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Seasonal variation of phosphorus release from the sediments of Shallow Lake Balaton (Hungary)
Wat. Res. Vol. 22, No. 12, pp. 1473-1481, 1988 Printed in Great Britain. All rights reserved
0043-1354/88 $3.00+0.00 Copyright © 1988 Pergamon Press plc
SEASONAL VARIATION OF PHOSPHORUS RELEASE FROM THE SEDIMENTS OF SHALLOW LAKE BALATON (HUNGARY) VERA ISTVANOVICS Balaton Limnological Institute of the Hungarian Academy of Sciences, H-8237 Tihany, Hungary (First received April 1987; accepted in revised form May 1988)
Abstract--Phosphorus release was low from intact sediment cores of the mesotrophic area of the lake throughout the year, and amounted to 0.3 mg P m -2 day-~ during autumn in short-term incubations. In the hypertrophic area maximum release (2.8 mgPm-2day -I) was measured during summer. Phosphorus release showed a rapid increase from long-term incubated intact sediment cores with the increasing pH of the overlying water. At the ecologically real maximum pH the release may amount to 0.8 and 4.0mgPm-2day -~ in the mesotrophic and hypertrophic areas, respectively. A release of 2.0-3.9 mg P m 2day-~ was estimated from sediment suspensions of the hypertrophic area within a pH range of 8-9. These values are similar to the external phosphorus loadings of the respective areas. The most important phosphorus mobilizing factors are pH and the decomposition of the organic matter in the sediments. Redox conditions may play a significant indirect role in the regulation of the internal loading. A positive feedback is hypothesized between the internal phosphorus loading and primary production both processes being affected by the external loading in different ways. Key words--calcareous lake, mesotrophic and hypertrophic sediments, fractional composition of phos-
phorus, internal loading, pH, primary production
INTRODUCTION
A part of phosphorus accumulating in lake sediments remains potentially mobile and, recycling to the water column, may support high primary production in spite of the decreased external loading (Ryding, 1981). The amount of phosphorus released is determined by various mobilization and transport processes. The relative importance of different environmental factors governing the mobilization (release and transfer between different forms) of sediment phosphorus depends primarily on its fractional composition (Bostr6m, 1984), The transport of both particulate and dissolved phosphorus from the sediments to the water is strongly affected by the hydrodynamical conditions in the lake, and therefore can hardly be simulated in laboratory experiments. A possible approach to estimate the internal phosphorus loading is to analyse changes in mobility of different phosphorus fractions induced by the most important environmental factors (Bostr6m, 1984), and to couple these findings with hydrodynamical models (Lijklema et al., 1983). Balaton is a large (596km2), shallow lake (~ = 3.4 m), as a consequence of the morphometric and loading conditions, the southwestern part of the elongated water body is hypertrophic, while its northeastern part is mesotrophic (Herodek, 1983). The concentration of orthophosphate is below I-2 mg PO4-Pm -3 throughout the year (Istvfinovics and Herodek, 1985). In connection with the large-scale 1473
management measures having been taken recently (Ling, 1986), it is increasingly important to estimate the internal loading and to study the factors modifying its magnitude. The sediments of Lake Balaton are highly calcareous, magnesium calcite making up 30-60% of dry weight (Miiller, 1969). Concentrations of hyroxylapatite, iron, aluminium and organic matter are low (Gelencs6r et al., 1982). Total phosphorus increases from 500-600 to 700-800/~g P g - i dry wt in the surficial sediments along the trophic gradient (Pettersson and Istv~novics, 1988). The bulk of both the inorganic phosphorus and the isotopically exchangeable sediment phosphorus is bound to calcium (Ca-P). The concentration of reducible iron-bound phosphorus (Fe-P) is below 20/~gP g-~ in the mesotrophic area, and two times higher in the hypertrophic one (Pettersson and Istv~novics, 1988). The sediment surface is usually aerobic. Anaerobic conditions were observed only for short periods in the hypertrophic area (Olfih et al., 1983). These facts suggest that pH can be one of the dominant factors in the mobilization of sediment phosphorus. The aim of the present study was to determine the range of seasonal variation of phosphorus release, and to quantify the effects of the increasing pH on the mobilization of sediment phosphorus. Possible interactions between external loading, primary production and internal loading are discussed.
1474
VERA ISTV,~NOVICS
MATERIALS AND METHODS The range of seasonal variations of phosphorus release was studied using intact sediment cores. Three to six samples were simultaneously collected with glass tubes of 4.4 cm dia in the mesotrophic and hypertrophic areas of the lake (November 1979-October 1980 and May~3ctober 1980, respectively). The year-to-year variation was tested in the summer of 1981 in the hypertrophic part, when the highest release rates were measured in the previous year. The overlying water was carefully changed for 200ml Whatman GF/C filtered lake water. The samples were incubated for 2-4 days in the dark, at the actual water temperature. The overlying water was bubbled with watersaturated compressed air as close to the sediment/water interface as possible without resuspending the sediments. Soluble reactive phosphorus (SRP) was measured in the water before and after the incubation (Murphy and Riley, 1962). The original lake water never contained more than 6 mg PO4-P m 3 as SRP. The pH-dependent phosphorus release was studied with long-term incubation of intact sediment cores and suspensions. Twelve cores were taken both in the mesotrophic and hypertrophic areas (January and February 1986, respectively). The pH of triplicate samples was adjusted with NaOH additions. The samples were incubated for 28 days at 5~'C, air-bubbled. Every 2-4 days 100 ml of the water was pipetted off and changed for fresh, filtered lake water containing half the amount of NaOH added initially. Concentrations of SRP and total dissolved phosphorus (TDP; Mackereth et al., [978) as well as pH were measured both in the fresh and removed water. The suspensions (5 g wet sediment/200 ml filtered lake water) were prepared from the upper 3cm layer of the sediment taken in the hypertrophic area in April 1986 with a Hargrave dredge. The pH was adjusted with NaOH as above, and the samples incubated for 16 days at 15°C, shaken manually twice a day. The suspensions were centrifuged every few days. SRP, TDP and pH were determined in the supernatants. The sediments were resuspended in fresh lake water containing the same amount of NaOH as initially. In order to follow transfers between different forms of sediment phosphorus, triplicate samples were fractionated before and after the incubation of the suspensions using the method of Hieltjes and Lijklema (1980) as well as the slightly modified method of Psenner et al. (1985). In the modified version of the latter procedure the initial distilled water step was replaced by NH4C1 extractions. Extractions with NH4CI yield the "loosely-absorbed" phosphorus (NH4C1-RP), NaOH removes the "iron- and aluminiumbound" phosphorus (NaOH-RPa_L), while HCI extraction gives the "apatite"-phosphorus (HC1-RP) (Hieltjes and Lijklema, 1980). The procedure of Psenner et al. (1985) intends to differentiate reducible iron- and aluminium-bound phosphorus with bicarbonate/dithionite extraction applied before the NaOH-step (BD-RP and NaOH-RPps, respectively). The NaOH-extracts also contain non-reactive phosphorus (NaOH-nRP), which is organic (Furumai and Ohgaki, 1982). Total phosphorus contents of dried sediments (105°C, 24h) were measured after perchloric acid/persulfate digestion. Concentrations of magnesium and calcium were determined with atomic absorption in the acidified extracts of the original sediments in the Research Centre for Water Resources Development.
the sediment/water interface of different lakes, or spatial and temporal variations within the same lake. It was repeatedly shown that the measured rates underestimated the in situ ones (Andersen, 1974; Ryding and Forsberg, 1977; Bostr6m et al., 1982), since the laboratory set-up would necessarily change some conditions prevailing in the lake. Moreover, the release rates vary with the time interval chosen for the basis of their calculation, as the time vs release relationship is not linear. Therefore, seasonal variation of phosphorus release was characterized with the initial release rates measured after 2-4 days of incubation. The initial phosphorus release rate was low throughout the year from the intact cores of the mesotrophic area. The highest rates ( 0 . 3 m g P m -2 day ~) were measured during early autumn. In the same period the release was two times higher from the hypertrophic cores, where the maximum (2.8 mg P m -2 day -~) was observed during summer. The range of the release rates was similar during the summers of 1980 and 1981 (Fig. 1). The maximum rates measured corresponded to about one half of the biologically available external phosphorus loadings of the mesotrophic and hypertrophic areas (0.6 and 3.8 mg P m - 2 d a y ~, respectively; Jolfinkai and Dfivid, 1983). This is clearly demonstrative of a difference in the mobilizable phosphorus contents in the sediments of the two areas, not directly related to their total phosphorus contents. This is on account of the dilution of the surface sediments by the precipitating biogenic and abiogenic calcium carbonate, the amount of which is higher by at least one order of magnitude in the hypertrophic area (Lijklema et al., 1983). The different mobilities of phosphorus is also reflected in the 2-5 times higher isotopic exchangeability of phosphorus (Herodek and Istvfinovics, 1986; Pettersson and Istwinovics, 1988), as well as the higher concentrations of CaCO3-P, Fe-P and easily degradable organic-P in the hypertrophic 2.8 2.4 2.0 ~:~ 1.6
,~
E Q.
jJJ
~.2 G8
E 0.4 0.0
I
RESULTS AND DISCUSSION The seasonal cycle o f phosphorus release Phosphorus release of intact sediment cores allows only a rough comparison of diffusive fluxes through
I
I
I
N D d FM 1979
I
I
AM
I
I
I
I
d JASON 1980
l
[~1
I
I
I
I
I
I
d d A S 0 1981
Fig. I. Seasonal variation of phosphorus release from intact sediment cores in the mesotrophic ( x ) and hypertrophic (Q) areas of Lake Ba]aton.
Seasonal variation of phosphorus release
sediments (Gelencsrr et al., 1982; Pettersson and Istv~inovics, 1988). Assuming 2 x 10-2-2 x 10-3m thickness, 10-gm 2 s-~ diffusion coefficient and 50-250 mg PO4-P m -3 interstitial SRP concentration, a release rate of 0.2-10 mg P m -2 day-J was estimated by Lijklema et al. (I 986). The rates measured were in agreement with these estimates. The seasonal cycle of phosphorus release from intact sediment cores seemed to be related to the bacterial numbers in the surficial sediments (Figs I and 2; Gorzr, 1982). The number of bacteria was low throughout the year in the mesotrophic area, reaching its maximum in September (105 ind. ml-t). Their number was higher by an order of magnitude in the hypertrophic sediments, showing maxima in April, July and during the autumn (Fig. 2).
1475
9 x 105
E "0 ¢1 6x10 5 c "0
.¢_ .0 t-
3 x 10 5
O L. o 0 m
pH-dependency of phosphorus release
x~ x . x x. I x + x + x ' l "x I Ix'~" /
I
In simple batch experiments, sediments of Lake Balaton behaved in a manner intermediate between redox-sensitive and -unsensitive categories (Bostrrm and Pettersson, 1982). The anaerobic release from intact sediment cores was low (Ol~th et al., 1977) in comparison to many other lakes (Table 1). Taking the low Fe-P and high CaCO3-P contents of the sediments into account (Pettersson and Istwlnovics, 1988), it could be supposed that the pH-conditions modified by primary production and mineralization may play the most important role in the regulation of the phosphorus mobilization. Since Gelencsrr et al. (1982) studied the pH-towering effect caused by bacterial decomposition taking place in the sediments, the effects of the increasing pH were studied in some detail with long-term incubation of intact sediment cores and suspensions. The pH showed minor fluctuations in the water during the incubation of the intact cores and suspensions (Table 2). The initial release rates increased rapidly with the pH [Fig. 3(a)] and were followed by a slower saturation at almost each pH level (Fig. 4). The mesotrophic cores released only SRP, therefore TDP values are not shown. In the hypertrophic cores
N D d
F MA
1979
Anaerobic
Aerobic
12.3 17.3 1.2 52
- 1.4 -2.0 0.2 --
Shagawa Glaningen Ramsjrn Ryssbysjrn
7.0 18 20 20
-2.0 0.3 0.7
Chub Balaton mesotrophic hypertrophic reed stand
*Present study.
I
I
A S 0
IX'l'Xl N D
1980
and suspensions SRP release made up more than 95% of TDP with the exception of the highest pH of the suspensions. As discussed by Bostrrm (1984), losses of phosphorus in forms other than SRP cannot be considered as released with certainty, since colloidal or fine particulate matter may be lost during samplings due to the slow sedimentation. The total amount of SRP liberated during the period of the incubation also showed an increase with the pH [Figs 3(b) and 4]. During summer the maximum pH of the water reaches 8.8 and 9.2 in the mesotrophic and hypertrophic areas, respectively. At these pH values the rate of release may be estimated as 0.8 and 4 . 0 m g P m - Z d a y -1 from the sediments of the respective areas. This is similar to the release rates measured under anaerobic conditions (Table 1). In contrast to the phosphorus release from intact sediment cores, pH had minor effect on the SRP
Release (mg P m-2 day i)
Esrom Fures~ Gribso Mendota
d
Fig. 2. Seasonal variation of bacterial numbers in the surficial sediments in the mesotrophic ( x ) and hypertrophic (O) areas of Lake Balaton. (After Gorz6, 1982).
Table 1. Phosphorus release from intact sediment cores in some lakes
Lake
M d
×/
1.5
--
1.0 4.0 13.2
0.0-0.8* 0.0-4.0* --
Reference Kamp-Neilsen (1974) Kamp-Nielsen (1974) Kamp-Nielsen (1974) Holdren and Armstrong (1980) Sonzogni et al. (1977) Ryding and Forsberg (1977) Ryding and Forsberg (1977) Niirnberg et al. (1986) Ol6h et al. (1977) Istvhnovics, unpublished
VERA ISTV.~NOVICS
1476
Table 2. The pH of the overlying water during the incubation. (A) Mesotrophic cores; (B) hypertrophic cores; (C) hypertrophic suspensions. Averages of triplicates Day from the beginning of the incubation Addition ml 2N NaOH [A)
.
.
.
.
.
.
.
13
16
20
23
27
8.03 8.15 8.26 9.33
8.00 8.46 8.74 9.16
8.17 8.31 8.5t 8.70
8.00 8.30 8.53 8.72
8.00 8.32 8.54 8.74
8.26 8.32 8.53 8.77
8.13 8.40 8.56 8.77
8.27 8.38 8.58 8.77
4
7
11
14
18
21
25
28
8.33 8.53 8.72 8.86
8.33 8.51 8.69 8.81
8.36 8.54 8.71 8.84
8.36 8.55 8.73 8.86
8.36 8.56 8.79 8.94
8.36 8.60 8.71 8.89
8.42 8.54 8.73 8.86
8.42 8.73 8.77 8.84
3
6
9
12
16
0.0 0.2 0.5
8.25 8.91 9.95
8.22 9.06 10.06
8.22 9,22 10.41
8.39 9.27 10,42
8.27 9.25 10.39
1.0
I 1.06
I 1.33
11.57
11.57
11.53
(a)
(b)
4.0
10
/
3.5
/
i
o~ 2.5 ¸
?
•
E
Q. 2.0
E
18-
/
30 T
.
9
0.0 0.5 1.0 1.5
(C)
.
5
0.0 0.2 0.5 1.0
t B)
.
2
/
/
1.5 1.0
/
/
16-
7~
14-
/
E
i4o /./
5T
~ 10 -
g_
/x
E
4
8
3
6
-
~/~
4
o///2" /
). f
2
0
0 8.0
8.5
9.0
9.5
10.0
10.5
'11.0
85
8.0
,
x
/
./
./
/
/
12-
0..
o ~//o . / /
0.5
45 i
8
/
30 lo
25 T
./
2O 15 -
l
I
l
I
I
9.0
9.5
10.0
105
11.0
pH
I0
pH
Fig. 3. Initial (a) and total (b) SRP release from intact sediment cores and suspensions as a function of the pH. [Figure 3(b) refers to a time period of 27, 28 and 16 days in the case of the mesotrophic cores, hypertrophic cores and hypertrophic suspensions, respectively.] O, Mesotrophic cores; O, hypertrophic cores; . × - - - hypertrophic suspensions.
(a)
4
5
20
I--
(b) ,o" ~o ~ ' ° ~ e ~
15
.e/e
,°"° 4
I (c) ~ o---o 50~ //
40
/
e/°''"
,../
¢J i
E
n
o
2-
/
7'
/.7
E 1
IT
~e/e o/°
l~e
lO
O ~ o /
o l~l~'~rlll"~lq I n I n I n I n I 2 6 10 14 18 22 26
Days
2
6
10
14
18
22 26
Days
rgr-~'T I i I i I 6 10 14 18
Days
Fig. 4. The pH-dependent phosphorus release from intact sediment cores in the mesotrophic (a) and hypertrophic (b) areas and from the hypertrophic sediment suspensions (c). Q, SRP; ©, total dissolved p h o s p h o r u s ; 1,2,3,4--increasing p H levels.
D..
::L
Seasonal variation of phosphorus release liberation from the suspensions in the pH range of 8-9 (Fig. 3). In the suspensions, similar to the resuspended sediments under in situ conditions, a given amount of phosphorus is present, which may be mobilized at the prevailing pH of the water. On the other hand, it was shown that the phosphorus mobilized in the deeper, reduced zones of the sediments diffuses upwards and is trapped in the oxidized surface layers (Carignan and Flett, 1981; Herodek and Istvfinovics, 1986). Phosphorus migrating upwards, however, may efficiently be bound in the surface layers only when the pH-conditions are favourable for its adsorption on the solid phases. In situ measurements evidenced that the average interstitial pH of 8 increased to 8.5 in the surficial sediment layers of the hypertrophic area, when the pH of the lake water increased from 8.6 to 9.2, as the result of the intense primary production (Gelencs6r et al., 1982). When this pH rise occurs in the interstitial water, the phosphorus having been trapped in the surficial layers may be mobilized and may diffuse to the water column. These processes account for the more definite pH-dependency of phosphorus release from intact cores in comparison to the suspensions. This indicates that the indirect redox-regulation of the internal phosphorus loading may be significant also in the calcareous sediments with relatively low share of Fe-P, while the direct effect of the redoxconditions is rather slight (Table 1, Bostr6m and Pettersson, 1982). Using intact sediment cores, Andersen (1975) found in calcareous Danish lakes that gross release of phosphorus was proportional to the increasing pH, while the net release decreased above pH = 9.5 as a result of hydroxylapatite formation. The significance of the same process was also verified in some other lakes (e.g. Kamp-Nielsen, 1974; Serruya et al., 1974). On the contrary, this was obviously not the situation in Lake Balaton (Figs 3 and 4; Pettersson and Bostr6m, 1984), where formation of hydroxylapatite is impeded by the high Mg 2+ and CO~- concentrations (Gelencs6r et al., 1982).
Changes in fractional distribution of phosphorus during the incubation of the suspensions Phosphorus mobilized under any conditions will in part be released and in part resorbed on other phases, depending on the adsorption capacity as determined by the composition of the sediments, as well as by the prevailing environmental conditions. Therefore, the fractional composition of phosphorus was also studied before and after the incubation of the hypertrophic sediment suspensions (Table 3). The total phosphorus content of the sediments was originally 721 ; t g P g -~ dry wt, 59 and 63% of this being inorganic according to the fractionation schemes of Hieltjes and Lijklema (1980) and Psenner et al. (1985), respectively. The bicarbonate/dithionite step in the latter scheme overestimated the amount of reducible iron-bound phosphorus due to a partial W R . 22/12--B
1477
dissolution of the HCI-RP fraction of the Hieltjes-Lijklema scheme, as it was shown by Pettersson and Istvhnovics (1988). The amount of Fe-P could be calculated as the difference of the NaOH-RP fractions of the two extraction schemes, and was found to be 70/~gPg -~ dry wt (Table 3). The average Ca/P ratios were 3300, 42 and 300, while Ca/Mg ratios were 24, 15 and 8 in the NH4CI, BD and HC1 extracts of the original sediments, respectively. In the NaOH-extracts Ca and Mg contents were negligible. A possible explanation for the highly different ratios would be that NH4CI removes phosphorus adsorbed on the freshly formed CaCO3, while HC1 from the high magnesium calcite formed by a slow ion exchange of about 10% Mg 2+ into the pure CaCO 3 precipitate (Szilfigyi, 1985). In the original sediments 41% of the residual-P (the difference between total and total reactive phosphorus contents) could be extracted with NaOH as NaOH-nRP according to both fractionation schemes used (Table 3). The SRP released during the incubation amounted to 1.1, 1.4, 3.3 and 5.1% of the total sediment phosphorus at the increasing pH levels, respectively (Fig. 4 and Table 3). This is low in comparison to many other lakes (Bostr6m, 1984), and is in agreement with the results of previous studies (Pettersson and Bostr6m, 1984). The amount of inorganic phosphorus increased during the incubation due to a loss from the NaOHnRP fraction (Table 3). The increase of the NH4CI-RP content could be a consequence of increased phosphate adsorption onto calcite with increasing pH. A similar explanation could be given to the observed increase of the HC1-RP fraction, since two NH4CI extractions did not remove the total amount of CaCO3-P from the sediments of Lake Balaton (Pettersson and Istv~inovics, 1988). The decrease of the BD-RP and NaOH-RP fractions was in agreement with the desorption of Fe- and AI-P at alkaline pH values (Table 3). The amount of NaOH-nRP showed a dramatic decrease during the incubation (with 100-110/~g P g-~ dry wt, Table 3). Supposing that this decrease was caused by the microbial utilization of the organic phosphorus, the rate of which was constant during the incubation, a consumption rate of 6.3 #g P g-i day-~ could be estimated in the hypertrophic suspensions, resulting in a turnover of 19 days for the easily degradable organic phosphorus. The corresponding values were also calculated for the uppermost layers of the intact sediment cores, which were fractionated according to the scheme of Hieltjes and Lijklema (1980) (results not shown), and compared to those calculated from the data of Pettersson (1984) (Table 4). The rate of consumption seemed to increase both with the concentration of the NaOHnRP fraction and the temperature. The different rates in the mesotrophic and hypertrophic cores were in agreement with the one order of magnitude higher
1478
VERA ISTV,~NOVICS
Table 3. Fractional composition of phosphorus in the hypertrophic sediments before and after the incubation of the suspensions. (A) Fractionation according to Hieltjes and Lijklema (1980); (B) fractionation according to Psenner et aL (1985). Averages of triplicates. NaOH-nRP----organic P extracted with NaOH; ZRP--total reactive phosphorus extracted; res-P = TP - ~RP; Fe-Pcak--the difference of the NaOH-RP fractions of the two fractionation schemes Samples
NH4CI-RP
BD-RP
NaOH-RP
NaOH-nRP HCI-RP (/~g P g- ~dry wt)
'~RP
res-P
Fe-P,ae
SRP,~I
(A)
Originally pH = 8.25 pH = 9.25 pH = 10.40" pH = 11.50
47 56 58 90 89
------
105 60 54 61 55
113 10 13 12 12
273 333 334 354 310
425 450 445 503 454
296 263 264 192 211
------
-8 I1 24 37
(B)
Originally pH = 8.25 pH = 9.25 pH = 10.40" pH = 11.50
47 56 58 90 89
135 127 108 108 I 11
36 23 23 18 14
117 8 8 6 4
233 268 285 262 250
451 473 474 478 463
270 239
70 37
-8 11 24 37
234
32
219 205
43 41
*One sample only.
Table 4. Rate of consumption and turnover of the NaOH-nRP fraction during the incubation of sediments NaOH-nRP (/~g g- ~) Before
After
Rate of consumption (,ag Pg i day t)
95
57-90
0.2-1.4
66-428
Reference Present study
215
115-145
2.5-3.6
59-86
Present study
117
17
6.3
t9
Present study
102
62
2.5
41
Pettersson (1984)
102
97
0.2
469
Pettersson (1984)
30 g/150 ml suspension, 20°C, 16 days, +4gNO3-NI -~ 30 g/150ml suspension, 10°C, 23 days, + 4 g N O : N 1 -~
1085
585
31.0
35
Pettersson (1984)
1085
805
12.6
86
Pettersson (1984)
30 g/150 ml suspension, 20°C 16 days, +4gNO3-NV ~ 30g/150ml suspension, 10°C, 23 days, +4gNO3-NI -t
630
450
11.0
56
Pettersson (1984)
630
520
4.8
132
Pettersson (1984)
Lake
Experimental conditions
Balaton, mesotrophic Balaton, hypertrophic
Intact upper Intact upper
Balaton, hypertrophic Balaton, hypertrophic Balaton, hypertrophic
5 g/200 ml suspension, 15°C, 16 days 30 g/150ml suspension, 20'C, 16 days, +4gNO3-NV ~ 30 g/150 ml suspension, 10'C, 23 days, +4gNO3-NI -I
Finjasj6n Finjasj6n Vallentunasj6n Vallentunasj6n
cores, 5°C, 27 days 1.6 cm cores, 5°C, 28 days, 1.8 cm
bacterial n u m b e r s in t h e h y p e r t r o p h i c s e d i m e n t s ( G o r z 6 , 1982). P e t t e r s s o n (1984) s u g g e s t e d to a c c o u n t for t h e difference b e t w e e n t h e d e c r e a s e o f t h e r e s i d u a l - P a n d N a O H - n R P f r a c t i o n s as a n i n c r e a s e in t h e b a c t e r i a l P. T h i s i n c r e a s e r a n g e d b e t w e e n 6 0 - 8 0 / ~ g P g-~ d r y wt in t h e h y p e r t r o p h i c s u s p e n s i o n s , w i t h t h e exception of the highest pH, where a part of the organic-P w a s lost by s a m p l i n g ( T a b l e 5). B a c t e r i a l b i o m a s s m i g h t i n c r e a s e d u e to t h e a d d i t i o n o f lake w a t e r c o n t a i n i n g 6 0 0 m g N O 3 - N m -3. A s s u m i n g t h a t t h e upper 3 cm layer of the sediments with 80% water content contains 7 kg dry sediment per square meter,
Turnover (day)
t h e rate o f t h e i n o r g a n i c p h o s p h o r u s p r o d u c t i o n m a y be e s t i m a t e d as 4 - 1 6 m g P m 2 d a y - ~ d u r i n g t h e dec o m p o s i t i o n t a k i n g p l a c e in t h e h y p e r t r o p h i c sedim e n t s . T h i s is s i m i l a r to t h e rate o f o r g a n i c p h o s phorus supply of the sediments from the water (2-14mgPm-2day-~), estimated on the basis of p r i m a r y p r o d u c t i o n , a n d a c c o u n t s for t h e low org a n i c p h o s p h o r u s c o n t e n t o f t h e s e d i m e n t s in c o m p a r i s o n to o t h e r l a k e s ( B o s t r 6 m a n d P e t t e r s s o n , 1982; P e t t e r s s o n , 1984). In o r d e r to e s t i m a t e m g m - 2 d a y -~ p h o s p h o r u s release f r o m t h e s u s p e n s i o n s , f u r t h e r c o n s i d e r a t i o n s m u s t be t a k e n into a c c o u n t . A c c o r d i n g to t h e s i m p l e
Table 5. The fate of NaOH-nRP during the incubation of the hypertrophic sediment suspensions. Averages of triplicates. Bacterial-P increase---the difference between the losses of the residual-P and NaOH-nRP fractions. Residual-P--the difference of the total and total reactive phosphorus contents
Samples pH = pH = pH = pH = *One
8.25 9.25 10.40" 11.50 sample only.
P-loss in forms other than SRP (#g p g-t dry wt) 0 1 2 19
Increase of Increase of bacterial-P SRP released 5~Rp (% of the NaOH-nRP decrease) 74 70 60 48
8 10 22 32
21 21 24 I1
Increase of bacterial-P 71 69 51 41
Seasonal variation of phosphorus release hydrodynamical model of Somlyrdy (1986), the rate of resuspension is 170 g sediment m-2day -t at an average wind velocity of 5 m s- i. Five g wet sediment, which was used for the preparation of the suspensions, represents the daily resuspension from a bottom area of 0.03 m 2. Taking into account the initial phosphorus release and the two orders of magnitude higher sediment/water ratio in comparison to the natural ones, a release of 2.0-3.9 mg P m-: day -~ can be estimated in the pH range of 8-9. Similar estimates were given for phosphorus release via desorption from the resuspended sediments in earlier studies [0.8-1.7 mg P m -2 dayfor the whole lake by Lijklema et aL (1983); 0.1-0.9 and 0.2--4.7 mg P m -2 day -1 for the mesotrophic and hypertrophic areas, respectively by Pettersson and Bostr6m (1984)]. Interactions between external and internal loadings and primary production
The available data indicate the existence of a positive feedback between the internal phosphorus loading and primary production, both being affected by the external loading in different ways. Any excess of biologically available phosphorus entering the water column is rapidly taken up by the planktonic microorganisms competing efficiently for the limiting amounts of phosphorus. Biological uptake is the only significant process keeping PO4 concentrations at their constantly low level (below 1-2 mg m-3), and transporting the biologically available phosphorus from the water to the sediments (Istv~novics and Herodek, 1985; Istvfinovics et al., 1986). Therefore, both the biologically available external and internal phosphorus loadings enhance the primary production in a direct manner. Primary production is proportional to the sum of these loadings (Somly6dy, 1983). The most important phosphorus mobilizing factors are more or less closely related to the rate of the primary production. Due to the low organic content of the sediments, the rate of mineralization depends, among other factors, on the organic matter supply from the water column. In the course of the bacterial decomposition taking place in the sediments, inorganic phosphorus is produced directly, as discussed previously. Redistribution or release of the inorganicP are controlled by the pH-dependent phosphate adsorption capacity of the sediments as well as by the rate of both mineralization and sorption. The pH is decreased by the CO2 production during the course of decomposition. A part of CaCO3 dissolutes resulting in mobilization of CaCO3-P. The rate of this process can be estimated as 0.2-0.5 and 0.8-2.4mg P m - : d a y -1 in the mesotrophic and hypertrophic areas, respectively, based on the amount of organic matter decomposing in the sediments (Vrr6s et al., 1983), as well as on the concentration of the CO2-soluble sediment phosphorus (60-180/~g P g-
1479
dry wt; Gelencsrr et al., 1982). On the other hand, there is an increase in the phosphate adsorption capacity of the metal hydroxides and clay minerals when the interstitial pH is lowered. Due to the pH rise during the period of intense primary production, calcium carbonate precipitates in proportion to the photosynthesis. About two thirds of it dissolutes in the water as a result of decomposition taking place there (Lijklema et al., 1983). The amount of the sedimenting CaCO3 can be estimated as 1.2 and 5.7gm-2day -1 in the mesotrophic and hypertrophic areas, respectively. CaCO3 adsorbs phosphate only in the sediments, where the interstitial SRP concentration is higher by 2-3 orders of magnitude as compared to the lake water (30-250 mg PO4-Pm-3; Ol~ih et al., 1977; Dobolyi, 1980; Gelencs~r et al., 1982; Sz~isz et al., in preparation). Taking into account the rate of CaCO3 precipitation and the concentration of the CaCO3-P fraction (60-180/~gPg -1 dry wt), the rate of PO4 adsorption on CaCO3 may be estimated as 0.6-2.4 and 4.0-14.8 mg P m -2 day -~ in the mesotrophic and hypertrophic areas, respectively. On the other hand, increasing pH of the interstitial water following the pH rise of the lake water results in a desorption of phosphorus from the iron- and aluminiumcompounds. The latter process also takes place when the sediments are resuspended into the lake water. As the structure of the sediments shows microscopic inhomogeneities, the opposite processes of the adsorption and desorption of PO4 on/from the carbonates and metal hydroxides, as well as precipitation and dissolution of CaCO 3 take place simultaneously. During the periods of intense primary production these result in a rapid increase of the internal phosphorus loading at the increasing pH of the lake water (Figs 3 and 4, Table 3). A similar regulatory role of the pH was also found to be important in a number of shallow, eutrophic lakes with different compositions of sediments (Drake and Heaney, 1987; Istv~inovics and Pettersson, in preparation). Independent of the primary production, the internal loading is also modified by the external one by means of a change of the adsorption capacity of the sediments. On the one hand, the adsorption capacity of the iron- and aluminium-compounds decreases gradually as their surfaces become saturated with phosphorus during prolonged periods of external loading (Bostr6m and Pettersson, 1982; Bostr6m, 1984). It was shown that saturation of the adsorption capacity of the NaOH-RP fraction is 70% in the hypertrophic sediments of Lake Balaton, in contrast to the 30% saturation in the mesotrophic sediments (Istv/movics et al., in preparation). On the other hand, the influent waters are supersaturated in respect to CaCO3. Precipitation of calcium carbonate
i
taking place within the lake continually increases the adsorption capacity of the sediments (Istv/movics et al., in preparation).
1480
VERA ISTV.~NOVICS CONCLUSIONS
The total external phosphorus loadings of the mesotrophic and hypertrophic areas are 0.9 and 7.3 m g P m - 2 d a y -1, respectively (Jol~nkai and Dfivid, 1983). The magnitude of the internal loading is similar in both areas of Lake Balaton. The most important phosphorus mobilizing factors are pH and the decomposition of the organic matter taking place in the sediments. The internal phosphorus loading increases rapidly with the increasing pH of the lake water during the periods of intense primary production. A positive feedback exists between the internal phosphorus loading and primary production, accounting for the observed seasonal cycle of phosphorus release from the sediments, as well as for the internal loading higher by about a factor of five in the hypertrophic area. F r o m the point of view of lake management, this feedback means that the response of the lake may be more beneficial to the reduction of the external phosphorus loading than would be expected without this relationship. Acknowledgements--1 am grateful to Dr Lhszlo Somly6dy and Dr Prter Gelencsrr for their valuable comments, to Mr I. B~ithory for the field assistance and to Mrs E. Kozma for laboratory assistance. REFERENCES
Andersen M. J. (1974) Nitrogen and phosphorus budgets and the role of sediments in six shallow Danish lakes. Arch. Hydrobiol. 74, 528-550. Andersen M. J. (1975) Influence of pH on the release of phosphorus from lake sediments. Arch. Hydrobiol. 76, 411-419. Bostr6m B. (1984) Potential mobility of phosphorus in different types of lake sediments. Int. Revue ges. Hydrobiol. Hydrogr. 69, 457-474. Bostrrm B. and Pettersson K. (1982) Different patterns of phosphorus release from lake sediments in laboratory experiments Hydrobiologia 92, 415-429. Bostrfm B., Jannson M. and Forsberg C. (1982) Phosphorus release from lake sediments. Arch. Hydrobiol. 18, 5-59. Carignan R. and Flett R. J. (1981) Postdepositional mobility of phosphorus in lake sediments. Limnol. Oceanogr. 26, 361-366. Dobolyi E. (1980) Data on the bottom sediment in Lake Balaton. Proc. 2nd Joint MTA /IIASA Task Force Meet. on Lake Balaton Modeling 2, 66-81. Drake J. C. and Heaney S. I. (1987) Occurrence of phosphorus and its potential remobilization in the littoral sediments of a productive English lake. Freshwat. Biol. 17, 513 523. Furumai H. and Ohgaki S. (1982) Fractional composition of phosphorus forms in sediments related to release. Wat. Sci. Technol. 14, 215-226. Gelencsrr P., Szilhgyi F., Somly6dy L. and Lijklema L. (1982) A study on the influence of sediment in the phosphorus cycle for Lake Balaton. IIASA Collab. Paper CP-82-44, Laxemburg, Austria. Gorzo Gy. (1982) Quantitative bacteriological studies in Lake Balaton (1978-1981) MTA Biol. Oszt. Ki~zl. 25, 289-305 (in Hungarian with an English summary). Herodek S. (1983) Biochemical processes in Lake Balaton. IIASA Collab. Paper CP-83-$3, pp. 101-147. Laxemburg, Austria.
Herodek S. and Istvfinovics V. (1986) Mobility of phosphorus fractions in the sediments of Lake Balaton. Hydrobiologia 135, 149-154. Hieltjes A. H. M. and Lijklema L. (1980) Fractionation of inorganic phosphates in calcareous sediments. J. envir. Qual. 9, 405-407. Holdren G. C. and Armstrong D. E. (1980) Factors affecting phosphorus release from intact sediment cores. Envir. Sci. Technol 14, 79-87. Istvfinovics V. and Herodek S. (1985) Orthophosphate uptake of planktonic microorganisms in Lake Balaton. Hydrobiologia 122, 159-166. Istv~movics V., Vrr6s L., Herodek S., T6th G. L. and Tfitrai I. (1986) Changes of phosphorus and nitrogen concentrations and of phytoplankton in enriched lake enclosures. Limnol. Oceanogr. 31, 798-811. Jol~.nkai G. and Dfivid L. (1983) Nutrient loads and watershed development. IIASA Collab. Paper CP-83-$3, pp. 45.81. Laxemburg, Austria. Kamp-Nielsen L. (1974) Mud-water exchange of phosphate and other ions in undisturbed sediment cores and factors affecting the exchange. Arch. Hydrobiol. 73, 218-237. L~ing I. (1986) Impact of policymaking: Background to a government decision. In Modeling and Managing Shallow Lake Eutrophication (Edited by Somlyrdy L. and van Straten G.), pp. I10-123. Springer, Berlin. Lijklema L., Gelencsrr P. and Szilhgyi F. (1983) Sediments and sediment-water interaction. IIASA Collab. Paper CP-83-$3, pp. 81-101. Laxemburg, Austria. Lijklema L., Gelencsrr P., Szilhgyi F. and Somlyrdy L. (1986) Sediment and its interaction with water. In Modeling and Managing Shallow Lake Eutrophication (Edited by Somly6dy L. and van Straten G.), pp. 156-183. Springer, Berlin. Mackereth F. J. H., Heron J. and Talling J. F. (1978) Water analysis. FBA Sci. Publ. No. 36. Mfiller D. (1969) Sedimentbieldung im Plattensee, Ungarn. Naturwissenschaften 56, 606-615. Murphy T. P. and Riley J. P. (1962) A modified single solution method for the determination of phosphate in natural waters. Analytica chim. Acta 27, 31-36. Niirnberg G. K., Dillon P. J. and McQueen D. J. (1986) Internal phosphorus load in an oligotrophic Precambrian Shield lake with an anoxic hypolimnion. Can J. Fish Aquat. Sci. 43, 574-580. Ol~h J., Abdelmoneim M. and T6th L. (1983) Nitrogen fixation in the sediment of shallow Lake Balaton, a reservoir and fishpond. Int. Revue ges. Hydrobiol. Hydrogr. 68, 13-45. Olfih J., Trth O. E. and Trth L. (1977) Phosphorus metabolism of Lake Balaton MTA Biol. Oszt. Krzl. 20, I11-139 (in Hungarian with an English summary). Pettersson K. (1984) Transfers between forms of sedimentary phosphorus induced by nitrate treatment Verh. int. Verein theor, angew. Limnol. 22, 233-238. Pettersson K. and Bostr6m B. (1984) Phosphorus exchange between sediment and water in Lake Balaton. 3rd int. Syrup. Interactions between Sediment and Water pp. 80--84. Geneva, Switzerland. Pettersson K. and Istv~inovics V. (1988) Sediment phosphorus in Lake Balaton--forms and mobility. Arch. Hydrobiol. J30, 25-41. Psenner R., Pucsko R. and Sager M. (1985) Die Fraktioniearung organischer und anorganischer Phosphorverbindungen von Sedimenten. Arch. Hydrobiol. (Suppl.) 70, 111-155. Ryding S.-O. (1981) Reversibility of man-induced eutrophication. Experiences of a lake recovery study in Sweden. lnt. Revue ges. Hydrobiol. Hydrogr. 66, 449-502. Ryding S.-O. and Forsberg C. (1977) Sediments as a nutrient source in shallow polluted lakes. In Interactions Between Sediments and Fresh Water (Edited by Goiterman H. L.). Junk, The Hague.
Seasonal variation of phosphorus release Serruya C., Edelstein M., Pollingher U. and Serruya S. (1974) Lake Kinneret sediments: nutrient composition of the pore water and mud-water exchanges. Limnol. Oceanogr. 19, 489-508. Somly6dy L. (1983) Lake eutrophication management models. IIASA Collab. Paper CP-83-$3, pp. 207-251. Laxemburg, Austria. Somly6dy L. 0986) Wind induced sediment resuspension in shallow lakes. Int. Conf. on Water Qual. Modelling in the Inland Natural Environment. BHRA, The Fluid Engineering Centre, Cranfield, England. Sonzogni W. C., Larsen D. P., Malueg K. W. and Schuldt
1481
M. D. (1977) Use of large submerged chambers to measure sediment-water interactions. War. Res. 11, 461~,64. Szilfi.gyi F. 0985) A study on the interaction between sediments and water in Lake Balaton (in Hungarian) Reports of tlie Research Center for Water Resources Development, Budapest, Hungary. V6r6s L., Vizkelety l~., T6th F. and N6meth J. (1983) The trophic conditions of the Keszthely basin in 1979. Hidrol. K6zl. 63, 390-398 (in Hungarian with an English summary).
0043-1354/88 $3.00+0.00 Copyright © 1988 Pergamon Press plc
SEASONAL VARIATION OF PHOSPHORUS RELEASE FROM THE SEDIMENTS OF SHALLOW LAKE BALATON (HUNGARY) VERA ISTVANOVICS Balaton Limnological Institute of the Hungarian Academy of Sciences, H-8237 Tihany, Hungary (First received April 1987; accepted in revised form May 1988)
Abstract--Phosphorus release was low from intact sediment cores of the mesotrophic area of the lake throughout the year, and amounted to 0.3 mg P m -2 day-~ during autumn in short-term incubations. In the hypertrophic area maximum release (2.8 mgPm-2day -I) was measured during summer. Phosphorus release showed a rapid increase from long-term incubated intact sediment cores with the increasing pH of the overlying water. At the ecologically real maximum pH the release may amount to 0.8 and 4.0mgPm-2day -~ in the mesotrophic and hypertrophic areas, respectively. A release of 2.0-3.9 mg P m 2day-~ was estimated from sediment suspensions of the hypertrophic area within a pH range of 8-9. These values are similar to the external phosphorus loadings of the respective areas. The most important phosphorus mobilizing factors are pH and the decomposition of the organic matter in the sediments. Redox conditions may play a significant indirect role in the regulation of the internal loading. A positive feedback is hypothesized between the internal phosphorus loading and primary production both processes being affected by the external loading in different ways. Key words--calcareous lake, mesotrophic and hypertrophic sediments, fractional composition of phos-
phorus, internal loading, pH, primary production
INTRODUCTION
A part of phosphorus accumulating in lake sediments remains potentially mobile and, recycling to the water column, may support high primary production in spite of the decreased external loading (Ryding, 1981). The amount of phosphorus released is determined by various mobilization and transport processes. The relative importance of different environmental factors governing the mobilization (release and transfer between different forms) of sediment phosphorus depends primarily on its fractional composition (Bostr6m, 1984), The transport of both particulate and dissolved phosphorus from the sediments to the water is strongly affected by the hydrodynamical conditions in the lake, and therefore can hardly be simulated in laboratory experiments. A possible approach to estimate the internal phosphorus loading is to analyse changes in mobility of different phosphorus fractions induced by the most important environmental factors (Bostr6m, 1984), and to couple these findings with hydrodynamical models (Lijklema et al., 1983). Balaton is a large (596km2), shallow lake (~ = 3.4 m), as a consequence of the morphometric and loading conditions, the southwestern part of the elongated water body is hypertrophic, while its northeastern part is mesotrophic (Herodek, 1983). The concentration of orthophosphate is below I-2 mg PO4-Pm -3 throughout the year (Istvfinovics and Herodek, 1985). In connection with the large-scale 1473
management measures having been taken recently (Ling, 1986), it is increasingly important to estimate the internal loading and to study the factors modifying its magnitude. The sediments of Lake Balaton are highly calcareous, magnesium calcite making up 30-60% of dry weight (Miiller, 1969). Concentrations of hyroxylapatite, iron, aluminium and organic matter are low (Gelencs6r et al., 1982). Total phosphorus increases from 500-600 to 700-800/~g P g - i dry wt in the surficial sediments along the trophic gradient (Pettersson and Istv~novics, 1988). The bulk of both the inorganic phosphorus and the isotopically exchangeable sediment phosphorus is bound to calcium (Ca-P). The concentration of reducible iron-bound phosphorus (Fe-P) is below 20/~gP g-~ in the mesotrophic area, and two times higher in the hypertrophic one (Pettersson and Istv~novics, 1988). The sediment surface is usually aerobic. Anaerobic conditions were observed only for short periods in the hypertrophic area (Olfih et al., 1983). These facts suggest that pH can be one of the dominant factors in the mobilization of sediment phosphorus. The aim of the present study was to determine the range of seasonal variation of phosphorus release, and to quantify the effects of the increasing pH on the mobilization of sediment phosphorus. Possible interactions between external loading, primary production and internal loading are discussed.
1474
VERA ISTV,~NOVICS
MATERIALS AND METHODS The range of seasonal variations of phosphorus release was studied using intact sediment cores. Three to six samples were simultaneously collected with glass tubes of 4.4 cm dia in the mesotrophic and hypertrophic areas of the lake (November 1979-October 1980 and May~3ctober 1980, respectively). The year-to-year variation was tested in the summer of 1981 in the hypertrophic part, when the highest release rates were measured in the previous year. The overlying water was carefully changed for 200ml Whatman GF/C filtered lake water. The samples were incubated for 2-4 days in the dark, at the actual water temperature. The overlying water was bubbled with watersaturated compressed air as close to the sediment/water interface as possible without resuspending the sediments. Soluble reactive phosphorus (SRP) was measured in the water before and after the incubation (Murphy and Riley, 1962). The original lake water never contained more than 6 mg PO4-P m 3 as SRP. The pH-dependent phosphorus release was studied with long-term incubation of intact sediment cores and suspensions. Twelve cores were taken both in the mesotrophic and hypertrophic areas (January and February 1986, respectively). The pH of triplicate samples was adjusted with NaOH additions. The samples were incubated for 28 days at 5~'C, air-bubbled. Every 2-4 days 100 ml of the water was pipetted off and changed for fresh, filtered lake water containing half the amount of NaOH added initially. Concentrations of SRP and total dissolved phosphorus (TDP; Mackereth et al., [978) as well as pH were measured both in the fresh and removed water. The suspensions (5 g wet sediment/200 ml filtered lake water) were prepared from the upper 3cm layer of the sediment taken in the hypertrophic area in April 1986 with a Hargrave dredge. The pH was adjusted with NaOH as above, and the samples incubated for 16 days at 15°C, shaken manually twice a day. The suspensions were centrifuged every few days. SRP, TDP and pH were determined in the supernatants. The sediments were resuspended in fresh lake water containing the same amount of NaOH as initially. In order to follow transfers between different forms of sediment phosphorus, triplicate samples were fractionated before and after the incubation of the suspensions using the method of Hieltjes and Lijklema (1980) as well as the slightly modified method of Psenner et al. (1985). In the modified version of the latter procedure the initial distilled water step was replaced by NH4C1 extractions. Extractions with NH4CI yield the "loosely-absorbed" phosphorus (NH4C1-RP), NaOH removes the "iron- and aluminiumbound" phosphorus (NaOH-RPa_L), while HCI extraction gives the "apatite"-phosphorus (HC1-RP) (Hieltjes and Lijklema, 1980). The procedure of Psenner et al. (1985) intends to differentiate reducible iron- and aluminium-bound phosphorus with bicarbonate/dithionite extraction applied before the NaOH-step (BD-RP and NaOH-RPps, respectively). The NaOH-extracts also contain non-reactive phosphorus (NaOH-nRP), which is organic (Furumai and Ohgaki, 1982). Total phosphorus contents of dried sediments (105°C, 24h) were measured after perchloric acid/persulfate digestion. Concentrations of magnesium and calcium were determined with atomic absorption in the acidified extracts of the original sediments in the Research Centre for Water Resources Development.
the sediment/water interface of different lakes, or spatial and temporal variations within the same lake. It was repeatedly shown that the measured rates underestimated the in situ ones (Andersen, 1974; Ryding and Forsberg, 1977; Bostr6m et al., 1982), since the laboratory set-up would necessarily change some conditions prevailing in the lake. Moreover, the release rates vary with the time interval chosen for the basis of their calculation, as the time vs release relationship is not linear. Therefore, seasonal variation of phosphorus release was characterized with the initial release rates measured after 2-4 days of incubation. The initial phosphorus release rate was low throughout the year from the intact cores of the mesotrophic area. The highest rates ( 0 . 3 m g P m -2 day ~) were measured during early autumn. In the same period the release was two times higher from the hypertrophic cores, where the maximum (2.8 mg P m -2 day -~) was observed during summer. The range of the release rates was similar during the summers of 1980 and 1981 (Fig. 1). The maximum rates measured corresponded to about one half of the biologically available external phosphorus loadings of the mesotrophic and hypertrophic areas (0.6 and 3.8 mg P m - 2 d a y ~, respectively; Jolfinkai and Dfivid, 1983). This is clearly demonstrative of a difference in the mobilizable phosphorus contents in the sediments of the two areas, not directly related to their total phosphorus contents. This is on account of the dilution of the surface sediments by the precipitating biogenic and abiogenic calcium carbonate, the amount of which is higher by at least one order of magnitude in the hypertrophic area (Lijklema et al., 1983). The different mobilities of phosphorus is also reflected in the 2-5 times higher isotopic exchangeability of phosphorus (Herodek and Istvfinovics, 1986; Pettersson and Istwinovics, 1988), as well as the higher concentrations of CaCO3-P, Fe-P and easily degradable organic-P in the hypertrophic 2.8 2.4 2.0 ~:~ 1.6
,~
E Q.
jJJ
~.2 G8
E 0.4 0.0
I
RESULTS AND DISCUSSION The seasonal cycle o f phosphorus release Phosphorus release of intact sediment cores allows only a rough comparison of diffusive fluxes through
I
I
I
N D d FM 1979
I
I
AM
I
I
I
I
d JASON 1980
l
[~1
I
I
I
I
I
I
d d A S 0 1981
Fig. I. Seasonal variation of phosphorus release from intact sediment cores in the mesotrophic ( x ) and hypertrophic (Q) areas of Lake Ba]aton.
Seasonal variation of phosphorus release
sediments (Gelencsrr et al., 1982; Pettersson and Istv~inovics, 1988). Assuming 2 x 10-2-2 x 10-3m thickness, 10-gm 2 s-~ diffusion coefficient and 50-250 mg PO4-P m -3 interstitial SRP concentration, a release rate of 0.2-10 mg P m -2 day-J was estimated by Lijklema et al. (I 986). The rates measured were in agreement with these estimates. The seasonal cycle of phosphorus release from intact sediment cores seemed to be related to the bacterial numbers in the surficial sediments (Figs I and 2; Gorzr, 1982). The number of bacteria was low throughout the year in the mesotrophic area, reaching its maximum in September (105 ind. ml-t). Their number was higher by an order of magnitude in the hypertrophic sediments, showing maxima in April, July and during the autumn (Fig. 2).
1475
9 x 105
E "0 ¢1 6x10 5 c "0
.¢_ .0 t-
3 x 10 5
O L. o 0 m
pH-dependency of phosphorus release
x~ x . x x. I x + x + x ' l "x I Ix'~" /
I
In simple batch experiments, sediments of Lake Balaton behaved in a manner intermediate between redox-sensitive and -unsensitive categories (Bostrrm and Pettersson, 1982). The anaerobic release from intact sediment cores was low (Ol~th et al., 1977) in comparison to many other lakes (Table 1). Taking the low Fe-P and high CaCO3-P contents of the sediments into account (Pettersson and Istwlnovics, 1988), it could be supposed that the pH-conditions modified by primary production and mineralization may play the most important role in the regulation of the phosphorus mobilization. Since Gelencsrr et al. (1982) studied the pH-towering effect caused by bacterial decomposition taking place in the sediments, the effects of the increasing pH were studied in some detail with long-term incubation of intact sediment cores and suspensions. The pH showed minor fluctuations in the water during the incubation of the intact cores and suspensions (Table 2). The initial release rates increased rapidly with the pH [Fig. 3(a)] and were followed by a slower saturation at almost each pH level (Fig. 4). The mesotrophic cores released only SRP, therefore TDP values are not shown. In the hypertrophic cores
N D d
F MA
1979
Anaerobic
Aerobic
12.3 17.3 1.2 52
- 1.4 -2.0 0.2 --
Shagawa Glaningen Ramsjrn Ryssbysjrn
7.0 18 20 20
-2.0 0.3 0.7
Chub Balaton mesotrophic hypertrophic reed stand
*Present study.
I
I
A S 0
IX'l'Xl N D
1980
and suspensions SRP release made up more than 95% of TDP with the exception of the highest pH of the suspensions. As discussed by Bostrrm (1984), losses of phosphorus in forms other than SRP cannot be considered as released with certainty, since colloidal or fine particulate matter may be lost during samplings due to the slow sedimentation. The total amount of SRP liberated during the period of the incubation also showed an increase with the pH [Figs 3(b) and 4]. During summer the maximum pH of the water reaches 8.8 and 9.2 in the mesotrophic and hypertrophic areas, respectively. At these pH values the rate of release may be estimated as 0.8 and 4 . 0 m g P m - Z d a y -1 from the sediments of the respective areas. This is similar to the release rates measured under anaerobic conditions (Table 1). In contrast to the phosphorus release from intact sediment cores, pH had minor effect on the SRP
Release (mg P m-2 day i)
Esrom Fures~ Gribso Mendota
d
Fig. 2. Seasonal variation of bacterial numbers in the surficial sediments in the mesotrophic ( x ) and hypertrophic (O) areas of Lake Balaton. (After Gorz6, 1982).
Table 1. Phosphorus release from intact sediment cores in some lakes
Lake
M d
×/
1.5
--
1.0 4.0 13.2
0.0-0.8* 0.0-4.0* --
Reference Kamp-Neilsen (1974) Kamp-Nielsen (1974) Kamp-Nielsen (1974) Holdren and Armstrong (1980) Sonzogni et al. (1977) Ryding and Forsberg (1977) Ryding and Forsberg (1977) Niirnberg et al. (1986) Ol6h et al. (1977) Istvhnovics, unpublished
VERA ISTV.~NOVICS
1476
Table 2. The pH of the overlying water during the incubation. (A) Mesotrophic cores; (B) hypertrophic cores; (C) hypertrophic suspensions. Averages of triplicates Day from the beginning of the incubation Addition ml 2N NaOH [A)
.
.
.
.
.
.
.
13
16
20
23
27
8.03 8.15 8.26 9.33
8.00 8.46 8.74 9.16
8.17 8.31 8.5t 8.70
8.00 8.30 8.53 8.72
8.00 8.32 8.54 8.74
8.26 8.32 8.53 8.77
8.13 8.40 8.56 8.77
8.27 8.38 8.58 8.77
4
7
11
14
18
21
25
28
8.33 8.53 8.72 8.86
8.33 8.51 8.69 8.81
8.36 8.54 8.71 8.84
8.36 8.55 8.73 8.86
8.36 8.56 8.79 8.94
8.36 8.60 8.71 8.89
8.42 8.54 8.73 8.86
8.42 8.73 8.77 8.84
3
6
9
12
16
0.0 0.2 0.5
8.25 8.91 9.95
8.22 9.06 10.06
8.22 9,22 10.41
8.39 9.27 10,42
8.27 9.25 10.39
1.0
I 1.06
I 1.33
11.57
11.57
11.53
(a)
(b)
4.0
10
/
3.5
/
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.
9
0.0 0.5 1.0 1.5
(C)
.
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t B)
.
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/
1.5 1.0
/
/
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7~
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85
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,
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./
./
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o ~//o . / /
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8
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30 lo
25 T
./
2O 15 -
l
I
l
I
I
9.0
9.5
10.0
105
11.0
pH
I0
pH
Fig. 3. Initial (a) and total (b) SRP release from intact sediment cores and suspensions as a function of the pH. [Figure 3(b) refers to a time period of 27, 28 and 16 days in the case of the mesotrophic cores, hypertrophic cores and hypertrophic suspensions, respectively.] O, Mesotrophic cores; O, hypertrophic cores; . × - - - hypertrophic suspensions.
(a)
4
5
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(b) ,o" ~o ~ ' ° ~ e ~
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lO
O ~ o /
o l~l~'~rlll"~lq I n I n I n I n I 2 6 10 14 18 22 26
Days
2
6
10
14
18
22 26
Days
rgr-~'T I i I i I 6 10 14 18
Days
Fig. 4. The pH-dependent phosphorus release from intact sediment cores in the mesotrophic (a) and hypertrophic (b) areas and from the hypertrophic sediment suspensions (c). Q, SRP; ©, total dissolved p h o s p h o r u s ; 1,2,3,4--increasing p H levels.
D..
::L
Seasonal variation of phosphorus release liberation from the suspensions in the pH range of 8-9 (Fig. 3). In the suspensions, similar to the resuspended sediments under in situ conditions, a given amount of phosphorus is present, which may be mobilized at the prevailing pH of the water. On the other hand, it was shown that the phosphorus mobilized in the deeper, reduced zones of the sediments diffuses upwards and is trapped in the oxidized surface layers (Carignan and Flett, 1981; Herodek and Istvfinovics, 1986). Phosphorus migrating upwards, however, may efficiently be bound in the surface layers only when the pH-conditions are favourable for its adsorption on the solid phases. In situ measurements evidenced that the average interstitial pH of 8 increased to 8.5 in the surficial sediment layers of the hypertrophic area, when the pH of the lake water increased from 8.6 to 9.2, as the result of the intense primary production (Gelencs6r et al., 1982). When this pH rise occurs in the interstitial water, the phosphorus having been trapped in the surficial layers may be mobilized and may diffuse to the water column. These processes account for the more definite pH-dependency of phosphorus release from intact cores in comparison to the suspensions. This indicates that the indirect redox-regulation of the internal phosphorus loading may be significant also in the calcareous sediments with relatively low share of Fe-P, while the direct effect of the redoxconditions is rather slight (Table 1, Bostr6m and Pettersson, 1982). Using intact sediment cores, Andersen (1975) found in calcareous Danish lakes that gross release of phosphorus was proportional to the increasing pH, while the net release decreased above pH = 9.5 as a result of hydroxylapatite formation. The significance of the same process was also verified in some other lakes (e.g. Kamp-Nielsen, 1974; Serruya et al., 1974). On the contrary, this was obviously not the situation in Lake Balaton (Figs 3 and 4; Pettersson and Bostr6m, 1984), where formation of hydroxylapatite is impeded by the high Mg 2+ and CO~- concentrations (Gelencs6r et al., 1982).
Changes in fractional distribution of phosphorus during the incubation of the suspensions Phosphorus mobilized under any conditions will in part be released and in part resorbed on other phases, depending on the adsorption capacity as determined by the composition of the sediments, as well as by the prevailing environmental conditions. Therefore, the fractional composition of phosphorus was also studied before and after the incubation of the hypertrophic sediment suspensions (Table 3). The total phosphorus content of the sediments was originally 721 ; t g P g -~ dry wt, 59 and 63% of this being inorganic according to the fractionation schemes of Hieltjes and Lijklema (1980) and Psenner et al. (1985), respectively. The bicarbonate/dithionite step in the latter scheme overestimated the amount of reducible iron-bound phosphorus due to a partial W R . 22/12--B
1477
dissolution of the HCI-RP fraction of the Hieltjes-Lijklema scheme, as it was shown by Pettersson and Istvhnovics (1988). The amount of Fe-P could be calculated as the difference of the NaOH-RP fractions of the two extraction schemes, and was found to be 70/~gPg -~ dry wt (Table 3). The average Ca/P ratios were 3300, 42 and 300, while Ca/Mg ratios were 24, 15 and 8 in the NH4CI, BD and HC1 extracts of the original sediments, respectively. In the NaOH-extracts Ca and Mg contents were negligible. A possible explanation for the highly different ratios would be that NH4CI removes phosphorus adsorbed on the freshly formed CaCO3, while HC1 from the high magnesium calcite formed by a slow ion exchange of about 10% Mg 2+ into the pure CaCO 3 precipitate (Szilfigyi, 1985). In the original sediments 41% of the residual-P (the difference between total and total reactive phosphorus contents) could be extracted with NaOH as NaOH-nRP according to both fractionation schemes used (Table 3). The SRP released during the incubation amounted to 1.1, 1.4, 3.3 and 5.1% of the total sediment phosphorus at the increasing pH levels, respectively (Fig. 4 and Table 3). This is low in comparison to many other lakes (Bostr6m, 1984), and is in agreement with the results of previous studies (Pettersson and Bostr6m, 1984). The amount of inorganic phosphorus increased during the incubation due to a loss from the NaOHnRP fraction (Table 3). The increase of the NH4CI-RP content could be a consequence of increased phosphate adsorption onto calcite with increasing pH. A similar explanation could be given to the observed increase of the HC1-RP fraction, since two NH4CI extractions did not remove the total amount of CaCO3-P from the sediments of Lake Balaton (Pettersson and Istv~inovics, 1988). The decrease of the BD-RP and NaOH-RP fractions was in agreement with the desorption of Fe- and AI-P at alkaline pH values (Table 3). The amount of NaOH-nRP showed a dramatic decrease during the incubation (with 100-110/~g P g-~ dry wt, Table 3). Supposing that this decrease was caused by the microbial utilization of the organic phosphorus, the rate of which was constant during the incubation, a consumption rate of 6.3 #g P g-i day-~ could be estimated in the hypertrophic suspensions, resulting in a turnover of 19 days for the easily degradable organic phosphorus. The corresponding values were also calculated for the uppermost layers of the intact sediment cores, which were fractionated according to the scheme of Hieltjes and Lijklema (1980) (results not shown), and compared to those calculated from the data of Pettersson (1984) (Table 4). The rate of consumption seemed to increase both with the concentration of the NaOHnRP fraction and the temperature. The different rates in the mesotrophic and hypertrophic cores were in agreement with the one order of magnitude higher
1478
VERA ISTV,~NOVICS
Table 3. Fractional composition of phosphorus in the hypertrophic sediments before and after the incubation of the suspensions. (A) Fractionation according to Hieltjes and Lijklema (1980); (B) fractionation according to Psenner et aL (1985). Averages of triplicates. NaOH-nRP----organic P extracted with NaOH; ZRP--total reactive phosphorus extracted; res-P = TP - ~RP; Fe-Pcak--the difference of the NaOH-RP fractions of the two fractionation schemes Samples
NH4CI-RP
BD-RP
NaOH-RP
NaOH-nRP HCI-RP (/~g P g- ~dry wt)
'~RP
res-P
Fe-P,ae
SRP,~I
(A)
Originally pH = 8.25 pH = 9.25 pH = 10.40" pH = 11.50
47 56 58 90 89
------
105 60 54 61 55
113 10 13 12 12
273 333 334 354 310
425 450 445 503 454
296 263 264 192 211
------
-8 I1 24 37
(B)
Originally pH = 8.25 pH = 9.25 pH = 10.40" pH = 11.50
47 56 58 90 89
135 127 108 108 I 11
36 23 23 18 14
117 8 8 6 4
233 268 285 262 250
451 473 474 478 463
270 239
70 37
-8 11 24 37
234
32
219 205
43 41
*One sample only.
Table 4. Rate of consumption and turnover of the NaOH-nRP fraction during the incubation of sediments NaOH-nRP (/~g g- ~) Before
After
Rate of consumption (,ag Pg i day t)
95
57-90
0.2-1.4
66-428
Reference Present study
215
115-145
2.5-3.6
59-86
Present study
117
17
6.3
t9
Present study
102
62
2.5
41
Pettersson (1984)
102
97
0.2
469
Pettersson (1984)
30 g/150 ml suspension, 20°C, 16 days, +4gNO3-NI -~ 30 g/150ml suspension, 10°C, 23 days, + 4 g N O : N 1 -~
1085
585
31.0
35
Pettersson (1984)
1085
805
12.6
86
Pettersson (1984)
30 g/150 ml suspension, 20°C 16 days, +4gNO3-NV ~ 30g/150ml suspension, 10°C, 23 days, +4gNO3-NI -t
630
450
11.0
56
Pettersson (1984)
630
520
4.8
132
Pettersson (1984)
Lake
Experimental conditions
Balaton, mesotrophic Balaton, hypertrophic
Intact upper Intact upper
Balaton, hypertrophic Balaton, hypertrophic Balaton, hypertrophic
5 g/200 ml suspension, 15°C, 16 days 30 g/150ml suspension, 20'C, 16 days, +4gNO3-NV ~ 30 g/150 ml suspension, 10'C, 23 days, +4gNO3-NI -I
Finjasj6n Finjasj6n Vallentunasj6n Vallentunasj6n
cores, 5°C, 27 days 1.6 cm cores, 5°C, 28 days, 1.8 cm
bacterial n u m b e r s in t h e h y p e r t r o p h i c s e d i m e n t s ( G o r z 6 , 1982). P e t t e r s s o n (1984) s u g g e s t e d to a c c o u n t for t h e difference b e t w e e n t h e d e c r e a s e o f t h e r e s i d u a l - P a n d N a O H - n R P f r a c t i o n s as a n i n c r e a s e in t h e b a c t e r i a l P. T h i s i n c r e a s e r a n g e d b e t w e e n 6 0 - 8 0 / ~ g P g-~ d r y wt in t h e h y p e r t r o p h i c s u s p e n s i o n s , w i t h t h e exception of the highest pH, where a part of the organic-P w a s lost by s a m p l i n g ( T a b l e 5). B a c t e r i a l b i o m a s s m i g h t i n c r e a s e d u e to t h e a d d i t i o n o f lake w a t e r c o n t a i n i n g 6 0 0 m g N O 3 - N m -3. A s s u m i n g t h a t t h e upper 3 cm layer of the sediments with 80% water content contains 7 kg dry sediment per square meter,
Turnover (day)
t h e rate o f t h e i n o r g a n i c p h o s p h o r u s p r o d u c t i o n m a y be e s t i m a t e d as 4 - 1 6 m g P m 2 d a y - ~ d u r i n g t h e dec o m p o s i t i o n t a k i n g p l a c e in t h e h y p e r t r o p h i c sedim e n t s . T h i s is s i m i l a r to t h e rate o f o r g a n i c p h o s phorus supply of the sediments from the water (2-14mgPm-2day-~), estimated on the basis of p r i m a r y p r o d u c t i o n , a n d a c c o u n t s for t h e low org a n i c p h o s p h o r u s c o n t e n t o f t h e s e d i m e n t s in c o m p a r i s o n to o t h e r l a k e s ( B o s t r 6 m a n d P e t t e r s s o n , 1982; P e t t e r s s o n , 1984). In o r d e r to e s t i m a t e m g m - 2 d a y -~ p h o s p h o r u s release f r o m t h e s u s p e n s i o n s , f u r t h e r c o n s i d e r a t i o n s m u s t be t a k e n into a c c o u n t . A c c o r d i n g to t h e s i m p l e
Table 5. The fate of NaOH-nRP during the incubation of the hypertrophic sediment suspensions. Averages of triplicates. Bacterial-P increase---the difference between the losses of the residual-P and NaOH-nRP fractions. Residual-P--the difference of the total and total reactive phosphorus contents
Samples pH = pH = pH = pH = *One
8.25 9.25 10.40" 11.50 sample only.
P-loss in forms other than SRP (#g p g-t dry wt) 0 1 2 19
Increase of Increase of bacterial-P SRP released 5~Rp (% of the NaOH-nRP decrease) 74 70 60 48
8 10 22 32
21 21 24 I1
Increase of bacterial-P 71 69 51 41
Seasonal variation of phosphorus release hydrodynamical model of Somlyrdy (1986), the rate of resuspension is 170 g sediment m-2day -t at an average wind velocity of 5 m s- i. Five g wet sediment, which was used for the preparation of the suspensions, represents the daily resuspension from a bottom area of 0.03 m 2. Taking into account the initial phosphorus release and the two orders of magnitude higher sediment/water ratio in comparison to the natural ones, a release of 2.0-3.9 mg P m-: day -~ can be estimated in the pH range of 8-9. Similar estimates were given for phosphorus release via desorption from the resuspended sediments in earlier studies [0.8-1.7 mg P m -2 dayfor the whole lake by Lijklema et aL (1983); 0.1-0.9 and 0.2--4.7 mg P m -2 day -1 for the mesotrophic and hypertrophic areas, respectively by Pettersson and Bostr6m (1984)]. Interactions between external and internal loadings and primary production
The available data indicate the existence of a positive feedback between the internal phosphorus loading and primary production, both being affected by the external loading in different ways. Any excess of biologically available phosphorus entering the water column is rapidly taken up by the planktonic microorganisms competing efficiently for the limiting amounts of phosphorus. Biological uptake is the only significant process keeping PO4 concentrations at their constantly low level (below 1-2 mg m-3), and transporting the biologically available phosphorus from the water to the sediments (Istv~novics and Herodek, 1985; Istvfinovics et al., 1986). Therefore, both the biologically available external and internal phosphorus loadings enhance the primary production in a direct manner. Primary production is proportional to the sum of these loadings (Somly6dy, 1983). The most important phosphorus mobilizing factors are more or less closely related to the rate of the primary production. Due to the low organic content of the sediments, the rate of mineralization depends, among other factors, on the organic matter supply from the water column. In the course of the bacterial decomposition taking place in the sediments, inorganic phosphorus is produced directly, as discussed previously. Redistribution or release of the inorganicP are controlled by the pH-dependent phosphate adsorption capacity of the sediments as well as by the rate of both mineralization and sorption. The pH is decreased by the CO2 production during the course of decomposition. A part of CaCO3 dissolutes resulting in mobilization of CaCO3-P. The rate of this process can be estimated as 0.2-0.5 and 0.8-2.4mg P m - : d a y -1 in the mesotrophic and hypertrophic areas, respectively, based on the amount of organic matter decomposing in the sediments (Vrr6s et al., 1983), as well as on the concentration of the CO2-soluble sediment phosphorus (60-180/~g P g-
1479
dry wt; Gelencsrr et al., 1982). On the other hand, there is an increase in the phosphate adsorption capacity of the metal hydroxides and clay minerals when the interstitial pH is lowered. Due to the pH rise during the period of intense primary production, calcium carbonate precipitates in proportion to the photosynthesis. About two thirds of it dissolutes in the water as a result of decomposition taking place there (Lijklema et al., 1983). The amount of the sedimenting CaCO3 can be estimated as 1.2 and 5.7gm-2day -1 in the mesotrophic and hypertrophic areas, respectively. CaCO3 adsorbs phosphate only in the sediments, where the interstitial SRP concentration is higher by 2-3 orders of magnitude as compared to the lake water (30-250 mg PO4-Pm-3; Ol~ih et al., 1977; Dobolyi, 1980; Gelencs~r et al., 1982; Sz~isz et al., in preparation). Taking into account the rate of CaCO3 precipitation and the concentration of the CaCO3-P fraction (60-180/~gPg -1 dry wt), the rate of PO4 adsorption on CaCO3 may be estimated as 0.6-2.4 and 4.0-14.8 mg P m -2 day -~ in the mesotrophic and hypertrophic areas, respectively. On the other hand, increasing pH of the interstitial water following the pH rise of the lake water results in a desorption of phosphorus from the iron- and aluminiumcompounds. The latter process also takes place when the sediments are resuspended into the lake water. As the structure of the sediments shows microscopic inhomogeneities, the opposite processes of the adsorption and desorption of PO4 on/from the carbonates and metal hydroxides, as well as precipitation and dissolution of CaCO 3 take place simultaneously. During the periods of intense primary production these result in a rapid increase of the internal phosphorus loading at the increasing pH of the lake water (Figs 3 and 4, Table 3). A similar regulatory role of the pH was also found to be important in a number of shallow, eutrophic lakes with different compositions of sediments (Drake and Heaney, 1987; Istv~inovics and Pettersson, in preparation). Independent of the primary production, the internal loading is also modified by the external one by means of a change of the adsorption capacity of the sediments. On the one hand, the adsorption capacity of the iron- and aluminium-compounds decreases gradually as their surfaces become saturated with phosphorus during prolonged periods of external loading (Bostr6m and Pettersson, 1982; Bostr6m, 1984). It was shown that saturation of the adsorption capacity of the NaOH-RP fraction is 70% in the hypertrophic sediments of Lake Balaton, in contrast to the 30% saturation in the mesotrophic sediments (Istv/movics et al., in preparation). On the other hand, the influent waters are supersaturated in respect to CaCO3. Precipitation of calcium carbonate
i
taking place within the lake continually increases the adsorption capacity of the sediments (Istv/movics et al., in preparation).
1480
VERA ISTV.~NOVICS CONCLUSIONS
The total external phosphorus loadings of the mesotrophic and hypertrophic areas are 0.9 and 7.3 m g P m - 2 d a y -1, respectively (Jol~nkai and Dfivid, 1983). The magnitude of the internal loading is similar in both areas of Lake Balaton. The most important phosphorus mobilizing factors are pH and the decomposition of the organic matter taking place in the sediments. The internal phosphorus loading increases rapidly with the increasing pH of the lake water during the periods of intense primary production. A positive feedback exists between the internal phosphorus loading and primary production, accounting for the observed seasonal cycle of phosphorus release from the sediments, as well as for the internal loading higher by about a factor of five in the hypertrophic area. F r o m the point of view of lake management, this feedback means that the response of the lake may be more beneficial to the reduction of the external phosphorus loading than would be expected without this relationship. Acknowledgements--1 am grateful to Dr Lhszlo Somly6dy and Dr Prter Gelencsrr for their valuable comments, to Mr I. B~ithory for the field assistance and to Mrs E. Kozma for laboratory assistance. REFERENCES
Andersen M. J. (1974) Nitrogen and phosphorus budgets and the role of sediments in six shallow Danish lakes. Arch. Hydrobiol. 74, 528-550. Andersen M. J. (1975) Influence of pH on the release of phosphorus from lake sediments. Arch. Hydrobiol. 76, 411-419. Bostr6m B. (1984) Potential mobility of phosphorus in different types of lake sediments. Int. Revue ges. Hydrobiol. Hydrogr. 69, 457-474. Bostrrm B. and Pettersson K. (1982) Different patterns of phosphorus release from lake sediments in laboratory experiments Hydrobiologia 92, 415-429. Bostrfm B., Jannson M. and Forsberg C. (1982) Phosphorus release from lake sediments. Arch. Hydrobiol. 18, 5-59. Carignan R. and Flett R. J. (1981) Postdepositional mobility of phosphorus in lake sediments. Limnol. Oceanogr. 26, 361-366. Dobolyi E. (1980) Data on the bottom sediment in Lake Balaton. Proc. 2nd Joint MTA /IIASA Task Force Meet. on Lake Balaton Modeling 2, 66-81. Drake J. C. and Heaney S. I. (1987) Occurrence of phosphorus and its potential remobilization in the littoral sediments of a productive English lake. Freshwat. Biol. 17, 513 523. Furumai H. and Ohgaki S. (1982) Fractional composition of phosphorus forms in sediments related to release. Wat. Sci. Technol. 14, 215-226. Gelencsrr P., Szilhgyi F., Somly6dy L. and Lijklema L. (1982) A study on the influence of sediment in the phosphorus cycle for Lake Balaton. IIASA Collab. Paper CP-82-44, Laxemburg, Austria. Gorzo Gy. (1982) Quantitative bacteriological studies in Lake Balaton (1978-1981) MTA Biol. Oszt. Ki~zl. 25, 289-305 (in Hungarian with an English summary). Herodek S. (1983) Biochemical processes in Lake Balaton. IIASA Collab. Paper CP-83-$3, pp. 101-147. Laxemburg, Austria.
Herodek S. and Istvfinovics V. (1986) Mobility of phosphorus fractions in the sediments of Lake Balaton. Hydrobiologia 135, 149-154. Hieltjes A. H. M. and Lijklema L. (1980) Fractionation of inorganic phosphates in calcareous sediments. J. envir. Qual. 9, 405-407. Holdren G. C. and Armstrong D. E. (1980) Factors affecting phosphorus release from intact sediment cores. Envir. Sci. Technol 14, 79-87. Istvfinovics V. and Herodek S. (1985) Orthophosphate uptake of planktonic microorganisms in Lake Balaton. Hydrobiologia 122, 159-166. Istv~movics V., Vrr6s L., Herodek S., T6th G. L. and Tfitrai I. (1986) Changes of phosphorus and nitrogen concentrations and of phytoplankton in enriched lake enclosures. Limnol. Oceanogr. 31, 798-811. Jol~.nkai G. and Dfivid L. (1983) Nutrient loads and watershed development. IIASA Collab. Paper CP-83-$3, pp. 45.81. Laxemburg, Austria. Kamp-Nielsen L. (1974) Mud-water exchange of phosphate and other ions in undisturbed sediment cores and factors affecting the exchange. Arch. Hydrobiol. 73, 218-237. L~ing I. (1986) Impact of policymaking: Background to a government decision. In Modeling and Managing Shallow Lake Eutrophication (Edited by Somlyrdy L. and van Straten G.), pp. I10-123. Springer, Berlin. Lijklema L., Gelencsrr P. and Szilhgyi F. (1983) Sediments and sediment-water interaction. IIASA Collab. Paper CP-83-$3, pp. 81-101. Laxemburg, Austria. Lijklema L., Gelencsrr P., Szilhgyi F. and Somlyrdy L. (1986) Sediment and its interaction with water. In Modeling and Managing Shallow Lake Eutrophication (Edited by Somly6dy L. and van Straten G.), pp. 156-183. Springer, Berlin. Mackereth F. J. H., Heron J. and Talling J. F. (1978) Water analysis. FBA Sci. Publ. No. 36. Mfiller D. (1969) Sedimentbieldung im Plattensee, Ungarn. Naturwissenschaften 56, 606-615. Murphy T. P. and Riley J. P. (1962) A modified single solution method for the determination of phosphate in natural waters. Analytica chim. Acta 27, 31-36. Niirnberg G. K., Dillon P. J. and McQueen D. J. (1986) Internal phosphorus load in an oligotrophic Precambrian Shield lake with an anoxic hypolimnion. Can J. Fish Aquat. Sci. 43, 574-580. Ol~h J., Abdelmoneim M. and T6th L. (1983) Nitrogen fixation in the sediment of shallow Lake Balaton, a reservoir and fishpond. Int. Revue ges. Hydrobiol. Hydrogr. 68, 13-45. Olfih J., Trth O. E. and Trth L. (1977) Phosphorus metabolism of Lake Balaton MTA Biol. Oszt. Krzl. 20, I11-139 (in Hungarian with an English summary). Pettersson K. (1984) Transfers between forms of sedimentary phosphorus induced by nitrate treatment Verh. int. Verein theor, angew. Limnol. 22, 233-238. Pettersson K. and Bostr6m B. (1984) Phosphorus exchange between sediment and water in Lake Balaton. 3rd int. Syrup. Interactions between Sediment and Water pp. 80--84. Geneva, Switzerland. Pettersson K. and Istv~inovics V. (1988) Sediment phosphorus in Lake Balaton--forms and mobility. Arch. Hydrobiol. J30, 25-41. Psenner R., Pucsko R. and Sager M. (1985) Die Fraktioniearung organischer und anorganischer Phosphorverbindungen von Sedimenten. Arch. Hydrobiol. (Suppl.) 70, 111-155. Ryding S.-O. (1981) Reversibility of man-induced eutrophication. Experiences of a lake recovery study in Sweden. lnt. Revue ges. Hydrobiol. Hydrogr. 66, 449-502. Ryding S.-O. and Forsberg C. (1977) Sediments as a nutrient source in shallow polluted lakes. In Interactions Between Sediments and Fresh Water (Edited by Goiterman H. L.). Junk, The Hague.
Seasonal variation of phosphorus release Serruya C., Edelstein M., Pollingher U. and Serruya S. (1974) Lake Kinneret sediments: nutrient composition of the pore water and mud-water exchanges. Limnol. Oceanogr. 19, 489-508. Somly6dy L. (1983) Lake eutrophication management models. IIASA Collab. Paper CP-83-$3, pp. 207-251. Laxemburg, Austria. Somly6dy L. 0986) Wind induced sediment resuspension in shallow lakes. Int. Conf. on Water Qual. Modelling in the Inland Natural Environment. BHRA, The Fluid Engineering Centre, Cranfield, England. Sonzogni W. C., Larsen D. P., Malueg K. W. and Schuldt
1481
M. D. (1977) Use of large submerged chambers to measure sediment-water interactions. War. Res. 11, 461~,64. Szilfi.gyi F. 0985) A study on the interaction between sediments and water in Lake Balaton (in Hungarian) Reports of tlie Research Center for Water Resources Development, Budapest, Hungary. V6r6s L., Vizkelety l~., T6th F. and N6meth J. (1983) The trophic conditions of the Keszthely basin in 1979. Hidrol. K6zl. 63, 390-398 (in Hungarian with an English summary).