Iron as a Limiting Nutrient for Primary Productivity

Three major oceanic regions (20 % of the world s open ocean) are characterized by high-nitrate and low-chlorophyll (HNLC) concentrations. The PP of the Southern Ocean (Broecker et al., 1982), the equatorial Pacific (Chavez and Barber 1987) and the Gulf of Alaska (McAllister et al. 1960) are obviously not limited by nitrate. Alternatively, as atmospheric dust loads in the Antarctic and equatorial Pacific are the lowest in the world (Prospero 1981 Uematsu et al. 1983) the importance of iron as limiting micronutrient for PP became increasingly discussed. 

The effect of added atmospheric dust to clean sea water from HNLC-regions were studied in 

A total volume of 15,600 1 of iron solution (450 Kg Fe) were distributed by ship over an area of 64 km which increased the original dissolved iron concentration of 0.06 nM to 4 nM. Iron 

If iron is a limiting micronutrient for present-day PP, it may be an important link to explain glacial-interglacial climatic cycles of the past. Martin (1990) postulated the iron hypothesis which explains decreased atmospheric CO concentrations during glacial times with increased iron deposition by aeolian input resulting in increased PP and thus increased CO - 

So far, dissolved iron has been discnssed with respect to the assimilation by phytoplankton. The chemical state of bioavailable dissolved species is presently a matter of intensive stndies and discussions. Due to thermodynamic reasons concentrations of free ions of dissolved iron are extremely low under oxic and pH-nentral conditions. A discussion paper by Johnson et al. (1997) reviews regional distributions and depth profiles of dissolved iron and points ont that at greater depth the iron concentrations always remain constant of 0.6 nM. Other elements with such short residence time (100 to 200 years) typically continuously decrease with depth and age. This suggests a substantial decrease in the iron removal rate below this concentration. As organic ligands with a binding capacity of 0.6 nM Fe have been found (Rue and Bruland 1995 Wn and Luther 

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