Continental Shelf:
• Area of the ocean found between the
bottom of the intertidal zone and the
shelf break
CONTINENTAL SHELVES
• Average width – 150 km (80 miles)
• Broad along passive margins
• Narrow along active margins
• Average depth 150 m (500 ft)
Passive Margin –
continental margin on the
edges of diverging plates
Active Margin – continental
margin facing converging or
sliding plates
continental shelf
slope
rise
abyssal plain
abyssal hills
ridge
Life on the Continental Shelf
• Biologically rich
– major portion of the fisheries global catch is caught over
the continental shelf
• Rich in potential natural resources
– 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 (230 land miles) off their
coast – this includes the continental shelf and its
resources
• Pelagic – water column away from bottom or
shore
• Epipelagic – sea surface to depth of about 200
meters – divided into:
– neritic waters over the continental shelf
– oceanic waters that are past the shelf
SUBTIDAL ECOSYSTEMS
• Marine areas that are
never exposed during low
tide
• always submerged
SUBTIDAL ABIOTIC FACTORS
• rich in nutrients
– autotrophs
– Decaying marine organic matter
– brought in from rivers
Mississippi Delta
Nile River Delta
SUBTIDAL ABIOTIC FACTORS
• Influenced by sedimentation processes
(especially shallow areas)
- lithogenic / terrigenous sediments
- physical and chemical weathering of rocks (turbidities,
volcanic ash, red clay)
- brought in by rivers, reworked by waves
- biogenic sediments
- shells and skeletons of marine organisms
Abiotic/Physical Features
– temperature – varies from place to place; affects
distribution of organisms
– more affected by waves and currents than deep
water
• turbulence – water motion – mixes up nutrients so
they don’t accumulate on bottom = highly productive
– substrate
• sandy or muddy dominate
• rocky
SUBTIDAL ORGANISMS
Pelagic
• Plankton (floaters)
• Nekton (swimmers)
Benthic
• Demersal (hover)
• Epifauna (on)
• Infauna (in)
Distribution of marine life
Pelagic
Benthic
Sampling the Benthic Habitat
•Grabs
•Bottom trawls
•Dredges
SUBTIDAL COMMUNITIES
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Soft-bottom Subtidal
Seagrass Beds
Hard-bottom Subtidal
Kelp Forests
SOFT-BOTTOM SUBTIDAL
COMMUNITIES
Dominant substrate sand & mud
Soft-Bottom Subtidal Communities
• Dominates the world’s continental
shelves
– Animals are distributed in this
community based on:
•
•
•
•
sediment particle size
sediment stability
salinity
light and temperature
– Organisms usually occur in patches,
due to planktonic settlement
• Metamorphose and “tasting settlement
sites”
Soft-Bottom Subtidal Communities
• Notable absence of large
plants and algae
– Occasional green algae
• Main producers are
diatoms
• Detritus important food
source
– there is little benthic
primary production
– filter and suspension
feeders rely on detritus
brought in from estuaries
• Deposit feeders and bacteria
dominate fine sediments due
to high organic content
SOFT-BOTTOM SUBTIDAL ORGANISMS
Mostly infauna, some epifauna, almost no sessile
organisms (nothing to hold onto)
Infauna – benthic organisms that bury themselves in
the sediment
Epifauna – organisms that inhabit the surface of the
bottom sediment
No. of subtidal species > intertidal (more stable, no desiccation)
Distribution of organisms influenced by particle size (mud or
sand)
Soft-bottom subtidal communities
Infauna:
• live within the sediment, mostly soft bottom;
• mostly clams and worms (polychaetes)
• burrow tubes for food scavenging and oxygen
supply
• primary producers: algae, mostly benthic
diatoms and dinoflagellates
• detritus important food source
Infauna
Epifauna
Soft-bottom subtidal communities
Life on the Continental Shelf
• Meiofauna – live
between sediment
grains
• 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
• Thousands of animals consume nutrients on
the 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 like cephalopods, fish, and marine
mammals
Soft bottom subtidal communities
32,000 polychaetes in sand/m2
vs
50-500 earth worms in soil/m2
Polycheate Ecological Role:
• clean sediments
• aerate sediments
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 stingrays as well as sperm whales are
well known for their activity as bioturbators
Carnivore feeders
Herbivore feeders
Deposit-feeders
Suspension-feeders
Soft-bottom subtidal communities
Generalized food web
Seagrass beds
• Areas that are
carpeted by flowering
plants
• They develop best in
sheltered shallow
areas along the coast
• Eel grass is the most
common
representative
Seagrasses
• Thick mats of eel grass
provide lots of habitat for
animals to hide
• By stabilizing the sediment
it decreases the turbidity
– Dead sea grasses
provide lots of detritus
that benefit suspension,
deposit and filter
feeders
• Many sessile animals
and epiphytic algae
Soft-bottom subtidal communities
Epibionts
Seagrasses support many
sessile animals and
epiphytic algae
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
• Seagrass beds
– Zostera or eelgrass
prefers cooler,
temperate waters
– Like turtle grass, it can
be directly consumed
by herbivores
– Both species are often
important to the food
chain after decaying to
form detritus
Sea Grass Food Web
Hard-Bottom Subtidal Communities
• Less common than soft-bottomed
• Often, 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 urchins grazing on seaweed in a hard bottomed subtidal community
HARD-BOTTOM SUBTIDAL
COMMUNITIES
• Most important organisms are the
sea weeds (able to settle on
rocks/hard substrate)
PRODUCERS
• Most important communities - seaweeds
• Strong competition
• Amount of light influence distribution of
seaweeds
• Seaweeds found in these areas have
higher chlorophyll concentration
GRAZERS
• invertebrates that move
slowly: sea urchins, limpets,
chitons, abalone
• defense against predators
- by having food that is not
tasty
- fast regeneration
- calcification (formed of
calcium carbonate)
Generalized food web for hard-bottomed subtidal community
Kelp Forests
• Animals that are present
have the same
characteristics as those of
the rocky intertidal
• Kelp is a macroalgae which
can reach amazing sizes
Kelp and Kelp Forests
Air sack
holdfast
Life on the Continental Shelf
• Kelp community– Kelps are macroalgae
that are restricted to
cold water distributions
– 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 - species of kelp common in the North
Atlantic, North Pacific and the Asiatic coast of the
Pacific
– one of the smaller species of kelp at maximum heights of
only around 10 feet
– its rapid growth provides an enormous amount of primary
production in the areas where it is found
– 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
– 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
Kelp Communities
Several species of kelp-community
fishes sheltering near giant kelp, Macrocystis.
© Galina Barskaya/ShutterStock, Inc.
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
Kelp Communities
• Below the effects of waves and tides, kelp
communities dominate in temperate areas
General structure of a West Coast kelp forest, with a complex
understory of plants beneath the dominant Macrocystis or Nereocystis.
Hard-bottom subtidal communities
Examples of N. Atlantic Kelp
Geographic distribution of kelp
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
seastars, and sea
otters
Sea Urchins
• What leads to population explosions?
– Absence of their predators
– Overfishing, less seals & sea lions, killer whales eat
sea otters, more urchins
Pictoral Food Web Example
PRIMARY PRODUCERS
PRIMARY CONSUMERS 2ND LEVEL CONSUMERS 3RD LEVEL CONSUMERS
KELP
SEA URCHINS
SEA OTTERS
MICROSCOPIC ALGAES
SMALL FISH
LARGE FISH
KILLER WHALES