Published March, 2001
Phosphorus Mobilization from Various Sediment Pools in Response
to Increased pH and Silicate Concentration
J. Koski-Vähälä,* H. Hartikainen, and P. Tallberg
ABSTRACT
anions for sorption sites. Both anions are sorbed by the
same specific ligand exchange mechanism onto Fe and
Al oxides (Hingston et al., 1967; Obihara and Russell,
1972). The ability of silicate to compete with phosphate
is, however, highly pH-dependent since the pKa value
of silicic acid (H4SiO4; 9.7) is higher than that of orthophosphoric acid (H3PO4; 2.1).
The aim of this study was to assess the effects of
elevated pH and Si enrichments on the fate of P in
sediment in order to more thoroughly understand the
processes involved in internal loading of surface waters.
The contribution of different inorganic P pools in the
solid phase to the soluble P in the interstitial water
was studied by a sequential P fractionation analysis.
Labeling with radiotracer 33P was adopted to improve
the sensitivity of the procedure and to corroborate the
responses obtained for native P.
Phosphorus (P) release from sediment particles to the interstitial
water has been studied extensively, but the contribution of different
inorganic P pools in sediment under differing environmental conditions is not fully understood. This study was undertaken to get more
detailed information about the chemical mobilization mechanisms.
Phosphorus mobilization from reserves bound by Al, Fe, and Ca
compounds in response to increased pH and to inorganic silicon (Si)
enrichments was investigated using a sequential fractionation analysis
and an isotope-labeling technique. The aerobic sediment of Lake
Vesijärvi had a high P retention capacity, and Fe-bound P was the
largest inorganic P pool as well as the main source of released P.
High Si addition (47 mg Si L⫺1 sediment) released more P to the
interstitial water than did the elevation of pH from 6.6 to 9.5, since
Si lowered the resorption of released P onto hydrated Al oxides. This
finding reveals that P equilibrium between Fe-bound and Al-bound
P in sediments regulates P net mobilization to the interstitial water
under aerobic conditions. Furthermore, elevated pH combined with
high Si enrichment had a positive synergistic effect, resulting in the
most substantial P mobilization. This synergism may cause a selffueled increase in the internal loading of P. It accentuates the effect
of diatom sedimentation on P fluxes in eutrophic lakes with high pH
and may favor the appearance of bloom-forming cyanobacteria.
MATERIALS AND METHODS
Sediment Material
The sediment used in the experiment was collected in May
1997 from the surface layer (0–2 cm) at 9 m depth in the
Enonselkä basin of Lake Vesijärvi (southern Finland). The
Enonselkä basin (surface area 26 km2, mean depth 6.8 m,
maximum depth 32 m) was eutrophicated by loading of industrial waste waters and domestic sewage until 1976, when the
sewage loading was diverted (Keto, 1982). After that the basin
started to recover, but cyanobacterial blooms ceased as late
as 1989 concomitantly with the mass removal of fish (Horppila
et al., 1998). Following the intensive fishing in 1989–1993, the
Enonselkä basin changed from a highly eutrophic and turbid
system to a mesotrophic and more clearwater system (Kairesalo et al., 1999). In the early 1990s, the mean summertime
cyanobacterial biomass dropped below 0.4 g m⫺3, the chlorophyll a concentration to 10 ␮g L⫺1, the total P concentration
to 30 ␮g L⫺1, and the transparency of the water had increased
to 3.5 m (Horppila et al., 1998; Koski-Vähälä et al., 2000).
The sediment was sampled just after overturn in spring,
when both the water column and the sediment were well
aerated. The oxygen conditions in the Enonselkä basin deteriorate only in the deep areas of the lake, and in our sampling
area the oxygen concentration does not decrease during the
summer (Koski-Vähälä et al., 2000; J. Koski-Vähälä, unpublished data, 1993–1995). The characteristics of the sediment
are given in Table 1. The water content was determined by
drying the fresh sediment at 105⬚C and loss on ignition at
550⬚C. The other characteristics were determined using sediment dried at 60⬚C. The total carbon and nitrogen were determined with a LECO (St. Joseph, MI) CHN-900 analyzer. Exchangeable cations (Ca, Mg, Na, and K) were extracted with
four portions of 1 M NH4Cl at a dried sediment to solution
ratio of 1:15 (w/v) and analyzed with an atomic absorption
spectrophotometer (AAS). The effective cation exchange capacity (ECEC) was calculated as the sum of exchangeable
I
nternal nutrient loading is a problem in many eutrophic lakes where P is the key element controlling
primary production. Chemical and biological factors affecting the P exchange between sediment, interstitial
water, and water column have been investigated intensively (e.g., Boström et al., 1982; Søndergaard, 1989).
Nevertheless, the contribution of different P pools in
sediment material and particularly their response to
changing, multivariate limnological conditions are not
fully understood (Wildung et al., 1977; Søndergaard,
1989).
The importance of high pH as a factor affecting the
release of sediment P has been intensively investigated
(MacPherson et al., 1958; Rippey, 1977; Ryding and
Forsberg, 1977; Jacoby et al., 1982), and the enhanced
P release has been interpreted to result from an increase
in the negative charge of hydrous oxides and from competition between hydroxide (OH⫺) and phosphate (H2
PO4⫺) anions for sorption sites (Andersen, 1975; Lijklema, 1980; Boström et al., 1982). On the other hand,
the effect of Si on P exchange in sediment has received
only little attention (Brinkman, 1993; Hartikainen et al.,
1996; Tuominen et al., 1998), although the silicate anion
(H3SiO4⫺) also is capable of competing with phosphate
J. Koski-Vähälä and P. Tallberg, Dep. of Limnology and Environmental Protection, P.O. Box 27, FIN-00014 Helsinki Univ., Finland. H.
Hartikainen, Dep. of Applied Chemistry and Microbiology, P.O. Box
27, FIN-00014 Helsinki University, Finland. Received 3 Feb. 2000.
*Corresponding author ([email protected]).
Published in J. Environ. Qual. 30:546–552 (2001).
Abbreviations: AAS, atomic absorption spectrophotometer.
546
547
KOSKI-VÄHÄLÄ ET AL.: P MOBILIZATION FROM VARIOUS SEDIMENT POOLS
cations. Hydrated oxides of iron, aluminum, and manganese
were extracted for 2 h with 0.05 M NH4–oxalate (pH 3.3) at
a dried sediment to solution ratio of 1:20 (w/v) and determined
by AAS. According to Hartikainen et al. (1996), the amount
of organic P in the Enonselkä basin sediment is 0.3 g kg⫺1,
about two-thirds of which was extractable by sequential fractionation analysis.
Experimental Design
The whole experiment consisted of isotope-labeled and unlabeled samples, which were set up according to a factorial
design with pH manipulation (two levels) and inorganic Si
enrichment (three levels) (Fig. 1). Each treatment was carried
out with three replicates. One sample in each treatment was
used for pH measurement (Schott CG 837 pH meter; SchottGeraete GmbH, Hofheim, Germany), since according to a
preliminary test, pH results showed no significant variation.
For the isotope-labeled samples, 20-mL polyethylene scintillation bottles were filled with 10 mL of fresh sediment and
covered with punctured parafilm membranes. All the subsamples were labeled by adding 250 ␮L of deionized water
containing 38 kBq 33PO4–P (in diluted HCl, carrier free, radioactive concentration 370 GBq L⫺1; Amersham, Buckinghamshire, UK) and left to equilibrate for 2 d in the dark at
20 ⫾ 1⬚C by stirring continuously (170 rpm) on an orbital
shaker. This equilibration period is taken to be long enough,
since the main exchange reactions are shown to take place
even within a few minutes (House et al., 1995). The calculated
amount of added 33P was only 9 ⫻ 10⫺9 g kg⫺1 dry sediment.
In an effort to understand the effect of increased pH on the
P exchange, after the 2-d equilibration 40 ␮L 4 M NaOH was
added to half of the sediment samples to elevate pH from
the unmanipulated level of 7 to approximately 9.5. Both sets
(unmanipulated pH, elevated pH) were divided into three
groups and used to study the effect of Si addition on the
reactions of P. The first group was not enriched with Si, while
the other groups were enriched with 0.5 mL of inorganic Si
solution (as Na2SiO3) containing 6.9 (low Si enrichment) or
940 mg Si L⫺1 (high Si enrichment), corresponding to concentrations of 0.35 or 47 mg Si L⫺1 sediment, respectively. Thereafter, all the samples were incubated in the dark at 11 ⫾ 1⬚C
under continuous stirring (170 rpm) on an orbital shaker.
To monitor the fate of the added 33P, four inorganic P
fractions were extracted from the samples 1 d after the pH
manipulation and Si enrichment using the Chang and Jackson
method modified by Hartikainen (1979). A subsample of 1 mL
of the labeled fresh sediment was extracted sequentially with
5 mL of 1 M NH4Cl, 0.5 M NH4F (pH 8.5), 0.1 M NaOH, and
0.25 M H2SO4 solution (i.e., at sediment to solution ratio [v/
v] of 1:5). NH4Cl extracts soluble P, NH4F extracts P bound
by hydrated Al oxides, NaOH extracts P bound by hydrated
Fe oxides, and H2SO4 extracts P bound by Ca. Another subsample was used to collect interstitial water by centrifugation
(2350 ⫻ g ), whereafter the supernatant was filtered through
a 0.2-␮m cellulose nitrate filter (Sartorius AG, Goettingen,
Germany). One milliliter of each extract and the interstitial
water was used to measure the 33P activity using a liquid scintillation counter (Wallac [Turku, Finland] 1411) and OptiPhase
HiSafe3 scintillation cocktail (Wallac). Because the NaOH
extract was more colored by humus than the other extracts,
two separate fine-tuned protocols with different quenchings
were used in the activity measurements; one for the NaOH
extract and another for the rest of the extracts and for the
interstitial water. All the results were half-life corrected.
A set of unlabeled sediment samples of 15 mL volume
was manipulated in the same way as the labeled ones and
Table 1. Characteristics of the sediment of Lake Vesijärvi.
Parameter
Value
Unit
Water content
Loss on ignition
Total C
Total N
Exchangeable Ca
Fe oxides
Al oxides
Mn oxides
ECEC†
908
120
47.9
5.9
2.3
213
35
32
16.3
g kg⫺1
g kg⫺1
g kg⫺1
g kg⫺1
g kg⫺1
mmol kg⫺1
mmol kg⫺1
mmol kg⫺1
cmolc kg⫺1
† Effective cation exchange capacity.
sequentially fractionated as described above. The various extracts (obtained at extraction ratio [v/v] of 1:5) were analyzed
for P and Si. Because of the very high P concentrations in
the extracts, these were analyzed for P by an ammonium
molybdate–stannochloride method (Kaila, 1955). To analyze
P in the interstitial water, a separate subsample of sediment
was centrifuged (2350 ⫻ g ), whereafter the supernatant was
filtered through a 0.2-␮m cellulose acetate filter (Schleicher &
Schuell GmbH, Dassel, Germany) and the dissolved P measured by a molybdenum blue–ascorbic acid method (Murphy
and Riley, 1962) with an autoanalyzer (Lachat [Milwaukee,
WI] QuickChem 8000). The interference of Si in the analyses
of P was tested separately for both methods. In the ammonium
molybdate–stannochloride method Si did not cause any interference, but the results from the molybdenum blue–ascorbic
acid method were corrected for interference by Si. The Si
concentration of the extracts and interstitial water was measured by AAS at a wavelength of 251.6 nm.
Data Processing
The amounts of native P and Si in the different pools were
determined as g kg⫺1 dry sediment or as mg L⫺1 for the interstitial water, and also as percentages of the total fractionable P
and Si. The distribution of 33P between the different P pools
was calculated as the percentage of the total 33P recovered
(excluding the interstitial water). In both methods, the water
content of the fresh sediment (908 g kg⫺1) was used to make
the interstitial water results commensurable with the results
of the fractions. Because in fractionation the interstitial water
in the fresh sediment samples was removed with the NH4Cl
extractant, the results of this fraction were also volume-corrected by the water content. It should be noted that the total
percentage exceeded 100% because the interstitial water was
included, as mentioned above, in the first extraction in the
fractionation analysis. Hereafter, the P and Si extracted in
the sequential fractionation analysis will be termed P or Si
fractions, and their sum fractionable P or Si.
The effects of the treatments were tested using analysis of
variance (ANOVA) and Tukey’s studentized range test by
the General Linear Models (GLM) procedure of SAS (SAS
Institute, 1989). Test values of p ⬍ 0.05 were regarded as
statistically significant.
RESULTS
Changes in the Inorganic Phosphorus and Silicon
Pools at Elevated pH and Silicon Enrichments
In the control sediment without pH manipulation and
Si enrichments pH was 6.6, but the low Si enrichment
increased pH to 7.0 and the high Si enrichment to 7.4.
At the elevated pH the differences in pH between the
treatments were very small (9.5–9.6). The NaOH frac-
548
J. ENVIRON. QUAL., VOL. 30, MARCH–APRIL 2001
Fig. 1. The experimental design and the laboratory procedure. Three replicates were used.
tion (Fe-bound P) was the largest inorganic P pool,
amounting to 49% of the total fractionable P reserves
(1.4 g kg⫺1; Table 2). The second largest P pool was the
H2SO4 fraction (Ca-bound P), constituting 40% of the
total fractionable P. The NH4F fraction (Al-bound P)
amounted to 11% of the fractionable P. The NH4Cl
fraction, which gives an estimate of soluble P reserves,
was almost 10-fold that of the interstitial water P.
At the elevated pH, the percentage of NaOH–P de-
creased 2 %-units and that of H2SO4–P 3 %-units, while
the percentage of NH4F–P increased almost 5 %-units
(Table 2). These changes doubled the percentage of P
in the NH4Cl fraction and increased the percentage of
P found in the interstitial water about 25-fold.
The low Si enrichment affected the distribution of P
between the various pools only slightly (Table 2). The
only statistically significant changes were the decrease
in percentage of NH4F–P and the increase in that of
549
KOSKI-VÄHÄLÄ ET AL.: P MOBILIZATION FROM VARIOUS SEDIMENT POOLS
Table 2. The dissolved P and Si concentration in the interstitial water, the amount of inorganic P and Si in each extraction, and the
percentages of the total extracted P and Si (excluding interstitial water) in the different treatments. The results are from unlabeled
samples; mean and standard deviation (SD) are given. na ⫽ not available.
Sequentially extracted with
Si enrichment
mg
Phosphorus extracted
Unmanipulated pH
ambient
low Si
high Si
Elevated pH
ambient
low Si
high Si
Silicon extracted
Unmanipulated pH
ambient
low Si
high Si
Elevated pH
ambient
low Si
high Si
NH4Cl
Interstitial water
L⫺1
%
g
kg⫺1
NH4F
%
g
kg⫺1
mean
SD
mean
SD
mean
SD
mean
SD
mean
SD
mean
SD
0.06
0.01
0.02
na
6.92
0.29
1.53
0.69
1.73
0.07
16.16
0.60
0.04
0.01
0.01
na
4.84
0.20
1.07
0.48
1.06
0.04
9.64
0.36
0.01
0.00
0.00
0.00
0.05
0.00
0.01
0.00
0.01
0.00
0.10
0.01
0.4
0.08
0.2
0.01
3.4
0.09
0.9
0.02
0.9
0.27
6.5
0.13
0.15
0.01
0.17
mean
SD
mean
SD
mean
SD
mean
SD
mean
SD
mean
SD
6.15
0.21
4.89
na
36.91
6.51
8.71
0.95
4.62
na
43.94
3.71
0.50
0.02
0.34
na
2.61
0.46
0.69
0.07
0.34
na
3.01
0.25
0.21
0.05
0.20
na
0.80
0.08
0.26
0.02
0.27
na
0.86
0.03
1.8
0.35
1.5
na
6.0
0.76
2.2
0.30
2.1
na
6.2
0.39
H2SO4–P at the elevated pH. The high Si enrichment
without pH manipulation caused a similar but even
more drastic redistribution of fractionable P as did the
elevated pH. The percentage of NaOH–P decreased 6
%-units, while that of NH4F–P increased by 4 %-units.
There was no statistically significant change in the percentage of H2SO4–P (p ⬎ 0.05). As a result of the high
Si enrichment, the percentage of P in the NH4Cl fraction
increased nearly 10-fold and that of P in the interstitial
water more than 100-fold. The effect of the high Si
enrichment on the redistribution of fractionable P was
further enhanced at the elevated pH.
Although the total amount of fractionable Si in the
control samples was more than eightfold that of P, the
distribution of these elements between the different
fractions was rather similar (Table 2). The distribution
of Si and P differed most remarkably in the NH4Cl
fraction and in the interstitial water, where the percentages of Si were more than 4-fold and 10-fold that of
P, respectively. The effects of the elevated pH on the
distribution of Si between the different pools and on
the total amount of fractionable Si were not statistically
significant (p ⬎ 0.05). The high Si enrichment increased
Si in all pools, but only the changes in the NH4Cl fraction
and interstitial water were statistically significant. The
fractionation analysis was not sensitive enough to assess
changes at the low Si enrichment.
Distribution of added Phosphorus-33 at Elevated
pH and Silicon Enrichments
In the control samples without pH manipulation and
Si enrichment, only 0.13% of the total 33P activity was
NaOH
%
g
kg⫺1
H2SO4
%
g
kg⫺1
Total
%
g kg⫺1
0.20
0.01
0.21
0.02
0.20
0.01
0.26
0.02
10.8
0.71
10.6
0.12
14.8
0.73
15.8
0.73
13.2
0.36
16.4
0.70
0.69
0.02
0.77
0.03
0.57
0.03
0.63
0.02
0.72
0.00
0.61
0.01
48.8
0.36
49.3
0.80
42.4
0.80
46.5
0.84
46.4
0.65
38.3
0.12
0.57
0.02
0.62
0.01
0.54
0.02
0.50
0.03
0.61
0.01
0.62
0.01
40.0
0.48
39.9
0.74
39.4
0.21
36.8
0.68
39.5
0.03
38.8
0.66
1.42
0.03
1.56
0.04
1.36
0.05
1.35
0.07
1.55
0.03
1.59
0.04
1.67
0.16
1.61
na
1.93
0.06
1.86
0.08
1.72
na
2.04
0.04
14.2
1.05
11.9
na
14.5
1.15
15.4
0.46
13.4
na
14.9
0.17
6.04
0.18
7.57
na
6.81
0.39
6.13
0.62
6.81
na
6.72
0.22
51.4
1.10
55.6
na
50.8
1.79
50.8
4.33
53.2
na
48.4
0.73
3.83
0.23
4.22
na
3.86
0.50
3.81
0.52
3.99
na
4.25
0.24
32.6
1.71
31.0
na
28.8
2.39
31.6
3.84
31.2
na
30.6
1.04
11.75
0.41
13.60
na
13.41
0.69
12.05
0.59
12.79
na
13.87
0.39
found in the interstitial water and 0.40% in the NH4Cl
fraction (Table 3). The highest 33P activity was found in
the NaOH fraction, which contributed 65% of the total
activity. About 22 and 13% of the total activity was
recovered in the NH4F and H2SO4 fractions, respectively.
The elevated pH diminished the 33P activity in the
NaOH fraction by about 6 %-units (Table 3), but in the
H2SO4 fraction the effect was statistically insignificant
(p ⬎ 0.05). Concomitantly, the 33P activity in the NH4F
fraction increased by 4 %-units, in the NH4Cl fraction
5-fold, and in the interstitial water more than 50-fold.
As with the unlabeled samples, the low Si enrichment
caused only a slight redistribution of the fractionable
33
P, although some differences between the treatments
were statistically significant (Table 3). Like the elevated
pH, the high Si enrichment also decreased the 33P activity
in the NaOH fraction by 6 %-units and increased it in
the NH4F fraction by 3 %-units, whereas in the H2SO4
fraction the 33P activity was not significantly affected
(p ⬎ 0.05). Nevertheless, in the other pools the increase
was more drastic than with the elevated pH; the 33P
activity in the NH4Cl fraction increased 10-fold and in
the interstitial water 100-fold.
Compared with the control, the elevated pH in combination with the high Si enrichment diminished the
33
P activity in the NaOH fraction by as much as 20
%-units (Table 3). Consequently, in the NH4F fraction
the activity increased 5 %-units, but this change was
the same as when pH alone was elevated ( p ⬎ 0.05).
This redistribution was reflected as an increase in 33P
40-fold in the NH4Cl fraction and 300-fold in the interstitial water.
550
J. ENVIRON. QUAL., VOL. 30, MARCH–APRIL 2001
Table 3. The recovery of the added 33P in the interstitial water and in each extraction in the different treatments. The values are
percentages of the total extracted activity (excluding interstitial water); mean and standard deviation (SD) are given.
33
Si
enrichment
Unmanipulated pH
ambient
low Si
high Si
Elevated pH
ambient
low Si
high Si
P extracted sequentially with
Interstitial
water
NH4Cl
NH4F
NaOH
H2SO4
0.13
0.08
0.00
0.01
13.23
0.77
7.28
0.65
7.12
1.46
36.39
2.45
0.40
0.01
0.07
0.04
4.30
0.18
2.05
0.20
1.71
0.07
15.73
0.45
%
21.90
0.25
21.58
0.34
24.50
0.44
26.25
0.17
26.03
0.62
26.78
0.13
65.00
0.19
67.29
0.29
58.76
0.66
58.76
0.71
59.85
1.08
45.24
0.32
12.70
0.33
11.03
0.17
12.45
0.15
12.95
0.37
11.99
0.22
12.25
0.18
mean
SD
mean
SD
mean
SD
mean
SD
mean
SD
mean
SD
DISCUSSION
The Effect of pH on Phosphorus Mobilization
The low native P concentration and the low recovery
of added 33P in the interstitial water in the control samples indicate high P retention capacity of the aerobic
sediment of Lake Vesijärvi. The high native P and especially the high 33P found in the NaOH and NH4F fractions could be, in turn, attributed to the very high Fe
and Al oxide contents representing potential sorption
sites for P (Hartikainen et al., 1996). Consequently, the
increase in P in the interstitial water in response to
elevated pH was related to the changes in these P reserves bound by ligand exchange. At elevated pH, P was
desorbed from the NaOH fraction because the ability of
the Fe oxides to retain P was reduced. On the other
hand, the increase in NH4F–P reveals that part of the
P released from the Fe oxides was resorbed by the Al
oxides. This reaction pattern is explained by the fact
that Fe3⫹ has a lower pKa value than Al3⫹ and, thus, at
a given pH Al3⫹ is surrounded by a higher number
of undissociated H2O groups, which are more easily
replaced by H2PO4⫺ anions than are OH⫺ groups (Hartikainen, 1981). In fact, the water-soluble P in soils is
mainly controlled by the P saturation on Al oxide surfaces (Hartikainen, 1982). Thus, the increased P saturation on the Al oxide surfaces further contributed to an
increase in NH4Cl–P and interstitial water P.
The elevated pH also slightly increased the amount
of Si in the loosely bound NH4Cl fraction and in the
interstitial water (p ⬎ 0.05). The cause was probably
dissolution of sedimentary biogenic Si, since this process
increases at high pH (e.g., Lewin, 1961; Spencer, 1983).
The effect of the elevated pH on the mobilization of P
was thus intensified by a simultaneous increase in the
amount of dissolved biogenic Si competing for sorption sites.
The Effect of Silicon Enrichment
on the Phosphorus Mobilization
The high Si enrichment seemed to cause a competition between Si and P for sorption sites, which resulted
in an increased P concentration in the interstitial water.
This conclusion is supported by the sorption of the
added Si to the NaOH and NH4F fractions. However,
in agreement with some previous studies (Obihara and
Russell, 1972; Faria et al., 1987; Tuominen et al., 1998),
only the high Si enrichment was able to enhance P
desorption, indicating that a threshold concentration of
Si is needed to get any effect. On the other hand, a
simultaneous slight increase in pH obviously contributed to the P-mobilizing effect of the high Si enrichment.
The decrease in Fe-bound reserves was largest at the
elevated pH combined with the high Si enrichment due
to the positive synergistic effect of these factors. This
reaction pattern caused the highest P concentration in
the interstitial water. In fact, the ability of Si to compete
with P increases with rising pH (Brinkman, 1993) because the dissociation of silicic acid to anion form is
promoted. Thus, through increased competition the
high Si enrichment further enhances the OH⫺–evoked
desorption of P from the Fe oxides.
A Synthesis of Phosphorus Mobilization
Mechanisms
The relative effects of the various treatments on the
desorption of native P and 33P from the NaOH-soluble
fraction and on the recovery of dissolved P in the various
pools are compiled in Fig. 2. The high Si enrichment
increased the 33P activity in the interstitial water more
than did the elevated pH, even though the desorption
of 33P from the NaOH fraction (Fe-bound) by the elevated pH was as intensive as that induced by the high
Si enrichment (Fig. 2A). These differences were attributable to the resorption of 33P to the NH4F fraction (Albound), which was more intensive at the elevated pH
than at the high Si enrichment. Similarly, in the unlabeled samples the high Si enrichment increased P in the
interstitial water and decreased resorption of P to the
NH4F fraction more than did the elevated pH (Fig. 2B).
These results indicate that, when added in high quantities, Si tends to prevent the resorption of released P
onto Al oxides by occupying the sorption sites, which
can be inferred from the increased amount of Si in the
NH4F and NaOH fractions. Tuominen et al. (1998) have
shown that when Si and P are added simultaneously, Si
is more effective at reducing sorption than at enhancing
desorption of P. Thus, the P concentration in the interstitial water is a net result of P desorption and resorption.
When the high Si enrichment was combined with an
KOSKI-VÄHÄLÄ ET AL.: P MOBILIZATION FROM VARIOUS SEDIMENT POOLS
551
to be bound by the specific ligand exchange mechanism
(Hartikainen, 1979). Thus, this fraction did not react to
the changing conditions.
The higher sensitivity of the measurements of 33P compared with the unlabeled P can be demonstrated mathematically by using the NH4F fraction as an example. In
order to obtain a detectable response (detection limit
of the fractionation analysis ⫽ 0.005 g kg⫺1; for analytical
procedure see Hartikainen, 1979) in the NH4F fraction,
the amount of P in the interstitial water needed to
change by at least 0.55 mg L⫺1. Such a change is very
high, since the interstitial P concentration in aerobic
lake sediment commonly is only a few dozen micrograms per liter, as in Lake Vesijärvi. Thus, in our control
samples the required change would be almost 10-fold.
For the labeled 33P samples, the standard deviations in
the control samples (0.25 %-units in the NH4F fraction)
can be used as an approximation of the detectable response. This means that only a twofold change in the 33P
percentage in the interstitial water is needed to obtain
a detectable change in the NH4F fraction. The higher
sensitivity was also reflected by the higher statistical
significances between the treatments, supporting the rationale for using isotope label.
CONCLUSIONS AND IMPLICATIONS
Fig. 2. The effect of the different treatments on the desorption of
labeled 33P (A, top) and unlabeled P (B, bottom) from the NaOH
fraction, and on the recovery of released P in the NH4F fraction
and in the interstitial water. Values are percent units of the P
amount in the NaOH fraction in the control sample without pH
manipulation and Si enrichment. High Si ⫽ high Si enrichment,
elevated pH ⫹ high Si ⫽ elevated pH combined with high Si enrichment.
elevated pH, a peak 33P activity in the interstitial water
was obtained because the desorption of Fe-bound P was
further enhanced and the resorption to the Al-bound
form diminished (Fig. 2A). However, in this treatment
the relative contribution of the resorption was lower
than at the high Si enrichment only. The unlabeled measurements supported these observations (Fig. 2B). It is
noteworthy that the synergistic effect of the elevated
pH and the high Si enrichment was caused by separate
mechanisms; increasingly dissociated Si compounds
competed for sorption sites, which further enhanced the
OH⫺–evoked desorption.
Phosphorus-33 Labeling as a Monitoring Tool
The overall responses to the pH and Si treatments
obtained by the two parallel monitoring methods corresponded well with each other, which improves the reliability of the findings. The relative distribution varied to
some extent due to proportionally very low retention
of 33P by the the H2SO4 fraction, where P is not supposed
The net P release from sediment in response to increased pH and Si enrichment is controlled by the desorption and resorption reactions. The Fe-bound P was
the largest P pool and the main source of P mobilized
from the sediment of Lake Vesijärvi, but the resorption
to the Al-bound fraction markedly affected the P concentration in the interstitial water. However, the presence of competitive ligands can restrict this resorption
and maintain a higher P concentration in the interstitial
water. At a given P mobilization from the Fe-bound
fraction, the resorption to Al-bound forms counteracted
the final release into the soluble phase more effectively
at the elevated pH than at the high Si enrichment. Furthermore, the synergistic effect of the elevated pH and
the high Si enrichment on the P mobilization emphasizes
the importance of the presence of competitive ligands,
such as silicates, on the internal nutrient loading in lakes.
The solubility of mineral silicates is very low, even at
high pH, and their effect on P dynamics is thus negligible
in natural environments. The only biogeochemically active Si resource in freshwater bottom deposits consists
of sedimented biogenic Si (diatoms) and its dissolution
products. Field data from Lake Vesijärvi indicate that
the theoretical pulses of dissolved Si (16–2500 mg Si
L⫺1 fresh sediment; Tallberg, 1999) caused by the diatom
flux to the sediment in spring would be in the range
where P desorption can be induced. The sedimentation
of diatoms in spring has been assumed to act as a sink
for both P and Si, but the observed interactions between
Si and P make the phenomenon far more complex. If
the removal of Si from the water column in fact increases
the availability of sediment P, nonsiliceous phytoplankton, such as bloom-forming cyanophytes, may be favored. Phytoplankton blooms raise pH, and both the
552
J. ENVIRON. QUAL., VOL. 30, MARCH–APRIL 2001
OH⫺–evoked and the Si-induced release of P as well as
the competitiveness of cyanophytes (Paerl and Ustach,
1982; Shapiro, 1990) are enhanced. The result may thus
be a self-fueled, drastic increase in the internal loading
of P, especially in shallow lakes. However, further research is needed to determine the threshold concentration of Si affecting P mobilization and the factors influencing the Si effect on the ecosystem scale.
ACKNOWLEDGMENTS
We are indebted to Antti Uusi-Rauva and Kaj-Roger
Hurme (Isotope Section of the Faculty of Agriculture and
Forestry, Helsinki University) for technical assistance. This
study was financially supported by the Maj and Tor Nessling
Foundation, the Jenny and Antti Wihuri Foundation, the Finnish Cultural Foundation, and J. Koski-Vähälä ltd.
REFERENCES
Andersen, J.M. 1975. Influence of pH on release of phosphorus from
lake sediments. Arch. Hydrobiol. 76:411–419.
Boström, B., M. Jansson, and C. Forsberg. 1982. Phosphorus release
from lake sediments. Arch. Hydrobiol. Beih. Ergebn. Limnol. 18:
5–59.
Brinkman, A. 1993. A double-layer model for ion adsorption onto
metal oxides, applied to experimental data and to natural sediments
of Lake Veluwe, the Netherlands. Hydrobiologia 253:31–45.
Faria, E.A., V. Hernando, and M.T. Pardo. 1987. Effect of silicate
and pumice stone on the P adsorption capacity of variable charge
soils. Agrochimica 31:309–316.
Hartikainen, H. 1979. Phosphorus and its reactions in terrestrial soils
and lake sediments. J. Sci. Agric. Soc. Finl. 51:537–624.
Hartikainen, H. 1981. Effect of decreasing acidity on the extractability
of inorganic soil phosphorus. J. Sci. Agric. Soc. Finl. 53:16–26.
Hartikainen, H. 1982. Water soluble phosphorus in Finnish mineral
soils and its dependence on soil properties. J. Sci. Agric. Soc.
Finl. 54:89–98.
Hartikainen, H., M. Pitkänen, T. Kairesalo, and L. Tuominen. 1996.
Co-occurrence and potential chemical competition of phosphorus
and silicon in lake sediment. Water Res. 30:2472–2478.
Hingston, F.J., R.J. Atkinson, A.M. Posner, and J.P. Quirk. 1967.
Specific adsorption of anions. Nature 215:1459–1461.
Horppila, J., H. Peltonen, T. Malinen, E. Luokkanen, and T. Kairesalo.
1998. Top-down or bottom-up effects by fish: Issues of concern in
biomanipulation of lakes. Restor. Ecol. 6:20–28.
House, W.A., F.H. Denison, and P.D. Armitage. 1995. Comparison
of the uptake of inorganic phosphorus to a suspended and stream
bed-sediment. Water Res. 29:767–779.
Jacoby, J.M., D. Lynch, E. Welch, and M. Perkins. 1982. Internal
phosphorus loading in a shallow eutrophic lake. Water Res. 16:
911–919.
Kaila, A. 1955. Studies on the colorimetric determination of phosphorus in soil extracts. Acta Agric. Fenn. 83:25–47.
Kairesalo, T., S. Laine, E. Luokkanen, T. Malinen, and J. Keto. 1999.
Direct and indirect mechanisms behind successful biomanipulation.
Hydrobiologia 395/396:99–106.
Keto, J. 1982. The recovery of L. Vesijärvi following sewage diversion.
Hydrobiologia 86:195–199.
Koski-Vähälä, J., H. Hartikainen, and T. Kairesalo. 2000. Resuspension in regulating sedimentation dynamics in Lake Vesijärvi. Arch.
Hydrobiol. 148:357–381.
Lewin, J. 1961. The dissolution of silica from diatom walls. Geochim.
Cosmochim. Acta 21:182–198.
Lijklema, L. 1980. Interaction of orthophosphate with iron(III) and
aluminum hydroxides. Environ. Sci. Technol. 14:537–540.
MacPherson, L.B., N.R. Sinclair, and F.R. Hayes. 1958. Lake water
and sediment. III. The effect of pH on the partition of inorganic
phosphate between water and oxidized mud or its ash. Limnol.
Oceanogr. 3:318–326.
Murphy, J., and J.P. Riley. 1962. A modified single-solution method
for the determination of phosphate in natural waters. Anal. Chim.
Acta 27:31–36.
Obihara, C.H., and E.W. Russell. 1972. Specific adsorption of silicate
and phosphate by soils. J. Soil Sci. 23:105–117.
Paerl, H., and J. Ustach. 1982. Blue-green algal scums: An explanation
for their occurrence during freshwater blooms. Limnol. Oceanogr.
27:212–217.
Rippey, B. 1977. The behaviour of phosphorus and silicon in undisturbed cores of Lough Neagh sediments. p. 349–353. In H.L. Golterman (ed.) Interactions between sediments and fresh water. Proc.
Int. Symp., Amsterdam, the Netherlands. 6–10 Sept. 1976. Dr. W.
Junk B.V. Publishers, The Hague, the Netherlands.
Ryding, S.O., and C. Forsberg. 1977. Sediments as a nutrient source
in shallow polluted lakes. p. 227–234. In H.L. Golterman (ed.)
Interactions between sediments and fresh water. Proc. Int. Symp.,
Amsterdam, the Netherlands. 6–10 Sept. 1976. Dr. W. Junk B.V.
Publishers, The Hague, the Netherlands.
SAS Institute. 1989. SAS/STAT user’s guide. Version 6, 4th ed., Vol.
2. SAS Institute, Cary, NC.
Shapiro, J. 1990. Current beliefs regarding dominance by blue-greens:
The case for the importance of CO2 and pH. Verh. Int. Verein.
Limnol. 24:38–54.
Spencer, C. 1983. Marine biogeochemistry of silicon. p. 101–142. In S.
Aston (ed.) Silicon geochemistry and biogeochemistry. Academic
Press, London.
Søndergaard, M. 1989. Phosphorus release from a hypertrophic lake
sediment. Experiments with intact sediment cores in a continuous
flow system. Arch. Hydrobiol. 116:45–59.
Tallberg, P. 1999. The magnitude of Si dissolution from diatoms at
the sediment surface and its potential impact on P mobilization.
Arch. Hydrobiol. 144:429–438.
Tuominen, L., H. Hartikainen, T. Kairesalo, and P. Tallberg. 1998.
Increased bioavailability of sediment phosphorus due to silicate
enrichment. Water Res. 32:2001–2008.
Wildung, R.E., R.L. Smith, and R.C. Routson. 1977. The phosphorus
status of eutrophic lake sediments as related to changes in limnological conditions. Phosphorus mineral components. J. Environ.
Qual. 6:100–104.