Hydrobiologia 302 : 147-161, 1995 .© 1995 Kluwer Academic Publishers. Printed in Belgium.

Forms and distribution of inorganic phosphorus in sediments of two shalloweutrophic lakes in Florida

O. G. Olila*, K . R. Reddy & W. G. HarrisUniversity of Florida, Institute of Food and Agricultural Sciences, Soil and Water Science Department,Gainesville, FL 32611, USA (*author for correspondence)

Received 10 March 1993 ; in revised form 1 December 1993 ; accepted 2 June 1994

Key words : phosphorus, chemical fractionation, synthetic P compounds, minerals, sediments, eutrophic lakes

Abstract

Phosphorus (P) reactivity and bioavailability in lake sediments may be determined by different forms of P and theirdistribution. Reactive and nonreactive P pools in two shallow subtropical lake sediments (Lake Apopka and LakeOkeechobee) were determined by sequential chemical extraction using 1 M NH 4CI (pH 7.0), 0.1 M NaOH, and0.5 M HCI, reportedly representing loosely-bound P, Fe- and Al-bound P, and Ca- and Mg-bound P respectively .The sequential P fractionation was tested using pure P compounds and selected P minerals . The scheme effectivelyseparated Fe- and Al-P from Ca-P fractions in an FePO4-A1PO4-Ca3(PO4)2 mixture . Readily available P, defined asthe sum of water-soluble P and NH4CI-extractable P, in the unconsolidated gyttja (UCG) layer (surface 0-30 cm)of Lake Apopka sediments accounted for 10.1 to 23 .7% of total P (TP). This sediment P fraction constitutes a largereservoir which may act as a source of P to the overlying water . In subsurface marl layers (134-148 cm depth)of Lake Apopka, NH4CI-P constituted <I% of TP whereas Ca-Mg-bound P and highly resistant P (residual P)accounted for 35 and 64% of TP respectively . Results suggest that 1 M NH4CI (pH 7 .0) and 0.5 M HCI, reportedto dissolve carbonate-bound P and Ca-Mg-bound P, respectively, may not be extracting distinct pools of P . LakeOkeechobee mud sediments had low concentrations of readily available P (2% of TP) and were dominated byCa-Mg-bound P (HCl-P>_ 58% of TP) . Sediments in the littoral and peat areas of Lake Okeechobee, however, hadhigh concentrations of readily available P (9 .7 and 17.4% of TP respectively) ; hence, these sediments may playan important role in internal P cycling . The NaOH-P (Fe-Al-P) concentrations for Lake Okeechobee sedimentswere strongly correlated with amorphous and poorly-crystalline Fe (p< 0 .01), suggesting that some P reactions inthese sediments may be sensitive to changes in physico-chemical conditions such as redox potential and sedimentresuspension .

Introduction

Phosphorus fractions in soils and sediments are char-acterized by their differential solubilities in variouschemical extractants . The early fractionation schemes(Chang & Jackson, 1957 ; Petersen & Corey, 1966)grouped soil P into four pools (Williams et al ., 1971) :(i) orthophosphate ions sorbed onto the surface of P-retaining components (non-occluded P), (ii) P presentwithin the matrices of P-retaining components (occlud-

* Florida Agricultural Experiment Station Journal Series No . R-03406 . This work was supported in part by South Florida Water Man-agement District and St . Johns River Water Management District .

147

ed P), (iii) P present in discrete phosphate mineralssuch as apatite, and (iv) P present as organic esters ofphosphoric acid (organic P) .

The sequential extraction procedure of Chang &Jackson (1957) involved a series of solvents as follows :0.5 N NH4F (Al-P), 0.1 N NaOH (Fe-P), 0.5 N H2SO4(Ca-Mg-P), Na2S2O4-citrate (reductant-soluble Fe-P),0.5 N NH4F (occluded Al-P), and 0 .1 N NaOH (occlud-ed Fe-Al-P). In more recent fractionation schemesdeveloped for sediments, the redoxsensitive P formssuch as occluded and reductant-soluble P are no longerdistinguished from the other forms of inorganic P(Hieltjes & Lijklema, 1980 ; van Eck, 1982) . Some

1 48

problems reported for the Chang and Jackson (1957)method were poor reproducibility, particularly thereductant-soluble P fraction (Frink, 1969), and pre-cipitation of CaF2 in calcareous systems during NH4Ftreatment (Williams et al., 1971). The newly formedCaF2 is capable of resorbing inorganic P released inthe succeeding extractions and hence, could result inunderestimation of one fraction and overestimation ofothers (Syers et al., 1973). In addition, the use ofdithionite-citrate reagent was found to be ineffective inseparating Ca-P from Fe-P since citrate is a strong com-plexing agent for Ca (Hieltjes & Lijldema, 1980) .

Phosphorus removed by the first one or two extract-ing solutions in a sequential fractionation methodis usually considered readily available (Gunatilaka,1988 ; Psenner et al., 1988) . Depending on strengthof extracting solution and fractionation method, read-ily available P has been estimated from such extractsas: 1 M NH4C1 (Hieltjes & Lijklema, 1980 ; van Eck,1982), 0.1 M NaOH (Wildung et al., 1977 ; Hieltjes &Lijklema, 1980 ; Ostrofsky, 1987), citrate-bicarbonate-dithionite (Williams et al., 1971), and nitrolotriaceticacid (NTA) (Golterman et al., 1969; Gunatilaka et al.,1988). The Ca-bound P (HCl-P) such as apatite wasfound to be unavailable (Pettersson, 1986 ; Gunatilaka,1988), while the redox-sensitive Fe-bound P (NaOH-P) may become available under anaerobic conditions(Wildung et al., 1977 ; Furumai & Ohgaki, 1982 ; Hoso-mi et al., 1981) .

Results obtained from existing fractionationschemes are recognized to be analytical estimates ofdifferent forms of P. However, an appropriate P frac-tionation method may provide a quantitative measureof reactive and non-reactive P in sediments . Chemicalcharacterization of sediment P in eutrophic lakes pro-vides information on the distribution of various P formsand their potential mobility and bioavailability underprevailing redox potential (Eh) and pH. Lake systemsdominated by Fe-bound P, for example, are predictedto release P under anaerobic conditions (Wildung et al.,1977; Hosomi et al., 1981). Similarly, P solubility inlake systems dominated by Ca-bound P are sensitive tochanges in pH . Quantification of reactive and nonreac-tive P in lake sediments, therefore, could be useful inpredicting P responses to physicochemical changes andin understanding internal P cycling . This research wasfocused on quantifying various P forms in sedimentsof two shallow, eutrophic lakes (Lake Apopka andLake Okeechobee) in Florida . Lake Apopka is a hyper-eutrophic lake showing persistent primary productiv-ity (Aldridge et al., 1993) whereas Lake Okeechobee

is a eutrophic lake showing sporadic algal blooms inits northwest (littoral zone) and southern (peat zone)areas. It was hypothesized that, like the eutrophic lakesof Hungary (Pettersson & Istvanovics, 1988), Sweden(Tiren & Pettersson, 1985), and Netherlands (van Eck,1982), the concentrations of water-soluble P (WSP)and loosely-bound P (NH4C1-extractable P) in LakeApopka and Lake Okeechdbee sediments comprise asizeable fraction of the reactive P pool and hence, playan important role in internal P cycling . To test thishypothesis, a P fractionation scheme was adopted toevaluate the various forms of sediment P. Preliminaryexperiments were also conducted to confirm the valid-ity of the selected fractionation scheme. The specif-ic objectives of this study were : (i) to compare thesolubility of pure P compounds and selected miner-als in an NH4C1-NaOH-HCl sequential fractionationscheme, (ii) to quantify the reactive and nonreactivepools of P in sediments of two sub-tropical eutrophiclakes, and (iii) to evaluate the relationships among Pfractions and selected sediment properties .

Site description

Lake ApopkaLake Apopka is located in Central Florida forming theheadwaters of the Oklawaha basin of lakes (Fig . 1) .The lake has a total surface area of 125 km 2 and amean water depth of 2 m. The surficial water of thelake has a total N concentration of 0 .35 (±0.06) mM Nand a total P concentration of 6.5 (±2) pM P (Reddy& Graetz, 1991) which categorizes it as a hypereu-trophic lake (Forsberg & Ryding, 1980) . The surficialsediment (0-30 cm) consists of unconsolidated gytt-ja (UCG), defined as a coprogenous sediment mixtureof remains of all particulate organic matter, inorganicprecipitation, and minerogenic matter (Wetzel, 1975) .The subsurface sediment layer (35-78 cm) is a consol-idated gyttja (CG) consisting of partially decomposedalgal cells, detritus from aquatic macrophytes, and par-ticulate organic matter. The CG layer is underlain byeither clay, marl/sand, or peat .

Lake OkeechobeeLake Okeechobee is a major aquatic resource forwater storage, wildlife, and recreation in South Florida(Fig. 1), with a total surface area of about 1800 km 2and an average depth of 3 m (Canfield & Hoyer, 1988) .The lake, classified as eutrophic, has mean lakewatertotal N and P concentrations of 0 .17±0.05 mM N and

1 49

Fig. 1. Maps showing the sampling sites for Lake Apopka and Lake Okeechobee . Group I for Lake Apopka represents sediments with claysublayers . Groups II and III represent sediments with marl and peat sublayers respectively .

150

4.5±1 .2 pM P (Brezonik et al., 1979). Lake Okee-chobee sediments were categorized into four majorzones : mud (44% of the total lake surface area), sandand rock (28%), littoral (19%), and peat (9%) (Red-dy et al., 1991) as shown in Fig . 1 . Littoral areaswere defined as the region along the northwestern andwestern side of the lake, dominated by macrophytegrowth (Reddy et al., 1991). Peat refers to the par-tially carbonized plant tissues formed by incompletedecomposition in water .

Materials and methods

Synthetic P compounds and selected minerals

Synthetic P compoundsCompounds of Fe-P, and Al-P were synthesized in thelaboratory according to the procedures described byWang (1990) . Iron-P was prepared by boiling a 600-ml aqueous solution containing 10 ml of 1 M FeC13and 30 ml of 1 M NaH2PO4 on a hot plate for 3days. Aluminum P was prepared by boiling a 600-m1aqueous solution containing 30 ml of 1 M AIC13 and90 ml of 1 M NaH2PO4 on a hot plate for 16 h . Waterwas added frequently to maintain the volume of themixtures during boiling . The precipitates were washedwith water two times, centrifuged, and air-dried. Nomineralogical analysis was performed on the syntheticcompounds prepared in this preliminary study ; how-ever, the precipitates prepared by Wang (1990) usingthe same procedure were identified as phosphosiderite(FePO4 2H20) for Fe-P and variscite (AlP04 2H20)for Al-P.

Phosphorus fractionationPhosphorus compounds used in this study included :(i) tricalcium phosphate (Ca3(PO4)2, Fisher Scientific),(ii) a mixture of P compounds (consisting of syntheticFe-P and Al-P, as previously described, and Ca3(PO4)2in a molar P ratio of 2:5 :5), (iii) variscite (Mont-gomery County, Arkansas), (iv) wavellite (Fairfield,Utah), (v) crandallite, and (vi) apatite (Cantley, Que-bec). The minerals (variscite, wavellite, crandallite,and apatite) were obtained from the Wards NationalScience Establishment, Inc . (Rochester, NY) . The Pcompounds were fractionated for P according to theprocedures described by Hieltjes & Lijklema (1980)(Fig. 2). The minerals were ground to pass througha 200-mesh sieve and a solid :solution ratio of 1 :400

(w/v) was used, due to the high P content of the mate-rials. Results of an earlier attempt using a 1 :100 ratiowere unsatisfactory, showing only 40 to 60% P recov-ery (data not shown) .

Sediment sampling

Lake ApopkaIntact sediment cores were obtained from selected sta-tions (Fig . 1) using a piston corer specifically designedfor soft sediments (Fisher et al ., 1992) . Samples weretaken from 3 layers: i) unconsolidated gyttja layer (0-X30 cm), ii) consolidated gyttja layer (:30-80 cm),and iii) underlying layers of either clay (I), marl (II), orpeat (III) . Samples from each layer were homogenizedand placed in sealed Nalgene mason jars fitted withrubber septa . The samples were purged with N2 andstored at 4 °C until used in the experiments . The sedi-ments were processed within 7 days after sampling.

Lake OkeechobeeIntact sediment cores from three stations on LakeOkeechobee (Fig . 1) were obtained by divers . Threereplicates of sediment cores were taken from each ofthe mud, littoral, and peat areas. Each core was sec-tioned into 0-5, 5-10, and 10-20 cm increments, usingthe sediment-water interface as the reference point .The samples were placed in 250-m1 centrifuge bottles,purged with N2, and stored at 4 °C until used in theexperiments . Sample holding time was 7 days .

Chemical fractionation

The sediments were fractionated for P using a modifiedHieltjes & Lijklema (1980) scheme shown in Fig. 2 .The original fractionation scheme was extended bydetermining P in both digested and undigested 0 .1 MNaOH extracts and calculating the moderately resistantorganic P by difference . A more extensive P fraction-ation method, proposed by van Eck (1982), was usedin the UCG layers of Lake Apopka sediments to quan-tify the carbonate-associated P and labile organic P(Fig . 3) .

Wet sediment samples were placed in de-aeratedpolycarbonate tubes and centrifuged at 3620 g for15 min prior to removal of porewater . Phosphorus inporewater, filtered through 0 .45 pm membrane, wasreferred to as water-soluble P (WSP). Water contentwas determined separately by oven-drying subsam-ples at 80 °C for at least 48 h . Wet sediments were

WET SEDIMENT

NH 4 C1-P(Exchangeable P)

RESIDUE

RESIDUE

1 M NH4C1(pH 7)2 extraction, 2 h shaking each

0.1 M NaOH17 h ghnlcing

0.5 M HC124 h shaking

RESIDUAL P(Highly Resistant P)

HC1-P(Ca/Mg-bound P)

extracted sequentially for inorganic P using the proce-dure described in Fig . 2. The sediment : solution ratiowas maintained between 1 :60 and 1 :80 (w/v), based oncalculated dry weight.

Phosphorus fractionation was done under anoxicconditions . Subsampling was carried out inside a glovebag continuously purged with N2 . Extracting solutionswere added or withdrawn using syringes and filtered(0.45 µm Gelman filter membrane) using an anaero-bic filtration set up, purging with N2 . Except for the0.1 M NaOH and 0.5 M HC1 extracts, the filtrates wereacidified to about pH 2 .0 with one drop of pure con-centrated H2SO4 (Baker ultrapure) and stored at 4 °Cuntil assayed for P.

NaOH-P(Fe/A1-bound P)

Fig. 2 . Phosphorus fractionation scheme used in this study (Modified from Hieltjes & Lijklema, 1980) .

Analytical methods

Soluble reactive phosphorus (SRP)Phosphorus in all extracts, except that of NH4C1, wasdetermined by the ascorbic acid method (Murphy &Riley, 1962) using an automated analyzer . Phospho-rus assay in NH4Cl extracts was slightly modified byheating the sample-reagent mixture to 60 °C for 5 minon a hot plate (Folsom et al., 1977). The mixture wasallowed to cool at room temperature, and the blue colorwas read at 880 nm using a UV VIS spectrophotome-ter.

Total phosphorus (TP)Wet sediment samples were oven-dried (80 °C) andground using a mortar and pestle. Total P in sedimentwas determined by ignition (Saunders & Williams,1955) whereas TP in the 0.1 M NaOH extract,described in Fig. 2, was determined by block digestion

Digested

(NaOH-TP - NaOH-P)

NaOH-T?

NaOH-OP(Moderately-Resistant OP)

1 5 1

152

WET SEDIMENT(Subsample 2)

RESIDUE

RESIDUE

WET SESEDIMENT(Subsa

1 M NH 4C1 (pH 7)2 extraction

(Watanah. & Olsai DRP - NaCl-P)2 h shaking each

0.1 M NaOH17 h shaking

0.5 M HC124 h shaking

RESIDUAL P(Highly Resistant P)

RESIDUE(Discard)

0.5 M NaC130 min shaking

Watanabe & Olsen DRP

(Murphy & Riley DRP - Carbonatebound P - NaCl-P)

Murphy & Riley DRP

(Murphy and Riley '1'DP-DRP)

Murphy & i ilcy Oki ,

NaCl-P(Exchangeable P)

Carbonate-boundP

Labile OrganicP

NaOH-P(Fe-Al-bound P)

NaOH-OP(Moderately-Resistant OP)

HC1-P(Ca-Mg-bound P)

Fig. 3. Phosphorus fractionation scheme proposed by van Eck (1982) . Watanabe and Olsen DRP is dissolved reactive P (DRP) assayed bythe Watanabe & Olsen (1961) method, Murphy & Riley DRP is DRP assayed by the Murphy & Riley (1962) method . TDP is total dissolved Pdetermined by digestion methods (USEPA, 1983) .

with H2SO4 and HgSO4 at 380 °C (USEPA, 1983),

citrate-dithionite-bicarbonate (CDB)-extractable Fefollowed by automated analysis for P

and Al (Mehra & Jackson, 1960), 1 M HCl-extractableFe, Al, Ca, and Mg, and 1 M KCl-extractable Ca

Extractable metals

and Mg. The metals were determined using an atomicThe sediments were analyzed for ammonium oxalate

absorption spectrophotometer.extractable Fe and Al (McKeague & Day, 1966),

Table 1 . Inorganic P fractions of synthetic and pure P compounds and selected natural minerals extracted using the scheme byHieltjes & Lijklema (1980) .

% ofTOSynthetic compounds :

Fe-P 0.9±0 .01 94.1±2Al-P 3.0±0.17 93.8±2FeP04-A1P04-Ca3(PO4)2 mixture 7.9±0.59

74.3±4(2 :5 :5 molar P ratio)

Natural minerals :Variscite 0.2±0 .01 47.7±7Wavellite 0.1±0 .01 101 .1±2Crandallite 0.1±0 .01 7 .8±1Apatite

0.2±0.01

tr -Pure compound :

Ca3 (P04)2 4.1±0 .31

tr = trace, not detected (detection limit = 0 .01 µM P)t Percent of total Pt Variscite (A1P04 .2H20), wavellite (A13(PO4)2(OH)3 .5H20), crandallite (CaA13(PO4)2(OH)5 .H20), apatite (Ca10(P04 ) 6 F 2 )

Other analysesAlkalinity of porewater was determined using stan-dard methods (#2320 B ; APHA, 1992) and electricalconductivity (EC) was measured using a conductivi-ty meter. Inorganic carbon (IC) in sediment was ana-lyzed by coulometry (Huffman, 1977) whereas totalcarbon was measured using a Carlo-Erba CNS analyz-er. Sediment minerals were identified by x-ray diffrac-tion (XRD) using a computer-controlled XRD system .The mounts were scanned at 2 ° 20 per min with CuKaradiation at 35 kV and 20 mA (Wang, 1990) .

Statistical methods

Statistical analyses were performed using SAS, Ver-sion 5 (SAS Institute Inc., 1985) . Phosphorus fraction-ation data were tested for correlation with other sed-iment properties using Pearson correlation coefficients .

Results and discussion

Solubility of synthetic phosphorus compounds andselected minerals in NH4Cl-NaOH-HClfractionation scheme

The P fractionation scheme proposed by Hieltjes &Lijklema (1980) (Fig . 2) recovered 96.9 to 101 .1% of

HCI-P

P extracted Residual P Total P(Ca-P)

by the 3

(Ignition)solutions

(mol P kg-1 )

1 .9±0 .1 96 .9 3 .2+0.5 6.8±0 .10.6±0 .1 97 .4 3 .1±0.5 6.6±0 .118.9±1 .1

101.1

tr -

6.5±0.1

4.2±0 .2 52 .1 44.1±2 .0 0.9±0 .01 .5±0 .0 102 .7 tr - 1 .0±0 .0

12.2±1 .5 20 .1 76.5±1.0 3.4±0 .197.8±4 .5

98.0

2.3±0.5

5.7±0.1

1.0±0

89.9±3 .7

95.0

6.2±1.0

5.7±0.0

153

total P (TP) from pure tricalcium P and synthetic Fe-Pand Al-P compounds (Table 1) . The scheme, however,extracted only 52.1 and 20.1% of TP from varisciteand crandallite minerals respectively . The undissolvedP fraction was recovered in the final residue (residualP), indicating only a partial dissolution of the min-eral during sequential extraction . Incomplete dissolu-tion of crandallite was also reported by Wang (1990) .Phosphorus recoveries from the softer minerals suchas wavellite and apatite were 102 .7 and 98% of TPrespectively.

The low P recovery from variscite and crandallitemay be attributed to physical hardness of the min-erals. The variscite and crandallite minerals used inthis study were found to be harder than wavellite andapatite, based on difficulty in pulverization and grind-ing. Since each mineral was passed through a 200-mesh sieve, the observed variation in solubilities ofthese minerals in dilute alkali and acid solutions couldbe due to structural differences . All minerals, howev-er, dissolved completely when analyzed for total P byfusion with Na2CO3 (Syers et al., 1968) which is amore drastic treatment than the sequential fractiona-tion .

The Hieltjes & Lijklema (1980) scheme was effec-tive in separating Fe-Al-P from the Ca-P fractions . Thescheme extracted 96% of Fe-Al-P (NaOH-P) and 106%

Materials NH4CI-P NaOH-P(Loosely- (Fe-Al-P)bound P)

154

Table 2 . Some physico-chemical properties of Lake Apopka (mean of 8 stations) and LakeOkeechobee (mean of 3 cores) sediments . UCG is unconsolidated gyttja. All mass units werebased on a calculated dry weight except when indicated .

of Ca-P (HCI-P) from an FePO4-A1PO 4-Ca3(PO4)2mixture . The 0.1 M NaOH solution, which was expect-ed to extract 77 .5% of TP from the mixture, dissolvedonly 74.3% of TP (Table 1) . The 0 .5 M HCl solution,which was expected to extract 17 .8% of TP from themixture, dissolved 18 .9% of TR

Physico-chemical properties of sediments

Lake Apopka sediments had lower bulk densities(0.03 g cm -3 ) than Lake Okeechobee sediments(>0.14 g cm-3) (Table 2) . The pH of porewater inthese sediments ranged from 7 .0 to 7 .5. Total P con-centrations varied among sediment types, with LakeApopka (UCG) giving the highest TP concentration(42.2 mmol P kg-1 ) followed by Lake Okeechobeemud sediment (31 .3 mmol P kg-1 ) . Lowest TP con-centration (10.9 mmol P kg -1 ) was observed in Lake

tr = trace, not detected (detection limit = 0 .2 µM Fe ; 0.3 µM Al ; 0 .1 % Carbon)

Okeechobee littoral sediments . Sediments in littoralareas of Lake Okeechobee contained some fine shell(gastropod and bivalve) fragments and sand, in addi-tion to particulate organic matter and floc (aggregateof suspended particles) with macrophyte remains.

Lake Apopka had an average porewater P con-centration of 40 .6 pM P, about 16 times higher thanporewater P concentration of Lake Okeechobee mudsediments (Table 2). Lake Apopka sediments also hadhigher water-soluble Ca concentration, alkalinity, andelectrical conductivity (EC) . The sediments exhibit-ed wide differences in 1 M HCI-extractable Fe, Ca,and Mg concentrations . The concentration of HC1-extractable Al for Lake Apopka (22 mmol Al kg -1 ),was higher than those in peat and littoral sedimentsof Lake Okeechobee . Organic carbon concentration inLake Okeechobee sediments decreased in the order :peat>mud>littoral (Table 2). Organic carbon concen-

Lake OkeechobeeProperties Lake Apopka

(UCG)Mud(K8)

Littoral(E12)

Peat(M17)

Physical :Thickness (cm) 32±18 20 20 22Bulk density (g cm-3) 0.03±0 .01 0 .15 0 .64 0 .14Water content (g wet kg-1 ) 960±1 840 600 870

Porewater :pH 7.0±0 .2 7 .3 7 .5 7 .3Alkalinity (mM CaC03) 4.2±1 1 .8 1 .9 1 .5EC (µS cm -1 ) 802±158 540 550 600P (µM P) 40.6±23 2 .6 0 .1 1 .9Ca (MM) 2.7±0 .5 1 .5 1 .8 2 .1Mg (MM) 1.2±0 .7 0 .8 0.8 1 .1Fe (µM) tr - 5 .4 5 .4 trAl (µM) tr - 2 .2 1 .9 tr

SedimentTotal P (mmol P kg-1 ) 42.2±6 31 .3 10.9 21 .5CarbonInorganic (g kg- ' ) 12±0.7 24 1 13Organic (g kg- ' ) 290±5 150 40 412Total (g kg- ' ) 302±8 174 41 425

1 M HCI-extractable12±0.5 97 75 20Fe (mmol Fe kg 1 )

Al (mmol Al kg- ' ) 22±1 .5 38 2 8Ca (mmol Ca kg' ) 580±64 2237 326 2442Mg (mmol Mg kg- ' ) 82±17 720 145 234

Table 3 . Readily available P for Lake Apopka and Lake Okee-chobee sediments calculated as the sum of water-soluble P (WSP)and NH4 C1-P pools . UCG(I) is unconsolidated gyttja underlain byclay ; UCG(II) is underlain by marl ; UCG(III) is underlain by peat.

tration in Lake Apopka sediments was 290 g kg-1 ,

about twice the concentration of organic carbon in LakeOkeechobee mud sediments .

Results of x-ray diffraction analysis (XRD) indicat-ed the presence of carbonate minerals such as calciteand dolomite in both Lake Apopka and Lake Okee-chobee sediments (Fig . 4) . Based on relative peakintensities, the clay fraction of Lake Apopka sedi-ments contained higher calcite content than the clayof Lake Okeechobee sediments . Calcite was moreabundant in silt than in clay fractions of Lake Okee-chobee sediments . Dolomite was also abundant in LakeOkeechobee sediments particularly in fine silt fraction(Fig. 4) .

Other minerals identified in the clay fractionsof both lakes were smectites and quartz. Sepiolite(Si12Mg8O3o(OH)4(OH2)4 8H2O) was strongly indi-cated by XRD for the Okeechobee clay fraction where-as palygorskite (Si8Mg5O20(OH)2(OH2)4 4H20) waspossibly present in the Apopka clay fraction . No crys-talline Ca-P mineral was detected in the sediment sam-ples from either Lake Apopka or Lake Okeechobeesediments .

Phosphorus distribution in sediments

Readily available P concentrations, defined as the sumof porewater P and NH4C1-extractable P (Gunatilaka,1988 ; Psenner et al., 1988), for Lake Apopka sedi-ments ranged from 10.1 to 23 .7% of total P (Table 3) .

155

Based on Pettersson's (1986) studies using severaleutrophic lakes in Sweden, Lake Apopka sedimentscould be ranked among the first five subgroups hav-ing the highest NH4C1-P concentrations (i.e., >8% ofTP). The unconsolidated gyttja (UCG) sediments inLake Apopka can potentially supply dissolved P to theoverlying water by either diffusion or P release dur-ing sediment resuspension . Phosphorus release fromthe sediments, however, can not be explained sole-ly by the inorganic P components since the reservoirof nutrients is made up of both organic and inorgan-ic forms. Van Eck (1982) emphasized the presence oflabile organic P which, in addition to carbonate-boundP and loosely-bound inorganic P, is extractable with1 M NH4C1 (pH 7.0) . Labile organic P concentrationfor Lake Apopka sediments accounted for about 5 .8 to9.4% of TP whereas carbonate-bound P ranged from1 .8 to 5.4% of TP (Table 4) . The presence of carbonate-bound P is supported by high calcite content in LakeApopka sediments (Fig . 4). It is uncertain, howev-er, whether all the carbonate-bound P was dissolvedby two consecutive extractions with 1 M NH4C1 asreported in literature (van Eck, 1982) . The marl layer,which had a TP concentration of 20 .2 mmol P kg - ' ,was expected to have high concentrations of carbonate-bound P; however, it showed NH4C1-P concentrationsof only < 1 % of TP (Table 5, group II) . This may indi-cate that I M NH4C1 was not effectively selective ofC03-bound P. A large portion of P in marl layer con-sisted of HCl-P (35% of TP) and residual P (64% ofTP) fractions and XRD results confirm the presence ofappreciable amounts of calcite and dolomite . Resultssuggest that I M NH4C1 and 0 .5 M HCl may not beextracting distinct pools of P. The NH 4CI-extractionstep in the sequential fractionation scheme needs fur-ther investigation . Future study should include x-raydiffraction analysis of the immediate NH4CI residuesto verify the extent of Ca- and Mg-carbonate removalafter two extractions .

Concentrations of readily available P in Lake Okee-chobee mud sediments were only about 2% of TP andwere 5 to 10 times lower than those in Lake Apop-ka sediments (Table 3). Readily available P concen-trations in Lake Okeechobee littoral and peat sedi-ments, however, were 9.7 and 17.4% of TP respec-tively, and were comparable to those in Lake Apopkasediments. These results are consistent with reportedseasonal increases in nutrient concentrations of the lit-toral and peat areas of Lake Okeechobee (Brezoniket al., 1979) .

Sediment (depth)Readily available P(WSP ± NH4CI-P)(% of TP)

Total P(Ignition)(mmol P kg -1 )

Lake Apopka:(0-30 cm)

UCG(I) 23.7±4 .0 41.3±6UCG(II) 12.0±2 .1 31.2±4UCG(III) 10.1±0 .8 28.2±4

Lake Okeechobee:(0-10 cm)

Mud 2.0±0 .3 35.3±3Littoral 9.7±0 .8 29.8±4Peat 17 .4±1 .3 11 .2±2

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Fig. 4. X-ray diffraction patterns for (I) clay, Lake Apopka sediment, (II) clay, Lake Okeechobee sediment, (III) fine silt (2-5 µm), LakeOkeechobee sediment, and (IV) coarse silt (20-50 µm), Lake Okeechobee sediment. (sm is smectite, p is palygorskite, sp is sepiolite, w iswedellite, q is quartz, c is calcite, and d is dolomite) . Most of the minor peaks could be accounted for by wedellite, quartz, calcite, and dolomite .

Table 4 . Phosphorus fractions of Lake Apopka sediments showing the subfractions of exchangeable P, C03-bound P, and labileorganic P in 1 M NH4C1 extracts according to van Eck (1982) scheme . UCG(I) is unconsolidated gyttja underlain by clay ;UCG(II) is underlain by marl; and UCG(III) is underlain by peat . Descriptions in parentheses are the traditional P fractionsattributed to the extracts (Hieltjes & Lijklema, 1980 ; van Eck, 1982) .

Table 5. Phosphorus fractions of Lake Apopka sediments extracted according to Hieltjes & Lijklema (1980) scheme .UCG is unconsolidated gyttja; CG is consolidated gyttja . Descriptions in parentheses are the traditional P fractionsattributed to the extracts (Hieltjes & Lijklema, 1980 ; van Eck, 1982) .

High NH4C1-P (loosely-bound P and labile organ-ic P) concentrations in the littoral and peat sediments(Table 6) could be attributed to the presence of macro-phytes capable of mobilizing and releasing P (Bre-zonik & Fox, 1976 ; Brezonik et al., 1979). Largemacrophyte communities which occupy the littoral(western shores) and peat (South Bay) areas of LakeOkeechobee can mobilize P from the sediments viaroot absorption and translocation into shoots (Bre-zonik et al., 1979). The translocated P may be releasedinto the water and/or surface of the sediments through

157

excretory processes (McRoy et al., 1972) or tissuedecay (Barko & Smart, 1980) .

The NaOH-P fraction, which represents P sorbedonto oxides and hydroxides of Fe and Al (Psenneret al., 1988), has been reported as bioavailable (Son-zogni et al., 1982), particularly under anaerobic condi-tions where Fe(III)-P may be reduced to a more solu-ble Fe(II)-P (Wildung et al., 1977; Furumai & Ohgaki,1982). As observed in our preliminary study involv-ing synthetic P compounds and selected minerals, the0.1 M NaOH solution extracted 96% of inorganic Fe-

Group Depth(cm)

Inorganic P NaOH-OPResidual P(Highly-resistant P)

Total P(Ignition)(mmol P kg- 1 )

NH4C1-P

NaOH-P(Loosely-

(Fe-Al-P)bound P)

HCI-P

(Moderately-(Ca-Mg-P) resistant

Organic P)

% of TPIUCF 0-28 19.9+2.9 15 .7 ± 6.1 28 ± 2 1 .3 36 42.4±6CF 28-96 17.4 ± 9 .8 6.0±3.8 37 ± 15 3 .2 32 21 .0 ± 4

Clay 96-110 1 .3 ± 0 .7 1 .8 ± 1 .1 33 ± 12 0 .5 63 6.6 ± 1

IIUCF 0-34 9.3 ± 2 .5 6 .3 ± 2.4 31 ± 7 1 .4 52 32.2±4CF 34-134 2.1 ± 0 .2 2 .4 ± 0.2 47 ± 2 0 .5 48 17 .2 ± 3

Marl 134-148 0.8 ± 0 .5 0 .5 ± 0.4 35 ± 9 0 .1 64 20.2 ± 5

IIIUCF 0-21 8.3±1.0 7 .9 ± 0.9 40±7 6 .8 37 27.9 ± 4CF 21-60 0.3±0.1 1 .4 ± 0 .2 48 ± 2 1 .5 49 20.9 ± 1

Peat 60-83 0.7 ± 0.1 1 .2 ± 0 .4 23 ± 14 1 .6 73 10 .8 ± 1

Group Depth NaCI-PNH4C1-P NaOH-OP

HCI-P

(Moderately- Residual l' Total P(Ignition)

(C03- (Labile NaOH-P(cm) (Exchangeable bound P) organic P) (Fe-Al-P)

-P)(Ca-Mg-P) Resistant

Organic P)(Highly-Resistant P) (mmol P kg- 1 )

UCG(I) 0-28 % of TP

42.4 ± 63 .1 ± 0 .1 5 .4 ± 0 .1 9 .4 ± 0 .1 15 .1 ± 3 .9 30.3±2.6 2 .2 34 .5UCG(II) 0-34 0 .5 ± 0 .6 2 .1 ± 0 .8 6 .7 ± 1 .1 6 .6 ± 2.9 32 .5 ± 9.6 2 .1 49 .5 32.2 ± 4UCG(III) 0-21 1 .2 ± 0 .4 1 .8 ± 0 .4 5.8+0 .1 8 .0 ± 0.3 36.7±2.4 5 .9 40 .6 27 .9 ± 4

1 58

Table 6 . Phosphorus fractions of Lake Okeechobee sediments as determined by Hieltjes & Lijklema (1980) scheme. Descrip-tions in parentheses are the traditional P fractions attributed to the extracts (Hieltjes & Lijklema, 1980 ; van Eck, 1982) .

Al-P and effectively separated Fe-Al-P from Ca-P frac-tions . However, the alkali solution may also extractorganic matter (Levesque & Schnitzer, 1966 ; Moshiet al., 1974) as well as organic P (van Eck, 1982) ;hence, caution must be used in the interpretation ofNaOH-P fraction in soils and sediments high in organ-ic carbon .

The high concentrations of NaOH-P (6 .3 to 15.7%of TP) in the UCG sediments of Lake Apopka (Table 5)may include hydrolyzable organic P. Concentrationsof extractable Al and Fe for Lake Apopka sedimentswere only 22 and 12 mmol kg -1 compared to 38 and97 mmol kg -1 for Lake Okeechobee mud sediments(Table 2). The low extractable Fe and Al concentra-tions in Lake Apopka sediments suggest that mostof the P extracted by 0 .1 M NaOH could be in anorganic form (van Eck, 1982). It is likely that someorganic P in undigested NaOH extract was releasedvia acid hydrolysis during colorimetric determina-tion which requires a pH of about 3 .5 (Murphy &Riley, 1962) . This is in agreement with the findingsreported by van Eck (1982) for sediments in Hol-lands Diep/Haringvliet, The Netherlands . For littoraland mud sediments of Lake Okeechobee, however,the NaOH-P concentrations were positively correlatedwith oxalate-extractable Fe (p<0.01), indicating thatthe NaOH-P pool may be dominated by moderately-resistant organic P and Fe-P fractions . Results suggestthat some sediment P reactions in both littoral and mud

areas in Lake Okeechobee may be sensitive to chang-ing redox (Eh) conditions .

The HCl-P concentrations for Lake Apopka sedi-ments ranged from 23 to 47% of TP (Table 5) whereasthose of Lake Okeechobee sediments varied from 24 to79% of TP (Table 6). This P pool, which represents Ca-Mg-bound P, has been considered unavailable in manystudies (Pettersson, 1986; Gunatilaka, 1988; Burruset al ., 1990) .

Results obtained in this study are comparable tothose published in literature . Total P concentrations inLake Apopka and Lake Okeechobee sediments werecomparable to TP in eutrophic lakes of Hungary (Tiren& Pettersson, 1985) and Sweden (Pettersson, 1986) .Total P concentrations in Lake Apopka and LakeOkeechobee sediments, however, were lower thanthose in highly eutrophic lakes such as the HollandsDiep/Haringvliet in The Netherlands (van Eck, 1982)(Table 7). The NH4C1-P (loosely-bound P) fractions forLake Apopka UCG sediments and Lake Okeechobeelittoral and peat sediments, were comparable to those inLake Balaton (Hungary), Lakes Nieuwkoop and Hol-lands Diep/Haringvliet (The Netherlands), and LakeVallentunasjon in Sweden (Table 7) . Results showedthat the sequential NH4C1-NaOH-HC1 scheme (Hielt-jes & Lijldema, 1980) might be a useful technique forP fractionation in calcareous sediments . Caution, how-ever, must be applied in the interpretation of results,particularly the NH4C1-P and NaOH-P fractions .

Group Depth(cm)

Inorganic P NaOH-OPResidual P(Ignition)

Resistant P)

Total P(mmol P kg-1 )

NH4C1-P(Loosely-

bound P)

NaOH-P(Fe-Al-P)

HCI-P

(Moderately-

(Ca-Mg-P) Resistant (Highly-

Organic P)

%of TP

Mud 0-5 2+0.4 7 ± 1 .5 58 ± 0 .4 0 .9 32 38.6+3

5-10 2+0.2 4+0.1 70+1 .6 0 .4 24 32.0+3

10-20 2+0.1 4+0.6 79+3.9 0 .5 14 25.9+1

Littoral 0-5 10 ± 0 .5 21 ± 0 .9 29 + 2 .2 16 .0 24 35 .8 + 11

5-10 9+0.1 16 ± 2 .0 24+2.3 9 .2 42 23.7+ 1

10-20 4 ± 0 .8 16 ± 4 .1 31+0.2 1 .7 47 5.7±0.4

Peat 0-5 12 ± 0 .6 4+0.2 65+6.3 0.8 21 14.5+2

5-10 14+2.3 7+0.9 31 ± 4 .3 0.4 48 7.8+1

10-20 29+5.1 11 ± 0 .3 28+2.7 2.0 30 4.7+0.3

Table 7. Total P and inorganic P fractions (based on Hieltjes & Lijklema (1980) scheme) in sediments of selected lakes reported in literature .

Conclusions

The sequential NH4C1-NaOH-HCI fractionationscheme proposed by Hielbes & Lijklema (1980)showed high P recoveries for pure, inorganic P com-pounds (tricalcium phosphate, and synthetic phospho-siderite and variscite) and soft minerals such as wavel-lite and apatite. The technique was effective in sepa-rating Fe-AI-P from Ca-P fractions in a mixture thatcontained the three pure, inorganic P compounds . Thescheme, however, extracted only about 20 to 52% ofTP in hard minerals such as variscite and crandallite

159

used in this study. The undissolved P fractions for theseminerals were recovered from the final residue . Thisindicates that, contrary to the assumption that `residualP' fractions are mostly `organic P', final residues froma fractionation procedure may contain highly resistantand insoluble inorganic P pools .

The shallow, hypereutrophic Lake Apopka hadhigh concentrations of NH4C1-P that were compara-ble to NH4C1-P concentrations in Lake Balaton (Hun-gary), Lake Nieuwkoop (The Netherlands), and LakesVallentunasjon and Norrviken (Sweden) . Lake Apop-ka, having a thick layer (0-30 cm) of unconsolidated

Location/Lakes

Total P(mmol P kg - t)

NH4C1-P

NaOH-P HCI-P(Loosely-bound) (Fe-Al-P) (Ca-Mg-P)

Residual P References

. . . . . . . . . . . . . . . . . . . . . % of TPConnecticut :Bantam 112 0.8

19 Ostrofsky, 19878 72.2Waramaug 115 3.8

43 10 43 .2 Ostrofsky, 1987Hungary :Balaton 20 10.5

6 47 36.5 Tiren & Pettersson, 1985Balaton 22 8.6

9 33 49.4 Pettersson & Istvanovics, 1988Netherlands :Nieuwkoop 29 22.1

46 8 23.9 Klapwijk et al., 1982Hollands Diep/Haringvliet 226 9.0

41 3 47.0 van Eck, 1982New York :Silver 59 1 .2

21 20 57 .8 Ostrofsky, 1987Ontario :Brewer 78 0.1

34 7 58.9 Ostrofsky, 1987Pennsylvania :Sugar 98 1.7

54 9 35.3 Ostrofsky, 1987Sweden :Osten 41 1 .7

30 36 32 .3 Tiren & Pettersson, 1985Vallentunasjon 58 8 .1

14 17 60 .9 Pettersson, 1986Norrviken 57 7 .9

14 21 57 .1 Pettersson, 1986Vermont:Shelburne 69 1 .3

11 8 79.7 Ostrofsky, 1987Carmi 73 1 .2

10 19 69.8 Ostrofsky, 1987

ApopkaZone 1 42 20.2

16 28 35 .8 this studyZone 2 32 8 .8

7 31 53.2 this studyZone 3 28 8 .6

8 40 43.4 this studyOkeechobeeMud 39 1 .9

5 65 28.0 this studyLittoral 36 9 .6

18 26 46.3 this studyPeat 15 17 .3

6 45 31 .6 this study

160

gyttja (UCG), has a large sediment P reservoir thatcould serve as source of P to the overlying water . Thisimplies that internal P loading is an important factor innutrient cycling in Lake Apopka .

Phosphorus distribution in Lake Okeechobee sedi-ments varied with sediment type. The mud sediments,which constitute about 44% of the total lake sur-face area, had lower concentrations of readily avail-able P than the sediments in Lake Apopka . Calcium-and Mg-bound P fractions dominated the mud sedi-ments of Lake Okeechobee, accounting for 65% of TP .The NaOH-P fractions in both littoral and mud sedi-ments were correlated with the amorphous and poorly-crystalline Fe (oxalate-extractable Fe), suggesting thatsome P reactions in these sediments may be sensitiveto changing redox conditions. Sediments in littoral andpeat areas of Lake Okeechobee, having high concen-trations of loosely bound P, may be an important con-tributor to internal P cycling .

Acknowledgments

We thank Drs C . T. Johnston, N. Comerford, andP. Gale for their suggestions and comments . Appre-ciation is extended to Dr P. C. M. Boers and anony-mous reviewers for their critique on the manuscript .We are grateful to Yoeng K . Ann and Keith Hollienwho helped us during the preparation of synthetic Pcompounds and x-ray diffraction analysis .

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