Geology Reference
In-Depth Information
encouraging the restoration of affected lakes. That this has not always
been successful, however, can be attributed in many cases to the release/
recycling of phosphorus previously deposited to and incorporated with-
in the bottom sediments of the lake systems in question. 138
The potential mobility and bioavailability of sedimentary phosphorus
are to a large extent governed by the chemical associations and interac-
tions of phosphorus with different sedimentary components. 139 Bearing in
mind the removal or transport of phosphorus to the sediments, especially
important phases are likely to be 'organic phosphorus' from deposited,
dead, decaying biota, and 'sorbed orthophosphate' on inorganic particu-
lates (e.g. iron (hydr)oxides). Many sequential extraction schemes have
been developed to investigate phosphorus fractionation in lake sediments.
Perhaps the most sophisticated is that of Psenner et al. 140 who identify,
operationally define, and separate the following fractions: 'labile, loosely
bound or adsorbed' (NH 4 Cl-extractable); 'reductant-soluble, mainly from
iron hydroxide surfaces' (buffered dithionite-extractable); 'adsorbed to
metal oxides (e.g. Al 2 O 3 )' (NaOH-extractable), subsequently distinguish-
able from 'organic' (also NaOH-extractable); 'apatite-bound' (HCl-ex-
tractable); and 'residual' (persulfate-digestible). Pardo and co-workers 141
have reviewed such approaches in the light of the development by the
European Commission of a harmonized protocol for the fractionation of
sedimentary phosphorus.
Many such studies of sedimentary phosphorus profiles, also incorpor-
ating pore water measurement of soluble reactive phosphate, have
demonstrated that redox-controlled dissolution of iron (hydr)oxides
under reducing conditions at depth releases orthophosphate to solution.
This then diffuses upwards (and downwards) from the pore water
maximum to be re-adsorbed or co-precipitated with oxidized Fe III in
near-surface oxic sections. The downwards decrease in solid phase
'organic' phosphorus indicates increasing release of phosphorus from
deposited organic matter with depth, some of which will become
associated with hydrous iron and other metal oxides, added to the pool
of mobile phosphorus in pore water or contribute to 'soluble unreactive
phosphorus'. The characteristic reactions involving inorganic phos-
phorus in the sediments of Toolik Lake, Alaska, are shown in
Figure 7. 142 If, at depth, the concentrations of Fe 21 and phosphate are
high enough, authigenic vivianite (Fe 3 (PO 4 ) 2 .8H 2 O) may precipitate out.
With redox control largely responsible for phosphorus mobility in
sediments, what might the consequences of oxygen depletion in the
hypolimnion be? If conditions in the surface sediments are not su -
ciently oxidizing to precipitate iron (hydr)oxides and thereby adsorb the
phosphate (i.e. the redox boundary for iron may be in the overlying
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