266
The Spanish conquistadors found Aztec
chinampa or ‘fl oating-garden’ agriculture in the
swampy lakeshore areas in the Valley of
Mexico. The name fl oating garden was given
because the seedlings were raised on floating
mats of vegetation and peat which could be
towed to the planting site and transferred to the
chinampa in peat blocks. Hence, it was named
as ‘fl oating gardens’. A similar agricultural
practice was in vogue in the Vale of Kashmir in
the Himalayas.
Water meadows are managed as grazing and
hay-producing grasslands in parts of lowland
Britain and northern Europe. The lands are
fl ooded with nutrient-rich silt-laden water (when
the rivers are in spate) using sluices, ditches and
embankments. During the spring months, such
enrichments promote nitrogen fi xation and stimulate an early herbage crop. Few such areas now
survive under the infl uence of modern mechanised agriculture. And, where they persist, their
species-rich grassland fl ora is further supplemented by many marshland and other wetland
species to form a vegetation type of great interest
and conservational importance.
The different wetland ecosystems in the world
are briefl y described below.
18.2 Coastal Wetland Ecosystems
In coastal areas, different types of wetlands are
infl uenced by alternate fl oods and ebbs of tides.
The salinity of the water approaches that of the
ocean near the coastlines. However, the tidal
effects may remain signifi cant further inland,
even when the salinity is that of freshwater. The
total area of coastal or estuarine wetlands is 3.2
million ha in the USA (including Alaska). Out of
this, 1.9 million ha is salt marsh and 0.5 million
ha is mangrove. The estuarine wetlands of Alaska
were estimated to cover 0.86 million ha (Hall
et al. 1994 ). Out of this, c 16.9 % (0.146 million
ha) are vegetated and, thus, presumably, salt
marsh. However, the vast majority of estuarine
wetlands in Alaska had been classifi ed as ‘nonvegetated’ (Hall et al. 1994 ).
18.3 Tidal Salt Marshes
Salt marshes are found throughout the world
along protected coastlines in the middle and high
latitudes. These are complex ecosystems which
are in dynamic balance with their surroundings.
These marshes fl ourish wherever the accumulation of sediments is equal to or greater than the
rate of land subsistence and where there is adequate protection from destructive waves and
storms. Some of the important physical and
chemical variables which determine the structure
and function of the salt marshes are tidal fl ooding
frequency and duration, soil salinity, soil permeability and nutrient limitation particularly by nitrogen. The vegetation of the salt marshes is primarily
salt-tolerant grasses and rushes. The salt marshes
are often dominated by the grass Spartina alternifl ora in the low intertidal zone in the eastern coast
of the USA. However, S. patens and the rush
Juncus often dominate the upper intertidal zone.
The plants and animals in these systems have
adapted themselves to the stresses of salinity,
periodic inundation and extremes in temperature.
They develop in identifi able zones in response to
these and possibly other factors. The heterotrophic communities are generally dominated by
detrital food chains with the grazing food chain
being much less trophically signifi cant. Salt
marshes are quite abundant in the USA along the
eastern coast from Maine to Florida and on to
Louisiana and Texas along the Gulf of Mexico.
Salt marshes are considered as productive
ecosystems in the world. The regional differences are believed to be related to available solar
energy and, to some extent, to available nutrient
imports by large rivers. The decomposition of
dead vegetation in the salt marshes is carried out
by fungi and bacteria. They enhance the protein
content of the detrital mixture for other marsh
estuarine organisms.
Beeftink ( 1977a , b ) defi ned a salt marsh as a
‘natural or semi-natural halophytic grassland and
dwarf brushwood on the alluvial sediments bordering saline water bodies whose water level
fl uctuates either tidally or non-tidally’ .
18 Classifi cation of Wetlands
The Spanish conquistadors found Aztec
chinampa or ‘fl oating-garden’ agriculture in the
swampy lakeshore areas in the Valley of
Mexico. The name fl oating garden was given
because the seedlings were raised on floating
mats of vegetation and peat which could be
towed to the planting site and transferred to the
chinampa in peat blocks. Hence, it was named
as ‘fl oating gardens’. A similar agricultural
practice was in vogue in the Vale of Kashmir in
the Himalayas.
Water meadows are managed as grazing and
hay-producing grasslands in parts of lowland
Britain and northern Europe. The lands are
fl ooded with nutrient-rich silt-laden water (when
the rivers are in spate) using sluices, ditches and
embankments. During the spring months, such
enrichments promote nitrogen fi xation and stimulate an early herbage crop. Few such areas now
survive under the infl uence of modern mechanised agriculture. And, where they persist, their
species-rich grassland fl ora is further supplemented by many marshland and other wetland
species to form a vegetation type of great interest
and conservational importance.
The different wetland ecosystems in the world
are briefl y described below.
18.2 Coastal Wetland Ecosystems
In coastal areas, different types of wetlands are
infl uenced by alternate fl oods and ebbs of tides.
The salinity of the water approaches that of the
ocean near the coastlines. However, the tidal
effects may remain signifi cant further inland,
even when the salinity is that of freshwater. The
total area of coastal or estuarine wetlands is 3.2
million ha in the USA (including Alaska). Out of
this, 1.9 million ha is salt marsh and 0.5 million
ha is mangrove. The estuarine wetlands of Alaska
were estimated to cover 0.86 million ha (Hall
et al. 1994 ). Out of this, c 16.9 % (0.146 million
ha) are vegetated and, thus, presumably, salt
marsh. However, the vast majority of estuarine
wetlands in Alaska had been classifi ed as ‘nonvegetated’ (Hall et al. 1994 ).
18.3 Tidal Salt Marshes
Salt marshes are found throughout the world
along protected coastlines in the middle and high
latitudes. These are complex ecosystems which
are in dynamic balance with their surroundings.
These marshes fl ourish wherever the accumulation of sediments is equal to or greater than the
rate of land subsistence and where there is adequate protection from destructive waves and
storms. Some of the important physical and
chemical variables which determine the structure
and function of the salt marshes are tidal fl ooding
frequency and duration, soil salinity, soil permeability and nutrient limitation particularly by nitrogen. The vegetation of the salt marshes is primarily
salt-tolerant grasses and rushes. The salt marshes
are often dominated by the grass Spartina alternifl ora in the low intertidal zone in the eastern coast
of the USA. However, S. patens and the rush
Juncus often dominate the upper intertidal zone.
The plants and animals in these systems have
adapted themselves to the stresses of salinity,
periodic inundation and extremes in temperature.
They develop in identifi able zones in response to
these and possibly other factors. The heterotrophic communities are generally dominated by
detrital food chains with the grazing food chain
being much less trophically signifi cant. Salt
marshes are quite abundant in the USA along the
eastern coast from Maine to Florida and on to
Louisiana and Texas along the Gulf of Mexico.
Salt marshes are considered as productive
ecosystems in the world. The regional differences are believed to be related to available solar
energy and, to some extent, to available nutrient
imports by large rivers. The decomposition of
dead vegetation in the salt marshes is carried out
by fungi and bacteria. They enhance the protein
content of the detrital mixture for other marsh
estuarine organisms.
Beeftink ( 1977a , b ) defi ned a salt marsh as a
‘natural or semi-natural halophytic grassland and
dwarf brushwood on the alluvial sediments bordering saline water bodies whose water level
fl uctuates either tidally or non-tidally’ .
18 Classifi cation of Wetlands