The NO 3 - concentrations in some rivers have been increased by human activities
to concentrations as high as 500mmol l-^1 , although the greatest changes have gen-
erally been in small rivers which make a small contribution to the total river flux
to the oceans. If, as an extreme case, we suppose that 10% of the world river flow
increased to this high concentration, we can calculate how long such a situation
would take to double the oceanic NO 3 - concentration, i.e. introduce 41¥ 1015 mol.
The calculation is simplified by ignoring NO 3 - removal to sediment. In reality,
increased NO 3 - inputs would result in increased biological activity and increased
NO 3 - removal from the oceans, since the main sink for nitrogen and phosphorus
is burial in organic matter.
If 10% of the world’s rivers have NO 3 - concentrations increased to
500 mmol l-^1 , total riverine NO 3 - inputs become:
eqn. 6.22
The time needed to double the oceanic nitrate inventory, assuming no removal
at all (hence a minimum estimate), is then:
eqn. 6.23
Thus, even drastic perturbations of the freshwater NO 3 - input to the oceans
cannot alter the seawater concentration rapidly because of the huge oceanic
reservoir of this element. This contrasts strongly with regional seas where NO 3 -
concentrations have increased in pace with the enhanced inputs and in some cases
have had significant ecological effects.
6.9 Further reading
Berner, K.B. & Berner, R.A. (1987) The Global Water Cycle.Prentice Hall, Englewood
Cliffs, NJ.
Broeker, W. & Peng, T.-H. (1982) Tracers in the Sea.Lamont Doherty Geological Obser-
vatory, Palisades, New York.
Chester, R. (2000) Marine Geochemistry.Blackwell, Oxford.
Drever, J.I., Li, Y-H. & Maynard, J.B. (1988) Geochemical cycles: the continental crust
and oceans. In: Chemical Cycles in the Evolution of the Earth, ed. by Gregor, C., Garrels,
R.M., Mackenzie, ET. & Maynard, J.B., pp. 17–53. Wiley, New York.
Falkowski, P.G., Barber, R.T. & Smetacek, V. (1998) Biogeochemical controls and feed-
backs on ocean primary production. Science281, 200–206.
Humphris, S.E., Zierenberg, R.A., Mullineaux, L.S. & Thomson, R.E. (1995) Seafloor
Hydrothermal Systems: Physical, Chemical, Biological and Geological Interactions.Geophysical
Monograph 91.American Geophysical Union, Washington, DC.
Libes, S. (1992) Marine Biogeochemistry.Wiley, New York.
Turner, B., Clark, W., Kates, R., Richards, J., Mathews, J. & Meyer, W. (1990) The Earth
as Transformed by Human Action.Cambridge University Press, Cambridge.
41 10
245 10
16 700
15
3
12 3 1
¥
¥
=
mol NO
mol NO yr
years
.
20 10 3 6 10 1 0 9
500 10 3 6 10 0 1
245 10
6 3 1161
6
3
1161
12
3
1
[]¥ ()¥¥()¥
+¥[]()¥¥()¥
=¥
--- -
--- -
--
mol NO l
mol NO l
mol NO yr
..
..
.
yr
1 yr
The Oceans 237
