The Chemistry of the Oceans and Coral Reefs

This topic submitted by Jennifer Guard ( at 9:15 AM on 6/12/04.

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When someone thinks about the ocean or coral reefs they think of the immense amount of species in it. The biology of the oceans or coral reefs is just one aspect of them. Truly, the most important aspect of coral reefs, and any body of water, is its chemistry. Knowing about how the carbon and other cycles in the ocean work can help people to better understand the biological processes. The chemicals and nutrients in the water can help or hinder the growth of marine life. So, I will be teaching the class about the chemistry of coral reefs and the oceans. I believe this topic is essential in understanding any of the processes occurring in the oceans.

Most species are very dependent on their environment. A few degrees hotter or colder can be life threatening to an organism, just as putting most saltwater creatures into freshwater or freshwater creatures into saltwater. Some people may believe that the reason you can not put a saltwater creature into freshwater, is because it is not its environment. The truth is, the saltwater creature has a higher or hypertonic solution of salts inside its body and by placing it into freshwater, it swells from taking in all the freshwater. Unless it has an internal system of getting rid of the freshwater or keeping the salts inside, it will eventually burst and die.

Most marine science majors, or at least at my college, do not see the importance of understanding the chemistry of bodies of water. To them it is some boring, tough subject that they do not even want to understand. What they do not understand and know is that just knowing one aspect of marine science is not enough. One aspect can not explain everything. How could you explain coral and zooxanthellae’s symbiotic relationship without understanding that the corals provide zooxanthellae with carbon dioxide and ammonia, and they intern provide nitrogen for the corals as well as help in producing aragonite (calcium carbonate) for the corals skeleton? (Davidson, 16-7) The chemistry distinguishes the tests of diatoms from foraminifera, one being calcareous and the other made on silica. Most importantly to geologists, knowing the chemical make-up or signature of a core sample or fossil can help determine the approximate age of the sample, or in core samples, it can date each layer. As far as corals, they are dated by either finding out about the chemistry of their bands, or by measuring the concentrations of radioisotopes in their skeleton at the time of accretion and after (Druffel, 8356).

Marine scientists can benefit from understanding the chemistry because the chemical make-up of bodies of water helps to tell what drives them. Also in better understanding the chemistry, you do not only have a better understanding for what is happening to, around, and within an organism, but a better understanding of the geological, physical, and biological processes. Chemistry is linked to everything, and everything is linked to chemistry. Chemistry makes up the DNA in every cell, and the food consumed. It is the start on every food chain, photosynthesis, and is one of the last things in an organism when it dies.

There are many important and much talked about cycles in the ocean, which are also affected by the atmosphere. Cycles in the ocean are not just exclusively devoted to the ocean. The concentrations of chemicals in the atmosphere will eventually affect the amount of chemicals in the oceans. Also for most, if not all the cycles in the oceans, there needs to be a source of the chemicals outside of the ocean like run-off from land or the flushing of rivers.

Of all the cycles in the ocean the two most talked about are the carbon cycle and the oxygen cycle. More particular to the carbon cycle is the carbon dioxide cycling. Carbon dioxide readily exchanges between the air, water, and solids due to its chemistry and participation in biogeochemical cycling (Shaw). Carbon dioxide in the ocean is controlled by its ability to form multiple species in seawater. Because it dissociates in the ocean, it can make up a quantity called alkalinity, specifically the carbonate alkalinity. The carbonate system in surface waters is controlled by carbon dioxide solubility, carbon dioxide uptake in photosynthesis, and carbonate uptake as calcite and aragonite (Shaw). The net removal of carbon dioxide from the surface waters is controlled by the preservation of particulate carbon. This particulate carbon may be preserved in an organism like a foraminifera that has a calcareous test. In the deep sea, the carbonate system controls the net sink for atmospheric carbon dioxide as buried calcium carbonate (Shaw). The preservation of calcium carbonate is truly controlled by the saturation index.

Knowing that all organisms and even nonliving organisms, rocks, or minerals, have an amount of carbon in them, they are able to be dated. Typically to date these, radioactive isotopes are looked at like Carbon-14, Radon-228, or Thorium-230. Once an organism dies, the amount of carbon, nitrogen, and other chemicals in it start breaking down and are lost. So by looking at the half life of different isotopes in carbon and the other chemicals, scientists are able to give an approximate age of the nonliving organism, rock, or mineral.

The oxygen cycle, though very important, is more readily understood. The oxygen cycle is like a complement to the carbon cycle, in that during photosynthesis oxygen is received from the initial carbon dioxide, and carbon dioxide received after respiration. Typically, if the total amount of carbon is known, scientists can use the Redfield Ratio that says on average for every 106 carbon there should be 138 oxygen. This ratio also compares nitrogen and phosphate to the carbon and oxygen.

Sodium and chloride are two lesser cycles known to most people. While most people know that sodium and chloride make up most forms of salt and are the cause for the salinity in the oceans, they do not know that they have a long residence times in the oceans. The reason for sodium having a typical residence time of about seventy million years and chloride a hundred million years is due to them not being actively used in biological processes (Pilson, 53,60). Some mammals have concentrations of sodium and chloride in the blood, but not enough to change the total amount in the oceans greatly. Also as salts enter the oceans from rivers and run-off, they are not taken up and start contributing to the already large concentration.

Silicate cycling is very important to like diatoms that have siliceous tests. At the surface waters, silica is depleted by biological production, and then eventually return to the deep waters by decomposition and respiration of sinking materials (Shaw). Unlike other cycles, the silicate cycle has a very consist concentration from about 1000 meters to the bottom. Silica has major control on the phytoplankton community structure in the oceans. The estimates for the silica budget in the water columns has a total input of about 17.5E14 grams per year, while at the same time biological uptake to form organisms tests is about 144E14 grams per year (Shaw). Silica is preserved in ocean sediments when it becomes saturated in sediment pore waters. Though it seems that silica is an essential nutrient, it actually is not. It may not be essential, but the export of particulate organic carbon from surface waters varies as a function of silica, and blooms associated with the upwelling of silica result in high particulate organic carbon export (Shaw).

Nitrate is another important and essential nutrient in oceans. Unlike silica there is less nitrate in the waters, and from about 1000 meters on down the amount of nitrate is lower. Nitrogen can be compared to carbon and oxygen in water columns, in that for every 106 Carbon there are about 16 Nitrogen. Nitrogen is controlled in the oceans also by the participation in biological systems, and as an electron acceptor. Sources of nitrogen come from places like rivers and groundwater, and even from volcanic activity. The removal of nitrogen is controlled by its burial in its particulate organic form and by denitrification (Shaw). Nitrogen is also fixed in the ocean by organisms when they intake nitrogen and release ammonium. Also there is denitrification that takes nitrate and converts it into nitrogen. Typically nitrogen is thought to be a limiting nutrient because of its importance to organisms and its low overall concentrations.

Another limiting nutrient in the oceans is phosphorus. Typically, phosphorus is much like nitrate in that it has higher concentrations above 1000 meters, than below. Compared with the other essential nutrients phosphate has the lowest concentrations. The Redfield ratio can also be used with phosphate in that for every 106 Carbon there are only one phosphate. The phosphorous cycle in the ocean is controlled by mineral sources and biogeochemical remove mechanisms (Shaw). In seawater inorganic phosphorous will make ion pairs with calcium and magnesium. High amounts of phosphorous in the water columns cause phytoplankton blooms, which are not always good.

The chemistry of the ocean has been basically stated above. Besides these nutrients and chemicals, there are trace elements. Trace elements do not play as a vital role as do the nutrients, but can be used as past tracers of environments, and help geologists reconstruction what the atmosphere and environments used to be like. Like stated above, the most important nutrients and chemicals in the oceans that drive phytoplankton and other organisms is carbon, oxygen, nitrogen, phosphorous, and silica. While normally carbon and oxygen are in high concentrations, limiting nutrients like silica, nitrogen, and phosphorous are not and are very essential to the underwater environments. In the oceans there are excess nutrients and chemicals like the salts made from sodium and chloride. These excesses are seen by high residence times.

Corals are made up of a few generally known chemicals. The most known being aragonite, because aragonite, which is calcium carbonate, makes up the skeleton of the corals. In order to grow, corals calcify with the help of the zooxanthellae in them. Studies have been done that show that corals with zooxanthellae in them calcify nineteen times faster than corals without them (Druffel, 8356). The calcification of the corals is greatly affected by the carbon dioxide chemistry and the nutrient concentrations in the seawater (Marubini, 117). From the study done by Marubini, he found that the effects of low concentrations of carbonate and pH on corals are not permanent and that when introduced into normal waters the effects were immediate and reversible (Marubini, 117). The carbon cycle of coral reefs is driven by biological processes.

While carbonate is very important to corals and the reefs, the nutrients in the reefs need to be very low in order for the corals to obtain their maximum growth. The nutrients are able to be in such low concentrations in reefs because before the nutrients enter the reef they are filtered out by mangroves, who take up most of them, and sea grass beds (Davidson, 65-71). There are some reefs though that are thriving even though they are nutrient loaded. Fringing reefs can take up atmospheric carbon dioxide due to their enhanced organic carbon production (Kawahata, 261).

Just like trees, the rings of coral sections can be counted and analyzed to date the corals. The corals are also dated by measuring the concentrations of radioisotopes in their skeleton (Druffel, 8356). The levels of trace or minor elements in corals like calcium can be used to reconstruct past concentrations of the elements in the seawater (Druffel, 8356). Like Davidson put it, corals are “treasure troves” and “historical climatic recorders”.

Overall, the chemistry of the oceans and coral reefs is essential to know. Knowing just one aspect of the environment is not enough. All chemicals and nutrients, be it trace, limiting, essential, or excess are vital in the processes of the oceans. With a better understanding of the chemical processes, people can better understand the ways of the oceans and its biologics.

Davidson, Osha Gray. The Enchanted Braid. 1998. John Wiley and Sons, Inc.,
New York. Pp. 16-7, 65-71.
Druffel, Ellen R. M. Geochemistry of corals: Proxies of past ocean chemistry,
ocean circulation, and climate. August 1997. Proc. Natl. Acad. Sci. USA. 94 (16), 8354-8361.
Kawahata, H., Suzuki, A. and Goto, K. Coral reef ecosystems as a source of
atmospheric CO2: evidence from PCO2 measurements of surface waters.
1997. Coral Reefs. 16, Pp. 261-266.
Marubini, F. and Atkinson, M. J. Effects of lowered pH and elevated nitrate on
coral calcification. 1999. Marine Ecology Progress Series. 188, Pp. 117-
Pilson, Michael. E. Q. An Introduction to the Chemistry of the Sea. 1998. Prentice
Hall, New Jersey. Pp. 102-258.
Shaw, Timothy. Lecture Notes. 2004. University of South Carolina.

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