• Coastal embayment where fresh and salt
water mix: connection of sea to fresh
water source at least part of the year
• Geomorphology, geologic history and
climate create differing chemical and
physical m conditions.
• dictate types of estuaries
Types of Estuaries
• Coastal plain – most common, rising SL
flooded river valleys [chesapeake, hudson]
• Tectonic – similar: sea invades subsiding
land [SF Bay]
• Lagoon - sandbars parallel coastline and
cut off embayment; salinity varies (river?
climate: evap/rainfall?) [NC, NL TX]
• Fjord – valley cut by glaciers then flooded
by sea, characteristic sill at mouth restricts
bottom water exchange [chile, scotland,
alaska, bc, hudson]
Salinity classification
• Gradient from FW to SW
• Density differences – FW < SW
• Shape, tides, rainfall:evap , river
discharge, affect FW-SW mixing
• Also seasonal changes in climate
Estuary Continuum
• Types form a continuum from
– little mixing (salt wedge), to
– moderate mixing, weak wedge (partially
mixed) to
– Fully mixed or homogenous, marine
dominated or neutral estuaries
– Negative (reversed salt wedge)
• Where on continuum depends on
– Mixing
– Tidal regime, basin geometry, river flow
– Seasonal variations in rainfall, wind regimes,
evap rate
Positive or Salt Wedge estuary
• Where FW input >>evap, FW moves
across the surface, mixing with SW, dec
salinity but leaving deep water unmixed
• Isohalines slant upstream at bottom
• Vertical profile: salinity always least at
• Horizontal –
decreasing upstream
Partially mixed and homogenous
• Partial – indistinct or variable salt wedge
• Homogenous - Complete mixing or where
evap rate = FW inflow
Negative or Evaporate Estuary
• Deserts, where FW input low, evap high,
• SW enters and mixes with limited FW.
Evap causes hypersalinity at surface
• Sinks, moves out as bottom current
• Isohalines slant opposite: downstream at
• Vertical profile reversed: salinity always
greatest at surface
• Horizontal – increased salinity upstream
Seasonal or Intermittent Estuary
• Where marked wet and dry seasons occur
• Wet – rainfall, open to sea
• Dry - little or no inflow, outlet often
• Salinity varies temporally not spatially
Physical Characteristics: Salinity
• Fluctuation dominant feature
• Gradient always occurs but varies w/tide,
basin topography, amt of freshwater
• Affects water column salinity much more
than interstitial water
• Tide – isohalines displaced up and down
stream, region with max salinity
• Coriolis effect – No. hemisphere, deflects
outflow of FW to right looking down a N-S
oriented estuary; SW flowing in deflected
to right looking up estuary from sea
• Seasonal effect - Change in evaporation or
FW inflow or both. Change in FW moves
salt wedge down or upstream
• Flushing time – water entry and exit: amt
of time for a given mass of FW to be
• Net depositional environment (dredging)
• Highly variable, most soft and muddy
• Depends on geology and recent sediment
transport (eg. fjord)
• Suspended particles in FW mix with SW,
ions cause flocculation and settling
• SW: estuary is sheltered, less energy,
suspended particles settle out
• Currents and particle size:
– larger settle out faster than smaller,
– currents=energy: more keeps larger particles
• SW and FW drop coarse particles first: coarse
sediments at mouth and upper reaches
Mixing zone with finest mud
Terrestrial and marine organic material: food
Fine particles high surface:volume ratio bacterial
Catastrophic events important
– deposition and removal of sediment
– Permanent alteration of volume, topography
– Prolonged salinity change
• Smaller volume, large surface: heats, cools more
rapidly (not fjords)
Surface waters most variable
FW inflow – FW more temperature variable than
– Estuary colder in winter and warmer in summer than
nearby sea
– Tidal change – vary temperature between river and
sea temp range
– Mid estuary greatest tidal temp effect
– Annual temp. variation least at mouth, increases up
estuary to max at head
• Limited fetch and shallow depth limits size
of potential waves;
• Narrow mouth and shallows dissipate sea
• Calm promotes sediment deposition and
rooted SAV
• tides and river flow,
limited to channels
Velocity highest in middle
of channel where friction
least, and where flows
Flow regimes control
sediment and larval
High velocity areas –
erosion, not deposition;
high larval recruitment,
high productivity
High flows = flux of food
for filter feeders, inc. gas
• Particles in suspension, max at mouth, at
time of max river inflow, decreases down
estuary, lowest at mouth
• Phytoplankton concentration and wind
speed are factors in lagoon systems
• Ecol effect - reduce light penetration,
reducing primary production Severe –
primary production by emergent plants
Down estuary:
Turbidity decline
Nutrients still elevated
Algal bloom
• FW, SW influx, mixing – usually sufficient
• Hypoxia -summer thermocline and
vertical salinity stratification, little vertical
• Isolation of deep water, plus high organic
loading, long flushing times may lead to
hypoxia, anoxia
• Substrate also low oxygen – organics plus
high bacterial numbers, fine particles, low
exchange rate – anoxic (also fertilizer)
• Key - Bioworking by Callianassa,
Balanoglossus oxygenates sediment
Substrate also low oxygen – organics plus high
bacterial numbers, fine particles, low exchange
rate – anoxic
Key - Bioworking oxygenates sediment
• Marine – most species; stenohaline (>25
psu) and euryhaline (15-30 psu)
• Brackish – 5-18 psu, mid region only; both
physical and biotic factors limit distribution
• Freshwater - < 5 psu, upper only
• Transitional –
– Migratory fishes (salmon, eels)
– Part of life in estuary (penaeid shrimp)
– Feeding only - bull sharks, birds
• Fewer species than FW or SW
• Origin marine, not FW – like other
transitional zones: Intertidal fauna origin
marine, not terrestrial
• No true estuarine species, low species
• Why? Theories:
– Extreme salinity range difficult to adapt to
– Estuaries are “young” environments
– Both??
• Subtidal - Limited by substrate availability,
– Sea grasses
• L imited green algae
• Intertidal
• Mud flats
– abundant benthic diatoms, blue green algae
• Emergent – salt marshes, mangroves
Morphological adaptation
Highly variable oxygen, temperature, salinity
• Burrowing – setae stop silt clogging
• Fish - Smaller body size
• Plants –
– Aerenchyma - anoxia
– salt glands – excess salt
– root carbohydrate stores – energy
– “succulance strategy” – buffer water loss form
– Small leaves, few stomata, photosyn stems
Reduce water loss
Physiological adaptation
• Maintain ionic balance when salinity
• Marine - most osmoconformers, internal
salt conc. > estuarine envt. ; barrier
• Estuarine – osmoregulators, function with
varying internal salt conc., barriers to
• Osmoregulators
– move water
– Move ions
– Adjust internal water-ion balance
• Burrowing – less change, buffered from
salinity and temp change
• Osmoregulatroy adults but vulnerable
larva – reproduce in or migrate to SW
• Burrowing and ability to tolerate low
salinity- predator avoidance
• Adaptable larvae- high nutrient sources up
Ecology of estuaries
• Internal primary production not high
• Role of primary production reduced: few
• Sink for primary production elsewhere –
terrestrial, salt marsh
• Detritus carbon system
• European type - – large mud flats, little
Large benthic, plankton diatom primary
Energy from outside (allocthonous) – sea or
river source
Support large populations because they are
effective detritus sinks
Net energy receivers
• American estuary – dominated by extensive
emergent vegetation
• Huge marsh productivity (~6850 kcal/m²/yr
vs diatoms - ~1600 kcal/m²/yr
• Excess carbon producer –
Detritus based food web
• Organic particles, bacteria, protozoa, algae
• Estuary water – 110 mg dry organic mater
per liter vs 1-3 open ocean
• Bottom up - salt marsh plant detritus
production controlled by physical factors
• Top down – consumers control production
• Sea grass contribution, nutrients – human
• Fertilizer use, coastal development (loss of
buffers), organic wastes
• Promotes macro algae growth, loss of
other productivity
• Excess phytoplankton growth - stops
light transmission, loss of sea grass
Structure and salinity
• Horizontal banding – assume physical
control but untested
• Plant communities distribution – each
does best in own salinity
• Research - All marsh plants do better in
FW, but salt marsh plants poor
competitors (comp exclusion, salt
adaptation a “refuge “)
• Obstructions – accelerate flow, increase flux of
larvae to site, influx of particles for filter
feeders, increase efficiency of gas exchange
on leaves
• Increases photosynthesis and metabolic rates
of vascular plants, algae
• Decreases importance of consumers
• Predators ineffective – hard to move, poor
olfactory cues
• High flow rates – less deposition, coarse
substrate, high density of organisms with
fast growth rates.
• Oyster beds, mussels, sea grass
• Low flow – low larvae, low food
availability, low gas exchange, more
effective predators
• Maine –
– high flow= mussel beds,
– low flow = unpalatable algae canopy, bare
Food webs
• primarily detritus based ? – low water
column productivity, few herbivores, large
amts of detritus
• Small detritus consumed by suspension
feeders, deposit feeders (size selected)
Both consumed by predators –
• Invertebrates: polychaetes, blue crabs,
Busycon whelks - keystone
• Fish and birds – consume detritus feeders
and predators
• Trophic relay – move estuarine production
Consumer control
• Fish – specialize on prey type and size,
and specialize with age
• Shore Birds – consume huge numbers of
prey (4-20% of invertebrate production)
• Shore bird predation keeps benthic density