229
tage of the rich detrital food chains supported by
the upper marsh.
Wetlands have considerable ability to ‘sink’
pollutants of all kinds, including nitrogen and
phosphorus, from sewage and agricultural
drainage. Wetlands also act as sink for heavy metals and artifi cial products, such as chlorinated
hydrocarbons. Chapman (1977) cited their annual
value for this purpose as several times that of an
alternative use such as oyster culture. Many ecologists are worried not only about the duration of
the sink activity but also the biological consequences for marshes of scientifi c value.
15.3.2 Nutrients
The nutrients act as fertilisers for plant groups,
and their supply determines the quantity and
quality of plants. The most common are nitrates
and phosphates and they are often collectively
called nutrients.
15.3.2.1 Nutrient Cycling
Nutrients are generally carried into wetlands by the
hydrologic inputs of precipitation, river fl ooding,
tides, surface and groundwater infl ows, etc. On the
other hand, outfl ows of nutrients are primarily controlled by the outfl ow of water. These hydrologic/
nutrient fl ows are also important determinants of
wetland productivity and decomposition.
Notwithstanding the above, the hydro-period
of a wetland has a signifi cant impact on nutrient
transformation, on the availability of nutrients
to plants and on the loss of gaseous nutrients
from the soil. Thus, in wetlands, nitrogen availability and loss are affected by the reduced conditions which result from waterlogged soil. In
addition, fl ooding of wetland soil, by altering
both the pH and the redox potential, a measure
of the intensity of oxidation or reduction of a
chemical or a biological system, could be ascertained. This could expose the state of oxidation
and availability of a number of nutrients.
Incidentally, phosphorus is known to be more
soluble under anaerobic conditions. This could
partly be due to the hydrolysis and reduction of
ferric and aluminium phosphates to more soluble compounds. Further, the hydrologic conditions in the wetlands are known to affect the
availability of major ions, e.g. Fe, Mn and S.
15.3.3 Wetlands as Sources,
Sinks and Transformers
of Nutrients
There have been much amount of works done on
whether wetlands are sources of nutrients, sinks
or transformers. It is pertinent to note here that a
wetland is considered as a ‘sink’, if it has a net
retention of an element or a specifi c form of that
element (organic or inorganic), i.e. if the inputs
are greater than the outputs. Conversely, if a
wetland exports more of an element or material
to a downstream or adjacent ecosystem than
would occur without those wetlands, it is considered as a ‘source’. Further, if a wetland transforms a chemical form, say, dissolved to
particulate form but does not change the amount
going into or out of the wetland, it is considered
to be a ‘transformer’.
Notwithstanding the above, there is a misconception that all wetlands are highly productive
ecosystems with high nutrient content. It may be
noted here that wetlands like the tidal marshes and
riparian forest wetlands could have rich supply of
nutrients. Hence, they may be highly productive.
But there are many types of wetlands which have
low supply of nutrients. The terms ‘eutrophic’
(rich in nutrients) and ‘oligotrophic’ (not rich in
nutrients) are usually used to denote the trophic
states of lakes and estuaries. Nevertheless, these
terms have been found to be quite appropriate for
wetlands too. In fact, the use of these terms had
originated in the classifi cation of peat lands
(Hutchinson 1973 ) and was later adopted for open
bodies of water.
15.3.3.1 Reed Swamp, Fen and Marsh
A rather variable ration of shoot to underground
rhizome is found amongst the reed swamp plants,
such as the temperate common reed ( Phragmites
australis ) or the tropical papyrus ( Cyperus papyrus ) because they rarely suffer from drought.
In northern Europe, their transpiration is near to
15.3 Wetland Productivity
tage of the rich detrital food chains supported by
the upper marsh.
Wetlands have considerable ability to ‘sink’
pollutants of all kinds, including nitrogen and
phosphorus, from sewage and agricultural
drainage. Wetlands also act as sink for heavy metals and artifi cial products, such as chlorinated
hydrocarbons. Chapman (1977) cited their annual
value for this purpose as several times that of an
alternative use such as oyster culture. Many ecologists are worried not only about the duration of
the sink activity but also the biological consequences for marshes of scientifi c value.
15.3.2 Nutrients
The nutrients act as fertilisers for plant groups,
and their supply determines the quantity and
quality of plants. The most common are nitrates
and phosphates and they are often collectively
called nutrients.
15.3.2.1 Nutrient Cycling
Nutrients are generally carried into wetlands by the
hydrologic inputs of precipitation, river fl ooding,
tides, surface and groundwater infl ows, etc. On the
other hand, outfl ows of nutrients are primarily controlled by the outfl ow of water. These hydrologic/
nutrient fl ows are also important determinants of
wetland productivity and decomposition.
Notwithstanding the above, the hydro-period
of a wetland has a signifi cant impact on nutrient
transformation, on the availability of nutrients
to plants and on the loss of gaseous nutrients
from the soil. Thus, in wetlands, nitrogen availability and loss are affected by the reduced conditions which result from waterlogged soil. In
addition, fl ooding of wetland soil, by altering
both the pH and the redox potential, a measure
of the intensity of oxidation or reduction of a
chemical or a biological system, could be ascertained. This could expose the state of oxidation
and availability of a number of nutrients.
Incidentally, phosphorus is known to be more
soluble under anaerobic conditions. This could
partly be due to the hydrolysis and reduction of
ferric and aluminium phosphates to more soluble compounds. Further, the hydrologic conditions in the wetlands are known to affect the
availability of major ions, e.g. Fe, Mn and S.
15.3.3 Wetlands as Sources,
Sinks and Transformers
of Nutrients
There have been much amount of works done on
whether wetlands are sources of nutrients, sinks
or transformers. It is pertinent to note here that a
wetland is considered as a ‘sink’, if it has a net
retention of an element or a specifi c form of that
element (organic or inorganic), i.e. if the inputs
are greater than the outputs. Conversely, if a
wetland exports more of an element or material
to a downstream or adjacent ecosystem than
would occur without those wetlands, it is considered as a ‘source’. Further, if a wetland transforms a chemical form, say, dissolved to
particulate form but does not change the amount
going into or out of the wetland, it is considered
to be a ‘transformer’.
Notwithstanding the above, there is a misconception that all wetlands are highly productive
ecosystems with high nutrient content. It may be
noted here that wetlands like the tidal marshes and
riparian forest wetlands could have rich supply of
nutrients. Hence, they may be highly productive.
But there are many types of wetlands which have
low supply of nutrients. The terms ‘eutrophic’
(rich in nutrients) and ‘oligotrophic’ (not rich in
nutrients) are usually used to denote the trophic
states of lakes and estuaries. Nevertheless, these
terms have been found to be quite appropriate for
wetlands too. In fact, the use of these terms had
originated in the classifi cation of peat lands
(Hutchinson 1973 ) and was later adopted for open
bodies of water.
15.3.3.1 Reed Swamp, Fen and Marsh
A rather variable ration of shoot to underground
rhizome is found amongst the reed swamp plants,
such as the temperate common reed ( Phragmites
australis ) or the tropical papyrus ( Cyperus papyrus ) because they rarely suffer from drought.
In northern Europe, their transpiration is near to
15.3 Wetland Productivity