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The Ocean
At the present time, the world ocean covers approximately 70% of the earth’s surface with an average depth of about 2.4 miles. The
world ocean has been separated into four major ocean basins: the Atlantic, Pacific, Indian, and Arctic oceans. Other smaller oceans and seas,
such as, the Bering and Mediterranean Sea, project from the margins of the larger oceans.
Properties of Seawater
Many properties of seawater are crucial to the survival and well-being of the ocean’s inhabitants. Water accounts for about 80-90%
of the bulk of most marine organisms. It provides buoyancy and body support for swimming and floating organisms, thereby reducing the
need for a heavy skeletal structure. Water is also the medium for most chemical reactions needed to sustain life. Organisms living in the
ocean are strongly influenced by salinity of ocean water, distribution of light and density stratification caused by surface heating of the ocean,
and water motions. An understanding of these and other physical factors in essential for our study of marine biology. The characteristics of
pure water and seawater differ in some respects and are similar in others. For this reason it is helpful to consider first the basic properties of
pure water, then to study the effects of dissolved substances on these properties.
Pure Water
Water is the only substance that exists naturally in all three states on the earth’s surface; solid, liquid, and gas. Water has some other
remarkable properties that we must understand to fully appreciate the ocean environment. Water has the greatest heat capacity of any
naturally occurring liquid on the earth; abnormally high melting and boiling points for a substance of its molecular mass; a high surface
tension; and a great ability to dissolve other substances. Perhaps its most peculiar property is that of being less dense in its solid form than in
its liquid state.
All properties listed above result from the unique shape of the water molecule. It is composed of one atom of oxygen (O) and two
atoms of hydrogen (H). The hydrogen atoms are separated by a 105 angle. This configuration produces an asymmetry of the water
molecule, with an oxygen atom dominating one end of the molecule and the hydrogen atoms dominating the other end. This makes the
oxygen end slightly negative and the hydrogen end slightly positive. When two or more water molecules bond together they form what is
called a hydrogen bond.
Viscosity and Surface Tension
Hydrogen bonding between adjacent water molecules within the fluid mass tends to resist external forces that would separate these
molecules. This property is known as viscosity and has a significant effect on floating and swimming marine organisms. The viscosity of the
water reduces the tendency of some organisms to sink by increasing the frictional resistance between themselves and nearby water molecules;
but at the same time it magnifies problems of drag that must be overcome by actively swimming animals.
The mutual attraction of water molecules at the air-sea boundary creates a flexible molecular “skin” over the water surface. This is
surface tension of water, and is sufficiently strong to support the full weight of many small organisms such as the water strider.
Heat Capacity
The high heat capacity (ability to absorb heat) and the large amount of heat required for evaporation enable large bodies of water to
resist extreme temperature fluctuations. Heat is absorbed slowly by water during warm periods and gradually given up during cold periods.
This process provides a temperature-moderating effect for the marine environment and adjacent land areas.
Seawater is the accumulated product of millions of years of water solvent action on rocks, soil, organisms, and the atmosphere.
About 3.5% of seawater is composed of dissolved compounds from these sources. The other 96.5% is pure water. Salt accounts for the
majority of dissolved substances in seawater. The total amount of dissolved salts in seawater is referred to as salinity, which is measured in
parts per thousand (% ). Average salinity is approximately 35 % . Salinity values range form nearly zero at river mouths to over 40 % in
some areas of the Red Sea. Yet in open ocean areas away from the coastal influences, the salinity varies only slightly over large distances.
Salinity is altered by processes that add or remove salts or water from the sea. The primary mechanisms of salt and water addition or
removal are evaporation and precipitation, river runoff, and freezing and thawing of sea ice. When evaporation exceeds precipitation, it
removes water form the sea surface, thereby concentrating the remaining salts and increasing the salinity. Excess precipitation decreases
salinity by diluting the sea salts. Freshwater runoff from rivers has the same effect. When seawater freezes, only the water molecules are
incorporated into the developing ice crystal. The dissolved salts are excluded, thus increasing the salinity of the remaining seawater. The
process is reversed when ice melts.
There is more than just salt found in seawater. The major constituents account for over 98% of the total salt concentration in
seawater. Seawater is a complete chemical medium for life, it provides all the substances necessary for the growth and maintenance of plant
and animal tissue. Magnesium, calcium, bicarbonate, and silicate are important components of the hared skeletal parts of marine organisms.
Nitrate and phosphate are required by plants for the synthesis of organic material. In addition, a vital similarity exists between the chemical
composition of seawater and that of body fluids of marine plants and animals.
Physical Ocean Questions
1. What percent of the earth’s surface is covered by Ocean?
2. What is the biological importance of each of the following unique properties of water?
a. Boiling Point b. Freezing Point c. Surface Tension d. Density of Solid e. Latent Heat of Evaporation f. Solvent Power
3. What are the 3 states of water on this planet and give an example of where each can be found in nature?
4. Draw the structure of a water molecule and several water molecules hydrogen bonded together.
5. What percent of seawater is composed of dissolved ions?
6. What are the 2 major ions in saltwater?
7. List 2 processes that increase the salinity of the ocean and list 2 processes that decrease the salinity of the ocean.
8. What is bicarbonate used for by marine organisms?