Transcript Intertidal Zones, Tides and the Coastline

Intertidal Zones, Tides
and the Coastline
Overview:
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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.
• 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
• Coastal rocks are uplifted from sea floor.
(Emergent Coastline)
– North Coast Franciscan sedimentary Rocks about 100150 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
• 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
• 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
Higher-High tide
Lower-High
Sea level
Lower-Low
tide
HigherLow tide
Tides
• 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 !!
Spring tide
• Spring and
Neap Tides
Neap tide
Pull of moon and Centrifugal force
Phases of the moon
From Spring to Neap Tide:
A typical Month in the spring
This April 2007
April – May 2008
Why we have to be there early
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)
• 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
Vertical Zonation
• Total number of
hours exposed
to air increase
towards top of
rock
• Predation
changes
• Heat
• Desiccation
• Wave action
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
• Pisaster (Sea Star) defends tide pool from
being taken over by mussels, barnacles.
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
Weak competitors stuck higher up in the intertidal
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
• 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
• Dunes
• Three zones:
– Fore dune – the beach area
– Crest – highest point on dunes
– Back dune- protected areas behind
front dunes
Dune Plant adaptations
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prostrate growth- rhizomes
gray hairs on leaves deflect light, air
nitrogen-fixing bacteria
thick waxy cuticle
deep roots
mycorrhizae
Fore dune
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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
– Strongest wind
– European beach grass
• Back dune–
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sheltered by crest
warmer
calm, less breeze
beginnings of soil (organic debris)
mature trees, shrubs