters tend to have fewer aquatic plants than clear water. Likewise, waters containing low nitrogen and phosphorus concentrations will have fewer aquatic plants than fertile waters with high concentrations of nitrogen and phosphorus.

The overall biological productivity of water depends on its trophic state, which is controlled by the abundance and availability of biologically useful nutrients. For example, a lake with few nutrients tends to be clear, while a lake with many nutrients may have green water because of an abundant phytoplankton population. When nutrients are too abundant, phytoplankton populations dominate the plant community, reducing light penetration, which results in fewer or an absence of other types of aquatic plants.

Further, overabundant phytoplankton populations may produce harmful algal blooms. In such cases, the water surface may be covered by what looks like foam, mats, scum, or paint. Blooms can cause the water to appear green, blue, red, brown, or another color. Conversely, rooted plants can thrive in ponds with low nutrients in the water because the water is clear, allowing ample sunlight penetration, and they get their nutrients from the soil.

Benefits of Aquatic Plants

Phytoplankton are the primary producers at the base of the aquatic food chain. This means phytoplankton production controls production of higher trophic levels, such as sport fish populations. Through photosynthesis, phytoplankton produce most of the energy for higher levels in the food chain and the dissolved oxygen critical to life in natural waters. In photosynthesis, plants use the energy in sunlight to convert water and carbon dioxide into sugar and oxygen.

Aquatic plants are a food source and provide attachment sites and habitat for other organisms. Many species of fish feed on the invertebrates (i.e., zooplankton, insects, snails) that live on and around aquatic plants, and turtles and crayfish eat various parts of the plants. Aquatic plants increase habitat complexity, provide shade, and furnish cover that young fish use to hide from predators. Aquatic plants can improve spawning success by protecting fish nests from wave action and sedimentation, which are harmful to fish eggs and small fish. Some popular sport fish, such as bluegill and largemouth bass, have a strong affinity for aquatic plants.

Aquatic plants provide habitat for other wildlife such as aquatic insects, fur-bearing animals, frogs, turtles, reptiles, amphibians, waterfowl, and water birds. Some aquatic plants have colorful flowers that are aesthetically pleasing and attract pollinating insects, while others are food sources for a variety of wildlife species. Aquatic plants can also improve water quality, reduce shoreline erosion by limiting wave action, and stabilize bottom sediments.

Considerations for Aquatic Plant Control

The decision on whether to control the amount of aquatic plant coverage in a water body depends on several factors, including how the landowner uses the water body, if fish are present, where the plants are located, water chemistry, and if invasive species are present. Each landowner should decide which aquatic plant species they are willing to accept, where those plants are located, and how much plant coverage they want or can tolerate.

How the water body is used will determine if aquatic plants are desirable. If the appearance of a golf-course pond is important, then all aquatic plants should be treated until they are eliminated. If plants interfere with or prevent a preferred water use such as swimming, skiing, or boating, treat the plants with the appropriate herbicide to restore those uses. If aquatic plants prevent bank fishing from some preferred locations, treat the plants in those areas. On the other hand, if fish production and habitat diversity are valued, then some aquatic plants are beneficial. Always control invasive species because failure to do so may result in total coverage of the water body, which could cause a fish kill.

Plant control can be expensive and carries inherent risks, so financial and biological factors must be considered. Does the landowner have the money, ability, and equipment for the recommended aquatic herbicides, or can they afford to hire a consultant to treat their aquatic plants? Several treatments and herbicides may be necessary to eliminate all plants, so it is common to underestimate the total cost of treatment. Further, once plants are treated, they will decompose. This uses dissolved oxygen, and a dissolved oxygen depletion may result if aquatic plants are abundant and treated simultaneously. A consequence can be a fish kill, which begs the question: Are the risks worth the reward?

Treatment methods also depend on how quickly plant control is desired. Chemical herbicides usually provide plant control in a short period, often days or weeks. An alternative to chemical control for many submersed aquatic plants is biological control with grass carp. However, this is a much slower process because plant coverage reductions depend on several factors, including the feeding rate of the grass carp, the number of grass carp stocked, the age of the grass carp, the amount of plant coverage, and the growth rates of the plant species. Stocking rate is particularly difficult to estimate. In most cases, grass carp will either eliminate all aquatic plants (too many) or have minimal control (too few). Too many stocked requires the landowner to remove excess grass carp by fishing (with plants or grapes for bait), bowfishing, or seining. Too few stocked wastes time and requires additional stocking.

Other control methods can also be considered. Physical methods include shading (e.g., dyes), using synthetic liners, deepening ponds, draining/scraping, and more. Mechanical methods include harvesting, raking, seining, removing by hand, or other forms of physically removing plants from the water body. Some of these approaches can be extremely expensive, time consuming, and often incompatible with the intended uses of the water body.

Considerations for Establishing Aquatic Plants

Aquatic plants serve many ecological roles and can be desirable to some landowners. Aquatic plants help attract wildlife by providing a diversity of habitats beneficial for both aquatic and terrestrial wildlife species. They provide oxygen, food, shade, shelter, and reproductive habitat for many aquatic species. Some species have large, colorful flowers that are visually appealing.

Landowners may choose to introduce aquatic plants for a variety of reasons. Some plants can be very attractive and are added to improve aesthetics. While many attractive plants can be invasive under the right conditions, there are some species that are unlikely to be problematic. Plants may also be introduced to provide fish habitat, serve as forage for waterfowl, or stabilize shorelines. In all cases, choosing the best species is important.

It is illegal to release any non-native aquatic plant species into a public or private water body in Mississippi. Only native aquatic plant species may be introduced. However, many native plants can exhibit invasive tendencies under ideal conditions. Only species that are less likely to become invasive are recommended in this text; even then, some control may be required.

Plant Growth Forms

Aquatic “plants” generally include two groups: algae and vascular plants. The algae are primitive species lacking true roots, stems, or leaves, and they do not produce flowers or seeds; however, algae are often included with aquatic plants because both groups use photosynthesis. Vascular plants are a higher, more advanced group of species that have roots (usually), stems, and leaves, and produce flowers and seeds.

Algae come in many forms but can be generally classified as:

• planktonic – single- or few-celled organisms suspended in the water or floating on the surface as “scums.”
• filamentous and colonial – long strands, mats, clumps, or webs of algae that may start growing from the pond bottom and then rise to the surface to form mats.
• macroalgae – a more advanced group that resembles vascular plants in growth habits.

Vascular plants typically exhibit one or more of three potential growth forms—submersed, emergent, and floating. This reference book uses these criteria to categorize vascular plants.

Submersed plants spend their entire life cycle at or below the surface of the water. The flower parts of the plants will extend above the water surface during the reproductive season. Submersed plants are often rooted in the soil, but masses of plants may tear loose and float free in the water. Some submersed plants may appear to be emergent or floating plants, particularly when support structures for flowers are present. Some of the most noxious exotic plant species are submersed.

Emergent plants are rooted in the bottom soil, and their leaves, stems, and flowers will extend above the surface of the water. Many can grow in both aquatic and terrestrial environments. These plants are often rigid and do not require the water for support. Many emergent plants may appear as submersed plants during the early growing season before they reach the water’s surface, and a few species may remain submersed indefinitely. In addition, some species may form extensive floating mats and appear to be floating plants. Emergent plants are typically found in marginal or shoreline areas except in water bodies that have extensive shallow water, or in cases where they form mats that extend out to deeper water. Some species have leaves that float on the surface and long stems that attach to roots in bottom sediments (sometimes called floating-leaved plants). These can have characteristics similar to submersed plants, although a few species may mature to have leaves that extend well above the surface, making them appear more like emergent plants.

Floating plants are species that are not rooted or attached to the bottom soil and draw their nutritional needs directly from the water. These plants float freely on the surface and are not restricted by sunlight penetration; thus, they can often form nuisance mats capable of covering entire water bodies.