Introduction

Ecology is the study of factors controlling the distribution and abundance of organisms. A thorough knowledge of aquatic plant ecology provides many benefits for aquatic plant management, including: 1) improving the effectiveness of management techniques, 2) evaluating the effectiveness of management techniques, 3) evaluating the impacts of management techniques on nontarget species and environmental quality, and 4) preventing new infestations of nonnative aquatic plants. Information has been included below which provides some insights into the relationship between aquatic plant ecology and ecological techniques to aquatic plant management.

Overview of Aquatic Plant Ecology

Several factors have been implicated as limiting aquatic plant growth (Table 1). Although a hierarchy of which factors are most significant is difficult to establish, light is typically considered the most significant single factor limiting both distribution and abundance of submersed plants. However, all factors interact in a complex manner to determine a given plant community distribution (where they are found) and abundance (how much plant material is found).

Table 1

Light Availability

Light is the single most significant factor to limit plant distribution and abundance. The amount of light available to submersed aquatic plants is typically dependent on both the transparency and the depth of water. In highly turbid systems, submersed plants may be limited to less than 1 meter (m) depth. In one study, rooted submersed plants were found to have maximum depths ranging from 1.5 m in a turbid lake to over 11 m in a clear lake (Vant et al. 1986).

Transparency

Light is a major limiting factor for submersed aquatic plants (Barko et al. 1986). These limitations are expressed both in the maximum depth to which plants may grow, as well as different species that may colonize and thrive under differing regimes of light. Light penetration is typically measured with a Secchi disk, though more accurate measurements can be made with an underwater irradiometer (from which light attenuation coefficient estimates are generally made). The more turbid the water, the less light that is transmitted. Low transparency may be the result of suspended inorganic solids (e.g., silt or clay particles), phytoplankton growth, and dissolved organic material.

Depth

Depth is most significant as a cofactor with transparency or light attenuation of the water to reduce total light level to plants. It had been historically asserted that pressure directly limited plant growth, but this was refuted experimentally (Bodkin et al. 1980, Dale 1984). In situations where the maximum depth of plants is not regulated by light availability, cold water temperatures of the hypolimnion typically restrict vascular plant growth (Pip 1989).

Water Chemistry

The two most significant components to water chemistry for plant growth are inorganic carbon (dissolved carbon dioxide, carbonate, and other forms) and dissolved plant macro- or micronutrients. Of these, inorganic carbon is the most significant. Because substances diffuse 10-4 more slowly in water than air, thus the concentrations of dissolved carbon dioxide are substantially lower in water than air, therefore carbon for photosynthesis is severely limiting (Barko et al. 1986). Some plants overcome this limitation by utilizing bicarbonate as a carbon source (Raven 1970, Bowes and Salvucci 1989). Some plant macronutrients, such as potassium, are taken up by submersed plants predominately from the water (Barko 1982). However, most of these nutrients are readily available in surface waters. It is rare that a rooted submersed aquatic plant is limited by the supply of a nutrient (other than carbon dioxide) available only from the water. Most commonly, rooted plants are limited by nitrogen availability and, more rarely, by phosphorus; both of which are more typically taken up predominantly from the sediment (Barko and Smart 1981b).

Sediment Chemistry

Like their terrestrial counterparts, most rooted submersed plants are limited by nitrogen or phosphorus from the sediment (Carignan and Kalff 1979, 1980; Anderson and Kalff 1986). Nitrogen is typically the nutrient most limiting to submersed plant growth. Water column phosphorus or nitrogen are generally not important to the growth of rooted submersed plants, other than what might settle to the sediment. Another aspect of the sediment, the texture and composition, are also significant to plant growth. High organic content has been shown to reduce rooted plant growth (Barko and Smart 1983). Also, the particle size and physical properties have a significant bearing on both the nutrient-holding capability and stability as a rooting medium.

Temperature

Temperature is an important limiting factor to plant growth, both for the rate of growth (Barko et al. 1982, Spencer and Ksander 1991) and to initiate growth in the spring or induce dormancy in the fall (Madsen and Adams 1988a,b; Barko and Smart 1981a).

Competition

Unfortunately, competition has not been extensively studied in submersed aquatic plants, but is an important factor in explaining why one species is present and another is not (McCreary 1991). The clearest example of this factor at work is the replacement and suppression of submersed native plant communities by nonnative species (Madsen et al. 1991b, Madsen 1994).

Disturbance

Several mechanisms are lumped under disturbance, which can be either abiotic or biotic in origin. Abiotic disturbances include wave action, which is the most important factor limiting the inshore distribution of submersed aquatic plants (Chambers 1987). In addition to human activity, biotic disturbances can include herbivory (Lodge 1991) or agitation of the bottom for feeding (e.g., common carp, Crivelli 1983) or nesting (e.g., sunfish, Carpenter and McCreary 1985).

Stress

Stress factors include a wide range of candidates, but the most common example in aquatic environments is probably salinity. Most freshwater plants are severely restricted by even moderate levels of sodium chloride in the water (Twilley and Barko 1990, Haller et al. 1974, Hammer and Heseltine 1988).

Maximum Depth Distribution of Plants

As light transparency increases, submersed aquatic plants can colonize deeper areas. This relationship has been explored by several investigators using Secchi disk depth as a measure of light penetration. For over 100 north temperate lakes, Canfield and others (1985) developed the following expression:

in which zc = maximum depth of macrophyte colonization (m), and zSD = Secchi disk depth (m).

For 90 lakes, Chambers and Kalff (1985) found the relationship:
 (2)

This relationship was found to be further moderated by other optical properties of lakes, such as color (Chambers and Prepas 1988). A third study relevant to this discussion is that by Vant and others (1986), in which New Zealand lakes were studied. They found light attenuation strictly limited the maximum depth of macrophyte colonization, which they expressed using the attentuation coefficient, Kd :