X-Y plots of N and P in the entire ocean
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We extracted nitrate (N) and phosphate (P) at the surface (0 m), pycnocline (750 m), and deep (3000 m) in the entire oceans from climatological normal (annual average) data from World Ocean Atlas, and plotted on XY-coordinate. The black bold line means N/P=16. What would it read?
There are almost all plots below the line of N/P=16 (black bold line). (N/P<16)
Characteristics of Surface (10 m)
At the surface of 10 m (×), there is a noticeable surplus of phosphate where nitrate concentrations are low (NO3- < 1 μmol/L). Since primary production is nitrogen-limited in many areas, likely, phosphate consumption will not proceed in those areas.
Characteristics of Pycnocline (750 m)
Even at a depth of 750 m (♢), there is a marked excess of phosphate (N/P<16) relative to the Redfield ratio of N/P=16. At the pycnocline (750m depth), the decomposition of organic matter is predominant and oxygen concentration is minimum. The decomposition of organic matter is particularly advanced in the oxygen minima of the North Pacific subarctic and eastern North Pacific subtropics (yellow-green box in the figure above). There, N/P = 13.3 (Phosphate = 3 μmol/L, Nitrate=40 μmol ). Why is the N/P ratio as low as 14 at the pycnocline (oxygen minimum layer) in the subarctic and eastern subtropical North Pacific? Here is my (Ohki's) own explanation.
Consider the reason for NO3-/ PO43- = 13.3 at the oxygen minimum layer in the subarctic and eastern subtropical North Pacific
① When diatoms reproduce at large in nutrient-rich conditions, they are known to absorb nutrients at a ratio of N/P< 14. Diatoms that rapidly grow in nutrient-rich environments have high P ratios because they store more lipids in their bodies. If a large number of diatoms were transported to the deep sea, NO3- and PO43- would be remineralized in seawater with N/P<14. The N/P ratio of 13.3 in seawater in the oxygen minimum layer of the subarctic North Pacific is probably due to the large reproduction of diatoms in the surface layer and the remineralization of nutrients by decomposition of organic matter that has settled at the pycnocline.
② It is known that nitrate-reducing bacteria, which reduce NO3- to N2, inhabit anoxic waters where organic matter decomposition is predominant. In the pycnocline, particulate organic matter is retained and organic matter decomposition and oxygen consumption proceed. The pycnocline will be anoxic and, furthermore, will approach anoxic conditions locally in the organic particles. NO3-(nitrate) reduction can occur in the organic matter particles retained at the pycnocline. If NO3- is lost from seawater due to nitrate reduction, the N/P ratio will decrease. There is circumstantial evidence that nitrate reduction is taking place. In the nitrate reduction reaction, N2 and N2O are generated simultaneously. Analyzing the distribution of N2O, which shows traces of the nitrate reduction reaction, reveals that a maximum concentration of N2O is in the pycnocline (200 to 1000 m) in the subtropical eastern North Pacific.Characteristics of Deep (3000 m)
In the deep North Atlantic near the start of the deep circulation (3000 m), at low concentrations of NO3- and PO43-, the N/P= approaches 16. At higher concentrations, the N/P ratio is decreasing. The lowest N/P ratio in the deep layer is N/P = 13.3 (Phosphate = 3 μmol/L, Nitrate = 40 μmol), just in the deep layer of the eastern North Pacific subtropical zone, connected to the plot of the pycnocline (750 m) in the same figure.
Low values of N/P<14 in deep water are found in the Antarctic Ocean, the Southern Ocean, the Pacific Ocean off North and South America, and the eastern North Pacific subarctic region. These are highly productive regions. In highly productive regions, there is probably more transport of lipid (P)-rich organic particles into the deeper layers. The subarctic region of the western Pacific is highly productive but shows a slightly higher ratio of N/P=14-15. The reason for this is not known at this time.
The Atlantic Ocean, where nutrient concentrations are lower in the deep layer, shows a high ratio of N/P=15-16. It is thought that what was N/P=15-16 at the start of the deep circulation is gradually altered by the organic matter transport from the upper layers while flowing under various ocean regions.
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