Cause 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 Online.

Zonation | Intertidal Pools | Collection Etiquette

Kelp & Algae | Fungi | Porifera | Cnidaria

Mollusca | Echinodermata | Osteichthyes

Tides | References