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Journal of Environmental Quality 30:546-552 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
SURFACE WATER QUALITY

Phosphorus Mobilization from Various Sediment Pools in Response to Increased pH and Silicate Concentration

J. Koski-Vähäläa, H. Hartikainenb and P. Tallberga

a Dep. of Limnology and Environmental Protection, P.O. Box 27, FIN-00014 Helsinki Univ., Finland
b Dep. of Applied Chemistry and Microbiology, P.O. Box 27, FIN-00014 Helsinki University, Finland

Corresponding author (jukka.koski-vahala{at}helsinki.fi)

Received for publication February 3, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS AND IMPLICATIONS
 REFERENCES
 
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 self-fueled 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.

Abbreviations: AAS, atomic absorption spectrophotometer


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS AND IMPLICATIONS
 REFERENCES
 
INTERNAL 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 (H2PO-4) 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 (H3SiO-4) also is capable of competing with phosphate 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.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS AND IMPLICATIONS
 REFERENCES
 
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 µg L-1, the total P concentration to 30 µg 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 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.


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  		Table 1. Characteristics of the sediment of Lake Vesijärvi

 
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; Schott-Geraete GmbH, Hofheim, Germany), since according to a preliminary test, pH results showed no significant variation.



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  		Fig. 1. The experimental design and the laboratory procedure. Three replicates were used

 
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 µL 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 x 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 µL 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 x g), whereafter the supernatant was filtered through a 0.2-µm 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 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 x g), whereafter the supernatant was filtered through a 0.2-µm 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
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS AND IMPLICATIONS
 REFERENCES
 
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 fraction (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.


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  		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

 
At the elevated pH, the percentage of NaOH–P decreased 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 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 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.


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  		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

 
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 33P, 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 33P 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.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS AND IMPLICATIONS
 REFERENCES
 
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 H2PO-4 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 (Al-bound), 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).



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  		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

 
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 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 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
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS AND IMPLICATIONS
REFERENCES
 
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 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
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS AND IMPLICATIONS
 REFERENCES
 




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