Hs. Jensen et B. Thamdrup, IRON-BOUND PHOSPHORUS IN MARINE-SEDIMENTS AS MEASURED BY BICARBONATE-DITHIONITE EXTRACTION, Hydrobiologia, 253(1-3), 1993, pp. 47-59
A sequential five-step extraction scheme for phosphorus pools in fresh
water sediment was modified for use in marine sediments. In the second
step phosphate bound to reducible forms of iron and manganese ('iron-
bound P') is extracted by a bicarbonate buffered dithionite solution (
BD-reagent). The extraction scheme was tested on sediment from 16 m wa
ter depth in Aarhus Bay, DK and used in two other marine sediments: Ka
ttegat at 56 m and Skagerrak at 695 m depth. By comparing the BD-extra
ctable P-pool with both the pool of iron in the BD-fraction and the po
ol of oxidized, amorphous or poorly crystalline iron (am. FeOOH), high
ly significant correlations (p < 0.001) were observed in all three sed
iments. Thus, we conclude that the BD-reagent was very specific for ir
on-bound P. Further evidence for this came from two experiments: 1) En
hanced BD treatment did not result in additional phosphate extraction
and 2) by sequential extraction of phosphorus pools in pure cultures o
f diatoms and cyanobacteria no phosphate was recovered in the BD-fract
ion. The pool of am.FeOOH was very important for controlling porewater
phosphate concentration which was inferred from the significant inver
se relationships between the two parameters (p < 0.001) in all sedimen
ts studied. Further, an isotopic exchange experiment with (PO43-)-P-32
revealed that BD-extractable P was by far the most exchangeable P-poo
l even deep in the sediment where the pool size was small. Iron-bound
P made up 33-45% of total P in the surface sediments. The ratio betwee
n iron-bound phosphate and am.FeOOH was 8-11 in Aarhus Bay and Kattega
t. In Skagerrak the ratio was 17, which may indicate that the iron min
eral extracted from this sediment is less capable of adsorbing phospha
te or less saturated with phosphate.