CHAPTER 3

AQUATIC PLANT BIOLOGY

It is not the facts which the student pursues as his ultimate aim, but the general laws underlying the facts. He is an unfortunate man who sees the trees, but cannot perceive the forest, who can see the stones of which the cathedral is constructed and show how they were lifted to their places, but cannot perceive the beauty of the structure as it stands in its exquisite proportions, its massive masonry and wealth of sculptured detail only serving to express the idea of beauty and harmony in the master mind of the architect.C. Dwight Marsh -Scientist

Introduction

Most aquatic plant research has been stimulated by the need to control nuisance species such as Eurasian watermilfoil (Myriophyllum spicatum), elodea (Elodea canadensis), coontail (Ceratophyllum demersum), curly-leaf pondweed (Potamogeton crispus), water hyacinth (Eichhornia crassipes), hydrilla (Hydrilla verticillata), and alligator-weed (Alternanthera philoxeroides). Understanding aquatic plant biology is important to the immediate problems of managing aquatic plants and aquatic ecosystems and it makes the development of new management techniques, the application of present techniques, and the assessment of environmental impacts more efficient. Interest in restoring and restructuring macrophyte communities and an appreciation for the littoral zone (littoral zone is that portion of a water body extending from the shoreline lakeward to the greatest depth occupied by rooted plants) is growing. There is also a need to make management results more predictable, especially when considered in a long-term ecosystem context.

The development of effective and environmentally acceptable aquatic plant management programs also requires some knowledge of lake and reservoir limnology (limnology is the scientific study of the physical, chemical, geological, and biological factors that affect aquatic productivity and water chemistry in freshwater ecosystems-lakes, reservoirs, rivers, and streams). Many limnological processes affect the kind and distribution of aquatic plants and aquatic plants also impact, nutrient, chemical and temperature regimes and other biota in a lake or reservoir, especially in the littoral zone.

A single chapter cannot review all the macrophyte biology, ecology, and limnology (Hutchinson 1975; Wetzel 1983; Cole 1983) that might be relevant to aquatic plant management and it is not our intent to do so. We will provide information that is most applicable to plant management efforts including information about the basic types of aquatic plants, the habitat factors that determine plant distribution, the influence aquatic plants have on the littoral zone, and the relationship between aquatic plants and other organisms including epiphytes, macroinvertebrates, fish, and wildlife.

Types of Aquatic Plants

The type of aquatic and wetland plants (macrophytes) of interest to most aquatic plant management programs can be classified into four groups: Emergent, Floating-leaved, Submersed, and Freely floating. Aquatic macrophytes, by definition, are the macroscopic (large enough to be observed by the naked eye) forms of aquatic and wetland plants found in water bodies. The term aquatic macrophytes refers to a diverse group of aquatic plants and encompasses flowering vascular plants, mosses, ferns, and macroalgae.

Emergent macrophytes (plants that are rooted in substrate with the tops of the plant extending into the air) grow on periodically inundated or submersed soils. Most emergent macrophytes are perennials (plants or plant parts living for greater than one year). They are typically rooted in the bottom, have their base portions submersed in water, and have their tops elevated into the air. This is ideal for plant growth. Nutrients are available from the sediment, water is available from the sediment and overlying water, carbon dioxide and sunlight are available to the emergent portions of the plant.

Emergent plants have to be strongly rooted and much energy is put into producing a strong structure to withstand the wind and waves in the shallow water zone. Many species need mud flats to spread by seeds but they can spread into deeper water by sprouting from an expanding roots or underground stem (rhizome) systems. In northern climates the dry dead stems often supply oxygen for root respiration during the winter when the lakes are locked under ice. Cutting off dead stems below the water surface before the lake freezes limits oxygen supplies and sometime kills rhizomes - a potentially useful management technique.

Common emergent macrophytes include plants such as reeds (Phragmites spp.), bulrushes (Scirpus spp.), cattails (Typha spp.), and spikerushes (Eleocharis spp.). Some emergents, wild rice (Zizania spp.) for example, have submersed or floating leaves before mature aerial leaves form.

Floating-leaved macrophytes (plants that are rooted to the lake bottom with leaves that float on the surface of the water) generally occur in areas of a lake that do not occasionally dry out. Common representatives include waterlilies (Nymphaea spp.), spatterdock (Nuphar spp.), and watershield (Brasenia spp.). Floating leaves are attached to roots or rhizomes with a flexible, tough stem (actually in many cases a leaf stalk). Some floating-leaved macrophytes, like Nuphar spp., can exist in a submersed form for a considerable time. Many floating-leaved species form large colonies from spreading underground rhizomes. In northern climates, under winter drawdown conditions, frost will often "heave" the rhizomes out of the bottom, which helps thin dense stands.

Floating leaves live in two extremely different habitats, water on the bottom, air on top. A thick, waxy coating protects the top of the leaf from the aerial environment this makes herbicidal control difficult without the addition of special chemicals called adjuvants (wetting agents) to help the herbicide stick to and penetrate the waxy surface. Adjuvants are also used on many kinds of emergent and free-floating species when treating with herbicides for the same reason. Floating leaves can be ravaged by wind and waves so these plants are usually found in protected areas.

Submersed macrophytes (plants that grow completely submersed under the water) are a diverse group that includes quillworts (Isoetes spp.), mosses (Fontinalis spp.), muskgrasses (Chara spp.), stoneworts (Nitella spp.) and numerous vascular plants. Many submersed plants, such as widgeon-grass (Ruppia maritima), various pondweeds (Potamogeton spp.), and tape-grass (Vallisneria spp.), are native to the United States. Others like hydrilla, Eurasian watermilfoil and curly-leaf pondweed are not native and cause some of the worst aquatic nuisances.

Submersed species face special problems obtaining light for photosynthesis and they must obtain carbon dioxide from the water where it is much less available than it is in air. Submersed species invest much less energy into structural support because they are supported by water and water accounts for about 95% of the weight of these submersed species.

Free floating macrophytes (plants that typically float on or just under the water surface with their roots in the water and not in sediment) are a diverse group of aquatic plants. Small free floating plants include duckweeds (Lemna spp.), mosquito fern (Azolla caroliniana) and water fern (Salvinia spp.). Larger surface floating plants include water hyacinth (Eichhornia crassipes), frog's bit (Limnobium spongia).

Free floating species are entirely dependent on the water for their nutrient supply. In fact, some (e.g., water hyacinth) have been used in wastewater treatment to remove excess nutrients. If nutrient limitation will work for macrophyte management, this is the group where it is most likely to work. The location of these plants is at the whims of wind, waves, and current so they will likely be found in quiet locations and embayments.

The above is a general description of aquatic plant groups and some of the biology pertinent to their management. One excellent resource for this type of information is the Aquatic Plant Information Retrieval System at the University of Florida, Center for Aquatic Plants, 7922 N. W. 71st Street, Gainesville, Florida (http://aquat1.ifas.ufl.edu/). Control tactics are often species specific and as management plans are devised you will want to know exactly what species are present, where they are located, and in what abundance. This takes some technical knowledge but help is usually readily available through natural resource agencies, universities, museums, natural history surveys, and private consultants.

The Littoral Zone

Rooted aquatic plants inhabit the littoral zone, the interface between dry land and open water of lakes and reservoirs. The littoral zone is defined by where plants will grow. It is the area from the lake's edge to the maximum water depth where plant growth occurs. Because most lakes and reservoirs in the United States are relatively small and shallow the littoral zone often contributes significantly to a water body's productivity and it can be a major factor regulating lake or reservoir ecosystems (Wetzel 1983). The littoral zone has traditionally been divided into four rather distinct transitional zones: the eulittoral, upper littoral, middle littoral, and lower littoral (Wetzel 1983).

The eulittoral constitutes that part of the shoreline that lies between the highest and lowest seasonal water levels. It often contains many wetland plants. The upper littoral is commonly called the emergent plant zone because it is dominated by emergent plants like cattails and bulrushes. This zone generally extends from the waters edge to depths of 3 to 6 feet (1 to 2 m).

The middle littoral is dominated by floating-leaved plants like fragrant waterlily (Nymphaea odorata), yellow waterlily (Nymphaea mexicana) and American lotus (Nelumbo lutea). The middle littoral zone extends lakeward from the upper littoral zone to water depths of 3 to 9 feet (1 to 3 meters).

The lower littoral zone is where most submersed plants are found and typically extends from the floating-leaved plant zone down to the limits of the photic zone (photic zone is the area of a lake where photosynthesis can occur and is defined by the depth to which at least 1 percent of the surface light intensity penetrates) where light tends to be insufficient for macrophyte growth. In clear waters, submersed vascular macrophytes can colonize to depths of 30 to 45 feet (10 to 15 m).

Factors Regulating Plant Growth

Different species of aquatic plants live in different "worlds" - sediment, water, and air in different combinations. Most aquatic plants are secondarily adapted to life in the water having once lived on land and gradually evolving mechanisms to deal with a watery world.

The most important environmental factors affecting the abundance and distribution of aquatic macrophytes in lakes are light availability (Spence 1975; Chambers and Kalff 1985; Nichols 1992; and Canfield et al. 1985), lake trophic characteristics as they relate to nutrients and water chemistry, (Hutchinson 1975; Beal 1977; Kando 1982; Hoyer et al. 1996), substrate (sediment) characteristics (Pearsall 1920; Barko et al. 1986; Nichols 1992), and wind energy (Duarte and Kalff 1986; Dieter 1990). Lake morphology (e.g., shape, depth etc.), size, and watershed characteristics are related to these factors independently and in combination (Pearsall 1917; Spence 1982; Duarte and Kalff 1986; Gasith and Hoyer 1997).

Light Availability

Aquatic plants require light for growth, thus light availability is often considered the single most crucial environmental factor regulating the maximum depth of plant growth (Pearsall 1920; Spence 1975; Chambers and Kalff 1985). Light availability is directly linked to water clarity (Canfield et al. 1985; Chambers and Kalff 1985). As water depth increases or water clarity decreases both the amount and the spectral quality of light for photosynthesis at the lake bottom diminishes. Generally, submersed macrophytes will grow to a depth of 2-3 times the Secchi depth (depth at which a black and white disk lowered into a lake disappears). Thus, lakes with the majority of their bottom exceeding 2-3 times the Secchi depth will have fewer aquatic macrophytes. Even shallow lakes, if they are turbid enough, will have sparse aquatic plant growth (Engel and Nichols 1994; Nichols and Rogers 1997, in press).

Water clarity in most lakes is controlled by phytoplankton, organic color and both organic and inorganic suspended particles (Jones and Bachmann 1978; Canfield and Hodgson 1983; Hoyer and Jones 1983). Lakes with low phytoplankton concentrations and low color values have high water clarity. As phytoplankton and color levels increase, there is a rapid reduction in water clarity, aquatic macrophytes become light limited, and the size of the littoral zone can decrease. Conversely, the size of the littoral zone can increase if phytoplankton or color levels decrease which can occur where management efforts reduce nutrient levels in a lake.

The amount of non-algal suspended particles (suspended solids) in a lake is determined by the continuous processes of tributary input, sedimentation and resuspension. Shallow lakes with substantial layers of soft sediments and open to the wind often have high suspended solids concentrations due to wind mixing of bottom sediments. Suspended solids limit light for plant growth and decrease littoral zone size. Boat traffic, shoreline erosion, and biotic factors like common carp can increase suspended sediment.

Plant Nutrition, Trophic Status, and Water Chemistry

All things being equal, nutrient poor lakes are less productive than nutrient rich lakes (Hutchinson, 1975). A primary factor determining the trophic status (i.e., nutrient richness) of a lake is the geologic region where the lake occurs (Jones and Bachmann 1978; Canfield and Hoyer 1988). Watershed management practices and direct human-caused nutrient additions can also be important.

There are few substantiated reports of nutrient related growth limitation for aquatic plants (Barko et al. 1986). Nutrients supplied from sediments combined with those in solution are generally adequate to meet nutritional demands of rooted aquatic plants, even in oligotrophic (nutrient poor) systems (Barko et al. 1986). While this information suggests that nutrients do not limit growth of aquatic plants in oligotrophic lakes, these lakes generally maintain less total biomass of aquatic plants and usually different species than eutrophic (nutrient rich) lakes (Canfield and Hoyer 1992).

Rooted macrophytes usually fulfill their phosphorus (P) and nitrogen (N) requirements by direct uptake from sediments (Barko et al. 1986). The role of sediment as a direct source of P and N for submersed macrophytes is ecologically quite significant, because available forms of these elements are normally in very low concentration in the open water of aquatic systems during the growing season. Likewise, the availability of micronutrients in the open water is usually very low but they are relatively available in most sediments. However, the preferred source of some nutrients such as potassium (K), calcium (Ca), magnesium (Mg), sulfate (SO4), sodium (Na), and (Cl) appears to be the open water.

Submersed macrophytes make use of both aqueous and sedimentary nutrient sources, and sites (roots vs. shoots) of uptake are related, at least in part, to nutrient-specific differences in sediment compared to overlying water nutrient availability. In other words, submersed plants are operating like good opportunistic species should operate; they take nutrient supplies from the most available source.