Introduction

The Earth’s surface is dominated by water. In fact, water accounts for 70.8% of the surface of this planet. The majority of this water is contained in the world’s oceans. Without water, there would be no life. Many physical properties of ocean water control the distribution and diversity of organisms.

Density of water is an important abiotic (non-living) factor. Temperature, salinity, and depth are determining factors in the density of seawater. Density increases with decreasing temperature (cold water is denser than warm water) and increasing salinity and depth. Much of the ocean is divided into three density zones: the surface zone or mixed layer, the pycnocline—an area where density increases rapidly with increasing depth—and the deep zone where there is little increase in density with increasing depth (Garrison, 1993).

Light is another important factor in the distribution of life in the oceans. Once sunlight passes the ocean surface, it is scattered and absorbed, limiting the depth to which it can penetrate. As a result, there is only a relatively thin layer of lighted surface water, known as the photic zone, within which all food production by photosynthetic marine plants occurs. In the open ocean, the photic zone is generally about 100 meters in depth, although in clear, tropical waters, it may extend to a depth of 200 meters (Garrison, 1993).

The scattering and absorption of light also results in the ocean waters' appearing blue. Absorption of visible light in the ocean is greater for longer wavelengths; therefore, red, orange, and yellow light are absorbed quickly. Fifty five percent of the light entering the water is absorbed in the first meter or so of water, and the remaining 45% is mostly comprised of green and blue wavelengths. Clear ocean waters appear blue because blue light penetrates the water far enough to be scattered back through the surface to our eyes (Garrison, 1993).

Ocean currents, which can move vast quantities of water, are driven by a number of different forces. Primary forces, which start water moving and determine its velocity, include wind stress at the water surface, thermal expansion and contraction, and differences in water density. Secondary factors include the Coriolis effect, the shape and size of ocean basins, gravity, and friction (Garrison, 1993).

Changes in typical atmospheric and oceanic circulation can affect global weather patterns. An example of this is an El Niño event. In normal years, trade winds blow from east to west from a region of higher pressure over the eastern Pacific, toward an area of lower pressure in the western Pacific generally centered over Indonesia. As these trade winds blow over the surface of the water, they drag cool water from the South American

coast westward. Sunlight and the atmosphere heat this water, resulting in surface water along the equator that is cooler in the eastern Pacific and warmer in the western Pacific. However, every few years, there is a breakdown in the usual atmospheric pressure patterns. Air pressure decreases in the eastern pacific and increases in the west. This results in the weakening of the trade winds. In strong pressure reversals, the normal easterly winds are replaced by west winds ( that is, winds blow from west to east.) As a result, a broad area of warm, nutrient-poor, tropical Pacific waters moves eastward toward South America. Large quantities of fish, particulary anchovies, and marine plants that thrived in cold, nutrient rich waters, may die. Birds that feed on the fish may also die, littering the waters and Peruvian beaches with their carcases. The impact on the fishing industry in the area can be catastrophic.

The impact of an El Niño event is not limited to South America. The large area of abnormally warm water fuels the atmosphere with additional moisture and warmth and can have an effect on global wind patterns. The actual mechanism of this relationship between changes in surface ocean temperatures and wind patterns is not fully understood, but the results can be observed worldwide. The major El Niño in 1982-83 resulted in droughts in Indonesia, southern Africa, and Australia and record rains and flooding in Ecuador and Peru. In the Northern Hemisphere storms pounded areas of the United States from California to the Gulf States. It is estimated that worldwide damage exceeded $8 billion (Ahrens, 1991).

Tides and waves are other factors that move ocean waters. Tides are created by the gravitational attraction between the sun, moon, and Earth. The moon’s gravitational pull on the Earth results in water's being pulled toward the moon, causing a “bulge” in the water. Inertia causes a similar bulge of water on the opposite side of the Earth. These “tidal bulges” are, in essence, the crest of planet-sized waves and are the cause of high tides. Low tides are associated with the troughs.

Twice each month, when the sun, moon, and Earth are in alignment, the gravitational pull combines to produce spring tides or tides with the greatest tidal range (highest high tides and lowest low tides.) When the moon is in its first quarter or third quarter phase, the sun, moon, and Earth form a right angle. The resulting tides, known as neap tides, have a smaller than average tidal range (lowest high tides and highest low tides).

The most familiar types of waves are produced by wind. The size of waves produced depends on wind speed, the length of time the wind is blowing at a particular speed, and the fetch—the unobstructed distance of the sea over which the wind blows (Bearman, 1989). As a wave approaches shore, it encounters shallower water. When the depth of the water is approximately 2 of the wavelength, the wave begins to “feel” the bottom and friction causes the wave to slow and wavelength to decrease (wave period remains the same.) As depth continues to decrease, the top of the wave—the wave crest—is moving faster than the wave bottom. The wave will eventually “break.” This generally occurs at a 3:4 ratio of wave height to water depth, that is, a 3-meter wave will break in 4 meters of water (Garrison, 1993).

The chemical composition and quality of water also dictate which organisms can survive in a given region. Salinity or, very simply, the salt content of the water, is an important abiotic factor. Salinity is measured in parts per thousand (0/00). The average salinity of ocean water is between 30 and 35 0/00. However, salinity fluctuates with the amount of freshwater influx through precipitation, the melting of snow and/or glaciers, river runoff, and evaporation rate. Salinity generally increases with a decrease in depth. The area of rapidly changing salinity is known as the halocline.

Water temperature is another important component of water quality. Water temperatures vary greatly by region. Water is warmed by solar energy; therefore, surface waters are generally warmer than bottom waters. Ocean water is mixed through a variety of mechanisms, including currents and wave action. The area where the warmer surface waters meet the colder bottom waters is known as the thermocline. The temperature of the water within the thermocline decreases rapidly with depth.

The pH of water is also an integral factor in the distribution of organisms on this planet. The pH of a solution is determined by measuring the concentration of hydrogen ions and is measured on a scale from 0 to 14. A solution with a high concentration of hydrogen ions is acidic, with a pH between 0 and 7. A solution with a large number of hydroxyl ions is an alkaline or basic solution with a pH ranging from 7 to 14 (Greene, 1998). Ocean water generally has a pH between 8 and 9 and is, therefore, basic. Freshwater is typically more acidic. Smaller bodies of water, such as lakes and ponds, are often affected by acid precipitation, which causes the pH to become more acidic. The larger volume of water in an ocean helps to dilute the effects of precipitation. Chemicals in the ocean water known as buffers also help to control pH (Greene, 1998). The carbonate radical (CO3=) is an example of one buffer in the ocean. The carbonate radical can accept hydrogen ions, causing pH to increase, or it can release hydrogen ions, causing the water to become more acidic. Photosynthesis and respiration alter the amount of CO2 in the water, which also affects pH. Photosynthesis removes CO2 from the water, which forces buffers to remove hydrogen ions, increasing pH. Respiration, at night, when photosynthesis does not occur, adds CO2 to the water, resulting in an increase in the number of hydrogen ions, thereby lowering the pH (Greene, 1998).

The amount of oxygen dissolved in water also controls the distribution of organisms. The majority of oxygen produced on Earth is the result of photosynthesis by plants and algae living in the upper portions of the water column. The amount of dissolved oxygen (DO) in water is measured in parts per million (ppm). Ocean water is generally between 1 and 9 ppm (Greene, 1998). Dissolved oxygen generally decreases with depth as the plants and algae that produce oxygen require sunlight. Waves and currents help mix surface waters and bring oxygen to the depths of the water column. Areas with very low DO levels are known as hypoxic, while areas with no usable DO are referred to as anoxic.