Transcript estuary - Teaching Web Server

Pei-Yuan Qian
Chapter 8: 328-369
Outline of Lecture
 Definition
 Classification of estuaries
 Physical characteristics of estuary
environment
 Animal adaptation
 Food-web of estuaries
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
Pearl River Estuary
Rivers
Discharge
(m3 s-1)
Rivers
Discharge
(m3 s-1)
Amazon
175,000
Tocantins
11,000
Congo
39,640
Aur
10,300
Orinoco
33,950
MacKenzie
9,710
Yangtze Kiang
22,000
Columbia
7,960
Brahmaputra
19,200
Indus
7,550
Mississippi
17,800
Magdalena
7,500
Yenissei
17,800
Zambezi
7,080
Lena
16,300
Danube
6,530
Parana
14,900
Yukon
6,220
Mekong
14,900
Niger
6,090
St. Lawrence
14,160
Uruguay
5,500
Irrawady
13,560
Ogoove
4,710
Pearl River
12,500
Huang Ho
3,900
Ob
12,200
Sepik
3,800
Ganges
11,600
Frazer
3,540
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
Classification of estuary
Based on the origination
1. Coastal plain estuary
2. Tectonic estuary
3. Semienclosed bay or
lagoon
4. Fjord
Classification of estuary
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.
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.
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
Fjord
Fjord -- Valleys
have been
deepened by
glacial action and
are then invaded
by the sea. Very
common in British
Columbia.
The general pattern of fw and
sw mixing in an estuary
Freshwater
from river
Seawater from ocean
Well-mixed estuary
Stratified estuary
Typical estuary
Salinity changes in estuary
High Tide
30
or
25
20
15
5
River Flow
Increase
or
Low Tide
20
10
15
10
River Flow
Decrease
5
0
River
Estuarine Plume
Salt Wedge
Oceanic Water
Flood-HHW
Estuarine Front
Riverine Front
Estuarine Plume
Riverine Plume
Entrainment
Nutrient-rich water
Ebb-LLW
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.
Classification of estuary
FW input>E
FW input<E
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
This is the riverine front (color front) between the riverine plume and seawater.
Riverine
front
Salt water
Riverine plume
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
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

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.
A summary of physical-biological
coupling processes in an estuary
Nutrients (N, Si, DOM)
Salinity
Chl a
Turbidity
Light penetration
Estuarine Plume
River Outflow
Salt Wedge
Oceanic Water
Features of faunal composition in estuary
Freshwater
species
Marine
species
Transitional
Euryhaline
marine
species
Stenohaline
species
Brackish
water
species
0
5
10
15
20
Salinity
25
30
35
Features of faunal composition
Stenohaline marine animals, usually restricted to the
mouth of estuaries, salinity > 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 > 5, are
restricted to upper reaches of estuaries
Transitional species: migratory fishes that pass through
the estuary
Faunal composition
 Marine – the largest in terms of numbers of
species, restricted to the mouths in salinity >25
 Freshwater – restricted to the upper reaches in
salinity <5
 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
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)
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.
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.
Adaptation in distribution pattern(con’t)
 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.
Adaptation in distribution pattern(con’t)
 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.
Adaptation in distribution pattern(con’t)
 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
Animal’s adaptation
Nutrient inputs
 River input--freshwater drainage delivers large
amounts of nutrients in dissolved and particulate form
 Oceanic import --Dissolved nutrients arrive with deepwater 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.
Foodweb of
estuary
Food-web of a typical estuary
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.