Chapter 13 Life on the Continental Shelf. Submerged edge of the continents Richest part of the ocean Includes world’s most important fishing grounds.

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 (Clymenella)

•Lugworms (Arenicola)

•Heart Urchins (Spatangus)

•Sand Dollars (Dendraster)

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

Trumpet worm

Bamboo Worm

LugwormSea Cucumber

Heart Urchin

Ghost Shrimp

Suspension Feeders (Filter Feeders)

•Clams

•Razor clams

•Quahog (Mercenaria mercenaria)

•Cockles

•Soft-shelled clam (Mya arenaria)

•Amphipods

•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 (Callinectes sapidus)

•Lady Crab (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

SoleRay

Flounder

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 (Diadema, Lytechinus)

•Parrotfishes (Sparisoma)

•Birds

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

Pen shell (Pinna carnea)

Queen conch (Strombus)

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 (Membranipora)

• Sponges

• Sea squirts

• Lobsters

• Crabs

• Hermit crabs

• Sea stars

• Abalones

• octopuses

Fishes of the Kelp Community

•Rockfishes (Sebastes)

•Kelp bass (Paralabrax clathratus)

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

•Surf perches (Rhacochilus, Brachyistius)

•Topsmelts (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 (S. purpuratus) and green sea urchin (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 …..