The role of iron as a nutrient in the oceans

Although abundant in the Earth's crust (see Fig. 1.3), iron is present in seawater at very low concentrations (about 1 nmol l-1or less) because the thermodynami-cally stable Fe(III) species is both insoluble (see Fig. 5.2) and particle reactive, being a highly charged small ion (Section 6.5.5). Despite this, iron is an essential component for a number of life-supporting enzyme systems including those involved in photosynthesis and nitrogen fixation. It may appear surprising that phytoplankton have evolved with an essential requirement for an element present at such low oceanic concentrations. However, the primitive algae that eventually gave rise to modern phytoplankton probably evolved at a time of much lower global oxygen concentrations in a mildly reducing ocean, when soluble Fe(II) was the dominant iron species.

It is only relatively recently that oceanographers have been able to measure accurately dissolved iron concentrations in the oceans; earlier efforts were hampered by a combination of the low dissolved iron concentrations and the ubiquitous sources of iron contamination that compromised sampling and analysis. The distribution of dissolved iron in the oceanic water column is now known to be mostly nutrient-like (Fig. 6.25). Surface waters can have concentrations below 0.1nmoll-1 due to phytoplankton uptake, increasing to about 1nmoll-1 in deep waters throughout the oceans. These higher deep-water concentrations are maintained by complexation of Fe(III) by strong organic ligands (Box 6.4), which prevent scavenging onto particles. Despite this, the residence time of dissolved iron is thought to be only a few hundred years, similar to that of other scavenged elements. Thus the behaviour of iron in seawater is similar to both nutrient and scavenged elements.

As much of the potential riverwater input of dissolved iron is rapidly stripped out of water during estuarine mixing, the main external source of iron to the open oceans is from the (very limited) dissolution of wind-blown soil and dust. This is material derived primarily from the great Asian, African and Middle Eastern deserts (Plate 6.2, facing p. 138) of the northern hemisphere. Dust has an atmospheric residence time of only a few days, much shorter than hemispheric atmospheric mixing times. This means that the southern hemisphere oceans receive much lower atmospheric dust inputs than those in the northern hemisphere. For example, dust inputs to the North Pacific are 11 times greater than those to the South Pacific.

In some areas of the oceans, it is now clear that the supply of dissolved iron from terrestrial-derived atmospheric dust and from upwelling is inadequate to

Fig. 6.25 Dissolved 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 Antarctica, 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 these areas has come from a recent experiment in which iron was added to a small patch of 'chemically labelled' surface water in the Southern Ocean. The chemical label (an artificial compound called SF6) was used to trace the movement of the iron-enriched patch of water, and within a few days, phytoplankton abundance had increased markedly (Fig. 6.26).

During the last ice age, approximately 20000 years ago, dust fluxes to the oceans were higher because the climate was drier and windier. It has been proposed recently that the resulting enhanced supply of dust-derived iron at this time increased ocean productivity directly by increasing photosynthesis. The high dust-derived iron flux may have also promoted enhanced productivity by bacterial nitrogen fixation in the nitrate-deficient central ocean areas, as this enzyme system has a requirement for iron (see above). If increased productivity did take place, it would have also consumed CO2 because of increased photosynthesis (see eqn. 5.19) lowering global atmospheric CO2 (see Section 7.2.2) and hence sustaining the glacial climate. The proposed linkage between the biogeochemical cycling of iron and CO2 is debated hotly between scientists and is the subject of much ongoing research.

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