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Chapter 12
Estuaries: Where Rivers
Meet the Sea
Estuary
• Semi-enclosed areas where fresh water
and seawater meet and mix
• Close interaction between land and sea
• Among the most productive
environments on earth
• Among the environments most effected
by humans
• Many cites are located along them (NY,
London, Tokyo)
Environmental Impacts
• Dredged or filled and transformed into
marinas, seaports, industrial parks,
cities and garbage dumps
Origins and Types of
Estuaries
• Scattered along the shores of all the
oceans and vary widely in origin, type
and size
• May be called lagoons, bays
• Drowned river valleys or coastal plain
estuaries (formed when the sea
invaded lowlands and river mouths)
• Bar built estuary – formed from the
accumulation of sediments that build
up and create sand bars and barrier
islands dividing fresh and salt water
• Tectonic estuaries – formed when the
land sank – San Fransicso Bay
• Fjords – created from deep cuts by
glaciers that later filled in by rivers
• Passive margins have broad well
developed estuaries
Physical
Characteristics
of Estuaries
• Have a unique combination of physical
and chemical characteristics from the
mixing of salt and freshwater
Salinity
• Fluctuates dramatically from place to
place and time to time
• Seawater 35 o/oo and freshwater 0 o/oo
so salinity is somewhere in between
• Decreases as you move upstream
• Varies with depth – salinity increases
as you go down because of density
• Salt wedge – seawater is more dense
and flows along the bottom with
fresher water on top – moves back and
forth with the tides
Substrate
• Sand to soft mud
• Particles are carried in by the river and
settle out as the river slows
• Mud (mix of silt and clay) is rich in
organic material
• Sediments are often anoxic – decay
bacteria use up the oxygen in the
interstitial water
Other Physical Factors
• Water temperature varies because of
shallow depths and surface area
• Large amounts of suspended
sediments are typical – reduces water
clarity – reduces light
Estuaries as
Ecosystems
• May look like a wasteland
• Really are tremendously productive
• Home to large numbers of organisms
that are of commercial importance
• Provide vital breeding and feeding
grounds
Living in an
Estuary
• Live revolves around the need to adapt
to extremes in salinity, temperature and
other physical factors
• Change rapidly and in many ways
• Life is not easy so few species have
successfully adapted to estuarine
conditions
Coping with Salinity
• Maintaining the proper salt and water
balance of cells and body fluids is one
of the greatest challenges facing
estuarine organisms
In General
• Marine fish – have a lower
concentration of salts in their blood as
compared to the seawater – constantly
drink water – secrete small volume of
very concentrated urine – salt also
excreted by gills
• Freshwater fish – concentration of salt
in their blood in greater than the
surrounding water so they constantly
gain water through osmosis over their
skin and gills
• Do not drink, secrete large volume of
dilute urine, salts are absorbed by gills
Osmosis in an estuary
• Most estuarine organisms are marine
species that have developed the ability
to tolerate low salinities
• Euryhaline – organisms that can
tolerate a wide range of salinities –
most estuarine organisms are
• Stenohaline – tolerate a narrow range
in salinity
Brackish
• Water is intermediate salinity
• Some organisms are adapted to this
type of environment
Dealing with Osmosis
• Since most estuarine organisms are
from a marine background they tend to
take on water through osmosis
• Move
• Osmoconformers – body fluids change
with the salinity (mollusks and some
polychaete worms)
• Osmoregulators – keep the salt
concentration of their body fluids more
or less constant
• Salinity lower than blood – get rid of
excess water through active transport
• Fishes, crabs and some mollusks
Adapting to Mud
• Nothing to hold on to
• Organisms must burrow or
live in permanent tubes
beneath the sediments
surface
• Hard to move in mud –
stationary or slow moving
• Salinity does not change as
much
• Oxygen concentrations are low from
the decay
• Pump oxygen rich water
• Blood that contains hemoglobin that
holds oxygen
Types of
Estuarine
Communities
• Consist of few species
• Species present are in large numbers
Open Water
• Type and abundance of plankton varies
tremendously with the currents, salinity
and temperature
• Murky water also limits photosynthesis
• Rich supply of fish and shellfish in or
near the estuary
• Nurseries for young – abundant food
Mudflats
• Bottoms of estuaries that become
exposed at low tide
• Can be extensive where there is a large
tidal range and a gently sloping bottom
Low tide in a mudflat
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Desiccation
Variations in temperature
Predation
Variations in salinity
Mudflat organisms
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Primary producers are not present
Benthic diatoms
Bacteria
Infauna – feed on detritius – deposit
and suspension feeders
• Bivalves (quahog - Mercinaria
mercenaria and soft shelled clam Mya
arenaria and razor clams Ensis), fiddler
crabs
Mya arenaria
Razor Clam
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Fishes and birds – important predators
Fishes – high tide
Birds – low tide
Important stopover and wintering areas for
many migratory birds
Salt Marshes
• Estuaries in temperate and subartic
regions usually bordered by extensive
grassy areas that extend inland from
the mudflats
• Also develop along sheltered open
coasts
• Partially flooded at high tide
• Also known as tidal marshes
• Wave action is minimal to allows the
accumulation of muddy sediments
• Tidal creeks, freshwater streams and
shallow pools frequently cut through
the marsh
• Have extremes in salinity, temperature
and tides like the mudflats
• Muddy bottom held together by the
roots
Plants of the Salt Marsh
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Hardy grasses dominate
Salt tolerant land plants
Pronounced zonation of plants
Cord grasses (Spartina alterniflora) –
occupy the fringe above the mean lowtide level
• Another cord grass (S. patens)
• Rushes and pickle weed
Spartina alterniflora
S. patens
Pickle Weed
Zonation results from:
• Effects of flooding
• Competition for space
• Increased soil salinity in warm areas
due to evaporation
• Effect of burrowing animals
Animals of the Salt Marsh
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Decay bacteria
Diatoms
Thick mats of filamentous green algae
Cyanobacteria
Nematodes
Small crustaceans
Larvae of land insects
Fiddler crabs
• Coffee bean snails (Melampus)
• Marsh periwinkles (Littorina, Littoraria)
• Ribbed or horse mussel (Geukensia
demissa)
• Killifish and juvenile silversides
Melampus
Mangrove Forests
• Tropical equivalents of salt marshes
• Mangroves – flowering land plants
adapted to live in the intertidal
• Mangel – dense forest of trees and
shrubs
• Typical in tropical and subtropical
areas
• Grow on protected coasts where
muddy sediments accumulate
• Mangroves must get rid of
salts from the water
• Salt glands on the leaves of
some species excrete salts
• Prop roots which branch
downward and support the
tree-like stilts
Mangrove Organisms
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Crabs
Mudskippers
Sponges
Polychaetes
Mud shrimps
Clams
Birds make their homes in the
mangrove branches
• Food on fishes, crabs and other prey
• Snakes, frogs, lizards, bats and other land
animals also live in the mangrove trees
Feeding
Interactions
among Estuarine
Organisms
Estuaries have high primary
production because:
1. Nutrients brought in by the tide and
rivers, together with those generated
by nitrogen-fixing organisms and the
decomposition of detritus are used by
plants, algae and bacteria the primary
producers
2. Nutrient trap in deep estuaries
3. Surplus detritus is exported to the
open and neighboring ecosystems in
a process known as outwelling –
valuable source of food and nutrients
to other ecosystems
The End ……