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Wetland Ecology
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Lectures 14-15-16
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Wetland Biogeochemistry
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What is biogeochemical cycling?
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Transport & Transformation of chemicals in an ecosystem, involving numerous
interrelated physical, chemical, & biological processes
Examples of movement include
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Water-sediment exchange
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Plant Uptake
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Organic exports
Two major categories of wetland biogeochemistry include:
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Intrasystem cycling through various transformation processes
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Exchange of chemicals between a wetland & its surroundings
Open vs Closed Ecosystems
OPEN
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BHF + Tidal Marshes have significant exchange of materials with surroundings
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River flooding + Tidal Exchange
CLOSED
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Bogs + Cypress Domes
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Little material exchange except for gaseous matter!
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Biogeochemically open vs closed
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BGC Open – Abundant exchange of materials with surroundings
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BGC Closed – Little movement of materials across the ecosystem boundary
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Tidally driven or riparian
Cypress swamps or very stagnant areas
So, are wetlands open or closed systems?
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They can be both!
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Wetland Soil
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Wetland soil is:
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Medium in which many of the wetland chemical transformations occur
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The primary storage of available chemicals for wetland plants
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Two major types of wetland soils
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Organic & Organic Mineral Soils
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Mineral
Organic Soil + Organic Mineral Soils
Defined under two saturation conditions
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1. Saturated with water for long periods
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Have > 18% organic carbon
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Have > 12% organic carbon if no clay present
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Have a proportional content of organic carbon (12 – 18%) if clay content (0 – 60%)
2. Soils are never saturated with water for more than a few days and have > 20% organic carbon
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%Corg = %OM/2
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%Corg = percentage of organic carbon
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%OM = percentage of organic matter
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Bulk density & porosity – have lower bulk densities (dry weight of soil/volume). Low due to high
porosity (peat soils ~ 80% air space)
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Hydraulic conductivity – Depends on degree of decomposition. Organic soils hold more water (do
not necessarily allow more water to pass)
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Nutrient availability – Organic soils have more minerals tied up in organic form.
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Cation Exchange Capacity – Organic soils have greater CEC (sum of exchangeable ions)
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Organic soils (What order?) are classified into four groups; 3 are hydric
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Saprists (muck) – > 2/3 of the material is decomposed, < 1/3 of plant fibers are
identifiable
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Fibrists (peat) - < 1/3 of material is decomposed and > 2/3 of plant fibers are identifiable
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Hemists (mucky peat or peaty muck) – Conditions fall between saprist & fibrist soil
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Folists – organic soils caused by excessive moisture (precip > evapotranspiration) that
accumulate in tropical & boreal mountains; not classified as hydric because saturated
conditions are the exception rather than the rule
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Mineral Wetland Soil
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When flooded for extended periods mineral soils develop certain characteristics that allow for
their identification
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Redoximorphic features (mediated by microbes)
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The rate these features are formed depend on three conditions (all must be present)
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Sustained anaerobic conditions
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Sufficient soil temp (5°C “biological zero”)
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Organic Matter (substrate for microbial activity)
Hydric mineral soils are characterized by:
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Gleying
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Oxidized Rhizosphere
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Mottles (aka Redox Concentrations)
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Current nomenclature
1. Redox concentrations – Accumulation of Fe & Mn in 3 different structures
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Nodules & Concretions (firm  extremely firm irregularly shaped bodies with diffuse
boundaries)
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Masses – formerly called ‘reddish mottles’
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Pore linings – Formerly included ‘oxidized rhizospheres’
2. Redox depletions – Low-chroma (<2) bodies with high values (>4) including:
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Iron depletions: “Gray mottles” or “Gley mottles”
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Clay depletions: Contain less Fe, Mn, and clay than adjacent soils
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3. Reduced matrices: Low-chroma soils
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REDOX in Wetlands
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When mineral or organic soils are flooded anaerobic conditions result. Water fills pore spaces
and rate of oxygen diffusion through soil is drastically reduced
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Rate of oxygen depletion depends on:
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Ambient Temperature
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Availability of organic substrates for microbial respiration
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Chemical oxygen demand from reductants such as ferrous Fe
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Resultant O2 deficiency prevents plants from normal aerobic root respiration and affect nutrient
availability and adds toxic materials in the soil
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Usually thin layer formed and is related to:
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Rate of O2 transport across the atmosphere-surface water interface
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Small population of O2 consuming organisms
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Photosynthetic O2 production by algae within water column
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Surface mixing by convection currents & wind action
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Eh ranges
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If DO is present, the redox potential range +400 to + 700mV
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If O2 disappears, Eh range from +400 down to -400mV
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As organic substrates in a water logged soil are oxidized (donating) the redox potential drops = a
sequence of reductions (gains) takes place
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1 reaction to occur after becoming anaerobic is the reduction of NO3- (nitrate) first to NO2(nitrite) and ultimately to N2O or N2
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Nitrate becomes an acceptor ~ 250mV
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At 225mV Mn is transformed from manganic to manganous
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At -75 to -150mV Fe is transformed from ferric to ferrous; while sulfates are reduced to
sulfides
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pH
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Soil and overlying waters of wetlands occur over a wide range of pH’s
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Organic soils – often more acidic
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Mineral soils – often neutral or alkaline
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Alkaline soils previously drained decrease in pH because of buildup of CO 2 then carbonic acid
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Acid soils previously drained increase in pH because of reduction of ferric iron hydroxide
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Nitrogen Transformations
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Nitrogen is often the “most limiting nutrient in flooded soils”
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Limitations reported in salt marshes, freshwater inland marshes, & freshwater tidal
marshes
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Involve complex microbial processes
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NH4 is the primary form of mineralized N in wetlands
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Mineralization
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Often referred to as ammonification
NH2CONH2 + H2O  2NH3 + CO2
NH3 + H2O  NH4 + OH
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Nitrification
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Once ammonia has formed, it can take several possible pathways
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Aerobic environment – ammonium is oxidized (nitrification) in two steps by Nitrosomonas
sp.
+
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2NH4 + 3O2  2NO2 + 2H2O + 4H + energy
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By Nitrobacter sp.
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2NO2 + O2  2NO3 + energy
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Denitrification
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Denitrification is carried out by facultative bacteria under anaerobic conditions – nitrate is the
terminal electron acceptor
C6H12O6 + 4NO3  6CO2 + 6H2O + 2N2
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Most significant path of nitrogen loss from wetlands
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Usually lost as N2 & N2O
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N fixation
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Conversion of N2 gas to organic nitrogen
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Favored by low oxygen concentrations
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Rhizobium species
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Cyanobacteria (blue-green algae) are also common in Louisiana, northern bogs, & in rice
cultures
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Fe & Mn Transformations
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Found in reduced forms in wetlands
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Readily available  Toxic levels
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More soluble
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Ferrous Fe  reduced form of Fe
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Ferric Fe  oxidized form of Fe
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Oxidized form creates barrier; may prevent plant from uptaking other nutrients
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Reduced form reacts with P  making it unavailable
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Sulfur
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Rarely limiting to plant or animal growth in wetlands
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Hydrogen sulfide (H2S) is toxic (rotten-egg smell)
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Ferrous sulfide is responsible for black color of wetland soils (highly reduced sediment)
Negative effects of sulfides on higher plants are attributable to a number of causes
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Direct toxicity of free sulfide as it comes into contact with plant roots
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Reduced availability of sulfur for plant growth because of its precipitation with trace
metals
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Immobilization of zinc & copper by sulfide precipitation
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Carbon Cycle
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Photosynthesis & aerobic respiration dominate the aerobic horizons
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Fermentation
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Methanogenesis
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Occurs when certain bacteria (methanogens) use CO2 as an electron acceptor for the production
of gaseous methane (CH4)
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Requires extremely reduced conditions
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Redox potential range from -250 to -350mV
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Phosphorus
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One of the most important chemicals in wetland systems
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Most limiting in northern bogs, freshwater marshes, & southern deepwater swamps
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Retention is one of the most important features for natural & constructed wetlands
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Principle inorganic form = orthophosphate
3-
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PO4
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H2PO4 (pH 2-7)
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HPO4 (pH 8-12)
(pH > 13)
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Predominant form dependent on pH
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P is not directly altered by Eh changes
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Affected by association with other elements especially Fe
P is rendered relatively unavailable to plants & microconsumers by:
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1. Precipitation of insoluble phosphates with ferric Fe, Ca, & Al under aerobic conditions
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2. Adsorption (particle surface) of phosphate onto clay particles, organic peat and ferric &
aluminum hydroxides & oxides
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3. Binding of phosphorous in OM as a result of its incorporation into the living biomass of
bacteria, algae, & vascular macrophytes
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Chemical Transport into Wetlands
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Geologic inputs – from weathering of rock
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Type of soil  direct reflection of parent material
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Biologic inputs – Photosynthetic uptake of carbon, nitrogen fixation, & biotic transport of materials
by animals (birds)
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Hydrologic inputs – Major inputs into wetlands
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Precipitation
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Burning of fossil fuels
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Increased [ ]’s of sulfates & nitrates in atmosphere
Streams, Rivers, & Groundwater
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As precip reaches the ground it will:
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Infiltrate into the ground
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Return to atmosphere via evapotranspiration
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Flow on surface as runoff
Groundwater influence:
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Chemical characteristics of streams & rivers depend on the degree to which the water
has previously come into contact with underground formations & types of minerals
present in those formations
Climate:
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Balance of precipitation & evapotranspiration
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Type of vegetation present
Geographic effects: amount of dissolved & suspended materials that enter streams, rivers, &
wetlands depend on:
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Size of watershed
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Steepness or slop of landscape
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Soil texture
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Streamflow/Ecosystem effects: The water quality of surface water runoff, streams, & rivers varies
seasonally
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Human effects: water that has been modified by humans through sewage effluent, urbanization, &
runoff from farms alters the chemical composition of streamflow & groundwater that enters
wetlands
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Drainage from agriculture fields:
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Higher [ ]’s of sediments, nutrients, herbicides, & pesticides might be expected
Drainage from urban & suburban areas:
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May have high [ ]’s of trace organics, oxygen demanding substances, & some toxic
materials
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Estuaries
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Quality differs from that of rivers
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Seawater chemical composition is fairly constant worldwide
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33 0/00 to 37 0/00
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Mass balances
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A quantitative account of the inputs, outputs, & internal cycling of materials in an ecosystem
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Mass balances help determine:
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Ecosystem functions
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Determine the importance of wetlands as sources, sinks, and transformers of chemicals
Inputs primarily through:
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Hydrologic
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Precipitation
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Surface & groundwater inflow
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Tidal Exchange
Biotic
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Atmospheric carbon fixation (photosynthesis)
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Atmospheric nitrogen (nitrogen fixation)
Exports:
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Surface water & groundwater
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Long-term burial of chemical in the sediments
Intrasystem cycling involves exchanges among various pools, or standing stocks of chemicals in
within a wetland. Involves pathways such as:
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Litter production
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Remineralization
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Chemical transformations
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Translocation of nutrients through plants
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Generalizations
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1. Wetlands serve as sources, sinks, or transformers of chemicals, depending on the wetland
type, hydrologic condition, & length of time the wetland has been subjected to chemical loadings
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2. Seasonal patterns of nutrient uptake & release
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Temperate regions retention is higher in growing season
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Increased microbial activity
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Higher macrophyte activity
3. Wetlands are frequently coupled to adjacent ecosystems through chemical exchanges that
affect both systems
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Downstream ecosystems benefit from retention or from exportation
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4. Wetlands are either highly productive (eutrophic) or low productivity (oligotrophic)
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5. Nutrient cycling in wetlands differs from aquatic & terrestrial systems
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More nutrients in sediment & peat
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Aquatic systems have autotrophic activity more dependent on nutrients in water column
than in sediments.
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Wetland plants obtain nutrients from sediment
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6. Anthropogenic changes have led to changes in nutrient cycling in many wetlands
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The capacity of wetlands to assimilate anthropogenic wastes from the atmosphere or
hydrosphere is not unlimited!