PeiYuan Qian

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Estuary Ecology

• Pei-Yuan Qian
• Chapter 8 328-369

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Outline of Lecture

• Definition
• Classification of estuaries
• Physical characteristics of estuary environment
• Animal adaptation
• Food-web of estuaries

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Definition

• Estuary is a semienclosed coastal body of water
  that has a free connection with the open sea and
  within which seawater is measurably diluted with
  freshwater. The decrease in salinity in an
  estuary precludes those species incapable of
  volume regulation. A critical salinity of 5 to
  8 precludes species incapable of extensive
  regulation of specific inorganic ion
  concentrations.
• Estuary is coastal bays in which freshwater and
  salt water mix, thus permanent impoundments

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Pearl River Estuary
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(No Transcript)
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Significance of estuaries

• Biologically productive, together with salt
  marshes and mud flat
• Nursery ground for fisheries
• Migration pathways for many important commercial
  fisheries
• A link for global geochemical cycling of elements
  between terrestrial water and oceans
• Gateways of inputs of both natural and
  anthropogenic substance (nutrients, metals, and
  contaminants)
• Entrance for riverine sediments from land to the
  sea
• Estuarine delta regions often become most
  populated cities in the world, Shanghai, San
  Francisco, New York

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Classification of estuary

• Based on the origination
• Coastal plain estuary
• Tectonic estuary
• Semienclosed bay or lagoon
• Fjord

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Classification of estuary
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Coastal plain estuaries on US east coast
Delaware Bay
Chesapeake Bay
Coastal plain estuary -- formed at the end of the
last ice age when the rising sea level invaded
low-lying coastal river valleys. Most common
estuaries, broad shallow, found along the
northern and central Atlantic coast, e.g.
Chesapeake Bay, Pearl River estuary in China.
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Tectonic Estuary
Tectonic estuary -- the subsidence of the land in
response to crustal movements allows the sea
water reinvading, e. g. the San Francisco Bay.
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Semienclosed bay or Lagoon
Semienclosed bay or lagoon Sand bars build up
parallel to the coastline and partially cut off
the waters behind them from the sea. Common in
USA, such as Matagorda and San Antonio Bays
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Fjord
Fjord -- Valleys have been deepened by glacial
action and are then invaded by the sea. Very
common in British Columbia.
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The general pattern of fw and sw mixing in an
estuary
Freshwater from river
Seawater from ocean
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Well-mixed estuary
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Stratified estuary
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Typical estuary
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Salinity changes in estuary
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High Tide
River Flow Decrease
or
River Flow Increase
or
Low Tide
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River
Estuarine Plume
Salt Wedge
Oceanic Water
Flood-HHW
Estuarine Front
Riverine Front
Riverine Plume
Estuarine Plume
Entrainment
Nutrient-rich water
Ebb-LLW
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Classification of estuary

• Based on physiograph
• Positive (highly stratified) estuary --
  freshwater flows downstream over a deeper layer
  of higher salinity ocean water. Highly
  stratified estuaries only exist where river flow
  strongly dominates over tidal motion.
• Negative estuary -- exist where tidal flow
  strongly dominates over river flow.
• Neutral estuary -- vertically homogeneous
  estuary--At low tide, the salinity is dominated
  by downstream river flow whereas at high tide the
  inrush of SW determines the salinity. Only
  estuaries of very small extent permit such
  domination by tidal motion.
• Seasonal estuary -- Seasonal variation in
  rainfall (or tropical storms and hurricanes)
  increase freshwater drainage and shift downstream
  lines demarking constant surface salinity
  (isohalines). During dry seasons isohalines
  shift upstream.

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Classification of estuary
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FW inputgtE
FW inputltE
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Physical processes in an estuary
Entrainment a process of vertical mixing between
two layers of water which flow at different
velocity in the water column. Riverine Plume a
surface layer of water which flows out of the
river channel into the estuary, usually distinct
in color (brown), and bounded by the riverine
front. Front an interface or zone between two
different water bodies which move toward each
other. Riverine Front a front where the river
outflow meets with saline water, usually distinct
in color, debris gathering line Salt Wedge A
body of saline water invading the river channel
underneath the freshwater flow Estuarine Plume
a surface layer of water which flows out of the
estuary into the open region of the coastal
ocean. Estuarine Front a front where the
estuarine plume meets with oceanic water. This
front is much wider than the riverine front
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This is the riverine front (color front) between
the riverine plume and seawater.
Riverine front
Salt water
Riverine plume
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Physical characteristics

• The freshwater derives from land drainage and
  tends to float as a low-density surface layer
  over denser seawater tidal mixing can reduce or
  obliterate this stratification.
• Large amount of nutrient input from land
• Salinity decreases toward land
• Restricted exchange allows rapid changes in
  salinity, temperature, nutrients, and sediment
  load
• Muddy substrata
• Depleted oxygen in substrata
• Limited wave action, highest current velocities
  occur in the middle of channels

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Effects of River outflow on coastal oceans

• Stabilization of the water column mixing
  increase the stability of water
  column(stratification mixing)
• Sedimentation turbidity due to suspended solids
  reduces light sedimentation affects coastal
  benthic communities
• Nutrients primary productivity high nutrient
  input leads to high productivity
• River delta formation or erosion
• Sedimentation sediment particles sinking to the
  bottom of the estuarine bed or coastal ocean
  bottom
• Hypoxia of near bottom water
• Muddy substrata
• High current velocities, different water masses
  (including different layers) flow in different
  directions or changing directions due to tidal
  cycles, river discharge and winds
• Pollutants

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Physical characteristics

• Variability in salinity due to seawater heavier
  than freshwater periodical tide events the
  Coriolis effect that causes a moving body to the
  deflected from a straight path.

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A summary of physical-biological coupling
processes in an estuary
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Features of faunal composition in estuary
Freshwater species
Marine species
Transitional
Stenohaline species
Number of species
Euryhaline marine species
Brackish water species
0
5
10
15
20
25
30
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Salinity
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Features of faunal composition
Stenohaline marine animals, usually restricted to
the mouth of estuaries, salinity gt 25, as they
are unable or barely able to tolerate salinity
changes. Euryhaline marine animals, more
tolerable to salinity change between 10-30, few
down to 5, and so can be found throughout the
range of estuarine salinities Brackish estuarine
species true estuarine animals found in salinity
between 5-18, not found in freshwater and not in
full seawater. Freshwater species can not
tolerate salinities gt 5, are restricted to upper
reaches of estuaries Transitional species
migratory fishes that pass through the estuary
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Faunal composition

• Marine the largest in terms of numbers of
  species, restricted to the mouths in salinity gt25
• Freshwater restricted to the upper reaches in
  salinity lt5
• Brackish water the middle reaches of the
  estuary in salinity between 5-18
• Limited in the number of large plants,
  seagrasses, macroalgae and diatoms can be common
• Diatoms are dominant phytoplanktons but
  composition and abundance varied with tubulence,
  turbidity, and flushing rate

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Terms

• Osmoregulators the organisms have physiological
  mechanisms to control the salt content of
  internal fluids (ture esturine species)
• Move water move ions
• Adjust internal water-ion balance
• Osmoconformers the organisms unable to regulate
  its internal fluid and salt balance, therefore
  ones with a varying internal salt concentration
  tolerate fluctuations in salinity without tissue
  damage (ture marine species)

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Biotic responses to the estuarine gradient

• Two transitions in estuaries
• The critical salinity This region encompasses
  an approximate salinity range of 5-8 and marks
  a pronounced minimum of benthic invertebrate
  species richness. A relatively rich fauna of
  bivalve mollusks and other invertebrates reside
  in freshwater. Freshwater species decrease,
  however, in numbers at a maximum salinity of
• The mouth and lower reaches of the estuary
  organisms must adjust physiologically to lowered
  salinity. Salinity can change from fresh to
  completely marine in these area. If salinity is
  tidally regulated, benthic organisms may
  experience fresh and saltwater in a single tidal
  cycle--more of a physiological challenge because
  of the time require for acclimation. Seasonal
  estuaries show seasonal shifts up- and
  down-estuary, but the rate of salinity change at
  any point is slow--permitting acclimation.

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Adaptation in distribution pattern

• Species richness generally diminishes steadily
  up-estuary and reaches a minimum at salinity of
  5-8. Species richness then increases again in
  freshwater.--Estuarine marine bivalves are also
  rare at this salinity but increase steadily in
  species richness with increasing salinity.
• The steady decrease in the estuary must be
  related to the steady reduction of species
  capable of extensive cell volume regulation.
• Changes in physiological adaptations
• Changes in body size and genetically determined
  morphological features. In many cases, the
  maximum size of bivalve mollusks decreases with
  decreasing salinity--may simply be related to the
  negative role of decreased salinity in growth.

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Adaptation in distribution pattern(cont)

• An increase in niche breadth as competitors
  disappear in the decreased salinity.
• The diminution of species richness in brackish
  water is accompanied by niche expansion of those
  species capable of invading the estuary or
  surviving in a brackish sea.
• A related phenomenon to niche expansion is the
  presence of enormous populations of a relatively
  few species--may be due to lack of competitors
  and the nutrient enrichment of estuaries. Large
  fluctuating populations of phytoplankton,
  invertebrates, and fishes dominate the
  estuary.--rich invertebrate fisheries
• Adaptations to avoid transport in the surface
  layer to the open sea. Estuarine flow results in
  a net transport of surface water to the open
  sea--dilute the larval populations of estuarine
  species. In many species, larvae are adapted to
  stay near the bottom during ebb tide and rise up
  into the water during the flood--counteract the
  seaward dilution effect.

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Adaptation in distribution pattern(cont)

• Genetic divergence from open marine conspecifics.
  Sharp differentiation (changes in gene
  frequencies) has been found over a short
  geographic space (from the mouths of estuaries to
  brackish water seas). Selection may act on
  variation at individual loci, but it is probable
  that estuarine populations have diverged broadly
  into genetically distinct races relative to their
  open marine conspecifics. Consequently,
  fine-scale adaptation to local estuarine
  conditions has resulted.

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Adaptation in distribution pattern(cont)

• Many fish species spawn offshore but spend some
  period feeding in estuaries (estuaries are
  therefore crucial to many species as nurseries.
  The abundance and fluctuating nature of the food
  supply have favored the following characteristics
  of the feeding ecology of juveniles of common
  marine fishes
• flexibility of feeding habits in time and space
• omnivore
• sharing a common pool of food resources among
  species
• exploitation of food chains at different levels
  by the same species
• ontogenetic changes in diet with rapid growth
• short food chains based on detritus-algal feeders

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Animals adaptation
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Nutrient inputs

• River input--freshwater drainage delivers large
  amounts of nutrients in dissolved and particulate
  form
• Oceanic import --Dissolved nutrients arrive with
  deep-water flow into estuaries. Vertical mixture
  with the surface water permits the phytoplankton
  to use this nutrient source benthic algae and
  sea grasses may also benefit.
• Regeneration from the bottom -- Bottom sediments
  consist of an active microbial community whose
  ability to decompose particulate organic matter
  permits the continuous recycling of nutrients
  between the bottom and the overlying water.

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Food-web of estuary
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Food-web of a typical estuary
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Characteristics of food webs in estuaries

• Highly dynamics--The extensive variation of size,
  nutrient input, and tidal exchange among
  estuaries suggests a large variation in the
  pattern of connections between primary and
  secondary production.
• Detritus is a major factor in the webs, and
  detritus pathways dominate webs.
• Detritivorous are the major consumers in many
  webs.
• Phytoplankton and seaweeds may still play
  important roles in the webs.
• Relative short food-chains.