Chapter 13 Life on the



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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)





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)











Suspension Feeders (Filter Feeders)

  • Clams

  • Razor clams

  • Quahog (Mercenaria mercenaria)

  • Cockles

  • Soft-shelled clam (Mya arenaria)

  • Amphipods

  • Polychaetes (parchment worms and terebellids)





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









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





  • 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











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







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









  • 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







Grazers



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 …..



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