225
mineral-defi cient biotope. Some groundwater fens
and reed swamps receive continuously replenished
mineral supply. They also maintain conditions
which are very favourable for nitrogen fi xation.
Some eutrophic wetland species, particularly
those of reed swamp and shallow water, have
fast growth rates. In favourable climates, they
are amongst the most biologically productive
ecosystems. Such prolifi c growth frequently
causes blockade of waterways and irrigation
channels, if they receive extra nitrogen and phosphorus from leached-out fertiliser and domestic
sewage. Similar response has also been used in
the tertiary treatment of sewage to remove the
offending nitrate and phosphate.
Nitrogen-fi xing microbes convert N 2 to organic
–NH 2 through the mediation of the molybdenumbased nitrogenase enzyme, which is confi ned to a
few free-living heterotrophic bacteria, some photosynthetic cyanobacteria (BGA) and a limited
group of plant symbionts.
Some –NH 2 may be leaked directly to plants,
in particular, from the symbionts. But the major
pathway of nitrogen fi xation is via decomposer
microbes, in which hydrolysis of proteins and
deamination liberates NH 3 of NH 4
+ to soil solution. Many wetland plants absorb their nitrogen
directly as NH 4
+ .
The nitrogen balance of rainfed peat soils
must be dominated by contributions from BGA
fi xation as they are too acid and calcium defi cient
to support most free-living bacterial fi xers except,
perhaps, for the tropical genus Beijerinckia . BGA
nitrogen fi xation is also predominant in some
forms of wetland rice culture.
15.1.4.2 Flowing Water and Tidal
Replenishment
Soluble nutrients are both exported and imported
by a constant fl ow of water in many wet soils. Also,
soil is oxygenated and soluble toxic products of the
reducing environment are removed. Freshwater
and tidal environments are ‘pulse stabilised’. The
seasonal or diurnal oscillations of water level are
regular physical and chemical disturbances. It provides the energy for rapid nutrient cycling. By disturbance, it maintains the ecosystem in an early
successional state. Much of its primary production
is defl ected to the support of a very diverse
consumer population.
Notwithstanding the above, it is still not well
known what conditions make wetlands net
importers or net exporters of nutrients in spite of
the considerable researches have been done in
this fi eld (Hasler 1975 ; Burton and Liss 1976 ;
Good et al. 1978 ).
For some elements, the import and export vary
seasonally in lotic systems. Thus, analyses of
marsh discharge show that plant uptake reduces
the concentration of elements, e.g. P, Ca and Mg,
during spring and early summer.
15.1.5 Species Composition
and Species Richness
Hydrology is a two-edged sword for species
composition and diversity in wetlands. It acts as a
limit or a stimulus to species richness, depending
on the hydro-period and physical energies.
Hydrology is said to stimulate diversity when the
action of water and transported sediments creates
spatial heterogeneity, thus opening up additional
ecological niches.
15.1.6 Methodology
of Studying AM
A Δ 1 m
2 wooden quadrant is generally used to
determine the macrophytic density. The submerged and emergent macrophytes are collected with the help of a barbed iron hawser.
The fl oating ones are usually grabbed with
hand. After collection, the AM are segregated,
the total number and size range are noted species-wise and the wet biomass are determined
on the spot by weighing them together wet with
a double pan balance. The sampling operations
are carried out in both littoral and limnetic
zones of the wetland at different randomly
selected spots. The data are analysed to evaluate the seasonal succession and phytosociology
of the AM communities. Herbaria for different
plants are prepared and transferred to the laboratory for further works (Kar 1990 ).
15.1 Wetland Flora (Aquatic Macrophytes) (AM)
mineral-defi cient biotope. Some groundwater fens
and reed swamps receive continuously replenished
mineral supply. They also maintain conditions
which are very favourable for nitrogen fi xation.
Some eutrophic wetland species, particularly
those of reed swamp and shallow water, have
fast growth rates. In favourable climates, they
are amongst the most biologically productive
ecosystems. Such prolifi c growth frequently
causes blockade of waterways and irrigation
channels, if they receive extra nitrogen and phosphorus from leached-out fertiliser and domestic
sewage. Similar response has also been used in
the tertiary treatment of sewage to remove the
offending nitrate and phosphate.
Nitrogen-fi xing microbes convert N 2 to organic
–NH 2 through the mediation of the molybdenumbased nitrogenase enzyme, which is confi ned to a
few free-living heterotrophic bacteria, some photosynthetic cyanobacteria (BGA) and a limited
group of plant symbionts.
Some –NH 2 may be leaked directly to plants,
in particular, from the symbionts. But the major
pathway of nitrogen fi xation is via decomposer
microbes, in which hydrolysis of proteins and
deamination liberates NH 3 of NH 4
+ to soil solution. Many wetland plants absorb their nitrogen
directly as NH 4
+ .
The nitrogen balance of rainfed peat soils
must be dominated by contributions from BGA
fi xation as they are too acid and calcium defi cient
to support most free-living bacterial fi xers except,
perhaps, for the tropical genus Beijerinckia . BGA
nitrogen fi xation is also predominant in some
forms of wetland rice culture.
15.1.4.2 Flowing Water and Tidal
Replenishment
Soluble nutrients are both exported and imported
by a constant fl ow of water in many wet soils. Also,
soil is oxygenated and soluble toxic products of the
reducing environment are removed. Freshwater
and tidal environments are ‘pulse stabilised’. The
seasonal or diurnal oscillations of water level are
regular physical and chemical disturbances. It provides the energy for rapid nutrient cycling. By disturbance, it maintains the ecosystem in an early
successional state. Much of its primary production
is defl ected to the support of a very diverse
consumer population.
Notwithstanding the above, it is still not well
known what conditions make wetlands net
importers or net exporters of nutrients in spite of
the considerable researches have been done in
this fi eld (Hasler 1975 ; Burton and Liss 1976 ;
Good et al. 1978 ).
For some elements, the import and export vary
seasonally in lotic systems. Thus, analyses of
marsh discharge show that plant uptake reduces
the concentration of elements, e.g. P, Ca and Mg,
during spring and early summer.
15.1.5 Species Composition
and Species Richness
Hydrology is a two-edged sword for species
composition and diversity in wetlands. It acts as a
limit or a stimulus to species richness, depending
on the hydro-period and physical energies.
Hydrology is said to stimulate diversity when the
action of water and transported sediments creates
spatial heterogeneity, thus opening up additional
ecological niches.
15.1.6 Methodology
of Studying AM
A Δ 1 m
2 wooden quadrant is generally used to
determine the macrophytic density. The submerged and emergent macrophytes are collected with the help of a barbed iron hawser.
The fl oating ones are usually grabbed with
hand. After collection, the AM are segregated,
the total number and size range are noted species-wise and the wet biomass are determined
on the spot by weighing them together wet with
a double pan balance. The sampling operations
are carried out in both littoral and limnetic
zones of the wetland at different randomly
selected spots. The data are analysed to evaluate the seasonal succession and phytosociology
of the AM communities. Herbaria for different
plants are prepared and transferred to the laboratory for further works (Kar 1990 ).
15.1 Wetland Flora (Aquatic Macrophytes) (AM)