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Intertidal Zones, Tides and the Coastline. Overview:. Coastline Tidal Cycle Intertidal Zones Sand Flow Coastal Strand Community. California Coastline. 1100 miles long. With varying inland climates from warm by San Diego to Cool in Humboldt.

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Intertidal Zones, Tides and the Coastline

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Intertidal zones tides and the coastline

Intertidal Zones, Tides and the Coastline



  • Coastline

  • Tidal Cycle

  • Intertidal Zones

  • Sand Flow

  • Coastal Strand Community

California coastline

California Coastline

  • 1100 miles long.

  • With varying inland climates from warm by San Diego to Cool in Humboldt.

  • Coast rarely freezes, or exceeds 100 °F. Temperature moderate year-round.

    • San Francisco January 51 °F average, and in September 62 °F.

    • Water is cold around 50 °F. Down South warms to about 60 °F.

    • Summer fog, fog drip.

    • Nutrient rich Upwelling off shore

Coastline geology

Coastline Geology

  • Coastal rocks are uplifted from sea floor. (Emergent Coastline)

    • North Coast Franciscan sedimentary Rocks about 100-150 MYA.

    • South 60 MYA sedimentary rocks.

  • Easily eroded, soft rocks. Cuts deep surf-cut shelves or benches.

    • With uplifting these shelves become terraces, but also erode away.

  • River mouths cut through valleys, form wide lagoons with a sand bars.

    • Form shoreline lakes dammed by sand

Tidal cycle

Tidal Cycle

  • Approximately Every “day” (24 hrs. 48 mins.) has two high tides and two low tides.

    • One high tide is higher than the other: Hi-Hi

    • One low tide is lower: Lo-Lo

  • Daily Tidal cycle:

    • Hi-Hi; Lo-Lo; Hi-Lo; Lo-Hi repeat…….

    • tides change every 6 hrs. 12 mins.

    • full cycle in 24 hrs. 48 mins. (one Lunar orbit)

    • offset by about an hour per day

Sea level

Sea level

  • Means slightly different things depending on how precise you need to know.

    • Mean Sea level- the average between hi’s and lo’s of tides over 19 years, measured in the water compared to land based points of reference

    • Surface of open ocean (used to calculate altitude for aviation) satellite

  • Also used is Mean Lower Low Water (MLLW) average lo-lo tide

Tidal sequence

Tidal Sequence

Higher-High tide


Sea level

Higher- Low tide

Lower-Low tide



  • Influence of gravitational pull from Sun and Moon. Moon’s pull is stronger because it is closer.

    • Centrifugal force also important.

  • Spring tides (Upwelling) every two weeks or so

    • Sun’s pull adds to moon’s pull

  • Neap tides (Scarce) every two weeks or so.

    • Moon’s pull partially counteracts sun’s pull.

  • Sun’s influence changes with season,

    • closer in June and December stronger tides.

  • Moon’s distance varies by 15,000 miles over a month

    • Perigee (2x month) is when moon is closest makes stronger tides

    • perigee with a spring tide increases by 40%

    • At Apogee (2x month) moon is farthest and weakens tides

  • Most extreme spring tides are at perigee in June and December !!

Intertidal zones tides and the coastline

Spring tide

  • Spring and Neap Tides

Neap tide

Pull of moon and centrifugal force

Pull of moon and Centrifugal force

Phases of the moon

Phases of the moon

From spring to neap tide

From Spring to Neap Tide:

A typical month in the spring

A typical Month in the spring

This april 2007

This April 2007

April may 2008

April – May 2008

Why we have to be there early

Why we have to be there early

Friday april 20th

Friday April 20th

  • Low tide is at 8:13 a.m.

  • Meet at Schoolhouse beach, 7:45 a.m., need to leave DVC at 6:00 a.m.

  • At Beach till 8:45

  • Dunes from 9:00- 10:00

  • Home by noon !!!

Intertidal zones from highest above to lowest in water

Intertidal Zones(from highest above to lowest in water)

  • Splash / Spray Zone wetted by surf

    • crustose lichens, sea lice

  • High Tide Zone - covered at high tide exposed most of the day

    • Limpets, sea lettuce, acorn barnacles, rockweed

  • Middle Tide Zone -exposed only for short periods

    • barnacles, mussels, sea stars, chitons, urchins

  • Low Tide Zone -exposed only at lowest tides

    • Top of lower intertidal is sea level for that area.

    • sea palm, surf grass

  • Sub-tidal Zone - never exposed

  • Highly productive

Fig 12 7

Fig 12.7

Vertical zonation

Vertical Zonation

  • Total number of hours exposed to air increase towards top of rock

  • Predation changes

  • Heat

  • Desiccation

  • Wave action

Intertidal succession

Intertidal Succession

  • Sever competition for space

  • Rocks over-turned over in storms, leave bare areas.

    • Filamentous and foliose green algae colonize bare rock

    • Crustose and Coralline algae take over

    • Acorn barnacles colonize by prying off algae as they take over

    • Gooseneck barnacles and mussels take over, growing over an preying off acorn barnacles.

    • Gooseneck barnacles and mussels remain as climax, unless predator invades or rock is over turned.

    • One-year sequence.

Keystone species

Keystone species

  • Pisaster (Sea Star) defends tide pool from being taken over by mussels, barnacles.

Sea otters maintain kelp forest

Sea Otters maintain Kelp forest

  • Otters are a Keystone species

  • Kelp are the base of the kelp forest community

  • Urchins eat kelp at their base

  • Otters eat urchins, keeping their numbers low.

  • Otter numbers along California are dropping.

  • Alaska- Orcas starting to eat otters, because seal numbers are dropping

  • No fish for seals

Competition for space on the rocks

Competition for space on the rocks

Weak competitors stuck higher up in the intertidal

Sand flow

Sand flow

  • Sand from weathered rocks up in mountains.

    • banks of Merced River in Yosemite

  • Moves out to sea - westward

  • Waves come form the NW moving sand southward and back towards shore.

  • Zig- Zag flow with seasonal cycles - beach erosion with storms.

  • In winter

    • beach (more narrow) smaller,

    • more sand moved farther off shore

  • In summer

    • Wide beaches

    • Sand pushed back on to shore, now farther south.

Erosion beach decline

Erosion & Beach decline

  • River of sand slows: damming up stream, beaches disappear

    • Bodega head pushes out into sea, traps southward sand flow forming dunes

  • Sea walls impede southward flow of sand from north.

  • Headlands erode in surf, waves wrap around and hit sides

  • Resistant rocks remain longer forming stacks and arches.

Coastal strand communities

Coastal Strand Communities

  • Dunes

  • Three zones:

    • Fore dune – the beach area

    • Crest – highest point on dunes

    • Back dune- protected areas behind front dunes

Dune plant adaptations

Dune Plant adaptations

  • prostrate growth- rhizomes

  • gray hairs on leaves deflect light, air

  • nitrogen-fixing bacteria

  • thick waxy cuticle

  • deep roots

  • mycorrhizae

Fore dune

Fore dune

  • shifting sands

  • heavy winds and sand-blasting

  • Lots of sun-light, uv radiation

  • drains quickly, dry substrate

  • Not salty! Rain, fog wash out salt spray residue (doesn’t bind to sand)

Crest and back dune

Crest and Back dune

  • Crest

    • Strongest wind

    • European beach grass

  • Back dune-

    • sheltered by crest

    • warmer

    • calm, less breeze

    • beginnings of soil (organic debris)

    • mature trees, shrubs

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