An Introduction to Environmental Chemistry

228 Chapter Six

Fe (nmol l–1)

Depth (km)

0

0 0.2 0.6 0.8

4.0

3.0

0.4

3.5

0.5

1.5

1.0

2.0

2.5

Fig. 6.25Dissolved iron concentrations in Gulf of Alaska (NE Pacific) seawater showing

nutrient-like profile (compare with Fig. 6.20). After Martin et al. (1989), with permission from

Elsevier Science.

sustain high levels of primary production. In these regions, dissolved iron rather

than the macronutrients nitrogen, DIP or silicon becomes the limiting nutrient.

This appears to be particularly the case in the Southern Ocean around Antarc-

tica, the ocean area furthest from dust sources. Oceanographers have puzzled for

many years over the problem of why dissolved Southern Ocean macronutrient

concentrations remain high in summer while phytoplankton growth rates are

relatively low. This situation contrasts strongly with other upwelling areas where

ocean mixing processes bring nutrient-rich deep water to the surface, promoting

very high rates of phytoplankton growth (Plate 6.3, facing p. 138). Iron limitation

of the Southern Ocean explains this puzzle. Clear evidence of iron limitation in