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Chapter 13
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Chapter 13
Life on the Continental Shelf
Life on the Continental Shelf
• The continental shelf is
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the submerged edge of a
continental plate.
The shelf extends from
the low tide line to an
area known as the shelf
break.
The slope of the
continental shelf is
normally more gradual,
whereas, the slope past
the shelf break is
normally very steep.
Life on the Continental Shelf
• The slope of the shelf, the depth of the water at
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•
the shelf break and the distance of the shelf
break from shore varies greatly by location
(remember active versus passive margins).
Remember from chapter 12 that estuaries are
much more well developed in areas of the world
where the continental shelf has a gradual slope
and is very wide.
This is the case on the East Coast of the US,
which is the reason for the well developed
estuaries there.
Life on the Continental Shelf
• The depth of water at the shelf break
varies greatly with an average depth of
around 490 feet.
• The width of the continental shelf also
varies greatly from 0.6 miles to 470 miles
with an average of 48 miles.
Life on the Continental Shelf
• The continental shelf is biologically the richest
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part of the ocean (more species present).
This is also an area of potential natural
resources such as oil and natural gas.
For this reason, countries wanted a way to
protect these resources from exploitation by
other countries.
In the 1950’s, the United Nations agreed that
each country shall have control over the marine
resources out to 200 nautical miles off their
coast – this includes the continental shelf and its
resources.
Life on the Continental Shelf
• A major portion of the fisheries global
catch is caught over the continental shelf.
• The shelf is an important area for larvae
and juveniles of many species as well as
adults.
• A wide variety of organisms can be found
on/in the continental shelf. Some of the
most common includes sponges,
cnidarians, worms, molluscs, fish and
echinoderms.
Life on the Continental Shelf
• Temperature:
– Temperature of the water over the
continental shelf varies much more than the
water in the deep sea (but less than in an
estuary).
– The temperature fluctuates more because the
water is shallower than in the deep sea and
because waves and currents keep the water
circulating. This means that warmer surface
waters are mixing readily with the rest of the
water column.
Life on the Continental Shelf
• Water turbulence created by waves and currents
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keep the water column well mixed.
This affects not only the temperature of the
water, but also affects the nutrients available in
the water.
Nutrients never have a chance to “rain down” on
the bottom and be lost the way they do in the
deep sea.
This constant water motion means the nutrients
are available to the benthic organisms as well as
those in the neritic zone.
Life on the Continental Shelf
• Water may have less clarity due to the many
•
nutrients in the water and due to suspended
sediments.
Nutrients come from many sources, including:
– Autotrophs
– Decaying marine organic matter
– Nutrients delivered from estuaries and terrestrial
ecosystems via rivers
Life on the Continental Shelf
• Thousands of animals consume nutrients on the
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seabed.
Several species of worms may share an area,
burrowing into the sediment and cementing the
grains together into tubes of all shapes and
sizes.
Other burrowing animals include clams, lobsters,
mud shrimp and fish.
The abundant life in the sediment attracts
predators.
Types of Communities on the
Continental Shelf
1.
2.
3.
4.
Soft-bottomed benthic
Seagrass beds
Kelp forests
Hard-bottomed benthic
Life on the Continental Shelf
• Soft-bottomed benthic– Dominant bottom type on the continental
shelf
– Infauna, epifauna and meiofauna are present
as well as neritic species above the benthos
– Sessile types are rare due to lack of “hard
substrate” for attachment (remember, limiting
resources????)
– There are more species present in this area
than in the adjoining intertidal, but the
distribution is often patchy
Life on the Continental Shelf
• Soft-bottomed
benthic-
– Larvae are often
attracted to areas
where adults reside
– This is thought to
be the result of
chemical cues in
the water
– If it’s good enough
for the adults, its
good enough for
the youngsters!
Life on the Continental Shelf
• Soft-bottomed
benthic-
– A few varieties
of infauna and
epifauna are
seen at right
– You may
notice that
many are
tube-builders
to assist with
the stability of
the habitat
Heart urchin
Mya arenaria
Heart Urchin Meoma
Mya arenaria – soft-shell clam
Life on the Continental Shelf
• A wide variety of
organisms also live in
the spaces between
the particles of
sediment- meiofauna
• Many of these species
are found no where
else
• As you can see at the
right, these species
have appendages and
attachment structures
to lock themselves to
the grains of
sediments
Life on the Continental Shelf
• Soft-bottomed
benthic-
– Many species in this
area are deposit
feeders (directly
getting nutrients from
the soft bottomed
material)
– Many other species
are suspension
feeders and filter
nutrients directly
from the water
– Both are pictured at
right
Life on the Continental Shelf
• Soft-bottomed benthic– Bioturbators (organisms that disturb bottom
sediments) constantly “churn” the bottom
– This churning helps to oxygenate benthic soils
– Bioturbators can include worms, mollucs, fish
and even whales
– Skates and stringrays as well as sperm whales
are well known for their activity as
bioturbators
Life on the Continental Shelf
• Seagrass Beds
– They are best developed in sheltered, shallow
waters with good water clarity
– There are over 50 known species
– Species have particular tolerance levels with
regards to temperature, water clarity and
nutrient availability that determines their
distribution
Life on the Continental Shelf
• Seagrass Beds
– Due to these tolerances,
species normally prefer
either cool waters OR
tropical/subtropical waters,
but not both normally
– They are often found in
combination with algal
species
– The density of individual
seagrass plants tend to be
very thick (high biomass) –
this can been seen at right
Life on the Continental Shelf
• Seagrass Beds
– Thalassia (turtle grass) is a common seagrass in
tropical and subtropical areas
– It is named turtle grass because it is often directly
consumed by green sea turtles
– In fact, patches of turtle grass have been shown to
be “farmed” by individual turtles
– In this farming, green turtles bite the tops off the
turtle grass
– When the tops grow back, they are much more
tender and easier to digest, which makes the turtles
happy!!!!
Life on the Continental Shelf
• Seagrass beds
– Zostera or eelgrass prefers cooler, temperate
waters
– It is mainly found in the Pacific and the North
Atlantic
– Like turtle grass, it can be directly consumed
by herbivores
– Both species are often important to the food
chain after decaying
Life on the Continental Shelf
• Hard-bottomed subtidal– Less common
– Often, it is the result of a submerged rocky
shoreline
– Rock formations attract a wide variety of
organisms including many sessile types that
cannot survive in soft bottomed communities
– Oyster reefs, worm tubes and calcareous
algae can all serve as hard-bottomed subtidal
communities as well
Life on the Continental Shelf
• Hard-bottomed subtidal– One of the best features of the hard-bottomed
community is the hiding places it provides for prey
and predators alike
– Insert fig 13.20
Life on the Continental Shelf
• Kelp community– Kelps are brown algae
that are restricted to
cold waters in
distribution
– In addition to requiring
cold water, kelps are
very nutrient-needy
– This is the result of the
fast growth seen in
these algae (up to 20
inches a day)
Life on the Continental Shelf
• Laminaria is a species of kelp common in the
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North Atlantic, North Pacific and the Asiatic coast
of the Pacific
This is one of the smaller species of kelp at
maximum heights of only around 10 feet
Like other species, its rapid growth provides an
enormous amount of primary production in the
areas where it is found
It can be found growing with larger kelp species
in the North Pacific primarily
Life on the Continental Shelf
• Macrocystis is a large species of kelp that
can grow heights of over 100 feet
• It is found mainly in the Pacific on the
coasts of North and South America
• This species forms thick forests that serve
as a MAJOR ocean ecosystem in the areas
where it is found
Life on the Continental Shelf
• Kelp community– Sea urchins are by far
the largest direct
consumer of kelp
– In turn, many species
feed on the sea
urchins including their
“cousins” the starfish
and sea otters
Life on the Continental Shelf
• The diagram
at the right
shows the
anatomy of a
kelp forest
• Thousands of
organisms are
dependent on
the kelp
forests for
shelter
(habitat) or
for trophic
needs