Macrophytes are the conspicuous plants that dominate wetlands, shallow lakes, and streams. Macroscopic flora include the aquatic angiosperms (flowering plants), pteridophytes (ferns), and bryophytes (mosses, hornworts, and liverworts). An aquatic plant can be defined as one that is normally found growing in association with standing water whose level is at or above the surface of the soil. Standing water includes ponds, shallow lakes, marshes, ditches, reservoirs, swamps, bogs, canals, and sewage lagoons. Aquatic plants, though less frequently, also occur in flowing water, in streams, rivers, and springs.
Macrophytes constitute a diverse assemblage of taxonomic groups and are often separated into four categories based on their habit of growth: floating unattached, floating attached, submersed, and emergent. Floating unattached plants are those in which most of the plant is at or near the surface of the water. Roots, if present, hang free in the water and are not anchored to the bottom. Floating attached plants have leaves which float on the surface, but their stems are beneath the surface, and their roots anchor the plant in the substrate. Submersed plants are found when the entire plant is below the surface of the water. Emergent plants are those whose roots grow underwater, but their stems and leaves are found above the water.
Types of Leaves
Leaves on aquatic plants may be of three basic types: aerial, floating, and submersed. Commonly, more than one type may be found on a single plant; this is called heterophylly.
Aerial leaves are similar to terrestrial plant leaves and are found on emergent and free-floating species. The underside surface of aerial leaves contains stomata (openings in the epidermis that allow gas exchange) and the epidermis has a waxy cuticle.
In contrast, the stomata of floating leaves are located on the top since upper surfaces are exposed to the air and lower surfaces to the water. Floating leaves require special adaptations in order to survive underwater; these include changes in leaf shape, leaf texture, internal air spaces, and petioles. Protection against tearing from wind and waves is provided by the large, circular shape of the leaf. Its tough, leathery texture guards against further tearing and puncture from rainfall, while the waxy upper surface sheds water to prevent immersion. Lacunae, or internal air spaces, are scattered throughout the mesophyll and act as miniature flotation devices. Petioles are much longer than normal to facilitate movement in accordance with fluctuating water levels and to allow leaf blades to spread out over the surface so as not to crowd or shade other plants. In the case of a flood, petioles immediately begin a period of rapid growth until leaf blades reach the surface again.
Because oxygen concentrations are lower in water than air, submersed leaves have evolved a very high surface-to-volume ratio to permit greater exchange of gases between plant tissue and water. The leaves are thin and pliable to protect them from mechanical damage in swift currents; their surface is smooth and lacks a waxy cuticle.
Aquatic macrophytes play a vital role in healthy ecosystems. They serve as primary producers of oxygen through photosynthesis, provide a substrate for algae and shelter for many invertebrates, aid in nutrient cycling to and from the sediments, and help stabilize river and stream banks.
Biological filtration is an increasingly popular method of sewage treatment; some aquatic plants are being used to remove nutrients and reduce concentrations of phosphorus and nitrogen from raw sewage or from the effluent sewage treatment facilities. Aquatic plants are also able to absorb other substances, including pollutants such as phenols.
Aquatic plants supply a wide variety of wildlife with food and suitable nesting habitats. Some, even help to control pest populations; duckweeds are known to reduce mosquito numbers, which has the added benefit of decreasing the incidence of certain insect-borne diseases.
However, humans do not always consider plants to be so beneficial. Flooding of agricultural land is a concern for many that farm on or near a watershed and plants can play a significant role in creating these problems. As macrophyte biomass increases, the mean water velocity of a river decreases. If river discharge is constant, such a reduction in velocity will raise the water level, thereby presenting the possibility of overflowing banks or raising water tables.
Fishing and navigation is another concern, as tall emergent plants can prevent access for shoreline fishing. Submerged species can also spoil the gravel spawning beds of some fish (salmonids, in particular) and high densities of photosynthesizing macrophytes are capable of causing large fluctuations in oxygen; this can stress many fish species. Similarly, fish mortality may ensue when photosynthesis does not exceed respiration (under prolonged hot and cloudy conditions), thus resulting in oxygen depletion.
While some aquatics deter certain disease-carrying organisms, others provide an ideal habitat. Several human diseases are transmitted through intermediate hosts that are either dependent upon certain macrophytes for completion of their life cycle or inhabit stagnant water resulting from the obstruction of water-courses by vegetation. Schistosomiasis (African sleeping sickness) is one example; the intermediate host is an aquatic snail that lives among aquatic vegetation.
In general, it is agreed that flowering plants evolved from primitive algae or algae-like ancestors, but that they evolved on land, not in the water. Today's aquatic spermatophytes are specialized forms that have reinvaded aquatic environments. Since aquatic angiosperms are closely related to terrestrials, it is not surprising that the morphology and anatomy of these two groups are basically the same. However, their differences become obvious when considering the adaptations which aquatic species have evolved to survive in their watery environments.
The flowers of most aquatic angiosperms must be elevated above the water in order for pollination to occur; entomophily is pollination by insects and anemophily is by wind; very rarely does pollen transfer occur underwater (hydrophily). Getting and keeping flowers at the surface requires special adaptations; water lilies, for example, have waxy, bowl-shaped flowers that float by themselves, much like a small boat; water milfoil develops rigid stems that protrude above the water and on which small flowers are borne.
Aquatic macrophytes tend to replace sexual reproduction with vegetative reproduction, which may be related to the difficulty in raising the flowers above the water for aerial fertilization. Vegetative, or asexual, reproduction is a vital key to survival among the aquatic plants. Some species rarely generate viable seeds and those that are produced serve more as a "back-up" to ensure the species' survival in the event of a disaster. Vegetative reproduction occurs primarily via stem fragmentation, but some species use the whole plant (Lemna, Eichhornia crassipes), shoot fragments (Ceratophyllum demersum), and specialized organs such as tubers (Hydrilla, Potamogeton).
Seeds are important dispersal agents for emergent macrophytes, which are less likely than other aquatic species to fragment. Flowers of emergents do not usually need to be modified from the terrestrial habitat and are wind- or insect-pollinated.
Floating-leaved species are ordinarily fertilized in the same manner as emergents, with their chief adaptation to the aquatic environment being the production of long peduncles (flower stalks) capable of lifting the flower above the water (e.g. Nymphaea). These peduncles must often be longer than the depth of the water to accommodate changes in water level and water velocity (in flowing waters).
Mechanisms of dispersal
Various dispersal mechanisms are exploited by aquatic macrophytes. Propagules (seeds) are transported downstream in water currents and the rate of dispersal is determined by the size and weight of the propagules. Flooding may carry propagules to adjacent waterbodies. Small seeds can be carried on the wind, as is the case with the aquatic grasses Phragmites and Phalaris and other emergents (e.g. Typha). Seeds are often consumed by birds and other animals and "hitch a ride" in their digestive tracts or attached to feathers and fur. Humans have had a great impact on plant ranges through the intentional or accidental introduction of whole plants or their propagules into foreign countries. Transport commonly occurs via water, sediments, crops, animal fur, wool, or desirable aquarium plants.
Aquatic macrophytes have served humans well over the centuries, providing food, medicines, and building materials. The ancient Egyptians regularly harvested water lilies (Nymphaea spp.) for human consumption. Herodatus, the Greek historian, described the practice in the fifth century BC; lilies were dried and seeds were pounded or ground into flour, which was used to make bread. Other parts were eaten raw. Various Nymphaea species are still cultivated in the Orient for their fruits, seeds, and rhizomes. In Africa, various tribes dig up the starch-laden rhizomes for food.
Water chestnuts are cultivated in the Orient. The familiar Chinese water chestnut is actually the corm of an Eleocharis sp., a member of the Cyperaceae family.
Wild rice is an annual grass and is not related to the cultivated rice that first comes to mind. Its seeds are regularly gathered and eaten in the USA and Canada.
Although an introduced species, water cress provides fresh foliage for salads and as a garnish. It originally comes from Europe, but has been naturalized throughout North America.
Giant reeds grow to a height of 3 meters, thus yielding a viable option for construction materials. They are frequently used in Europe for thatching roofs, building fences, making musical instruments, and in pulp mills for paper, cardboard, cellophane, insulation, fiberboard, and even building blocks.