Oceanography (2+1)

Nitrate in Oceans

Nitrate in the oceans, is the end product of decomposition of organic nitrogen. Decomposition proceeds through organic nitrogen, ammonia, nitrite and nitrate in successive stages. Plankton utilizes all the inorganic nitrogen forms for their live processes and especially during spring and summer they tend to deplete the inorganic nitrogen supply in surface waters. Regeneration of nitrogen is accomplished mainly by bacterial action although some chemical breakdown does occur.

Nitrate is found at all depths in the oceans in higher concentrations than the other forms of nitrogen. Maximum nitrate concentrations are found at greater depths than that of phosphorus, which suggests that nitrate generation is somewhat faster than that of phosphorus.

The distributions of nitrate and of phosphate are quite similar in the oceans. Figure 1 gives the nitrate distribution with depth for the Atlantic, Pacific, and Indian Ocenas. It is apparent from figure that the nitrate concentrations in the Atlantic Ocean are much lower than those in either the Indian or Pacific oceans. The Antarctic Ocean has about twice the nitrate concentration of Atlantic Ocean.

All deep waters of the Atlantic Ocean are relatively rich in nitrate while the surface waters of the South Atlantic are generally richer in nitrates than surface waters of the North Atlantic. The Northern Pacific Ocean waters are higher in nitrates than the Southern Pacific waters. In the Antarctic Ocean the nitrate concentrations may reach 0.5ppm, but surface waters in the tropical areas are nearly devoid of both phosphate and nitrate. The table shows the variation in nitrate along the 30°W longitude line and points out clearly the high polar nitrate concentrations compared to the small nitrate concentrations found in the tropical surface water.s

Ammonium concentrations are most pronounced in the surface waters of the oceans but never are as high as the concentrations of nitrates. Surface waters contain their maximum amount of ammonium during winter. During the spring the minimum ammonium concentrations occurs; in fact, there may be complete depletion. In deep ocean layers ammonium, concentrations are around 0.002 ppm . Since ammonium compounds are a result of the first step in the decomposition of organic nitrogen, it is reasonable that the maximum concentrations would occur in winter. Except for anoxic basins, such as the Cariaco Basin, ammonia does not occur in significant quantities below the euphotic zone in the deep oceans.

Nitrite is found in very low concentrations in the oceans (on the order of a few milligrams per cubic meter) and almost entirely nil in the surface layers, the maximum nituite being found at depth  somewhere between 100 and 300m. Maximum nitrite concentrations are usually found just below the photic zone.