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# Ocean Tides

## Introduction

Tides are periodic changes in the height of the ocean surface at a certain location. Ocean tides are caused by a complicated combination of forces generated by the gravitational pull of the moon and sun, the rotation of Earth, and the geography of a particular location. Tides influence the ecology of many coastal ecosystems. They have also been used to generate power in places where tides are extreme.

## Historical Background and Scientific Foundations

Ocean tides are brief, episodic changes in the height of the ocean surface at a certain place caused by the gravitational pull of the moon and the sun as well as the spinning of Earth. The wavelength of tides is half Earth’s circumference. The average global height of ocean tides is about 7 ft (2 m), although they can vary from undetectable to 50 ft (15 m) depending on a collection of complex factors that influence tides in different locations.

The relationship between the position of the moon and the height of tides was first observed by Greek navigator Pytheas around 300 BC. Ancient Hawaiians and Chinese also recognized the tides and theorized about their source. Isaac Newton (1642–1727), English mathematician and physicist, put forward the first mechanistic description of tides. He showed that the pull of gravity between any two objects depends on their mass and the distance between the objects. Using this central finding, Newton developed the equilibrium theory of tides, which explains the major characteristics of tides. Newton’s ideas were later refined by French mathematician Pierre-Simon Laplace (1749–1827), who developed what has become known as the dynamic theory of tides.

To understand the tides, imagine Earth as a ball covered in water. The gravitational pull of Earth holds the water on its surface. The moon also exerts a gravitational pull on the water, creating a bulge in the water in the location that is the closest to the moon. The moon and Earth are fixed in a rotating system. Inertia, which explains why an object continues moving in the same direction if no force acts on it, produces an outward-flinging force in the direction opposite the moon. This results in a second bulge of water on the opposite side of Earth.

Earth also spins on its axis once a day. The bulges remain fixed in position relative to the moon. Imagine Earth spinning beneath the two bulges of water. A particular location on Earth passes below the bulges during the course of a day. The bulges create high tides and the positions between the bulges create low tides.

Although the moon is the major factor that drives tides, numerous other factors also impact their size and timing. In a similar manner to the moon, the sun exerts a gravitational force on the water on the planet. However, because it is much farther away, its impact is only about 40% as great as that of the moon. In addition, both the moon and the sun move north and south of the equator depending on the time of year and time of month. This movement offsets the positions of the bulges and influences both the size of the tides and their frequency. The depth of the ocean, the geography of coastlines, and the actual energy consumed by water friction due to tides all affect the timing and height of tides in any specific location. All in all, there are approximately 140 tide-generating forces and factors that are required to completely predict the tides in any location.

## Impacts and Issues

Coastal organisms are necessarily affected by tides. The tides influence the frequency and duration of time that organisms are covered in water or exposed to air. Some organisms require submergence for extended periods of time. These animals live in a zone below the low tide position. Other organisms can withstand only infrequent periods when they are submerged. They aggregate above the high tide mark. The effects of the tides create thin zones within the coastal region that are host to very different types of ecosystems.

Some organisms have developed adaptations that depend on changes in the tides. Certain species of diatoms, small plantlike organisms, rise to the surface of sandy beaches at low tides in order to perform photosynthesis. When the tide rushes in, they burrow down into the sand, protected from wave surge and predation. A species of fish called the grunion swim onto sandy beaches along the Pacific coast in large numbers during the highest tides. They lay their eggs in the sand, free from predation by aquatic animals. The eggs hatch nine

## WORDS TO KNOW

ESTUARY: Lower end of a river where ocean tides meet the river’s current.

GRAVITY: An attractive force that exists between all mass in the universe such as the moon and Earth.

INERTIA: The tendency of an object to continue in its state of motion.

days later at the return of the first tide that reaches the same point on the beach.

Ocean tides are used as a source of power in some places where tidal variation is extreme. On the estuary of the River Rance in France and along the Annapolis River in Nova Scotia, Canada, power plants have been built that harness the energy from tides. Energy is harvested as the water rushes inland as the tide becomes high and again as the water rushes seaward as the tide becomes low. Proposals for tidal plants in Scotland, New York City, San Francisco Bay, and in several inlets in Australia have also been proposed. In some cases tidal power plants can negatively affect the ecosystem by changing the turbidity and salinity of the enclosed bay. In addition, fish may swim through turbines and become injured or killed. As a result, there are often significant environmental objections to building tidal-powered electrical facilities.

## BIBLIOGRAPHY

Books

Garrison, Tom. Oceanography: An Invitation to Marine Science, 5th ed. Stamford, CT: Thompson/Brooks Cole, 2004.

Web Sites

Renewable Energy and AEoogle. “Tidal Power.” http://www.alternative-energy-news.info/technology/hydro/tidal-power/ (accessed March 7, 2008).

Tulane University. “Coastal Zones.” April 9, 2007. http://www.tulane.edu/~sanelson/geol204/coastalzones.htm (accessed February 8, 2008).

Juli Berwald