Intertidal Zones, Tides and the Coastline

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Intertidal Zones, Tides and the Coastline
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Overview

• Coastline
• Tidal Cycle
• Intertidal Zones
• Sand Flow
• Coastal Strand Community

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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

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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

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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

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Sea level

• Means slightly different things depending on how
  precise you need to know.
• Mean Sea level- the average between his and los
  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

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Tidal Sequence
Higher-High tide
Lower-High
Sea level
Higher- Low tide
Lower-Low tide
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Tides

• Influence of gravitational pull from Sun and
  Moon. Moons pull is stronger because it is
  closer.
• Centrifugal force also important.
• Spring tides (Upwelling) every two weeks or so
• Suns pull adds to moons pull
• Neap tides (Scarce) every two weeks or so.
• Moons pull partially counteracts suns pull.
• Suns influence changes with season,
• closer in June and December stronger tides.
• Moons 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 !!

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Spring tide

• Spring and Neap Tides

Neap tide
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Pull of moon and Centrifugal force
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Phases of the moon
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From Spring to Neap Tide
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A typical Month in the spring
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This April 2007
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April May 2008
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Why we have to be there early
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Friday April 20th

• Low tide is at 813 a.m.
• Meet at Schoolhouse beach, 745 a.m., need to
  leave DVC at 600 a.m.
• At Beach till 845
• Dunes from 900- 1000
• Home by noon !!!

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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

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Fig 12.7
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Vertical Zonation

• Total number of hours exposed to air increase
  towards top of rock
• Predation changes
• Heat
• Desiccation
• Wave action

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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.

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Keystone species

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

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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

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Competition for space on the rocks
Weak competitors stuck higher up in the intertidal
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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.

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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.

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Coastal Strand Communities

• Dunes
• Three zones
• Fore dune the beach area
• Crest highest point on dunes
• Back dune- protected areas behind front dunes

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Dune Plant adaptations

• prostrate growth- rhizomes
• gray hairs on leaves deflect light, air
• nitrogen-fixing bacteria
• thick waxy cuticle
• deep roots
• mycorrhizae

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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
  (doesnt bind to sand)

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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