Hydrobiologia 228: 55-63, 1992.H. L. Golterman (ed.), Sediment-Water Interaction.© 1992 Kluwer Academic Publishers. Printed in Belgium.

Phosphate and calcium carbonate saturation in a stratified coastallagoon

55

P. L6pez & J. A. MorguiDept. Ecology, Univ. Barcelona, Avda. Diagonal 645, E-08071 Barcelona, Spain

Key words: calcium carbonate saturation, dissolved phosphate, sedimentary phosphate, apatite,stratification

Abstract

Seasonal variation of phosphate concentration and saturation index for calcite in water of a smallstratified coastal lagoon have been studied. In surface waters, where salinity was low and pH high, thesaturation index increased to values near 20, whereas in bottom water, with high salinity and low pH,they were usually lower. The ionic product for H3PO4 was strongly correlated with the ionic productof Ca(OH)2 in surface and bottom waters, and with the ionic product of CaCO3 in bottom, whichsuggested that chemical composition was mainly controlled by a calcium-phosphate solid phase.

The low concentrations of phosphate in surface were due to chemical precipitation and organicsedimentation, whereas in bottom, calcium phosphate redissolved and organic matter was mineralizedproducing high concentrations of soluble phosphate (> 60 #imol 1 - ).

Decrease of calcium-bound phosphate in the upper layers of sediment was in agreement with adiminution of calcium-phosphate precipitation, probably due to a lower influence of seawater in the past.

Introduction

Calcium carbonate precipitation has usually beenassumed to be an important factor controllingconcentration of dissolved phosphate, o-P, intemperate hardwater lakes and estuarine systems.Laboratory work has shown that phosphate isadsorbed onto calcite and aragonite surfaces andinhibits crystal growth rates (Kuo & Lotse, 1972;De Kanel & Morse, 1978; Walter, 1986; De Jonge& Villerius, 1989). Phosphate adsorption byCaCO 3 is nearly independent of pH and redoxpotentials. Yet, if the concentration of o-P re-mains high, a conversion of the formed CaHPO4to apatite Cao1 (PO4) 6(OH)2 will occur (Stumm &Leckie, 1971; Jacobsen, 1978). Avnimelech (1980,

1983) suggested that the main product of the re-action between CaCO3 and dilute o-P is aCaCO 3H3PO4 surface complex, which is kineti-cally favoured over apatite.

The degree of CaCO 3 supersaturation (or sat-uration index), , depends on many factors:pH is probably the most important (Otsuki &Wetzel, 1974; Golterman & Kouwe, 1980; Golter-man & Meyer, 1985; De Jonge & Villerius, 1989;Kuchler-Krischun & Kleiner, 1990). Temperature(Brunskill, 1969), biological activity (Green et al.,1985; Kuchler-Krischun & Kleiner, 1990), salin-ity (Stumm & Morgan, 1981) and presence ofhigh Mg concentrations (Berner, 1975) play a sig-nificant role under natural conditions as well. Onthe other hand, the mechanisms of adsorption/

56

desorption of o-P on CaCO3 particles are influ-enced by salinity (Stirling & Wormald, 1977; DeJonge & Villerius, 1989) and phosphate specia-tion: aragonite precipitation rates are inhibited byPO3 - concentrations whereas calcite rates areinfluenced by concentrations of both PO - andHPO - (Burton & Walter, 1990). Moreover,phosphate speciation is pH dependent and influ-enced by specific and non-specific interactions(Kester & Pytkowicz, 1967).

The study of phosphate dynamics in estuarinewaters shows a decrease of o-P at high salinityand pH due to adsorption on calcite precipitates.At low salinity and pH calcite dissolves and o-Pwill be released (De Jonge & Villerius, 1989). Highconcentrations of o-P are also associated withanaerobic redox potential in marine sediments(Enstch et al., 1983).

La Massona, a small, hypereutrophic coastallagoon located on the Northeastern coast of Spain(Fig. 1), provides a model for the study of therelationship between phosphate and saturationindex with respect to CaCO3. Previous studies(Armengol et al., 1983; Lopez et al., 1984) showedthat the lagoon remained stratified throughout theyear because of a strong salinity gradient due toseawater input through the sand barrier. Thehypolimnion was anoxic, with low pH values(near 7) while the concentrations of major ionswere similar to those of seawater. In the epilim-nion, water was well oxygenated, oligomesohalineand pH values were usually high.

We have studied f) in both surface and bottomwaters, in order to relate it to observed variationsin phosphate concentrations. We have also stud-ied the distribution of sedimentary phosphatespecies along a vertical profile of sediments, whichallows us to establish the chemical relationsbetween water and sediment composition.

Methods

Chemical analysis

Water samples were collected monthly from dif-ferent depths at the deepest point of the lagoon

O 10O

Fig. 1. Location and bathymetry of La Massona. Arrow in-dicates sampling site.

(Fig. 1). A sediment core was also taken at thesame point and divided into subsamples for fur-ther analysis.

Water analyses were carried out according toStandard Methods (APHA, 1975) (alkalinity byacidic titration and further correction for the con-tribution of sulfide; Ca by atomic absorptionspectrometry using flame technique) (APHA,1975). o-P was determined with the Murphy &Riley method in a filtered sample (0.5 Im diam-eter), in an unfiltered sample for Tot-Preac andafter persulphate digestion of an unfiltered sam-ple for Tot-P (Strickland & Parsons, 1965).

For sediments, major elements and Psed wereanalyzed by X-ray fluorescence spectrometry.Phosphate fractions, NaOH-P (phosphate ex-tracted with NaOH, which supposedly corre-

57

sponds to phosphate associated to metals) andHCl-P (phosphate extracted with HCl, whichmainly corresponds to calcium bound phosphate)were analyzed following the method of Hieltes &Lijklema (1980) with slight modifications. Resid-ual phosphate, (R-P mainly organic and refrac-tory forms) was calculated by difference.

Equilibrium calculations

We calculated Q (calcite saturation index) for sur-face waters (ionic strength <0.5) with the equa-tion:

f = IPoal/Kcal,

where KcaI is a constant dependent on tempera-ture (Table 1) and IPca, (ionic product forcalcite) = [Ca2+ ]calcium[ CO2f ]ccarbonate Theactivity coefficients oc were determined with theDavies equation (Stumm & Morgan, 1981). Con-

centrations of free ions [Ca +] and [CO2f ] werededuced from the ion-pair distribution, which wascalculated from constants for H2CO3 and the sta-bility constants for the main ion-pairs (Table 1).[PO4 -] was calculated from the dissociationconstant for H3 PO4, K3 (Table 1), assuming thatSRP HPO42- (Stumm & Morgan, 1981).

For bottom waters with an ionic strength > 0.5,Davies' equation is not useful. So, we applied adifferent approach following Burton & Walter(1990):

Q' = IPAcal/CKcal,,

IPAcal (apparent ionic product) =

2 [Ca2 + ] [CO2-],

where CKcal is-the apparent solubility constant forcalcite at seawater salinity (Table 1). [Ca 2 + ]isthe observed concentration of Ca and [CO32-]was deduced from the apparent dissociation con-stants for the carbonic system (Table 1). Simi-

Table 1. Logarithms of the equilibrium constants for carbonic and phosphoric acid systems, calcium carbonate solubility andstability constants for the main ion-pairs. (1): Stumm & Morgan, 1981; (2): Mehrbach et al., 1973; (3): Golterman & Meyer, 1985;(4): Mucci, 1983; (5): Kester & Pytkowicz, 1967; (6): Morel, 1983.

T Infinite dilution Seawater

H2CO3K, 10 - 6.46 (1)15 -6.42 (1)20 -6.38 (1)25 - 6.35 (1) - 6.00 (2)30 -6.33 (1)

HCO 3 K2 - 10.625 + 0.0145T- 1.089T 2 (3) -9.11 (2)HPO4- K3 25 - 12.35 (1) - 8.65 (4)CaCO3K' 10 - 8.36 (1)

15 -8.37 (1)20 -8.39 (1)25 - 8.42 (1) - 6.36 (2)

NaCO- 25 - 9.06 (6)NaHCO ° 25 - 0.25 (6)NaSO_ 25 + 1.06 (6)KSO- 25 + 0.96 (6)CaCO ° 25 - 7.13 (6)CaHCO- 25 + 1.26 (6)CaSO ° 25 + 2.31 (6)MgCO ° 25 - 6.93 (6)MgHCO' 25 + 1.16 (6)MgSO ° 25 + 2.36 (6)

58

larly [PO'-] was calculated from the apparentdissociation constant of H3 PO4 in seawater (Ta-ble 1).

Following Avnimelech (1983), the solubility ofphosphate was considered using two possiblemechanisms: control by a calcium-carbonate-phosphate solid phase, and control by a calcium-phosphate phase. If composition was determinedby the first mechanism, the reaction between elec-troneutral components can be formulated as

3CaCO3 + H 3PO4-*Ca 3(HCO 3) 3PO4 .

At the equilibrium, logarithm of H3 PO4 activitiesshould be correlated with those of calcium car-bonate activities, because

3log(CaCO3) + log(H 3PO4) = constant,

where values in parentheses represent the activi-ties.

Similarly if the second mechanism controlsphosphate concentration, a significant correla-tion between logarithm of H3PO4 activity andlogarithm of Ca(OH) 2 should be found.CaCO3, H 3PO4 and Ca(OH)2 activities areequivalent to ionic products (i.e. log (CaCO3 ) =log IPcal; log (H 3PO 4 ) = log [PO - ] + 3 log[H + ]log Ca(OH) 2 = log [Ca + + 2log[OH]). Inthis study, we have used the apparent ionic prod-ucts for bottom waters in order to avoid both,specific and non-specific interactions.

Results

Annual variation of Q, Q' and phosphate con-centration in surface and bottom waters are givenin Figs 2 and 3 respectively. Values of alkalinity,Ca and pH used for Q calculations are given inTable 2. Surface water was generally suprasatu-rated with respect to CaCO3, Q rising to valuesnear 20 on several occasions. Only in early sum-mer and autumn the water became undersatu-rated (Fig. 2). Near the bottom, Q' approachedthe equilibrium, except in spring (March-May)and August, when it reached about 10 and 30respectively (Fig. 3).

0

a.O N D J F M A M J J A S O N

Fig. 2. Annual variation of phosphate concentration (lowergraph) and saturation index for calcite (upper graph) at sur-face waters. Horizontal dotted line indicates a saturation in-dex= 1.

zz0FZ

0

a-

ON D J F M A M J J A SO N

Fig. 3. As Fig. 2 but for bottom waters.

The ionic product for calcite was strongly re-lated to pH in bottom water (r = 0.9849, n = 16)and, to a lesser extent, in surface water (r = 0.7348,

59

Table 2. Values of pH and concentrations of total alkalinity, calcium and sulfide in meq 1- , in surface (S) and bottom (B) wa-ters. (N.D. not detectable. N.A. not available).

pH Alk Ca2 + HS- '

2 Oct SB

16 Oct SB

4 Nov

25 Nov

16 Dec

16 Jan

SB

SB

SB

SB

SB

SB

SB

SB

12 Mar

1 Apr

15 Apr

4 May

17 Jun SB

22 Jul

10 Sep

10 Oct

SB

SB

SB

SB

2 Nov

8.77.5

8.57.4

8.57.7

8.57.8

8.67.5

8.37.4

8.78.1

48.78.0

8.48.0

8.07.2

7.07.0

8.78.4

7.56.8

7.46.8

8.68.3

4.455.85

4.115.52

4.744.31

5.123.39

4.345.31

4.475.29

5.075.23

4.906.94

4.946.18

5.226.02

4.367.99

3.536.39

4.176.43

3.948.27

3.717.88

9.120.0

N.D.0.50

7.421.9

7.123.9

10.329.6

13.424.8

6.418.2

8.318.0

8.718.7

6.218.7

7.617.8

9.623.9

4.819.7

3.019.7

1.816.9

2.418.8

N.D.0.96

N.D.0.56

N.D.0.30

N.D.1.30

N.D.1.20

N.D.0.18

N.D.0.41

N.D.0.41

N.D.0.74

N.D.0.90

N.D.N.A.

N.D.0.50

N.D.0.63

N.D.0.65

n = 16). It did not show a correlation with tem-perature nor with SRP.

Phosphate concentration presented a differentpattern of variation in surface and bottom waters.In surface water, SRP was low (< tmol 1- ')reaching a peak in summer (1.2 #mol - ). Tot-P and Tot-Preac varied simultaneously: they roseto a maximum of 4 and 3 tmol I- ' respectively inDecember, decreasing until May. A second peak

was observed in summer, although the exceptionalvalues attained in August might be due to anepisodic input of wastewater. At bottom water(Fig. 3), SRP showed its minimum value(3p moll ') in December, then rose slowlyreaching levels near 40 #mol 1- ' in spring-summer. Tot-P and Tot-Preac concentrations weregenerally very similar to those of SRP, howevertwo exceptions must be noted: from December to

60

Table 3. Parameters of the regression lines, y = Ax + B, for the logarithms of H3PO4 ionic product against Ca(OH)2 and CaCO3ionic products. The error of the estimate is given in parentheses. See text for explanation.

H3 PO4 vs CaCO3

A B r n

Surface -1.14 (0.410) -43.42 (1.067) 0.6414 13Bottom - 1.92 (0.317) - 39.61 (0.633) 0.8864 12

H3PO4 vs Ca(OH)2Surface - 0.98 (0.095) - 48.45 (0.427) 0.9516 13Bottom -1.07 (0.111) -44.34 (0.425) 0.9506 12

April Tot-P and Tot-Prac rose more quickly thanSRP, reaching 40 #mol 1- in February; in Au-gust Tot-P and Tot-Preac did not decreasewhereas SRP attained a second minimum.

The relationship between H3 PO4 and CaCO3is plotted in Figure 4. Parameters of the regres-sion lines for surface and bottom waters are givenin Table 3. As can be observed, the slopes wereclose to 1, the correlation coefficient being signif-icant for bottom water. The relationship withCa(OH)2 (Fig. 5, Table 3) also showed slopesnear 1. The correlation are strongly significant forboth surface and bottom water.

34

30

264 5 6 7 8 9 10

Fig. 4. Logarithm of H3PO4 activity, log[PO' ] + 3log[H + ](y axis) against logarithm of calcium carbonate activity, IPcal,(x axis). Dotted line: regression line for surface samples.Dashed line: regression line for bottom samples.

Vertical variations of Psed, NaOH-P, HCI-Pand RP in a sediment profile of La Massona arepresented in Fig. 6. Psed varied from 16.88 to23.91 Mmol g - . The main fraction was RP, how-ever it decreased from 65 % of Psed to 45-50 % indeeper layers. HCI-P showed the opposite behav-ior increasing from 30% in sediment surface to50% at depth. NaOH-P was always a very smallfraction of Psed (< 1%)

Correlation coefficients between these formsand other chemical components of sedimentswere also calculated (Table 4). NaOH-P was notcorrelated with any other variable. HCI-P wasmainly associated with Fe, Al, Ca and Ti, show-ing a negative correlation with silicium. R-P wasonly related to Psed

34-

30-

12 13 14 15 16 17Fig. 5. As Fig. 4, but versus Ca(OH)2 activity.

A

A Surface ·

0 Bottom .

I .

[] --

oArV I I I

61

A B

0 10 20 0 20 40 60 80 100

o o NaOH-P A-A RPo-u HCI-P o o TP

Fig. 6. Concentrations of phosphate species along a vertical profile of sediment. A: values in Imo g- 1. B: values in %/ of Ps,,.

Table 4. Coeficients of correlation between sedimentaryphosphate species and major consistuents of sediment(*p<O0.00).

NaOh-P HCI-P RP TP

Fe - 0.28Al - 0.17Mn 0.00Ti - 0.35Mg 0.23Ca - 0.40K -0.15Si 0.36TPs - 0.03

0.82*0.81*0.310.90*0.430.74*0.70*

-0.74*0.22

-0.05-0.17

0.06-0.30

0.06- 0.48

0.080.480.76*

0.530.400.300.310.41

-0.010.600.001.00

Discussion

Results observed in La Massona indicate thatphosphate concentration in water was mainlycontrolled by a solubility product of a calcium-phosphate solid phase. Although the brushite has

a higher solubility than the observed, the slope ofthe H3PO4 - Ca(OH)2 being close to 1, is inagreement with the existence of CaHPO4 ratherthan hydroxyapatite. Several authors have pos-tulated that this compound is precipitated as ametastable phase (Stumm & Morgan, 1981). Thesignificant dependence of H3PO4 on CaCO3 ac-tivities in bottom water indicates that chemicalcomposition was also controlled by a calcium-carbonate-phosphate phase. Avnimelech (1980)interpreted the dependence of H3 PO4 on bothCaCO3 and Ca(OH)2 as imposed on the systemwhen CO2 is constant. When CO 2 is not con-stant, he suggests that the solution may be con-trolled by both, calcium-carbonate-phosphateand calcium-phosphate, the two solid phasesbeing in equilibrium with the water during winterand spring periods in lake Kinneret (Avnimelech,1983).

Values of Q observed in surface water in thelagoon are among the highest values reported for

62

eutrophic hardwater lakes (Effiler, 1984; Effier &Driscoll, 1985; Kuchler-Krischun & Kleiner,1990) and can be related to high pH values. De-pendence of Q on pH has been widely reported(Golterman & Kouwe, 1980; Golterman &Meyer, 1985; De Jonge & Villerius, 1989;Kuchler-Krischun & Kleiner, 1990) and is in agree-ment with the high correlation coefficient found inLa Massona. Calcium-phosphate precipitationcaused the low SRP values observed in surfacewaters. Tot-Preac and Tot-P were mainly influ-enced by biological activity, their maximum val-ues being simultaneous to the maximum develop-ment of phytoplanktonic populations (F.A.Comin, pers. com.).

In bottom waters, Q was lower than 40, a valuereported for seawater with high concentrations ofo-P (Burton & Walter, 1990). There is evidencethat phosphate inhibits calcite precipitation ratesdepending on the pH (De Kanel & Morse, 1978;Walter, 1986; De Jonge & Villerius, 1989; Burton& Walter, 1990). Conditions in the hypolimnionof La Massona, low pH and high SRP concen-tration, suggest low rates of calcite precipitation.Thus, phosphate transported from surface as ametastable phase could redissolve, whereas phos-phate formed by decomposition of organic matterremained in solution. From December to Aprilsedimentation rates exceeded decompositionrates, causing an increase in the difference Tot-P- SRP.

Phosphate and calcite dynamics in a watercolumn of La Massona were regulated by antag-onistic mechanisms: precipitation and sedimen-tation at surface, where salinity was low and pHwas high; redissolution and decomposition atbottom where salinity was high and pH low. Themain factor controlling the process seems to bepH. This hypothesis is strengthened by observa-tions in lakes and estuarine systems, where pre-cipitation takes place at high pH, but at lowsalinity too, and redissolution occurs at low pHand high salinity (De Jonge & Villerius, 1989).

The high RP levels observed in sediment andthe significant correlation with Psed are in agree-ment with an important sedimentation of organicmaterial.

The high phosphate levels observed in La Mas-sona might be partially due to sediment redisso-lution. The decrease of HCl-P (mainly related tocalcium-bound phosphate) can be interpreted asthe result of two mechanism: A) redissolution ofcalcium-phosphate minerals, because at pH near7 the calcium phosphate can dissolve; B) decreaseof precipitation rates of calcium-phosphate forms.HCl-P was also correlated with iron, aluminumand titanium, concentrations of which usuallyhigher in freshwater sediments than in seawatersediments. Similarly, silica, a common element inseawater sediments, presented a negative corre-lation with HCI-P in La Massona. Becausecalcium-phosphate precipitation was clearly as-sociated to low salinity in the lagoon, the hypo-thesis of a diminution in sedimentation rates, dueto a lower influence of seawater in the past, seemsto be the most probable. However, a better com-prehension of the relation between water chem-istry and sediment composition needs the studyof the characteristics of the interstial water.

Conclusions

Our study indicates that phosphate dynamics inLa Massona involves four components: H2 0,

H3 PO4 , Ca 2+ and CO _ and two solid phases:a calcium-phosphate phase and a calcium-carbonate-phosphate phase. pH is the key factorcontrolling the system as a whole.

Ca and o-P chemically precipitate at surfaceand then partially redissolve at bottom. Verticaltransport of o-P is also due to sedimentation oforganisms and further decomposition of particu-late material. The accumulation of o-P in the hy-polimnion seems to be due to low precipitationrates rather than high rates of phosphate releasefrom sediments.

Acknowledgements

The helpful comments of Mr Th. Moutin and DrH. L. Golterman are kindly acknowledged. Wealso thank Mr R. Rycroft for correcting the Eng-

63

lish. This work was partially supported by a grantof 'Fundaci6 Caixa de Barcelona'.

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