268
(d) At times, salt marshes have been considered
to be both sources and sinks of nutrients,
particularly nitrogen.
18.3.9 Ecosystem Models
A number of conceptual and mathematical models have been developed to describe the structure
and function of the salt marsh ecosystem. Models
like Teal’s energy fl ow diagram and the diagram
of a salt marsh food web represent some of the
conceptual models of the salt marsh.
18.4 Tidal Freshwater Marshes
The freshwater (FW) coastal wetlands are unique
ecosystems which combine many features of
both salt marshes and FW inland marshes. They
are believed to act in many ways like the salt
marshes. This type of wetlands are situated inland
from the tidal salt marshes but still close enough
to the coast to experience tidal effects. Their
biota refl ect the increased diversity which is
made possible by the reduction in the salt stress
found in salt marshes. Plant diversity is high.
These wetlands are dominated by a variety of
grasses and by annual and perennial broad-leaved
aquatic plants. More birds use these marshes than
any other marsh type. However, they are more
prone to human interference due to their proximity to urban areas. The tidal FW marshes could be
grouped into three major types, namely , (a)
‘mature marshes’ (these are usually with tidal
ranges of 1–2 m, such as those found on the
Atlantic coast of the USA), (b) mature coastal
fl oating marshes (these are found on the northern
coast of the Gulf of Mexico) and (c) newly emergent marshes (these are found in the prograding
deltas). However, broadly and primarily, they are
found along the Middle and South Atlantic coasts
and along the coasts of Louisiana and Texas. In
the USA, estimates of tidal freshwater wetlands
range from 164,000 ha along the Atlantic coast
(W.E. Odum et al. 1984 ) to 820,000 ha for the
conterminous USA (Field et al. 1991 ). Tidal FW
marshes may be described as intermediate in the
continuum from coastal salt marshes to FW
marshes. They are believed to be often very productive ecosystems because they are tidally infl uenced but lack the salinity stress of salt marshes.
The tidal FW marshes are interesting because
they receive the same ‘tidal subsidy’ as coastal
salt marshes but without the salt stress.
18.4.1 Geographical Extent
Adequate rainfall or river fl ow is one of the physical conditions required for tidal FW marsh
development and to maintain fresh conditions.
These conditions are said to occur where major
rivers debouch into the coastal waters. It had
been reported that the tidal FW marshes of
the Atlantic coast cover about 164,000 ha
(WE Odum et al. 1984 ). Conversely, the FW
marshes on the Gulf of Mexico cover about
383,000 (Gosselink et al. 1998 ).
18.4.2 Geomorphology
and Hydrology
Coastal FW wetlands are known to occur on different types of substrates. However, their recent geological history is similar in spite of regional
differences (WE Odum et al. 1984 ). It may be noted
here that the contemporary coastal marshes are said
to be recent (Holocene) in origin. They occurred in
the river valleys which were isolated during the
Pleistocene periods of lowered sea levels.
Subsequently, when the sea level rose after the last
glaciation (15,000–5,000 BP), FW coastal marshes
had possibly expanded rapidly. However, in view of
the recent changes, with the rising sea level along
most of the world’s coasts, it is quite pertinent to
consider whether tidal freshwater marshes are submerging or whether vertical accretion is keeping
pace with sea level rise. High sedimentation rates
often exceed land submergence, especially during
the years with major spring fl oods, as in the case of
new marshes of the Mississippi River delta (Roberts
and Van Hurden 1992 ).
18 Classifi cation of Wetlands
(d) At times, salt marshes have been considered
to be both sources and sinks of nutrients,
particularly nitrogen.
18.3.9 Ecosystem Models
A number of conceptual and mathematical models have been developed to describe the structure
and function of the salt marsh ecosystem. Models
like Teal’s energy fl ow diagram and the diagram
of a salt marsh food web represent some of the
conceptual models of the salt marsh.
18.4 Tidal Freshwater Marshes
The freshwater (FW) coastal wetlands are unique
ecosystems which combine many features of
both salt marshes and FW inland marshes. They
are believed to act in many ways like the salt
marshes. This type of wetlands are situated inland
from the tidal salt marshes but still close enough
to the coast to experience tidal effects. Their
biota refl ect the increased diversity which is
made possible by the reduction in the salt stress
found in salt marshes. Plant diversity is high.
These wetlands are dominated by a variety of
grasses and by annual and perennial broad-leaved
aquatic plants. More birds use these marshes than
any other marsh type. However, they are more
prone to human interference due to their proximity to urban areas. The tidal FW marshes could be
grouped into three major types, namely , (a)
‘mature marshes’ (these are usually with tidal
ranges of 1–2 m, such as those found on the
Atlantic coast of the USA), (b) mature coastal
fl oating marshes (these are found on the northern
coast of the Gulf of Mexico) and (c) newly emergent marshes (these are found in the prograding
deltas). However, broadly and primarily, they are
found along the Middle and South Atlantic coasts
and along the coasts of Louisiana and Texas. In
the USA, estimates of tidal freshwater wetlands
range from 164,000 ha along the Atlantic coast
(W.E. Odum et al. 1984 ) to 820,000 ha for the
conterminous USA (Field et al. 1991 ). Tidal FW
marshes may be described as intermediate in the
continuum from coastal salt marshes to FW
marshes. They are believed to be often very productive ecosystems because they are tidally infl uenced but lack the salinity stress of salt marshes.
The tidal FW marshes are interesting because
they receive the same ‘tidal subsidy’ as coastal
salt marshes but without the salt stress.
18.4.1 Geographical Extent
Adequate rainfall or river fl ow is one of the physical conditions required for tidal FW marsh
development and to maintain fresh conditions.
These conditions are said to occur where major
rivers debouch into the coastal waters. It had
been reported that the tidal FW marshes of
the Atlantic coast cover about 164,000 ha
(WE Odum et al. 1984 ). Conversely, the FW
marshes on the Gulf of Mexico cover about
383,000 (Gosselink et al. 1998 ).
18.4.2 Geomorphology
and Hydrology
Coastal FW wetlands are known to occur on different types of substrates. However, their recent geological history is similar in spite of regional
differences (WE Odum et al. 1984 ). It may be noted
here that the contemporary coastal marshes are said
to be recent (Holocene) in origin. They occurred in
the river valleys which were isolated during the
Pleistocene periods of lowered sea levels.
Subsequently, when the sea level rose after the last
glaciation (15,000–5,000 BP), FW coastal marshes
had possibly expanded rapidly. However, in view of
the recent changes, with the rising sea level along
most of the world’s coasts, it is quite pertinent to
consider whether tidal freshwater marshes are submerging or whether vertical accretion is keeping
pace with sea level rise. High sedimentation rates
often exceed land submergence, especially during
the years with major spring fl oods, as in the case of
new marshes of the Mississippi River delta (Roberts
and Van Hurden 1992 ).
18 Classifi cation of Wetlands