What's the Ocean Study Guide (page 2)

Updated on Sep 26, 2011

Chemistry In the Oceans

The ocean is not just water, as any mouthful that you get while swimming at the beach can tell you. There are chemicals in seawater. When you evaporate the water from a cup of seawater, the salt remains. It is typically about 3% by weight of the seawater, quite a significant amount.

Here is the breakdown of the contents of the ocean's salt:

      Chloride (chlorine ions) 55%
      Sodium ions 30%
      Sulfate ions 8%
      Magnesium ions 4%
      Calcium ions 1%
      All the other ions 2%

So just the top five ions equal 98% of the total salt. The top two—chloride and sodium—make 85% of the salt, which is why we typically call salt sodium chloride. But if you actually use sea salt, manufactured by evaporating salt water, then the other ions are present as well. Indeed, dozens of other ions are found in the 2% of "all others" in the list above. Some of these lower-ranking ions are crucial to life, which we will see in a moment. But first, you'll need some more information about the ocean's salt.

The ocean does vary a bit from place to place in its salinity. Rather than speaking of percent salt, oceanographers use a related term called parts per thousand. After all, percent is actually parts per hundred. So if the ocean's salinity is 3%, that is the same as 30 parts per thousand. The symbol for parts per thousand adds another zero to the denominator of the percent sign. Therefore, 30 parts per thousand is written as 30‰. Much of the ocean's salinity is around 34, but it varies from about 33‰ to as high as 36‰. What causes this variation?

Some local variation is caused by the fresh water of rivers that flow into the ocean—for example, the Amazon. But much of the variation in salinity of the open ocean, away from the coast, is caused by different balances of the two processes that add or subtract water from the ocean's surface. These two processes, respectively, are precipitation and evaporation.

While precipitation (rain and snow) and evaporation occur everywhere across the ocean, they are not always in balance. Where precipitation exceeds evaporation, that area of the ocean will become a little fresher, or less salty. Where evaporation exceeds precipitation, the region will become more salty, or have a higher salinity. No area of the ocean is isolated, as we have seen, so no zone gets ever saltier or fresher through time. Zones where evaporation exceeds precipitation will just have a higher salinity than the ocean average.

The Mediterranean Sea is especially salty. Rainfall is low, and the sea doesn't have that many freshwater rivers flowing into it. But the Mediterranean Sea has plenty of evaporation. It has some of the saltiest water in the world, and it flows out into the Atlantic Ocean through the Straits of Gibraltar.

The Arctic Ocean is relatively fresh. It is cold and thus evaporation is low. It does receive snow and rain, but another major factor is that it receives large amounts of fresh water from the rivers in Russia that flow north and empty into the Arctic Ocean. In essence, the Arctic Ocean gets the precipitation that falls on Russia.

The ocean's equatorial belts have a relatively high salinity. Sure, it rains a lot over the tropical oceans because of the Hadley cells. But that rain came from the ocean, from evaporation and then condensation into clouds. The important point is that some of the evaporated water is transported by winds to the continents, feeding rain to the tropical rainforests. Thus, more water is evaporated from the tropical ocean than falls back to the ocean as rain. Both evaporation and precipitation is high in these regions, but evaporation exceeds precipitation, making the tropical ocean belts saltier than the world ocean average.

Now we will look at two of the minor ions of the ocean's chemistry: phosphate and nitrate (with the chemical formulas: PO43–and NO3). These are present in tiny amounts in the ocean overall and even less so in the surface ocean, where most of the ocean's life exists. Photosynthesizing algae and bacteria, which together form the phytoplankton, are a major component of the life in the surface ocean.

Phosphate and nitrate are taken up from the ocean's water by the phytoplankton during photosynthesis, the process that uses sunlight to convert chemicals from the environment into living bodies. Two of the substances absolutely essential for all living things are phosphorus (P) and nitrogen (N). These are present in the ocean ions phosphate and nitrate. Thus, the plankton, when living in the sun-drenched upper portion of the ocean, actively take phosphate and nitrate into their bodies.

During this uptake, phosphate and nitrate are removed from the ocean water. The living things actually change the chemistry of the water. In fact, the phytoplankton can be so highly active that the amounts of phosphate and nitrate are reduced to nearly zero across the world ocean, on average. You can see this effect in Figure 13.2, where the amounts of P and N near the ocean's surface are close to zero.

Figure 13.2

You can also see in Figure 13.2 that the amounts of P and N increase with depth, reaching levels of nearly 80 for P and more than 500 for N at a depth of 1 km, then decreasing a bit but remaining quite abundant all the way to the bottom of the ocean at the average 4 kilometer depth.

The difference in the concentration of phosphorus and nitrate at different depths is caused, as noted, by the consumption of these two nutrient elements by the phytoplankton at the ocean's surface. But another biological factor is at work as well. As the phytoplankton die, their bodies fall downward in the ocean (while alive, the phytoplankton have ways to remain buoyant). Also, other creatures, such as tiny swimming crustaceans, feed on the phytoplankton and excrete wastes. These wastes also float downward into the deep ocean.

The dead cells and various kinds of wastes (from fish, too) carry bacteria on them. The bacteria feed on the falling wastes, and, as they feed, convert the wastes back into phosphate and nitrate, both of which are returned (regenerated) to the seawater by the bacteria. This is the reason that phosphate and nitrate are so abundant in the deep water; they are regenerated back into the water by the bacteria who feed on the falling waste products of the surface life. We see in this example how life in the ocean influences the actual chemistry of our huge expanse of salty water.

Practice problems of this concept can be found at: What's the Ocean Practice Questions

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