Chapter 13 Life on the Continental Shelf
Continental Shelf 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
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 Scavengers 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
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
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
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 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
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
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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