Simply said, ocean water is salty. Ocean water contains salts that were carried from rivers or from erupting volcanoes that make the water salty. The “saltiness” of the ocean is termed salinity. Salinity is the total salt content measured in parts per thousand (ppt). The average salinity of the Earth’s ocean is 35ppt, which means that there are 35 pounds of salts per 1000 pounds of water. Ocean water contains magnesium, sulfate, potassium, calcium, and other minor elements. But the most abundant salt ions are sodium and chloride, which make up 85% of the dissolved salts in ocean water.
Sea water is about 96.5% pure water and 3.5% dissolved solids and gases. These dissolved solids change the chemical properties of the water, thus changing some of the water’s characteristics. One such example is that it takes less energy to evaporate sea water than pure water. The dissolved solids also inhibit the formation of the crystal structure during freezing. The saltier the water, the lower the freezing point. Therefore, as seawater freezes, the ice formed is almost pure water.
Most organisms can not use sea water as drinking water. Marine organisms have special adaptations to utilize the water found in the oceans. But terrestrial and freshwater organisms can not use salt water for consumption. Consuming salt water will actually make the organism dehydrated.
To help solve the problem of water shortages, scientists have begun to examine different ways of taking the salt out of salt water. Desalination is the process of removing salt from ocean. One way to remove the salt from ocean water is to evaporate the seawater and collect the freshwater that condenses from the resulting steam. When the water evaporates, the salt will be left behind separate from the pure water collected.
Another way to desalinate water is to use glaciers. When sea water freezes, the ice formed is nearly fresh water. The salt once in the water remains under the ice, making the remaining water very dense and salty, but the ice is nearly pure.
Power of Hydrogen
pH is known as the degree of acidity or alkalinity of a liquid based on hydrogen ion concentration. The pH of a substance is measured on a scale from 0 to 14. This scale is called the pH scale. When water disassociates it forms positive ions and negative ions.
The negative ions are the OH- ions, which cause a solution to become alkaline and the positive ions are the H+ ions, which cause a solution to become acidic.
Pure water has a pH of 7, but most ocean water is, on average, about 8, which means it is slightly alkaline. Ocean water is slightly alkaline because there are more hydroxyl ions (OH-) than hydrogen ions (H+) in the water.
Ocean water also contains buffers, which are chemicals to help stabilize the pH of water. The definition of a buffer is “a substance that lessens the tendency of a solution to become too acidic or too alkaline.” One such buffer found in ocean water is carbonate (CO3–). Carbonate is formed by the reaction of carbon dioxide and water. Look at the chemical reaction below. The carbonate buffer can absorb free hydrogen ions to stabilize the ocean water at a pH of 8.
CO2 + H2O H2CO3 HCO3– + H+ CO32- + 2H+
Dissolved Oxygen (DO) is the amount of oxygen dissolved in the water. DO is measured in units of parts per million (ppm) or milligrams per liter (mg/L). DO levels in ocean water range from about 1 to 12 ppm. Just for comparison, the amount of O<sub>2</sub> in the air is 200 ppm!
The minimum DO level needed by fish and other aquatic animals to breathe is 4 ppm; below this level, a condition called hypoxia occurs. Hypoxia occurs when oxygen levels in the water decrease causing fish and other aquatic animals to suffocate. Hypoxia can result from algal blooms (anaerobic bacteria) and sludge dumping.
There are two major chemical reactions that contribute to maintaining levels of dissolved oxygen in ocean water. These chemical reactions are the same reactions that contribute to maintaining oxygen levels within the atmosphere; photosynthesis and cellular respiration.
Oxygen is a diatomic gas, which means gaseous oxygen moves in pairs as O2. Photosynthetic algae and plants take in carbon dioxide, water, and solar energy to produce carbohydrates and oxygen. Marine organisms then consume this oxygen either by diffusion or through gills. These organisms use oxygen and carbohydrates for cellular respiration and release carbon dioxide. Once again, the plants and algae take up the carbon dioxide and the cycle continues.
As depth increases, the DO decreases. The DO levels will continue to decrease with depth until the measured DO is zero. The depth at which there is the lowest amount of oxygen is called the O2 minimum zone. This is typically found at a depth of 1000 meters.