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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. • 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 • • • • • • prostrate growth- rhizomes gray hairs on leaves deflect light, air nitrogen-fixing bacteria thick waxy cuticle deep roots mycorrhizae 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 – Strongest wind – European beach grass • Back dune– – – – – sheltered by crest warmer calm, less breeze beginnings of soil (organic debris) mature trees, shrubs