• Wetland Ecology

• Lectures 14-15-16

• Wetland Biogeochemistry

• What is biogeochemical cycling?

– Transport & Transformation of chemicals in an ecosystem, involving numerous interrelated physical, chemical, & biological processes

• Examples of movement include

– Water-sediment exchange

– Plant Uptake

– Organic exports

• Two major categories of wetland biogeochemistry include:

– Intrasystem cycling through various transformation processes

– Exchange of chemicals between a wetland & its surroundings

• Open vs Closed Ecosystems

OPEN

• BHF + Tidal Marshes have significant exchange of materials with surroundings

– River flooding + Tidal Exchange

CLOSED

• Bogs + Cypress Domes

– Little material exchange except for gaseous matter!

• Biogeochemically open vs closed

• BGC Open – Abundant exchange of materials with surroundings

– Tidally driven or riparian

• BGC Closed – Little movement of materials across the ecosystem boundary

– Cypress swamps or very stagnant areas

• So, are wetlands open or closed systems?

– They can be both!

• Wetland Soil

• Wetland soil is:

– Medium in which many of the wetland chemical transformations occur

– The primary storage of available chemicals for wetland plants

• Two major types of wetland soils

– Organic & Organic Mineral Soils

– Mineral

• Organic Soil + Organic Mineral Soils

Defined under two saturation conditions

• 1. Saturated with water for long periods

– Have > 18% organic carbon

– Have > 12% organic carbon if no clay present

– 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

– %Corg = %OM/2

– %Corg = percentage of organic carbon

– %OM = percentage of organic matter

• Bulk density & porosity – have lower bulk densities (dry weight of soil/volume). Low due to high porosity (peat soils ~ 80% air space)

• Hydraulic conductivity – Depends on degree of decomposition. Organic soils hold more water (do not necessarily allow more water to pass)

• Nutrient availability – Organic soils have more minerals tied up in organic form.

• Cation Exchange Capacity – Organic soils have greater CEC (sum of exchangeable ions)

• Organic soils (What order?) are classified into four groups; 3 are hydric

– Saprists (muck) – > 2/3 of the material is decomposed, < 1/3 of plant fibers are identifiable

– Fibrists (peat) - < 1/3 of material is decomposed and > 2/3 of plant fibers are identifiable

– Hemists (mucky peat or peaty muck) – Conditions fall between saprist & fibrist soil

– 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

• Mineral Wetland Soil

• When flooded for extended periods mineral soils develop certain characteristics that allow for their identification

– Redoximorphic features (mediated by microbes)

– The rate these features are formed depend on three conditions (all must be present)

• Sustained anaerobic conditions

• Sufficient soil temp (5°C “biological zero”)

• Organic Matter (substrate for microbial activity)

• Hydric mineral soils are characterized by:

– Gleying

– Oxidized Rhizosphere

– Mottles (aka Redox Concentrations)

– Current nomenclature

• 1. Redox concentrations – Accumulation of Fe & Mn in 3 different structures

– Nodules & Concretions (firm extremely firm irregularly shaped bodies with diffuse boundaries)

– Masses – formerly called ‘reddish mottles’

– Pore linings – Formerly included ‘oxidized rhizospheres’

• 2. Redox depletions – Low-chroma (<2) bodies with high values (>4) including:

– Iron depletions: “Gray mottles” or “Gley mottles”

– Clay depletions: Contain less Fe, Mn, and clay than adjacent soils

• 3. Reduced matrices: Low-chroma soils

• REDOX in Wetlands

• When mineral or organic soils are flooded anaerobic conditions result. Water fills pore spaces and rate of oxygen diffusion through soil is drastically reduced

• Rate of oxygen depletion depends on:

– Ambient Temperature

– Availability of organic substrates for microbial respiration

– Chemical oxygen demand from reductants such as ferrous Fe

• Resultant O2 deficiency prevents plants from normal aerobic root respiration and affect nutrient availability and adds toxic materials in the soil

• Usually thin layer formed and is related to:

– Rate of O2 transport across the atmosphere-surface water interface

– Small population of O2 consuming organisms

– Photosynthetic O2 production by algae within water column

– Surface mixing by convection currents & wind action

• Eh ranges

• If DO is present, the redox potential range +400 to + 700mV

• If O2 disappears, Eh range from +400 down to -400mV

• As organic substrates in a water logged soil are oxidized (donating) the redox potential drops = a sequence of reductions (gains) takes place

• 1st reaction to occur after becoming anaerobic is the reduction of NO3- (nitrate) first to NO2-

(nitrite) and ultimately to N2O or N2

– Nitrate becomes an acceptor ~ 250mV

– At 225mV Mn is transformed from manganic to manganous

– At -75 to -150mV Fe is transformed from ferric to ferrous; while sulfates are reduced to sulfides

• pH

• Soil and overlying waters of wetlands occur over a wide range of pH’s

– Organic soils – often more acidic

– Mineral soils – often neutral or alkaline

• Alkaline soils previously drained decrease in pH because of buildup of CO2 then carbonic acid

• Acid soils previously drained increase in pH because of reduction of ferric iron hydroxide

• Nitrogen Transformations

• Nitrogen is often the “most limiting nutrient in flooded soils”

– Limitations reported in salt marshes, freshwater inland marshes, & freshwater tidal marshes

• Involve complex microbial processes

• NH4 is the primary form of mineralized N in wetlands

• Mineralization

• Often referred to as ammonification

NH2CONH2 + H2O 2NH3 + CO2

NH3 + H2O NH4 + OH-

• Nitrification

• Once ammonia has formed, it can take several possible pathways

– Aerobic environment – ammonium is oxidized (nitrification) in two steps by Nitrosomonas

sp.

2NH4+ + 3O2 2NO2

- + 2H2O + 4H

+ + energy

– By Nitrobacter sp.

2NO2- + O2 2NO3

- + energy

• Denitrification

• Denitrification is carried out by facultative bacteria under anaerobic conditions – nitrate is the terminal electron acceptor

C6H12O6 + 4NO3 6CO2 + 6H2O + 2N2

• Most significant path of nitrogen loss from wetlands

• Usually lost as N2 & N2O

• N fixation

• Conversion of N2 gas to organic nitrogen

– Favored by low oxygen concentrations

– Rhizobium species

– Cyanobacteria (blue-green algae) are also common in Louisiana, northern bogs, & in rice cultures

– Fe & Mn Transformations

• Found in reduced forms in wetlands

• Readily available Toxic levels

• More soluble

– Ferrous Fe reduced form of Fe

– Ferric Fe oxidized form of Fe

• Oxidized form creates barrier; may prevent plant from uptaking other nutrients

• Reduced form reacts with P making it unavailable

• Sulfur

• Rarely limiting to plant or animal growth in wetlands

– Hydrogen sulfide (H2S) is toxic (rotten-egg smell)

– 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

– Direct toxicity of free sulfide as it comes into contact with plant roots

– Reduced availability of sulfur for plant growth because of its precipitation with trace metals

– Immobilization of zinc & copper by sulfide precipitation

• Carbon Cycle

• Photosynthesis & aerobic respiration dominate the aerobic horizons

• Fermentation

• Methanogenesis

• Occurs when certain bacteria (methanogens) use CO2 as an electron acceptor for the production of gaseous methane (CH4)

• Requires extremely reduced conditions

– Redox potential range from -250 to -350mV

• Phosphorus

• One of the most important chemicals in wetland systems

• Most limiting in northern bogs, freshwater marshes, & southern deepwater swamps

• Retention is one of the most important features for natural & constructed wetlands

• Principle inorganic form = orthophosphate

– PO43-

(pH > 13)

– H2PO4- (pH 2-7)

– HPO4—

(pH 8-12)

• Predominant form dependent on pH

• P is not directly altered by Eh changes

– Affected by association with other elements especially Fe

• P is rendered relatively unavailable to plants & microconsumers by:

– 1. Precipitation of insoluble phosphates with ferric Fe, Ca, & Al under aerobic conditions

– 2. Adsorption (particle surface) of phosphate onto clay particles, organic peat and ferric & aluminum hydroxides & oxides

– 3. Binding of phosphorous in OM as a result of its incorporation into the living biomass of bacteria, algae, & vascular macrophytes

• Chemical Transport into Wetlands

• Geologic inputs – from weathering of rock

– Type of soil direct reflection of parent material

• Biologic inputs – Photosynthetic uptake of carbon, nitrogen fixation, & biotic transport of materials by animals (birds)

• Hydrologic inputs – Major inputs into wetlands

• Precipitation

– Burning of fossil fuels

– Increased [ ]’s of sulfates & nitrates in atmosphere

• Streams, Rivers, & Groundwater

– As precip reaches the ground it will:

– Infiltrate into the ground

– Return to atmosphere via evapotranspiration

– Flow on surface as runoff

• Groundwater influence:

– 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:

– Balance of precipitation & evapotranspiration

– Type of vegetation present

• Geographic effects: amount of dissolved & suspended materials that enter streams, rivers, & wetlands depend on:

– Size of watershed

– Steepness or slop of landscape

– Soil texture

• Streamflow/Ecosystem effects: The water quality of surface water runoff, streams, & rivers varies seasonally

• 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

• Drainage from agriculture fields:

– Higher [ ]’s of sediments, nutrients, herbicides, & pesticides might be expected

• Drainage from urban & suburban areas:

– May have high [ ]’s of trace organics, oxygen demanding substances, & some toxic materials

• Estuaries

• Quality differs from that of rivers

• Seawater chemical composition is fairly constant worldwide

– 33 0/00 to 37 0/00

– Mass balances

• A quantitative account of the inputs, outputs, & internal cycling of materials in an ecosystem

• Mass balances help determine:

– Ecosystem functions

– Determine the importance of wetlands as sources, sinks, and transformers of chemicals

• Inputs primarily through:

– Hydrologic

• Precipitation

• Surface & groundwater inflow

• Tidal Exchange

– Biotic

• Atmospheric carbon fixation (photosynthesis)

• Atmospheric nitrogen (nitrogen fixation)

• Exports:

– Surface water & groundwater

– 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:

– Litter production

– Remineralization

– Chemical transformations

– Translocation of nutrients through plants

• Generalizations

• 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

• 2. Seasonal patterns of nutrient uptake & release

– Temperate regions retention is higher in growing season

• Increased microbial activity

• Higher macrophyte activity

• 3. Wetlands are frequently coupled to adjacent ecosystems through chemical exchanges that affect both systems

– Downstream ecosystems benefit from retention or from exportation

• 4. Wetlands are either highly productive (eutrophic) or low productivity (oligotrophic)

• 5. Nutrient cycling in wetlands differs from aquatic & terrestrial systems

– More nutrients in sediment & peat

– Aquatic systems have autotrophic activity more dependent on nutrients in water column than in sediments.

– Wetland plants obtain nutrients from sediment

• 6. Anthropogenic changes have led to changes in nutrient cycling in many wetlands

• The capacity of wetlands to assimilate anthropogenic wastes from the atmosphere or hydrosphere is not unlimited!