Salinity-Temperature-Density Relationships Handout



Salinity-Temperature-Density Relationships Handout

Questions 9-28

Seawater density is a function of both temperature and salinity. The density increases with either a temperature decrease or a salinity increase. Under normal ocean conditions, temperature fluctuations exert a greater influence on seawater density because the range of marine temperatures values is much greater (-2( C to 30( C) than the range of open-ocean salinity.

Some generalizations may also be made about the vertical distribution of ocean temperature, salinity, and density. The most dense water is found o the bottom; but the physical processes that create this dense water (evaporation, freezing, or cooling) are strictly ocean surface features. Therefore, dense bottom water must sink originally from the surface. This sinking process is the only mechanism available to drive circulation of water in the deep portions of ocean basins. An obvious feature of in most oceans is a thermocline, a subsurface zone of very rapid temperature (and density) change (fig. 1.18). The large density difference on either side of the thermocline effectively separates the oceans into a two-layered system: a thin, well-mixed surface layer above the thermocline overlying a heavier, cold, thick, stable zone below. The thermocline inhibits exchange of gases, nutrients, and sometimes even organisms between the two layers. In temperate and polar regions, the thermocline is a seasonal feature. During the winter months, the surface water is cooled to the same low temperature as the deeper water. This causes the thermocline to disappear and allows seasonal mixing between the two layers. Warmer marine climates are characterized by well-developed, permanent thermoclines.

Dissolved Gases

The solubility of gases in seawater is a function of temperature. Greater solubility occurs at lower temperatures. However, the solubility of atmospheric gases in water in limited. Nitrogen, carbon dioxide, and oxygen are the most abundant gases dissolved in seawater. Carbon dioxide and water are utilized by green plants in photosynthesis to produce oxygen and high-energy organic compounds like sugar. Respiration by plants and animals reverse the results of photosynthetic process to release the usable energy incorporated in the organic components of the organisms food. Conversely, oxygen is used in respiration, and carbon dioxide is given off.

Carbon dioxide(CO2) is abundant in most regions of the sea, and concentrations too low to support plant growth do not normally occur. Seawater has an unusually large capacity to absorb CO2 because most dissolved CO2 does not remain as a gas but combines with water to produce a weak acid.

Oxygen in the form of O2 is necessary for the survival of most organisms (the major exceptions are some species of anaerobic microorganisms). The abundance or lack of O2 in seawater strongly influences the distribution of marine life. Oxygen is utilized by organisms in all areas of the marine environment, including the deepest trenches. However, the transfer of oxygen from the atmosphere to seawater and the production of excess oxygen by marine plants are the only available methods of introducing oxygen into seawater. Both of these processes are limited to the near-surface region of the ocean. Oxygen consumed near the bottom in deep areas can be replaced only with oxygen from the surface. If replenishment is not rapid enough, available oxygen supplies may be reduced to critically low concentration or removed completely. Oxygen replenishment occurs by very slow diffusion processes from the oxygen-rich surface layers downward and also by downward vertical water movements that carry oxygen-rich water to deep ocean basins. At intermediate depths, animal respiration and bacterial decomposition use O2 as fast as it is replaced, creating an O2 minimum zone. Figure 1.20 illustrates a vertical profile of dissolved oxygen from the surface to the bottom of the sea.

Dissolved Nutrients

Nitrate and Phosphate are the fertilizers of the sea. These and smaller amounts of other nutrients are utilized in the photosynthetic process by plants living in the near-surface waters and are excreted back into the water at all depths as waste products of the animals and bacteria that had consumed the plant material. This sinking of plant and animal material eventually removes nutrients from near-surface waters and increases their concentrations in deeper waters.

The vertical distribution pattern for dissolved nutrients is opposite that for dissolve oxygen, which is normally high in surface waters and low in deep waters. This is because oxygen is normally produced by near surface plants and consumed by animals, while nutrients are utilize near the surface by plants and excreted by animals at all depths.

The Ocean in Motion

Ocean water is constantly in motion, providing a near uniform medium for living organisms. Such motion enhances mixing to minimize vertical variations in salinity and temperature characteristics. Oceanic circulation processes also serve to disperse swimming and floating organisms and their reproductive products. Toxic body wastes are carried away, while food, nutrients, and essential elements are replenished. The driving force behind oceanic circulation processes is heat from the sun. These circulation processes, so beneficial to all forms of marine life, are wave action, currents, and vertical water movements.

Waves

Differential solar heating of various regions of the earth’s atmosphere produces winds. Winds that blow across the sea surface in turn produce waves and surface currents. Waves are periodic disturbances of the sea surface, typically traveling in a repeating series of alternating wave crests and troughs. The size and energy of waves produced are dependent on the wind’s velocity, duration, and fetch (the distance over which the wind blows in contact with the sea surface).

Once generated, waves move away from the area of formation. However, it is only the wave shape that advances, transmitting the energy forward. The water particles themselves do not advance horizontally. Instead, their paths approximate vertical circles with little or no forward motion (fig. 1.8 & 1.22). Waves provide an important mechanism to mix the near-surface layer of the sea.

Waves entering shallow water behave differently than open-ocean waves. When the water becomes shallow, bottom friction begins to slow the forward speed of the waves causing the waves to “bunch up” becoming higher and steeper. At a certain height, the wave becomes unstable, pitch forward, and break. Breaking waves release tremendous amounts of energy on shorelines (and on the organisms living there) and are major forces in shaping the character of the seashore.

Surface Currents

Measurable ocean surface currents occur in regions where winds blow over the ocean with a reasonable constancy of direction and velocity. Unlike wave motion, the surface currents represent a large-scale horizontal movements of water molecules. The momentum imparted to the sea by these major wind belts drives regular broad, slow, relatively shallow ocean surface currents. Some currents transport more than one hundred times the volume of water carried by all of the earth’s rivers combined. Currents of such magnitude greatly affect the distribution of marine organisms and the rate of heat transport from the tropical to polar regions. (Fig. 1.10)

Vertical Water Movements

Vertical water movements are produced by sinking and upwelling processes. Such processes tend to break down the vertical stratification established by the thermocline. Seawater sinks when its density increases. The physical processes hat increase Seawater density are strictly surface features. Thus dense seawater, derived from the surface, is usually highly oxygenated. This water transports dissolved oxygen to deep areas of the ocean basins, which would otherwise by anaerobic (without 02). The chief areas of sinking are located in the colder latitudes where sea surface temperatures are low.

Rising water masses are accounted for by different processes, usually lumped under the term upwelling. They bring deeper nutrient-rich water to the surface. The continuos availability of plant nutrients from deep water accounts for the high productivity characteristics of regions of upwelling. Several of the world’s important fisheries are based in upwelling areas. (Fig. 1.12)

Light

Marine biologists are concerned only with the visible spectrum and the infrared and ultraviolet regions on either side of it. We experience the visible spectrum as light, the longer-wavelength infrared radiation s heat, and the shorter-wavelength ultraviolet radiation as the penetrating rays that cause sunburn.

Only infrared and visible light penetrate beneath the ocean surface. The infrared is absorbed by a depth of 1m, and blue light may penetrate to 1,000m, although less that 0.1% of the incoming visible radiation remains below a depth of 100m. Throughout the ocean this is the maximum depth at which photosynthesis can occur (photic zone). (Fig 7.6)

In the clearest open tropical oceans where there is little organic material in the water, the ocean takes on a deep blue color. The reason for this is that the small, molecular size particles characteristic of these waters scatter blue wavelengths of light more than any other wavelength. In nutrient-rich coastal waters, sediment form continental runoff and high concentrations of microscopic marine organisms increase the average particle size so that the scattering of green wavelengths of light becomes greater and the ocean appears green in color. In some coastal waters, dense populations of microscopic plants cause the water to become brownish or red in color, producing what is called a red tide.

Physical Properties of Ocean Questions Cont.

9. What are the two main factors that affect seawater density

10. What three physical processes create dense water?

10. Describe the layer above the thermocline and below the thermocline.

11. Are gases more soluble(ability to dissolve into) in cold or warm water?

12. What are the three most abundant gases in the ocean?

13. What two processes introduce oxygen into the ocean?

14. What two processes replenish oxygen to the deep ocean basins?

15. What role do nitrate and phosphate play in the oceans?

10. Give four reasons why currents are so important to marine organisms.

16. What is the driving force behind oceanic circulation and what are the three major types of circulation processes.

17. What three things determine the size of a wave?

18. Describe how waves move or push things in the open ocean.

19. What causes waves to break on shore?

20. What is the major difference between surface currents and waves when it comes to water movements?

21. What direction do currents move in the northern hemisphere and southern hemisphere?

22. Why is the vertical movement of water so important to deep sea dwelling organisms?

23. Why are major fisheries located near upwellings?

24. What are the three types of light marine biologists are concerned about and what does each do?

25. What is the photic zone?

26. Why are oceans different colors?

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