Supporting Information For

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 -2 carbon (nmol Corg cm ) (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) 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 3 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

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 4 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.

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 -1 [NO3] µmol L 29.6 23.2 23.2 [SRP] nmol L-1 8.4 5.3 5.3 -1 [SO4] µmol L 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 other lakes has been reported in Tessier et al.(Tessier et al., 1996) (Corg/Fe = 1.3 - 2.3).

11 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

Surface complexation reactions log Kint Site type Calculation method + + ≡FeOH + H = ≡FeOH2 7.29 s,w EXP ≡FeOH = ≡FeO- + H+ -8.93 s,w EXP 2- + - ≡FeOH + SO4 + H = ≡FeSO4 + H2O 7.78 W EXP 2- 2- ≡FeOH + SO4 = ≡FeOHSO4 0.79 W EXP 2- + - ≡FeOH + SiO3 + H = ≡FeSiO3 + H2O 15.9 W LFER 2- 2- ≡FeOH + SiO3 = ≡FeOHSiO3 8.3 W LFER 3- + ≡FeOH + PO4 + 3H = ≡FeH2PO4 + H2O 31.29 W EXP 3- + - ≡FeOH + PO4 + 2H = ≡FeHPO4 + H2O 25.39 W EXP 3- + 2- ≡FeOH + PO4 + H = ≡FePO4 + 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 ≡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

13 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 3- + + - ≡SOH + PO4 + 3H = ≡SOH2 - H2PO4 25.1 3- + + 2- ≡SOH + PO4 + 2H = ≡SOH2 - HPO4 19.6 3- + 2- ≡SOH + PO4 + H = ≡SPO4 + H2O 29

14 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 diffuse- mottled 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.

15 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.

16 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 two- fold 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

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

17 station EB on August 14 and September 17 whereas the Corg content of the Mnox samples did not vary significantly among stations (Figure SM4).

A Feox CS Feox EB 500 Feox SB Mnox CS

) Mnox EB 2 - 400 Mnox SB m c

o 300 m n (

n 200 M

0 3500 B

3000 )

2 2500 - m c

2000 l o m n (

1500 g r o C 1000

0 July 30 August 14 September 17 October 6 Date

18 Figure SM3 Quantity of manganese oxyhydroxides (nmol Mn cm-2) (A) and organic

-2 carbon (nmol Corg cm ) (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.

19 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.

Station CS Station EB Station SB Date Feox accrual rate Feox accrual rate Feox accrual rate nmol Fe cm-2 d-1 nmol Fe cm-2 d-1 nmol Fe cm-2 d-1 Mean ± SD Mean ± SD Mean ± SD July 30 8.0 0.7 0.0 0.0 0.0 0.0 August 14 11.1 2.7 3.0 1.6 8.6 4.0 September 17 13.7 1.1 5.2 1.4 6.4 2.5 October 6 nd nd 23.4 12.1

Date Mnox accrual rate Mnox accrual rate Mnox accrual rate nmol Mn cm-2 d-1 nmol Mn cm-2 d-1 nmol Mn cm-2 d-1 Mean ± SD Mean ± SD Mean ± SD July 30 3.0 0.2 August 14 2.1 0.3 0.7 0.1 3.1 0.5 September 17 4.5 0.9 1.5 0.4 2.9 0.0 October 6 4.6 1.6 0.9 0.1 2.8 0.1

20 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.

21 25 A Feox CS Feox EB )

1 Feox SB

- 20 l

o Mnox CS

m Mnox EB

o Mnox SB m 15 % (

o i t a r

a 10 l o m

0 2000 B )

1 1750 - l o m

l 1500 o m

% 1250 (

o i t a

r 1000

r a l o

500 g r o C 250

0 July 30 August 14 September 17 October 6 Date

22 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

23 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

24 13. Two hour soluble reactive phosphorus exchange experiment

1 - e F

l 3 o m

l o 2 m m (

e s a e l Mnox EB e r 1 Mnox SB P R S

0 0.30 B ) 2

- 0.25 m c

R 0.20 S

l o m n (

0.15 e s a e l e

r 0.10

P R S 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] =

25 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 non- linear least-square fits (Marquardt-Levenberg).

26 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).

27 0 A 1

4 Fe CS POC CS )

m Fe EB c ( 5 POC EB h t Fe SB p

e POC SB d

n 6 e 0 m 0 1000 2000 3000 4000 7000 8000 9000 i d e

Mn CS 4 P CS Mn EB P EB 5 Mn SB P SB

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.

28 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

29 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

(n-3) of the total arsenate concentration ([HnAsO4 ]total) added to solution and the uncomplexed

(n-3) (to natural organic matter or NOM) arsenate concentration ([HnAsO4 ]free), the organic

(n-3) (n-3) matter-arsenate ternary complexes concentration ([NOM-Me-HnAsO4 ] = [HnAsO4 ]total

(n-3) - [HnAsO4 ]free) can be determined. Adding the organic matter-metal complex concentration ([NOM-Mez+]), we can calculate a conditional binding constant of arsenate to

z+ NOM-Me (KNOM-Me-As):

KNOM-Me-As = [NOM-Me-As] (1)

z+ (n-3) [NOM-Me ] [HnAsO4 ]

30 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).

-7 KNOM-Me-As = (2 x 10 M) (2)

(1.08 x 10-5 M) (8 x 10-7 M)

4 yielding KNOM-Me-As = 2.31 x 10

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.

n-3 Reactions of inorganic phosphorus species (HnPO4 ) with cations adsorbed to the particulate organic matter coatings ({POM-Mez+}) on the Fe Mn oxyhydroxides yielding

(n-3) the ternary complex ({POM-Me-HnPO4 }) can be described by the following simplified equations:

31 (n-3) z+ (n-3) [HnPO4 ] + {POM-Me }= {POM-Me-HnPO4 } (3) with the conditional binding constant

(n-3) KPOM-Me-P = {POM-Me-HnPO4 } (4)

(n-3) z+ [HnPO4 ] {POM-Me }

(n-3) Where [HnPO4 ] 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).

(n-3) {POM-Me-HnPO4 } is the excess phosphorus concentration measured in the authigenic oxyhydroxide materials at station EB or SB.

(n-3) z+ To estimate KPOM-Me-P, we determined the quotient {POM-Me-HnPO4 }/{POM-Me } by dividing the mean sheet-measured P/Fe molar ratios by the measured (Ca+Mg+Al)/Fe

(n-3) ratios. The latter value was then divided by SRP [HnPO4 ]:

32 Station EB:

KPOM-Me-P = 8.0 (5)

(37.8 x 10-9 M) (11.5)

7 KPOM-Me-P = 1.84 x 10

Station SB:

KPOM-Me-P = 6.9 (6)

(39.5 x 10-9 M) (11.0)

7 KPOM-Me-P = 1.59 x 10

7 The estimated conditional equilibrium constants (KPOC-Me-P ≈ 2 x 10 ) would be about 3

4 orders of magnitude higher than those determined for arsenate (KNOM-Me-Arsenate = 2.31 x 10 ).

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.

33 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

34 %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 t student p

% % Redfeild ratio measured ratio predicted ratio

0.1 99.9 6.625 19.78 19.77

1.0 99.0 6.625 19.78 19.65 0.12 0.90

10.0 90.0 6.625 19.78 18.46 1.18 0.24

15.0 85.0 6.625 19.78 17.81 1.76 0.086

17.0 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

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.

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 %).

36 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.

37 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 min-1;

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, 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.

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