SUPPLEMENTARY MATERIAL FOR
Phosphorus mobilization at the sediment-water interface in softwater Shield lakes: the
role of organic carbon and metal oxyhydroxides.
M. Lavoie and J.-C. Auclair
Eight supplementary tables and six figures
1
Table of Contents – Supplementary Material
1. Description of the study site and bathymetric map ........................................................ 6
Figure SM1: The location of the study site and sampling stations: Central (CS), Echo
Bay (EB) and South Basin (SB) stations. Bathymetric map provided by (Tremblay et
al., 2001). ........................................................................................................................ 7
2. Surface complexation modeling of elements adsorbed onto iron and manganese
oxides. ................................................................................................................................. 8
3. Chemical composition of water overlying sediments at the three stations. ................... 9
Table SM1: Anions and cations measured in water above the sediment-water interface
at the three sites measured on 17 September 2008. ........................................................ 9
4. Modeling adsorption of organic carbon on iron oxyhydroxides by using Langmuir
isotherms........................................................................................................................... 10
5. Intrinsic surface complexation constants used for estimating the adsorption of various
anions and cations on iron oxyhydroxides ....................................................................... 13
Table SM2: Intrinsic surface complexation constants (Log Kint) calculated
experimentally (EXP) or by linear free-energy relationships (LFER) for weak (w) and
strong (s) site type used for estimating the adsorption of various anions and cations on
iron oxyhydroxides by using the two layer surface complexation model (values from
(Dzombak and Morel, 1990)). The constants were derived from ferrihydrite and
assume a molecular weight of 89 g FeOOH mol-1, a specific surface area of 600 m2 g-1,
a concentration of weak and strong sites of 0.2 and 5 x 10-3 mol mol-1 of iron
oxyhydroxides respectively .......................................................................................... 13
6. Intrinsic surface complexation constants describing phosphate adsorption on γMnO2
using the triple layer model. ............................................................................................. 15
Table SM3: Intrinsic surface complexation constants (derived experimentally by (Yao
and Millero, 1996) ) describing phosphate adsorption on γMnO2 using the triple layer
model. Those constants were obtained with aged manganese dioxide (γMnO2)
characterized with a specific surface area of 206 m2 g-1; a surface site density of 18
sites nm-2 and an inner and outer layer capacitance of 2.4 and 0.2 F m-2 respectively. 15
7. Teflon sheet image showing the Fe and the Mn-Fe oxyhydroxide layers. ................... 16
2
Figure SM2: Teflon sheet image retrieved from the deep station CS on July 30. The
lower yellow-orange layer constitutes the iron oxyhydroxide (the “Feox” layer)
deposit whereas the upper plate section was characterized by brownish Mn and Fe
nucleation sites (the “Mnox” layer) extending over the superior region of the plate. .. 17
8. Mnox and organic carbon amounts per Teflon sheet surface area ............................... 18
Figure SM3 Quantity of manganese oxyhydroxides (nmol Mn cm-2) (A) and organic
carbon (nmol Corg cm-2) (B) retrieved from the Feox or the Mnox layers of the Teflon
sheets. The sheets were incubated at 3 stations in Lac St-Charles (CS: Central station;
EB: Echo Bay; SB: South Basin) and sampled on 4 different occasions during the
summer 2008. Note that Teflon sheets from stations CS and EB were not located on 6
October and the Feox layer on the sheets from stations EB and SB only began to be
clearly visible on August 14. Error bars are the standard deviations of three replicate
Teflon sheets. ................................................................................................................ 20
9. Iron and manganese oxyhydroxide accrual rates .......................................................... 21
Table SM4: Iron (nmol Fe cm-2 d-1 ± 1SD) and manganese (nmol Mn cm-2 d-1 ± 1 SD)
oxyhydroxide accrual rates by date and sampling station (CS: Central Station; EB:
Echo Bay; SB: South Basin)......................................................................................... 21
Teflon sheets from station CS and SB could not be located on October 6. Accrual rates
are computed relative to June 16 (Day 0), when sheets were initially deployed. ........ 21
10. Evolution over time of the P:Fe and the Corg:Fe molar ratios measured in the Feox or
the Mnox layers harvested on Teflon sheets. ................................................................... 22
Figure SM4: Phosphorus to iron oxyhydroxide (P:Fe) (A) and organic carbon (Corg) to
iron oxyhydroxide (Corg:Fe) (B) molar ratios (% mol:mol) harvested from the Feox or
the Mnox layers deposited on Teflon sheets after different incubation times (insertion
on June 16 and retrieval on July 30, August 14, September 17 and October 6 2008) in
lake sediments at three sampling sites (CS: Central Station, EB: Echo Bay and SB:
South Basin). Note that Teflon sheets from stations CS and EB were not found on 6
October and that the Feox layer on the sheets from station EB and SB began to be
clearly visible only on August 14. Error bars are the standard deviations of three
replicate Teflon sheets. ................................................................................................. 22
11. Organic carbon to organic nitrogen ratios as a function of sampling time and stations
.......................................................................................................................................... 24
Table SM5: Organic carbon to organic nitrogen (Corg:Norg) mean molar ratios (± SD,
n=3) measured in the Feox and Mnox layer material at Central (CS), Echo Bay (EB)
3
and South Basin (SB) stationss on August 14, September 17 and October 6 2008.
Teflon sheets at stations CS and EB could not be located on October 6. ................... 24
12. Correlations between adsorbed elements on the Teflon sheets .................................. 25
Table SM6: Total and partial Pearson correlation coefficients between organic carbon
and elements determined by digestion of Teflon sheet material. Total coefficients are
all significant to p < 0.001 except where noted. Partial coefficients were only
significant where probabilities are indicated. n= 43 sheets. ......................................... 25
13. Two hour soluble reactive phosphorus exchange experiment .................................... 26
Figure SM5: Soluble reactive phosphorus (SRP) concentrations (A: mmol SRP·mol
Fe-1; B: nmol SRP·cm-2) measured as a function of time (t in min.) in 500 mL water
overlying pieces of Teflon sheets with adsorbed Mnox retrieved from Echo Bay (EB)
and South Basin (SB) stations on July 30th 2008. Data for “EB Mnox” (A: [SRP] =. 26
14. Sediment elemental composition ................................................................................ 28
Figure SM6: Total solid concentrations (µmol g-1) of Fe and particulate organic
carbon (POC) (A) as well as Mn and P (B) as a function of sediment depth (cm)
sampled at three stations (CS, EB and SB) in Lac St-Charles. .................................... 29
15. Is phosphorus bound through cation bridging at the littoral stations? ........................ 30
16. Sensitivity analysis: Simulating the effect of a minor bacterial or labile organic
carbon pool on the Corg:Norg and P:Fe measured molar ratios on the Teflon sheets......... 35
Table SM7: Effect of different proportions (%) of bacterial organic carbon on the
Corg:Norg molar ratios measured (Corg occurring mainly as humic substances, HS) on
the Teflon sheets of Feox or Mnox deposits. We assumed a Redfield C:N ratios as an
estimate of living microorganism composition. The Corg:Norg predicted ratio take into
account the given bacterial organic carbon proportion (%) on the Teflon sheets. The
predicted Corg:Norg ratios were compared statistically to the measured Corg:Norg ratio
(19.78 ± 7.2) by using the one-sample t-test yielding a t value and a p value. ............ 37
Table SM8: Effect of a low amount of biological organic carbon (17.3%) of different
P:Corg ratios (%) on the measured P:Fe ratios on the Teflon sheets yielding a predicted
P:Fe molar ratios. We chose a modeled P:Fe ratio of 0.01, which is close to the
modeled P:Fe ratios at stations EB and SB. We also used a measured Corg:Fe ratio of
10 as a representative estimate at stations EB and SB. The biological Corg: Fe ratios
were computed by multiplying the proportion of biological organic carbon (0.173) on
the sheets by the measured Corg:Fe ratio (10). The calculation yield a predicted P:Fe
4
molar ratios (biological Corg:Fe ratio multiplied by biological P:Corg) for a given
bacterial P:Corg ratios. ................................................................................................... 38
17. Comparing the theoretical labile organic matter degradation rate with the soluble
reactive phosphorus fluxes measured experimentally at littoral stations ......................... 39
18. Literature cited ............................................................................................................ 41
5
1. Description of the study site and bathymetric map
Lac St-Charles is the main drinking water reservoir for the 200 000 person
population living in Quebec City. The total area and volume of the lake is 3.6 km2 and 14
967 km3 respectively. The average rate of hydraulic renewal is only 23 days due to the
large watershed surface area (165.8 km2) with respect to the lake area (3.6 km2). Farmland
accounts for 1 km2. The vast majority of the drainage basin is covered with mixed
deciduous forest (84.5 %) while residential and agricultural lands constitute 11.5% and less
than 1% of the drainage basin respectively (Tremblay et al., 2001) and the MSc thesis cited
therein). Although this lake is still considered mesotrophic (Tremblay et al., 2001), the
progressive disappearance of brook trout (Salvelinus fontinalis), an increase in the
phosphorus loading (Légaré, 1997) as well as repetitive cyanobacterial blooms (Apel,
2008) during the last few years suggest that the lake may be transitioning toward becoming
more eutrophic and increased nutrient recycling from the sediment could take place.
6
CS
EB
S2
SB
Figure SM1: The location of the study site and sampling stations: Central (CS), Echo Bay
(EB) and South Basin (SB) stations. Bathymetric map provided by (Tremblay et al., 2001).
7
2. Surface complexation modeling of elements adsorbed onto iron and
manganese oxides.
Note that modeled cation/anion adsorption onto Fe or Mn oxyhydroxide obtained
with chemical equilibrium calculations should be viewed as approximate estimates as the
equilibrium constants were obtained from synthetic amorphous ferrihydrite and aged
manganese oxides under laboratory conditions. Hence, the possible presence of more
crystallized iron forms (e.g. lepidocrocite) or less crystallized manganese oxide such as
hydrous manganese oxide is not accounted for.
8
3. Chemical composition of water overlying sediments at the three
stations.
Table SM1: Anions and cations measured in water above the sediment-water interface at
the three sites measured on 17 September 2008.
Station
Station
Station
CS
EB
SB
[Al] µmol L-1
1.1
1.3
1.2
[Ba] µmol L-1
0.24
0.08
0.06
[Ca] µmol L-1
91
118
121
[Cl] µmol L-1
423
336.2
336.2
[Fe] µmol L-1
5.9
4.9
4.3
[K] µmol L-1
12.4
16.7
17.6
[Mg] µmol L-1
70.9
51
53.6
[Mn] µmol L-1
5.9
0.6
0.3
[Na] µmol L-1
209
265
270
[NO3] µmol L-1
29.6
23.2
23.2
[SRP] nmol L-1
8.4
5.3
5.3
[SO4] µmol L-1
46.9
45.0
45.0
[Si] µmol L-1
82.5
87
83.1
[Zn] µmol L-1
0.44
0.12
0.22
[DOC] µmol L-1
235
267
285
pH
6.13
6.45
6.45
9
4. Modeling adsorption of organic carbon on iron oxyhydroxides by using
Langmuir isotherms
Adsorption of organic carbon (Corg) onto iron oxyhydroxide could not be calculated with
the double or triple layer surface complexation model due to the lack of appropriate
intrinsic surface complexation constants. We thus used existing Langmuir adsorption
isotherms with metal oxides (Tipping, 1981) to model Corg adsorption (occurring mainly as
humic substances or SH on our Teflon sheets) to iron and manganese oxyhydroxides.
Various Langmuir isotherms (see equation 1) describing the adsorption of aquatic
HS on goethite at pH ranging from 5.0 to 8.5 (with increments of 0.5 units of pH) were
published in (Tipping, 1981). Strong linear relationships between Langmuir parameters and
pH in the range 5.5 to 7.0 were found (K: sorption affinity constant, R2 = 0.997 and n:
maximum site concentrations, R2 = 0.985), allowing us to calculate Langmuir parameters at
the measured pH at the three stations (i.e. pH = 6.13, 6.45 and 6.45 for station CS, EB and
CS respectively). The HS concentration overlying the sediments at each station was
estimated by assuming that all of the dissolved organic carbon present (Table SM1) is
humic material and that organic carbon represents 50% (w/w) of this humic material,
(Tipping, 1981). The amount of HS theoretically adsorbed on goethite (α) was calculated
with equation 1 for each station. The molar ratio of HS (expressed as carbon organic
content) adsorbed per mol of goethite (mol HS/mol Fe) at station CS, EB and SB was 0.09,
0.08 and 0.08 respectively.
10
α = (n K c) (1 + K c)-1
(1)
where: α represents the amount of humic substances adsorbed on pure iron or manganese
oxides (mg g-1)
n is the value of α at saturation or the maximum site concentration
K is a measure of the affinity of the oxide surface for the humics or the sorption affinity
constant (L mg SH-1)
c is the humic substances concentration (mg L-1)
Since Fortin et al. (1993) established that most iron oxyhydroxides collected on
Teflon sheets, incubated from three to twelve months in the sediments of several
oligotrophic lakes, were ferrihydrite, the amount of HS adsorbed onto our field collected
iron oxyhydroxide may be better modeled by using Langmuir parameters defined with
ferrihydrite compared to goethite. However, the only Langmuir parameters available in the
literature for ferrihydrite were derived at pH 7.2 (Tipping, 1981). These Langmuir
isotherms demonstrated that ferrihydrite adsorbs 8.5 to 24 fold more HS than goethite at pH
7.2. Assuming a similar relationship at slightly lower pH found at our sampling stations, it
can be estimated that ferrihydrite would adsorb 0.77 - 2.16, 0.68 – 1.92 and 0.69 – 1.94 mol
C/mol Fe at stations CS, EB and SB respectively. The Corg/Fe molar ratio measured on the
Feox diagenetic material across stations (Table 1) falls within or near this range. Similar
organic carbon content on authigenic iron oxyhydroxide collected by the same method in
11
other lakes has been reported in Tessier et al.(Tessier et al., 1996) (Corg/Fe = 1.3 - 2.3).
Moreover, Fe-rich particles formed in the water column of a seasonally anoxic lake were
characterized by Corg to Fe molar ratios between 1.2 and 2.5 (Tipping and Cooke, 1981).
We also attempted to model the adsorption of humic substances onto manganese
oxyhydroxide collected in the Mnox layer. The only dataset describing HS adsorption on
Mn oxyhydroxide (aged) was found in Tipping and Heaton (1983). The authors have
determined Langmuir parameters for the adsorption of Esthwaite Water HS on Mn3O4 at
pH 6.7. Using these parameters, we estimated HS/Mn molar ratios of 0.45, 0.47 and 0.48 at
stations CS, EB and SB respectively. When accounting for the proportion of HS
theoretically adsorbed on ferrihydrite in the Mnox layer, the computed HS/Mn+Fe molar
ratios were 0.65 – 1.48, 0.57 – 1.16 and 0.56 – 1.03 at stations CS, EB and SB respectively.
The predicted HS/Mn+Fe molar ratios tend to underestimate the adsorption of HS onto
Mnox diagenetic material albeit they remain close to the maximum predicted molar ratios
at station CS (measured HS/Mn+Fe molar ratios = 1.45 ± 0.46, 7.06 ± 2.84 and 3.37 ± 0.52
at stations CS, EB and SB respectively) (Table 1).
12
5. Intrinsic surface complexation constants used for estimating the
adsorption of various anions and cations on iron oxyhydroxides
Table SM2: Intrinsic surface complexation constants (Log Kint) calculated experimentally
(EXP) or by linear free-energy relationships (LFER) for weak (w) and strong (s) site type
used for estimating the adsorption of various anions and cations on iron oxyhydroxides by
using the two layer surface complexation model (values from (Dzombak and Morel, 1990)).
The constants were derived from ferrihydrite and assume a molecular weight of 89 g
FeOOH mol-1, a specific surface area of 600 m2 g-1, a concentration of weak and strong
sites of 0.2 and 5 x 10-3 mol mol-1 of iron oxyhydroxides respectively
log Kint
Site type
Calculation method
≡FeOH + H+ = ≡FeOH2+
7.29
s,w
EXP
≡FeOH = ≡FeO- + H+
-8.93
s,w
EXP
≡FeOH + SO42- + H+ = ≡FeSO4- + H2O
7.78
W
EXP
≡FeOH + SO42- = ≡FeOHSO42-
0.79
W
EXP
≡FeOH + SiO32- + H+ = ≡FeSiO3- + H2O
15.9
W
LFER
≡FeOH + SiO32- = ≡FeOHSiO32-
8.3
W
LFER
≡FeOH + PO43- + 3H+ = ≡FeH2PO4 + H2O
31.29
W
EXP
≡FeOH + PO43- + 2H+ = ≡FeHPO4- + H2O
25.39
W
EXP
≡FeOH + PO43- + H+ = ≡FePO42- + H2O
17.72
W
EXP
≡FeOH + Mn2+ = ≡FeOMn+ + H+
-0.4
S
LFER
≡FeOH + Mn2+ = ≡FeOMn+ + H+
-3.5
W
LFER
≡FeOH + Ca2+ = ≡FeOHCa2+
4.97
S
EXP
≡FeOH + Ca2+ = ≡FeOCa+ + H+
-5.85
W
EXP
Surface complexation reactions
13
≡FeOH + Ba2+ = ≡FeOHBa2+
5.46
S
EXP
≡FeOH + Ba2+ = ≡FeOBa+ + H+
-7.2
W
LFER
≡FeOH + Zn2+ = ≡FeOZn+ + H+
0.99
S
EXP
≡FeOH + Zn2+ = ≡FeOZn+ + H+
-1.99
W
EXP
14
6. Intrinsic surface complexation constants describing phosphate
adsorption on γMnO2 using the triple layer model.
Table SM3: Intrinsic surface complexation constants (derived experimentally by (Yao and
Millero, 1996) ) describing phosphate adsorption on γMnO2 using the triple layer model.
Those constants were obtained with aged manganese dioxide (γMnO2) characterized with a
specific surface area of 206 m2 g-1; a surface site density of 18 sites nm-2 and an inner and
outer layer capacitance of 2.4 and 0.2 F m-2 respectively.
Surface complexation reactions
log Kint
≡SOH + PO43- + 3H+ = ≡SOH2+ - H2PO4-
25.1
≡SOH + PO43- + 2H+ = ≡SOH2+ - HPO42-
19.6
≡SOH + PO43- + H+ = ≡SPO42- + H2O
29
15
7. Teflon sheet image showing the Fe and the Mn-Fe oxyhydroxide layers.
Upon first sampling on July 30, after 44 days of deployment, Teflon plate retrieval revealed
two distinct bands of deposited material. An area, just below the sediment-water interface
was characterized by a well defined thin yellow-orange band, of diagenetically formed
amorphous iron-oxide, referred as the “Feox layer”. Above this layer was a more diffusemottled zone (referred to as the “Mnox layer”), characterized by amorphous Mn and Fe
nucleation sites extending over the entire plate surface from the sediment-water interface to
approximately 8 to 10 cm above the sediment surface. As shown in Figure SM2, an
intermediate “mixed” layer was present at the deep station (CS), although this layer was
never very distinct in plates recovered from littoral zone stations. Material from both layers
was digested separately to determine chemical composition, abundance on a plate areal
basis, and orthophosphate exchange kinetics. Finally, it should be noted that plate surfaces
buried in the sediment, below the Feox layer, were very similar to clean new plates, and
visibly- free of sediment and organic matter.
16
Figure SM2: Teflon sheet image retrieved from the deep station CS on July 30. The lower
yellow-orange layer constitutes the iron oxyhydroxide (the “Feox” layer) deposit whereas
the upper plate section was characterized by brownish Mn and Fe nucleation sites (the
“Mnox” layer) extending over the superior region of the plate.
17
8. Mnox and organic carbon amounts per Teflon sheet surface area
Even though Fe represents a major component of the Feox diagenetic layer relative to other
elements (Fe:Mn molar ratios around 30, data not shown), that layer could be slightly
contaminated by manganese oxyhydroxides as suggested by the Mn:Fe measured ratios
consistently higher by two orders of magnitude than the ones predicted by thermodynamic
modeling of Mn adsorption onto Feox (Table 1; Figure SM4). The lower modeled
Mn:Corgratios than the measured Mn:Corg ratios also suggest that Mn enrichment occurred
in Feox layer with respect to the Corg (Table 1).
The Mnox layer above the Feox deposit was enriched with both Fe and Mn. Iron
oxyhydroxide content per surface area in the Mnox layer tended to be much higher at
station CS than at the two other stations on August 14 and September 17 and was also twofold greater at station SB than at station EB on September 17. For all sampling dates, the
Mnox content per surface area varied in a similar manner at the three stations, that is to say
a greater Mnox deposition (by three to five-fold) at station CS than at station EB (p<0.01)
as well as a two to four-fold higher Mnox level at station SB than at station EB (p<0.05)
and a greater but insignificant Mnox deposition at station CS than at station SB (Figure
SM4).
The Corg content per surface area deposited on Feox and Mnox was more variable (larger
error bars within replicates) and was similar among stations or oxyhydroxide types. For the
Feox layer, Corg content per surface area was around three-fold higher at station CS than at
18
station EB on August 14 and September 17 whereas the Corg content of the Mnox samples
did not vary significantly among stations (Figure SM4).
600
A
Mnox (nmol Mn cm-2)
500
400
Feox CS
Feox EB
Feox SB
Mnox CS
Mnox EB
Mnox SB
300
200
100
0
3500
B
Corg (nmol C cm-2)
3000
2500
2000
1500
1000
500
0
July 30
August 14
September 17
October 6
Date
19
Figure SM3 Quantity of manganese oxyhydroxides (nmol Mn cm-2) (A) and organic
carbon (nmol Corg cm-2) (B) retrieved from the Feox or the Mnox layers of the Teflon
sheets. The sheets were incubated at 3 stations in Lac St-Charles (CS: Central station; EB:
Echo Bay; SB: South Basin) and sampled on 4 different occasions during the summer 2008.
Note that Teflon sheets from stations CS and EB were not located on 6 October and the
Feox layer on the sheets from stations EB and SB only began to be clearly visible on
August 14. Error bars are the standard deviations of three replicate Teflon sheets.
20
9. Iron and manganese oxyhydroxide accrual rates
Table SM4: Iron (nmol Fe cm-2 d-1 ± 1SD) and manganese (nmol Mn cm-2 d-1 ± 1 SD)
oxyhydroxide accrual rates by date and sampling station (CS: Central Station; EB: Echo
Bay; SB: South Basin).
Teflon sheets from station CS and SB could not be located on October 6. Accrual rates are
computed relative to June 16 (Day 0), when sheets were initially deployed.
Date
July 30
August 14
September 17
October 6
Date
July 30
August 14
September 17
October 6
Station CS
Feox accrual rate
nmol Fe cm-2 d-1
Mean
± SD
8.0
0.7
11.1
2.7
13.7
1.1
nd
Mnox accrual rate
nmol Mn cm-2 d-1
Mean
± SD
2.1
4.5
4.6
0.3
0.9
1.6
Station EB
Feox accrual rate
nmol Fe cm-2 d-1
Mean
± SD
0.0
0.0
3.0
1.6
5.2
1.4
nd
Mnox accrual rate
nmol Mn cm-2 d-1
Mean
± SD
0.7
1.5
0.9
0.1
0.4
0.1
Station SB
Feox accrual rate
nmol Fe cm-2 d-1
Mean
± SD
0.0
0.0
8.6
4.0
6.4
2.5
23.4
12.1
Mnox accrual rate
nmol Mn cm-2 d-1
Mean
± SD
3.0
0.2
3.1
0.5
2.9
0.0
2.8
0.1
21
10. Evolution over time of the P:Fe and the Corg:Fe molar ratios measured
in the Feox or the Mnox layers harvested on Teflon sheets.
Figure SM4: Phosphorus to iron oxyhydroxide (P:Fe) (A) and organic carbon (Corg) to iron
oxyhydroxide (Corg:Fe) (B) molar ratios (% mol:mol) harvested from the Feox or the Mnox
layers deposited on Teflon sheets after different incubation times (insertion on June 16 and
retrieval on July 30, August 14, September 17 and October 6 2008) in lake sediments at
three sampling sites (CS: Central Station, EB: Echo Bay and SB: South Basin). Note that
Teflon sheets from stations CS and EB were not found on 6 October and that the Feox layer
on the sheets from station EB and SB began to be clearly visible only on August 14. Error
bars are the standard deviations of three replicate Teflon sheets.
22
25
P : Fe molar ratio (% mol mol-1)
A
Feox CS
Feox EB
Feox SB
Mnox CS
Mnox EB
Mnox SB
20
15
10
5
0
2000
Corg : Fe molar ratio (% mol mol-1)
B
1750
1500
1250
1000
750
500
250
0
July 30
August 14
September 17
October 6
Date
23
11. Organic carbon to organic nitrogen ratios as a function of sampling
time and stations
Table SM5: Organic carbon to organic nitrogen (Corg:Norg) mean molar ratios (± SD, n=3)
measured in the Feox and Mnox layer material at Central (CS), Echo Bay (EB) and South
Basin (SB) stationss on August 14, September 17 and October 6 2008. Teflon sheets at
stations CS and EB could not be located on October 6.
Corg:Norg molar ratios
August 14
September 17
October 6
CS Feox
15.2 ± 2.3
14.9 ± 2.6
-
EB Feox
15.5 ± 7.1
15.9 ± 4.8
-
SB Feox
21.0 ± 1.6
16.1 ± 3.0
18.5 ± 3.4
CS Mnox
18.8 ± 6.8
25.1 ± 8.3
-
EB Mnox
17.2 ± 1.2
21.1 ± 4.4
-
SB Mnox
25.0 ± 4.1
18.5 ± 2.7
21.0 ± 2.0
24
12. Correlations between adsorbed elements on the Teflon sheets
Table SM6: Total and partial Pearson correlation coefficients between organic carbon and
elements determined by digestion of Teflon sheet material. Total coefficients are all
significant to p < 0.001 except where noted. Partial coefficients were only significant where
probabilities are indicated. n= 43 sheets.
Element
Total correlation
Partial correlation
Al
0.65
0.004
Ca
0.85
0.55 (p<0.001)
Fe
0.65
-0.09
K
0.58
-0.19
Mg
0.71
-0.21
Mn
0.28 (p<0.06)
-0.01
Na
0.36
-0.14
P
0.77
0.09
Si
0.61
-0.01
Zn
0.74
0.13
25
13. Two hour soluble reactive phosphorus exchange experiment
4
SRP release (mmol SRP mol Fe-1 )
A
3
2
Mnox EB
Mnox SB
1
SRP release (nmol SRP cm-2)
0
0.30
B
0.25
0.20
0.15
0.10
0.05
0.00
0
20
40
60
80
100
120
140
Time (minutes)
Figure SM5: Soluble reactive phosphorus (SRP) concentrations (A: mmol SRP·mol Fe-1;
B: nmol SRP·cm-2) measured as a function of time (t in min.) in 500 mL water overlying
pieces of Teflon sheets with adsorbed Mnox retrieved from Echo Bay (EB) and South
Basin (SB) stations on July 30th 2008. Data for “EB Mnox” (A: [SRP] =3.6±0.12 (1 –
0.93±0.01t) or B: [SRP] = 0.254±0.009 (1 – 0.93±0.01t) and “SB Mnox” (A: [SRP] =
26
1.97±0.04 (1 – 0.91±0.01t) or B: [SRP] = 0.16±0.003 (1 – 0.92±0.01t). R2 ≥0.94 in all nonlinear least-square fits (Marquardt-Levenberg).
27
14. Sediment elemental composition
Solid phase sediment concentrations show Mn ~ 40 to 80%) and Fe (~20 to 40%) surface
enrichment relative to the concentration found at 5-cm depth at the three stations.
Moreover, total iron and manganese concentrations in sediment surface reached a rather
high molar fraction of particulate organic carbon (POC) (Fe:POC = 24 to 40%; Mn:POC =
0.2 to 0.7%). Since complexation modeling with WHAM predicted Fe:DOM and Mn:DOM
ratios around 0.01%, the bound fraction of Mn or Fe to POC (assuming similar behavior of
DOM and POC) is supposed to be of minor importance with respect to total metal
concentrations The above arguments thus suggest that both Fe and Mn oxyhydroxides
represent a major fraction of total metal surficial sediments (Figure SM6).
28
0
A
1
2
3
Sediment depth (cm)
4
Fe CS
POC CS
Fe EB
POC EB
Fe SB
POC SB
5
6
0
0
1000
2000
3000
4000
7000 8000 9000
B
1
2
3
Mn CS
P CS
Mn EB
P EB
Mn SB
P SB
4
5
6
0
20
40
60
80
100
Element concentrations (µmol g-1)
Figure SM6: Total solid concentrations (µmol g-1) of Fe and particulate organic carbon
(POC) (A) as well as Mn and P (B) as a function of sediment depth (cm) sampled at three
stations (CS, EB and SB) in Lac St-Charles.
29
15. Is phosphorus bound through cation bridging at the littoral stations?
Using the last complete data set obtained on September 17, the molar ratios of Ca
and Mg to Corg measured in the Feox or the Mnox layers at the three stations were shown to
be within a factor of 3 of the modeled ratios to dissolved organic carbon.
However, the Ca adsorption modelling to iron oxyhydroxyde yielded predicted
molar ratios several orders of magnitude lower that the Ca:Fe measured ratios in the Feox
or Mnox sheet layer samples (Table 1). Partial correlation analysis (Table SM6) reveals that
the excess Ca is mainly associated with organic carbon, suggesting that this base cation is
adsorbed to organic matter rather than to Fe or Mn oxyhydroxide, as has been shown in
other circumneutral or acidic lakes (Feyte et al., 2010, Tessier et al., 1996). Laboratory
studies have revealed that Ca electrostatically-bound to organic matter may in turn increase
the surface charge of iron (Tipping, 1981) and manganese oxides (Tipping and Heaton,
1983) and thus enhance anionic adsorption onto metal oxides.
A ternary complexation mechanism, in which cationic metals mediate the association
between organic matter functional groups and phosphate was one hypothesis that might
explain the higher measured than predicted P:Fe molar ratios discovered at the littoral
stations. We thus decided to explore the possibility of such a mechanism. Quantitative
binding to cation-rich dissolved organic matter is documented for another oxyanion,
arsenate (Redman et al., 2002). In addition, chemical equilibrium constants of cation/
arsenate complexes (Nordstrom and Archer, 2003, Whiting, 1992) or cation/phosphate
complexes (Martell et al., 2004) are very similar (Luengo et al., 2007). So, we would
30
expect that the importance of ternary surface complexes, through cation bridging, between
phosphate and cation-bound organic matter would be similar to arsenate-cation binding.
Redman et al (2002)measured the aqueous complexation of natural organic matter,
collected in several rivers, with arsenate oxyanions at pH 6 (I = 10 mM). Total “free” (or
uncomplexed arsenate to organic matter) arsenate concentration decreased by 20% in
aqueous solutions (pH 6, I=10 mM) of natural dissolved organic matter samples taken from
the Inangahua River and the Upper Peninsula Stream (unpolluted rivers). From knowledge
of the total arsenate concentration ([HnAsO4(n-3)]total) added to solution and the uncomplexed
(to natural organic matter or NOM) arsenate concentration ([HnAsO4(n-3)]free), the organic
matter-arsenate ternary complexes concentration ([NOM-Me-HnAsO4(n-3)] = [HnAsO4(n3)
]total - [HnAsO4(n-3)]free) can be determined. Adding the organic matter-metal complex
concentration ([NOM-Mez+]), we can calculate a conditional binding constant of arsenate to
NOM-Mez+ (KNOM-Me-As):
KNOM-Me-As =
[NOM-Me-As]
(1)
[NOM-Mez+] [HnAsO4(n-3)]
31
Where K NOM-Me-Arsenate is conditional to pH 6 at I=10 mM. [NOM-Me] was computed from
the total metal content completely complexed by NOM samples (Redman et al., 2002).
KNOM-Me-As =
(2 x 10-7 M)
(2)
(1.08 x 10-5 M) (8 x 10-7 M)
yielding KNOM-Me-As = 2.31 x 104
We need to compare this conditional binding constant to a constant calculated from
the excess phosphorus bound to particulate organic matter (total phosphorus minus the
phosphorus predicted to be adsorbed on iron oxyhydroxides) harvested on Teflon sheets
(Mnox samples) at station EB and SB. We assume that: 1) organic matter coatings in the
diagenetic material behave similarly to dissolved or natural organic matter (DOM) (Davis,
1982); 2) all Corg on the Teflon sheets is humic substance (HS) with a ratio of [FA]:[HA] of
9:1, and 3) phosphate and arsenate affinities to form ternary complexes with organic matter
are similar.
Reactions of inorganic phosphorus species (HnPO4n-3) with cations adsorbed to the
particulate organic matter coatings ({POM-Mez+}) on the Fe Mn oxyhydroxides yielding
the ternary complex ({POM-Me-HnPO4(n-3)}) can be described by the following simplified
equations:
32
[HnPO4(n-3)] + {POM-Mez+}= {POM-Me-HnPO4(n-3)}
(3)
with the conditional binding constant
KPOM-Me-P =
{POM-Me-HnPO4(n-3)}
(4)
[HnPO4(n-3)] {POM-Mez+}
Where [HnPO4(n-3)] is obtained from the measured soluble reactive phosphorus
concentration measured in the overlying water.
{POM-Mez+} is the concentration of metal-particulate organic matter complexes inferred to
be similar to dissolved-metal organic matter complexes modeled with WHAM 6 (where
Ca2+, Mg2+ and Al3+ are the principal cations complexed to DOM).
{POM-Me-HnPO4(n-3)} is the excess phosphorus concentration measured in the authigenic
oxyhydroxide materials at station EB or SB.
To estimate KPOM-Me-P, we determined the quotient {POM-Me-HnPO4(n-3)}/{POM-Mez+} by
dividing the mean sheet-measured P/Fe molar ratios by the measured (Ca+Mg+Al)/Fe
ratios. The latter value was then divided by SRP [HnPO4(n-3)]:
33
Station EB:
KPOM-Me-P =
8.0
(5)
(37.8 x 10-9 M) (11.5)
KPOM-Me-P = 1.84 x 107
Station SB:
KPOM-Me-P =
6.9
(6)
(39.5 x 10-9 M) (11.0)
KPOM-Me-P = 1.59 x 107
The estimated conditional equilibrium constants (KPOC-Me-P ≈ 2 x 107) would be about 3
orders of magnitude higher than those determined for arsenate (KNOM-Me-Arsenate = 2.31 x
104). This would imply that inorganic phosphorus species would have a much higher
affinity compared to the predicted arsenate affinity; a very improbable result. Therefore,
from the foregoing, we suggest that ternary complexes between inorganic phosphorus and
adsorbed organic matter to iron/manganese oxyhydroxides cannot explain the enhanced
phosphorus concentrations measured at stations EB and SB.
34
16. Sensitivity analysis: Simulating the effect of a minor bacterial or labile
organic carbon pool on the Corg:Norg and P:Fe measured molar ratios on
the Teflon sheets.
We compared the organic carbon to organic nitrogen molar ratios (Corg:Norg) measured in
the Feox or the Mnox deposits across all stations on August 14, September 17 and October
6. Statistical analyses reveal that the Corg:Norg ratios remain similar across all stations and
sampling times (19.78 ± 7.2, n = 41, p > 0.05). We thus simulated the effect of a bacterial
organic carbon on the mean Corg:Norg ratio measured on the Teflon sheets (Table SM7). To
do so, we used the following equation yielding a predicted Corg:Norg ratio for a given
proportion of bacterial organic carbon (assuming the Redfield C:N ratio ≈ 6.6:1):
Corg:Norg predicted = (C:NRedfield x %Corg bacterial) + (Corg:Norg measured x %C org humic)
eq. 1
Where,
Corg:Norg predicted = Predicted Corg:Norg ratio for a given bacterial organic carbon
amount.
C:NRedfield = Redfield C:N ratio as an estimate of bacterial Corg:Norg ratio = 6.6
35
%Corg bacterial = Relative proportion of bacterial organic carbon with respect to total
measured Corg on Teflon sheets (in %) (%Corg bacterial = 100 - %C org humic)
Corg:Norg measured = Mean measured Corg:Norg ratio (19.78 ± 7.2)
%C org humic = Relative proportion of humic substances carbon with respect to total
measured Corg on Teflon sheets (in %) (%C org humic = 100 - %Corg bacterial)
Corg bacterial
Corg HS
Redfeild Corg:Norg
Corg:Norg HS
Corg:Norg
%
%
Redfeild ratio
measured ratio
predicted ratio
0.1
99.9
6.625
19.78
19.77
1.0
99.0
6.625
19.78
10.0
90.0
6.625
15.0
85.0
17.0
t student
p
19.65
0.12
0.90
19.78
18.46
1.18
0.24
6.625
19.78
17.81
1.76
0.086
83.0
6.625
19.78
17.54
2.00
0.052
17.2
82.8
6.625
19.78
17.52
2.02
0.051
17.3
82.7
6.625
19.78
17.50
2.04
0.048
18.0
82.0
6.625
19.78
17.41
2.12
0.040
20.0
80.0
6.625
19.78
17.15
2.35
0.024
25.0
75.0
6.625
19.78
16.49
2.94
0.005
36
Table SM7: Effect of different proportions (%) of bacterial organic carbon on the Corg:Norg
molar ratios measured (Corg occurring mainly as humic substances, HS) on the Teflon
sheets of Feox or Mnox deposits. We assumed a Redfield C:N ratios as an estimate of
living microorganism composition. The Corg:Norg predicted ratio take into account the given
bacterial organic carbon proportion (%) on the Teflon sheets. The predicted Corg:Norg ratios
were compared statistically to the measured Corg:Norg ratio (19.78 ± 7.2) by using the onesample t-test yielding a t value and a p value.
This sensitivity analysis shows that a proportion of bacterial organic carbon greater or equal
to 17.3% yielded Corg:Norg ratios significantly different to the mean measured Corg:Norg ratio
on the Teflon sheets. It follows that a proportion lower than 17.3% of bacterial organic
carbon would not have been detectable by measuring the Corg:Norg ratios on the Teflon
sheets but could have influenced their phosphorus content. We thus simulated the effect of
a low amount of bacterial organic carbon (17.3%) of different P:Corg ratios on the measured
P:Fe ratios on the Teflon sheets (Table SM8). This analysis show that a biological organic
carbon source comprising a mere 17% of total organic carbon on the Teflon sheets would
need to be relatively phosphorus rich (approximately 5% P:Corg) in order to fully account
for the measured P:Fe molar ratios at station EB (9.9 ± 1.6 %) and station SB (8.9 ± 2.8 %).
37
P:Corg biological
measured Corg:Fe
modeled P:Fe
Corg biological Corg biological:Fe
P:Fe
%
Ratio
Ratio
%
ratio
Predicted ratio
0.1
10
0.01
17.3
1.73
0.00
1
10
0.01
17.3
1.73
0.02
2
10
0.01
17.3
1.73
0.03
5
10
0.01
17.3
1.73
0.09
7
10
0.01
17.3
1.73
0.12
10
10
0.01
17.3
1.73
0.17
20
10
0.01
17.3
1.73
0.35
30
10
0.01
17.3
1.73
0.52
40
10
0.01
17.3
1.73
0.69
50
10
0.01
17.3
1.73
0.87
Table SM8: Effect of a low amount of biological organic carbon (17.3%) of different
P:Corg ratios (%) on the measured P:Fe ratios on the Teflon sheets yielding a predicted P:Fe
molar ratios. We chose a modeled P:Fe ratio of 0.01, which is close to the modeled P:Fe
ratios at stations EB and SB. We also used a measured Corg:Fe ratio of 10 as a
representative estimate at stations EB and SB. The biological Corg: Fe ratios were computed
by multiplying the proportion of biological organic carbon (0.173) on the sheets by the
measured Corg:Fe ratio (10). The calculation yield a predicted P:Fe molar ratios (biological
Corg:Fe ratio multiplied by biological P:Corg) for a given bacterial P:Corg ratios.
38
17. Comparing the theoretical labile organic matter degradation rate with
the soluble reactive phosphorus fluxes measured experimentally at littoral
stations
In order to evaluate whether organic matter degradation of a relatively small labile organic
carbon pool on the Teflon sheets could account for the SRP release flux measured among
stations, we calculated the labile organic matter degradation rate (expressed as P release) at
littoral stations by using the labile organic matter degradation rate constant (k = 40 y-1)
derived in another Shield lake by Carignan and Lean (1991) and by assuming that 1), at
most, 17.3% of organic carbon on the Teflon sheets is labile organic matter (as estimated
by sensitivity analysis above) and 2) Porg:Corg molar ratio is near 5% (as estimated by
sensitivity analysis). The computed P release rates at both littoral stations were then
compared to the measured SRP release rate at those stations.
Our computed P release flux due to labile organic carbon mineralization (SB station: 5.9 ±
0.8 pmol cm-2 min-1; EB station: 6.3 ± 1.1 pmol cm-2 min-1) on the Teflon sheets is slightly
lower than the measured SRP fluxes at both littoral stations (SB station: 11 pmol cm-2 min1
; EB station: 15 pmol cm-2 min-1). This analysis thus suggests that organic matter
mineralization is a plausible explanation for the SRP release observed among stations,
39
sampling time and oxide type since degradability of even a low amount of labile organic
matter approximates the highest SRP release at the littoral stations.
40
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42