Chapter 13

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Chapter 13

Life on the

Continental Shelf

Continental Shelf

Submerged edge of the continents

Richest part of the ocean

Includes world’s most important fishing grounds (90% of total global catch)

Oil and minerals have been found on it

Profoundly affected by pollution on other activities of humans on land

Physical Characteristics of the

Subtidal Environment

Subtidal or Littoral Zone

Never exposed at low tide

Extends from the low tide level on shore to the shelf break (outer edge of continental shelf)

Benthos of the continental shelf live in the subtidal zone

Plankton and nekton over the continental shelf are part of the nertic zone

Factors that affect subtidal organisms are linked to two of the shelf’s fundamental characteristics:

1. Shallow Water

• temperature varies from place to place one of the most important factors effecting distribution of organisms

Bottom affected by waves and currents – prevents stratification and nutrients do not concentrate in the bottom layer

2. Proximity to Land

Nutrients are brought in by rivers

Water over the continental shelf is far more productive and plankton rich than the open ocean

Water has a greenish tint from the phytoplankton and the decaying organic matter

Freshwater runoff can lower the salinity

Sedimentation

Of great influence because of the proximity to land and the shallow water

The settling of sediment particles from the water

Most sediments are lithogenous (sediment that comes from the physical and chemical breakdown of rocks on land)

1.Wind transported

2. Shore erosion

3. Transport by rivers

4. Transport by ground water

5. Yields of biological and chemical processes

6. Transport by icebergs and floating ice

7. Volcanic eruptions

Water Clarity

Abundant phytoplankton and the sediment from rivers and stirred up by waves and currents make shelf water murkier than the open ocean

Light does not penetrate as deeply which reduces the depth that primary producers can live

Continental Shelf Bottom

Communities

The type of substrate is very important in determining which particular organisms inhabit the floor of the continental shelf

Soft-Bottom Subtidal Communities

Sandy and muddy substrates dominate the world’s continental shelves

There are distinct communities whose distribution is greatly influenced by such factors as the particle size and stability of the sediments, light and temperature

Infauna predominate, some epifauna, sessile organisms are rare

There are higher numbers of organisms on soft bottoms in the subtidal zone as compared to the intertidal zone

Reasons for Higher Diversity

Desiccation is not a problem

No drastic temperature changes

Minimal salinity changes

Stable environment

Distribution of organisms

Patchy

Organisms are in distinct clumps

Caused by different sediment types

Unvegetated Soft-Bottom Communities

Lack significant amounts of seaweeds or sea grasses

Main primary producers are diatoms

Detritus is a very important food source for many inhabitants

Deposit Feeders

Polychaets

Trumpet worms ( Pectinaria ) •

Bamboo worms (

Lugworms (

Clymenella

Arenicola )

)

Heart Urchins (

Sand Dollars (

Spatangus

Dendraster )

)

Echiurans, peanut worms, sea cucumbers and ghost shrimps (Callianassa)

Trumpet worm

Lugworm

Bamboo Worm

Sea Cucumber

Heart Urchin

Ghost Shrimp

Suspension Feeders (Filter Feeders)

Clams

Razor clams •

Quahog (

Cockles

Mercenaria mercenaria )

Soft-shelled clam (

Amphipods

Mya arenaria )

Polychaetes (parchment worms and terebellids)

Quahog

Soft Shell clam

Distribution of burrowing deposit and suspension feeders is influenced by several factors:

1. Type of Substrate

Deposit feeders predominant in muddy sediments

Suspension feeders – sandy bottoms

2. Type of Organisms present affects the establishment of others

Deposit feeders exclude suspension feeders

Bioturbators – move sediment while burrowing or feeding

Epifaunal Invertebrates

Deposit Feeders

Brittle stars amphipods •

Scavengers

Shrimps ( Penaeus ) •

Predators

Whelks ( Nassarius

Moon snails (

)

Polinices )

More Predators

Blue crab (

Lady Crab (

Callinectes sapidus

Ovalipes ocellatus )

)

Hermit crabs

Lobsters • octopuses

Sea Stars ( Astropecten ) prey on:

Clams

Brittle stars

Polychaetes

Predatory amphipod

Bottom Dwelling fishes of the soft bottom community

Rays

Skates

Flounders

Halibuts

Soles

Tubots

Ray

Flounder

Sole

Skate

Skate

Halibut

Sea Grass Beds

Soft bottoms along the coast are occasionally carpeted by seagrasses

Flowering plants, grass-like in appearance but unrelated to true grasses

Develop best in sheltered, shallow water along the coast

Also found in estuaries and association with mangrove forests

50 to 60 species of sea grasses

Most are tropical and subtropical

Several species are common – Eelgrass

(

Zostera marina )

Form thick luxuriant beds

Their roots keep them anchored in the face of turbulence

Eel Grass

Stabilize the soft bottom

Leaves cut down wave action and currents

More and finer sediment can be deposited which affects colonization by other organisms

Sea grass beds have a higher primary production than anywhere else on soft bottoms •

Rank among the most productive communities in the entire ocean •

Part of the reason: true roots – able to absorb nutrients from sediment •

Increased by algae growing on the surface of the sea grass - epiphytes

Herbivores that eat seagrass:

Sea turtles

Mantees

Sea urchins (

Parrotfishes (

Birds

Diadema, Lytechinus

Sparisoma )

)

Diadema

Ways that animals take advantage of high primary production of seagrasses:

Feed on the large amounts of decaying leaves and seaweeds

Offer shelter

Animals live on the leaves: hydroids, snails, tiny tube dwelling polychaetes, amphipods, shrimps

Larger animals that live among the plants:

Queen conch ( Strombus )

Clams

Pen shell ( Pinna carnea )

Nurseries for commercially available species

Queen conch (Strombus)

Pen shell (Pinna carnea)

Hard-Bottom Subtidal Communities

Relatively small portion of the continental shelf

In some cases a significant component of the hard substrate is provided by calcareous algae, tubes of polychaete worms and oyster shells

Often called reefs

Rocky Bottoms

Never subject to desiccation

Wider variety of organisms

• are rich and productive

Seaweeds – most conspicuous inhabitants

Types of Seaweeds

Brown and red

Filamentous ( Chordaria, Ceramium )

Branched ( Agardhiella, Desmarestia

Thin and leafy ( Porphyra, Gigartina )

)

Encrusting ( Lithothamnion

All have holdfasts

)

Chordaria Agardhiella

Lithothamnion

One of the main problems for seaweeds and sessile animals in the subtidal is to find a place to attach

There is intense competition for living space

Rich epifauna and poor infauna

Organisms

Sponges

Hydroids •

Sea anemones

Soft corals •

Bryozoans

Tube-dwelling polychaetes •

Barnacles

Sea squirts

Bryozoans

Sea Squirts

Grazers

Small, slowing moving invertebrates

Sea urchins ( Arbacia, Diadema,

Stronglyocentrotus )

Chitons

Limpets

Sea hares

Abalones

Abalones

Seaweed defenses against grazing

Chemicals such as sulfuric acid and phenols

Can rapidly regrow

Tough and leathery

Calcareous algae ( Lithothamnion,

Clathromorphum, Halimeda ) deposit calcium carbonate in their cell walls

Carnivores

Feed on attached invertebrates

Sea urchins – seaweeds and flimsier attached invertebrates

Crabs

Lobsters

Fish

Grazers and predators strongly influence the composition of hard-bottom communities

Kelp Communities

Kelps are a group of large brown seaweeds that live in relatively cold water and are restricted to temperate and sub polar regions

True giants

Home to a vast assortment of organisms

Laminaria

North Atlantic and Asiatic coast

Blades 3 m or 10 ft long

Giant Kelp

Macrocystis

Pacific coasts of north and south America

Stipe – 20 to 30 m in length

Kelp beds – large dense patches of kelp

Kelp forests – when the fronds of the kelp beds float at the surface

Canopy – floating tops of kelps at the surface

Physical Factors that influence kelp

Communities

Temperature – must be cool

Do not do well where there is heavy wave action – fragile

Prefer to attach to deep bottoms where wave action is reduced

Kelps can grow very fast with the giant kelp growing as fast as 50 cm/day ( 20 in/day)

Kelp communities are very productive

Organisms found within the kelp beds

Polychaetes

Small crustaceans

Brittle stars

Tube-dwelling polychaetes •

Lace-like bryozoans (

Sponges

Sea squirts

Lobsters

Membranipora )

Crabs

Hermit crabs •

Sea stars

Abalones octopuses

Fishes of the Kelp Community

Rockfishes (

Kelp bass (

Sebastes )

Paralabrax clathratus )

California sheephead ( Semicossyohus pulcher ) eats sea urchins, crabs)

Surf perches (

Topsmelts (

Rhacochilus, Brachyistius

Atherinops ) – plankton

) feeders

Small Algae are grazed by:

Snails

Crabs

Sea urchins

Fishes

Few eat kelps

Sea urchins

Most important grazers in kelp communities

Most important species – red

( Strongylocentrotus franciscanus ), purple ( urchin (

S. purpuratus ) and green sea

S. droebachiensis)

Populations sometimes explode (known as plagues)

Normally urchins feed on drifting kelp

During a plague urchins eat attached kelp and can clear large areas – urchin barrens or urchin deserts

Sea otters can help maintain urchin populations

The End …..

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