a paint that sheds water and dirt.
Variability in Form
Aquatic plants are notoriously variable in form, which can make identification challenging. Variation in water depth is a major cause. For example, common bur-reed, which is normally an emergent plant with stiffly erect leaves, will grow in deeper water, but does not flower, and the leaves are less rigid and bend over and float at the tip. Several arrowheads (Sagittaria graminea and S. rigida) grow vegetatively as short sterile rosettes of narrow, pointed leaves in deep water. Sagittaria graminea can even flower under water, but in the absence of flowers it is impossible to tell the rosettes of these species apart visually.
Plants like the water-crowfoots (Ranunculus spp.), false-mermaid (Proserpinaca spp.), and most of the water-milfoils (Myriophyllum spp.) and pondweeds (Potamogeton spp.) have both underwater and floating or emersed leaves. The floating or emersed leaves are simpler and sturdier compared to the submersed leaves of the same plant. In addition, many of the pondweeds vary as to whether floating leaves are produced. Species such as Potamogeton bicupulatus and Potamogeton diversifolius can grow as submergents in deeper water, but often produce floating leaves in shallow water.
Reproduction
Sexual reproduction—Aquatic angiosperms (flowering plants) may produce their flowers under water, at the water surface, or elevated above the water surface. Pollination strategies vary accordingly. For species that hold their flowers above the water surface, insect or wind pollination can proceed as for terrestrial plants. Underwater flowers, such as those of the waternymphs (Najas spp.) and some pondweeds must depend on water currents to transport pollen, much like wind-pollinated plants on land. For annuals like the waternymphs (Najas spp.) and waterworts (Elatine spp.) seed production is essential to maintain populations from year to year. The pollination success rate for perennial species is not quite as critical, since the plants usually live from year to year. In addition, most aquatic perennials also have effective asexual or clonal growth mechanisms (see below).
Some aquatic plants have evolved unique pollination strategies utilizing the water surface as a stage. Species in the Frog’s-bit Family (Hydrocharitaceae) are especially interesting in this regard.
Water-celery (Vallisneria americana) has one of the most unusual pollination methods. Water-celery is dioecious, meaning that male and female flowers are produced on separate plants. Male flowers are released from an inflorescence at the base of the plant to float freely to the water surface where they are dispersed by wind. Meanwhile female flowers on long peduncles just reach the water surface where their tips produce a dimple that attracts the floating male flowers to where the pollen can be transferred directly from anther to stigma (Figure 1.2). We have seen lakes white with male flowers at the leeward end from a large blooming population of water-celery (Figure 1.3). Once pollination has occurred, the peduncle coils tightly, pulling the developing fruit farther under the water surface where it can mature.
The waterweeds (Elodea canadensis and E. nuttallii) and hydrilla (Hydrilla verticillata), also in the Frog’s-bit Family, have similar mechanisms. Pollen is released onto the water surface from male flowers, explosively in the case of hydrilla, where it is carried by wind or water currents to the stigmas of the female flowers which are positioned at the surface.
Figure 1.2. Pollination in water-celery (Vallisneria americana) occurs when free-floating male flowers drift into contact with female flowers positioned at the water surface.
Figure 1.3. Lake surface covered with white male flowers of water-celery (Vallisneria americana) in Sullivan County, PA.
Asexual reproduction—Asexual, or clonal, reproduction is very common in aquatic plants. Many perennial species produce rhizomes, modified stems that grow horizontally in the substrate, sending up shoots, leaves, or flowers at intervals. Water-lilies, many pondweeds, and most emergent species are in this category. Clonal colonies can expand to cover large areas, forming dense patches of genetically identical plants. In addition, pieces of rhizome that become detached can start a new colony at another location.
Fragmentation is not limited to rhizome segments. Pieces of the stems of many aquatic plants break off and float; these may continue to grow and even flower and fruit as floating fragments. In addition, through the formation of adventitious roots, these fragments can start new colonies. Water-milfoils (Myriophyllum spp.), waterweed (Elodea spp.), coontail (Ceratophyllum spp.), and bladderworts (Utricularia spp.) are examples of plants that reproduce this way.
The formation of dormant buds, or turions, is another way aquatic plants can reproduce. Turions are usually formed at the end of the growing season, resulting in a vegetative structure that can survive the winter or other periods of unfavorable growing conditions. Turions are generally dense and sink to the bottom. When the water warms in the spring they resume active growth. Species that form turions include duckweeds (Lemna spp.), waterweeds (Elodea spp.), bladderworts (Utricularia spp.), and some pondweeds (Potamogeton spp.).
Dispersal
Lakes and ponds are islands of habitat; although streams connect some to watersheds, others are completely isolated. Even streams contain a variety of growing conditions based on variables such as the degree of shading and speed of water movement. Each body of water, connected or not, seems to have a slightly different set of plants. The differences are partly due to the water chemistry (more on that later), but the random movement of seeds and plant fragments (propagules) is also involved.
How do aquatic plants move? Flowing water connects some lakes and ponds to watersheds and carries seeds or plant fragments downstream. Some seeds, such as those of the cat-tails (Typha sp.) are dispersed by wind. But waterfowl and other animals are important vectors of many aquatic species.
Charles Darwin was one of the first to study the potential for birds to disperse seeds (Browne 1995). More recent studies have documented that many waterfowl feed on the fruits and seeds of aquatic plants and serve as effective dispersal agents (Charalambidou and Santamaria 2002). Seeds can also be carried externally on the feet or feathers of birds. Small plants such as the duckweeds and watermeals can also adhere to birds or other animals such as beaver, otter, muskrats, or turtles and be carried from one site to another.
Humans too, spread plants from site to site. Some of this is inadvertent, through seeds or plant fragments that cling to the exterior of boots, boats, motors, oars, or fishing gear. Some of it is more deliberate, such as dumping the contents of an aquarium into a local stream, canal, or lake. Cultivated water gardens can also be a source of plant introductions resulting from overflow during heavy rains, the careless handling of garden waste, or bird-mediated dispersal. The deliberate introduction of species to lakes or ponds, usually for ornamental value, is yet another source.
ROLE OF PLANTS IN AQUATIC ECOSYSTEMS
Photosynthesis
Green plants play a basic role in aquatic ecosystems because of their ability to carry out photosynthesis. Plants, including macrophytes, phytoplankton, and filamentous algae, plus cyanobacteria, are the primary producers that drive aquatic food chains (Figure 1.4). Powered by the
