An Introduction to Environmental Chemistry

Section 5.5.1) have little effect on oceanic NO 3 - concentration, assuming that

NO 3 - is effectively mixed throughout the ocean volume (Section 6.8.3).

In addition to the actual nutrient elements, many other elements show

nutrient-like behaviour in the oceans, i.e. low concentrations in surface waters

and high concentrations at depth (Fig. 6.22). This distribution implies that bio-

logical removal rates from surface waters are rapid, although it does not prove

that these elements are limiting, or even essential, to biological processes. In the

case of some metals (e.g. zinc (Zn)), a clear biological function has been estab-

lished. However, for other metals (e.g. cadmium (Cd)), there is less evidence for

a biological role; cadmium is usually thought of as a poison, although not at the

extremely low concentrations (<0.1 nmol l-^1 ) found in seawater. Elements like

cadmium probably show nutrient-like behaviour (Fig. 6.22) because they are

inadvertently taken up during biological processes, or substitute for other ele-

ments when these are in short supply. The Cd^2 +ion has chemical similarities to

Zn^2 +, thus the nutrient-like cycling of cadmium may reflect inadvertent biologi-

cal uptake with—or substitution for—zinc.

Finally, we should be aware that even metals with a clear biological role (e.g.

Zn^2 +) can be toxic at sufficiently high concentrations (see Box 5.5). This reminds

222 Chapter Six

0 0 1 2 3 4

10 20 30 40

(a)

Depth (km)

Atlantic

Indian

Pacific

NO– 3 (mmol l–1)

0 1.0 2.0 3.0 4.0

(b)

Atlantic

Indian

Pacific

DIP (mmol l–1)

0 100 200

(c)

South

Atlantic

Indian

Northeast

Pacific

North

Pacific

H 4 SiO 4 (mmol l–1)

Fig. 6.20Vertical distribution of dissolved nitrate (a), phosphorus (b) and silicon (c) in the Atlantic, Pacific and
Indian Oceans. After Svedrup et al.(1941), reprinted by permission of Pearson Education Inc., Upper Saddle
River, NJ.