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Jensen et al. in press
Limnology and Oceanography (06/17/2009)
Title: Phosphorus release with carbonate dissolution coupled to sulfide oxidation in Florida
Bay seagrass sediments
Henning S.Jensen,1,* Ole I. Nielsen,2 Marguerite S. Koch,2 Inmaculada de Vicente,1, 3
1 Institute of Biology, University of Southern Denmark, Odense, Denmark.
2 Aquatic Plant Ecology Lab, Biological Sciences Department, Florida Atlantic University, Boca
Raton, Florida.
3 Present address: Water Research Institute, University of Granada, Spain.
* Corresponding author: hsj@biology.sdu.dk.
Running head: Phosphorus release in seagrass sediment
2
ACKNOWLEDGMENTS
We thank Stephanie Schopmeyer and students in the Florida Atlantic University, Aquatic
Plant Ecology Lab for their assistance in sampling. The manuscript was greatly improved by
qualified reviews from K. J. McGlathery and one anonymous reviewer. The research was funded by
the Critical Ecosystem Studies Initiative (CESI) program of the U.S. Department of Interior (DOI)
through Everglades National Park (Cooperative Agreement H5000 02 0409, J5284 05 0047) and
supplemented by the Danish Natural Science Research Council grants 21030013, 21040034, and
21020463 and by the project OAPN (Organismo Autónomo Parques Nacionales) 129 A/2003.
Everglades National Park’s Florida Bay Interagency Science Center in Key Largo, Florida, and the
South Florida Water Management District, West Palm Beach, Florida, provided field logistical
support.
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Abstract
We hypothesized that CaCO3 dissolution, coupled to sulfide oxidation, is an important
mechanism by which solid-phase inorganic P (iP) becomes available to seagrass in tropical
carbonate sediments. To examine this supposition, we measured field sulfate reduction rates and
simulated the acidity (10-50 µmol H2SO4 cm-3 sediment) generated by subsequent sulfide oxidation
from high (western) and low (eastern) total P (TP) sediments in Florida Bay. Dissolution
experiments were conducted using sediment slurries at field pH (porewater pH ~6.5 to 7.5). While
CaCO3 dissolution (max 1.3% of sediment dry weight) was evidenced at all sites by leaching of
Ca2+ into slurries, at 3 of the 4 sites, PO43- was primarily recovered in the sediment exchangeable
pools, and at a lower P:Ca ratio than observed in source sediments. In contrast, no PO43- was
recovered from an eastern bay site with the lowest TP and finest-grained sediments, suggesting that
PO43- was either tightly adsorbed or incorporated into the carbonate matrix post-acidification. The
potential for tight PO43- re-sorption by sediment from the low-TP site was also supported by the
rapid rate of isotopic exchange of 33PO43- into the sediment matrix. These adsorptive and
incorporation processes may explain the low PO43- in porewaters in the eastern versus western
regions of Florida Bay, even upon dissolution of carbonates. Carbonate dissolution coupled to
sulfide oxidation could potentially provide 1.8 and 23.5 µmol iP m-2 d-1 at eastern and western bay
sites, respectively, meeting 5% of eastern and 29% of western bay seagrass (Thalassia testudinum)
P requirements.
4
Introduction
Seagrass primary production in carbonate environments is frequently limited by phosphorus
(P) availability (Short et al. 1990; Fourqurean et al. 1992a,b) resulting from low terrestrial P loads
and high sediment affinity for phosphate (PO43-) in these systems. Phosphorus in carbonate
sediments can be strongly bound to the solid phase carbonate matrix, e.g., as carbonate fluoroapatite
(CFA) and other P- and fluoride (F-)-bearing carbonate phases (Kitano et al. 1978; Rude and Aller
1991; Millero et al. 2001), and also adsorbed onto carbonate surfaces (Jensen et al. 1998; Koch et
al. 2001; Zhang et al. 2004). Jensen et al. (1998) observed that both inorganic P (iP) and organic P
were being mobilized from the solid-phase pools by dissolution of the carbonate matrix in the
rooting depth of the tropical seagrass Thalassia testudinum. The same constituents, F-, PO43- and
dissolved organic P, were observed to be elevated in seagrass porewater and released when
carbonate sediments were gradually dissolved using sequential chemical extraction. The study by
Jensen et al. (1998) did not identify any mechanism behind the dissolution process. Ku et al. (1999)
and Burdige and Zimmerman (2002) proposed that O2 released from T. testudinum roots enhances
sulfide oxidation, as well as aerobic mineralization of organic matter in seagrass sediment. As both
of these metabolic oxidation processes generate acidity, they have the potential to dissolve
carbonate sediments, and release sediment-bound P in close proximity to the roots (Burdige and
Zimmerman 2002). Acid generation in seagrass carbonate sediments has been implied by low pH
measured in sediments with high (mmol L-1) porewater sulfide levels in Florida Bay (Ku et al.
1999; Koch et al. 2007) and lower pH in seagrass beds than in bare sediments in the Bahamas
(Morse et al. 1987) and Florida Bay (Long et al. 2008). Another possible process that can release P
from carbonates is the exchangeability of iP with organic acids exuded from growing roots as
shown by Long et al. (2008). The release of organic acids may also stimulate aerobic microbial
metabolism or enhance the coupled sulfate reduction (SR) – sulfide oxidation.
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Sulfate reduction to H2S and reoxidation by O2 to SO42- is an important biogeochemical
redox process in Florida Bay that has a high potential for acid generation (2H+ generated per H2S
oxidized) and thus for CaCO3 dissolution. Ku et al. (1999) showed that the porewater SO42-:Cl- ratio
was nearly uniform in the upper 30 cm of vegetated sediment in Florida Bay, while the enriched
isotopic signature of 18O and 34S in SO42- implied a rapid recycling of sulfide generated by SR. At
the same time the Ca2+ concentration in the porewater was elevated and model calculations
suggested that carbonate dissolution was stoichiometrically coupled to sulfide oxidation by a 1:2
relationship between H2S oxidized and Ca2+ mobilized to the porewater, assuming formation of
HCO3- from CaCO3 dissolution. Thus, when O2 is transported to the root zone, SR and CaCO3
dissolution may be coupled and, in principle, all sulfide produced in iron-poor carbonate sediments
has the potential to be re-oxidized given sufficient oxygen availability (Ku et al. 1999). Using the
stoichiometry of 1:2, Burdige and Zimmerman (2002) calculated that published rates of net
carbonate dissolution of 4-11 mmol m-2 d-1 in sediments from Florida Bay (Rude and Aller 1991)
could be accounted for by sulfide oxidation, requiring only ~15% of the daily net O2 production of
T. testudinum. Lee and Dunton (2000) also showed that diurnal porewater sulfide varies in seagrass
(T. testudinum) sediments indicating that sufficient O2 can be provided during the day to re-oxidize
most of the sulfide generated at night via SR. Based on these studies, there is indirect evidence that
SR in seagrass beds, and subsequent sulfide oxidation and acid generation, may promote carbonate
dissolution (Walter et al. 2007). Hu and Burdige (2007) suggested that organic matter oxidation
(CO2 production), as enhanced by O2 release from roots, may be more important than sulfide
oxidation for generating the acidity that drives carbonate dissolution in sediments from seagrass
beds in the Bahamas. This CO2 from oxidative respiration dissolves and reacts with water to
produce carbonic acid (H2CO3) which readily dissociates to bicarbonate (HCO3-) and a hydrogen
ion (H+) producing acidity. In a recent modeling study, Burdige et al. (2008) simulated both
6
advective O2 transport and seagrass mediated O2 transport, and concluded that carbonate dissolution
is elevated in vegetated as compared to bare sediments due to enhanced metabolism mediated by
seagrass O2 flux. The different importance of the two biogeochemical reactions for acid formation
in the Bahamas and in Florida Bay can be partly due to the larger grain size of sediment in the
Bahamas site (200-800 µm, Burdige et al. 2008) as compared with the small grain size (<63 µm) of
the Florida Bay site and lower organic matter in highly oligotrophic sediments in the Bahamas
Islands. The larger grain size and lower O2 demand may facilitate porewater advection and O2
transport to an extent where SR is being suppressed.
Since sulfide accumulation is evident in porewater from Florida Bay sites (Walter et al
2007, Koch et al. 2007), and there is isotopic evidence that there is close coupling between SR and
sulfide oxidation (Ku et al. 1999), we focused the present study on the acidity generated from
sulfide oxidation and its effects on P availability. We hypothesized that H2SO4 generated from
sulfide oxidation is neutralized by carbonate dissolution with concomitant release of solid phase iP
and F- with modest changes in porewater pH. By adding H2SO4 to sediment slurries we examined
carbonate sediment dissolution at four sites in Florida Bay at pH levels comparable to those
observed in the field (6.6 – 6.9; Koch et al. 2007). We quantified PO43- mobilization to slurry water,
as well as resorption to surface-bound P pools following CaCO3 dissolution. Fluoride release was
also tracked post-acidification, as its release is considered an indicator of the breakdown of the
tightly bound P associated with the carbonate matrices (e.g., CFA; Jensen et al. 1998). Phosphate
re-adsorption was further elucidated by adding 33PO43- to sediment slurries and following the rates
at which they became homogeneously labeled in the exchangeable and matrix-bound P pools. By
measuring site-specific sulfate reduction rates (SRR) and assuming complete recycling of sulfide to
H2SO4 we calculated a potential iP mobilization rate for eastern and western sites from SR-
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oxidation coupling and compare this P flux to P release rates based on literature estimates of
carbonate dissolution.
Methods
Sampling sites
Florida Bay is a shallow (< 2 m) semi-enclosed estuarine lagoon at the southern tip of the
Florida peninsula (Fig. 1). The sediment is predominantly aragonite (50-60%) and high-Mg calcite
(30-40%) of biogenic origin, generated internally by primarily calcareous algae and in situ
precipitation and composed of a very low silicate fraction (Rude and Aller 1991; Lyons et al. 2004;
Walter et al. 2007) and low Fe (Chambers et al. 2001; Zhang et al. 2004). Extensive seagrass
meadows of the tropical seagrass T. testudinum dominate primary production in the bay decreasing
in biomass and primary production from southwest to northeast as a consequence of P availability
(Fourqurean and Zieman 2002). This trophic gradient is well established by sediment P
concentrations in the fine fraction of the sediment (Zhang et al. 2004) and in the bulk sediment
(Koch et al. 2001, this study), sediment P adsorption isotherms (Nielsen et al. 2006), and leaf C:P
ratios (Fourqurean et al. 1992a). Sites chosen for this study (Fig. 1) include three sites in the
western bay, Buchanan Bank (BB; 24o54’55” N; 80o45’24” W), Green Mangrove Key (GMK;
24o54’55” N; 80o45’24” W) and Rabbit Key (RK; 24o58’50” N; 80o50’11” W), high sediment P
sites, and two sites in the northeastern bay, Eagle Key (EK; 25o09’32” N; 80o34’58” W) and Black
Betsy Key (BLBK; 25o08’23” N; 80o38’51” W), low sediment P sites, all dominated by T.
testudinum, although, with varying shoot biomass of 97, 41, 226, and 153 g dry wt-1 m-2 at EK,
BLBK, BB, and RK, respectively (Koch et al. 2007; Rosch and Koch 2009). The more enriched
western sites are also characterized as having relatively higher porewater sulfide concentrations (4-6
8
mmol L-1) relative to eastern sites (≤ 2 mmol L-1) (Barber and Carlson 1993; Carlson et al. 1994;
Koch et al. 2007).
P mobilization experiment
Sediment (2-15 cm) was collected in seagrasses at BB, RK, BLBK, and EK sites via coring
(March 2005; 16 cm diameter cores) and subsequently sieved (2 mm) to remove large rhizomes and
roots.The wet sediment (20 g) was added to ambient seawater (80 mL) from the sites. The resulting
sediment slurries were placed on a shaker table and pre-incubated for 16 h prior to the start of the
experiment in order to obtain equilibrium between PO43- in slurry water and PO4
3- adsorbed on
particle surfaces. To examine sediment carbonate dissolution and subsequent release of iP as pH
was lowered through acidification, four different amounts of sulfuric acid: 0, 150, 400, and 800
µmol H2SO4 (~0-50 µmol cm-3 sediment; n=3) were added to sediment from each site. The
maximum H2SO4 addition was chosen so that the observed slurry water pH would represent the
lower values observed in the field. The required amount was found from pre-runs of the experiment.
Since the experimental setup could cause other changes as a result of oxidation of porewater
sulfide or stimulation of aerobic mineralization, the ‘zero addition’ treatment was applied as a
control in the acid addition experiments. Sulfuric acid additions to the carbonate sediments in slurry
generated CO2, as observed by gas bubble formation, suggesting that some CO32- was titrated to
CO2 rather than HCO3-. Under these conditions, 2H+ would be consumed per mole of CaCO3,
probably accounting for a 1:1 relationship between acid treatment and Ca2+ released (see results).
Seawater alkalinity was measured by titration and the acid consumed by changing alkalinity
in the 80 mL seawater was taken into account when calculating the effective acid addition per g dry
wt sediment. Release of PO43-, F-, and Ca2+, as well as pH changes in the slurries, were monitored
throughout the experiment at time 0, 1, and 6 h. The volumes removed by sampling were taken into
9
account when calculating fluxes. The pH of the slurry was measured directly, while PO43-, F-, and
Ca2+ were measured after filtering (GF/F) a subsample (15 mL) of the sediment-water slurry.
Phosphate was determined using standard colorimetric methods (Koroleff 1983) with a detection
limit of 20 nmol L-1. Inorganic fluoride was determined using a combined F- electrode (Orion 96-
09) with 1:1 addition of low-level total ionic strength adjusting buffer (LL TISAB). LL TISAB was
prepared by adding 57 g NaCl and 57 g glacial acetic acid to 550 ml DI water, adjusting pH to 5.65-
6.0 with strong NaOH, and making up volume to 1 L with DI water. The resolution at ambient
seawater F- concentration was ±1.8 μmol L-1. Dissolved Ca2+ was determined by flame atomic
absorbance spectroscopy (Perkin Elmer) also using standard methods. Due to constraints in
volumes available for analyses, we omitted the measurement of organic P (oP) leaching from the
sediment matrix, although, increments in dissolved oP may be an additional indicator of matrix
dissolution (Jensen et al. 1998).
After acid treatments, slurry water was decanted and sequential extractions were conducted
with the wet sediment to quantify P and F that had adsorbed onto carbonates and not recovered in
the dissolved phase. The extraction procedure was a modification of the first two steps of the
extraction scheme used for carbonate sediment by Jensen et al. (1998). In the first step, the sediment
was transferred into 60 mL centrifuge tubes to which 30 mL of 1 mol L-1 MgCl2 was added and put
on a shaker table for 1 h. After centrifugation and decanting, the sediment was washed in 30 mL 0.5
mol L-1 NaCl. The NaCl wash supernatant was added to the MgCl2 supernatant and brought up to
volume with H2O (100 mL). The second sequential step was a 1 h sediment extraction with a
Bicarbonate-Dithionite (BD) reagent (0.11 mol L-1 Na-dithionite and 0.11 mol L-1 NaHCO3)
followed by two BD washes and one NaCl wash. The wash supernatants were again brought to
volume with H2O (200 mL). The first extraction step extracts loosely adsorbed iP while the second
step extracts both Fe-bound and more tightly-bound iP on the surface of carbonates (Jensen et al.
10
1998). In siliclastic temperate sediments MgCl2 extracts loosely adsorbed surface bound iP with BD
extraction, considered highly specific for Fe-bound P (Ruttenberg 1992; Jensen and Thamdrup
1993). However, Jensen et al. (1998) showed that in carbonate sediment BD extracts both Fe and
surface-bound iP without concurrent release of Ca2+. Zhang et al. (2004) also found no correlation
between reactive Fe oxides and BD-extractable iP in the fine-grained fraction of Florida Bay
sediments. PO43- and F- concentrations in the extracts were measured as stated above using standard
curves prepared in the specific matrix extract solutions. While the sediment treatments were similar
on a sediment volume basis, differences in the bulk densities across sites resulted in slightly
variable amounts of MgCl2 and BD amendments per g dry wt sediment. Since eastern sites has
higher bulk densities (see results) this may cause a small, relative, underestimation of the size of
surface-bound P-pools at eastern bay sites.
Sediment characteristics
Porosity, dry:wet wt ratios and % of small grain size (<63 μm) were determined on sediment
from all sites. All parameters except porosity were determined on sediment that was passed through
a 2 mm mesh after removal of larger seagrass debris and seagrass tissue. Total Ca, F, and P
concentrations were quantified by combusting the dried sediment (550°C, 3 h) and subsequently
dissolving the pellet in 1 mol L-1 HCl (120°C, 1 hour). Dissolved Ca2+, F-, and PO43- were analyzed
as above with the exception of F- measurements which required neutralization (NaOH) prior to
analyses. The solid inorganic P fraction in the bulk sediment was determined by dissolving the
sediment in cold 0.5 mol L-1 HCl and analyzing the extract for PO43-.
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Sediment P-pool labeling experiment
We used a unique radiotracer technique to examine sediment P exchange in eastern fine-
grained P-poor sediment from EK and western coarser P-rich sediment from GMK. This technique
is based on the principle that 33PO43-, when added to sediment slurries in trace amounts under
conditions of equilibrium between surface adsorbed and slurry water 31PO43-, will undergo isotopic
exchange with any surface-bound or solid fraction 31PO43- reservoir. The exchange rates will depend
on the exchangeability potential of a particular iP pool (Jensen et al. 2005). After wet sediment (20
g) was kept in suspension on a shaker table for 16 h in P-free artificial seawater (100 mL), carrier-
free 33PO43- (~42,000 Bq) was added to the slurry. The activity of 33PO4
3- in the slurry water was
followed for 4 h, after which the distribution of the tracer in the exchangeable and matrix-bound
fractions (MgCl2-extractable, BD-extractable, and HCl-extractable P, respectively) was determined
by sequential extraction at time 4.66, 7.66, 33, and 55 h. 33PO43- in solution was measured on a
GF/F filtrate while solid pool tracer distribution was measured on the particles retained on the filter.
The experiments were run in triplicates. Radioactivity was measured on a Beckman LS 6500 Liquid
Scintillation Counter using standard techniques and correcting for background, decay and
quenching with known activity.
Sulfate reduction rates (SRR)
SRR were determined on seagrass sediment cores (diameter 3.5 cm; n=4) from EK, BLBK,
BB, and RK in October 2005 and July 2006 to relate dissolution of carbonates to potential SR and
coupled sulfide oxidation. Intact cores from each site were injected with 35SO42- (37,000 Bq in 2
µL) through silicon stopper holes at 2 cm intervals down to 14-18 cm depth (after Jørgensen 1978).
Cores were incubated under anoxic conditions for 2 h at in situ temperature (26oC October and 31oC
July). Post-incubations, cores were carefully sliced into 2 cm intervals and immediately transferred
12
into zinc acetate (10 mL of 1 mol L-1) for sulfide fixation. Subsequently, the sediment was
centrifuged, decanted and an additional 10 mL of zinc acetate (1 mol L-1) added in order to bind
remaining sulfides and allow removal of excess 35SO42-. Reduced 35S-pools were retrieved using a
two-step distillation method described by Fossing and Jørgensen (1989) and radioactivity was
immediately counted on a scintillation counter. SRR were calculated as the percent recovery of
radiolabelled sulfur in the acid volatile sulfide (AVS) and chromium reducible sulfide (CRS) pools.
Porewater sulfate was measured using a separate non-radioactive core at 2 cm intervals. Sediment
was transferred into centrifuge tubes and centrifuged to extract porewater that was stored frozen
until analysis of sulfate using ion-chromatography.
Statistics
Differences between PO43-, F-, and Ca2+ fluxes with acid treatments relative to control
treatments and release of adsorbed F- and PO43- in chemical extracts were compared using paired t-
tests. Linear regression analyses were used to determine slopes that defined the relationship
between acid addition and ion flux (Sigma Stat version 3.1). Significant differences are reported at
the p < 0.05 level unless otherwise stated.
Results
Sediment characteristics - solid phase
Total sediment phosphorus (TP) and inorganic solid phase phosphorus (iP) (Table 1) closely
followed the established trophic gradient in the bay, depicting greater P enrichment in the western
(TP = 4.40 ± 0.03 and 9.48 ± 0.25) than eastern (TP = 1.77 ± 0.06 and 2.22 ± 0.05) bay sediments.
The importance of the inorganic Ca-bound P pool is shown by this fraction (iP) representing
approximately 50±2% of the total P pool at all four sites (Table 1). Organic matter content,
13
measured as loss on ignition (LOI) also showed an east-west gradient with LOI = 7.8% and 7.6% at
eastern sites and 10.3% and 22.2% at western sites (Table 1). Total Ca concentration (Table 1)
reflect the LOI numbers, as Ca levels can be multiplied by the molar weight of 100 to give the
weight of CaCO3 in the dry sediment. The fact that the two values make up ~100% of dry weight at
all sites, confirms that insignificant amounts of other mineral phases than CaCO3 were present.
Total F- levels were similar across sites with the exception of BB where it was nearly two
times higher. The molar ratios of fluoride to calcium (Table 1) are indicative of sediment dominated
by easily dissolvable aragonite and high-Mg calcite in support of the supposition that dissolution
could be an important mechanism of P recycling in Florida Bay sediments. Carbonate dissolution at
western bay sites has the potential to release greater amounts of sediment P than eastern bay sites
based on a 4-fold higher iP:Ca ratio and an almost 3-fold higher iP concentration in western
compared to eastern bay sediments. The grain size distribution showed that EK had the largest
fraction (83%) of small particles (<63 µm), while this fraction made up 69%, 63%, and 59% at
BLBK, BB, and RK, respectively (Table 1).
Sulfuric acid addition experiment
After 6 h post-acid treatment, pH levels stabilized at all sites to approximately 6.6 to 7.1. As
pH dropped in response to sediment acidification, Ca2+, F-, and PO43- were released. A portion of
the F- and PO43- recovered was in the seawater slurry water (Fig. 2), while another portion was
recovered from the adsorbed pool with MgCl2 and BD extractions (Table 2). The proportionality of
dissolved versus re-adsorbed F- and PO43- was very site specific.
The CaCO3 dissolution, calculated from the increase of Ca2+ in slurry water, amounted to
1.3% (maximum) of the sediment solid phase pool in response to the acid treatments simulating
field pH conditions (Table 2, Fig. 2). Despite Ca2+ mobilization with acid treatments were evident at
14
all sites, PO43- dissolved into the slurry water was only significantly different from controls at
Rabbit Key (Fig. 2, Table 2), the site with the highest sediment TP and iP in the bulk sediment
(Table 1). Sequential extraction of the surface bound iP pools of acidified sediment was necessary
to recover a significant amount of the dissolved iP at several of the sites. These data indicate that a
portion of the mobilized iP became surface bound, as either the loosely adsorbed MgCl2 fraction or
the more strongly adsorbed BD fraction (Table 2). A significant increase in extractable iP in one or
both of the surface-bound pools was found at RK, BB, and BLBK (Table 2). Only at EK were the
changes in surface bound iP insignificant across all acidification treatments and were not recovered
in the slurry water or through sequential extractions with MgCl2 and BD (Table 2).
If we examine the total amount of iP extracted as dissolved and recovered as surface
adsorbed relative to controls, there was significantly more P recovered in western bay sediments
(Table 2) with higher total sediment P and iP in the bulk sediment (Table 1). Fluoride mobilization
to the slurry water with acidification followed the same trend as observed for PO43- with the greatest
F- release at the western sites, RK and BB, and with a significant flux also at BLBK, relative to
controls (Table 2). F- was also mobilized to both the slurry water and recaptured as a surface-bound
pool on the carbonate sediment. While no significant release of F- to the slurry water was found at
EK, all four stations had significant F- recovery in the adsorbed exchangeable pools (Table 2).
The amount of iP dissolved versus the sulfuric acid added correlated positively for RK (p <
0.05; R2 = 0.74) and BLBK (p < 0.05; R2 = 0.44) while there was only a tendency for a positive
relationship at BB (p = 0.08; R2 = 0.33). No iP mobilization was observed for sediments collected
from EK (Fig. 3). For the three sites with observed iP mobilization, PO43- flux as a linear function
of H2SO4 amended was 0.058, 0.105, and 0.359 nmol iP per µmol H2SO4 at BLBK, BB, and RK,
respectively, the same ranking of sites as for iP concentration in the bulk sediment (Table 1). In
general, the amount of Ca2+ and F- dissolved and the sulfuric acid concentrations added showed a
15
strong correlation among sites (Fig. 3). The response of F- to acid treatment was similar at EK,
BLBK and RK sites with slopes of 1.23, 1.37, and 1.25 nmol F- per µmol H2SO4, respectively. In
contrast, BB had a higher slope of 1.61, in correspondence with the higher concentration of F in the
bulk sediment at this site (Table 1). Leaching of Ca2+ with acid treatment did not differ significantly
among sites but there was a tendency for less Ca2+ to be mobilized from eastern (slopes of 0.77 and
0.84 µmol Ca2+ per µmol H2SO4 at EK, and BLBK, respectively) than western sites (slopes of 0.93
and 0.97 µmol Ca2+ per µmol H2SO4 at BB, and RK, respectively). This observation may indicate
that western bay sediments are more easily dissolved than eastern bay sediments. The near 1:1
relationship between Ca2+ leached and H2SO4 added points toward an approximate equimolar
reaction between sulfuric acid and carbonate sediment mineralization (see methods).
The molar ratio between F- and Ca2+ dissolved upon acidification (Table 3) was significantly
lower than F:Ca ratios of the bulk sediment (Table 1), indicating preferential dissolution of a F--
poor carbonate fraction applying modest acid treatments that achieved field pH conditions.
Likewise, less iP was mobilized and subsequently recovered relative to Ca2+ mobilization (Table 3)
than predicted from the bulk sediment iP and Ca distribution among sites (Table 1). This either
indicates dissolution of a relatively P-poor carbonate fraction, or more likely, that a large re-
adsorption and incorporation into solid phase P pools occurred that was not recovered by the
sequential extraction scheme applied. Re-adsorption was less pronounced for F- than for PO43-.
Nevertheless, P:F ratios in the leachates (including slurry water and adsorbed pools) were 65% and
115% higher than in the bulk sediment at the two western sites (BB and RK, respectively), while
similar to the ratio in bulk sediment at BLBK.
16
Tracer experiment
33PO43- amended to sediment slurries rapidly adsorbed to the carbonate particles with 0.35%
and 1.1% left in the water after 60 min at EK and GMK, respectively (Fig. 4). At the eastern bay
site (EK), 33PO43- was distributed in proportion with the cold phosphate after 4 h with 25% found in
the surface-adsorbed P pools, 2.6% in the MgCl2 and 22.4% in the BD extractions, and 75% within
the carbonate matrix (Table 4). In the western bay at GMK, 33PO43- was primarily found in the
surface-bound pools from where it only slowly re-distributed into the HCl-extractable carbonate
matrix pool. Even after 55 h, 53% of the 33PO43- was recovered in the surface adsorbed pools which
only contributed 25% of the solid fraction iP. Despite the difference in rate of incorporation and
despite the clear gradient in sediment iP concentration (Table 1) the proportion between different iP
pools in the sediment, as evaluated by sequential chemical extraction, appeared quite similar in the
eastern EK and the western GMK sediment (Table 4). The results suggest that both 31PO43-
adsorption and the subsequent incorporation into the solid matrix will proceed considerably faster at
EK than at GMK, indicating a very different rate of solid phase iP turnover at eastern versus
western bay sediments.
Sulfate reduction rates
The highest SRR (up to 800 nmol cm-3 d-1) were measured in the western bay sediments,
and in the upper 0-2 cm of sediment (Fig. 5). Depth integrated SRR in the top 14 cm were 2-fold
higher at RK (45.4 ± 18.1 in July and 85.4 ± 19.4 mmol m-2 d-1 in October) compared to BLBK and
EK, while BB had moderate SRR rates, falling between rates measured at RK and the eastern sites
(Table 5). The different SRR among sites resembled the pattern of LOI (Tables 5 and 1). Only at
Rabbit Key did we observe different SRR during the two sampling occasions with rates in the fall
almost twice rates in July, although high variance was also observed (Fig. 5; Table 5). The higher
17
autumn rates could be due to higher DOC availability from senescence of T. testudinum leaves and
a reduction in shoot density (Koch et al. 2007, Rosch and Koch 2009).
Discussion
Our results show that carbonate dissolution caused by sulfuric acid production can mobilize
significant amounts of PO43- at near neutral pH, one mechanism by which seagrass growing in
carbonate sediments can access P bound in the solid matrix. Recycling of sulfide to sulfuric acid
through O2 mediated oxidation, and its coupling to carbonate dissolution, has been established in
seagrass sediments of Florida Bay (Ku et al. 1999; Walter et al. 2007). When sulfide oxidation is
coupled to O2 released from seagrass roots, the iP released is in close proximity of roots. Other
processes like bioturbation and porewater advection (Burdige et al. 2008) can also transport O2
down into the sediment and potentially cause sulfide oxidation. However, the oxidation-reduction
biogeochemical processing around the root zone of seagrass may be the most active because of
concomitant root exudation of organic acids that can stimulate SR, or potentially compete with
PO43- for sorption sites on sediment surfaces (Long et al. 2008). Our study also indicates that
mobilization of carbonate-bound P through dissolution, and its subsequent availability to porewater
and seagrass, is not solely a function of the P:Ca ratio in the bulk sediment. Rather, the capacity of
the sediment to re-sorb PO43- can modify the net mobilization of P during dissolution and control iP
availability in the root zone. We evaluate below the potential magnitude of iP mobilization coupled
with SR and subsequent sulfide oxidation, and provide estimates of its significance for seagrass
nutrition in Florida Bay.
18
Potential rate of P mobilization by sulfuric acid formation
Under our H2SO4 treatments, pH values dropped in the sediment slurry to 6.6-6.8, on par
with those found in porewaters of Florida Bay (6.6-6.9; Koch et al. 2007; Walter and Burton 1990).
These results indicate that the acidification levels generated in this study by H2SO4 represent those
that can occur over time in the bay generated in part by SR and reoxidation of sulfides. As a first
approximation, we estimate potential iP dissolution rates in Florida Bay carbonate sediments based
on sulfuric acid formation applying our field SRR and assuming unity between SR and sulfide
oxidation. Our SRR determined on cores collected from seagrass sites in the field give us values of
187-610 nmol cm-3 d-1. These data compare to those determined by Walter et al. (2007) for
sediments of mudbank and island fringe sites in central Florida Bay (70-579 nmol cm-3 d-1) and
reported by Holmer et al. (2006) for tropical seagrass sediments in East Asia. Using our SRR, with
the assumption that all sulfides generated from SR are re-oxidized in a 1:1 ratio, sulfide oxidation
would contribute the equivalent of 1.4±0.66 and 0.27±0.09 µmol H2SO4 g dry wt-1 d-1 at western
and eastern bay sites, respectively.
Applying the potential daily acid generation with experimental results on PO43- release with
acidification in this study, a maximum of 23.5 and 1.8 µmol P m-2 d-1 would be regenerated at the
western RK and eastern BLBK sites, respectively (Table 6a). These iP mobilization rates are within
the range calculated from literature values for Ca2+ and F- mobilization in Florida Bay (Table 6).
Since we mobilized Ca2+ with a 1:1 ratio between H2SO4 addition and Ca2+ flux, where the ratio
theoretically should be closer to 1:2, we suggest that the range given in Table 6a is a conservative
estimate of the potential effect that sulfide oxidation might have on iP mobilization. We reach
similar rates of iP mobilization using the gross carbonate dissolution rates estimated by Rude and
Aller (1991) in Florida Bay (Bob Allen Key Bank) applying our iP:Ca efflux ratios (Table 6b).
These estimates increase 2-fold when a 2:1 molar ratio between carbonate dissolution and sulfuric
19
acid formation (Ku et al. 1999) is assumed, or if we apply the calculated F- efflux from Rude and
Aller (1991) with our observed site specific iP:F efflux ratios (Table 6c and 6d). A slightly more
conservative estimate for iP mobilization rate is calculated when using the lower carbonate
dissolution rate reported by Walter and Burton (1990) and our site specific iP:Ca efflux ratios
(Table 6e). Averaging (±SE) the estimates presented in Table 6 give us an iP turnover rate of
3.5±1.0, 6.8±1.8, and 28.9±8.4 µmol P m-2 d-1 from BLBK, BB, and RK, respectively. These iP
turnover rates are site specific with an increase at sites with high sediment TP and seagrass
productivity in Florida Bay.
Seagrass P nutrition from potential iP dissolution
We calculated potential seagrass P nutrition attainable from carbonate dissolution by
applying the estimates of iP mobilization in Table 6 and site specific T. testudinum total P
production in leaves, the tissue accounting for the greatest allocation and turnover of nutrients in
this seagrass species. Annual average (±SE) T. testudinum leaf P requirements at BLBK and RK are
34±8 and 80±18 µmol P m-2 d-1, (Rosch and Koch 2009) comparable to estimates for the bay as a
whole (70 µmol P m-2 d-1; Fourqurean 1992a), but clearly showing the higher biomass production at
RK (3.3 g dry wt m-2 d-1) than BLBK (1.0 g dry wt m-2 d-1) during summer months when leaf
elongation rates are maximal (Koch et al. 2007). Based on the leaf TP requirements and average
estimates for iP mobilization from carbonate dissolution (Table 6), we estimate approximately 38%
of the P requirements can be met by iP from carbonate sediment dissolution at RK, while only 10%
at BLBK, clearly indicating that rapid recycling of this solid-phase pool probably assists in
promoting greater seagrass biomass development at western Florida Bay sites.
Our measured SRR also provide an estimate of P mobilization from organic matter
mineralization using the C:P molar ratio of belowground tissue at RK and BLBK (~2290 and 2960,
20
respectively; Rosch and Koch 2009). Applying a respiration quotient of 2 for SR (1 mol SO42-
reduced per 2 mol C oxidized), P recycled from organic matter mineralization would be
approximately 18 µmol P m-2 d-1 at BLBK and 57 µmol P m-2 d-1 at RK. Thus, based on our results,
solid phase inorganic P cycling is 19% and 51% of the organic P recycling generated from SR at
BLBK, and RK. Combining this inorganic and organic P turnover, we estimate that only 64% of the
P requirements for T. testudinum is being met by iP dissolution and SR at the eastern bay BLBK
site, while in contrast, 100 percent (108%) of the P needs are calculated to be satisfied in the
western bay at RK. However, this dissolution effect and iP release can be significantly affected by
the sediment potential for PO43- sorption, which is also site specific.
Phosphate resorption and incorporation by carbonates
Independent of the mechanism that may cause dissolution of the solid carbonate matrix, the
PO43- flux to the porewater is modified by resorption processes. Eagle Key sediments in the
northeastern bay, the site with the lowest sediment iP and the largest fraction of fine particles, was
found to strongly bind iP, even to the point where no PO43- recovery was made from the dissolved
or adsorbed P pools after treatment with 53 µmol H2SO4 g dry wt-1. This result can be explained in
two ways. One, P was not released with acid dissolution, which appears unlikely based on the fact
that CaCO3 was dissolved, or two, P was rapidly (< 1 h) re-adsorbed and incorporated into a non-
exchangeable P pool.
In a previous experiment by the authors on PO43- adsorption-desorption equilibrium (Nielsen
et al. 2006), the crossover concentration (Ce0) where porewater PO43- is in equilibrium with
adsorbed pools and can no longer be taken up, was extremely low (0.017 µmol L-1) at EK. These
results indicate a high capacity for the eastern bay sediments to resorb PO43-, even at low
concentrations. On the contrary, at a western bay site (GMK), Ce0 was 10-times higher (0.292 µmol
21
L-1), suggesting a higher porewater PO43- equilibrium is maintained at this site and higher inorganic
P availability to seagrass roots embedded in the sediment compared to the EK site. Small carbonate
grain sizes (<10 µm) have been shown to be important in P adsorption (Suess 1973) and Florida
Bay sediments have been defined as having high surface energies resulting from small particle size
(Walter et al. 2007). One mechanism behind the high PO43- affinity at EK could be the large
proportion of fine particles, but it is also possible that the degree of iP saturation on and in the
carbonate grains modified PO43- affinity. McGlathery et al. (1994) observed a slower rate of PO4
3-
adsorption (6 h experiment) in more P-enriched carbonate sediments from Bermuda without
detectable differences in grain-size distribution. For Florida Bay such a mechanism would imply
that eastern bay sediment, low in TP, would have higher affinity for PO43- than western bay
sediment, given the same grain size. In the McGlathery et al. (1994) study, however, PO43- was
amended at high concentrations and the median grain size was > 200 µm in the Bermuda sediments.
In contrast, the Nielsen et al. (2006) study in Florida Bay examined affinity at low field porewater
PO43- concentrations with median grain sizes < 63 µm. Thus, the role of grain size cannot be ruled
out as a potential influence on sediment P adsorption contrasting finer sediments in the eastern
compared to coarse grains in the western regions of the bay, although these site differences are
confounded by a P enrichment gradient making it difficult to distinguish between these two effects
on PO43- adsorption.
The potential for tight resorption of iP released from EK sites with fine grains is also
supported by the rapid rate of isotopic exchange of 33PO43- into the HCl-extractable iP pool at this
site. Thus, while the apportionment of iP in the various pools in the bay are more similar based on
31P (Zhang et al. 2004, this study), as revealed through traditional low pH extractions (0.5 mol L-1
HCl), the rates of incorporation and binding efficiency (Nielsen et al. 2006) is quite variable, with
stronger binding of iP in the finer, low TP eastern bay sediments. The fluxes of PO43- to the
22
porewater from carbonate dissolution and re-mineralization may be counteracted by re-adsorption
onto carbonates and/or incorporation into the sediment matrix. Incorporation would be driven either
by diffusion (Froelich 1988) or by re-crystallization (Rude and Aller 1991; Hu and Burdige 2007,
Walter et al. 2007). These adsorptive and incorporation processes may explain the low PO43- in
porewaters in the eastern versus western regions of the bay (Fourqurean et al. 1992b), and the
inability to recover PO43- from EK sediments in this study, even with a clear dissolution of
carbonates.
Because of the importance of resorption of PO43- by carbonates, porewater PO4
3-
concentrations, rather than TP pools, are probably a good proxy for P availability for seagrass
nutrition. This idea is partially supported by the positive correlation between porewater PO43- and
tissue P concentration (Fourqurean et al. 1992a, b), and by PO43- adsorption experiments (Nielsen et
al. 2006) that imply P-limitation of seagrasses in eastern Florida Bay, resulting from low porewater
PO43- concentrations. Also, T. testudinum root PO4
3- uptake kinetics responds linearly as a function
of porewater PO43- concentration in the low ranges found in the bay; and although this uptake can
sequester phosphate down to ~0.030 µmol L-1, at these low concentrations, the plants cannot meet
their P requirements (Nielsen et al. 2006). As porewater PO43- in tropical carbonate seagrass
sediments has a rapid turnover rate of ~2 days (Jensen et al. 1998), it can be reasoned that the
loosely adsorbed and solid phase iP pools, with several orders of magnitude higher P levels than
porewater, are critical in supporting highly productive tropical seagrasses. We contend that this
turnover is driven primarily by dissolution processes associated with O2 enhanced acid formation.
Phosphate availability may be further enhanced by root exudation of organic acids.
In laboratory studies Long et al. (2008) observed that moderate addition of di- and trivalent
organic acids, shown to be released by the roots of T. testudinum, caused significant PO43- leaching
without concomitant F- leaching. No effect was found with HCl at the same acid strength of the
23
organic acid exudates measured in the field. Since this acidity from organic acids added by Long et
al. (2008) to Florida Bay sediments was approximately an order of magnitude lower than in our
experiment herein applying H2SO4, the results suggest that the organic acids leached at the root tips
affected the equilibrium between porewater PO43- and the surface-bound iP pool. However, these
organic acids appeared to generate inadequate acidity to dissolve the solid-phase iP carbonate pool,
again evidenced by a lack of F- mobilization. Indirectly, organic acid exudates can stimulate
carbonate dissolution by enhancing acid generation from oxidative mineralization processes. This is
supported by field observations of increased P and F- along with high concentrations of organic
acids in the root zone of T. testudinum (Long et al. 2008).
Earlier studies (Jensen et al. 1998, Nielsen et al. 2006) provided strong indications that the
loosely adsorbed and solid phase iP pools are potentially critical in supporting highly productive
tropical seagrass in carbonate environments where P availability is low. Here we forward the
supposition that this solid phase iP pool is turned over during the dissolution of carbonate minerals
that occur when sulfuric acid from sulfide oxidation is buffered at near neutral pH. We also show
that subsequent to dissolution, P availability to seagrass roots may be highly dependent on sediment
adsorptive capacities, which are controlled by sediment P loads and characteristics at a particular
site (e.g., grain size). Future research is needed to more clearly define how site differences in
sediment characteristics, nutrient loads, and seagrass plant productivity influence O2 flux into the
sediments (Burdige et al. 2008), and sulfide oxidation, as well as di- or trivalent organic acids,
potential to drive biogeochemical processes and/or assist tropical seagrass in accessing the surface
adsorbed PO43-.
24
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29
Table 1. Sediment total calcium (Ca), fluoride (F), and inorganic (iP), and total P (TP), molar ratios of F to Ca and iP to Ca and F, and bulk
density (bd), porosity, and % sediment < 60 µm at Eagle Key (EK), Black Betsy Key (BLBK), Buchanan Bank (BB), Rabbit Key (RK), and
Green Mangrove Key (GMK) in Florida Bay. Elemental and ratio values represent means ± SD (n=3). Sediment bd and porosity are depth
averages (0-14 cm). Not all parameters were measured for GMK.
(mmol g dry wt-1) (μmol g dry wt-1) Molar ratios (x 10-3) g cm-3 %
__________________________________ ___________________ _______________
Site Ca F iP TP F:Ca iP:Ca iP:F bd porosity <63 µm LOI
EK 9.23 28.8 ± 1.3 0.81 ± 0.02 1.77 ± 0.06 3.11 0.09 28 0.70 0.73 83 7.8 ± 0.03
BLBK 9.29 31.5 ± 3.1 1.12 ± 0.02 2.22 ± 0.05 3.39 0.12 36 0.91 0.66 69 7.6 ± 0.07
BB 8.90 58.4 ± 6.3 2.35 ± 0.03 4.40 ± 0.03 6.57 0.26 40 0.34 0.84 63 10.3 ± 0.15
RK 7.80 35.6 ± 3.9 4.66 ± 0.16 9.48 ± 0.25 4.56 0.60 131 0.25 0.89 59 22.2 ± 0.07
GMK 8.87 1.69 ± 0.13 3.44 ± 0.17 0.19 0.42 0.82 46 12.2 ± 0.18
30
Table 2. Inorganic phosphorus (ΔPO43-), fluoride (ΔF-), and calcium (ΔCa2+) mobilized in seawater (SW) after acidification treatments (150,
400, and 800 µmol H2SO4) and ΔiP and ΔF- re-adsorbed but recovered in specific sequential extracts with MgCl2 (loosely adsorbed) and BD
(strongly adsorbed). Values are represented as changes in concentration compared to control treatments with no acid dissolution; (*) represent
significant changes (p < 0.05). Σ, in bold, represents the sum of increments in iP or F- in the three sequential fractions (SW, MgCl2, and BD).
Calcium was only measured in slurry water.
150 µmol H2SO4 400 µmol H2SO4 800 µmol H2SO4
_____________________________________________ ________________________________________________ ___________________________________________________
Site SW MgCl BD Σ SW MgCl BD Σ SW MgCl BD Σ
ΔiP (nmol g dry wt-1)
EK 0.04 0.08 10.47 10.60 0.43 0.07 -16.50 -16.00 0.08 0.09 -9.96 -9.79
BLBK -0.08 -0.60 1.66* 0.98* -0.16 -0.07 2.26 2.03 -0.16 -0.01 4.46* 4.29*
BB -0.62 0.28 3.43 3.10 -0.43 1.33* 8.78 9.67 0.65 2.59* 8.78 12.13
RK 0.07 1.94 34.66* 36.66* 2.29* 5.26 48.62* 56.17* 12.10* 22.11 48.67* 82.87*
31
ΔF- (nmol g dry wt-1)
EK -0.07 7.48 12.72* 20.13 10.84 6.54 28.23 45.61* 13.66 9.06 58.52* 81.23*
BLBK 2.91 2.63 -17.01 -11.47 9.16 7.95 4.33 21.44 21.81* 12.26* 19.26 53.34
BB 17.15 0.64 18.31 36.10 43.86* 2.93 39.54 86.33* 93.83* 8.16* 63.40* 165.39
RK 34.19 0.32 -22.52 11.98 104.28* 16.61* -38.19 82.69* 213.63* 27.17* -72.89 167.91
ΔCa2+ (μmol g dry wt-1)
EK 7.6* 31.0* 50.6*
BLBK 16.9* 28.9* 54.9*
BB 19.4 48.6 94.1
RK 29.6 70.9 152.8
32
Table 3. Ratios between iP, F-, and Ca2+ mobilized
after sulfuric acid additions to carbonate sediments
from Eagle Key (EK), Black Betsy key (BLBK),
Buchanan Bank (BB), and Rabbit Key (RK). The
ratios are calculated from the slopes generated from
Fig. 3.
Atomic ratios (x 10-3)
_______________________
F:Ca iP:Ca P:F
EK 1.45 ns ns
BLBK 1.78 0.07 41
BB 1.71 0.11 66
RK 1.29 0.37 282
No significant (ns) iP mobilization was found at Eagle Key.
33
Table 4. Distribution of 33P (%) in various P-pools as a function of time following addition of trace
amounts of 33PO43- to sediment slurries from the western Florida Bay (Green Mangrove Key;
GMK) and the northeastern Florida Bay (Eagle Key; EK) sites. The percentage distribution of
31PO43- in the sediments is also shown. Values represent mean ± SD (n=3).
Relative 33P distribution (%) 31P dist (%)
______________________________________________ __________
4.66 h 7.66 h 33 h 55 h
GMK MgCl2 26.8 ± 2.9 22.8 ± 3.7 16.1 ± 1.7 16.1 ± 0.4 5.4 ± 1.2
BD 42.7 ± 1.3 44.4 ± 2.2 39.5 ± 0.9 36.9 ± 0.2 20.1 ± 0.9
HCl 30.5 ± 1.6 32.8 ± 1.5 44.4 ± 2.6 47.0 ± 0.2 74.5 ± 2.0
EK MgCl2 3.8 ± 0.0 3.5 ± 0.1 2.3 ± 0.1 2.6 ± 0.2 5.7 ± 0.4
BD 22.8 ± 0.3 24.2 ± 0.2 20.6 ± 1.3 22.9 ± 1.9 17.1 ± 1.8
HCl 73.5 ± 0.3 72.1 ± 0.3 77.1 ± 1.2 74.5 ± 1.8 77.2 ± 1.7
34
Table 5. Seasonal variations in sulfate reduction rates (SRR) integrated to
14 cm depth and mean SRR normalized to sediment volume in seagrass
sediments from Eagle Key (EK), Black Betsy Key (BLBK), Buchanan
Bank (BB), and Rabbit Key (RK) in Florida Bay. Values are mean ± SD
(n=3 cores).
ΣSRR0-14 cm SRR
Site Date (mmol m-2 d-1) (nmol cm-3 d-1)
EK Oct 2005 34.9 ± 3.1 249.3 ± 21.9
Jul 2006 30.3 ± 4.3 216.1 ± 23.2
BLBK Oct 2005 26.2 ± 4.6 187.3 ± 33.1
Jul 2006 28.0 ± 5.5 199.6 ± 26.4
BB Oct 2005 42.7 ± 3.6 304.8 ± 95.2
Jul 2006 50.7 ± 14.6 362.5 ± 104.5
RK Oct 2005 85.4 ± 19.4 610.3 ± 138.8
Jul 2006 45.4 ± 18.1 324.2 ± 129.2
35
Table 6. Potential inorganic solid-phase P (iP) mobilization
rates in Florida Bay (FB) carbonate sediments estimated by
sulfate reduction rates and equivalent acid generation
assuming a (a) 1:1 molar ratio (Fig. 3a) and (b) a 1:2 molar
ratio (Ku at al. 1999) of H2SO4 produced to Ca2+ dissolved,
literature estimates of CaCO3 dissolution in FB sediments
applying our ratios of iP:Ca and iP:F mobilization (Table
3), based on model predicted (Rude and Aller 1991) gross
(c) Ca2+ mobilization of 53 mmol m-2 d-1, (d) F- efflux of
170 µmol m-2 d-1, and (e) based on CaCO3 dissolution rates
of 13.7 mmol m-2 d-1 (Walter and Burton 1990).
iP mobilization estimates*
(µmol P m-2 d-1)
Black Betsy Key Buchanan Bank Rabbit Key
1.8a 5.0a 23.5a
3.8b 10.3b 48.4b
3.7c 5.8c 19.6c
7.0d 11.2d 47.9d
1.0e 1.5e 5.1e
*No estimates were made for Eagle Key because of a lack
of P recovery (dissolved and exchanged) after sediment
acidification.
36
Figure Legends
Figure 1. Study sites in Florida Bay, a semi-enclosed lagoonal estuary at the southern terminus of
the Florida Peninsula, bounded to the north by the Everglades, to the south by the Florida Keys and
Gulf of Mexico to the west. Sediment for SRR and dissolution experiments were collected from
eastern bay sites: Eagle Key (EK) and Black Betsy Key (BLBK) and western bay sites: Rabbit Key
(RK) and Buchanan Bank (BB). Radiotracer experiment with 33PO43- was conducted with sediment
from EK and Green Mangrove Key (GMK). In the bay numerous mangrove islands (dark grey) and
mudbanks (light grey) are noted.
Figure 2. pH changes and efflux of phosphate (PO43-), fluoride (F-), and Ca2+ into the seawater
sediment slurry (dissolved phase) at three different acidification treatments (see legend) and control
over time in the 6 h incubation (n=3).
Figure 3. Relationship between sulfuric acid additions and release of (a) inorganic P (PO43-), (b)
fluoride (F-), and (c) calcium (Ca2+), including both the seawater sediment slurry (dissolved ions)
and the exchangeable (MgCl2 and BD extractable) pools. All fluxes and acid additions are
calculated per g dry wt-1 sediment and the acid treatment is corrected for the acid consumption by
the alkalinity of the added 80 mL seawater.
Figure 4. Changes in the 33P radioactivity over time in the aqueous phase of the sediment slurry,
using sediment from Eagle Key and Green Mangrove Key, eastern and western bay sediments,
respectively.
37
Figure 5. Depth profiles of microbial sulfate reduction rates incubated at in situ temperatures in
intact sediment cores October 2005 (27oC) and July 2006 (31oC) from western (Rabbit Key and
Buchanan Bank) and eastern (Black Betsy Key and Eagle Key) Florida Bay.
38
Figure. 1
Florida Bay
BB
EK
Everglades National Park
25o 0
7′00″N
24o 5
5′20″N
10 km
GMK
North
BLBK
RK
Gulf of Mexico
81o0′0″W 80o30′0″W
39
Figure 2.
6,6
6,8
7,0
7,2
7,4
7,6
7,8
Eagle KeyBuchanan BankRabbit Key
pH
Control 150 μmol H2SO4 400 μmol H2SO4 800 μmol H2SO4
Black Betsy Key
0,0
0,5
1,0
1,5
2,0
iP (μ
mol
L-1)
30
40
50
60
70
F- (μm
ol L
-1)
0 1 2 3 4 5 6 71,0x104
1,5x104
2,0x104
2,5x104
3,0x104
3,5x104
Ca
(μm
ol L
)
Time (h)
0 1 2 3 4 5 6 7
Time (h)
0 1 2 3 4 5 6 7
Time (h)0 1 2 3 4 5 6
Time (h)
PO43-
(μm
olL-1
)
7.87.6
7.27.4
7.06.86.6
2.0
1.5
1.0
0.5
0.0
15
20
25
3035
Ca2+
(mm
olL-1
)
30
40
50
60
70
F- (μm
olL-1
)
control150 μmol H2SO4400 μmol H2SO4800 μmol H2SO4
pH
Rabbit Key Buchanan Bank Black Betsy Key Eagle Key
Time (h)1 2 3 4 5 6
101 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
40
Figure 3.
0 20 40 60 80 100 120 140
0.00
0.02
0.04
0.06
0.08
0.10
0.12
[iP] (
μmol
g d
w)
[H2SO4] (μmol g dw-1)
a
PO43-
(μm
olg
dry
wt-1
)
0 20 40 60 80 100 120 140
0.00
0.05
0.10
0.15
0.20
0.25
[F] (
μmol
g d
w-1)
[H2SO4] (μmol g dw-1)
b
F- (μm
ol g
dry
wt-1
)
0 20 40 60 80 100 120 1400
25
50
75
100
125
150
175 Rabbit Key Buchanan Bank Black Betsy Key Eagle Key
[Ca]
(μm
ol g
dw
-1)
[H2SO4] (μmol g dw-1)
Ca2+
(μm
ol g
dry
wt-1
)
c
(μmol g dry wt-1)
41
Figure 4.
0 50 100 150 200 2500.0
1.0x103
2.0x103
3.0x103
4.1x104
4.2x104
4.3x104
Slur
ry w
ater
33P
(Bq)
Time (min)
Eagle Key Green Mangrove Key
42
Figure 5.
BB
October 2005 July 2006
RK
BLBK EK
0
2
4
6
8
10
12
14
16
18
0
2
4
6
8
10
12
14
16
18
20
Dep
th (c
m) Oct 05
Jul 06
250 750 1250 250 1250750SRR (nmol cm-3 d-1)
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