Chemical and Physical Features of the World Ocean

Chemical and Physical Features of the World Ocean

Impacts of Tides on Marine Organisms: Intertidal Zonation

• Supralittoral (Supratidal Zone)

  • Also known as the spray zone.

  • Located above the highest high tide mark.

  • Organisms include: Rock louse ($Ligia$), Periwinkle ($Littorina$).

• Littoral (Intertidal Zone)

  • Located between the highest high tide and the lowest low tide.

  • Characterized by periods of exposure to air and submergence by water.

  • Subdivided into:

    • High Tide Zone: Above the lowest high tide.

      • Organisms include: Limpet ($Acmaea$), Periwinkle ($Littorina$), Buckshot barnacle ($Chthamalus$).

    • Middle Tide Zone: Between lowest high tide and lowest low tide.

      • Organisms include: Buckshot barnacle ($Balanus$), Chiton ($Nuttalina$), Limpet ($Acmaea$), Mussel ($Mytilus$/$Modiolus$), Hermit crab ($Pagurus$), Goose barnacles ($Pollicipes$), Sea star ($Asterias$/$Pisaster$).</p></li></ul></li><li><p><strong>Low Tide Zone</strong>: Below the lowest low tide.</p><ul><li><p>Rich in diversity, with many species of animals and plants.</p></li><li><p>Organisms include: Acorn barnacles ($Balanus$), Sea anemone ($Authopleura$).</p></li></ul></li></ul></li></ul></li><li><p><strong>Sublittoral (Subtidal Zone)</strong></p><ul><li><p>Always submerged beneath the lowest low tide mark.</p></li></ul></li></ul><h4 id="4cad23e2-b951-4ca5-9f2e-9eb9adbc3a5c" data-toc-id="4cad23e2-b951-4ca5-9f2e-9eb9adbc3a5c" collapsed="false" seolevelmigrated="true">NOAA CO-OPS Representation of Tidal Datums</h4><ul><li><p><strong>Tidal Datums</strong>: Standard elevations defined by a certain phase of the tide, used as vertical references for marine charting, coastal engineering, and other purposes.</p></li><li><p><strong>Key Tidal Datums</strong>:</p><ul><li><p><strong>Highest Observed</strong></p></li><li><p><strong>MHHW (Mean Higher High Water)</strong>: The average of the higher high water height of each tidal day over a$19$-year period.</p></li><li><p><strong>MHW (Mean High Water)</strong>: The average of all high water heights over a$19$-year period.</p></li><li><p><strong>MTL (Mean Tide Level)</strong>: The arithmetic mean of MHW and MLW, or the midpoint of the range of tide.</p></li><li><p><strong>MLW (Mean Low Water)</strong>: The average of all low water heights over a$19$-year period.</p></li><li><p><strong>MLLW (Mean Lower Low Water)</strong>: The average of the lower low water height of each tidal day over a$19$-year period. Often used as the chart datum for navigation.</p></li><li><p><strong>Lowest Observed</strong></p></li></ul></li></ul><h4 id="d93ba9e7-db08-43cf-81ac-89f4fae8f9c7" data-toc-id="d93ba9e7-db08-43cf-81ac-89f4fae8f9c7" collapsed="false" seolevelmigrated="true">Types of Tides and Global Distribution</h4><ul><li><p><strong>Semidiurnal Tides (Figure 4a)</strong>:</p><ul><li><p>Characterized by <strong>two high tides and two low tides of approximately equal heights</strong> per tidal day.</p></li><li><p>Example: Woods Hole, MA (August$25$).</p><ul><li><p>Water level (ft) fluctuates. First low tide ~$4 ext{am}$, first high tide ~$9 ext{am}$, second low tide ~$4 ext{pm}$, second high tide ~$9 ext{pm}$. Tidal range about$2 ext{ ft}$.</p></li></ul></li></ul></li><li><p><strong>Diurnal Tides (Figure 4c)</strong>:</p><ul><li><p>Characterized by <strong>one high tide and one low tide</strong> per tidal day.</p></li><li><p>Example: Dauphin Island, AL (March$1$).</p><ul><li><p>Water level (ft) fluctuates. Low tide ~$8 ext{am}$, high tide ~$8 ext{pm}$. Tidal range about$0.8 ext{ ft}$.</p></li></ul></li></ul></li><li><p><strong>Mixed Semidiurnal Tides (Figure 4b)</strong>:</p><ul><li><p>Characterized by <strong>two high tides and two low tides of unequal heights</strong> per tidal day.</p></li><li><p>Example: Seattle, WA (Puget Sound) (May$1$).</p><ul><li><p>Water level (ft) fluctuates significantly. First high tide ~$4 ext{am}$, second high tide ~$6 ext{pm}$. First low tide ~$11 ext{am}$, second low tide ~$11 ext{pm}$. Tidal ranges of$10 ext{ ft}$and$4 ext{ ft}$$.

  • Global Distribution (Figure 5):

    • Semidiurnal tides are common in the Atlantic Ocean.

    • Mixed semidiurnal tides are prevalent in the Pacific Ocean.

    • Diurnal tides are found in regions like the Gulf of Mexico and parts of the Indian Ocean.

  Tide-Generating Forces: Equilibrium Theory vs. Dynamic Theory

  • Equilibrium Theory of Tides: A simplified model assuming a global ocean covering the Earth uniformly, with no landmasses and no friction. It explains the ideal tidal bulges.

  • Dynamic Theory of Tides: A more realistic model that considers the effects of continents, ocean depth, basin shape, and the Coriolis effect, leading to more complex and varied tidal patterns observed globally.

  Factors Influencing Tides

  • Gravitational Force from Sun and Moon (Figure 20a):

    • The Moon's gravity primarily pulls water on the side of Earth closest to it, creating a bulge.

    • The Sun also exerts gravitational pull, but its effect is less pronounced due to its greater distance.

  • Centrifugal Force (Figure 20b):

    • As the Earth and Moon orbit a common center of mass (