Anaerobic conditions in wetland soils
require adaptations to overcome the
problems associated with respiration and high concentrations of toxins. Relatively
few plant species possess these adaptations, although many species typical of
seasonally flooded wetlands avoid physiological stress by passing the flood period
in a state of dormancy. Some wetland plants can respire anaerobically for short
periods, but all will require oxygen eventually. Most species overcome the problems
of low oxygen concentrations in the soil by development of aerenchyma (aerated
tissues), creating interconnected air spaces within the root system, either within
existing roots or, upon flooding, in new roots. The mangrove genus Avicennia
develops roots which bend upwards, growing out of the soil to allow direct gaseous
exchange with the atmosphere. Shoots, too, grow in some semi-aquatic species in
response to flooding, to restore contact with the open air. Oxygen transported to
roots reacts with metal ions, transferring them into oxidised, and therefore less
toxic, forms, but some wetland plants tolerate high concentrations of toxins by
incorporating them into their tissues.
Animals, too, have to be adapted to the
special conditions of wetlands. Seasonally
flooded wetlands will support aquatic species when inundated and terrestrial
species when dry, these animals either moving away, becoming dormant or even
metamorphosing into a terrestrial life stage when conditions are unfavourable.
Fringe wetlands and other permanently flooded areas support a community
reminiscent of lakes except that it tends to be based more on detritus than on
phytoplankton. Stagnant water containing large quantities of decomposing organic
matter reduces oxygen concentrations but many invertebrates overcome this by
breathing air, either rising to the surface to breathe or transporting and storing
bubbles of air under water; others pierce aquatic macrophytes and breathe air in
the aerenchymae. The greatest effect of wetland conditions is upon organisms
living within the substrate, which have to overcome the same problems of oxygen
shortage and toxicity faced by plant roots.
Zonation
Vertical zonation is commonly seen in
wetlands, where a change in elevation of only
a few centimetres will create a range of different conditions, each requiring different
strategies and therefore supporting different species. A series of zones, related to
the height of the water table with respect to the sediment surface, can be
recognised at the land-water interface, representing different sets of conditions to
which their inhabitants must be tolerant.
Zone 1 represents the fully terrestrial
environment, with freely draining, aerated
soils.
Plants living in zone 2 have roots which
are, for the most part, in aerated soil, but
may have to overcome problems of anoxia and toxins at depth.
Plants living in zones 3 to 6 must be
tolerant of permanently waterlogged
sediments; in zone 3, their above-ground parts are above the water and, in zone 4,
the water is shallow enough to allow emergent species to grow.
Zones 5 and 6 are too deep for emergent
species, and their flora is fully aquatic
(euhydrophytes).
Zone 7 represents those parts of the water
body beyond the littoral zone which are
too deep for light to penetrate to the bed, and so are devoid of attached
macrophytes. This type of zonation is particularly clear in fringe wetlands, but can
be found in most wetland types.
Zonation of species analogous to that
of marine algae occurs in coastal wetlands,
where water level fluctuations are predictable yet frequent. Mangroves occur in
tropical coastal zones from mean sea level to the highest spring tide level, within
which are clear zones, each with its dominant species. Species-poor Central
American mangals are dominated by red mangrove (Rhizopora mangle) on the
seaward side, where tidal inundation is daily, black mangrove (Avicennia
tomentosa) where tidal inundation is less frequent, and white mangrove
(Lagunalaria racemosa) at levels flooded only by spring tides. Indomalayan
mangals are much more speciose, but similar zonations occur).
Saltmarsh can generally be divided into
three vertical zones. Low marsh supports
very few macrophyte species and can contain large areas of bare mud, although in
eastern North America this zone may be dominated by the grass Spartina
alterniflora, and in the British Isles increasingly by S. townsendii, a hydrid of the
alien S. alterniflora and the native S. maritima. Above the MHW is middle marsh, or
turf, extensively vegetated with halophytic grasses, while above MHWS is high
marsh, supporting halophytes and some freshwater species, such as common reed
(Phragmites australis), which will persist so long as freshwater inputs are great
enough to flush out occasional saltwater incursions.
Floating vegetation mats
A consequence of anoxia in flooded wetlands
is floating mats of vegetation, formed
when an interconnected mass of vegetation, generally dominated by a single
species, breaks free from the underlying mineral substrate. Such mats float
because their tissues are less dense than water and also, as they develop,
because anaerobic decomposition within the mat generates trapped bubbles of
methane, which will keep the mat afloat even if living tissue on the surface is
removed.
Floating mats are especially prevalent
in equatorial Africa, where they are known
as 'sudd'. They are normally dominated by papyrus (Cypems papyrus), which can
form extensive rafts of vegetation held together by interconnected rhizomes, with
plants such as Vossia growing on the fringes. These mats are created when
floating swamp on the edge of a water body breaks free during stormy conditions
or when there is a sudden rise in the water table.
