Palaemonetes

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Palaemonetes glass shrimp
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Boundary Habitats

• Estuaries

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Questions

• What are estuaries?
• Physical parameters
• Biological classification
• What are patterns of mixing in estuaries?
• What are the challenges of estuaries?
• How are estuaries and ocean systems linked?

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Estuary refers to a physical condition

• A partially enclosed coastal area where fresh and
  sea water mix
• Mixing creates high variability in salinity
• Variable salinity is unusual in most other ocean
  systems
• Definition excludes inland, saline waters
• Geological derivation leads to finer
  classification of estuaries

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Drowned river valleys(Coastal plain estuaries)

• Formed after the last ice age to 3000 years bp
  (before present)
• Usually in temperate zone, with low sediment
  loads
• Process of sedimentation is slower than
  inundation (otherwise one finds deltas)

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A drowned river valley estuary
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coastal plain estuary
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Bar-build Estuaries

• Shallow basins, with barrier islands inlets
• Sedimentation
• inundation
• May be shallow with extensive lagoons, marshes

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Fjords

• Once covered by ice sheets
• Glaciers cut typical U shaped valleys rock
  sills at mouths
• Norway, Canada, Chile, New Zealand

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Tectonic estuaries

• Created by faulting and land subsidence

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Patterns of mixing for fresh and sea water

• Mechanisms influencing pattern
• Fresh water has lower density floats on sea
  water if turbulence is low
• River flow is out, tidal flux is both out and in
• Mixing patterns are sensitive to flow rates
• Salinity can be highly variable in space and time

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Fresh and salt water mixing regimes
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Positive or Salt-wedge estuaries

• High flow of fresh water from river
• Fresh water flows outward, over deeper salt water
• Wedge of salt water changes position with tides
• Moderately stratified
• Also called river-dominated estuaries

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Marine-dominated estuaries

• Very low flow of fresh water
• Promotes more complete vertical mixing
• Tidal flux, winds, and shape of basin influence
  mixing

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Negative/evaporite estuaries

• In hot climates, high evaporation is combined
  with low fresh-water flow
• Creates hypersaline estuaries (higher salinity
  than surrounding sea water)

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Fluctuations in salinity in the water column are
buffered in sediments
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Other important physical parameters

• Turbidity
• Sediment load delivered by river
• Dynamics of settling and resuspension
• Temperature, which can be highly variable in
  shallow water
• Oxygen levels, also more variable than in typical
  ocean habitats

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Interactions among physical parameters
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Characteristics of biological communities Marsh
habitats

• Generally, high density but low diversity
• Physiological stresses
• Low topographic substrate variability
• But very high potential for productivity due to
  nutrient input

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Components

• Macrophytes (seagrasses, sedges, rushes,
  cordgrasses)
• Epiphytic algae (macro- and micro-)
• Benthic macroalgae and microalgae
• Phytoplankton

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Estuary edge zonation in marshes