and Effect of Tides
Tides are caused mainly by the gravitational pull of the
Moon on the waters of the ocean. The Moon revolves around the
Earth in the exact same direction as the Earth rotates on its
axis, but they travel at different speeds. High tides exist in
a location when the Moon is either directly overhead or on the
opposite side of the Earth from the same location. The attraction
of the Moon is strongest on the side of the earth which it is
facing. This gravitational pull causes the waters to bulge out
toward the Moon in a high tide. When the moon is not directly
overhead, low tides result, the lowest of which occurs when the
Moon is at a right angle from any given location. When there
is a full moon or a new moon it causes the Earth to have the
best low and high tides.
Depending on the relative position of the Moon and the Sun
to the Earth it causes different types of tides. A "Spring
Tide" is caused when the Earth, Sun and Moon are lined up.
Because of this greater centrifugal pull on the earth, the tides
have the most variation between high to low. When the Sun and
the Moon are not lined up with the Earth, we have a "Neap
Tide" or the least variation between high to low.
In southern California, in a period just over 24 hours, the
rotating Earth passes through these liquid bulges and produce
two high tides and two low tides. Here is Los Angeles this phase
is known as a mixed semidiurnal tide schedule. A mixed semidiurnal
tide means two low and two high tides of different heights in
a 24 hour period. A semidiurnal tide schedule is when the two
low and two high tides are both approximately the same height
in each 24 hour period. The water recedes and comes up about
the same length each day. Due to the location of places like
the Caribbean, being closer to the equator, they have what is
known as a diurnal tide. In a 24 hour period the tide only produces
one high and one low tide.
The different tides have different effects all over the Earth.
In rocky intertidal zones, the tide pools can be effected greatly
with the change in these tides. The temperature and salinity
in the tide pools can increase and decrease quickly. As the water
warms and evaporates the salinity level of the water goes up.
There can also be an increase in the UV light when the organisms
are out of water. All of these effects cause the organisms to
adapt or die.
Most of the effects of the tides on organisms living in these
areas are due to the exposure to air. Another main problem is
dehydration. Some strategies for survival in this area are to
move to a moist area in order to maintain water. Also to move
to a sheltered area protected from the sun and the crashing of
the waves. The bivalves "clam up"
in order to keep themselves from drying out. Limpets use their
home scars to hide in sometimes trapped for life. The snails
use their operculum like a tiny door to close themselves up inside.
Some organisms cover their body with a protective layer.
The sea anemones secrete a mucus to help
them keep moist. Some also attach shell fragments to themselves.
Worms like the polychaete worm and sand castle
worm live in tubes.
Organisms that have light colored shells don't absorb the
heat as fast but rather reflect it. Having ridges on the shells
also keep the organisms cooler as the texture of haveing a ridged
shell helps reflect most light and sun rays.
Organisms that live in the pounding surf have found ways
to adapt too. Algae uses its holdfast
to attach to the rocks an keep it in place. It's stem is flexible
and moves with the wave motion. Mussels attach by their byssal
threads and the members of gastropods use their foot to secure
them in place. The clingfish uses a modified fin, on its pelvic
area, like a suction cup. Having a compact size and a hydrodynamic
shape can also help to hold colonies of organisms like barnacles
and chitons in place.
Changes in food availability can be a major problem. Barnacles
out of water can not feed since they are filter feeders and need
the water to obtain their food. There is a competition for space
in these tide pools as organisms fight for shady areas. Larval
settlement is an effect of the tides. These babies need a place
to attach to begin their advancement to adulthood. All of these
effects vary according to the duration of exposure, the time
of day during the exposure and the season during exposure. California
does not have as much of a problem as does places like Alaska
where seasons are shorter, water is colder and daylight can be
scarce or non existent in winter.
Good resource of these tide schedules for the United States
can be found by going to Tides