216
of the establishment and maintenance of specific
types of wetlands and wetland processes.
The starting point for the ‘hydrology’ of a
wetland is the climate and basin morphology.
The second significant factor is the geomorphology of the landscape and basin. Steep terrain
tends to have fewer wetlands than flat or gently
sloping landscapes. Isolated basins have different
potential for wetlands than do tidal-fed or riverfed environments. When climate, basin geomorphology and hydrology are considered as one
unit, it is referred to as wetland’s ‘hydrogeomorphology’. The hydrology of a wetland directly
modifies and changes its ‘physico-chemical environment’, particularly oxygen availability and
related chemistry, such as nutrient availability,
pH and toxicity. Hydrology also transports sediments, nutrients, etc., into wetlands. It, thus, further influences the physico-chemical environment.
It may be noted here that modifications of the
physico-chemical environment, in turn, have a
direct impact on the ‘biota’ in the wetland. The
biota may respond with massive changes in species composition and richness and in ecosystem
productivity when hydrologic conditions in wetlands change even slightly. Biota, e.g. the emergent aquatic plants, adapt themselves to the
anoxia in the sediments. Further, the level of
nutrients in the sediments determines productivity and which species will dominate that productivity. Animals adapted to shallow water and the
vegetation cover therein will, in turn, flourish.
Microbes, which are able to metabolise in anoxic
conditions, dominate the reduced sediments.
Conversely, the aerobic microbes survive in a
thin layer of oxidised sediments and in the water
column, if oxygen is present there. When hydrologic patterns remain similar from year to year,
a wetland’s biotic structural and functional integrity
may persist for several years.
14.2 Biotic Control of Wetland
Hydrology
Wetland biota are not passive to their hydrologic
conditions. They exert feedback (cybernetic)
control over their physical environments just as
in many other ecosystems. Biotic components of
wetlands may, thus, exert control over the hydrology and chemistry of their environment through a
variety of mechanisms. Microbes, in particular,
catalyse virtually all chemical changes in wetland soils. Thus, they control nutrient availability
to plants and even to production of phytotoxins,
such as sulphides. Plants, animals and microbes
which use these essential biological feedback
mechanisms have been formally recognised as
‘ecosystem engineers’ in the ecological literature
(Jones et al. 1994; Alper 1998). Plants cause
changes in their physical environment through
processes, such as peat building, sediment trapping, nutrient retention, water shading and transpiration. Wetland vegetation influences the
hydrologic conditions and the physico- chemical
environment by binding sediments to reduce erosion, by trapping sediments, by interrupting
water flows and by building peats. Accumulated
sediments and organic matter, in turn, interrupt
water flows and may eventually decrease the
duration and frequency by which the wetlands
are flooded. Bogs build peats to the point at which
they are no longer influenced at the surface by the
inflow of mineral waters. Some of the trees in
some southern swamps save water by their deciduous nature, their seasonal shading, and their
relatively slow rates of transpiration. In more
temperate climates, trees which invade shallow
marshes and vernal pools may decrease water
levels during the growing season by increasing
transpiration. Thus, they may allow the more
woody plants to take over. Removal of these trees
in what appears to be dry forest may sometimes
cause standing water and marsh vegetation to
re-appear (Golet et al. 1993).
Concomitant to above, a number of animals
are particularly noted for their contributions to
hydrologic modifications and subsequent changes
in wetlands. The role of the beavers in much of
North America and Asia (particularly India) in
both creating and destroying wetlands are quite
well known. They may build dams on streams,
backing up water across great expanses and creating wetlands where none had existed before.
They are sometimes considered as significant
causal force in the creation of the Great Dismal
14 Wetland Hydrology
of the establishment and maintenance of specific
types of wetlands and wetland processes.
The starting point for the ‘hydrology’ of a
wetland is the climate and basin morphology.
The second significant factor is the geomorphology of the landscape and basin. Steep terrain
tends to have fewer wetlands than flat or gently
sloping landscapes. Isolated basins have different
potential for wetlands than do tidal-fed or riverfed environments. When climate, basin geomorphology and hydrology are considered as one
unit, it is referred to as wetland’s ‘hydrogeomorphology’. The hydrology of a wetland directly
modifies and changes its ‘physico-chemical environment’, particularly oxygen availability and
related chemistry, such as nutrient availability,
pH and toxicity. Hydrology also transports sediments, nutrients, etc., into wetlands. It, thus, further influences the physico-chemical environment.
It may be noted here that modifications of the
physico-chemical environment, in turn, have a
direct impact on the ‘biota’ in the wetland. The
biota may respond with massive changes in species composition and richness and in ecosystem
productivity when hydrologic conditions in wetlands change even slightly. Biota, e.g. the emergent aquatic plants, adapt themselves to the
anoxia in the sediments. Further, the level of
nutrients in the sediments determines productivity and which species will dominate that productivity. Animals adapted to shallow water and the
vegetation cover therein will, in turn, flourish.
Microbes, which are able to metabolise in anoxic
conditions, dominate the reduced sediments.
Conversely, the aerobic microbes survive in a
thin layer of oxidised sediments and in the water
column, if oxygen is present there. When hydrologic patterns remain similar from year to year,
a wetland’s biotic structural and functional integrity
may persist for several years.
14.2 Biotic Control of Wetland
Hydrology
Wetland biota are not passive to their hydrologic
conditions. They exert feedback (cybernetic)
control over their physical environments just as
in many other ecosystems. Biotic components of
wetlands may, thus, exert control over the hydrology and chemistry of their environment through a
variety of mechanisms. Microbes, in particular,
catalyse virtually all chemical changes in wetland soils. Thus, they control nutrient availability
to plants and even to production of phytotoxins,
such as sulphides. Plants, animals and microbes
which use these essential biological feedback
mechanisms have been formally recognised as
‘ecosystem engineers’ in the ecological literature
(Jones et al. 1994; Alper 1998). Plants cause
changes in their physical environment through
processes, such as peat building, sediment trapping, nutrient retention, water shading and transpiration. Wetland vegetation influences the
hydrologic conditions and the physico- chemical
environment by binding sediments to reduce erosion, by trapping sediments, by interrupting
water flows and by building peats. Accumulated
sediments and organic matter, in turn, interrupt
water flows and may eventually decrease the
duration and frequency by which the wetlands
are flooded. Bogs build peats to the point at which
they are no longer influenced at the surface by the
inflow of mineral waters. Some of the trees in
some southern swamps save water by their deciduous nature, their seasonal shading, and their
relatively slow rates of transpiration. In more
temperate climates, trees which invade shallow
marshes and vernal pools may decrease water
levels during the growing season by increasing
transpiration. Thus, they may allow the more
woody plants to take over. Removal of these trees
in what appears to be dry forest may sometimes
cause standing water and marsh vegetation to
re-appear (Golet et al. 1993).
Concomitant to above, a number of animals
are particularly noted for their contributions to
hydrologic modifications and subsequent changes
in wetlands. The role of the beavers in much of
North America and Asia (particularly India) in
both creating and destroying wetlands are quite
well known. They may build dams on streams,
backing up water across great expanses and creating wetlands where none had existed before.
They are sometimes considered as significant
causal force in the creation of the Great Dismal
14 Wetland Hydrology