Water Quality and Wetland Function in the Northern Prairie Pothole Region
WQ-1313, August 2006
Bruce Seelig, Extension Water Quality Program Coordinator,
Department of Soil Science
Shawn DeKeyser, Lecturer/Research Specialist, Department of
Animal and Range Sciences
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Introduction
Wetland Classification and Water Quality
- Water Movement and Water Quality
- The Surface Water-Groundwater Connection
Hillslope Interflow and Wetlands
-
- Wetland Processes and Water Quality
- Sediment
Carbon and Organic Matter
- Nutrients: Nitrogen
Nutrients: Phosphorus
- Salts
Selenium
- Mercury and Other Trace Metals
Man-made Organic Chemicals
-
- Modifications to Wetland Systems
- Drainage
Sediment Trapping
- Reduced Soil Quality in Wetland Catchment Soils
-
- Assessment of Wetland Function and Biotic
Integrity
- Hydrogeomorphic Model (HGM)
Index of Plant Community Integrity (IPCI)
-
- Soil Management and Wetland Restoration
- Tillage
Crop Rotations
- Conservation Practices and Structures
Chemical Inputs
- Water Inputs
Grazing Systems
-
- Wetland Basin Management
- Mowing
Cultivation
- Restored Wetlands
Idled Wetlands
References by Section
Introduction
Wetlands are unique areas that have characteristics of both land and water,
as the name implies. Wetlands are considered transitional areas between aquatic
systems and upland terrestrial systems. They can be inundated with water for
long periods of time, or can lack free water on the surface for lengthy periods.
Ponding and drying often is a regular, annual cycle of wetness.
The transitional nature of wetlands is characterized as a zone of dynamic water
movement. Water levels may fluctuate from a few feet above to several feet below
the surface in a single season. The fluctuation in soil moisture content and
aeration is highly variable, compared with aquatic systems that are saturated
continuously or upland terrestrial systems where soils never are saturated.
Cycles of wetness may occur in relatively predictable seasonal patterns or longer-term
climatic patterns. However, within these patterns, the variability of weather
also contributes to extreme conditions of wetness or dryness within a given
wetland basin.
The term wetland is somewhat misleading because it suggests a narrow range
of properties and functions within wet areas. In reality, wetland areas are
transitional zones that exhibit extremely different properties and functions
that vary with both space and time. Determining the relationship between wetland
function and water quality within the context of an ecological system or landscape
requires understanding of the range of chemical, physical and biological processes
that likely are to occur. Soil properties of the wet-extreme are very different
from the dry-extreme. Within a landscape context, wetland soil properties and
processes vary along a continuum, often in a predictable pattern. The processes
that characterize the wetland extremes generally are coupled and need to be
viewed in tandem to fully appreciate wetland function, particularly as it relates
to water quality. For example, the wetter areas of a wetland allow conversion
of nitrate to gaseous forms of nitrogen that are released into the atmosphere.
However, drier areas of the same wetland will produce nitrate through mineralization
of organic matter. The ultimate effect on nitrate levels in groundwater or surface
water will depend on the balance of these two processes.
Wetlands have several major functions: 1) biologic diversity/integrity; 2)
water storage; 3) water exchange between surface water and groundwater; 4) surface
water filtration; 5) vapor and gas exchange with the atmosphere; 6) chemical
attenuation and transformation. The capacity of wetland systems to perform these
functions has a direct bearing on water quality.
Natural wetlands are quite variable, thus the capacity of a given wetland to
perform a given function also is variable. To help us understand the range in
wetland properties and function, wetland classification schemes have been developed.
Addressing the issue from the perspective of the more common classification
systems is helpful to understand the relationships between wetland function
and water quality.
Wetland Classification and Water Quality
Cowardin et al. (1979) developed a classification for wetlands and aquatic
systems in the United States that still is in use today. The Cowardin classification
system is composed of five major categories that are subdivided into many subclasses.
Wetlands fall into the major category of “Palustrine.” In North
Dakota, the palustrine systems often occur adjacent to the major categories,
“Riverine” and “Lacustrine,” which represent the aquatic
habitats of streams and lakes, respectively.
The stream channel separates the stream and wetland systems. A somewhat arbitrary
depth of 6 feet has been used to separate aquatic systems of lakes from permanent
wetlands with shallower water. Another arbitrary measure often used to separate
lakes from wetlands is size. Areas of permanent water less than 20 acres generally
are considered wetlands unless active wave cut features are present along the
shore.
The wetlands of the palustrine system form a transitional zone between the
uplands and aquatic systems that serves as a filter or buffer with respect to
water quality. Water chemistry is one of the properties used to modify the wetland
type. Salinity and pH are the two water chemistry parameters used to categorize
wetlands (Tables 1 and 2).
Table 1. Cowardin system wetland water salinity classes.
--------------------------------------------------------
Modifier EC (dS/m)
--------------------------------------------------------
Hypersaline > 60
Saline 45-60
Near-saline 30-45
Moderately saline 8-30
Slightly saline 0.8-8
Fresh < 0.8
--------------------------------------------------------
Table 2. Cowardin system wetland water pH classes.
--------------------------------------------------------
Modifier pH
--------------------------------------------------------
Acid < 5.5
Circumneutral 5.5-7.4
Alkaline > 7.4
--------------------------------------------------------
Water salinity that exhibits extreme variability may be described by adding
the prefix “poikilo” to the salinity class name, or vice versa,
water salinity exhibiting little variability may be described by adding the
prefix “homio” to the salinity class name.
Stewart and Kantrud (1972) developed a classification system specifically for
ponds and lakes in glaciated prairies. This system was the basis, in part, for
Cowardin’s classification and remains in use today. It probably is referred
to most often when wetlands are categorized in the northern prairie pothole
region (NPPR). The parameters used for classification were permanence of standing
water and salinity as correlated to plant communities. Seven different vegetation
zones were recognized, as shown in Table 3. Areas greater than 50 acres were
considered lakes. No attempt was made to differentiate aquatic from wetland
systems using water depth. However, “open water” was one of the
seven vegetation zones and is considered to have relatively deep water that
would be a characteristic of the aquatic system.
Table 3. Stewart and Kantrud wetland classes.
---------------------------------------------------------
Central
vegetation
Wetland class zone Common plant*
---------------------------------------------------------
I-ephemeral Low prairie Kentucky bluegrass
II-temporary Wet meadow Fowl bluegrass
III-seasonal Shallow marsh Slough sedge
IV-semipermanent Deep marsh Cattail
V-permanent Open water Western widgeongrass
VI-alkali Intermittent Western widgeongrass/
alkali salt flat
VII-fen Alkaline bog Aquatic sedge
---------------------------------------------------------
*General plant indicators that may be present, depending
on salinity or disturbance
The Stewart and Kantrud classification recognized that changes in water level
and cultivation affect plant communities; therefore, drawdown and tillage phases
were defined for certain wetland classes. The plant community also reflects
water salinity and is recognized as a subclass (Table 4) similar to the Cowardin
system.
Table 4. Stewart and Kantrud wetland salinity subclasses
--------------------------------------------------------
Subclass designation EC (dS/m)
--------------------------------------------------------
Saline >45
E-Subsaline 15-40
D-Brackish 5-15
C-Moderately brackish 2-5
B-Slightly brackish 0.5-2
A-Fresh < 0.5
--------------------------------------------------------
Both of the previously described wetland classifications emphasize indicators
of wetness, as one would expect. Wetness is an aspect of hydrology, but lacks
any indication of the direction or type of water flow. Understanding of water
flow can be helpful in determining important wetland processes and estimating
the capacity to perform those processes or functions. Arndt and Richardson (1988)
combined knowledge of groundwater flow in the northern prairie pothole region
with the Stewart and Kantrud classification. They found that soil chemical indicators,
such as salinity, could be used to classify wetlands into three major groundwater
flow categories (Figure. 1). Groundwater flow can be predominantly away from
(recharge) or into (discharge) the wetland basin. When groundwater flows away
from the wetland basin and is balanced by flow into the basin, a flow-through
wetland exists.
Figure 1. Recharge-Flow-through-Discharge
(7KB)
The relationship among wetland hydrology, soils and the Stewart and Kantrud
system is illustrated with a summary of the data gathered by Arndt and Richardson
(1988) (Table 5).
Table 5. Soil properties in wetlands with different groundwater hydrology
--------------------------------------------------------------------------------
Pond Soil salinity
Wetland class salinity Soil class dS/m Calcite % Gypsum %
--------------------------------------------------------------------------------
Recharge hydrology
Seasonal Fresh Argiaquolls and 0.4 –0.6 0 0
Haplaquolls
--------------------------------------------------------------------------------
Flow-through hydrology
Seasonal – Slightly – Calciaquolls and 2 – 8 3 - 16 .02 - 15
Semipermanent moderately Haplaquolls
brackish
--------------------------------------------------------------------------------
Discharge hydrology
Semipermanent Subsaline Fluvaquents and 11 – 26 7 – 11 2 - 6
Haplaquolls
--------------------------------------------------------------------------------
Note: Soil samples were collected from the upper 21 inches in the wet meadow
and shallow marsh vegetation zones.
Salinity is an obvious water quality characteristic that influences the wetland
environment. It is relatively predictable and has been incorporated in the major
wetland classification schemes. Wetland salinity by itself probably is not a
critical water quality issue. However, because of the connection among wetlands,
streams, lakes and aquifers, it may be a first indicator of possible changes
to these water resources.
The need to predict the fate of various chemicals as they pass through the
wetland environment is what elevates the importance classification systems.
Chemical transport processes are dependant on the wetland conditions that are
defined and categorized in a classification system. Classification is the basis
for wetland inventory and monitoring activities that help predict chemical transformations
and transport on a landscape scale. Effective management of our natural resources
depends on accurate assessment of wetland processes and the functions they serve.
In other words, different classes of wetlands have different functions with
respect to water quality, and these differences need to be recognized in our
management strategies.
Water Movement and Water Quality
To understand the relationship between wetlands and water quality of streams,
lakes and aquifers, the flow of water to and from the wetlands must be viewed
from the perspective of the whole landscape. The permeability of the surface
soil affects water runoff from areas adjacent to wetlands. Under prairie vegetation,
soils have relatively higher amounts of organic matter, greater aggregate stability
and a higher percentage of macropores, compared with cultivated fields. Because
of this, prairie soils have higher infiltration rate capacities and less potential
to generate surface runoff to wetlands. Under these conditions, the predominant
source of water for wetland ponds is precipitation that falls directly on the
pond.
The major impacts to wetlands adjacent to cultivated uplands are increased
pond levels immediately after rainfall events, increased sedimentation and increased
nutrients. From landscape analyses, we know wetlands occur in positions that
function as natural filters or sediment basins. Considering the natural role
wetlands play as an environmental filter, what are the water quality implications
related to wetlands in cultivated areas? The answer is complicated by the fact
that different wetland classes provide different functions on the landscape.
Also, impacts of surface runoff from cultivated areas are due to increased rates
of natural processes, not new phenomenon. If the rate of input of sediment,
organic matter, nutrients and other natural or manmade contaminants exceeds
the functional capacity of a given wetland to absorb and transform, then one
would expect contaminants to be passed along to adjacent streams or lakes. The
question is not if these things are getting into wetlands, because they are
and always will. The real question is, when does the rate of input exceed the
functional capacity?
An important point regarding wetlands and water quality is the proximity to
aquatic systems (streams and lakes). It is a critical factor that must be addressed
when prioritizing management activities. Wetlands adjacent to streams and lakes
will provide the most immediate water quality benefits. If adjacent wetlands
lose their functional capacity to absorb and transform contaminants, impacts
to streams and lakes would be expected shortly thereafter. Evidence shows that
isolated wetlands also are connected to other water resources, but in a more
subtle and indirect fashion. The long-term loss of isolated wetlands actually
may have greater consequences on other water resources than the loss of adjacent
wetlands.
The Surface Water-Groundwater Connection
The discussion of water flow, wetlands and water quality should not proceed
too far before interjecting the connection between surface water and groundwater.
The concept of recharge-flow-through-discharge already has been introduced.
Soil infiltration controls surface water flow in all three of these groundwater
flow scenarios discussed above. However, the interconnection between surface
water flow and groundwater flow is relatively different for each of the three
wetland-hydrology scenarios. In turn, the implications for water quality also
are quite different.
The ratio between the wetland catchment that contributes runoff and the size
of the wetland basin decreases from an average of about 8 for recharge wetlands
to about 2 for discharge wetlands. The relatively larger catchments for recharge
wetlands emphasizes the dominance of surface water flow to their basins, which
are relatively smaller, compared with flow-through and discharge wetlands. The
leached nature of the soils in recharge basins as shown earlier (Table 5) indicates
that pooled surface water in recharge basins eventually percolates through the
wetland soil to the groundwater. This is the dominant groundwater recharge process
in the prairie pothole region and has been coined as “depression focused
recharge.”
Groundwater flow is mostly in the vertical direction immediately beneath recharge
wetlands, but eventually begins to turn laterally with distance and feeds into
flow-through wetlands and finally discharge wetlands (Figure 1). The horizontal
permeability of the till generally is no greater than in the vertical direction
because of the relatively fine texture, high bulk density and lack of horizontal
stratification. This means that directly beneath recharge wetlands, groundwater
removes salts to depths of nearly 30 feet. However, little salt removal occurs
beneath upland areas only short distances from where focused recharge occurs.
Groundwater flow then slowly redistributes leached salts from the beneath the
recharge wetland to flow-through and discharge wetlands. Salts become more concentrated
in pond water and soils of wetlands that receive proportionately greater contributions
of groundwater and have no outflow either to groundwater or surface water (Table
5). Discharge wetlands essentially are evaporation basins that have such high
loads of salts that some have been mined commercially. The rate of salt transport
to wetlands is substantially greater when groundwater flow encounters geologic
materials with high horizontal permeability, such as coarse-textured sands and
gravels. The highest levels of salinity generally are associated with discharge
wetlands that occur in glacial outwash.
Two salient points must be made regarding the relationship between water quality
and groundwater hydrologic regime in the NPPR. First, groundwater recharge is
focused in depressions, some of which do not meet jurisdictional criteria for
wetland designation. Large numbers of these areas (class I – III wetlands)
are farmed actively. Because these areas recharge groundwater, they have the
potential to transmit contaminants, and recent studies show they do indeed influence
nitrate concentrations in shallow aquifers. Farming practices that influence
the balance between water infiltration and runoff will affect groundwater recharge.
Second, groundwater recharge is closely coupled with shallow groundwater discharge
to lower-lying wetlands (class IV-VII). In this case, the main water quality
issue related to groundwater is the concentration of salts in the pond water.
Changes to the hydrologic system that increase groundwater discharge or impede
either surface and/or groundwater flow-through will result in increased salinization
in these wetlands. Consequently, adjacent aquatic systems of streams and lakes
also would be expected to exhibit higher salt loads. Specifically, sodium concentrations
would be of concern.
Hillslope Interflow and Wetlands
In the NPPR, the importance of transitory lateral flow or interflow from adjacent
hill slopes to adjacent wetlands is open to debate. Interflow is a well-established
phenomenon that regularly occurs through hillslope soils in more humid environments,
being controlled by a combination of antecedent moisture, infiltration capacity
and horizontal hydraulic conductivity. Physical differences in soil horizons
have been observed to play an important role in this type of groundwater flow.
Conditions for interflow are not as likely to occur in drier environments largely
due to lower potential for high levels of antecedent moisture. However, evidence
for interflow through hillslopes under prairie vegetation in the NPPR does exist
and its relative importance to surficial hydrology probably has been underestimated.
Saline seepage is a well-documented hydrologic process that occurs in the northern
Great Plains (Figure 2) and is similar to hillslope interflow. Excess water
that escapes vertically through the soil profile on summer-fallowed land initiates
the seepage process. This water is diverted laterally by impermeable geologic
strata and picks up soluble salts. It moves back toward the soil surface on
hillslopes that truncate the impermeable geologic strata, where it evaporates
and deposits its load of salts. The hydrologic change is directly related to
a drastic change of land use in the upland soils (prairie to a cultivated crop
– summer-fallow rotation). Although summer fallow contributes to lower
infiltration, it causes increased percolation losses due to the lack of plant
transpiration during the fallow period. The capacity of the soil to store water
in the rooting zone is exceeded by the amount of water that infiltrates. The
saline seepage phenomenon demonstrates that changes to the soil condition in
upland areas can have significant impact on the hydrology in areas further down-slope.
Figure 2. Hillslope saline seepage
(38KB)
Differences in the period of ponding between wetlands of the same class receiving
the same amount of rainfall have been explained by differences in interflow
related to soil management. The correlation between reduced soil aggregate stability,
decreased infiltration capacity, and increased surface runoff and erosion is
well-known. Soil conditions in adjacent uplands that promote runoff to wetlands
decrease the potential for subsurface interflow. Wetlands with an active interflow
component, compared with those filled solely by runoff, will exhibit lower ponding
levels that exist for longer periods of time. Greater sediment loading in runoff-fed
wetlands, compared with wetlands fed by interflow, is the primary difference
related to water quality. However, the slower but more continuous contribution
of interflow water also contributes to the maintenance of class VII wetlands
or fens. Short periods of high-level ponding in runoff-fed wetland basins accompanied
by long periods of dry conditions shifts the wetland environment to one of greater
chemical oxidation, e.g., greater release of carbon dioxide (CO2) and nutrients.
Wetland Processes and Water Quality
The wetlands and water quality discussion began with classification and hydrology
for a good reason. When addressing this subject, having some understanding regarding
the range of conditions commonly used to define wetlands is absolutely necessary.
The discussion on wetland classification was intended to provide that perspective
with an organized structure. The discussion on hydrology followed because it
determines when, where and how much of the key ingredient, water, is delivered.
Water quality depends not only on those things that are transported with water
to wetlands via different hydrologic pathways, but it also depends on those
things that are transformed within and transported out of wetlands (Figure 3).
The presence of water or lack of it influences the processes, such as oxidation-reduction,
that control chemical transformations and the mobilization/immobilization balance.
In general, when the balance swings in favor of mobilization, excessive amounts
of the mobile chemical may jeopardize water quality. Emphasis is on the concept
of balance because many transformation processes alternate relatively rapidly
between mobilization and immobilization or are operating contemporaneously due
to spatial variability within the wetland.
Figure 3. Environmental role of a semi-permanent
wetland (42KB)
Sediment
Wetlands act as natural filters for sediment. Erosion and sedimentation are
natural, ongoing geologic processes that explain the textural continuum of coarse
to fine material proceeding into the wetland from the edge. Geologic rates of
sedimentation are relatively slow and allow soil-forming processes to incorporate
recent sediment into the existing soil profile of horizons. However, sedimentation
rates may exceed natural rates of soil formation when increased by accelerated
erosion from adjacent uplands. Water quality impacts begin with the loss of
capacity for wetland soils to stabilize sediment deposits adequately. The filter
no longer functions to protect adjacent streams and lakes. Suspended sediment
levels increase, which contributes to warmer water with less dissolved oxygen.
Associated effects are related to organic matter, nutrients, pesticides and
other chemicals that are intricately associated with chemically active clay-sized
particles. Rather than being transformed in the wetland, they pass through to
streams and lakes, where they contribute to eutrophication or conditions toxic
to aquatic organisms.
Secondary impacts of excessive sedimentation in wetlands are related to materials
that are substantially different than the organic-rich A-horizon of most wetland
soils. Organic matter stabilizes the soil surface by promoting aggregation,
soil structure and macroporosity. It helps create an environment for rapid exchange
of chemicals and serves as a source of energy and nutrients for biological life
cycles. Thick layers of recent sediment deposits generally lack the soil structure
found in natural soils, thus reducing the natural capacity for chemical transformation.
This may lead to increased levels of mobile chemicals available for transport
to adjacent streams and lakes or aquifers below class I – III wetlands.
Carbon and Organic Matter
The wetland environment usually is a system of high plant productivity (sequestration
of CO2) and low decomposition due to the anaerobic conditions that saturated
soils create. Although wetlands occupy only 6 percent of the world’s surface,
they contain an estimated 14 percent of the terrestrial carbon pool. Stored
organic carbon in wetlands is transformed and released as methane gas (CH4)
through anaerobic oxidation. Therefore, wetlands serve as both a sink and source
for atmospheric carbon.
As mentioned before, organic matter serves to stabilize soil aggregates and
promotes structural integrity that is important to water movement through soils.
It also is a source for nutrients and energy required for biotic growth. Organic
matter exerts a positive influence on wetland soils’ capacity to transform
chemicals. However, when organic matter is transferred beyond the wetland boundary,
it has negative impacts to aquatic environments. Transport of organic matter
in water may occur as a dissolved or suspended phase. Organic matter in an aquatic
environment provides the energy for microorganisms, such as bacteria, to thrive.
Microbial oxidation of organic matter releases energy to the microorganisms
and consumes dissolved oxygen (biological oxygen demand, or BOD). Aquatic organisms,
such as fish, need relatively high levels of dissolved oxygen, so high levels
of BOD will have a negative effect on their survival. Organic matter also is
a source of nutrients that further contribute to excessive growth of lower groups
of aquatic organisms, such as algae. Maintenance of a wetland’s ability
to assimilate organic matter depends on the rate at which it is added. As with
sediment, the functional capacity of a wetland will depend on runoff and erosion
from adjacent upland soils.
Dissolved organic matter may or may not be utilized easily as an energy source.
Some soluble forms of organic matter, such as tannins, are quite resistant to
biodegradation and give the water a yellow to dark brown color. Biologic decomposition
of organic matter is greater under oxidizing conditions, compared with conditions
of low to no oxygen. Under these circumstances, large amounts of organic matter
in various stages of decomposition may accumulate. Wetland environments that
have developed under exceptionally high rates of groundwater inflow (springs)
are characterized by soils that are composed predominantly of organic matter.
The organic soils often occur as a “floating” mass above the free
water surface and are classified as fens (class VII). The water in these systems
is alkaline and has high amounts of calcium. Fens are an uncommon type of wetland
on the prairies and often have rare and endangered plant species due to their
unique niche in the prairie environment.
Nutrients: Nitrogen
Nitrogen is distributed among the atmospheric, biologic, geologic and hydrologic
pools. The processes of nitrogen exchange among these pools are commonly referred
to as the “nitrogen cycle.” The most active pool of nitrogen transformation
is the biologic, where nitrogen is a component of myriad organic compounds.
However, the amount of nitrogen in this pool at any given time accounts for
only 2 percent of the Earth’s total nitrogen.
Oxidation of the organic matter releases inorganic forms of nitrogen and is
referred to as mineralization. The reverse of mineralization occurs when inorganic
nitrogen is incorporated into biologic tissue in a reduced-oxidation state and
is referred to as immobilization. Both of these processes involve the exchange
of eight electrons that results in gaseous, mineral and organic compounds composed
of nitrogen with different oxidation states (Table 6). Mineralization and immobilization
occur simultaneously and provide continuous movement between the organic and
inorganic pools of nitrogen.
Table 6. Natural forms of nitrogen in the
environment
--------------------------------------------
Oxidation
N form Chemical name state of N
--------------------------------------------
-CH2-NH2 Organic N -3
NH4+ Ammonium ion -3
NH3 Ammonia -3
N2 Dinitrogen gas 0
NO2 Nitrous oxide gas +1
NO2- Nitrite ion +3
NO3- Nitrate ion +5
--------------------------------------------
Mineralization of organic nitrogen includes two processes, ammonification and
nitrification. Ammonification releases ammonia gas that quickly is hydrolyzed
to the ammonium ion. Nitrification occurs when the ammonium ion is oxidized
through a series of reactions to nitrate. Nitrate is an extremely active chemical
in the environment because of its high availability for biologic uptake and
mobility. Although nitrification is not the only process that contributes nitrate
to the soil, it is the predominant process. Because many types of reactions
and organisms are involved with mineralization and immobilization, changes in
environmental conditions have varying impacts on the forms and quantity of soil
nitrogen at any given time.
The major source of nitrogen that accumulates in soils is biological fixation
of elemental-N (N2) from the atmosphere. Organisms capable of fixing N2 are
thought to have developed early in Earth’s history. The symbiotic relationship
between legumes’ Rhizobia bacteria is the culmination of millions of years
of evolution. Legumes have the potential to fix as much as 500 pounds/acre/year
of nitrogen. Nonsymbiotic organisms, such as Azotobacter and Clostridia bacteria,
and autotrophic blue-green algae also fix atmospheric N2. Blue-green algae synthesize
toxins that have potential for serious impacts to aquatic systems when rapid
growth (algal bloom) is triggered.
When oxygen is depleted in the soil, reduction of nitrate by anaerobic bacteria
is called denitrification. The process results in volatilization of nitrous
oxide (N2O) and N2 if the reaction is completed. Through volatilization of N2O
and N2, the denitrification process removes nitrogen from soil environment,
particularly in wetlands. Having denitrification and nitrification occur simultaneously
is not uncommon in a soil profile with extreme spatial variation in organic
material and oxygen content. Denitrification also depends on the presence of
an oxidizable substrate to furnish the energy that denitrifying bacteria require.
Organic carbon usually serves as the source of energy in the denitrification
process, but in some cases, other chemicals such as iron sulfide or pyrite also
may serve that role. The process of denitrification, coupled with plant uptake
of nitrate in wetlands, make these systems powerful natural tools for environmental
control of nitrogen.
Secondary chemical reactions linked to the denitrification process also have
water quality implications. If sulfate produced by denitrification in the deeper
part of the wetland sediments moves upward into the wetland soil, it will be
reduced to hydrogen sulfide gas. The sodium and calcium associated with the
sulfate are converted to carbonatic salts. A relative excess of sodium, compared
to calcium ions, can lead to the formation of sodium carbonate salts that are
highly corrosive due to a pH often well above 8.5.
In addition to N2O and N2 produced by denitrification, ammonia also may be
produced under highly reduced conditions in some wetland soils. If ammonification
becomes the dominant process, conditions of high temperature, high pH and low
cation exchange capacity will allow toxic quantities of dissolved ammonia to
persist.
Nitrogen levels in NPPR wetlands, lakes and tributaries have been observed
to vary seasonally. Generally the highest concentrations of nitrites and nitrates
are found during spring runoff. These concentrations subside substantially by
biological activity as temperatures increase later in the spring and summer.
Total nitrogen concentrations in NPPR lakes are lowest in the fall, increase
in the winter, remain the same or decrease in the spring and increase in the
summer. The periods of highest total nitrogen concentrations are the summer
and winter. In the summer, the predominant form of nitrogen is organic due to
flourishing populations of aquatic organisms. In the winter, the predominant
form of nitrogen is ammonia. This is because decomposition of organic material
only proceeds through the ammonification step of mineralization due to the reduced
environment. By the end of winter, toxic levels of ammonia may become a water
quality problem, particularly in smaller lakes.
Nutrients: Phosphorus
Phosphorus belongs to the same group (periodic chart) of elements as nitrogen.
It shares many similarities with nitrogen, but also has distinct differences.
Compared with nitrogen, phosphorus is substantially more abundant in earth materials.
However, in just the soil, phosphorus is a minor constituent, averaging 0.05
percent. Phosphorus deficiency in plants is a common soil fertility problem.
Phosphorus also is distinctly less mobile in the environment, compared with
nitrogen. Phosphorus does not have gaseous phases that compare to nitrogen in
terms of importance related to its environmental cycle. Phosphorus can exist
in oxidation states ranging from –3 to +5 (Table 7). Unlike nitrogen,
phosphorus does not shift readily between oxidation states, with the most prevalent
state being +5.
Table 7. Natural forms of phosphorus in the environment
--------------------------------------------------------------
Oxidation
P form Chemical name state of P
--------------------------------------------------------------
Ca3P2 calcium phosphide -3
P4 elemental phosphorus 0
PO3-3 (salt of H3PO3) phosphite +3
PCl3 phosphorus trichloride +3
PCl5 phosphorus pentachloride +5
PO4-3 (salt of H3PO4) orthophosphate +5
PO4-3 (ester of H3PO4) organic orthophosphate +5
--------------------------------------------------------------
Most phosphorus compounds (inorganic and organic) are derivatives of phosphoric
acid (H3PO4). These compounds are formed in combination with the PO4-3 anion,
commonly referred to as orthophosphate. PO4-3 exists in soils in an exchange
continuum that runs from being weakly bound or adsorbed to being strongly bound
within a crystal matrix, such as the mineral apatite [Ca5(PO4)3]. Because of
this, the availability and mobility of PO4-3 is subject to both exchange and
chemical solubility reactions.
PO4-3 that is present in solution is the most mobile state and also available
for uptake by plants and microorganisms. Adsorbed PO4-3 is associated with hydroxyl
(OH) groups that are a chemical component of organic matter and many minerals.
The bond is weak and readily releases PO4-3 to solution as replacement for soluble
phosphorus that plants and microorganisms consume. Adsorbed PO4-3 often is referred
to as “exchangeable” or “labile” phosphorus.
Some of the PO4-3 adsorbed on crystal lattices becomes incorporated into the
crystal during a period of time with an accompanying loss of exchangeability.
Crystalline-PO4-3 is released only during long periods of time due to the relative
insolubility of phosphate minerals. Both exchange and solubility reactions are
dependant on pH and redox potential; consequently, so is the release of PO4-3
to solution.
The term orthophosphate has become synonymous with soluble phosphate. This
is somewhat of a misnomer that can lead to confusion when discussing water resource
impacts from phosphorus. Most of the phosphate present in solution is in the
ionic form of HPO4-2 or H2P04-1. Actually little soluble phosphorus occurs as
PO4-3 under normal pHs. Technically, almost all insoluble phosphorus, both inorganic
and organic, also is orthophosphate. Most organic forms of phosphorus, such
as nucleic acids, nucleotides and sugar phosphates, are esters of phosphoric
acid. Phosphorus is released when these organic chemicals are mineralized similarly
to the release of nitrogen. Mineralization is an extremely important step in
both the nitrogen and phosphorus cycles. In both cases, the chemical form released
has the same oxidization state as the organic form. However, beyond this point,
the nitrogen and phosphorus cycles diverge in similarity. Throughout its cycle,
the chemical form of phosphorus remains basically the same as PO4-3, a component
of many relatively insoluble compounds. Among these phosphorus-bearing materials,
large differences in the release of PO4-3 to solution control environmental
impacts to water resources.
Because both phosphorus and nitrogen are major nutrients required for biological
growth, they work in tandem with respect to the process of eutrophication. In
terms of biological activity, the nutrient in least supply will be the limiting
factor for biological growth. Generally, aquatic systems with a ratio of N to
P of less than 10:1 indicates an “N-limited” system and greater
than 10:1 indicates a “P-limited” system. In other words, availability
of the limiting nutrient will control aquatic biomass growth, algae in particular.
Small additions of the limiting nutrient may result in noticeable increases
in aquatic biomass production. To have nutrient limitations fluctuate seasonally
between nitrogen and phosphorus is not uncommon in some northern prairie lakes.
Generally in this region, a combination of high amounts of organic matter in
the topsoil and unweathered minerals containing phosphorus contribute to high
phosphorus concentrations in water resources. Under these conditions, nitrogen
often is the limiting nutrient; however, control of phosphorus inputs may be
the most manageable. If phosphorus is chosen for control as a nonlimiting nutrient,
improvements to water quality will occur only during an extended period of time.
An important aspect of phosphorus control is related to the release of PO4-3
from lake sediments. This is known as internal nutrient loading. Anoxic or low
redox potentials in lake or wetland sediments will contribute to environmental
conditions that maintain soluble PO4-3 in the water at relatively high levels.
The oxidation state of iron in iron oxides is reduced when the redox potential
is lowered. Under these conditions PO4-3 is not readily adsorbed to iron oxide
surfaces and is released to solution. Mineralization also continues to release
PO4-3 from organic matter. Therefore, aquatic systems that have accumulated
a significant layer of eroded sediment likely will not see much reduction in
PO4-3 concentrations for extended periods after the implementation of management
practices.
Salts
Salinity, as discussed in previous sections, is an integral characteristic
that helps us classify wetlands. Recharge wetlands have naturally low concentrations
of salts, whereas discharge wetlands have high concentrations. The type and
composition of salts in wetlands and adjacent soils may vary depending on parent
material and hydrology. However, under most circumstances, high levels of salinity
in the northern prairie pothole region are associated with sulfatic salts. Epsomite
(MgSO4•7H2O), mirabilite (Na2SO4•10H2O) and thenardite (Na2SO4)
are commonly found in salt crusts formed on dry wetland soil surfaces. Deposition
of sodium sulfates in some discharge areas in northwestern North Dakota and
southern Saskatchewan has been great enough to produce economically extractable
quantities. Plants and animals have evolved to flourish under all levels of
salinity; therefore, environmental protection or stabilization may require maintenance
of rather high concentrations of salts.
Changes in the total salinity and the composition of ions provide an indication
of changes in wetland function. Salinity is a good indicator of changes in the
wetland environment because of the mobility of soluble ions. Climatic or man-induced
changes in hydrology can cause substantial change in salt concentration and
composition within relatively short periods of time (from one season to another).
Hydrologic differences naturally cause soluble salts to be depleted in some
areas and concentrated in others. Therefore, soluble salts are an indicator
of hydrologic conditions and can be used as a surrogate to track hydrologic
change or trends. Evidence also indicates that changes in the minerologic composition
of evaporative salts can be used to gauge long-term trends in salinization.
An increased ratio of Mg2+:Ca2+ has been related to increased salinity in soils
with high concentrations of carbonatic salts.
Wetlands serve as sinks for salt deposition both within the wetland and the
adjacent edge soils. Wetland edges are active hydrologically due to groundwater
flow into and from a shallow water table. Lateral water flow through wetland
edges contributes to salt concentration in edge soils. This process contributes
to the formation of edge soils with high concentrations of relatively insoluble
CaCO3 (e.g., lime). As the groundwater salt load increases in flow-through and
discharge wetlands, more soluble salts, such as CaSO4•2H2O (gypsum), MgSO4•7H2O
and Na2SO4•10H2O, also concentrate in edge soils (Figure 4). Increased
contributions of surface water to wetlands with large concentrations of soluble
salts stored in edge soils can contribute to further expansion of salinity into
adjacent areas. Evidence of this effect occurs throughout the prairie pothole
region where road ditches and lagoons that serve as created wetlands have caused
soil salinization of adjacent soils that were nonsaline prior to the man-induced
change in hydrology.
Figure 4. Salt deposition and soil water
evolution in soils on wetland margins (33KB)
The salt-sink function of wetlands has important consequences related to wetland
drainage. As stated previously, wetlands buffer the aquatic systems. Connection
of saline discharge wetlands to streams or lakes with surface drainage ditches
will transfer the salt load to these systems. Surface drainage of flow-through
wetlands likely will cause increased evapotranspiration and salt concentration
from a shallow water table below the drained soil. The reverse process of impounding
water, or interrupting surface drainage with dams, also has consequences related
to salt concentration. The flow of water through natural surface streams and
channels is the mechanism that exports weathered materials, such as salts, from
the landscape. Impeding the flushing mechanism of a stream may result in a man-made
discharge wetland with increased potential for salinization of impoundment water
and edge soils.
Selenium
Selenium (Se) is an essential trace element for animals, including human beings.
Deficiency of selenium in livestock is exhibited when forages have low selenium
content due to leached acidic soils. Selenium deficiency is likely to occur
if food contains less than 50 parts per billion (ppb). Symptoms of selenium
toxicity are known to occur when concentrations in food exceed 3,000 to 5,000
ppb. These levels in food may be reached in forages or grains grown on soils
that have high accumulations of selenium (seleniferous).
Selenium is chemically similar to sulfur and occurs naturally along with sulfur
(S). When elemental selenium is oxidized, it eventually forms selenic acid (H2SeO4)
comparable to sulfuric acid (H2SO4). The selenate anion (SeO42-) reacts with
a cation to form a soluble salt similar to the sulfate anion (SO42-). In waterlogged
soils, Se and S behave similarly and are reduced to less soluble forms.
Sulfur is present in much larger quantities, but the chemical similarity of
selenium allows it to move through the environment in an analogous fashion.
Therefore, conditions that allow sulfur to accumulate as sulfatic salts also
potentially will allow selenium to accumulate as selenatic salts. Selenium has
been found to occur at higher concentrations in discharge wetlands and some
drained wetlands where sulfates also were accumulating. Selenium toxicity in
wildlife inhabiting some wetlands in the San Joaquin Valley of California has
been related to hydrologic changes caused, in part, by irrigated agriculture.
Mercury and Other Trace Metals
Mercury exists in the environment due to emissions that are both natural and
man-induced. Most mercury in the atmosphere is in the elemental vapor form.
In water, soil, sediments and organisms, mercury occurs in the form of inorganic
salts and organic complexes. Recent estimates indicate that 40 percent to 75
percent of the atmospheric mercury is from combustion of fossil fuels and wastes.
The elemental form of mercury vapor remains in the atmosphere for an average
of one year and may be carried for long distances before it is oxidized and
deposited. After oxidized mercury is deposited, it is concentrated in water
resources through surface flow processes. Once in the soil-water environment,
some of the oxidized form of mercury will be complexed in methylmercury compounds,
particularly under anoxic conditions. This form of mercury is highly available
to fish through the aquatic food web, and accordingly may result in human exposure
to high levels of mercury.
Although the mercury cycle is not as well understood as other elements, such
as nitrogen, wetlands obviously play an important role in the delivery of mercury
to aquatic systems. Oxidation-reduction reactions related to wetness and adsorption-desorption
processes related to sediments are influential wetland processes that may release
or bind mercury in the wetland environment. Hydrologic change to a wetland,
such as drainage, may contribute to increased concentration of mercury in aquatic
systems through less volatilization of elemental mercury and increased mobilization
of adsorbed mercury through surface runoff.
In North Dakota, mercury concentrations in fish tissue from Devils Lake are
elevated. Results of lake sediment analyses from Devils Lake are consistent
with atmospheric deposition of mercury concentrated by surface water flow. Although
the link has not been proven, extensive drainage of wetlands in the Devils Lake
basin may contribute to these elevated levels of mercury.
Other trace metals also have high ratios of man-made to natural levels in the
atmosphere and include lead, cadmium, zinc, vanadium, nickel, arsenic and copper.
The ratio for mercury actually is relatively low, compared with many of the
other trace metals. As described for mercury, the general mechanism of atmospheric
deposition and subsequent concentration through surface water flow also applies
to these other trace metals. Wetland hydrology may play an important role in
determining whether toxic levels of trace metals are reached in various aquatic
environments.
Some studies on individual species indicate that reaction to heavy metal contamination
is variable among wetland plants. Overall, only limited study has been done
on the relationship between these contaminants and the wetland plant community.
Man-made Organic Chemicals
Organic chemicals produced as a result of man’s activities are ubiquitous
in the environment. Pesticides, polychlorinated biphenyls (PCBs), dioxins, polycyclic
aromatic hydrocarbons (PAHs) and antibiotics are a few of the thousands of organic
chemicals that are released regularly through various pathways into the environment.
Although the chemistry of each of these compounds is unique, they have the common
characteristic of being carbon-based. This means they are subject to microbial
attack or degradation and they are attracted to organic matter in soils and
sediments.
The attraction of these chemicals to organic matter allows them to follow the
movement of water-transported soil sediments. They may be adsorbed and degraded
in the soils that they contact, or they may be transported to other locations
with eroded sediment. Transported sediment with its load of adsorbed chemicals
often is deposited in wetland systems. The fact that many pesticides and other
man-made organic chemicals are degraded in wetland environments is well-documented.
In this case, healthy, functioning wetlands serve as environmental filters and
protect aquatic systems. However, if the rates of addition exceed the capacity
of the wetland to perform chemical transformations, toxic concentrations may
result. The consequences may be twofold: 1) deterioration of the wetland biotic
system, causing a reduction in function; and 2) elevated chemical concentrations
in adjacent aquatic systems due to reduced wetland function. Herbicides used
in and around wetlands to control weeds in row-crop situations can decrease
production and species richness of wetland plant species. In certain instances,
a whole wetland plant community can be eliminated by herbicides, such as when
used to control weeds on summer-fallowed land.
As noted with other potential water contaminants, the role of wetlands as an
environmental filter is closely connected to the maintenance of natural hydrologic
conditions. The balance between sink and source changes with major shifts in
hydrology, either natural or man-induced. Generally, losses of surface water
impoundment and lowered water tables will result in reduced capacity of wetlands
to attenuate and transform man-made organic chemicals.
Modifications to Wetland Systems
Drainage
Permanently removing water from the soil surface and lowering the water table
changes the hydrologic regime to one more favorable to development and agricultural
production. Saturated and/or inundated soils make the operation of most equipment
cost prohibitive and cannot physically support structures. The lack of oxygen,
cool soil temperatures and physical challenges of working saturated soils make
crop production unlikely except for a few exceptions, such as rice and cranberries.
Drainage of hydric (wet) soils, either by surface or subsurface (tile) drains,
not only removes water, but also changes the soil environment from anaerobic
to aerobic conditions (Figure 5). A practical benefit of the change is increased
mineralization of stored soil organic matter and release of nutrients for crop
production. Changing to aerobic conditions has been observed to have detrimental
effects, such as lowered pH caused by oxidation of iron pyrite; however, this
is not a likely scenario in the calcareous tills in the NPPR. More likely, the
processes of increased mineral precipitation and adsorption of soluble phosphorous
might contribute to plant deficiency.
Figure 5. Drainage changes the aeration
and chemistry of hydric soils (53KB)
Introduction of an aerobic environment to a wetland soil, combined with steady
removal of surface water, shifts the carbon and nutrient equilibrium. The wetland
becomes a source for the release of carbon dioxide (CO2), nitrate (NO3) and
phosphate (PO4). The wetland loses its capacity to remove nitrate through denitrification;
therefore, the increased nitrate produced by mineralization has higher potential
to be translocated with drainage water. The loss of the capacity to impound
water due to more efficient water removal, coupled with greater availability
of nutrients, increases the potential for delivery of nutrients to aquatic systems
downstream.
Sediment Trapping
Concave slopes often occupied by wetlands serve as sediment traps on the landscape.
The wetland serves as a filter for adjacent aquatic systems. When surface drainage
connects basins, the upper wetland basins lose some of their sediment-trapping
capacity. On landscapes where erosion has been accelerated substantially, drainage
actually may reduce excessive deposition in the wetland and improve wetland
function. However, the problem of sedimentation is passed downstream, eventually
impacting aquatic systems.
Sediment impacts to aquatic systems go far beyond the physical problems related
to turbid water and muddy bottoms. Wetlands have evolved to transform the soluble
and adsorbed chemical load delivered in surface runoff into nontoxic forms that
allow diverse biotic conditions to flourish. When wetlands are removed from
the landscape, soluble and adsorbed chemicals are delivered directly to aquatic
systems. Streams, rivers and lakes have not evolved the capacity to withstand
increased chemical inputs, particularly at the rates delivered due to accelerated
erosion. The result is hypereutrophic conditions and chemical toxicity that
reduces the biotic diversity and value of aquatic water resources.
Sedimentation in wetlands is a natural process on native prairie but is greatly
increased by soil erosion from cultivated catchment basins. The type of crop
planted directly influences the amount of sedimentation. Row crops will deliver
more sediment than small-grain crops. Not only does increased sedimentation
occur, but also more turbidity, greater amount of surface water entering the
wetland and the overall “life span” of the wetland could be affected.
Sedimentation and turbidity affect species composition, species richness, biomass
and cover ratio of wetlands. Submersed species are most affected by sedimentation
and turbidity in a wetland. Sedimentation reduces the germination and over-winter
survival of Eurasian watermilfoil and water celery. Increased sedimentation
decreases the amount and depth of water held within a wetland, and water is
not maintained during the entire growing season. Sedimentation and lower water
levels could cause upland species to encroach on smaller wetlands and overrun
the wet meadow zone. Decreased water depth may contribute to decreased species
richness by allowing certain plants, such as cattails, to dominate the wetland
plant community.
Nutrients often accompany sediment in the runoff of cultivated wetland catchment
basins. One of the most frequent effects of an increased nutrient load on wetland
plant species is an increase in the biomass and cover of emergent and floating
plants. Floating species, such as duckweed, also increase, but submersed plant
species decrease. High nutrient loads seem to eradicate Eurasian watermilfoil
and hornwort, but algae will bloom under these conditions. Moderate nutrient
loads do not appear to affect hornwort and bladderwort, even though the diversity
and productivity are diminished. Overall, severe nutrient loads will cause more
drastic changes in the vegetation, including loss of species, a change in the
total plant community and changes in individual species.
Reduced Soil Quality in Wetland Catchment Soils
Land use in the upslope areas adjacent to wetland basins may have considerable
influence on wetland hydrology. Runoff from the upslope soils will occur when
the rainfall rate surpasses the infiltration rate for a specific event. Soil
properties such as texture and structure control the amount and type of pores
that conduct water through the soil profile. Soil texture is determined by the
percentage of sand, silt and clay, and is a relatively permanent soil property.
On the other hand, soil structure is determined by how the primary particles
of sand, silt and clay are aggregated, and is subject to substantial change
during relatively short periods of time. Soil organic matter plays a major role
in determining the stability of soil aggregates. Land use practices that affect
soil organic matter also affect soil aggregation, porosity and water infiltration.
Soil pores range in size and distribution. Larger pores that allow water to
drain by gravity are greater than 30 microns in diameter and often referred
to as macropores. Macropores that connect with the surface have substantial
influence on infiltration. These pores exist due to the stacking of the larger
soil aggregates. The larger soil aggregates are stabilized or bonded by organic
materials that have the least resistance to deterioration, such as living biological
tissue (e.g., root hairs) and polysaccharides (sugars). Consequently, cultivation
causes a relatively rapid loss of large soil aggregates and macroporosity. Water
infiltration rates are reduced, causing increased runoff, erosion and deposition
of sediments and adsorbed chemicals in concave hillslope positions that wetlands
often occupy.
Deterioration of physical properties of surrounding soils can cause observable
hydrologic change to wetlands even though drainage has not been attempted. Increased
runoff during rainfall will contribute to high pond levels, but decreased subsurface
flow will allow pond levels to drop at a more rapid rate. The result is greater
instability created by wider, more rapid fluctuations of pond levels. Wide fluctuation
of water input to wetlands introduces longer periods of low water tables and
aerobic conditions. Increased mineralization and release of soluble nutrients
is one result. Decreased denitrification may be another result. Release of other
soluble phases of chemicals such as selenium also may follow.
Another consequence of hydrologic change in wetlands induced by upslope land
use is redistribution of shallow salts. The perimeter of a wetland is a zone
of active water movement that allows concentration of a variety of salts. The
distribution and location of the different types of salts is influenced by their
solubility. Calcium carbonate, commonly referred to as lime, is a salt with
low solubility that is concentrated during long periods of time in soils on
the edge of almost all wetlands. More soluble salts, such as sodium and magnesium
sulfate, also often are concentrated in soils on wetland edges, but are subject
to relatively rapid translocation created by short-term changes in hydrology.
Evaporation from the wetland edge is a mechanism that pulls salts from the
wetland upslope into the adjacent soils. Movement of water down-slope due to
subsurface flow is a mechanism that moves salts away the adjacent soils back
toward the wetland. When land use increases surface runoff at the expense of
subsurface flow, the balance of soluble salt movement is shifted in a lateral
direction outward from the wetland. Encroachment of soluble salts into adjacent
upslope soils causes a shift in the wetland plant community to species more
resistant to high osmostic pressure. It also reduces the productivity of crops
grown on adjacent soils for the same reason. Longer periods of low pond levels
also will result in higher pond salinity due to the concentration of salts as
pond water is evaporated.
Increased fluctuation in pond levels also will alter the balance between groundwater
recharge and discharge. Long periods of dry conditions will not only lower water
tables, but also can cause a wetland to change from a flow-through or discharge
condition to a recharge condition. Aerobic conditions during the dry period
will mobilize nutrients and other chemicals, whereby a sudden influx of runoff
into the wetland basin may push them into the groundwater below.
Note that large fluctuations in wetland pond levels also are associated with
climatic variation. In the NPPR, water in wetland ponds has risen to unprecedented
levels in the last 20 years. Clearly the local hydrologic conditions surrounding
most wetlands have changed in a relatively short period of time. This type of
natural climatic fluctuation is consistent with the continental climate of the
region. Natural fluctuations in water, sediment and chemical wetland budgets
result in changes to wetland plant communities, water quality and adjacent soils.
When addressing management issues related to water level fluctuations and land
use, the relationship to climatic variation also must be considered.
Assessment of Wetland Function and Biotic Integrity
Assessment of function provides an estimate of the capacity of a wetland to
participate in a given environmental process. Wetland function often is divided
into three major categories: 1) hydrologic; 2) biogeochemical; 3) habitat and
food web support. Function should not be confused with value, which is an estimate
of worth to society.
Hydrogeomorphic Model (HGM)
The three major categories of wetland function listed above have many specific
functions within them. There is a large amount of variability among individual
wetlands to perform each of these specific functions. The first step of wetland
assessment methods, such as the hydrogeomorphic (HGM) model developed by the
Natural Resource Conservation Service and U.S. Army Corp of Engineers, is a
classification of wetlands established on hydrogeomorphic factors such as geomorphology,
water source and transport, hydrodynamics, water chemistry and soil properties.
Basically, wetlands are standardized in groups based on similar abilities to
perform natural functions. Within these groups, reference wetlands that represent
relatively undisturbed conditions are used to set the specific criteria for
assessment of functionality (Figure 6). The algorithm containing the criteria
developed for each wetland group is referred to as a hydrogeomorphic model.
A regionalized “reference system” is created by sampling and comparing
functionality of a number of unimpaired and impaired wetlands within a class.
The level of function performed by a given wetland is determined by comparing
it to the unimpaired reference conditions. A hydrogeomorphic model for temporary
and seasonal wetlands in the NPPR has been developed.
Figure 6. Assessment with the hydrogeomorphic
model compares measured values of function in undisturbed wetlands to disturbed
wetlands of similar class (44KB)
Index of Plant Community Integrity (IPCI)
The integrity of the native biological components of a wetland is known to
be a reflection of the condition or health of a wetland. For example, plant
species have different ranges of tolerance to a variety of environmental factors,
such as inundation, wetness, salinity, pH, sedimentation, physical alterations,
etc. When wetland conditions are disturbed, one of the first indicators of reduced
condition or health is a change in the wetland plant community. As the intensity
of disturbance increases, the native perennial plant species’ richness
and diversity decrease in number and cover, and annual, biennial and exotic
species will increase. In many cases under intensive disturbance, such as cultivation,
the native plant community is totally decimated and weedy annuals or mono-cultural
stands of invasive exotics will dominate.
An index of plant community integrity (IPCI) has been developed for temporary,
seasonal and semipermanent wetlands for the majority of the NPPR. Similar to
the HGM model, the IPCI was developed to assess wetlands across a range of disturbance.
Plant community attributes (e.g., native perennial species richness; percent
of annual, biennial and introduced species, etc.) are quantified and compared
to the level of disturbance in upland catchments that range from well-managed
rangeland to intensively cultivated cropland. The relationships determined allow
plant community attributes to be used to predict disturbance intensity and/or
wetland condition. A value of wetland condition can be assigned by locating
its position on the disturbance gradient (e.g., low [native prairie] high [cultivation]).
Five categories of wetland condition are recognized and range from very good
to very poor.
Note that extrapolation of wetland assessment methods beyond the experimental
conditions or region of data evaluation is risky and not recommended.
Soil Management and Wetland Restoration
The degree of wetness controls wetland soil properties, which in turn controls
the biotic community or habitat. The balance between aerobic and anaerobic conditions
is critical to wetland function. This is a hydrological problem that needs to
consider periods and depth of wetland inundation. Hydrologic conditions in the
wetland depend on surface and subsurface flow to and from adjacent areas. Wetland
restoration and protection must account for the hydrologic connection with soils
in the adjacent areas that are not wet but transmit water.
Restoration of previous hydrologic conditions depends not only on reintroducing
impounded water, but also on return to the previous balance between inputs from
surface and subsurface flow. This requires changes in management to upslope
areas adjacent to the wetland. In this case, management techniques that provide
increased carbon sequestration need to be applied to improve the structural
stability and porosity of upslope soils (Figure 7). Increased infiltration comparable
to well-managed rangeland on similar soils is the goal.
Figure 7. Wetland condition depends
on basin water management and upland land use (44KB)
Management of upslope soils that are cropped will be more challenging than
range or pasture. The impact of cropland management to wetlands is highly variable,
depending on a host of different factors. However, some general practices can
be discussed in terms of their influence on wetland systems. Management effects
may be categorized into tillage, rotations, conservation practices and structures,
chemical inputs and water inputs.
Tillage
As discussed previously, tillage has profound effects on soil aggregate stability
and size, which causes a reduction in water infiltration. The impacts from tillage
can be minimized by using implements that expose the soil for relatively short
periods and leave relatively high amounts of plant residue on the surface. Moldboard
plowing, disking and black summer fallow are practices that cause extreme stress
to soil aggregates. Reduced-tillage practices will promote less mechanical stress
from implements, rain and wind on soil aggregates. At the same time, increased
return of plant residues and decreased mineralization will promote higher levels
of the types of organic matter responsible for stabilizing the larger soil aggregates.
Some of the challenges of reduced tillage are related to decreased soil temperatures
and higher amounts of soil moisture that are desirable during germination and
seedling growth. Increased application of nutrients may be required early in
the growing cycle. Greater amounts of available nutrients in the surface soil
can lead to higher losses to both surface and groundwater systems. Increased
levels of weed and disease infestations may require greater use of pesticides
and have consequent impacts on water resources.
Crop Rotations
The period of soil exposure to the elements has a strong influence on the amount
of stress placed on soil aggregates. The crop canopy and plant residues left
after harvest protect the soil surface from the impact of rain, water and wind
shear. The rotation that provides the least amount of protection is clean summer
fallow during the most erosive periods of the year. Row crops generally take
longer to develop a protective canopy, compared with small grains or other crops
grown without discernable rows. The inter-row area has the potential to focus
surface runoff, causing increased shear stress and erosion.
Some crops, such as wheat and barley, are more effective in utilizing applied
nutrients, compared with corn or potatoes. Other crops, such as alfalfa, are
more effective in extracting water and nutrients from deeper parts of the soil
profile. Rotations that include deep-rooted crops help scavenge nutrients and
water left by less efficient crops. Finally, crop rotation almost always increases
crop yield, compared with continuous cropping systems. This is probably related
to number of things that improve plant health, such as breaking disease cycles.
Vigorous plant growth provides greater soil protection through soil canopy development
and plant residues.
Conservation Practices and Structures
Removal and transport of sediments is a geologic process that controls the
formation of landforms. The geologic rate of landscape formation is relatively
slow. However, under extreme conditions that either increase the erosive forces
or reduce the ability of the soil surface to withstand those forces, erosion
rates may be accelerated substantially. A study of erosion rates in Iowa showed
increases of seven to 900 times after settlement. Conservation practices and
structures will not eliminate the natural processes of sediment removal and
deposition important to landscape formation, but if applied correctly will reduce
the rate of erosion to one closer to the natural level.
Farming practices such as contour tillage and planting, strip cropping, shelterbelts
and grassed waterways are designed to reduce the potential for detachment and
movement of sediment. Farming across, rather than down, hill slopes reduces
the potential for runoff to focus into rills and small channels where high-shear
stress develops. Using strips of crops, such as beans and alfalfa, in the same
field allows detached soil particles from the less protected soil (beans) to
be trapped in the protected crop strip (alfalfa). This practice works to reduce
off-site soil movement by both wind and water. Drainageways are stabilized to
allow the transport of water through the landscape without gully development.
Shelterbelts of trees and shrubs are used to break the velocity of the wind
and reduce wind shear.
Weather extremes often overwhelm conservation practices and more robust measures
are required to protect soil and water resources. Terraces, water diversions
and water impoundments require substantial effort to construct and maintain.
In some places, these structures are highly effective in protecting water resources
by providing sediment trapping and controlled transport of runoff through the
landscape.
Chemical Inputs
Controlled input of nutrients and pesticides is extremely important to plant
health and, subsequently, soil and water protection. Appropriate nutrient and
pesticide application depends on knowledge of the plant environment. The goal
is to apply enough nutrition to the plant so it meets its genetic potential
for yield without leaving excessive residual amounts of nutrients in the soil.
Efficient use of nutrients requires soil testing and sometimes plant analyses
of nutrient status. Useful soil testing requires implementation of a well-designed
sampling plan. Nitrogen status needs to be determined most regularly, compared
with less mobile nutrients such as phosphorus. Nitrogen testing may be required
every year under a highly intensive cropping rotation.
Pest monitoring or field scouting should be used to predict when levels of
infestation will have an economic impact worth treating. A comprehensive management
plan that includes all the options available for pest control, including application
of pesticides, should be developed. Integrated pest management systems have
been shown to economically control pests and also reduce the overall use of
pesticides.
Water Inputs
Water often is the most limiting nutrient with respect to plant growth. The
benefit of increased available water in the soil profile usually contributes
to improved plant growth and yields. As discussed previously, this leads to
increased levels of plant residue that helps promote protection from the erosive
forces of wind and water. Greater soil protection effectively provides more
efficient use of water inputs in many upland soils.
Not all upland soils, however, exhibit the same positive influence of practices
that promote increased levels of soil moisture. Too much of a good thing can
be detrimental to plant growth for a variety of reasons, such as reduced soil
temperatures and low levels of oxygen. Evidence indicates that timely removal
of water from these soils either by surface or subsurface drainage systems will
improve plant growth and increase soil protection through greater plant residues.
Also, soil drainage will increase water infiltration, thus reducing the potential
for runoff and water erosion. However, drainage allows low-intensity land use
with permanent plant communities to be replaced with higher-intensity land use
with intermittent vegetation. The result may be quite successful from an economic
point of view, but the potential for soil erosion also will increase. For example,
the wet soils in the Red River Valley are highly productive due to surface drainage,
but eroded sediment generated from these soils, particularly by wind, continues
to challenge resource managers.
Water input management is critical on irrigated soils for both economic and
environmental reasons. Application of water in excess of plant needs may contribute
to water table buildup, nutrient and salt leaching to aquifer resources, and
increased runoff and erosion due to surface soil saturation. Water applications
that don’t meet the crop-growth requirements will result in lower yields,
less plant residues and decreased protection of the soil surface from wind and
water.
Before attempting irrigation, the compatibility of the soil to water application
should be estimated. Soil properties such as slope, texture, internal drainage
and salts are considered with respect to the salinity and sodicity of the irrigation
water. Irrigation of soil with incompatible water can lead to substantial loss
of soil productivity due to salinization, sodification, erosion and water table
buildup. The damage to the soil resource will extend to deterioration of surface
and groundwater resources.
After determining soil and water compatibility, the single most useful recommendation
is adaptation of an accepted water scheduling method to the grower’s management
system. The “checkbook method” is popular in the northern Great
Plains and accounts for available soil water and crop water use. Matching crop
water requirements with water inputs is the ultimate goal. If this can be accomplished,
crops can be produced with the least amount of impact to soil and water resources.
Grazing Systems
Many wetlands in the NPPR are found adjacent to rangeland or pastureland, as
opposed to cultivated crops. In these areas, grazing animal management influences
soil condition and resultant impacts to wetlands. Grazing of prairie grasses
stimulates root growth and plays a key role in prairie soil development. The
salient feature of a prairie soil is a thick topsoil with a high amount of organic
matter and stable aggregates.
Although the productive qualities of prairie soils can be linked to grazing
animals, there is a limit. Studies show that many factors, such as soil type,
slope, aspect, plant type and range condition, will affect runoff and erosion
from prairie soils. However, the most important factor is grazing intensity.
When prairie grasses are grazed so intensely that the plant cannot recover nutrient
reserves lost during early grazing, the plant cover and root mass diminish.
The result is reduced infiltration of water and increased runoff that contributes
to sedimentation and hydrologic changes to wetlands.
Some impacts also are related directly to grazing of wetland plants. Wetlands
subjected to grazing contrast in dominant plant species, compared with wetlands
with other land uses, such as haying, cultivation or idle conditions. Depending
on its intensity, grazing can increase species diversity and diversity along
environmental gradients, as well as increase wetland plant community boundary
definition. Grazing decreases dead plant matter, and overgrazing can lead to
decreased primary production. Overgrazing and severe trampling will reduce plant
cover and increase the amount of bare soil. The effects of livestock trampling
occur mainly in the exterior zones of a wetland. Many wetland plant species
are adapted to grazing, or can withstand the grazing pressures. Overall, the
effects of grazing on wetland plant community composition, species richness
and cover can vary greatly with grazing intensity. Because grazing historically
has been a disturbance within the prairie wetland region, wetlands located on
well-managed rangeland should exhibit high-quality conditions.
Stocking rate management is critical to maintenance of good range and soil
condition. Generally, stocking rates never should exceed levels that allow the
plant cover to diminish below 30 percent. This usually can be achieved with
grazing management that utilizes 50 percent to 60 percent by weight of the above-ground
biomass. A rule of thumb often applied to grazing systems is “take half
– leave half.” Utilization charts for different grass species have
been developed for range managers to gauge biomass removal with grazed plant
height.
Timing of grazing also is an important element in maintaining range and soil
resources. Grasses are most vulnerable to stress in the early part of their
growth cycle. Usually grazing of native rangeland is not recommended prior to
May 20 in southeast North Dakota and June 1 in the rest of the state. Pastures
with introduced cool-season grasses, such as crested wheatgrass and smooth bromegrass,
can be used to extend grazing into April and May. Pastures with introduced warm-season
grasses, such as Altai and Russian wildrye, can be used to extend grazing into
the fall.
Rotation grazing and range improvement methods help maintain economic stocking
rates without causing deterioration of plant and soil resources. Range improvements,
such as fencing, water development, burning, mowing and mineral placement, help
provide a more even distribution of animals. Water development also helps place
watering sites upslope from depressional areas with wetlands. This reduces damage
to wet soils and also diminishes the consumption of stagnant water, a threat
to animal health. Other techniques, such as weed control, fertilization, seeding
and interseeding, and runoff entrapment, help improve plant health and range
condition.
Wetland Basin Management
Mowing
Mowing or haying is another common practice influencing NPPR wetlands. The
plant material removed is used as forage or bedding for livestock. Mowing also
occurs around urban wetlands, usually for aesthetic or management values. Haying
usually occurs in situations where the upland has been planted into an alfalfa
or grass alfalfa mixture. Haying wetlands is common where the uplands have been
cultivated but the wetland area is too wet to be cultivated. The most commonly
hayed areas of a wetland are the wet meadow and shallow marsh zones. These zones,
especially the wet meadow zone, usually are accessible to farm machinery in
the fall. Wetland researchers believe that extensive haying over time will increase
specific types of plant species, such as white top. Also, livestock owners often
introduce a species, such as reed canarygrass, into wetlands as a desirable
hay species. Mowing or haying can, therefore, affect the composition of wetland
plant communities through long-term haying or by physically planting a desirable
forage species.
Cultivation
One of the most severe types of disturbance affecting NPPR wetland vegetation
is cultivation. Principally, temporary and seasonal wetlands or the temporary
and seasonal zones of larger wetlands are cultivated. Cultivation favors weedy,
often introduced, annual and perennial species, and eradicates native perennial
vegetation. Invader species establish quickly after water levels fall. Stewart
and Kantrud (1971) provide a list of common species found in cultivated wetlands
in their “cropland drawdown phase” and “cropland tillage phase.”
The “cropland drawdown phase” consists of drawdown annual species
that usually are found on open, muddy areas of wetlands. The “cropland
tillage phase” consists of weedy annuals and row crop or small-grain species
that can be planted because of drier conditions. Cultivation also establishes
a unique uneven microtopography where upland weedy annuals, drawdown plant species
and naturally occurring wetland species exist close to one another. Cultivated
wetlands will differ in plant species assemblages from native situations.
Restored Wetlands
Many NPPR wetlands have been restored with backing from federal, state and
private organizations. Most of these have been restored through the Conservation
Reserve Program. Recently restored wetlands have fewer species than those found
in natural conditions. Compared with natural wetlands, restored wetlands often
have different species associations, distribution and abundance of species according
to elevation, and number of species and total number of seeds in the seed bank.
Also certain species, such as reed canarygrass and rice cutgrass, may invade
rapidly and inhibit the establishment of desirable species.
Idled Wetlands
Wetlands in the NPPR sometimes are left idle as a part of the Conservation
Reserve Program, waterfowl production areas, national refuges, state game management
areas or privately owned land. Idled wetlands often create opportunities for
invasion of cattails, willows and cottonwoods, particularly in areas that once
were under cultivation. Idling salt-marsh wetlands decrease species richness
and vegetation types. Common reed has been observed to increase in wetlands
left idle.
References by Section
Wetland Classification and Water Quality
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Water Movement and Water Quality
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Hillslope Interflow and Wetlands
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Sediment Trapping
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Service, Washington, D.C. 15 pp.
Kantrud, H.A., J.B. Millar and A.G. van der Valk. 1989a.
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van der Valk, ed. Northern Prairie Wetlands. Iowa State University Press,
Ames, Iowa.
Reimold, R.J., R.A. Linthurst and P.L. Wolf. 1975. Effects
of grazing on a salt marsh. Biological Conservation. 8:105-125.
Sedivec, K. 1992. Water quality the rangeland component.
NDSU Extension publication R-1028, North Dakota State University, Fargo.
Stewart, R.E., and H.A. Kantrud. 1972. Vegetation of Prairie
Potholes, North Dakota, in relation to quality of water and other environmental
factors. U.S. Geological Survey, Professional Paper 585-D.
Walker, B.H., and R.T. Coupland. 1968. An analysis of vegetation-environment
relationships in Saskatchewan sloughs. Canadian Journal of Botany 46:509-522.
Wetland Basin Management
Anderson, H., J.P. Bakker, M. Brongers, B. Heydemann and
U. Irmler. 1990. Long-term changes of salt marsh communities by cattle grazing.
Vegetatio 89:137-148.
Galatowitsch, S.M., and A.G. van der Valk. 1996. The vegetation
of restored and natural prairie wetlands. Ecological Applications 6(1):102-112.
Hellings, S.E., and J.L. Gallagher. 1992. The effects of
salinity and flooding on Phragmites australis. Journal of Applied Ecology
29:41-49.
Kantrud, H.A., and W.E. Newton. 1996. A test of vegetation-related
indicators of wetland quality in the Prairie Pothole Region. Journal of Aquatic
Ecosystems Health 5:177-191.
Kantrud, H.A., G.L. Krapu and G.A. Swanson. 1989b. Prairie
basin wetlands of the Dakotas: A community profile. U.S. Fish and Wildlife
Service Biological Report 85(7.28).
Stewart, R.E., and H.A. Kantrud. 1971. Classification of
natural ponds and lakes in the glaciated prairie region. U.S. Fish and Wildlife
Service, Resource Publication 92.
Stewart, R.E., and H.A. Kantrud. 1972. Vegetation of Prairie
Potholes, North Dakota, in relation to quality of water and other environmental
factors. U.S. Geological Survey, Professional Paper 585-D.
Stewart, R.E., and H.A. Kantrud. 1973. Ecological distribution
of breeding waterfowl populations in North Dakota. Journal of Wildlife Management
37(1):39-50.
Walker, B.H., and R.T. Coupland. 1968. An analysis of vegetation-environment
relationships in Saskatchewan sloughs. Canadian Journal of Botany 46:509-522.
Walker, B.H., and R.T. Coupland. 1970. Herbaceous wetland
vegetation in the Aspen Grove and Grassland Regions of Saskatchewan. Canadian
Journal of Botany 48:1861-1878.
For more information on this and other topics, see: www.ag.ndsu.edu
WQ-1313, August 2006
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