Ecological Roles of Aquatic Plants
Rooted aquatic plants, or “macrophytes” , collectively provide surfaces for benthic algae, help to control blooms of excess phytoplankton and metaphyton, provide shelter and feeding sites for invertebrates, and are used as cover and feeding sites by fish. As such, a diverse plant community is important to the overall productivity of a pond. Rooted plants are often light-limited in deeper ponds, relegated to near-shore areas and shaded out by the phytoplankton and metaphyton above them in deeper water. Extensive feeding by ducks or swans may also reduce aquatic plants to very low levels. Loss of plant cover may have a devastating impact on fish populations, and ponds without aquatic plants often experience obnoxious phytoplankton blooms.
Aquatic plants are often classified according to their growth form. “Emergent” species such as cattails and Sweet Flag (Acorus calamus) are common in near-shore areas in southeast Pennsylvania ponds. Cattails are native to this region, but may aggressively take over pond margins, especially if the shoreline is disturbed, creating a “ring around the pond” effect which can impede recreation and reduce plant diversity. One management option is to cut the stems beneath the waterline and “drown” the cattails.
“Floating-leaved, rooted plants” (e.g., waterlilies) are usually found in quiet water over soft, flocculent sediments. Their leaves are often waxy to shed water and are designed to resist damage due to currents and wave action. They are especially useful in providing cover for small fish and invertebrates, and for slowing down water movement.
“Submersed” species begin life on the bottom, rooted in the sediments. They may be absent in deeper water, below the compensation depth (where light levels are less than 1% of light at the surface, providing inadequate light to sustain most species. The stems and leaves of many submersed plants, however, may extend into the upper portion of the water column. Underwater leaves are usually designed for rapid gas exchange needed for life under water. Some species like Pondweeds that make it the surface actually have both thin underwater leaves and rounder, waxy surface leaves, on a single plant.
In contrast to most rooted aquatic plants, which obtain their nutrients primarily from the sediments, “free-floating” plants depend on nutrients from the water column, and are likely to become dominant only if nutrient concentrations are very high. Watermeal (Wolffia) takes the prize as the smallest flowering plant known (hundreds can fit on a finger dipped into the water). The flowers are tiny, and enclosed within the single leaf. Free-floating plants, when they become abundant, also shade out plants and algae below, often reducing oxygen levels and suppressing pond biodiversity. Ponds covered by duckweed (Lemna, Spirodela) and watermeal (Wolffia) may be candidates for management to reduce nutrients (Section II).
Algae and aquatic plants compete for light and nutrients. If phytoplankton densities are high, light penetration is reduced and rooted plants are suppressed. If instead plants are able to flourish, they reduce water movement and help protect zooplankton grazers, thereby reducing the abundance of phytoplankton. Most managers would agree that having a diverse plant community is the better option. As shown in the diagram below, plants simply support a more diverse community of animals. Managing a pond to support more plants and less phytoplankton can be assisted by adding largemouth bass (see biomanipulation above), or temporarily lowering water levels so that rooted plants can become established under conditions of higher light.