The reasons behind the lowering of the spectrophotometrically measurab
le silicon (Si) content in sediment interstitial and unfiltered lake w
ater samples after storage in frozen form were explored. The storage c
ontainer material had little effect on the immobilization, whereas low
ering the pH of interstitial water samples before freezing decreased t
he ''loss'' of Si. When interstitial water samples were frozen, both n
ative, naturally present Si and added standard solution Si were immobi
lized. The same reaction pattern was found for native Si in unfiltered
lake water samples. On the contrary, neither the native Si in filtere
d lake water nor added standard solution Si was immobilized when lake
water samples were frozen, indicating that high concentrations of part
iculate material, especially in the interstitial water, was the prime
cause of the immobilization phenomenon. The results obtained in a sequ
ential filtration (0.2 mu m, 100,000 and 10,000 daltons) experiment wi
th interstitial water samples supported the theory that Si is immobili
zed through polymerization or complex formation mediated by particulat
e material and molecules present in interstitial water. An incubation
test revealed that the immobilization process was not entirely reversi
ble. The amount, nature and behaviour of Si in interstitial water samp
les showed some seasonal variation. Copyright (C) 1996 Published by El
sevier Science Ltd