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Chapter 6

Intertidal Ecology

Intertidal Zones

Best known

Most accessible to humans

Studied intensely

Low tide observations

Greatest variations of environmental factors only cm’s away from each other

Diversity is great

Has not been invaded by terrestrial flora or fauna

Environmental Conditions


Important influencing factor

Without them, this zone does not exist

2 highs and 2 lows each day



Bay of Fundy in Nova Scotia

Cook Inlet in Alaska

Diurnal tides

Semidiurnal tides

Mixed tides

Duration of exposure

No tides in Mediterranean, Black, Baltic

Wind action pushing water

Critical Tide Levels


Oceans and seas so minimum ranges of temperature change because of its size

Rarely exceeds lethal limits

Intertidal do have this lethal situation

Death will occur later because of weakness from extreme temperature

Secondary causes

Marine organism – death by desiccation ( dry up )

Wave Action

Very influential or marine organisms


1. Mechanical – smashes and tears away objects

2. Wave action extends limits

Throws water higher on shore than tides

Organisms higher exposer

More chance of desiccation from high temperatures

Mixes atmospheric gases with water, higher O2

Never lacks O2



Changes can effect organisms

Exposure at low tide

Flooded by heavy rains or runoff from heavy rains

Reduces salinity

Organisms limited tolerance at low salinities

Extreme reduction cause swelling by osmosis , death

Tide Pools – retain seawater at low tide

Freshwater flooding

Reduce salinity, but evaporation creates high salinity

Other Factors


Far northern of southern shores


Catastrophic mortality by freezing / mechanical abrasion

Different substrates have different faunas and community structure




Adaptations of Intertidal Organisms

Avoiding or minimizing stresses of daily exposure to air

Major stress is wave action

Resistance to Water Loss

Maintenance of Heat Balance

Mechanical Stress



Salinity Stress



Composed of hard substrate

Densely inhabited with macroorganisms

Greatest diversity of animal and autotroph species

Almost barren of sand and mud surfaces

Marine biologists fascinated

Species interaction and maintain or change of community

Solitary or clonal

Atlantic and Pacific Rocky Shores

Differences Temperate Atlantic and Rocky Pacific

Atlantic lacks rocky intertidal south of Cape Code

Sedimentary dominate

Pacific dominated by rock from California up to Alaska

Sedimenatry uncommon

Range of aerial temperatures

Narrow over Pacific coast

Seasonal and daily over Atlantic

- 20 0 C winter to 40 0 C summer

Pacific, Canada, southern Alaska

rarely go below freezing in winter

Summer – cloudy and foggy


Atlantic – lower diversity than Pacific

Ice age - glaciers scoured Atlantic coastline

Widespread extinction of rocky intertidal organisms

Pacific intertidal

spared, older geologically, more speciation

Puget Sound

Five common chiton

Large size

New England

3 species

All small, uncommon, primarily subtidal

Green crab species introduced from Europe

Littorine – dominate herbivore


Horizontal banding of organisms

Succeed vertical

Mountain elevation

Km compared to meters

Vary in vertical extent

angle of slope

Gradual – zones are broad

Narrow zones

tidal range

exposure to wave action


Supralittoral fringe

Midlittoral zone

Infralittoral fringe

Infralittoral zone

Sublittoral area

Causes of Zonation

Physical and Biological factors

Physical factors

Tidal action

Exposure to air

Temperature extremes


Series of tidal curves

Critical Tide Levels

Doty ( 1946 )

Various places and several scientists

Gulf of Maine

Simple zonation


Kelp zone

Few mussels

Biological factors

Tropical Intertidal Shores



Horizontal Distribution Patterns

Age Structure

Interactions Among Factors: A Summary

Tidal Pools


Environmental Conditions

Adaptations of Organisms

Types of Organisms

Feeding Biology

Community Organization on Exposed Sand Beaches

Community Organization on Protected Sand Flats


Physical Factors

Adaptations of Organisms

Types of Organisms

Feeding Biology and Trophic Structure

Zonation and Community Structure



Low tides, they show up to feed

Sand and mudflats are the place the like to visit

Predators of infaunal invertebrates

Plenty of food available

Migratory birds in temperate zone

Infauna can recover

Tidal flats are not uniform in community


Grain size

Succession patterns that follow disturbances

Depth in sediment and tidal level

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