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Published online 16 October 2007
Published in J Environ Qual 36:1670-1677 (2007)
DOI: 10.2134/jeq2006.0559
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Surface Water Quality

Magnesium-Rich Minerals in Sediment and Suspended Particulates of South Florida Water Bodies: Implications for Turbidity

W. G. Harris*, M. M. Fisher, X. Cao, T. Osborne and L. Ellis

Soil and Water Science Dep., Univ. of Florida, Gainesville, FL 32611

* Corresponding author (apatite{at}ufl.edu).

Received for publication December 22, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Summary and Conclusions
 REFERENCES
 
Fine sediments in shallow water bodies such as Lake Okeechobee are prone to resuspension. Predominantly inorganic "mud" sediment that covers ~670 km2 of the lake has been recognized as a persistent source of turbidity. The objective of this study was to determine if mineral components of sediments in Lake Okeechobee and water conveyances of the northern Everglades also occur as suspended sediment and hence constitute a potential abiotic contributor to turbidity. Sediment samples were collected from nine stations within the lake and eight locations north of Water Conservation Area 2A in the Everglades. Water samples were also collected at selected locations. The silt and clay mineralogy of sediment and suspended particles was determined using X-ray diffraction, thermogravimetry, scanning-electron microscopy, energy-dispersive X-ray elemental microanalysis, and high-resolution transmission-electron microscopy. Clay fractions of the lake sediment contained the Mg silicate minerals sepiolite and palygorskite, along with smectite, dolomite, calcite, and kaolinite. Sediment silt fractions were dominated by carbonates and/or quartz, with smaller amounts of Ca phosphates and sepiolite. Mineralogy of the mud sediment was similar to that reported for geologic phosphate deposits. This suggests that the mud sediment might have accumulated by stream transport of minerals from these deposits. Suspended solids and mud-sediment mineralogy were similar, except that smectite was more abundant in suspended solids. Everglade samples also contained Mg-rich minerals. The small size, low density, and fibrous or platy nature of the prevalent mud sediment minerals make them an abiotic, hydrodynamically sensitive source of persistent turbidity in a shallow lake. Mitigation efforts focused exclusively on P-induced biogeochemical processes do not address the origin or effects of these minerals. Ecological management issues such as turbidity control, P retention, geologic P input, and suitability of dredging are related to mud-sediment properties and provenance.

Abbreviations: XRD, X-ray diffraction • SEM, scanning electron microscope • HRTEM, high-resolution transmission electron microscope • TG, thermogravimetry • EDS, energy-dispersive X-ray spectroscope • LOI, loss-on-ignition


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results
 Discussion
 Summary and Conclusions
 REFERENCES
 
DECLINING water quality in Lake Okeechobee and the Everglades has been a concern for several decades. This has been attributed largely to anthropogenic factors such as excessive P loading, high water levels, and expansion of exotic plants (Hoyer, 1988; Janus et al., 1990; Havens et al., 1996; Brezonik and Engstrom, 1998; Canfield and Hoyer, 1988; Havens and James, 2005; Engstrom et al., 2006). An important aspect of water quality is turbidity, which can be caused by eutrophication and/or sediment resuspension by wind. Fine sediments in shallow water bodies, such as Lake Okeechobee, are particularly prone to resuspension, and turbidity has been a water quality problem on the lake in recent years (Maceina and Soballe, 1991; James et al., 1997; Hanlon et al., 1998).

"Mud" sediment (predominantly <50-µm-sized particles) has accumulated over an extensive area (~670 km2) of the Lake Okeechobee bottom (Kirby et al., 1994; Fisher et al., 2001). The mud, historically attributed to P enrichment (e.g., Brezonik and Engstrom, 1998), has been linked to water-quality degradation. Studies have indicated that the sediment contributes significantly to the total P load to the lake (Moore et al., 1998; Fisher et al., 2005). Studies addressing feasibility of dredging have been sponsored by the South Florida Water Management District. The mud is composed predominantly of inorganic components (Olila et al., 1994; Brezonik and Engstrom, 1998; Engstrom et al., 2006), even though it has been referred to as "organic mud" (Havens and Gawlik, 2005) or "organic sediments" (Brezonik and Engstrom, 1998). Olila et al. (1994) determined that the clay fraction of the mud sediment at one sampling station (K8) contained abundant amounts of the Mg-rich minerals sepiolite (Mg silicate) and dolomite (CaMgCO3), along with lesser amounts of palygorskite (Mg silicate), calcite, and smectite (an expandable phyllosilicate that commonly contains Mg).

Havens (1995) has inferred that light attenuation in the pelagic region of the lake is mainly due to abiotic particles, not phytoplankton as is common in many lakes. Fluctuation in light intensity due to this material is a major factor in regulation of the phytoplankton community and the submerged aquatic vegetation of the lake. The sediments are, therefore, an important regulator of trophic conditions in the lake, but little is known of their mineralogy, origin, or age. These aspects of the sediment are relevant to management issues, such as sediment accumulation control and whether sediment dredging is advisable. The objective of this study was to determine the mineralogy of sediment and suspended particles at multiple locations in Lake Okeechobee and in canals that convey agricultural drainage and lake water to and from a storm water treatment area (STA). The STA is a constructed wetland designed to reduce P discharge into water conservation areas of the northern Everglades. This work documents the abundance of fine, Mg-rich, and other minerals prone to resuspension and that slowly settle in these aquatic systems. These minerals could not have originated from the P-induced biogeochemical processes that have been the primary focus of mitigation efforts.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 Summary and Conclusions
 REFERENCES
 
Nineteen sediment samples from nine stations and four water samples from four stations were collected at Lake Okeechobee (Fig. 1 ) in January 2001 and May 2006, respectively. Intact sediment cores were collected using plastic tubes and a piston corer (Fisher et al., 1992). Okeechobee water was sampled between 7 and 10 AM on the same day in 4.4-L high-density polyethylene containers that had been previously acid washed, soaked in DDI (distilled, deionized) water over night, and rinsed again in DDI before sampling. Samples were collected approximately 25 cm below the water surface, capped without air bubbles, and kept on ice in the dark until delivered to the laboratory where analyses were promptly performed. Water depths at the sampling locations were similar, ranging from 3.5 to 3.7 m.


Figure 1
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  		Fig. 1. Map of locations for samples designated in Tables 1–5GoGoGoGo.

 

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  		Table 1. Mass losson ignition of Lake Okeechobee mud sediment samples and mineralogy of the clay fraction analyzed by XRD (see also Fig. 2) and thermogravimetry. Calcite and aragonite are also reported quantitatively, as determined by thermogravimetry (Karathanasis and Harris, 1994). Dolomite was corroborated by thermogravimetry as well, but its lower temperature reaction was too close to the range of organic matter combustion to be confident in quantification. Qualitative assessment of relative mineral abundance is indicated by the number of "i" characters, with "i" = low but detectable and "iiii" = abundant.

 

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  		Table 2. Mineralogy of Okeechobee silt fractions for samples analyzed by XRD (see also Fig. 2) and thermogravimetry. Calcite, aragonite, and dolomite are also reported quantitatively, as determined by thermogravimetry (Karathanasis and Harris, 1994). Ca phosphate was not identified by XRD, but Ca-P particles were detected fairly often using elemental dot mapping under SEM (Fig. 4 and 5). Qualitative assessment of relative mineral abundance from XRD peak intensity is indicated by the number of "i" characters, with "i" = "low but detectable" and "iiii" = "abundant".

 

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  		Table 3. Total dissolved and suspended solids for Okeechobee water samples (mean for 3 replicates) collected at the lake surface near the four designated stations.

 

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  		Table 4. Mineralogy of suspended particles from Water Conservation Area 2A (WCA-2A) and Stormwater Treatment Area 2 (STA-2), located south of the Everglades agricultural area (Fig. 1). Qualitative assessment of relative mineral abundance is indicated by the number of "i" characters, with "i" = low but detectable and "iiiii" = abundant, based on XRD peak size.

 

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  		Table 5. Mineralogy of < 0.05-mm sediment (essentially the whole sediment with plant detritus removed) from three cells of Stormwater Treatment Area 2 (STA-2), located south of the Everglades agricultural area (Fig. 1). Qualitative assessment of relative mineral abundance is indicated by the number of "i" characters, with "i" = low but detectable and "iiiii" = abundant.

 
Eight sediment samples and eight corresponding water samples were taken at eight sites from the water conveyances leading to and leaving an STA (STA-2) north of Everglades Water Conservation Area 2A (Fig. 1). Everglade conveyance samples were collected in June 2003 (water samples) and October 2003 (sediment samples). Water samples were collected to target the upper few cm of the water column, using containers washed with phosphate-free detergent and rinsed with DDI water. Details of water sample collection are provided by Dierberg et al. (2006). Sediment core samples were collected with an Al tube. The top 2 cm of four replicate cores at each location were composited.

Sediments were refrigerated and maintained in their original moisture state in plastic bags and were not dried before particle-size fractionation. They were analyzed without pretreatment except for wet-sieving (<0.05 mm) to remove plant detritus; most of the sediment was <0.05 mm in particle size. However, duplicate sets of Everglades sediment and six selected Okeechobee sediment samples were treated for selective removal of carbonates (NaOAc buffered at pH 5) and organic matter (oxidation by H2O2) to concentrate silicates. Density fractionation into the specific gravity (g cm–3) ranges of >2.7, 2.7–2.2, and <2.2 was performed on pretreated silts using centrifugation and Na polytungstate as the high-density liquid. Various analyses were conducted on these selective dissolution aliquots and density fractions, including X-ray diffraction (XRD), electron microscopy, and solid-phase elemental assessments as described below. Clay (<2 µm) and silt (2–50 µm) fractions of the Okeechobee sediment samples were separated using centrifugation methods following removal of sand by wet sieving (Whittig and Allardice, 1986). Mass fractions of <2-µm material in eight Okeechobee sediment samples were determined using the same centrifugation procedure, passing separate 10-mL aliquots of the suspended clay through a 0.45-µm filter, drying the filtrate in a tarred beaker and determining the mass of dissolved solids (operationally defined as concentration of solids <0.45 µm). Mass fractions of <2-µm material were obtained by subtracting the mass of dissolved solids from the total mass of <2-µm material based on 10-mL aliquots of the suspended clay.

Okeechobee and Everglades water samples were filtered through a 0.45-µm membrane and retentates were dried for mineral characterization. Total, particulate, and dissolved solids contents were determined for water samples from Lake Okeechobee. Total solids content was measured by the residual mass in 10 mL of water after being dried at 100°C and placed in a desiccator for cooling, while total dissolved solids content was determined by the residual mass in 10 mL of water following 0.45-µm filtration. Total suspended solids content was calculated by the difference between total and dissolved solids.

Minerals in the sediment and suspended particles were identified by XRD using a computer-controlled X-ray diffractometer equipped with stepping motor and graphite-crystal monochromator. Oriented mounts of Okeechobee clay fractions were prepared by sedimentation on unglazed ceramic tiles under suction, and diagnostic cation saturations (Mg and K) and heat treatments were performed to aid in phyllosilicate identification (Whittig and Allardice, 1986). Silt fractions of Okeechobee sediment were mounted in recessed holders (cavity mounts) as air-dry powders. Samples of Everglades sediment (<0.05 mm) and suspended particulates and of Okeechobee suspended particulates were deposited as a slurry directly onto quartz-crystal mounts specially prepared to minimize background scatter of X-rays. The quartz mount was advantageous for cases of limited sample mass. Selected Everglades samples were also mounted on unglazed ceramic tiles and subjected to diagnostic cation saturations and heat treatments to help verify mineral identification. Samples of Okeechobee suspended particulates were mounted "as is" without diagnostic saturations, which were not needed for mineral identification (i.e., smectite was naturally expanded to an identifiable and diagnostic state). Samples were scanned at 2°2{theta} per min using CuK{alpha} radiation. No {theta}-compensating slit was used. Relative mineral abundance was qualitatively assessed using relative peak areas (i.e., above background and low-angle scatter) of the most prominent XRD peak for each mineral. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), in conjunction with energy-dispersive X-ray fluorescence elemental microanalysis (EDX) were also applied to determine whether Ca phosphate and other phases were present. Samples for SEM were prepared on a carbon stub as a dilute suspension and allowed to air dry under a cover. They were then coated with an ultra-thin C layer to conduct electrons from the sample surface. Samples for HRTEM were mounted on commercially obtained "holey carbon" grids that enable effective presentation of very fine sample materials to the electron beam (Gilkes, 1990).

Calcium carbonate in the form of calcite and aragonite was quantified in Everglades sediment samples (which contained significant amounts of calcite) using thermogravimetry (TG) (Karathanasis and Harris, 1994), based on mass loss from the thermally induced evolution of CO2 at 750 to 900°C. Organic matter content was also determined by TG, based on mass loss between approximately 300 and 600°C (depending on TG curve inflections). Attributing mass loss in this temperature range to evolved CO2 from organic matter combustion is reasonable because the Everglades sediment mineralogy did not suggest significant inorganic mass loss in that range. The mineral palygorskite, identified in some samples, does lose some mass from water evolution in that range, but it is probably not present in sufficient amounts to affect the organic matter determination to a significant extent on a relative basis (organic matter was abundant in most samples).

Loss-on-ignition (LOI) was determined for the Okeechobee sediment samples based on mass loss between 105 and 550°C. Because of mineral components in the Okeechobee mud sediment that would lose appreciable water when heated in that range (i.e., smectite, sepiolite, and palygorskite) LOI cannot be attributed exclusively to organic matter combustion.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
Discussion
 Summary and Conclusions
 REFERENCES
 
Lake Okeechobee
The mud sediment samples from Lake Okeechobee were predominantly inorganic (Table 1 ). This is significant because fine sediments from most Florida lakes are high in organic matter (Binford and Brenner, 1986). The sediment clay content (<2 µm) averaged 300 g kg–1 for eight samples, with a range of 130 to 660 g kg–1. Clay mineralogical compositions were very similar among samples, as inferred from XRD and TG results (Table 1 and Fig. 2 ). Sepiolite (Mg silicate), quartz, and dolomite [(CaMg(CO3)2] occurred in significant amounts in all samples, except for a quartz-dominated layer from station E9 at 15- to 22-cm depth. High-resolution transmission electron microscope imagery (Fig. 3 , left) showed fibrous particles that elemental microanalysis (EDX; data not shown) verified to contain Mg and Si, consistent with the presence of sepiolite. Smectite, an expandable mineral which commonly contains structural Mg as well as Al, was present in significant amounts in all samples except one. Palygorskite (Mg silicate) occurred in small amounts in all samples. Calcite (CaCO3) was detectable in small amounts in about one third of the samples. Clay mineralogy of these samples was similar to that reported by Olila et al. (1994) for a mud sediment sample from Lake Okeechobee. There were no obvious differences in clay mineralogy based on core locations or depth, except for two samples collected at station E9 (Table 1).


Figure 2
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  		Fig. 2. X-ray diffraction (XRD) patterns of Okeechobee mud sediment clay (Mg-saturated, glycerol solvated) and silt from specified stations and depths. Minerals indicated are sepiolite (Se), palygorskite (Pa), quartz (Qz), aragonite (Ar), calcite (Ca), and dolomite (Do). The peaks without a mark are from the XRD slide mount. Smectite was also present in most Okeechobee mud samples (including O7, above), but smectite XRD peaks were only marginally evident before K saturation (not shown) (Whittig and Allardice, 1986).

 

Figure 3
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  		Fig. 3. High-resolution transmission electron microscope (HRTEM) image of Mg silicates (fibrous particles) in Okeechobee mud sediment (left; station P9) and in suspended particulates from Everglades stormwater treatment area (Cell 3). Micro-elemental analysis confirmed the presence of Mg and Si (data not shown).

 
Silt fractions (Table 2 ) contained more calcite and/or quartz, but less smectite and sepiolite than did the clay fractions. Scanning electron microscope–EDX analysis revealed the presence of both Ca phosphate and iron sulfide in the six silt fractions analyzed (Fig. 4 and 5 ). The samples with iron sulfide contained subhedral crystals which suggested the presence of pyrite. There was also XRD evidence for FeS2 minerals (pyrite and/or marcasite) in the silt, especially after concentration of the dense >2.7 g cm–3 fraction. Similar peak positions for FeS2 minerals and aragonite (CaCO3) that were also concentrated in the dense fraction precluded positive identification of pyrite/marcasite by XRD. There was a small XRD peak for the >2.7 g cm–3 treated silt corresponding to the strongest peak for apatite (Ca phosphate), but evidence was insufficient to confirm the presence of apatite.


Figure 4
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  		Fig. 4. Secondary electron image (top) of Okeechobee sediment silt collected at station O7 (20–30 cm) (Scale bar = 20 µm). Below this image are scanning electron microscope-energy-dispersive X-ray spectroscope (SEM-EDS) elemental dot maps showing zones of Mg, Si, P, S, Ca, and Fe concentrations based on K{alpha} characteristic X-ray emissions. Dolomite is indicated by some correspondence between Mg and Ca. Silica bioliths (e.g., sponge spicule fragments, etc.) are evident in the sample, along with a Ca phosphate particle and an iron sulfide crystal. Similar Ca-P associations were confirmed for all six silt-fraction samples that were analyzed by SEM-EDS (Fig. 5).

 

Figure 5
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  		Fig. 5. Elemental dot maps of the silt fractions from several Okeechobee mud sediment samples, showing localized zones of Ca-P association (upper row is P, lower row is Ca). Phosphorus is not associated with Ca in a general or diffuse way, but only as localized concentrations, often as discrete grains.

 
Optical assessment of the sand and coarse silt of Okeechobee sediment revealed a prevalence of sponge spicules, shell fragments including intact snail shells, exoskeletons of arthropods, and quartz grains in varying proportions. Sponge spicules and diatoms (evident using SEM; images not shown) are biogenic silica components that are commonly present in lake sediments (Conley and Schelske, 1993). A subsurface layer (3–22 cm) from station E9 was dominated by quartz sand on a whole-sample basis.

Suspended particulates contained smectite, sepiolite, kaolinite, calcite, and dolomite, the same components found in the mud sediment clay (Fig. 6 ). However, the proportion of smectite relative to sepiolite was greater in the suspended particulates, based on relative intensities of the XRD peaks. Total suspended solids concentrations of the samples studied (45–66 mg L–1; Table 3 ) were within the range reported by Jin and Ji (2004).


Figure 6
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  		Fig. 6. X-ray diffraction (XRD) patterns of the suspended particles collected at Okeechobee stations K8, N6, M9, and O7. Minerals indicated are smectite (Sm), sepiolite (Se), kaolinite (Ka), quartz (Qz), calcite (Ca), dolomite (Do), and aragonite (Ar).

 
Everglades Stormwater Treatment Area
Calcite was the dominant crystalline phase for all samples of the Everglades water-suspended particles (Table 4 ). Palygorskite was identified in all samples based on evidence from XRD (Table 4) and HRTEM (Fig. 3, right). The image from HRTEM revealed fibrous crystals characteristic of palygorskite. Elemental microanalysis of areas rich in these fibers indicated them to be high in Si and Mg (data not shown), which are the major components of palygorskite. Hence, there are multiple lines of evidence for the presence of this mineral. Sepiolite, closely akin to palygorskite in composition and crystal habit, was also tentatively identified in suspended particles. Palygorskite and dolomite (another Mg-bearing mineral) appeared to be associated in terms of their relative prevalence in samples.

The same components identified in the water-suspended particles were also detected in the sediments of canals leading to and leaving from Everglades STA-2 (Table 5 and Fig. 7 ). Bulk sediment samples contained less palygorskite than the suspended particulates, but palygorskite was easily identified following selective removal of carbonates and density separation (Fig. 7). The sediment also contained less dolomite than the suspended materials.


Figure 7
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  		Fig. 7. X-ray diffraction (XRD) patterns of the sediment sample from Everglades Cell 3 inflow, showing concentration of palygorskite and quartz by selective dissolution of Ca carbonate minerals (calcite and aragonite).

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
Summary and Conclusions
 REFERENCES
 
All 12 water samples collected from diverse locations in Lake Okeechobee and managed northern Everglades conveyances contained suspended sepiolite and/or palygorskite. This is direct evidence that these minerals are resuspended in the water column. The small particle size, low density (specific gravity ~2.0–2.3 g cm–3), and fibrous crystal habit (Fig. 3) amount to a very high surface-area/mass ratio and would favor a very slow settling rate for sepiolite and palygorskite once dispersed and resuspended. Their surface chemistry may also favor dispersion. Surface charge would be elevated at the high pH (generally >7.0) consistently observed in these water bodies (e.g., Olila et al., 1994; Farve et al., 2004; pH of Okeechobee sediment porewater ranged from 8.5 to 9.0 for samples of this study), based on the amphoteric nature and probable low point of zero charge (PZC) of these minerals. Neaman and Singer (2000) reported a PZC range of 4.0 to 4.5 for palygorskite. Higher surface charge would likely favor dispersion and suspension over flocculation and settling, given relatively dilute concentrations of suspended particulates and low ionic strength (mean EC for Lake Okeechobee porewater as measured in this study was 0.58 mS cm–1, n = 13). Smectite would likely show similar colloidal behavior under these conditions due to its small particle size and platy habit. Hence the consistent prevalence of this mineral suite in the mud sediment is tantamount to an abiotic, hydrodynamically sensitive potential source of turbidity in a shallow lake. Also, the propensity of these minerals for suspension and transport in streams and canals is likely to be high.

The provenance of the mud sediment Mg-rich minerals is related to the sedimentary history of the region and has implications for Lake Okeechobee management. These minerals are not the result of anthropogenic P loading that has reportedly increased in the lake since 1910 (James et al., 1997; Havens and James, 2005). Accumulation of Mg-rich minerals has a direct bearing on turbidity because the particles can be resuspended into the water column. Identification of the Mg-rich minerals' provenance would help to distinguish sediment sources that influence the sediment accumulation rate. The suitability of prospective turbidity remediation strategies (e.g., mud removal by dredging) rests in part on the source of the mud and the prospect of continued accretion.

The mineralogy of the mud sediment suggests that a significant portion of the minerals in the mud could have been transported via stream flow from Miocene or Pliocene sediments that occur upstream from the lake and that also contain sepiolite, palygorskite, smectite, dolomite, calcite, and Ca phosphates (Compton, 1997). The largest stream flowing into the lake, the Kissimmee River, delivers about 5.3 billion L d–1 to the lake (Kenner et al., 1975). The Kissimmee River is a continuous sediment source for the lake, which raises the following questions: Is it a primary source of mud sediment in the lake and ultimately the northern Everglades? If so, did the channelization of the Kissimmee River initiate or accelerate the delivery of Mg-rich minerals to the lake?

An alternative origin of sepiolite, palygorskite, and dolomite would be that they precipitated within the lake environment itself. Sepiolite and palygorskite stabilities are favored by the alkaline conditions of Lake Okeechobee and Everglades (e.g., Olila et al., 1994; Farve et al., 2004), though these minerals are not common in soils of humid climates (Singer, 2002). Sepiolite has been reported to form in lacustrine (Singer et al., 1998; Mayayo et al., 1998) as well as marine (Isphording, 1973) environments. Dolomite is common in some Florida carbonate rocks (Randazzo, 1997). It has been thought to form primarily from replacement of calcite with the continued incursion of Mg over time rather than by direct precipitation from solution. However, recent studies support the microbially mediated precipitation of dolomite in the laboratory (Vasconcelos et al., 1995) and in lacustrine settings (Wright, 1999; del Cura et al., 2001).

The origin of minerals in the mud sediment also has implications for the nature of the inorganic pool of phosphate in the lake. For example, if the mud zone minerals are mainly derived from old marine (Miocene) phosphorites, a significant proportion of the inorganic P could be in the form of carbonate fluorapatite (the dominant phosphate mineral in the unweathered phosphorites). An unidentified Ca phosphate form was verified by EDX (Fig. 4 and 5). The phosphate must be to some degree bound in discrete phosphate mineral phases (indicated by elemental dot maps [Fig. 5]) rather than a labile, dispersed pool or as co-precipitate with CaCO3. Sepiolite, smectite, and palygorskite do not have a high surface affinity for phosphate sorption (Yadov et al., 1984; Shariatmadari and Mermut, 1999) and hence may be passive components in phosphate dynamics.


   Summary and Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Summary and Conclusions
REFERENCES
 
Mud sediment and suspended particles in Lake Okeechobee are consistently rich in Mg-bearing minerals, including sepiolite and palygorskite that are rare in humid regions. The sediment also contains discrete Ca phosphate particles. Mineralogical similarity between this sediment and the clay associated with nearby phosphatic geologic deposits raises the possibility that the sediment could have originated from these deposits via stream transport. Water conveyances of the northern Everglades also have detectable but lesser amounts of Mg-rich minerals in suspended particulates and sediment. Properties of these minerals are pertinent to the ecological management of the lake. The small size, low density, and fibrous or platy nature of the prevalent mud sediment minerals make them an abiotic, hydrodynamically sensitive source of persistent turbidity in a shallow lake. Surfaces of these minerals do not have a high affinity for P. It is important to know the provenance of the mud sediment because it bears on rate of future accumulation, advisability of dredging, and the possibility of natural phosphate sources to the lake. The prospect of the Kissimmee River being a significant source of the mud sediment raises the following questions: (i) does its channelization exacerbate turbidity-related problems in the lake and (ii) would continued sediment delivery by the river eventually negate any benefit from dredging? These questions could be answered by mineralogical examination of sediment cores from the river's original bed, constructed channel, and mouth.


   ACKNOWLEDGMENTS
 
Data presented in this paper were generated from projects funded in part by the South Florida Water Management District. We acknowledge contributions of Drs. Ramesh Reddy and John White who administered the Okeechobee dredging feasibility study through which the Okeechobee samples characterized in this paper were obtained, and Ms. Yu Wang, who coordinated chemical analyses of these samples. We are indebted to Dr. Woody Dierberg, who supervised collection of the Everglades STA samples, Mr. Scott Jackson for his sampling efforts and detailed record keeping, and Ms. Kerry Siebein who was very helpful in performing HRTEM analyses. We thank Dr. Dierberg, Dr. Tom James, Dr. Michael Thompson, and anonymous reviewers for their helpful feedback in reviewing this paper.


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 Materials and Methods
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