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Types of Estuaries
• Drowned river valleys
Chapter 12
Estuaries
– Most common type of estuary
– They were formed by the “drowning”
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 estuaryestuary– Built by the
accumulation of
sediments into sand
bars or barrier islands
– Ex: North Carolina
(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’
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
– Puget sound is a
type of glacial
fjord
Development of Estuaries
• Over millions of years canadian
glaciers formed and retreated at
least 4 times
• 20,000 years ago, glacier 1 mile
deep
• After glaciers was a large lake
• Sea levels eventually rose and
filled the estuary
• http://exhibits.pacsci.org/puget_s
ound/graphics/ps_glaciationsm.
mov
•
Physical Characteristics of
Estuaries
SalinitySalinity– 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
• Estuaries are the best developed in areas
where the coastal plain is flat and the
continental shelf is wide. i.e. East Coast
US
• The opposite is true in areas with steep,
narrow continental shelves and coastal
plains, i.e., Washington, west coast US
Dealing with Salinity Changes
• Organisms in the estuaries are normally
euryhaline (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
Osmoregulation in fishes
• Ion transport, and urine production
Dealing with Salinity Changes
• Flowering plants in the estuarine
community must either expel excess
solutes (such as Spartina and mangrove
trees using salt glands) or concentrating
solutes in specific tissues (such as in
Salicornia,
Salicornia, a.k.a. “pickleweed”
pickleweed” or glasswort
Substrate
Water Temperature
• 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)
• Like in the intertidal community (and some
areas of estuaries are intertidal),
temperature can vary greatly.
• Temperatures can vary especially at times
when water levels are low.
Water Clarity
Representative Inhabitants in a
Mudflat of an Estuary
• Water clarity is very poor in estuaries
• This is due to suspended sediments and
particles from rivers in the water
• This reduced clarity makes it more difficult
for photoautotrophs in the water
• Most primary production is the result of
flowering plants in the salt marsh as
opposed to algae or plankton
Mudflat ecology
• Only the upper few mm of the sediment is oxygenated,
the rest is anoxic and rich in hydrogen sulfide
• Very little hard substrate which is preferred by many
organisms
• Burrowing organisms can oxygenate their environment
and also provide oxygen for other organisms that live in
their burrows
– Still can have problems when exposed at low tide – anoxia and
shorebirds
• Seagrasses can provide substrate and also oxygenate
the sediment
• Species diversity tends to be higher in subtidal
sediments that are sand, and lower in mud
http://www.youtube.com
/watch?v=HK6IdWTodV
4
http://www.youtube.com
/watch?v=0PZwkx-YSbs
Communities within an Estuary
• “Oyster reefs”
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
Communities within an Estuary
Zostera marina Eel grass
• Saltmarsh
• The vegetated
area of an estuary
• Spartina species
predominate near
the water in most
estuaries (Juncus
(Juncus
or needlerush can
be found near the
water in low
salinity estuaries <
10 ppt)
seagrasses
Organisms living in eelgrass
meadow
Eelgrass distribution in puget
sound – 33% reduced
•
•
•
•
•
•
lack of beds in southern Puget
Sound
attributed to a combination of
high tidal amplitudes and
timing of low tides during the
summer
During low tide events,
especially during hot summer
middays, desiccation/heat
stress limits the upper
distribution,
at high tides, enough water
covers the plants to limit net
photosynthesis at depth.
the limit of distribution
corresponds to the 10.1-foot
tidal amplitude isobar.
The problem is exacerbated
by the fact that the timing of
extreme low tides in southern
Puget Sound is in midday,
when temperatures are the
highest.
Additional factors affecting eelgrass
•
Competitors
– Introduced brown seaweed Sargassum muticum
– the sand dollar (Dendraster excentricus) disturbs substrate
– crabs can uproot eelgrass
•
Excessive nutrients
– algal species such as sea lettuce (Ulva spp.) will
– overgrow eelgrass
– overgrowth by epiphytes on the blades, blocking light, nutrients and gas
exchange.
•
Communities within an Estuary
• Mangroves or mangals
• Found in tropical and subtropical coastal areas
• Replace saltmarshes in these areas normally
Facilitation
– Several herbivores (the snail Lacuna spp. and the marine isopod Idotea
spp.) can control epiphyte density and thereby benefit the underlying
eelgrass
•
Direct herbivory
•
Storms can bury or kill eelgrass by sand overwash
– usually not significant.
– Black Brandt, a small seagoose, can eat large quantities
Worldwide Distribution of
Saltmarshes and Mangrove
Forests
Communities within an Estuary
• Some mangrove tree
species possess
pneumatophores, or
vertical root
extensions, that assist
the plant with the
exchange of gases
(pictured at right)
Aerial View of a Mangrove