Aquatic plants are plants that have adapted to living in aquatic environments (saltwater or freshwater). They are also referred to as hydrophytes or macrophytes. A macrophyte is an aquatic plant that grows in or near water and is either emergent, submergent, or floating. In lakes and rivers macrophytes provide cover for fish and substrate for aquatic invertebrates, produce oxygen, and act as food for some fish and wildlife.
Macrophytes are primary producers and are the basis of the food web for many organisms. Macrophytes have a strong effect on soil chemistry and light levels. Aquatic plants slow down the flow of water and capture pollutants and other sediments; the plants will eventually absorb the pollutants into their tissue and the excess sediment will settle down to the bottom of the body of water. Macrophytes are different from algae because macrophytes are big enough to see with a naked eye, but microscopic algae cannot be seen with the naked eye. Seaweeds are not vascular plants and do not have a root system; rather they are multicellular marine algae, and therefore are not typically included among aquatic plants.
Aquatic plants require special adaptations for living submerged in water, or at the water's surface. The most common adaptation is aerenchyma, but floating leaves and finely dissected leaves are also common. Aquatic plants can only grow in water or in soil that is permanently saturated with water. They are therefore a common component of wetlands.One of the largest aquatic plants in the world is the Amazon water lily; one of the smallest is the minute duckweed. Many small aquatic animals use plants like duckweed for a home, or for protection from predators, but areas with more vegetation are likely to have more predators. Some other familiar examples of aquatic plants might include floating heart, water lily, lotus, and water hyacinth.
The principal factor controlling the distribution of aquatic plants is the depth and duration of flooding. However, other factors may also control their distribution, abundance, and growth form, including nutrients, disturbance from waves, grazing, and salinity. A few aquatic plants are able to survive in brackish, saline, and salt water.
Aquatic plants have adapted to live in either freshwater or saltwater. Aquatic vascular plants have originated on multiple occasions in different plant families; they can be ferns or angiosperms (including both monocots and dicots). The only angiosperms capable of growing completely submerged in seawater are the seagrasses. Examples are found in genera such as Thalassia and Zostera. An aquatic origin of angiosperms is supported by the evidence that several of the earliest known fossil angiosperms were aquatic. Aquatic plants are phylogenetically well dispersed across the angiosperms, with at least 50 independent origins, although they comprise less than 2% of the angiosperm species. Archefructus represents one of the oldest, most complete angiosperm fossils which is around 125 million years old. These plants require special adaptations for living submerged in water or floating at the surface.
Classification of Macrophytes
Based on growth form, macrophytes can be classified as:
- Rooted: rooted to the substrate
- Unrooted: free-floating in the water column
- Attached: attached to substrate but not by roots
An emergent plant is one which grows in water but pierces the surface so that it is partially in air. Collectively, such plants are emergent vegetation.
This habit may have developed because the leaves can photosynthesis more efficiently above the shade of cloudy water and competition from submerged plants but often, the main aerial feature is the flower and the related reproductive process. The emergent habit permits pollination by wind or by flying insects.
There are many species of emergent plants, among them, the reed (Phragmites), Cyperus papyrus, Typha species, flowering rush and wild rice species. Some species, such as purple loosestrife, may grow in water as emergent plants but they are capable of flourishing in fens or simply in damp ground.
Submerged macrophytes completely grow under water with roots attached to the substrate (e.g. Myriophyllum spicatum) or without any root system (e.g. Ceratophyllum demersum). Helophytes are plants that grows in a marsh, partly submerged in water, so that it regrows from buds below the water surface. Fringing stands of tall vegetation by water basins and rivers may include helophytes. Examples include stands of Equisetum fluviatile, Glyceria maxima, Hippuris vulgaris, Sagittaria, Carex, Schoenoplectus, Sparganium, Acorus, yellow flag (Iris pseudacorus), Typha and Phragmites australis.
Floating-leaved macrophytes have root systems attached to the substrate or bottom of the body of water and with leaves that float on the water surface. Common floating leaved macrophytes are water lilies (family Nymphaeaceae), pondweeds (family Potamogetonaceae).
Free-floating macrophytes are aquatic plants that are found suspended on water surface with their root not attached to substrate, sediment, or bottom of the water body. They are easily blown by air and provide breeding ground for mosquitoes. Example include Pistia spp commonly called water lettuce, water cabbage or Nile cabbage.
- Amphiphytes: plants that are adapted to live either submerged or on land
- Elodeids: stem plants that complete their entire lifecycle submerged, or with only their flowers above the waterline
- Isoetids: rosette plants that complete their entire lifecycle submerged
- Helophytes: plants rooted in the bottom, but with leaves above the waterline
- Nymphaeids: plants rooted in the bottom, but with leaves floating on the water surface
- Pleuston: vascular plants that float freely in the water
Functions of macrophytes in aquatic system
Macrophytes perform many ecosystem functions in aquatic ecosystems and provide services to human society. One of the important functions performed by macrophyte is uptake of dissolve nutrients (N and P) from water. Macrophytes are widely used in constructed wetlands around the world to remove excess N and P from polluted water. Beside direct nutrient uptake, macrophytes indirectly influence nutrient cycling, especially N cycling through influencing the denitrifying bacterial functional groups that are inhabiting on roots and shoots of macrophytes. Macrophytes promote the sedimentation of suspended solids by reducing the current velocities, impede erosion by stabilising soil surfaces. Macrophytes also provide spatial heterogeneity in otherwise unstructured water column. Habitat complexity provided by macrophytes like to increase the richness of taxonomy and density of both fish and invertebrates.
Uses and importance to humans
Some aquatic plants are used by humans as a food source. Examples include wild rice (Zizania), water caltrop (Trapa natans), Chinese water chestnut (Eleocharis dulcis), Indian lotus (Nelumbo nucifera), water spinach (Ipomoea aquatica), and watercress (Rorippa nasturtium-aquaticum).
A decline in a macrophyte community may indicate water quality problems and changes in the ecological status of the water body. Such problems may be the result of excessive turbidity, herbicides, or saliniation. Conversely, overly high nutrient levels may create an overabundance of macrophytes, which may in turn interfere with lake processing. Macrophyte levels are easy to sample, do not require laboratory analysis, and are easily used for calculating simple abundance metrics.
Potential sources of therapeutic agents
Phytochemical and pharmacological researches suggest that freshwater macrophytes, such as Centella asiatica, Nelumbo nucifera, Nasturtium officinale, Ipomoea aquatica and Ludwigia adscendens, are promising sources of anticancer and antioxidative natural products.
Hot water extracts of the stem and root of Ludwigia adscendens, as well as those of the fruit, leaf and stem of Monochoria hastata were found to have lipoxygenase inhibitory activity. Hot water extract prepared from the leaf of Ludwigia adscendens exhibits alpha-glucosidase inhibitory activity more potent than that of acarbose.
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