Aquatic plants require specialadaptations for prolongedinundation in water, and forfloating at the water surface. The most common adaptation is the presence of lightweight internal packing cells,aerenchyma, but floating leaves and finely dissected leaves are also common.[5][6][7] Aquatic plants only thrive in water or insoil that is frequentlysaturated, and are therefore a common component ofswamps andmarshlands.[8]
Aquatic plants have adapted to live in either freshwater or saltwater. Aquaticvascular plants have originated on multiple occasions in different plant families;[5][9] they can beferns or angiosperms (including bothmonocots anddicots). The onlyangiosperms capable of growing completely submerged in seawater are theseagrasses.[10] Examples are found in genera such asThalassia andZostera. An aquatic origin of angiosperms is supported by the evidence that several of the earliest known fossil angiosperms were aquatic. Aquatic plants arephylogenetically well dispersed across theangiosperms, with at least 50 independent origins, although they comprise less than 2% of the angiosperm species.[11]Archaefructus represents one of the oldest, most complete angiosperm fossils which is around 125 million years old.[12] These plants require special adaptations for living submerged in water or floating at the surface.[12]
Fully submerged aquatic plants have little need for stiff or woody tissue as they are able to maintain their position in the water using buoyancy typically from gas filled lacunaa or turgidAerenchyma cells.[13] When removed from the water, such plants are typically limp and loseturgor rapidly.[14]
Those living in rivers do, however, need sufficient structuralxylem to avoid being damaged by fast flowing water and they also need strong mechanisms of attachment to avoid being uprooted by river flow.
Many fully submerged plants have finely dissected leaves, probably to reduce drag in rivers and to provide a much increased surface area for interchange of minerals and gasses.[13]Some species of plants such asRanunculus aquatilis have two different leaf forms with finely dissected leaves that are fully submerged and entire leaves on the surface of the water.
Some still-water plants can alter their position in the water column at different seasons. One notable example iswater soldier, which rests as a rootless rosette on the bottom of the water body but slowly floats to the surface in late spring so that its inflorescence can emerge into the air. While it is ascending through the water column it produces roots and vegetative daughter plants by means ofrhizomes. When flowering is complete, the plant descends through the water column and the roots atrophy.
In floating aquatic angiosperms, the leaves have evolved to only havestomata on the top surface to make use of atmospheric carbon dioxide.[15] Gas exchange primarily occurs through the top surface of the leaf due to the position of the stomata, and the stomata are in a permanently open state. Due to their aquatic surroundings, the plants are not at risk of losing water through the stomata and therefore face no risk of dehydration.[15] For carbon fixation, some aquatic angiosperms are able to uptake CO2 frombicarbonate in the water, a trait that does not exist in terrestrial plants.[16] Angiosperms that useHCO 3- can keep CO2 levels satisfactory, even in basic environments with low carbon levels.[16] The many possible classifications of aquatic plants are based upon morphology.[5]
Aquatic plants are either aquatic macrophytes or aquatic microphytes.[17] Aquatic macrophytes are hydrophytes that are large enough to be seen with the naked eye.[18] Aquatic microphytes are hydrophytes that cannot be seen with the naked eye; they are microscopic.[17]
Anemergent plant is one which grows in water but pierces the surface so that it is partially exposed to air. Collectively, such plants areemergent vegetation.[20]
This habit may have developed because the leaves canphotosynthesize more efficiently in air 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 flyinginsects.[20][21]
Althoughseaweeds, which are largemulticellular marinealgae, have similar ecological functions to aquatic plants such asseagrass, they lack the specializedroot/rhizoid system of plants.[25] Instead, seaweeds haveholdfasts that only serve as anchors and have no absorptive functions.[26]
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 (familyNymphaeaceae), pondweeds (familyPotamogetonaceae).[27]
Free-floating macrophytes are found suspended on water surface with their root not attached to the substrate,sediment, or bottom of the water body. They be easily easily blown by air and some may provide breeding ground for mosquitoes. Examples includeLemna spp orPistia spp. the latter commonly called water lettuce, water cabbage or Nile cabbage.[27]
Phytoplankton , also known as microscopic algae.[19] Free-floating; drifting with water currents.[29]
Periphyton - a microphyte that lives and grows on the surface of rooted aquatic plants.[19][30]
Benthic algae - reltively immobile algae that inhabit the submerged substrate surface of freshwater on mud, stones, or other relatively stable material[31]. Algae may be single celled such asDiatoms orDesmids, or multi-celled such asSpirogyra orCladophora. A few such as some of the diatoms have limited abilities to move over their substrate.
Terrestrial plants may undergo physiological changes when submerged due to flooding. When submerged, new leaf growth has been found to have thinner leaves and thinner cell walls than the leaves on the plant that grew while above water, along with oxygen levels being higher in the portion of the plant grown underwater versus the sections that grew in their terrestrial environment.[33] This is considered a form ofphenotypic plasticity as the plant, once submerged, experiences changes in morphology better suited to their new aquatic environment.[33] However, while some terrestrial plants may be able to adapt in the short-term to an aquatic habitat, it may not be possible to reproduce underwater, especially if the plant usually relies on terrestrialpollinators.
Due to their environment, aquatic plants experience buoyancy which counteracts their weight.[34] Because of this, their cell covering are far more flexible and soft, due to a lack of pressure that terrestrial plants experience.[34] Green algae are also known to have extremely thin cell walls due to their aquatic surroundings, and research has shown that green algae is the closest ancestor to living terrestrial and aquatic plants.[35] Terrestrial plants have rigid cell walls meant for withstanding harsh weather, as well as keeping the plant upright as the plant resists gravity. Gravitropism, along with phototropism and hydrotropism, are traits believed to have evolved during the transition from an aquatic to terrestrial habitat.[36][37] Terrestrial plants no longer had unlimited access to water and had to evolve to search for nutrients in their new surroundings as well as develop cells with new sensory functions, such asstatocytes.
Submerged aquatic plants have more restricted access to carbon as carbon dioxide compared to terrestrial plants. They may also experience reduced light levels.[16] In aquatic plants diffuseboundary layers (DBLs) around submerged leaves and photosynthetic stems vary based on the leaves' thickness, shape and density and are the main factor responsible for the greatly reduced rate of gaseous transport across the leaf/water boundary and therefore greatly inhibit transport of carbon dioxide.[16] To overcome this limitation, many aquatic plants have evolved to metabolisebicarbonate ions as a source of carbon.[16]
Environmental variables affect the instantaneous photosynthetic rates of aquatic plants and the photosynthetic enzymes pigments.[38] In water, light intensity rapidly decreases with depth. Respiration is also higher in the dark per the unit volume of the medium they live in.[38]
Although most aquaticangiosperms can reproduce by flowering and setting seeds, many have also evolved to have extensiveasexual reproduction by means ofrhizomes,turions, and fragments in general.[6]
One of the largest aquatic plants in the world is theBolivian waterlily, which holds theGuinness World Record of having the largest undivided leaf at 3.2 m (10 ft 6 in) diameter; the smallest is therootless duckweed, which is only 1 mm (0.039 in) across. Many small animals use aquatic plants such as duckweeds andlily pads forspawning or as protective shelters against predators both from above and below the water surface.
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 dissolved nutrients including nitrogen and phosphorus.[40] Macrophytes are widely used in constructed wetlands around the world to remove excess N and P from polluted water.[46] Besides direct nutrient uptake, macrophytes indirectly influencenutrient cycling; especially N cycling through influencing the denitrifying bacterial functional groups that are inhabiting on roots and shoots of macrophytes.[47] Macrophytes promote the sedimentation of suspended solids by reducing the current velocities,[48] impede erosion by stabilising soil surfaces.[49] Macrophytes also provide spatial heterogeneity in otherwise unstructured water column. Habitat complexity provided by macrophytes tends to increase diversity and density of both fish and invertebrates.[50]
The additional site-specific macrophytes' value provides wildlife habitat and makes treatment systems of wastewater aesthetically satisfactory.[51]
World aquaculture production of food fish and aquatic plants, 1990–2016
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 excessiveturbidity,herbicides, orsalination. Conversely, overly high nutrient levels may create an overabundance of macrophytes, which may in turn interfere withlake processing.[3] Macrophyte levels are easy to sample, do not require laboratory analysis, and are easily used for calculating simple abundance metrics.[3]
Hot water extracts of the stem and root ofLudwigia adscendens, as well as those of the fruit, leaf and stem ofMonochoria hastata were found to havelipoxygenase inhibitory activity. Hot water extract prepared from the leaf ofLudwigia adscendens exhibitsalpha-glucosidase inhibitory activity more potent than that ofacarbose.[53]
Wastewater treatment
Macrophytes have an essential role in some forms of wastewater treatment, most commonly in small scalesewage treatment usingconstructed wetlands or in polishing lagoons for larger schemes.[51]
The principal factor controlling the distribution of aquatic plants is the availability of water. However, other abiotic factors may also control their distribution including nutrient availability, availability of carbon dioxide and oxygen, water temperature, characteristics of the substrate, water transparency,[54] water movement, and salinity.[8] Some aquatic plants are able to thrive inbrackish, saline, and salt water.[5] Also biotic factors like grazing,[8] competition for light,[54] colonization by fungi,[55] and allelopathy[56] are influencing the occurrence of macrophytes.
The introduction of non-native aquatic plants has resulted in numerous examples across the world of such plants becoming invasive and frequently dominating the environments into which they have been introduced.[57] Such species includeWater hyacinth which is invasive in many tropical and sub-tropical locations including much of the southern US, many Asian countries and Australia.New Zealand stonecrop is a highly invasive plant in temperate climates spreading from a marginal plant to encompassing the whole body of many ponds to the almost total exclusion of other plants and wildlife[58]
In 2012, a comprehensive overview of alien aquatic plants in 46 European countries found 96 alien aquatic species. The aliens were primarily native to North America, Asia, and South America. The most spread alien plant in Europe wasElodea canadensis (Found in 41 European countries) followed byAzolla filiculoides in 25 countries andVallisneria spiralis in 22 countries.[57] The countries with the most recorded alien aquatic plant species were France and Italy with 30 species followed by Germany with 27 species, and Belgium and Hungary with 26 species.[57]
The European and Mediterranean Plant Protection Organization has published recommendations to European nations advocating the restriction or banning of the trade in invasive alien plants.[61]
^Chambers, Patricia A. (September 1987). "Light and Nutrients in the Control of Aquatic Plant Community Structure. II. In Situ Observations".The Journal of Ecology.75 (3):621–628.Bibcode:1987JEcol..75..621C.doi:10.2307/2260194.JSTOR2260194.
^Hallin, Sara; Hellman, Maria; Choudhury, Maidul I.; Ecke, Frauke (2015). "Relative importance of plant uptake and plant associated denitrification for removal of nitrogen from mine drainage in sub-arctic wetlands".Water Research.85:377–383.Bibcode:2015WatRe..85..377H.doi:10.1016/j.watres.2015.08.060.PMID26360231.
^Horppila, Jukka; Kaitaranta, Joni; Joensuu, Laura; Nurminen, Leena (2013). "Influence of emergent macrophyte (Phragmites australis) density on water turbulence and erosion of organic-rich sediment".Journal of Hydrodynamics.25 (2):288–293.Bibcode:2013JHyDy..25..288H.doi:10.1016/S1001-6058(13)60365-0.S2CID120990795.
^Thomaz, Sidinei M.; Dibble, Eric D.; Evangelista, Luiz R.; Higuti, Janet; Bini, Luis M. (2007). "Influence of aquatic macrophyte habitat complexity on invertebrate abundance and richness in tropical lagoons".Freshwater Biology.53 (2):358–367.doi:10.1111/j.1365-2427.2007.01898.x.
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