Spatial distribution of sedimentary P pools in aMediterranean coastal lagoon ‘Albufera d’es Grau’

(Minorca Island, Spain)

P. Lopez �

Department of Ecology, University of Barcelona, Avda Diagonal 645, 08028 Barcelona, Spain

Received 19 July 2002; received in revised form 21 August 2003; accepted 18 September 2003

Abstract

The method for the sequential extraction of P proposed by Jensen et al. [Mcglathery, K.J., Marino, R., Howarth,R.W. (1998) Limnol. Oceanogr. 43, 799^810] was used to study the spatial distribution of sedimentary P in superficialsediments of a mesohaline coastal lagoon located in a watershed formed by carboniferous sandstone in the WesternMediterranean (Minorca Island, Spain). Dissolved inorganic phosphate (IP), dissolved organic phosphate (OP), Fe,Ca, Al, and F were analyzed in the extractions to assess adequacy of this method to clay sediments. The elementalcomposition of the solid phase (Al, Fe, K, Ti, Si, Mg and Ca) was also analyzed to relate concentrations of P pools tothe mineral composition. Samples from marine carbonated sediments, rock and some materials of biological origin(tubes of polychaete Ficopomatus enigmaticus, bivalve shell debris) were analyzed for comparative purposes. Thesequential procedure allowed to extract almost all sedimentary P from carbonate sediments and biogenic debris, butonly 70% of total phosphorus (TP) from clay sediments and rock. Main IP pools in the lagoon were Fe-bound P(16.6% of TP), CaCO3-bound P mainly from Ficopomatus tubes (12.0% of TP), and detrital carbonate fluorapatite(7.8% of TP). The most abundant P pool was refractory P (20.3% of TP), which appeared associated to the (Fe,K)Al-silicate fraction and to humic P. This indicated that clay^humic^organic P complexes were the main P reservoir in thelagoon. Spatial distribution of P pools reflected differential sedimentation of allochthonous materials, authigenicprecipitation of Fe-oxides, and Fe-bound P as well as the differential distribution of organisms such as Ficopomatus.@ 2003 Elsevier B.V. All rights reserved.

Keywords: phosphorus; sequential extraction; sediment composition; biogenic structures; coastal lagoons

1. Introduction

The cycle of phosphorus in coastal sedimentshas a dynamic nature that involves biological

and geochemical transformations of the incomingmaterial, with signi¢cant reorganization of P oc-curring during burial processes. The regenerationof dissolved P from organic matter (e.g. Billen etal., 1991; Howarth et al., 1995), the removal ofdissolved P by adsorption/coprecipitation ontoFe-oxides (e.g. Lucotte and D’Anglejan, 1988;Slomp et al., 1996), and the permanent burial ofP by diagenetic formation of carbonate £uorapa-

0025-3227 / 03 / $ ^ see front matter @ 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0025-3227(03)00333-5

* Corresponding author. Tel. : +34-93-4021515;Fax: +34-93-4111438.

E-mail address: mplopez@porthos.bio.ub.es (P. Lopez).

MARGO 3426 22-12-03

Marine Geology 203 (2004) 161^176

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tite (CFAP) (e.g. Ruttenberg and Berner, 1993;Filippelli, 1997; Louchouarn et al., 1997) are themain processes that determine the P cycle. Con-sequently, knowledge of the various solid P reser-voirs is necessary to understand P dynamics, inboth short- and long-term time scales.

The method commonly used to identify andquantify solid P phases is the sequential extractiontechnique. Although several procedures have beendeveloped (e.g. Williams et al., 1976; De Grootand Golterman, 1990; Penn et al., 1995), atpresent the most frequently used procedure incoastal and marine sediments is the procedureproposed by Ruttenberg (1990, 1992): the SE-DEX method. This method separates ¢ve mainP fractions: loosely adsorbed P, Fe-bound P, au-thigenic CFAP, detrital CFAP, and organic P.The SEDEX method helps to elucidate P geo-chemistry and has been used to study the varia-tion with depth of sedimentary P forms in marine(Filippelli and Delaney, 1996; Eijsink et al.,1997), coastal (Louchouarn et al., 1997; Vink etal., 1997), and inland environments (Baldwin,1996). However, the spatial variability of solid Pin super¢cial sediments of coastal areas has beenonly scarcely studied (Mccomb et al., 1998).

The nature of the phosphorus extracted at thedi¡erent SEDEX steps still su¡ers from some un-certainties. Signi¢cant errors in separating organicand inorganic solid phosphorus forms can arisefrom two sources. First, some organic phasesmay dissolve in the ¢rst steps (De Groot andGolterman, 1990), resulting in an underestimationof organic P and, second, some inorganic phasesmay not be dissolved in the inorganic steps result-ing in overestimation of the organic P (Vink et al.,1997). Moreover, organic P measured by the SE-DEX method does not discriminate betweenleachable and refractory organic P (Ruttenberg,1992). Such a distinction is important becauseleachable organic P may be a source of dissolvedphosphate to the water (Gachter and Meyer,1993; Jensen et al., 1995), whereas refractory or-ganic P is a sink of sedimentary P. Jensen et al.(1998) proposed a modi¢cation of the SEDEXmethod by including an 18-h NAOH extractionbetween the second step (CDB-extracted P) andthird step (acetate bu¡er-extracted P). NAOH is

assumed to extract IP associated with non-reduci-ble metal oxides, IP associated to humic acids,and leachable organic P (Jensen and Thamdrup,1993; Paludan and Jensen, 1995; Qiu andMcComb, 2000). The measurement of organic Pin the MgCl2 extraction and in the acetate bu¡erextraction gives, with the NaOH step, the largestpart of the leachable organic P.

Another source of uncertainty about the chem-ical signi¢cance of the P extracted at the di¡erentsteps of any sequential procedure comes from thematrix e¡ect. Some results suggest that Fe-boundP might be overestimated in carbonate-rich sedi-ments because of the dissolution of Ca-bound Pduring the CDB step (Pettersson and Istvanovics,1988; Jensen et al., 1998). Similarly, authigenicCFAP and refractory P may be overestimated inclay-rich sediments (Ruttenberg, 1992; Vink etal., 1997).

A useful approach to partially solve some ofthese uncertainties is to compare the amount ofphosphate extracted at each inorganic step withthe simultaneous extraction of the main elementsassociated with P, i.e. Ca, Fe, Al, and F3 (Lu-cotte and D’Anglejan, 1985; Jensen et al., 1998;Koch et al., 2001). Another approach to identifyP phases is the study of the correlation betweensedimentary phosphorus and mineral componentsor major elements of bulk sediment (Froelich etal., 1982; Moody et al., 1988).

Both approaches were used to study sedimenta-ry P in two Mediterranean environments: carbon-ate marine sediments from an oligotrophic bay,Andratx Bay (Majorca Island) and siliciclasticsediments from a mesohaline coastal lagoon, Al-bufera d’Es Grau (Minorca Island). P pools con-tained in tests of carbonate-building organismswere also studied since they have an importantpresence in sediments of coastal shallow environ-ments. In addition, a rock sample from Es Grauwatershed were also analyzed in the same waythat previous materials for comparative purposes.The aims of this study were:

(A) To assess the behavior of P during the dif-ferent steps of extraction proposed by Jensen etal. (1998) in sedimentary materials with di¡erentcontent of carbonates and clays and to evaluatethe interest and limitation of this technique.

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P. Lopez /Marine Geology 203 (2004) 161^176162

(B) To study the spatial variability of P formsin Es Grau lagoon and to contribute to theknowledge of the biogeochemical processes thatdetermined this variability in such as coastal sys-tems.

(C) To evaluate the contribution of carbonatedbiogenic debris to sedimentary P in a shallowcoastal environment.

2. Material and methods

2.1. Study area

Albufera d’Es Grau is a small lagoon locatedon the northeastern coast of the island of Minor-ca (Fig. 1). The watershed is mainly formed bycarboniferous sandstone. Little information existsabout the mineralogical composition, preliminaryanalysis of rock samples showing majority pres-ence of muscovite.

The lagoon has a maximum length of 1700 m, amaximum width of 880 m, and a maximum depthof 3 m, with a mean depth of 1.37 m and a vol-ume of 106 m3. The main input of freshwatercomes from Torrent d’Es Prat (point TEP inFig. 1A) with a relatively wide and £at catchmentarea. A secondary input is through Torrent de NaBona (point NB, Fig. 1A), £owing from a smallerand sharper drainage basin. The lagoon is con-nected to the sea by a winding narrow channelabout 300 m long, that has a £oodgate regulatingthe £ux between seawater and lagoon water. Thesystem prevents seawater from £owing into thelagoon, minimizing the role of seawater exchangein the turnover of water mass and avoidingsummer strati¢cation. Throughout the year, watersalinity is on the average 11.75 ppt (range 8.75^14.49 ppt). The lagoon has low anthropogenicinputs, the concentration of dissolved phosphatebeing usually low (mean annual value: 0.131 WM,range: 6 0.010^0.192 WM). Previous studies indi-

Fig. 1. Location of sampling sites in the studied areas. (A) Es Grau lagoon. Dotted lines indicate isolines for 1 and 2 m depth.(B) Andratx Bay. The dashed area indicates zone where samples were collected.

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P. Lopez /Marine Geology 203 (2004) 161^176 163

cated that the cycle of nutrients was strongly as-sociated with the annual cycle of phytoplanktonand macrophytic populations (Pretus, 1989).

Sediments are ¢ne-grained, with a porosity of60.4^77.7% (v/v), carbonate content range from0.5 to 5.9% dry weight, and C/N atomic ratiofrom 9 to 18 (Lo¤pez et al., 1996). The lagoon isinhabited by a dense population of the serpulidpolychaete Ficopomatus enigmaticus (Fauvel).This species is most abundant in shallow areas(1 m), but present up to a depth of 3 m. Macro-phytic vegetation was present in the littoral area(6 1.5 m), with Potamogeton pectinatus and Rup-pia cirrosa as the main species.

To study the spatial variability of sedimentcomposition in this lagoon 17 sampling pointswere selected on basis of a regular grid (stations1^17, Fig. 1). An additional sampling point (sta-tion 18, Fig. 1) was located on the channel con-necting lagoon with the sea. Samples of Ficopo-matus tubes, bivalve shells and rock fragmentwere collected at sites 16, 17 and point TEP re-spectively.

Andratx Bay is located at the southeasterncoast of Majorca Island (Fig. 1B). The system ishighly oligotrophic (Moya, personal communica-tion). Eight super¢cial sediment samples were col-lected in an area with no continental in£uence.Carbonate content of the samples ranged from

70 to 92% dw; calcite, aragonite and dolomitebeing the major minerals.

2.2. Materials and methods

Super¢cial sediments (2 cm depth) were col-lected with a modi¢ed Eckman grab in Es Graulagoon and Andratx Bay. Samples were kept at4‡C until analysis, within 48 h of collection. In thelaboratory, 2 g of wet sediment were subsampled,sieved through 1 mm mesh in a N2-¢lled glovebag and used for P pools replicate analysis. Theremaining sediment was used to for X-ray £uores-cence analysis (XRF).

Ficopomatus tubes, bivalve shell debris and rockfragment were rinsed with ¢ltered water to elimi-nate ¢ne material, dried to 70‡C and groundedwith a mortar. The powder obtained was thenanalyzed in triplicate as the sediment samples.

P pools were analyzed with the sequential pro-cedure proposed by Jensen et al. (1998), modify-ing the original scheme by including the treatmentof the NaOH extract (step III) indicated by Pa-ludan and Jensen (1995). The methodological def-initions of the 10 P pools obtained with the com-plete procedure are given in Table 1. Inorganicand organic phosphate in the leachates were mea-sured by standard colorimetric methods (Korole¡,1983) with a detection limit of 0.06 WM of phos-

Table 1Operational description of the 10 P pools obtained with the sequential procedure applied to the studied samples.

Step P pool Operational description Expected P-forms

I. MgCl2-IP IP extracted with 1 M MgCl2 Loosely adsorbed IPMgCl2-OP Organic P extracted with 1 M MgCl2 Loosely adsorbed organic P

Leachable organic PII BD-IP IP extracted with 0.11 M BD (bicarbonate/dithionite reagent) P-bound to reducible ironIII NaOH-IP IP into solution after extraction with 0.1 M NaOH and further

acidi¢cation of the extract to pH 1P-bound to Al oxides

P-bound to claysNaOH-OP Organic P into solution after acidi¢cation of the 0.1 M NaOH

extract.Leachable organic P

HA-P P particulate after acidi¢cation of the 0.1 M NaOH extract P-bound to humic acidsIV Acetate-IP IP extracted with acetate bu¡er Authigenic carbonate £uorapatite

IP bound to CaCO3

HydroxyapatiteAcetate-OP Organic P extracted with acetate bu¡er Leachable organic P bound to CaCO3

V HCl-IP IP extracted with 0.5 M HCl Detrital carbonate £uorapatiteVI Refractory-P P dissolved in hot acid after combustion of the residual pellet

at 450‡CRefractory organic P

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P. Lopez /Marine Geology 203 (2004) 161^176164

phate. Fe, Al, and Ca in the leachates were ana-lyzed by inductively coupled plasma (OES) (Ther-mo Jarrell Ash) and also using standard proce-dures (detection limit : 0.010 WM for Fe and Aland 0.025 WM for Ca). Fluoride was analyzed inthe acetate and HCl leachates with a Orion com-bination F3 electrode with 1:1 addition of low-level TISAB (Orion standard procedures). Thedetection limit of the electrode was 2.0 WM.

Major elements in the solid phase (Al, Fe, K,Si, P, Ca, Mg and Ti) were analyzed by XRFanalysis (Phillips PW2400).

2.3. Data treatment

The relationships among major elements andP pools were drawn from ratios between pairsof variables as indicated by Rollinson (1993), be-cause the use of correlation may give misleadingresults due to the constant sum e¡ect (Aitchinson,1986). The variation matrix (i.e. the variances ofthe log-ratios of each pair of variables) allowsidenti¢cation of elements that coexist in thesame mineral phase, since coexisting elementspresent very low values in this matrix. Ratios be-tween variables also re£ect the di¡erent geochem-ical processes that simultaneously act on sedimentcomposition, such as the nature of clays, the de-gree of erosion in the watershed, sedimentationacross the lagoon, the percentage of biogenic cal-careous debris, and other factors.

The spatial distribution of major elements andP pools in Es Grau lagoon were graphically rep-resented by using the software package Surfer-(Win32)V6.01 zGolden Software Inc. Isolineswere calculated by using the Kriging method.

3. Results

3.1. P pools

The sum of the di¡erent P pools agreed wellwith the parallel total phosphorus (TP) measure-ments by XRF, with a signi¢cant linear regressionbetween both methods (4P pools = 0.73*TP+1.78;R=0.88, P6 0.0001) (Fig. 2). The sequential pro-cedure extracted 70% of TP and 50% of solid Cain Es Grau sediments and rock, and 100% of TPand solid Ca in marine sediments and biogenicsamples (Table 2). The sequential procedure alsodissolved most of the solid Fe in biogenic debris,

Fig. 2. Relationship between 4P pools extracted with sequen-tial procedure and total sedimentary P (TP). CMS: Carbon-ate marine sediments; Es Grau: sediments of Es Grau la-goon; EGR: Es Grau rock; BS: bivalve shells debris; FT:Ficopomatus tubes.

Table 2Mean and standard deviation of the sum of P, Ca, and Fe recovered at the di¡erent steps and percentage of the total concentra-tion in solid phase

n P Ca Fe

Mean SD % Mean SD % Mean SD %

Es Grau sediments 18 23.39 6.27 71 1.64 0.99 49 0.148 0.046 30Marine sediments 8 8.98 1.32 95 8.25 1.11 108 0.032 0.011 43Es Grau rock 3 17.17 0.70 72 0.48 0.05 51 0.342 0.009 56Bivalve shells 3 23.06 2.30 109 11.25 0.70 110 0.025 0.001 102Ficopomatus tubes 3 45.16 1.05 107 8.72 1.56 100 0.030 0.004 105

P in Wmol g31. Ca and Fe in mmol g31.

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P. Lopez /Marine Geology 203 (2004) 161^176 165

but less than 50% of total Fe in marine sedimentsand Es Grau (Table 2).

Mean concentrations of P pools in the studiedsamples are given in Table 3. The sum of IP frac-tions accounted for 42% of TP in Es Grau sedi-ments, 48% of TP in marine sediments, 58% of TPin rock, and s 90% of TP in biogenic debris. BD-IP, acetate-IP, and HCl-IP were the major IPfractions, while MgCl2-IP, NaOH-IP and HA-Prepresented less than 10% of TP. The sum of or-ganic P pools (TOP) accounted for 42% of TP inmarine sediments, 26% of TP in Es Grau, 16% ofTP in biogenic debris and 6% of TP in rock sam-ples. Refractory-P was the most abundant organicP pool in Es Grau and rock (70% of TOP), butonly accounted for 14% of TOP in marine sedi-ments and it was not detected in biogenic debris.Acetate-OP was the main organic P pool in ma-rine sediments (60% of TOP), but was not de-tected in the other samples.

Table 3Mean concentration of phosphate, calcium, and iron extracted at the di¡erent steps of sequential procedure

Es Grau sediments Carbonate marinesediments

Es Grau rock Bivalve shells Ficopomatus tubes

Mean % Mean % Mean % Mean % Mean %

MgCl2-IP 0.11 0.3 0.61 5.6 0.11 0.4 0.31 1.5 2.22 5.3BD-IP 5.71 16.6 0.55 5.5 0.20 0.8 1.35 6.3 3.09 7.2NaOH-IP 1.03 2.8 0.47 4.9 0.20 0.8 0.09 0.4 0.30 0.7Acetate-IP 3.93 12.0 1.98 19.8 9.46 38.7 17.655 83.6 32.62 77.3HCl-IP 2.55 7.8 1.09 11.1 4.18 17.0 0.06 0.4 0.08 0.2HA-P 0.80 2.3 0.15 1.5 0.04 0.1 0.05 0.2 0.05 0.1MgCl2-OP 0.16 0.5 0.55 5.3 1.92 7.8 2.22 10.5 2.61 6.2NaOH-OP 2.13 5.7 0.49 5.4 n.d. 1.34 6.3 4.26 10.1Acetate-OP n.d. 2.52 25.5 n.d. n.d. n.d.Refractory-P 7.04 20.3 0.57 5.8 1.58 6.4 n.d. n.d.MgCl2-Fe n.d. 0.02 0.0 0.96 0.2 0.97 3.9 0.88 3.1BD-Fe 46.45 9.6 16.57 24.4 328.14 53.6 22.67 90.7 25.62 89.1NaOH-Fe 1.91 0.4 0.08 0.1 0.54 0.1 0.10 0.4 0.24 0.8Acetate-Fe 69.94 14.3 7.94 12.2 8.88 1.4 1.49 5.9 1.67 5.7HCl-Fe 27.74 5.6 4.11 5.6 3.96 0.6 0.15 0.6 1.87 6.5HA-Fe 1.309 0.3 0.25 0.3 n.d. n.d. n.d.MgCl2-Ca 0.27 9.2 0.43 5.7 n.d. 0.55 5.4 0.35 4.0BD-Ca 0.19 6.3 0.22 2.9 0.08 21.7 1.39 13.6 0.878 10.0NaOH-Ca 0.01 0.1 0.04 0.6 0.04 9.1 0.06 0.6 0.07 0.8Acetate-Ca 1.06 29.2 6.82 89.0 0.03 6.8 9.23 90.2 7.42 85.2HCl-Ca 0.10 3.3 0.75 10.2 0.05 12.7 0.02 0.2 0.02 0.2HA-Ca 0.02 0.7 n.d. n.d. n.d. n.d.

P and Fe in Wmol g31, Ca in mmol g31. Column % gives the average of the concentration expressed as % of total amount in sol-id phase. n.d.: values below the detection limit.

Fig. 3. Relationship between dissolved Fe and IP extractedwith bicarbonate/dithionite (Fe-bound P). Symbols as inFig. 2.

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P. Lopez /Marine Geology 203 (2004) 161^176166

3.2. Recovering of inorganic P, Fe, Ca, Al and F3

Concentrations of Fe and Ca recovered at thedi¡erent steps of the sequential procedure are alsogiven in Table 3.

Small amounts of Ca and only traces of Fewere leached in MgCl2, but no correlation withIP recovered at this step was observed.

BD reagent extracted signi¢cant amounts of Fe,

with only little concurrent dissolution of Ca. InFicopomatus tubes and bivalve shells, this step ex-tracts the majority of the total Fe. The regressionequation between BD-Fe and BD-IP was BD-IP= 0.11*BD-Fe-0.23; R=0.84, P6 0.0001 (Fig.3), the ratio BD-Fe/BD-IP being about 8 in EsGrau sediments and Ficopomatus tubes, 17 in bi-valve shells and 30 in marine sediments.

Signi¢cant amounts of Fe and Al were recov-ered as dissolved and precipitated forms afteracidi¢cation of the NaOH extract in Es Grau (Ta-ble 3; Fig. 4). NaOH-IP and HA-P were closelyrelated to NaOH-Al and HA-Al respectively(NaOH-IP= 0.10*NaOH-Al+0.1792; R=0.61,P6 0.005; HA-P= 0.06*HA-Al-0.09; R=0.81,P6 0.0001). Both P pools were also relatedto Fe, but the signi¢cance of regression equa-tions were lower than for Al (NaOH-IP=0.38*NaOH-Fe+0.31; R=0.56, P6 0.012; HA-P= 0.48*HA-Fe+0.17; R=0.57, P6 0.011). Themolar ratios Al/P and Fe/P were 8 and 1 respec-tively in the NaOH extract and 12 and 2 respec-tively in the HA extract.

Acetate bu¡er extracted the highest percentageof Ca and also substantial amounts of Fe from allsamples (Table 3). Since acetate bu¡er dissolvesauthigenic CFAP, F3 was also measured in thisextract in Es Grau samples. Concentrations of F3

ranged from 0.01 to 0.08 Wmol g31 (n=17). How-

Fig. 4. Relationship between Al and P obtained after acidi¢-cation to pH 1 of the NaOH extract A): in the particulatematter (HA-Al, HA-P), B): in the ¢ltrate (NaOH-Al, NaOH-IP).

Fig. 5. Relationship between dissolved £uoride and IP ex-tracted with HCl (detrital CFAP).

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P. Lopez /Marine Geology 203 (2004) 161^176 167

ever, no signi¢cant relationships between inor-ganic P, Ca, F3 and Fe in the acetate extractwere observed.

Concentrations of F3 were also measured inthe HCl extract, the regression equation betweenHCl-IP and HCl-F3 being: HCl-IP= 3.6*HCl-F3+1.8; R=0.68, P6 0.003 (Fig. 5). Mean valueof the HCl-IP/HCl-F3 agreed with that of CFAP.Ca and Fe were also recovered in the HCl extract(Table 3), but HCl-Ca and HCl-Fe were not re-lated to HCl-IP. Ratios HCl-Ca/HCl-IP (487 inmarine sediments, 39 in Es Grau) indicated thatCa from phases other that CFAP dissolved at thisstep and that, such as phases were especially im-portant in marine sediments.

3.3. Elemental composition and relationships withP pools

The elemental composition of the studied ma-terials is presented in Table 4. Sediments of EsGrau were characterized by the abundance ofAl, Fe, K and Si when compared with marineones. On the contrary, the Ca and Mg contentwere lower in Es Grau lagoon. Ficopomatus tubesand bivalve shells were mainly formed by Ca com-pounds, Ficopomatus tubes presenting higher con-centrations of P, Fe, and Mn than bivalve shells.

The variation matrix calculated for the wholeset of data allowed to identify Fe, Al, and K aselements coexisting in the same mineral phase,since variances of the log-ratios between themwere very low (Table 5a). The ratios K/Al andFe/K (0.3 and 0.9 respectively) were in agreementwith those reported for (Fe,K)Al-silicates (Bel-mans et al., 1993). This indicated that ¢ne alumi-nosilicates were the main Fe, Al and K mineralsin the studied sediments.

Si showed the lowest variance of log-ratio withTi and higher variances with Al, Fe, K (Table 5a),which indicated the existence of a Si-phase otherthan aluminosilicates. Since Ti is usually associ-ated to detrital materials, this second Si-phasecould be identi¢ed as coarse quartz grains fromdetrital origin.

Ca and Mg presented the lowest variance be-tween them and the highest variances with Al, Feand K (Table 5a). In marine sediments, the ratioT

able

4M

ean,

stan

dard

deviation

and

rang

eof

conc

entrations

ofmajor

elem

ents

inth

ematerials

stud

ied

EsGra

usedimen

tsCar

bona

temar

inesedimen

tsEsGra

uro

ckBivalve

shells

Ficopom

atus

tube

s

Mea

nSD

Ran

geM

ean

SDRan

geM

ean

SDRan

geM

ean

SDRan

geM

ean

SDRan

ge

Al

2.24

0.49

1.35

^2.99

0.24

0.14

0.07

^0.47

2.53

0.05

2.57

^2.59

0.01

0.00

0.01

0.07

0.24

0.05

^0.09

P35

.55

7.81

23.23^

51.39

10.21

1.99

8.45

^14.78

24.64

1.00

23.94^

25.46

21.12

1.15

20.35^

24.16

42.24

1.99

40.85^

43.96

K0.55

0.12

0.37

^0.75

0.08

0.05

0.02

^0.16

0.63

0.02

0.62

^0.64

0.00

0.00

0.00

0.03

0.01

0.02

^0.03

Ca

3.30

1.14

1.38

^5.52

7.62

0.96

6.20

^8.90

0.25

0.16

0.14

^0.36

10.23

0.03

10.20^

10.25

8.72

0.08

8.65

^8.79

Si4.92

1.21

2.64

^8.10

1.83

1.22

0.34

^3.44

11.89

0.19

11.58^

12.05

0.06

0.00

0.06

0.19

0.06

0.15

^0.24

Ti

56.88

11.96

33.13^

81.25

16.17

13.11

1.25

^35.63

86.25

1.77

85.12^

88.06

0.85

0.06

0.15

^0.95

1.25

1.77

1.02

^2.03

Mn

8.24

2.93

4.60

^14.95

6.90

0.00

6.90

1.15

0.00

1.15

27.60

0.00

27.60

Fe

517.08

117.2

311.9^

716.8

71.80

32.90

4056

^134

.38

613.15

11.88

604^

621

17.50

3.39

15.02^

20.25

28.75

3.38

26.36^

31.09

Mg

0.56

0.14

0.32

^0.92

1.15

0.15

0.82

^1.30

0.22

0.01

0.22

^0.23

0.07

0.01

0.06

^0.08

1.20

0.01

1.20

^1.21

Al,

K,Ca,

Si,an

dM

gin

mmol

g31.P,Ti,

Mn,

and

Fein

Wmol

g31.

MARGO 3426 22-12-03

P. Lopez /Marine Geology 203 (2004) 161^176168

Table 5Variation matrix, where Al-K, etc. means the variance of natural logarithm of the ratio Al/K, etc.

K Fe Ti Si Mg Ca

aAl 0.03 0.06 0.21 0.36 2.29 2.77K 0.04 0.14 0.25 1.88 1.12Fe 0.25 0.27 1.76 2.18Ti 0.13 1.77 2.26Si 1.11 1.46Mg 0.16

MgCl2 MgCl2 BD NaOH NaOH Acetate HCl Ref-P HA AcetateIP OP IP IP OP IP IP P OP

bAl 4,59 3,05 0,31 0,96 0,96 1,38 0,56 0,23 0,29 0,81K 4,03 2,53 0,36 0,73 1,05 1,21 0,42 0,29 0,20 0,86Fe 3,90 2,42 0,24 0,68 1,02 1,02 0,39 0,20 0,16 0,46Ti 3,75 2,39 0,76 0,94 1,21 1,38 0,38 0,70 0,41 2,17Si 2,78 1,73 0,72 0,71 1,49 0,84 0,10 0,74 0,33 1,14Mg 0,81 0,31 2,29 0,90 3,41 0,80 0,82 2,34 1,36 0,26Ca 0,99 0,49 2,58 1,39 3,88 0,87 1,05 2,71 1,80 0,22

a: Variation matrix for major elements. b: Variation matrix for ratios between major elements and P pools.

Fig. 6. Spatial distribution of major P pools in Es Grau lagoon. Concentrations in Wmol g31.

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Ca/Mg varied from 6 to 10, a value characteristicof magnesian calcite deposits. In Es Grau sedi-ments, Ca/Mg presented intermediate values be-tween those observed in Ficopomatus debris (Ca/Mg=7) and (Fe,K)-Al-silicates (Ca/Mg= 1). Thissuggested that these were the two main phases ofCa and Mg in the lagoon.

To establish the main relationships betweenP pools and the mineral composition of sedi-ments, the variation matrix was also calculatedfor ratios between P fractions and concentrationsof major elements (Table 5b). Acetate-IP, MgCl2-P (both inorganic and organic), NaOH-IP, andNaOH-OP presented high variances of the log-ratios with all the elements, so no association ofthese forms with a speci¢c mineral phase could beobserved.

BD-IP was associated with Fe (Table 5b), theratio Fe/BD-IP ranging from 45 to 140 with thehighest values in marine sediments. The highervariances observed in the BD-IP/K ratio indicatedthat BD-IP was not associated with the alumino-silicate fraction but with other Fe materials suchas metal oxides.

HCl-IP was strongly associated with Si. Sincedetrital CFAP ^ the target phase of the HCl ex-traction ^ naturally occurs as well-rounded grains(Ruttenberg and Berner, 1993), association be-tween HCl-IP and Si was probably due to thesimilar particle size of CFAP grains and quartzsands.

Refractory-P and HA-P showed low variancesof the log-ratios with elements related to (Fe,K)Al-silicates, indicating the association of theseP fractions with clay materials. The ratio K/re-fractory-P was higher in marine sediments(W150). It showed a notable decrease from rocksamples (W400) to Es Grau sediments (mean val-ues 78; range: 45^120).

3.4. Spatial variability of P pools in Es Grausediments

The widest range of spatial variability was ob-served for the most abundant P fractions: refrac-tory-P, acetate-IP, BD-IP and HCl-IP. Each ofthese fractions presented a characteristic patternof distribution.

Fig.7.

Spatialdistribu

tion

ofSi

and

ratios

Fe/BD-IP

and

K/refra

ctor

y-P

inEsGra

usedimen

ts.Con

cent

ration

sof

Siin

mmol

g31.Ratiosin

mol/m

ol.

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P. Lopez /Marine Geology 203 (2004) 161^176170

The highest concentrations of BD-IP were ob-served near the Torrent d’Es Prat input (site 1)and near the £oodgate (site 17). (Fig. 6a). Max-imum levels of BD-IP concurred with the lowest

values of the Fe/BD-IP (Fig. 7a). Acetate-IP pre-sented a complex pattern characterized by themaximum levels at site 3 and near the easternside (sites 16, 17) and the minimum levels close

Fig.8.

Spatialdistribu

tion

ofminor

Ppo

olsin

EsGra

ulago

on.Con

cent

ration

sin

Wmol

g31.

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P. Lopez /Marine Geology 203 (2004) 161^176 171

to point TEP (Fig. 6b). Spatial distribution ofHCl-IP (Fig. 6c) was close to that of Si (Fig.7b) and was characterized by high concentrationsat shallower areas and in the southern basin (sites14, 15) that receives materials from Torrent de NaBona (NB). Refractory-P was mainly accumu-lated at site 7 which was the western site with adepth over 2 m (Fig. 6d). Also near the £oodgate(site 17), refractory-P reached high levels. Maxi-mum concentrations concurred with the lowestratios K/refractory-P (Fig. 7c).

Some minor P pools (Fig. 8) presented a spatialdistribution close to those previously described:spatial variability of HA-P and NaOH-IP wassimilar to that of refractory-P while spatial vari-ability of MgCl2-OP was close to that of BD-IP.On the contrary, NaOH-OP and MgCl2-IP pre-sented a pattern of distribution quite di¡erent toany other element or P-pool. The ¢rst one wascharacterized by the highest concentrations atsites with high density of Ficopomatus (sites 14,12, 1) and also at site 7. The second was homoge-neous across the lagoon with concentrationsslightly higher at the southern sides and near the£oodgate.

4. Discussion

4.1. Identi¢cation of IP pools

MgCl2-IP is de¢ned as P sorbed onto surfacessuch as clays and CaCO3 (Ruttenberg, 1992), butthis pool does not include P sorbed onto ferricoxides which is recovered with BD reagent (Slompet al., 1996). Since Fe-oxides have higher e⁄-ciency absorbing phosphate than clays or CaCO3

(e.g. Brinkman, 1993; Sundareshwar and Morris,1999), concentrations of MgCl2-IP in the iron-richsediments of Es Grau were expected to be lowerthan those of carbonate marine sediments inagreement with observed results. These also sug-gested that CaCO3 rather than clays was the mainsource of MgCl2-Ca and probably MgCl2-IP in EsGrau and marine sediments, since MgCl2-Ca rep-resented similar percentage of total Ca in bothkinds of samples in spite of their di¡erent claycontent.

BD-IP was related to P bound to Fe as re-ported elsewhere (e.g. Lucotte and D’Anglejan,1985; Ruttenberg, 1992). Dissolution of P boundto Ca with BD reagent has also been reported(Pettersson and Istvanovics, 1988; Jensen et al.,1998), but our results showed that BD-IP was al-ways associated to Fe, but not to Ca. BD-IP andBD-Fe from Ficopomatus tubes and bivalve shellscould be related to the presence in these materialsof oxyhydroxide iron coatings (Sherwood et al.,1987; Ruttenberg, 1992), the higher amounts ofBD-IP and BD-Fe observed in Ficopomatus tubesbeing probably a consequence of its higher specif-ic surface with respect to bivalve shells. Forma-tion of iron oxyhydroxide surfaces has also beenrelated to bacterial activity in freshwater sedi-ments (Hupfer et al., 1995). This could explainthe close spatial pattern of BD-IP and MgCl2-OP observed in Es Grau lagoon.

IP extracted with NaOH was mainly related toAl. Paludan and Jensen (1995) proposed that IPrecovered in the precipitate obtained after acid-i¢cation to pH 1 of NaOH extract was mainlyhumic-P. The ratios Al/P and Fe/P obtained inEs Grau were in the range of those reported forpeat sediments (White and Thomas, 1981; Palu-dan and Jensen, 1995), and agreed with this iden-ti¢cation. IP that remains in solution after acid-i¢cation (NaOH-IP) has been proposed to beP bound to clays or bound to non-reducible metaloxides, such as Al2O3 (Jensen et al., 1998).P bound to Al-oxides also form complexes withfulvic acids, which do not precipitate at pH 1(Malcom, 1990; Ulrich, 1997), fulvic acids beingenriched in P relative to humic acids (Nissen-baum, 1979). Lower Al/IP ratios remaining intosolution after acidi¢cation relative to the ratio inthe precipitate agreed thus with identi¢cation ofNaOH-IP with Al-fulvic-P compounds.

The wide range of variation of the ratio Ca/IPin the acetate bu¡er extract indicated the presenceof di¡erent carbonate species in our samples. Ace-tate-IP has been assumed to be authigenic CFAPand it is considered the reactive sink of phosphatein marine sediments (e.g. Ruttenberg and Berner,1993; Louchouarn et al., 1997). However, sub-stantial amounts of acetate-IP in Es Grau sedi-ments were probably biogenic CaCO3-bound P,

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since most P from biogenic debris was recoveredin this form. Hydroxyapatite also dissolves in ace-tate bu¡er (Vink et al., 1997). This mineral couldcontribute to acetate-IP fraction in Es Grau la-goon, mainly at the deepest sites were the ace-tate-IP/Ca ratio attained values much higherthan those observed in biogenic debris. Whetherhydroxyapatite was authigenically formed in thelagoon or it was from allochthonous origin re-mained unclear.

The ratio IP/F3 in the HCl extract and thestrong association of HCl-IP and Si agreed withidenti¢cation of HCl-IP as detrital CFAP in bothbrackish sediments of Es Grau and marine sedi-ments as previously reported for calcareous ma-rine sediments (Ruttenberg, 1992).

4.2. Geochemical signi¢cance of sedimentaryorganic P

Identi¢cation of the organic P fractions can notbe performed from the study of relationships withother elements. However some considerationsabout their geochemical signi¢cance may bedone from the data obtained. Distinction betweenrefractory and leachable forms is a critical aspect,since refractory forms are sinks of P and leachableforms may return to the water as dissolved P.Previous studies indicate that in many systemsthe sum of OP in MgCl2 and NaOH extracts ac-counts for most of the leachable OP (Jensen andThamdrup, 1993; Thomsen, 1993; Baldwin, 1996;Ulrich, 1997). In carbonate marine sediments OPis also recovered in acetate bu¡er (Entsch et al.,1993; Jensen et al., 1998), but signi¢cance of thisform in brackish sediments has not been estab-lished. Our results showed that acetate-OP waspresent only in carbonate marine samples, the ma-jority of the OP present in the other samples beingrecovered in MgCl2 and NaOH extracts.

The concentrations of refractory-P reportedhere were in the range previously observed in es-tuarine and continental systems (Jensen et al.,1992; Vink et al., 1997; Mccomb et al., 1998).The association of refractory-P with elements re-lated to (Fe, K)Al-silicates indicated that refrac-tory-P was associated with clay minerals and ex-plained the highest levels observed in Es Grau

sediments. However, whether refractory-P corre-sponds to organic or IP remains unclear. On onehand, the substantial amount observed in rocksuggested that it may be mineral P occluded inthe siliceous matrix. This should involve an over-estimation of organic-P in Es Grau sediments andshould indicate an allochthonous origin of refrac-tory-P. Accumulation of refractory P at site 7 wasin agreement with this consideration, since it wasexpected to be the main point of sedimentation ofthe ¢ne clays transported to the lagoon from Tor-rent d’Es Prat. On the other hand, decrease of theratio K/refractory-P from rock to lagoon sedi-ments agreed with autochthonous formation oforganic P-clay complexes in the lagoon. The litto-ral area of Es Grau lagoon (above 2 m depth) wasdensely covered by macrophyte species. This sug-gested that at sites with high percentage of alumi-nosilicates organic-P from macrophyte originformed complexes with clays becoming a refrac-tory form of P, while at sites with lower concen-trations of clays, organic-P could remain as leach-able forms.

Finally, a signi¢cant amount of sedimentary Pwas not recovered with the sequential procedurein Es Grau sediments. Underestimation of TPmay result from physical loss of sample duringthe ¢rst steps ^ due to manipulation ^ but alsoto the existence of highly refractory P phaseswhich do not dissolve after 450‡C ashing and at-tack with strong acid (Ruttenberg, 1992). SinceTP was totally recovered form marine sedimentsand biogenic debris, non-extracted P in Es Grausediments and rock seems to re£ect the presencein these materials of P occluded in the siliceousmatrix, as previously proposed in other systems(Baldwin, 1996; McComb et al., 1998).

4.3. Factors determining spatial distribution ofP pools in Es Grau lagoon

Sedimentation of particles depending on par-ticle-size, chemical reactions occurring in thewater body and spatial distribution of biomassof carbonate building organisms appeared to bethe main biogeochemical factors that determinespatial variability of sedimentary P in Es Graulagoon. Data reported here also suggest some hy-

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pothesis on mechanisms involved in the sedimen-tary P cycle that need further investigation.

The spatial distribution of detrital CFAP and Passociated with clays was in agreement with theexpected pattern of sedimentation depending onparticle-size. Detrital CFAP that naturally occursas coarse material accumulated at shallower areas.P associated with the ¢nest materials such as claysand organic complexes mainly accumulated withdepth and also near the £oodgate. Di¡erences inthe characteristics of the two main drainage ba-sins also determined the distribution of theseP forms. The western side mostly receiving mate-rials from the £at catchement area of Torrentd’Es Prat presented higher levels of refractory-P,while the southern basin that received materialsfrom a sharper catchement area presented thehighest levels of detrital CFAP. Interpretation ofthe association of refractory-P, aluminosilicatesand P bound to fulvic and humic acids must bedone with caution. These forms could appearedrelated as a result of chemical complexationamong them (De Haan et al., 1990; Jonsson,1997). But association among them could be justas a consequence of the similar particle size oforganic complexes and highly weathered clays.

The abundance and distribution of BD-IPdepended basically on authigenic chemical reac-tions. At the western side, dissolved Fe trans-ported by Torrent d’Es Prat (Pretus, personalcommunication) is expected to precipitate as Fe-oxides when reached the brackish water mass oflagoon. Since freshly precipitated oxides havehigh capacity to adsorb P (Lijklema, 1980), BD-IP accumulated at this area with a low BD-Fe/BD-IP ratio. Accumulation at the eastern sidewas also consistent with previous studies report-ing that sediments of this area also present a highcapacity to adsorb P (Lo¤pez et al., 1996). How-ever, causes of the enhanced adsorption capacityat this zone remained unclear. The presence ofabundant Ficopomatus tubes with signi¢cantamounts of BD-IP suggested the hypothesis thatenhanced adsorption could be related with abun-dance of this material. Because of its tubular ge-ometry Ficopomatus tubes should increase theoxygen transport into the sediments as well asact as a nucleation surface for Fe-oxides deposits.

Further investigation on this hypothesis is thusdesirable.

The other major form of sedimentary P, ace-tate-IP, in Es Grau lagoon appeared to be mainlyrelated with distribution of the biomass of car-bonate building organisms. Consequently, estima-tion of the rates of diagenetic formation of CFAPfrom acetate-IP concentrations were not possiblein Es Grau lagoon. The possible presence of hy-droxyapatite suggested by the variation of theacetate-IP/acetate-Ca across the lagoon di⁄cultthe interpretation of the abundance and distribu-tion of acetate-IP. From a geochemical approach,distinction between autochthonous and allochto-nous contribution to hydroxyapatite contentshould be desirable. Improvement of separationof the di¡erent Ca-bound phosphates and studyof the conditions in porewater to assess chemicalprecipitation of Ca-P compounds may help to elu-cidate these uncertainties.

5. Conclusions

The modi¢cation of the SEDEX method pro-posed by Jensen et al. (1998) provides a goodapproach to the di¡erent organic and inorganicP pools in carbonate marine sediments fromWestern Mediterranean and also in biogenic car-bonate debris. In Es Grau lagoon the sequentialprocedure gives a good approach to the amountof detrital apatite from allochthonous origin, thatconsequently can be used as an estimate of water-shed erosion and particle transport. However, thesequential method underestimates sedimentary Pin the clay-rich sediments of Es Grau lagoon.

Since P associated to clay materials, humic andfulvic acids was the main fraction of sedimentaryP Es Grau lagoon the sediments of this systemsappeared as an important sink of P from bothallochthonous and autochthonous origin. Im-provement of characterization of refractory-P,P bound to fulvic and humic acids, and alsoknowledge of paths and reactivity of interactionswith clays are critical to an understanding theP-cycle not only in Es Grau sediments but alsoin other similar coastal systems.

P bound to reducible iron was the main form of

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autochthonous P in Es Grau lagoon. The signi¢-cant concentrations observed in super¢cial sedi-ments represented a P reservoir which could bereleased to water under anoxic conditions. In or-der to prevent water eutrophication, adequatemanagement of the lagoon should take this intoaccount by minimizing summer strati¢cation,mainly at the eastern and western sides of thelagoon.

P contained in biogenic CaCO3 signi¢cantlycontributed to sedimentary P in Es Grau lagoon,not only as CaCO3-bound P but also as P boundto reducible iron and leachable OP. Consequently,abundance and spatial distribution of carbonatebuilding organisms are important factors to beconsidered in the study of sedimentary P dynam-ics in shallow systems.

Acknowledgements

The author wishes to thank J.L. Pretus and J.Gonzalez for sample collection, X. Lluch for helpin laboratory analysis, H.S. Jensen for helpfulcomments, and R. Rycroft for improving theEnglish manuscript. Dr. A. McComb and ananonymous reviewer are also acknowledged fortheir valuable comments on the manuscript. Thisstudy was supported by a CYCIT grant (PB97-0953).

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