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Chapter 12
Estuaries
Types of Estuaries
• Drowned river valleys
– Most common type of estuary
– They were formed by the “drowning” of low land
around the mouths of rivers when sea level rose after
the last ice age
– Ex: Chesapeake Bay
Types of Estuaries
• Bar built estuary– Built by the
accumulation of
sediments into sand
bars or barrier islands
– Ex: North Carolina
barrier islands (seen in
lower part of the
photo to the right near
Cape Hatteras)
Types of Estuaries
• Tectonic estuaries –
– Formed as a result of land sinking due
to movements of the Earth’s crust
– Ex: San Francisco Bay
Types of Estuaries
• Fjords –
– Deep channels
cut in the coastal
zone as a result
of retreating
glaciers
– Ex: Alaska,
Norway
Physical Characteristics of Estuaries
• Salinity-
– Can vary from 5 – 30
ppt
– Salinity varies
according to distance
from saltwater (tides)
or freshwater (river)
input
– Can also vary as a
result of storms
– Depth also contributes
to salinity
– The diagram to the
right illustrates that
the salinity is not
uniform (saltwater is
heavier and sinks
below freshwater) –
this is known as a salt
wedge
Dealing with Salinity Changes
• Organisms in the estuaries are normally
________ (can tolerate a wide variety of
salinities)
• Some are osmoregulators that have
mechanisms for keeping their internal
concentration stable
• Others are osmoconformers with internal
concentrations that vary with their
surroundings
Types of species living in estuaries in relation to salinity
Substrate
• Mostly composed of sand or mud
• Mud is difficult to move through
• Mud or sand can also shift unlike hard
substrate which is a challenge for
organisms as well
• Particle sizes are so small that most areas
are actually anoxic (devoid of oxygen)
Communities within an Estuary
• Open water in bays
•
•
•
and tidal creeks
Predominant organisms
include phyto- and
zooplankton, floating
macroalgae, fish and
many larval forms
(crabs, shrimp, molluscs,
and fish)
At the right, this
diagram shows the
extreme importance of
estuaries as a “nursery”
area for larval species
Many marine species
spend at least a portion
of their life in an
estuary, mainly as
larvae.
Communities within an Estuary
• Saltmarsh
• The vegetated
•
area of an estuary
Spartina species
predominate near
the water in most
estuaries.
Halophytes occur
in higher elevation
parts of the
marsh.
Aransas National Wildlife Refuge
Salt marsh on Texas Coast with
endangered whooping cranes that
depend on the marsh ecosystem.
Blue crab and halophytes are their
main food sources.
Representative Inhabitants in a
Mudflat of an Estuary
Epifauna includes
deposit feeding snails
like mudsnails and
Predators like shorebirds
and moon snails.
Infauna includes clams,
crustaceans like ghost
shrimp, and
annelids.
Meiofauna is low
due to anoxy.
Communities within an Estuary
• Mangroves or mangals
• Found in tropical and subtropical coastal areas
• Normally replace saltmarshes in these areas
Worldwide Distribution of
Saltmarshes and Mangrove
Forests
Generalized Food Webs in
Estuarine Ecosystems
Food webs for salt mashes and mangals
Aerial View of a Mangal
Communities within an Estuary
• In Texas, mangrove
forest estuaries are
found at Boca Chica,
where the Rio Grande
River enters
(sometimes) the Gulf
of Mexico. It is rich in
tropical marine life
but like many
estuaries and bays is
becoming more
saline.
Communities within an Estuary
• Seagrass beds can be found in some estuaries if
•
the water clarity permits
Water clarity must be better than the average
estuary to support this growth
Communities within an Estuary
• “Oyster reefs” are
•
•
found in many estuaries
These congregations of
oysters are the platform
on which dozens of
other species grow and
thrive
The oysters provide the
only hard substrate in
many estuaries