An Introduction to Environmental Chemistry

with the organic tissue of organisms, while silicon and calcium (Ca) are cycled as
skeletal material. The decomposition of organic tissue is mainly by bacterial res-
piration, a rapid and efficient process. By contrast, skeletal material is dissolved
slowly (Sections 6.4.4 & 6.4.5). The effect of these different decomposition rates
is that the NO 3 - and DIP concentration profiles show rapid increase with depth,
implying shallower regeneration of material in the water column than silicon.
Nitrate and DIP distributions are therefore closely correlated (Fig. 6.21) with a
slope of approximately 16 : 1. This ratio reflects the relative proportions of nitrate
and DIP regeneration and utilization by phytoplankton. This ratio is often
referred to as the Redfield Ratio in honour of Alfred Redfield who first described
the close linking of these two ions in the ocean.
Biological cycling not only removessome ions from surface waters, it also trans-
forms them. The stable form of iodine (I) in seawater is iodate (IO 3 - ), but biologi-
cal cycling results in the formation of iodide (I-) in surface waters, because the
production rate of the reduced species is faster than the rate of its oxidation. Bio-
logical uptake of IO 3 - in surface water results in a nutrient-like profile, contrast-
ing with the conservative behaviour of most halide ions, for example Cl-and Br-.
The biological demand for NO 3 - also involves transformation. Phytoplankton
take up NO 3 - and reduce it to the –3 oxidation state (see Box 4.3 & Fig. 5.12) for

The Oceans 219

0

0.2

0.4

0.6

0.8

0 40 80 120 160

Molybdenum (nmol l–1)

0

0.2

0.4

0.6

0.8

0 0.02

Tungsten (nmol l–1)

0.04 0.06 0.08

1

2

3

4

5

Depth (km)

1

2

3

4

5

Depth (km)

Fig. 6.19Vertical distribution of dissolved molybdenum and tungsten in the North Pacific.
After Sohrin et al.(1987), with permission from Elsevier Science.