Introduction to biogeochemical cycles

Much of the discussion of environmental processes in Chapter 6 will deal with single environmental reservoirs, such as the atmosphere, or the oceans. Many important processes involving chemicals, some of which have given rise to significant environmental problems, have involved transfer of chemicals from one reservoir or compartment to another. The transfer of chemical elements and compounds between compartments, taking into account the important influences of the biota, is termed biogeochemical cycling. Much work has been carried out to elucidate the biogeochemical cycles of the elements, but much remains to be done to describe the cycles in adequate detail.

The major transfers taking place in a biogeochemical cycle appear in Figure 5.1. An element is washed from rock and soil and enters fresh water streams and lakes in runoff, and to a lesser extent, in water flowing from ground waters into surface waters. The major part of that entering the fresh waters is carried by streamflow into the oceans. Direct coastal discharges of pollutants can also be important, especially in fully and partially enclosed seas like the North Sea. Volatile elements (e.g. mercury) and compounds are degassed from land surfaces and the sea. Less volatile materials enter the atmosphere from the land surface in wind-blown dusts (and man-made emissions) and from the sea surface in sea spray, the smaller particles of which form marine aerosol.