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 sun, they fix carbon, producing energy-rich sugars and starches. Other members of the aquatic community that eat plants are termed herbivores. They include fish, turtles, snails, insects, and some birds. Carnivores in turn depend on the herbivores for food. Predatory birds such as osprey, eagles, herons, and egrets are at the top of the food chain in these systems (Figure 1.5). Directly or indirectly all are dependent on plants.
Sugars and starches are not the only product of photosynthesis; the process also releases oxygen. In aquatic systems the oxygen produced by green plants is important in meeting the respiratory needs of plants and animals.
Limiting factors—The need for light to drive photosynthesis limits the depth to which plants can grow in a lake; this is especially true for rooted submergent species. Light does not travel as far through water as through air. In addition, plankton and suspended sediments create turbid conditions that limit light penetration. The clearer the water, the greater the depths at which rooted submergent plants will be found. See also discussion of stem length and branching patterns in the section on growth habit above.
Emergent, floating-leaf, and free-floating plants have unimpeded access to sunlight. However, they can form a canopy that reduces the amount of light reaching lower layers of aquatic vegetation, just as in a forest.
Figure 1.4. An aquatic food chain: powered by the sun, plants and algae in and around the lake (primary producers) photosynthesize, producing sugar and starch. Plants are food for many small fish, insects, and other invertebrates such as snails (herbivores), which are in turn consumed by carnivores. Larger fish and birds of prey are often the top carnivores in freshwater systems.
Figure 1.5. The Great Blue Heron and other fish-eating birds are top predators in aquatic ecosystems.
Carbon dioxide is the source of carbon for photosynthesis in terrestrial plants; in aquatic systems the carbon source can be either dissolved carbon dioxide (CO2), bicarbonate
Dissolved CO2 is used by most aquatic macrophytes and algae. Some algae and submersed macrophytes, including mosses and quillworts, can use only CO2. Submersed, rosette-type plants, including water lobelia (Lobelia dortmanna) and quillworts (Isoetes spp.), obtain up to 90 percent of their carbon dioxide directly from lakebed sediments through their roots. Interestingly, these plants have stiff leaves with a cuticle, which prevents loss of CO2 through diffusion into the surrounding water. Other plants, such as waterweed (Elodea spp.), can switch to bicarbonate when free CO2 is in low supply; however, photosynthesis under those conditions is less efficient.
Another modification seen in hydrilla, Brazilian waterweed, and common waterweed is similar to the C4 photosynthesis seen in warm season grasses and other terrestrial plants in high light/high temperature conditions. This allows the plants to capture carbon dioxide in the dark, later converting it to typical C3 sugars (Casati et al. 2000).
Decomposers
Detritus feeders and decomposers that feed on decaying plants and other organic matter form another important link in the system. These organisms release minerals to be utilized in a new cycle of growth.