in the delta (Maltby, 1986). There are efforts to restore the
Mekong Delta, as evidenced by a project known as Plain
of Reeds (Beilfuss and Barzen, 1994). China has the largest extensions of Asian wetlands, covering more than
600,000 km
2
. Most of them provide food, habitat, and
recreation. Forty percent of these wetlands are included
in the Ramsar list as systems of international importance
(Parish and Elliot, 1990). The Yangtze, Zhujiang, and
Liaohe rivers have the most important wetlands, but they
are also the most populated areas. Some wetlands in these
areas have been converted to rice paddies, reed fields, or
fish ponds (Lu, 1990; Ma and Yan, 1989). Wetlands in
China serve as refuge and migratory corridors; more than
160 species of birds have been documented in these
systems (Lu, 1990).
American wetlands
In South America, river discharges into the Pacific are less
than in the Caribbean; therefore, wetlands are seasonal
along the Pacific. An example is the Palo Verde National
Park in Costa Rica which has had diminishing diversity
because of invasion by cattail (Typha domingensis) that
has displaced native types of vegetation (McCoy and
Rodriguez, 1994). The Orinoco River in Venezuela supports the Llanos wetland, the largest inland wetland in
South America (Dugan, 1993). Flora in this wetland is
the savanna type as a result of the dry summer seasons,
unlike the flora found in the Orinoco Delta (Junk, 1993).
Fauna characteristic of this zone are caiman (Caiman
sp.) and red piranha (Serrasalmus nattereri). Pantanal in
Paraguay is an inland wetland with an extension of more
than 130,000 km
2 that is flooded each year (Hamilton
et al., 1996). This wetland provides shelter and food to
more than 600 species of birds, including ibis, egrets, the
jabiru (Jabiru mycteria), emblem of Pantanal wetland
(Por, 1995), and capybara (Hydrochoerus hydrochaeris)
the largest rodent. The Amazon River has more than
300,000 km
2 of flooding forests. It is also subject to great
pressure from land clearing activities, threatening the survival and habitat quality of vast wetland areas (Junk,
1993). Some of the major wetlands in North America
include the Mississippi River Delta, Hudson Bay Lowlands, the Everglades, Prairie Potholes, Magdalena Bay,
Marismas Nacionales, Ría Lagartos, and Sian Ka’an
(Mitsch and Gosselink, 2000; CONABIO, 2009).
Types of wetlands
Tidal salt marshes are most abundant in temperate and
polar latitudes. Flora characteristic of these wetlands
include halophytes, plants that are tolerant of high salinities and partial submergence in marine water during the
tidal cycle; the genera Spartina, Juncus, and Salicornia
are examples. Tidal freshwater marshes are the transition
between coastal and freshwater wetlands. They are distant
from the coast but still influenced by the tides; however,
they do lack the stress associated with high salinities
found at the coast. Perennial and annual grasses and
aquatic plants dominate this ecosystem (Typha sp., Phragmites sp., Juncus sp., Hibiscus sp.). In tropical and subtropical latitudes, tidal salt marshes are replaced by
mangroves, a group of trees that have developed physiological and morphological strategies to thrive in saline to
hypersaline environments (Hogarth, 2007). In America,
mangroves are characterized by the genera Rhizophora
(red mangrove), Avicennia (black mangrove), and
Laguncularia (white mangrove). Mangroves in the tropics
form tall canopies, while in the subtropics the trees are
much smaller. The northern limit of mangroves along the
Pacific coast of the USA is 29
N in Bahia de Los Angeles
in the Gulf of California, while along the Atlantic Coast,
the northern limit for mangroves is 27
N in Florida
(Domínguez-Cadena et al., 2011). Smaller and less-dense
mangrove forests are found at their northwest limit along
the Pacific. Factors involved in this distribution are the
lack of permanent rivers or streams, scarce rainfall, and
low winter temperatures found in the Baja California Peninsula (Domínguez-Cadena et al., 2011).
Freshwater marshes are composed of grasses, sedges
(Cyperaceae), pickerelweed (Pontederia sp.), and arrowheads (Sagittaria sp.). They have low amounts of peat,
which is one of the key differences between freshwater
marshes and peatlands. The latter have the largest deposits
of peat, formed during the last glaciation. Bogs and fens
are two types of peatlands occurring in lake basins; they
lack large inflows or outflows, thus generating stagnant
hypoxic conditions. In peatlands, mosses like Sphagnum
sp. are common, as are carnivorous plants (Cephalotus
sp.). Unlike freshwater swamps, the aforementioned
inland wetlands (freshwater marshes and peatlands) lack
woody vegetation. Freshwater swamps are forested wetlands with water present nearly the entire year. Cypress
(Taxodium sp.) and tupelo (Nyssa sp.) are commonly
found here (Mitsch and Gosselink, 2000). Riparian systems are found along rivers and streams where flooding
occurs periodically during part of the year. They are rich
in woody vegetation, including oak (Quercus sp.), cedar
(Chamaecyparis sp.), and redwood (Sequoia sp.).
Summary
To understand the functioning of a wetland, it is necessary
to consider several aspects: (1) the hydrology of the system, (2) the variation of the water level throughout the
year, (3) the flow and its frequency whether perennial or
intermittent, and (4) the influence of the tides, if the wetland is close enough to the coast. The climatic condition
of a region is critical for the determination of the hydrologic characteristic of the wetland, as is the basin for its
geomorphology. The geologic origin of the basin affects
the steepness of the wetland. The amount of water that
a wetland contains is less for systems located on steeper
terrains which also are more likely to have coarser sediments. River deltas with nearly flat slopes are more likely
to harbor fine-grained wetlands. Another crucial aspect in
the hydrogeomorphology of a wetland is the physical
740
WETLANDS
Mekong Delta, as evidenced by a project known as Plain
of Reeds (Beilfuss and Barzen, 1994). China has the largest extensions of Asian wetlands, covering more than
600,000 km
2
. Most of them provide food, habitat, and
recreation. Forty percent of these wetlands are included
in the Ramsar list as systems of international importance
(Parish and Elliot, 1990). The Yangtze, Zhujiang, and
Liaohe rivers have the most important wetlands, but they
are also the most populated areas. Some wetlands in these
areas have been converted to rice paddies, reed fields, or
fish ponds (Lu, 1990; Ma and Yan, 1989). Wetlands in
China serve as refuge and migratory corridors; more than
160 species of birds have been documented in these
systems (Lu, 1990).
American wetlands
In South America, river discharges into the Pacific are less
than in the Caribbean; therefore, wetlands are seasonal
along the Pacific. An example is the Palo Verde National
Park in Costa Rica which has had diminishing diversity
because of invasion by cattail (Typha domingensis) that
has displaced native types of vegetation (McCoy and
Rodriguez, 1994). The Orinoco River in Venezuela supports the Llanos wetland, the largest inland wetland in
South America (Dugan, 1993). Flora in this wetland is
the savanna type as a result of the dry summer seasons,
unlike the flora found in the Orinoco Delta (Junk, 1993).
Fauna characteristic of this zone are caiman (Caiman
sp.) and red piranha (Serrasalmus nattereri). Pantanal in
Paraguay is an inland wetland with an extension of more
than 130,000 km
2 that is flooded each year (Hamilton
et al., 1996). This wetland provides shelter and food to
more than 600 species of birds, including ibis, egrets, the
jabiru (Jabiru mycteria), emblem of Pantanal wetland
(Por, 1995), and capybara (Hydrochoerus hydrochaeris)
the largest rodent. The Amazon River has more than
300,000 km
2 of flooding forests. It is also subject to great
pressure from land clearing activities, threatening the survival and habitat quality of vast wetland areas (Junk,
1993). Some of the major wetlands in North America
include the Mississippi River Delta, Hudson Bay Lowlands, the Everglades, Prairie Potholes, Magdalena Bay,
Marismas Nacionales, Ría Lagartos, and Sian Ka’an
(Mitsch and Gosselink, 2000; CONABIO, 2009).
Types of wetlands
Tidal salt marshes are most abundant in temperate and
polar latitudes. Flora characteristic of these wetlands
include halophytes, plants that are tolerant of high salinities and partial submergence in marine water during the
tidal cycle; the genera Spartina, Juncus, and Salicornia
are examples. Tidal freshwater marshes are the transition
between coastal and freshwater wetlands. They are distant
from the coast but still influenced by the tides; however,
they do lack the stress associated with high salinities
found at the coast. Perennial and annual grasses and
aquatic plants dominate this ecosystem (Typha sp., Phragmites sp., Juncus sp., Hibiscus sp.). In tropical and subtropical latitudes, tidal salt marshes are replaced by
mangroves, a group of trees that have developed physiological and morphological strategies to thrive in saline to
hypersaline environments (Hogarth, 2007). In America,
mangroves are characterized by the genera Rhizophora
(red mangrove), Avicennia (black mangrove), and
Laguncularia (white mangrove). Mangroves in the tropics
form tall canopies, while in the subtropics the trees are
much smaller. The northern limit of mangroves along the
Pacific coast of the USA is 29
N in Bahia de Los Angeles
in the Gulf of California, while along the Atlantic Coast,
the northern limit for mangroves is 27
N in Florida
(Domínguez-Cadena et al., 2011). Smaller and less-dense
mangrove forests are found at their northwest limit along
the Pacific. Factors involved in this distribution are the
lack of permanent rivers or streams, scarce rainfall, and
low winter temperatures found in the Baja California Peninsula (Domínguez-Cadena et al., 2011).
Freshwater marshes are composed of grasses, sedges
(Cyperaceae), pickerelweed (Pontederia sp.), and arrowheads (Sagittaria sp.). They have low amounts of peat,
which is one of the key differences between freshwater
marshes and peatlands. The latter have the largest deposits
of peat, formed during the last glaciation. Bogs and fens
are two types of peatlands occurring in lake basins; they
lack large inflows or outflows, thus generating stagnant
hypoxic conditions. In peatlands, mosses like Sphagnum
sp. are common, as are carnivorous plants (Cephalotus
sp.). Unlike freshwater swamps, the aforementioned
inland wetlands (freshwater marshes and peatlands) lack
woody vegetation. Freshwater swamps are forested wetlands with water present nearly the entire year. Cypress
(Taxodium sp.) and tupelo (Nyssa sp.) are commonly
found here (Mitsch and Gosselink, 2000). Riparian systems are found along rivers and streams where flooding
occurs periodically during part of the year. They are rich
in woody vegetation, including oak (Quercus sp.), cedar
(Chamaecyparis sp.), and redwood (Sequoia sp.).
Summary
To understand the functioning of a wetland, it is necessary
to consider several aspects: (1) the hydrology of the system, (2) the variation of the water level throughout the
year, (3) the flow and its frequency whether perennial or
intermittent, and (4) the influence of the tides, if the wetland is close enough to the coast. The climatic condition
of a region is critical for the determination of the hydrologic characteristic of the wetland, as is the basin for its
geomorphology. The geologic origin of the basin affects
the steepness of the wetland. The amount of water that
a wetland contains is less for systems located on steeper
terrains which also are more likely to have coarser sediments. River deltas with nearly flat slopes are more likely
to harbor fine-grained wetlands. Another crucial aspect in
the hydrogeomorphology of a wetland is the physical
740
WETLANDS