Chapter 13
Life on the Continental Shelf
Life on the Continental Shelf
• The continental shelf is
•
•
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
• Water turbulence created by waves and currents
•
•
keep the water column well mixed.
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 (the pelagic
environment above the continental shelf).
• Pelagic – water column away from bottom
or shore
• Epipelagic – sea surface to depth of about
200 meters
– The epipelagic can be divided into (1) neritic
waters over the continental shelf and (2)
oceanic waters that are past the shelf
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
– Benthos of infauna, epifauna and meiofauna
are present as well as _____ species (pelagic
species above continental shelf) above the
benthos
– Sessile types are rare due to lack of “hard
substrate” for attachment
– 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– Infauna like
snails, clams,
urchins, and
annelids as well
as epifauna like
crustaceans and
brittle stars.
– Many are tubebuilders to
assist with the
stability of the
habitat.
Life on the Continental Shelf
• Meiofauna
• Many of these species
•
are found no where
else and include
protozoa, cnidarians,
crustaceans,
nematodes, &
annelids
These species have
appendages and
attachment structures
to lock themselves to
sediment grains
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, mollusks, 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 and most
common genus is Zostera, commonly called
eelgrass
– Species have particular tolerance levels with
regards to temperature, water clarity and
nutrient availability that determines their
distribution
– Many sessile animals and epiphytic algae
Life on the Continental Shelf
• Seagrass Beds
– Because of their different
tolerances, seagrass
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 provides
large amounts of detritus
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 by green sea turtles
Life on the Continental Shelf
• Hard-bottomed subtidal– Less common
– Often, it is the result of a submerged rocky
shoreline creating lots of hiding places
– 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
communities as well
Sea _____ grazing on seaweed in a hard bottomed subtidal community
Generalized food web for hard-bottomed subtidal community
Life on the Continental Shelf
• Kelp community– Kelps are ____ 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
• 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
Geographic distribution of kelps
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