THE OCEAN SCHOOL
Oceanography - Ocean Water
As water heats up, the molecules vibrate faster. Molecules always vibrate, unless you are at absolute zero (-273 degrees Centigrade), the temperature where all movement stops, but the warmer a substance is, the faster the molecules vibrate. In order to move despite the stickiness of the hydrogen bonds, a lot of energy is necessary. Vaporization finally breaks the hydrogen bonds completely, and the molecules just free-float. Because of the vibration of the molecules, the density of liquid water decreases as it warms up. Therefore warmer water floats on top of colder water. In the summer this causes the surface of oceans and lakes warms up, while the cold water stays underneath, separated from the warm surface layer by a thin layer called the thermo cline.
Because water is polar, it is also extremely good at dissolving salts. Salts are generally made up of positive and negative ions. Water breaks those ions up and surrounds them. This is why the ocean is salty, and salt does not simply pile up on the sea floor. The world’s oceans contain approximately 50,000,000,000,000,000 tons of dissolved salt. Salinity (the amount of dissolved salt) in water varies. Freshwater contains very little salt, brackish water (where freshwater flows into saltwater) contains up to 3%, and open oceans contain about 3.5%. The capacity of water to dissolve salts is limited.
When water evaporates, salt is left behind and eventually crystallizes. This can be seen in areas where inland seas slowly evaporate, such as the Red Sea. Salt lakes and finally salt pans are the end stages of evaporated oceans. Humans have been known to speed up this process. The Aswan Dam on the Nile, once considered one of mankind’s greatest achievements, is in fact turning into one of the greatest environmental disasters. For millennia the river flowed swiftly to the ocean, with seasonal floods flushing out the fields. Now however the dam stops the flow. Water evaporates already in the reservoir, and the fields are constantly being irrigated, with irrigation water then evaporating completely on the fields, leaving behind the salt dissolved in it. Since the soils no longer are flushed clean by floods the salt builds up year after year – and less and less crops grow. Water that has a higher salinity is more dense than water with a lower salinity, leading to all kinds of interesting ocean processes.
Living organisms themselves are miniature oceans. Our bodies are roughly 65% water. The high heat of vaporization of water allows us to cool our bodies by sweating. The high heat capacity of water allows us to maintain a stable body temperature. And the ability of water to dissolve ions makes life processes in our cells possible. Our bodies are in fact slightly saltier than freshwater, and slightly less salty than ocean water. Membranes separate our bodies from the environment. Water can pass relatively easily through them, while dissolved ions cannot. This is called semi-permeability. Water tries to equalize concentrations of ions across membranes. The result is that if you swim in freshwater, water seeps into your body (it is saltier than the lake), while if you swim in saltwater, water is drawn out of your body (it is less salty than the ocean). This presents unique problems to aquatic organisms. Evolutionarily all life evolved in the oceans, then moved to fresh water and land, with some species later returning to salt water. Change in physiological systems took place over millions of years.
Some species of fish however switch between living in freshwater and living in saltwater several times during their life. Like oceans, living bodies cannot tolerate state changes of water. If water freezes inside a living cell, the resulting crystal ruptures the cell, injuring and eventually killing the organism. The freezing point of water is suppressed if more ions are dissolved (saltwater only freezes at –1.9 degrees Centigrade), which is the reason why lake ice forms before sea ice. Water below 0 degrees centigrade is considered supercooled, and living organisms in this environment have used similar mechanisms to develop anti-freeze substances which keep their body fluids from freezing. Evaporation likewise injures cells. As the concentration of ions increases, living systems cease to function. The organism dies of dehydration. Once the water fully evaporates, the ions come out of solution and once again form salt crystals. Those crystals damage any fine structure in which they are caught, which is why materials are quickly destroyed by evaporating salt water.
Gases such as oxygen and carbon dioxide also dissolve in water, which is essential for the life processes of respiration and photosynthesis. Colder water dissolves gases better than warmer water which is for example why fast-moving fish prefer cold water. The Antarctic ocean is so oxygen rich that a certain species of fish completely lacks a respiratory system – it simply takes up oxygen through its skin! As the temperature of water increases, less and less oxygen is dissolved, leading for example to summer fish kills.
Non-polar substances such as oils cannot dissolve in the polar water. All membranes of living systems are made of an oil-like substance (lipid). Oils also have a lower density than water. Living organisms use this property to increase their buoyancy by having large amounts of oil in their tissues. The same fact – that oil is less dense than water and does not dissolve – also creates a huge problem when oil is introduced into a marine environment. The oil will float on top of the water, spreading out into a huge slick which contaminates beaches and kills marine life. The incredible amount of about 3,500,000 tons of oil is spilled into the ocean every year. Some oils in fact spread out so well on water that they can cover the water with a single-molecule thick (monomolecular) film. A single gallon can cover one square mile! This film changes all kinds of ocean processes, including evaporation and wave formation and can dramatically affect phytoplankton, which provides 50% of our planet’s oxygen.
Study Topics:
Find a river that no longer reaches the ocean.
What is one of the great mysteries of salmon.
Test tiny amounts of oil in shallow tank.
Freeze and thaw a tomato
Evaporate salt water
Resources:
Oceanography, An Understanding of the Ocean Environment, The United States Power Squadron. 888.367.8777. www.usps.org.