Tidal salt marshes Chapter 9 Mitsch

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Tidal salt marshesChapter 9Mitsch Gosselink
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Basic properties of salt marshes

• Definition a halophytic grassland, alluvial
  sediments, bordering saline water bodies with
  variable water level (tidal or non-tidal)
• Salt marshes are generally considered
  outwelling systems
• - currents, migrations of transient species
• - trophic relays
• Highly productive, low diversity
• Many stresses and fluxes driving zonation patterns

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Geomorphology

• Extent of salt marsh related to
• tides
• sediment types
• freshwater inputs
• shoreline structure
• Gentle slopes allow for stability of vegetation
• Adequate protection needed from wave storm
  energy
• Sediments most from rivers, some from offshore
  reworked sediments

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Marine-dominated salt marshes

• Require shelter to ensure that sedimentation
  gt erosion
• Form at mouths of estuaries, behind spits bars
• Erosion at seawater end is balanced by sediment
  inputs at river/runoff end
• Balance between sediment accretion () erosion
  (-)
• tides bring in sediments to marsh surface
• sedimentation raises marsh above high tide level
• lowers tidal influence and sedimentation peat is
  oxidized used up
• marsh subsides
• back to tidal influence

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Biota have an impact

• Peat production builds up marshes
• Emergent plants slow waves
• Roots rhizomes stabilize sediments, slow
  erosion
• Surface films (algal / bacterial) trap fine
  particles on sediment surface
• Organisms slow flow, control deposition (e.g.
  oyster reefs slow flow, fiddler crabs change
  sediment permeability)
• Waterfowl, mammals feed cause erosion as they
  dig up or denude areas (eat outs)

oyster reefs
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Gulf of Mexico marshes

• Strong influence from the Mississippi River
• Mississippi has been channelized, carrying
  sediments deep far offshore
• Marshes are starved of sediments, sedimentation lt
  erosion
• Marsh subsidence has taken over

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Gulf of Mexico marshes

• Sequence of normal (natural) events
• - new lobe of Mississippi River forms, brings
  sediments
• - freshwater/brackish marsh forms
• - river moves away and marsh is inundated with
  seawater
• - plants are stressed, no new sediments are
  supplied
• - marsh subsides and converts back to open ocean

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Salt marsh hydrology

• Tides influence marsh species productivity
• sediment deposition
• scouring
• mineral organic influx efflux
• flush out toxins
• control sediment redox potential (add O2)
• Tidal range sets limits of lower upper marsh
• Low marsh, intertidal flooded daily
• High marsh irregularly flooded (often only on
  spring tides

wrack buildup
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Tidal creeks

• Channelize water flow
• Salinity levels similar to bay or estuary
• Stable structure due to bidirectional flow
• Creeks fill in as marsh matures and increases in
  elevation

marsh edges are important sites of trophic
relay
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Ecosystem function of salt marshes

• High annual gross net primary production (NPP)
• Benthic algae can have equal or higher
  productivity than macrophytes
• Grazing is low on macrophytes, high on benthic
  algae
• Detritivores fungi and bacteria (major impacts
  on energy flow!)
• Salt marshes can be either sources OR sinks for N

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Zonation patterns marsh buildup

• Tall-form Spartina alterniflora at tidal creek
  edges
• Short-form S. alterniflora away from tidal creeks
• Zonation patterns relative to creek bank
• affected by sediment, nutrient inputs
• affected by frequency, duration of inundation
• Salt pans where drainage is low evaporation is
  high