Inland Wetland Ecosystems

Mitsch and Gosselink Ch. 12, 13, 14, 15

 

Freshwater Marshes

 

Defining Characteristics

 

Freshwater marshes as described by Mitsch and Gosselink are diverse.  They are non-tidal, freshwater systems dominated by grasses, sedges and other emergent hydrophytes.

 

Some examples:  prairie potholes, near-coast marshes that are non-tidal, the Everglades, California vernal pools, Great Lakes marshes, riverine marshes, playas.

 

Geographical Areas

 

Freshwater marshes exist in the northeast and southeast because of extensive precipitation.  They occur in the Midwest and west from the prairie potholes of the north to the playas of the southwest.  Mountains have freshwater marshes associated with them because of elevated precipitation, depressed evapotranspiration, and presence of lots of localized depressions.  Freshwater marshes may be associated with rivers or lakes anywhere.  Areas of poor drainage in the west and northwest may become marshes.

 

Vegetation

 

Vegetation in freshwater marshes is characterized by graminoids such as the tall reeds Typha and Phragmites, the grasses Panicum and Glyceria,  the sedges Cladium and Carex, broad-leafed monocots such as Sagittaria, and floating aquatics such as Nymphaea and Nelumbo.

 

Hydrology

 

The hydroperiod of a freshwater marsh determines its ecological character.  The primary elements of the hydroperiod are 1) its annual response (fluctuating or stable; dries out or stays wet), and 2) its event response (how high the water get after a rainfall event, and how long it stays high).

 

Freshwater marshes can have excess water because rainfall exceeds potential evapotranspiration and the egress of surface water is slowed or stopped by impermeable soils, constricted outlets, retarded overland flow.  Where potential ET exceeds precipitation, wetlands may form where water is collected in streams, depressions, behind sills, at the base of cliffs.  The source of water may be surface water or ground water.

 

Biogeochemistry

 

The water and soil chemistry of freshwater marshes is dominated by mineral soils and inputs of organic vegetation.  Conductivity as a measure of dissolved salts in water varies from low conductivity  in very soft-water systems dominated by rainfall inputs, to moderately high conductivity in areas with extensive runoff from unvegetated land or from areas with a high productivity and leaching of nutrients and organics into water flowing into the wetlands..  Bacterial activity is usually high and organic material turnover is rapid.  pH is usually in the range of 6-9.

 

Nutrients

 

Nutrients are reflections of the kinds of sediments in the marsh.  Mineral sediments are associated with high phosphorus.  Total nitrogen is correlated with the organic content.

 

Vegetation

 

Freshwater marsh vegetation is the result of species occurring in rough zones on slight gradients (usually flooding or elevation gradients).

 

Fig 12.7 shows a model of species response to flooding, drying-down, and permanent flooded or saturated soils.  Wetlands that dry down may have a very large component of annual plants, which respond strongly to the time of the year that a particular zone becomes dry.  Some systems may flood-up during the growing season.  This may result in mature plants becoming inundated; in Chester Morse Lake, Carex species flower underwater after having started their vegetative growth in the absence of inundation.

 

Freshwater wetlands have been especially plagued by invasive plants: Lythrum salicaria, Phragmites communis, Phalaris arundinacea, Eichhornia crassipes, Salvinia molesta, Alternanthera philoxeroides.  Hydrilla and Myriophyllum have invaded standing-water wetlands.

 

Primary Productivity

 

Aboveground productivity is high, ranging upward from 1000 g m-2 yr-1.  Decomposition rates are fairly high; the detrital food chain dominates.  Export is highly dependent upon the hydrology of a particular wetland, with riverside and lakeside wetlands exporting most.  There is a highly seasonal pattern of biomass and nutrient storage, with rapid uptake and deficiencies of nitrogen and phosphorus during growing season.  Organic material and leached nutrients are then metered back into outflow waters during the winter and early spring.

 

Consumers

 

Because they are isolated by surrounding farmland, pasture or forest, freshwater wetlands have a rich diversity of consumer organisms and wildlife.  They are resource islands in an otherwise more homogeneous landscape.  Animals using this resource include nematodes, enchytraeids, invertebrates, crustaceans, numerous mammals and abundant birds.

 

Peatlands

 

Defining Characteristics

 

Bogs and fens are peatlands that occur primarily in the cool boreal zones of the world where there is excess moisture.  Precipitation may not be extremely high, but evapotranspiration is quite low (often water is tied up in ice and under snow for long periods).  In addition, some northern areas are quite flat because of their glacial origin, and water has a difficult time making its way out of these regions because of minimal topographic gradients.

 

Bogs are primarily rainfall-fed.  Fens have the characteristics of bogs but have input of waters that have had some contact with mineral soil (though it may have been very rocky or poor soil).  Nutrients are low.  pH is low, often because of  the excretion of H+ ions by Sphagnum moss.  Acidophilic vegetation is supported.

 

Once formed, bogs tend to be resistant to conditions that might alter the water balance and peat accumulation.  The perched water table, the water storage capacity of the peat, and the low pH create a stable microhabitat under fairly wide environmental fluctuations.

 

Geographical Areas

 

On a worldwide basis, extensive areas of bogs and fens occur in Scandinavia, eastern Europe, western Siberia, Alaska and Canada.  In North America, a very large percentage of the landscape is found in peatlands in the Hudson Bay lowlands.  In the U.S. , there are large areas of peat forming wetlands in Minnesota, Wisconsin and Michigan.  Peat deposition is common all along the boreal forest that runs from Labrador to Alaska.

 

Hydrology

 

For peatland development to occur, two processes must be in place.  There must be a positive water balance, and there must be peat accumulation.  Peatlands require a humid environment year-around.  South of Wisconsin, Minnesota and Michigan, summers are warm and dry enough to eliminate bog vegetation.

 

In a cool, moist maritime climate, peatlands will develop over most any surface, even hillslopes.  Hanging bogs of the west coastal areas of Scotland and Ireland are an example.

 

Bog Formation

 

Bogs may form through terrestrialization or paludification or intermediate processes that have characteristics of both.  In terrestrialization, floating bog vegetation will fringe, cover, then close over a lake.  Quaking bogs quake because they are afloat on water below.  Paludification occurs when a bog  encroaches on formerly dry land.  This could be the result of changes in soil permeability, beaver activity, climate change, forest practices, etc.

 

Biogeochemistry

 

Bogs characteristically have low nutrients, high acidity, high dissolved organics, low dissolved oxygen, and produce a dark brownish or blackish water that is characterized by low productivity and dissolved or particulate OM.

 

Nutrients

 

pH decreases as the OM increases with the shift from a minerotrophic to ombrotrophic system.  As a fen develops into a bog, the supply of metallic cations (Ca2+, Mg2+, Na+, K+) drops sharply as OM increases.  CEC (cation exchange capacity) increases, but hydrogen ions come to predominate the water chemistry.

 

Bogs are exceedingly deficient in available plant nutrients.  As a result, primary productivity is low, and plants that live in bogs have many special adaptations to low-nutrient conditions.

 

Vegetation

 

Bogs can be sphagnum moss, sphagnum-sedge, sphagnum-shrub, bog forests, or other combinations of acidophilic plants.  Sphagnum mosses are the primary peat-building plants in most bogs.  They may grow in association with cotton grass (Eriophorum), sedges (Carex), shrubs (Calluna, Chamaedaphne, Vaccinium, Ledum).  Trees such as pine, spruce and tamarack, extremely stunted, will grow in bogs.

 

Primary Productivity

 

Primary production is generally low, but decomposition is even more depressed, so peat accumulates.  Decomposition is depressed by the acidity of the water, by flooding (anaerobiosis), and by lowered biological rates caused by cold soils.  Annual productivity is often below 500 g m-2 yr-1

 

Vegetation Adaptations

 

Plants often have the usual wetland plant adaptations for life in waterlogged soils.  Sphagnum, on the other hand, is morphologically adapted to hold huge quantities of water and maintain waterlogging.

 

Sphagnum acidifies the water surrounding it.

 

Bog plants adapt to low nutrients by growing slowly, adopting an evergreen habit, minimizing grazing through adopting sclerophylly, and deep root systems.  Insectivorous plants are found in bogs, but this is not a major plant strategy.

 

Sphagnum tends to grow up and over other plants, forcing them to use adventitious roots or elongate their rhizomes.

 

Consumers

 

The populations of bog animals is low, and consumption is low.  There are a number of specialized bog dwellers among invertebrates.  Acidity tends to restrict the use of bogs as a habitat.

 

Freshwater Swamps

 

 

Defining Characteristics

 

Freshwater swamps are mineral soil-dominated, forested wetlands.  Primarily deepwater swamps dominated by bald cypress (Taxodium distichum) and tupelo (Nyssa aquatica) or pond cypress (Taxodium distichum var nutans) and black gum (Nyssa sylvatica) along the Atlantic coastal plain and the Mississippi embayment.  They can occur under nutrient-poor conditions (cypress domes) or nutrient-rich conditions (lake edge or river gallery forests).

 

Geographical Areas

 

Baldcypress swamps occur along the gulf coast from Texas to New Jersey.  Pond cypress is found along the eastern seaboard but barely makes it as far west as Louisiana.  White cedar (Chamaecyparis thyoides) swamps have a more spotty distribution along the Atlantic and Gulf coasts, and red maple (Acer rubrum) swamps are found in New England and the Mid-Atlantic states.

 

Geomorphology and Hydrology

 

Cypress domes are poorly drained to permanently wet depressions dominated by pond cypress.  Trees are larger in the middle and smaller on the edges.  This could be because of deeper peat in the middle, or repeated fire stunting on the edges.  Standing water in a dome is often dominated by rainfall.  Groundwater exchange is restricted.  They may dry down in the fall and spring.

 

Alluvial river swamps are made up of cypress and other trees across broad floodplains (especially in oxbows or sloughs) of southeastern streams.  They are nutrient-rich and are noted for seasonal pulses that bring water and nutrient-rich sediment.  They are almost continually flooded.

 

Biogeochemistry

 

1.  Swamps are acidic to neutral, depending on the accumulation of peat and the degree to which precipitation dominates the system.

 

2.  Rainfall-fed swamps can be nutrient-poor.  River-associated or groundwater-fed swamps can be nutrient-rich.

 

3.  A river swamp may have water quality quite different from that of the adjacent river.  They are often fed by both river water and groundwater.

 

Nutrients

 

Buffering capacity in cypress domes is low, and they have much in common with oligotrophic conditions in bogs.  Swamps open to river flow, on the other hand, are often rich in alkalinity (carbonates, bicarbonates, hydroxides), dissolved ions, and nutrients.

 

Vegetation

 

Trees have a hard time surviving under continuously flooded conditions.  Few do it; they have evolved ways to adapt.  Knees and pneumatophores are protrusions that project up from the root system to well above the average water level.  They both add to the rooted stability of swamp trees and provide for aeration channels.  Buttresses (stem hypertrophy) occur in Taxodium and Nyssa.  The buttressing usually extends from one to several meters above the soil.  Seeds of swamp trees are capable of dispersal by water.  Taxodium is long-lived with extremely resistant wood.

 

Primary Productivity

 

Net primary productivity is moderate (<600 g m-2 yr-1) in nutrient-poor, slow-moving waters, but high in alluvial systems (>1500 g m-2 yr-1).  Biological utilization is primarily through the detrital food chain, though decomposition is impeded by anaerobic conditions that prevail in sediments.  Export depends upon the hydrology.  Alluvial river swamps have high export, while cypress domes have low export.

 

Consumers

 

Swamp consumer communities are quite diverse and they are dependent upon the abundant detritus.  Crayfish, clams, oligochaete worms, snails, freshwater shrimp, midges, amphipods and various immature insects are common.

 

Fish are both temporary and permanent residents of alluvial river swamps.  Low DO in slower moving, more remote swamps limits fish use. 

 

Reptiles and amphibians are common users of swamps because of their ability to adapt to fluctuating water levels.

 

 

Riparian Ecosystems

 

Defining Characteristics

 

Riparian wetlands are ecosystems in which soils and soil moisture are influenced by adjacent streams.  They are unique because they are linear, and because the process large fluxes of energy and materials from upstream systems.  Major expanses of such systems are found in the eastern United States, though much of this land has been cleared for agriculture.  Ecologically critical riparian systems are found in the west and arid southwest, though water management, logging and livestock grazing have greatly modified them.

 

Geographical Areas

 

The floodplains of the Mississippi and of many southeastern and southern rivers and streams, have been and continue to be the site of vast bottomland hardwood forests.  Such forests are highly productive and diverse.

 

Western riparian forests are narrow, linear features that contrast with the broad, rolling, grass and shrub-dominated landscapes they dissect.  Dams and groundwater extraction have severely limited their water sources.

 

Geomorphology and Hydrology

 

Three gradients may be used to describe important environmental driving forces impacting riparian systems.

 

1.  A continental gradient, from east coast to west coast. (Mesic in east, arid in west.)

2.  An intrariparian gradient along the length of a stream. ( From erosional to transport to depositional.)

3.  A transriparian gradient across the riparian zone and stream valley. (Adjacent highlands to hillslope to stream terrace to floodplain to streambed.)

 

Biogeochemistry

 

Soils in erosional and transport reaches are coarse and poorly developed.  May be quite quick-draining.  Soil conditions may change quickly along a stream-course.  Soils may become anoxic quickly with flooding, and aerobic quickly when flooding abates.

 

Nutrients

 

Nutrients may be reasonably available in the sediments of stream in the southeastern U.S., but may be dependant upon pulses in western streams.

 

Vegetation

 

Southeastern riparian forests are diverse (Salix, Acer, Populus, Quercus, Carya, Fraxinus, Betula, etc.) 

 

Western tree species are phreatophytes.  Cottonwood and willow are common, but invasive phreatophytes have cause ecosystem damage (Tamarix, Eleagnus, Prosopis). 

 

Primary Productivity

 

Production can be high (>1000 g m-2 yr-1), though excessive flooding or inadequate annual re-supply of water can both decrease the productivity.

Pulsed systems often have the highest productivity.  Export is tied to the production, but also to the hydrology.  Greater outflows or more intense pulses of water will carry away more organic material and nutrients.  These are open systems.  They also are effective transverse filters of incoming nutrients and sediment.

 

Consumers

 

Four attributes of riparian systems that are important to animals:

 

1.  Predominance of woody plant communities ( relative islands, protection, roosting areas, favorable microclimate, snags, shade, soil stabilization, leaf litter).

 

2.  Presence of surface water and abundant soil moisture.

 

3.  Diversity and Interspersion of habitat features.

 

4.  Corridors for dispersal and migration.