The distribution and role of "available" iron in two estuaries Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9593tz351

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  • A detailed study of iron concentrations measured by the reducible, reactive ferrous, and extractable methods was conducted in Auke Bay, Alaska during June 1973 and in Hood Canal, Washington during October 1973 and April 1974. The concentrations by each method were generally highest at the bottom and decreased towards the surface with occasional pockets of high iron concentration water at the surface. In Hood Canal in October the maximum reactive ferrous iron concentration was observed at the bottom of the pycnocline. Large changes in the iron concentration were observed between successive surveys. The reducible iron method (range <0.8 to 187 μg Fe/L) measures colloidal ferric hydroxide plus easily dissolvable and reducible organically and inorganically bound iron. The reactive ferrous iron method (range <0.04 to 4.2 μg Fe/L) measures loosely bound organic and inorganic ferrous iron. The extractable iron method, as finally used, (range <4 to 170 μg Fe/L) measures that iron measured by reducible iron method plus up to twice the tightly bound organic iron. We have defined "available" iron (with quotes) as that iron which appears to be taken up by phytoplankton as determined by the correlation between the in situ iron concentration and the chlorophyll a concentration. Chlorophyll a concentration maxima generally occurred at the depth of the iron concentration minima, strongly supporting the idea that each method measures "available" iron. However, significant correlation coefficients for the regression of iron concentration versus chlorophyll a only support the "availability" of reactive ferrous iron and that phytoplankton prefer ferrous iron to ferric iron. This was one of the first extensive uses of Lewin and Chen's (1973) ferrous iron method and values observed were generally 2 to 100 times less than they reported. Changes occurring during water storage appear to be a function of p0₂. Major changes took place during the first five hours of storage. Interstitial water was greatly enriched in iron compared with the overlying water. The ferrous iron concentration was particularly high due to the low Eh of the sediments. Although the net flow of "available" iron is out of the sediment, the flux is small (approximately 3 x 10⁻⁵ μg Fe/m²sec). Most of the iron that appears to be coming from the sediment is probably being produced by bacterial decomposition as in the case of subsurface sources of iron associated with ammonia or urea maxima. Fresh water is a significant source of extractable and reactive ferrous iron into the well-mixed surface layer. Fresh water is also a source of organically bound iron. Zooplankton appear to produce "available" iron by eating or excretion. Light appears to cause the production of ferrous iron through an undetermined mechanism.
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