Wessam Neweshy

Dolors Planas

Elisabeth Tellier

Marie Demers

Remi Marsac

Raoul- Marie Couture
email: raoul.couture@chm.ulaval.ca

Response of sediment phosphorus partitioning to lanthanum-modified clay amendment and porewater chemistry in a small eutrophic lake

Sustained eutrophication of the aquatic environment by the remobilization of legacy phosphorus (P) stored in soils and sediments is a prevailing issue worldwide. Fluxes of P from the sediments to the water column, referred to as internal P loading, often delays the recovery of water quality following a reduction in external P loads. Here, we report on the vertical distribution and geochemistry of P, lanthanum (La), iron (Fe) and carbon (C) in the culturally eutrophied Lake Bromont. This lake underwent remediation treatment using La-modified bentonite (LMB) commercially available as Phoslock TM . We investigated the effectiveness of LMB in decreasing soluble reactive phosphorus (SRP) availability in sediments and in reducing dissolved fluxes of P across the sediment-water interface. Sediment cores were retrieved before and after LMB treatment at three sites representing bottom sediment, sediment influenced by lakeside housing and finally littoral sediment influenced by the lake inflow. Sequential extractions were used to assess changes in P speciation. Depth profiles of dissolved porewater concentrations were obtained after LMB treatment at each site. Results indicate that SRP extracted from the

Eutrophication of lake ecosystems is a pervasive global environmental problem. Excess loading of the nutrient phosphorus from human activities in catchment areas cause lakes to fall into eutrophic states from which natural recovery is slow. As such, measures designed to retain nutrients in sediments, and limit internal nutrient loads, are widely studied. Here, we document the response of sediment solid-phase and porewater geochemistry to a prevalent chemically based phosphorus remediation technique based the addition of lanthanummodified clay. We use field and modelling evidence to reveal the diagenetic partitioning of phosphorus with lanthanum and Fe (oxy)hydroxides. We suggest that the reported ability of dissolved organic carbon to lower treatment efficacy depends on the absence of ferrous iron.

Introduction

Harmful freshwater algae blooms negatively impact water quality, which is detrimental to public health and aquatic ecosystems globally [START_REF] De Figueiredo | Microcystin-producing blooms-a serious global public health issue[END_REF] . In lake ecosystems, inadequate management of external nutrient loads, namely phosphorus (P) and nitrogen (N), is often the culprit. Decisive actions on improving catchment nutrient loads are, however, not often met with immediate results and legacy nutrients from within the catchment and from within the lake's sediments can be released long after such appropriate actions are taken [START_REF] Orihel | The "nutrient pump:" iron-poor sediments fuel low nitrogen-to-phosphorus ratios and cyanobacterial blooms in polymictic lakes[END_REF] . It is generally accepted that internal nutrient loads are responsible for the observed decoupling between management actions and water quality responses [START_REF] Søndergaard | Internal phosphorus loading in shallow Danish lakes[END_REF][START_REF] Welch | Internal phosphorus loading in shallow lakes: importance and control[END_REF][START_REF] Schindler | Eutrophication of lakes cannot be controlled by reducing nitrogen input: Results of a 37-year whole-ecosystem experiment[END_REF][START_REF] Watson | The reeutrophication of Lake Erie: Harmful algal blooms and hypoxia[END_REF][START_REF] Randall | Sediment potentially controls in-lake phosphorus cycling and harmful cyanobacteria in shallow, eutrophic Utah Lake[END_REF] . Even though dual nutrient limitations by both P and N is widespread [START_REF] Paerl | Controlling eutrophication along the freshwater-marine continuum: Dual nutrient (N and P) reductions are essential[END_REF][START_REF] Paerl | It takes two to tango: when and where dual nutrient (N & P) reductions are needed to protect lakes and downstream ecosystems[END_REF] , lakes can be impacted more severely by excess P rather than by excess N [START_REF] Orihel | The "nutrient pump:" Iron-poor sediments fuel low nitrogen-to-phosphorus ratios and cyanobacterial blooms in polymictic lakes[END_REF] . For such lakes, managers are justified to focus their economic and logistical efforts on P reduction to limit excessive algal biomass growth and are often faced with internal P sources that endure after external sources are abated. In view of the pervasive nature of internal P load, the role of sediments in supplying P and the coupling of the biogeochemical cycling of P with that of other key elements is intensely studied [START_REF] Steinman | Internal Phosphorus Loading in Lakes[END_REF] . Internal sediment sources of P can be understood as pools with specific reactivities, susceptible to mobilization as bottom water chemistry fluctuates [START_REF] Orihel | The "nutrient pump:" Iron-poor sediments fuel low nitrogen-to-phosphorus ratios and cyanobacterial blooms in polymictic lakes[END_REF] . Organic P (P org ) can be accumulated as biomass or bound to sedimentary organic carbon (C org ) [START_REF] Mclaren | Complex Forms of Soil Organic Phosphorus-A Major Component of Soil Phosphorus[END_REF] . P org can be derived externally from vegetation or organic material leached from the watershed (allochthonous) or generated in situ (autochthonous) from algal blooms occurring, in particular during the spring and summer. Finally, inorganic P is scarce in the undisturbed catchment, as it is released from weathering, but abundant in the sediment porewater, when it is regenerated during mineralization of P org [START_REF] Hensen | Marine Geochemistry[END_REF] . Upon its release to the porewater, inorganic P can be adsorbed to redox-sensitive (e.g. Fe) or pH-sensitive (e.g. Al or Ca) mineral surfaces [START_REF] Reddy | Phosphorous cycling in the greater everglades ecosystem: legacy phosphorous implications for management and restoration[END_REF] .

A host of approaches have been proposed to artificially inhibit internal nutrient loading, with the aim of lowering nutrient fluxes from sediments to stem cyanobacterial growth and decrease eutrophication. Such approaches can be either physically or chemically based. Physical approaches include dredging and removal of sediment, hypolimnetic withdrawal, and hypolimnetic circulation [START_REF] Jilbert | Preface: Restoration of eutrophic lakes: current practices and future challenges[END_REF] . Chemical approaches include hypolimnetic oxygenation (along with mixing), flocculation [START_REF] Connor | An effiecient method of appling aluminum salts for sediment phosphorus in activation in lakes[END_REF][START_REF] Kuster | A comparison of aluminum dosing methods for reducing sediment phosphorus release in lakes[END_REF][START_REF] Huser | Longevity and effectiveness of aluminum addition to reduce sediment phosphorus release and restore lake water quality[END_REF] , and sediment capping [START_REF] Welch | Internal phosphorus loading in shallow lakes: importance and control[END_REF][START_REF] Spears | Lake responses following lanthanum-modified bentonite clay (Phoslock®) application: An analysis of water column lanthanum data from 16 case study lakes[END_REF] . Here, we focus on the outcome of a specific approach to sediment capping that relies on La-modified bentonite (LMB), commercially available as Phoslock TM 20,[START_REF] Douglas | the International Association Water Quality Conference on Diffuse Pollution[END_REF] . With this product, clay is modified by replacing cations within the exchangeable layer that resides between the two tetrahedral-octahedraltetrahedral structures by a layer of La 3+ . La 3+ is released upon LMB addition to the lake's water and captures P to form rhabdophane (LaPO 4 .nH 2 O (s) ), which ages into monazite (LaPO 4(s) ) (log

4

K= 10 -26 ) over time. LMB was introduced to the market by CSIRO [START_REF] Douglas | the International Association Water Quality Conference on Diffuse Pollution[END_REF][START_REF] Robb | Application of Phoslock an innovative phosphorus binding clay, to two Western Australian waterways: preliminary findings[END_REF] and has been used in many lakes worldwide [START_REF] Hickey | Lake sediment phosphorus release management-Decision support and risk assessment framework[END_REF] .

LMB is often applied using the recommended dose of 100:1 LMB :P or 1:1 La:P which is thought to be sufficient to immobilize soluble reactive P (SRP) [START_REF] Reitzel | Phosphate adsorption by lanthanum modified bentonite clay in fresh and brackish water[END_REF] . Further studies of LMB applications have suggested that the ratio of 100:1 may not always be sufficient due to the interference from other solutes at the P-binding sites of the LMB [START_REF] Spears | Lake responses following lanthanum-modified bentonite clay (Phoslock®) application: An analysis of water column lanthanum data from 16 case study lakes[END_REF][START_REF] Meis | Sediment amendment with Phoslock® in Clatto Reservoir (Dundee, UK): Investigating changes in sediment elemental composition and phosphorus fractionation[END_REF][START_REF] Reitzel | Phosphate adsorption by lanthanum modified bentonite clay in fresh and brackish water[END_REF][START_REF] Ross | The effect of pH and anoxia on the performance of Phoslock®, a phosphorus binding clay[END_REF][START_REF] Zamparas | Removal of phosphate from natural waters using innovative modified bentonites[END_REF][START_REF] Márquez-Pacheco | Phosphorous control in a eutrophied reservoir[END_REF][START_REF] Meis | Assessing the mode of action of Phoslock® in the control of phosphorus release from the bed sediments in a shallow lake (Loch Flemington, UK)[END_REF][START_REF] Lürling | Case study on the efficacy of a lanthanum-enriched clay (Phoslock®) in controlling eutrophication in Lake Het Groene Eiland (The Netherlands)[END_REF][START_REF] Van Oosterhout | Lanthanum from a modified clay used in eutrophication control is bioavailable to the marbled crayfish (Procambarus fallax f. virginalis)[END_REF] . Alkalinity 31 , pH [START_REF] Ross | The effect of pH and anoxia on the performance of Phoslock®, a phosphorus binding clay[END_REF] and DOC [START_REF] Spears | A meta-analysis of water quality and aquatic macrophyte responses in 18 lakes treated with lanthanum modified bentonite (Phoslock®)[END_REF][START_REF] Dithmer | Influence of dissolved organic carbon on the efficiency of P sequestration by a lanthanum modified clay[END_REF][START_REF] Lürling | Humic substances interfere with phosphate removal by lanthanum modified clay in controlling eutrophication[END_REF][START_REF] Li | Interactions of phosphate and dissolved organic carbon with lanthanum modified bentonite: Implications for the inactivation of phosphorus in lakes[END_REF] are identified as key factors that can decrease both LMB P-binding capacity as well as the efficiency of the treatment [START_REF] Lürling | Mitigating eutrophication nuisance: in-lake measures are becoming inevitable in eutrophic waters in the Netherlands[END_REF] .

DOC is as an important parameter that influences the binding capacity of La within LMB [START_REF] Spears | A meta-analysis of water quality and aquatic macrophyte responses in 18 lakes treated with lanthanum modified bentonite (Phoslock®)[END_REF][START_REF] Dithmer | Influence of dissolved organic carbon on the efficiency of P sequestration by a lanthanum modified clay[END_REF][START_REF] Lürling | Humic substances interfere with phosphate removal by lanthanum modified clay in controlling eutrophication[END_REF] . In the context of boreal lakes subjected to increasing DOC fluxes [START_REF] Algesten | Role of lakes for organic carbon cycling in the boreal zone[END_REF][START_REF] Finstad | From greening to browning: Catchment vegetation development and reduced S-deposition promote organic carbon load on decadal time scales in Nordic lakes[END_REF] , where the concentrations of DOC typically range from 4 to 20 mg L -1 39 with highest concentrations found in the sediment porewater, the role of DOC is particularly crucial. For example, Li et al. [START_REF] Li | Interactions of phosphate and dissolved organic carbon with lanthanum modified bentonite: Implications for the inactivation of phosphorus in lakes[END_REF] found that P uptake via LMB depended on the ratio of DOC to P, with uptake suppressed when the C:P molar ratio exceeded 9. To our knowledge, few field-based studies have accessed the geochemical conditions leading to the suppression of P fluxes from the sediments in the presence of DOC. [START_REF] Dithmer | Responses in sediment phosphorus and lanthanum concentrations and composition across 10 lakes following applications of lanthanum modified bentonite[END_REF] These studies point to a key interplay between La 3+ , DOC, and Fe [START_REF] Dithmer | Responses in sediment phosphorus and lanthanum concentrations and composition across 10 lakes following applications of lanthanum modified bentonite[END_REF][START_REF] Waajen | Effects of Dredging and Lanthanum-Modified Clay on Water Quality Variables in an Enclosure Study in a Hypertrophic Pond[END_REF][START_REF] Zhi | Influence of natural organic matter and pH on phosphate removal by and filterable lanthanum release from lanthanum-modified bentonite[END_REF] , which we set out to better understand using field data and thermodynamic modelling.

Few studies have measured the vertical distribution of P, La and associated elements in the sediment following treatment [START_REF] Meis | Sediment amendment with Phoslock® in Clatto Reservoir (Dundee, UK): Investigating changes in sediment elemental composition and phosphorus fractionation[END_REF][START_REF] Meis | Assessing the mode of action of Phoslock® in the control of phosphorus release from the bed sediments in a shallow lake (Loch Flemington, UK)[END_REF][START_REF] Yasseri | Analysis of the La:P ratio in lake sediments -Vertical and spatial distribution assessed by a multiple-core survey[END_REF][START_REF] Van Oosterhout | Lanthanum in Water, Sediment, Macrophytes and chironomid larvae following application of Lanthanum modified bentonite to lake Rauwbraken (The Netherlands)[END_REF] , which is key given that LMB relies entirely on sediment remediation of P via fixation by La. Similarly, porewater systematics has seldom been measured at an LMB-remediated site after treatment, despite porewater being sensitive indicators of the reactions taking place in the porous media. Here, we investigate the sequestration of P in the sediments post LMB applications using both sediment and porewater data. We discuss P speciation in the sediment before and after treatment and use porewater concentrations to calculate the speciation La. Specifically, our objectives are to: i) assess dominant P species in the sediment before and after treatment, ii) document P speciation at three sites under contrasting influences from the catchment and iii) interpret porewater profiles in terms of equilibrium La and P speciation, and iv) quantify the effect that DOC has on prevailing equilibria.

Material and method:

Study site.

Lake Bromont (45°15' N 72° 40' W, Fig. 1) is in the municipality of Bromont in southeastern Québec, Canada. It is part of the hydrographic network of the Yamaska River, which itself flows into St. Lawrence River at lake St. Pierre. Formerly a pond, Lake Bromont was formed following the construction of a dam in the 1950s to provide drinking water for the town of Cowansville.

The lake has a surface area of 4.6 km 2 at maximum water level of 7.2m. Its average depth is 4.8 m. It is thermally stratified in the summer and develops seasonal anoxia in the summer (up to 121 days in the warm 2010 summer [START_REF] Planas | Le programme de recherche sur les cyanobactéries: synthèse, conclusions et recommandation[END_REF] ) and likely under ice as many lakes do [START_REF] Fang | Simulated climate change effects on dissolved oxygen characteristics in ice-covered lakes[END_REF] .

Sediment and water sampling.

Before treatment, sediment cores were collected by the Phoslock TM Europe GmbH company in July 2014 [START_REF] Van Goethem | Sediment P-fractionation Lac Bromont -Final report[END_REF] , sliced from 0 to 4 and 4 to 10 cm and frozen until analysis using sequential extractions described below. After treatment, we collected sediment cores of 90 mm diameter and 60 cm length in September 2019 at three stations in Lake Bromont using a UWITEC gravity corer (Fig. 1) hereafter referred to as: Deep site, House site, and Inflow site. Before collecting the samples, we measured water column temperature and dissolved oxygen using an YSI EXO-II multiparameter probe. Three sediment cores were collected at each station. Core 1 was sectioned following the three-section scheme used by the consultants; here we document the first two layers of 0-4 and 4-10 cm for the purpose of before and after treatment comparison. We performed the analysis of the samples from this core in triplicates. Core 2 was extruded with a 1cm resolution from 0 to 40 cm depth and stored at -20 o C for solid-phase analyses. Core 3 was subsampled for porewater samples using micro rhizons (Rhizosphere product inc.) with a PES filter of 0.2µm pore size from 2 cm above the sediment-water interface down to 12 cm depth.

Porewater subsamples were distributed in vials according to the required analysis: 1) 4 ml was delivered in acid-washed PFA bottles with ultra-pure HNO 3 to measure major and trace dissolved elements; 2) 1.5 ml was delivered in Eppendorf tubes to measure the anions; 3) 4ml was delivered to alkaline-washed amber glass bottle acidified with HCl 2N to measure DOC, 4) 2 ml was delivered in 2 ml amber glass bottles containing Zn-acetate to measure total dissolved sulfides.

Discrete duplicate water samples were collected once a month from May to October from 2017 onward with a Van Dorn bottle at the deepest part of the lake at 0.5 m and 6.3 m for the surface and the bottom sample. P TOT was digested in an autoclave at 150 atm with persulphate solution [START_REF] Menzel | The measurement of total phosphorus in seawater based on the liberation of organically bound fractions by persulfate oxidation[END_REF] and determined using the molybdenum blue colorimetric method [START_REF] Murphy | A modified single solution method for the determination of phosphate in natural waters[END_REF][START_REF] Stainton | The chemical analysis of freshwater[END_REF] .

Solid phase analysis.

Frozen sediment was freeze-dried and homogenized in an agate mortar. Solid-phase P was extracted using a P fractionation scheme (sequential extraction; SEQ) based on Hupfer et al.

2009 [START_REF] Hupfer | Evaluation of a wellestablished sequential phosphorus fractionation technique for use in calcite-rich lake sediments: identification and prevention of artifacts due to apatite formation[END_REF] and modified from Huser et al. 2019 [START_REF] Huser | Sequential fractionation of phosphorus in sediment[END_REF] (Table 1). 200 mg of dry sediment was weighed and mixed at a 1:50 soil: solution mass ratio using a bench-top roller for the following extraction steps: i) 2 hrs H 2 O-step, associated with loosely bound P, ii) 1 hr bicarbonate/dithionite step (BD) associated with redox-sensitive P bound to Fe and/or Mn oxides, iii) 16 hrs NaOH-step associated with P bound to Fe and or Al oxides, as well as a fraction of organic P, iv) 16 hrs HCl-step associated with P bound to Ca (apatite). Extractions were carried out in acid-washed 15 ml PP centrifuge tubes and centrifuged at 3000×g for 10 minutes after each step. Afterward, each supernatant was pipetted into a syringe equipped with 0.45 µm PES membrane filter cartridge, delivered to an acid-washed 12 ml HDPE centrifuge tube and kept at 4 o C until analysis. Total P, Fe and La concentrations in the extracts were analyzed by triple-quadrupole ICP-MS (ICP-QQQ-MS, Agilent model 8900) while soluble reductive phosphorus (SRP) was determined using a Lachat QuikChem 8500 nutrient analyzer. Finally, total organic carbon (TOC) and nitrogen (TN) concentrations in the sediment were analyzed on homogenized freeze-dried sediment after decarbonatation using a CHNS elemental analyzer (Thermo-Fisher Flash Smart 2000). P speciation was further evaluated using [START_REF] Reitzel | The influence of lake water alkalinity and humic substances on particle dispersion and lanthanum desorption from a lanthanum modified bentonite[END_REF] P NMR on liquid solid-phase extracts. P was extracted from suspended samples based on Cade-Menun 2005 [START_REF] Cade-Menun | Refining 31P nuclear magnetic resonance spectroscopy for marine particulate samples: Storage conditions and extraction recovery[END_REF] . Briefly, ~3g of homogenized freezedried sediment sediments were extracted in 25 ml 0.5M NaOH and 0.1M EDTA for 16 hrs at ambient room temperature on a bench-top roller. The tubes were centrifuged at 3500 × g for 30 min and the supernatant filtered using a 0.45 PES filter cartridge. The resulting solution was then freeze-dried at -80 o C for 48hrs and the dried extract redissolved in a solution of NaOH and EDTA amended with D 2 O, then transferred into an NMR glass tube. Three thousand [START_REF] Reitzel | The influence of lake water alkalinity and humic substances on particle dispersion and lanthanum desorption from a lanthanum modified bentonite[END_REF] P NMR scans were obtained on an Agilent DD2 500 MHz NMR spectrometer equipped with a 5 mm broadband probe using a 90 o pulse, 0.7 s acquisition time and 5 s delay, requiring 12 hours per sample. P compounds and ratios were identified by their chemical shift using inorganic orthophosphoric acid standards. Peak area calculations and spectra processing were completed using Mnova v.14 (Mestrelab Research).

LMB treatment.

A 5-year study of the nutrient inputs to the lake and the influence of nutrients in supporting the proliferation of harmful algae began in spring 2007, following an intensive cyanobacteria bloom in the summer of 2006. Planktothrix agardhii and Aphanizomenon flosaquae, the cyanobacteria species causing the major bloom in the lake, were growing in the meta-hypolimnion of the lake, where P was abundant and light was sufficient for their photosynthesis requirements [START_REF] Pannard | Recurrent internal waves in a small lake: Potential ecological consequences for metalimnetic phytoplankton populations[END_REF][START_REF] Planas | Importance of climate change-physical forcing on the increase of cyanobacterial blooms in a small, stratified lake[END_REF] .

Despite its shallow depth, this lake forms an intermediate-deep layers as a result if its intense thermal stratification during the summer [START_REF] Pannard | Recurrent internal waves in a small lake: Potential ecological consequences for metalimnetic phytoplankton populations[END_REF] . The study of the lake's P budget based on massbalance inflow-outflow model established that around 80% of the concentration in the water column was supported by the P internal loading.

Given historical P inputs to the lake and the pool of reactive P species in the top layer of the sediments, LMB was recommended as a capping agent to control internal P fluxes in lake Bromont.

LMB typically contains approximately 5% La by weight [START_REF]Existing chemical secondary notification: Phoslock TM[END_REF] . The lake's physico-chemical characteristic, i.e. neutral to slightly alkaline pH of the water column, low DOC (< 4 mg/L) and deep-water anoxic conditions during the ice-free season were deemed suitable for such treatment.

In the fall 2017, 173.8 tons of LMB were dispersed in the water column to achieve a targeted La:P ratio of 1:1. The dosage aimed to sequester 1639 kg of potentially releasable P in the surface column. The dosage of LMB was calculated to target two P pools: P in the hypolimnion estimated in July 2014 and the releasable P in the sediment collected before treatment, assumed to comprise water-extractable P and BD-extractable P, and P org from NaOH extracts 47 51 .

Porewater analysis.

The concentration of major dissolved elements (i.e., Fe, Mn, Ca) were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES Thermo-Fisher duo iCAP 7400). Trace element (P, La) concentrations were determined by ICP-QQQ-MS as above. Anion (PO 4 3-, NO 3 -, SO 4 2-, Cl -) concentrations were measured by ion chromatography with UV detection (Thermo Dionex model integration HPLC equipped with AS-11 column). Total dissolved sulfide concentrations were analyzed using UV spectroscopy and the Cline reagent 57 on an Horiba Aqualog EEM spectrofluorometer. Finally, DOC analysis was performed using a TOC analyzer (Vario TOC-Cube, Elementar America).

Thermodynamic modelling.

Thermodynamic modelling was performed with PHREEQC v. 3 [START_REF] Parkhurst | Description of input and examples for PHREEQC version 3-A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations[END_REF] , a computer code based on an ion-association aqueous model, designed to perform speciation and saturation-index calculations in water. The thermodynamic database "Minteq.v4.dat" provided with PHREEQC was updated with generally accepted reactions and stability constants at 0 M ionic strength and 25°C for Lainorganic anion complexation relevant to the present work (Table 2and references therein).

Further, the humic-ion binding model "Model VII" [START_REF] Tipping | Humic Ion-Binding Model VII: a revised parameterisation of cation-binding by humic substances[END_REF] was implemented directly in PHREEQC to simulate metal ion binding by DOC. The use of humic substances as a surrogate for bulk dissolved organic matter (DOM) has been shown to be reasonable for the simulation of metal binding [START_REF] Lofts | The Chemical speciation of Fe(III) in freshwaters[END_REF] . It was assumed that DOM: (i) comprised 50% C, (ii) was 50% chemically active and (iii) was composed of humic acids (HA) only. The most recently optimized binding parameters for La and HA were taken from [START_REF] Marsac | Modeling rare earth elements binding to humic acids with model VII[END_REF] . The PHREEQC-Model VII coupling is described elsewhere 62, 63 . Briefly, Model VII assumes that the complexation of ions by HA occurs through various discrete groups (carboxylic and phenolic groups), which can form either mono-, bi-or tri-dentate binding sites. Because HA are large and negatively charged polyelectrolytes or colloids, electrostatic effects are also accounted for the quantification of metal ions-HA binding [START_REF] Tipping | Humic Ion-Binding Model VII: a revised parameterisation of cation-binding by humic substances[END_REF] . Cation competition was considered by including Mn 2+ , Ca 2+ , Al 3+ , Fe 2+ and Fe 3+ binding to HA. Measured aqueous concentrations and bottom water pH (pH = 6.43) were applied to the model to mimic in-situ conditions. The potential precipitation of several minerals was allowed (e.g., pyrite, magnetite, vivianite and MnHPO 4(s) ) in the model. Saturation indices (SI) for the porewater with respect to LaPO 4(s) phases were also calculated as a function of depth at the three different sampling sites, using the determined composition of the porewater (inorganic P, Fe, Mn, La, Ca, Na and S, with Cl added for electroneutrality). Finally, the combined effects of

Flux calculations across the sediment-water interface.

Inverse diagenetic modelling was used to objectively constrain fluxes of solutes across the sediment-water interface [START_REF] Couture | Arsenic, iron and sulfur co-diagenesis in lake sediments[END_REF][START_REF] Leclerc | Quantifying arsenic post-depositional mobility in lake sediments impacted by gold ore roasting in sub-arctic Canada using inverse diagenetic modelling[END_REF] . Porewater profiles were modelled assuming the absence of significant advective flux and at steady state (i.e., C/t = 0). The one-dimensional mass conservation equation provided by Boudreau 1997 67 at steady state was solved using the computer code PROFILE [START_REF] Berg | Interpretation of measured concentration profiles in sediment pore water[END_REF] inputs, to yield fluxes of La, P, and Fe across the sediment-water interface (SWI) at the three sampling sites.

Results.

3.1. Vertical distribution of P, Fe, La and C in the sediment.

The comparison of P partitioning was done on the basis of SEQ results performed before and after the LMB treatment using identical protocols at three sampling sites. Here, we present results for the sediment depths at 0-4 cm (Fig. 2) and 4-10 cm (Fig. S2), within which the excess solid-phase La is contained (Fig. 3b,f,j). P TOT extractable by BD and HCl more than doubled after treatment in surface sediment (0-4 cm). Meanwhile P TOT extracted by NaOH decreased by 11-21% depending on the site, with largest decrease observed at the Deep site and the smallest at the Inflow. SRP measurements in the extract follows similar patterns, but with smaller magnitudes, suggesting that P accumulated in the sediment after the treatment is not in the form of SRP. In contrast with the 0-4 cm depth layer, where SEQ before and after treatment lead to clear contrasts, SEQ results were closer together for the 4-10 cm depths layer (Fig. S2).

Nevertheless, P TOT extractable by BD and HCl increased slightly at the Deep and Inflow sites, and HCl-P doubled at the House site. As for the 0-4 cm layer, SRP measurements show similar patterns than P TOT .

P concentrations were measured at a 1-cm vertical resolution in the sediment after the LMB treatment. P TOT in the BD, NaOH and HCl fractions are displayed in Fig. 3(a,e,i). The profiles do not show pronounced subsurface peaks. P TOT, mostly comprising of SRP, is highest at the Deep site, followed by the House and Inflow sites. At the Deep site, it reaches a maximum of 79 mmol kg -1 at 11cm depth, of 69 mmol kg -1 at 5cm and 37.5 mmol kg -1 at 7cm at the House and Inflow sites, respectively. P TOT associated with the NaOH fractions dominates P speciation at the Deep Finally, [START_REF] Reitzel | The influence of lake water alkalinity and humic substances on particle dispersion and lanthanum desorption from a lanthanum modified bentonite[END_REF] P NMR indicates the presence of several organic P forms [START_REF] Shinohara | Changes in the composition of phosphorus (P) compound groups in sediment and P in sediment pore water in a shallow eutrophic lake: a 31P NMR study[END_REF] in addition to the dominant inorganic orthophosphate form. Orthophosphate monoesters, diesters, and pyrophosphates (Fig. 4), collectively referred to as P org , represent 15.7%, 43%, and 27% of the P TOT at the Deep, House and Inflow sites respectively (Fig. 4). The P org pool comprises between 70 and 85% of orthophosphates monoesters. As a general trend, P org content decreases with depth at all sites. along with the decrease in SO 4 2-, below the SWI at the House site to reach 10 M and 5 M at the Inflow site (Fig. 5). Finally, DOC concentrations were 325 µM at 7 cm, 432 µM at 1 cm, and 422 cm at the SWI, and minimum concentrations were 59 µM at SWI, 84 µM at 11 cm, and 94 µM at 4 cm for the Deep, House, and Inflow sites, respectively (Fig. 5 e,j,o).

Water-column total P, fluxes at the SWI and saturation indices.

Water-column P TOT remained between 14 and 25 g/L in the epilimnion of the lake (Fig. S1), and between 21 and 60 g/L in the hypolimnion, with the notable exception of the months of June, July and August 2017, that is, before the treatment, where it ranged between 96 and 180 g/L. Post treatment hypolimnetic P TOT concentrations never exceeded 60 g/L. Fluxes of solutes across the SWI were calculated after curve fitting of the measured profiles using the code PROFILE. At the Deep and Inflow sites, the sediment acts as a source of solutes to the water column. Conversely, it acts as a sink for P at the House site. Calculated P fluxes are seven orders of magnitude higher than La fluxes at all sites, and fivefold lower than Fe fluxes. Maximum values for diffusive fluxes are found at the Deep sites for all species, as shown on Table 3.

Finally, P fluxes before treatment were taken from Planas & Juneau (2019) [START_REF] Planas | Rapport technique: projet de restauration du lac Bromont[END_REF] .

Thermodynamic modelling (Fig. 6) of La speciation based on porewater chemistry (Fig. 5,S3 and S4) in the sediments suggests that the porewater are oversaturated with respect to LaPO 4(s) minerals below the SWI. However, when accounting for La 3+ (aq) binding by DOC, the model predicts that the porewater becomes undersaturated with respect to LaPO 4(s) at the House and Inflow sites, while they just reach saturation at the Deep site (-0.05 < SI > 0.05).

Discussion

Changes in P composition following LMB application.

Monitoring at the sites indicates that both water-column P TOT and cyanobacterial biomass responded to the LMB application [START_REF] Planas | Rapport technique: projet de restauration du lac Bromont[END_REF] , consistent with the outcome of other similar treatment and experiments [START_REF] Spears | Lake responses following lanthanum-modified bentonite clay (Phoslock®) application: An analysis of water column lanthanum data from 16 case study lakes[END_REF][START_REF] Copetti | Eutrophication management in surface waters using lanthanum modified bentonite: A review[END_REF][START_REF] Egemose | Chemical Lake Restoration Products: Sediment Stability and Phosphorus Dynamics[END_REF][START_REF] Lürling | Controlling toxic cyanobacteria: Effects of dredging and phosphorus-binding clay on cyanobacteria and microcystins[END_REF][START_REF] Lürling | Effects of lanthanum and lanthanum-modified clay on growth, survival and reproduction of Daphnia magna[END_REF][START_REF] Waajen | Management of eutrophication in Lake De Kuil (The Netherlands) using combined flocculant -Lanthanum modified bentonite treatment[END_REF][START_REF] Waajen | Geo-engineering experiments in two urban ponds to control eutrophication[END_REF][START_REF] Yin | Remediation of internal phosphorus loads with modified clays, influence of fluvial suspended particulate matter and response of the benthic macroinvertebrate community[END_REF] . Here, SEQ on vertical sections of sediment cores performed before and after treatment showed a transfer from redox and pH-sensitive P to the more recalcitrant HClextractable fraction (Fig. 2), likely consisting of LaPO 4(s) [START_REF] Spears | Lake responses following lanthanum-modified bentonite clay (Phoslock®) application: An analysis of water column lanthanum data from 16 case study lakes[END_REF][START_REF] Meis | Sediment amendment with Phoslock® in Clatto Reservoir (Dundee, UK): Investigating changes in sediment elemental composition and phosphorus fractionation[END_REF][START_REF] Reitzel | Phosphate adsorption by lanthanum modified bentonite clay in fresh and brackish water[END_REF][START_REF] Meis | Assessing the mode of action of Phoslock® in the control of phosphorus release from the bed sediments in a shallow lake (Loch Flemington, UK)[END_REF][START_REF] Dithmer | Responses in sediment phosphorus and lanthanum concentrations and composition across 10 lakes following applications of lanthanum modified bentonite[END_REF][START_REF] Dithmer | Characterization of Phosphate Sequestration by a Lanthanum Modified Bentonite Clay: A Solid-State NMR, EXAFS, and PXRD Study[END_REF] as suggested by the correlation between La and P at the Deep and the House sites. The low R 2 at the Inflow might be attributed to very low LMB dose in this site compared to other sites. Nevertheless, the redoxsensitive Fe-bound P increased after applying LMB, in particular at the Deep site, possibly linked to the bentonite being a source of Fe as well as of La. This suggests a partitioning of P within the LMB layer involving not only La but also redox-sensitive Fe oxyhydroxides, consistent with the close coupling between porewater P and Fe (Fig. 5). Reductive dissolution of Fe oxyhydroxides and the concomitant release of redox-sensitive P is also consistent with the observation made that anoxia could release P from LMB. [START_REF] Dithmer | Influence of dissolved organic carbon on the efficiency of P sequestration by a lanthanum modified clay[END_REF] SEQ with 1-cm vertical resolution (Fig. 3) showed that the Deep and House sites contain an important fraction of pH-sensitive P and organic-P [START_REF] Hupfer | Evaluation of a wellestablished sequential phosphorus fractionation technique for use in calcite-rich lake sediments: identification and prevention of artifacts due to apatite formation[END_REF] , data supported by results from [START_REF] Reitzel | The influence of lake water alkalinity and humic substances on particle dispersion and lanthanum desorption from a lanthanum modified bentonite[END_REF] P NMR (Fig. 4). Via a transfer from di-to mono-ester in the sediment [START_REF] Reitzel | Degradation rates of organic phosphorus in lake sediment[END_REF][START_REF] Ahlgren | Degradation of organic phosphorus compounds in anoxic Baltic Sea sediments: A 31P nuclear magnetic resonance study[END_REF] , P org is immobilized by LMB [START_REF] Kong | First attempt for in situ capping with lanthanum modified bentonite (LMB) on the immobilization and transformation of organic phosphorus at the sediment-water interface[END_REF] when given enough time, although the stability of this association remains poorly constrained given that P org is labile towards microbial degradation. Nevertheless, sharp solid-phase P peaks that parallel those of La (Fig. 3b,f,j) at the House site suggest a coupling in line with the formation of LaPO 4(s) species [START_REF] Spears | Lake responses following lanthanum-modified bentonite clay (Phoslock®) application: An analysis of water column lanthanum data from 16 case study lakes[END_REF][START_REF] Meis | Sediment amendment with Phoslock® in Clatto Reservoir (Dundee, UK): Investigating changes in sediment elemental composition and phosphorus fractionation[END_REF][START_REF] Reitzel | Phosphate adsorption by lanthanum modified bentonite clay in fresh and brackish water[END_REF][START_REF] Meis | Assessing the mode of action of Phoslock® in the control of phosphorus release from the bed sediments in a shallow lake (Loch Flemington, UK)[END_REF][START_REF] Dithmer | Characterization of Phosphate Sequestration by a Lanthanum Modified Bentonite Clay: A Solid-State NMR, EXAFS, and PXRD Study[END_REF] .

The remobilisation of La along with redox-sensitive Fe (Fig. 5) suggests an interplay with Fe oxyhydroxides, likely via the adsorption of the La 3+ cation. An enhanced mobility of La 3+ ions promoted by DOC complexation might explain this association with Fe. Such redox dynamics are absent from the littoral Inflow site. At this site, the sediment contains one tenth of the sedimentary La and Fe found at the other two sites, potentially indicating the presence of acidsoluble organics or carbonates in addition to La-bound P. The low La content is consistent with sediment accumulation from the material introduced by stream water and with smaller proportion of LMB settling at this littoral site after LMB application.

Coupling of P cycling to that of Fe, La and DOC.

At or below the SWI of all sites, dissolved P, Fe and La concentrations increase along sharp gradients (Fig. 5b, g, l), consistent with removal of aqueous species from the porewater. Thermodynamic calculations, accounting for the effect of DOC in complexing La 3+ , show that the porewater are oversaturated with respect to LaPO 4(s) at the SWI of all sites. Oversaturation suggests that the solid LaPO 4(s) may precipitate, but is either kinetically hindered, as has a been observed previously for stable minerals such as pyrite [START_REF] Rickard | Kinetics of pyrite formation by the H2S oxidation of iron (II) monosulfide in aqueous solutions between 25 and 125°C: The rate equation[END_REF] or that La colloids have passed through the 0.2 mm filter membranes, leading to an over estimation of dissolved La [START_REF] Reitzel | The influence of lake water alkalinity and humic substances on particle dispersion and lanthanum desorption from a lanthanum modified bentonite[END_REF] At the House and Inflow sites, which both display higher porewater DOC concentration at the SWI, accounting for DOC in the thermodynamic calculation shifts the SI by several orders of magnitude away from saturation with respect to LaPO 4(s) (Fig. 6), although this is limited to a zone just below the SWI. This is in line with previous studies that have suggested the competition between PO 4 3-and other anions for complexation with La 3+ (aq) , given that La-DOC is a thermodynamically favourable complex [START_REF] Marsac | Modeling rare earth elements binding to humic acids with model VII[END_REF] .

While our conclusion holds for the date of sampling given the model inputs, notably the measured dissolved Fe concentrations, it thus depends on variations in Fe 2+ caused by seasonal bottom water dissolved oxygen depletion. With the caveat that we did not ascertain vivianite formation [START_REF] Rothe | Evidence for vivianite formation and its contribution to long-term phosphorus retention in a recent lake sediment: a novel analytical approach[END_REF][START_REF] Rothe | Sedimentary sulphur : iron ratio indicates vivianite occurrence: A study from two contrasting freshwater systems[END_REF] , our calculation suggests that Fe 2+ (aq) binds strongly to DOC as it is present, in Lake Bromont porewater, at concentrations 2 to 3-fold higher than DOC. Fe 2+ (aq) could thus effectively negate DOC's effect on La 3+ by freeing the latter to bind to PO 4 3-and form LaPO 4(s).

The prevalence of Fe 2+ strongly depends on redox conditions. Under oxygenated conditions, Fe 2+ is rapidly oxidized to Fe 3+ and precipitates as Fe oxyhydroxides [START_REF] Schwertmann | Solubility and dissolution of iron oxides[END_REF] . In an oxic system dominated by Fe oxyhydroxides, DOC is available to bind La 3+ , with available PO 4 3-adsorbing on the freshly precipitated Fe 3+ oxyhydroxides [START_REF] Reitzel | Phosphate adsorption by lanthanum modified bentonite clay in fresh and brackish water[END_REF][START_REF] Dithmer | Influence of dissolved organic carbon on the efficiency of P sequestration by a lanthanum modified clay[END_REF][START_REF] Lürling | Humic substances interfere with phosphate removal by lanthanum modified clay in controlling eutrophication[END_REF][START_REF] Marsac | Modeling rare earth elements binding to humic acids with model VII[END_REF] . Visual inspection of the profiles on Fig. 5 suggests that the release of porewater P is closely coupled to that of Fe rather than La below the depth of LaPO 4(s) formation. Such coupling typically responds to seasonal oxygen depletion, which results in the recycling of PO 3 4-along with that of Fe (oxy)hydroxides, a chief cause of internal P loading 4-7, 86, 87 . Under such scenario where Fe fluxes exceed those of P across the SWI, well oxygenated bottom waters are desirable to maintain low P fluxes from the sediment to the bottom water [START_REF] Kleeberg | Redox sensitivity of iron in phosphorus binding does not impede lake restoration[END_REF] .

Dissolved Fe concentrations at depth in the porewater are tightly coupled to those of sulfides.

Evidence of SO 4 2-reduction (Fig. 5d,i,n) indicate that Fe titrate sulfides out of the porewater at the Deep site. S -II accumulate in the porewater at the House site and to a lesser extent at the Inflow site. Residual HS -may promote the reductive dissolution of Fe oxides, further releasing P adsorbed onto the oxide's surfaces [START_REF] Parsons | Sediment phosphorus speciation and mobility under dynamic redox conditions[END_REF][START_REF] Hyacinthe | An authigenic iron phosphate phase in estuarine sediments: composition, formation and chemical reactivity[END_REF] . While porewater P was stable with depth after its initial increase at the Deep and Inflow sites, it increased continuously at the House site (Fig. 5f). A layer of buried P org (Fig 3e) would be responsible for this supply to the porewater, testifying to historical inputs from septic tank overflow from the lakeside properties.

In a recent series of incubation experiments testing the effect of light and benthic photosynthesis on LMB efficacy, Li et al [START_REF] Li | Immobilization and Release Behavior of Phosphorus on Phoslock-Inactivated Sediment under Conditions Simulating the Photic Zone in Eutrophic Shallow Lakes[END_REF] found that the latter was lowered by photosynthesis, and ascribed it an increase in pH and to the release of DOC in the form of exudates from the algae. Through their reported concurrent increase in DO was not discussed as a factor in the reduction in LMB efficacy, we find it consistent with DO allowing DOC to freely bind La 3+ by lowering dissolved Fe 2+ . Thus, contrary to the intuition that oxic conditions prevent PO 4 3-mobility, LMB treated lakes could sequester P more effectively under anoxic and ferruginous conditions. This hypothesis is in line with the outcome of oxic vs anoxic laboratory experiments LMB, which

shows that it decreased P fluxes from the SWI under anoxic conditions, but not under oxic conditions [START_REF] Zeller | The efficacy of Phoslock® in reducing internal phosphate loading varies with bottom water oxygenation[END_REF] . Hence, we have generalized this hypothesis and used the thermodynamic model to illustrate (Fig. 7) that Fe 2+ and DOC are antagonistic with respect to their influence on LaPO 4(s)

formation. However, our interpretation does not account for kinetic aspects of the effect of DOC on LMB capacity, which remains unclear.

DOC can also hinder La 3+ uptake by LMB via the competitive sorption DOC as shown by Li et al. [START_REF] Li | Interactions of phosphate and dissolved organic carbon with lanthanum modified bentonite: Implications for the inactivation of phosphorus in lakes[END_REF] . These authors reported that DOC didn't influence the absorption of P when the C:P molar ratio was ~1.5 or higher. However, the influence of DOC was significant when the molar ratio exceeded 9. As for Lake Bromont, the C:P ratio exceeds this reported threshold, suggesting that DOC not only binds La 3+ directly in solution but also likely passivates the LMB. Dithmer et al. [START_REF] Dithmer | Influence of dissolved organic carbon on the efficiency of P sequestration by a lanthanum modified clay[END_REF] , in a long-term laboratory study, showed that the adverse effects of DOC on P updake by LMB is not permanent, as DOC can be degraded despite being bound to LMB, and that its complexation of La 3+ is fully reversible [START_REF] Li | Interactions of phosphate and dissolved organic carbon with lanthanum modified bentonite: Implications for the inactivation of phosphorus in lakes[END_REF] . As such, it is unclear which mechanism -passivation of LMB or complexation of La 3+ -best accounts for the role of DOC. In both cases, however, the reductive dissolution of Fe oxyhydroxides and the production of porewater Fe 2+ would be a desirable outcome, shifting the equilibrium away from the LMB-DOC and DOC-La 3+ complexes. Sources of uncertainty not accounted for here include further competition by cations such as Ca 2+ , which can be introduced to the lake via the use of CaCl 2 as a de-icing salt. On one hand, Ca 2+ has been shown to enable PO 4 3-binding to humics [START_REF] Tan | Humic matter in soil and the environment: Principles and controversies[END_REF][START_REF] Feng | Chemical and mineralogical controls on humic acid sorption to clay mineral surfaces[END_REF] , specifically via ternary complexes [START_REF] Audette | Phosphorus binding to soil organic matter via ternary complexes with calcium[END_REF] , which increases P mobility. On the other hand, the adsorption of Ca 2+ on oxides can increase surface potential, hence favouring PO 4 3-adsorption through electrostatic effect [START_REF] Antelo | Arsenate and phosphate adsorption on ferrihydrite nanoparticles. Synergetic interaction with calcium ions[END_REF] . Similarly, changing alkalinity was not accounted for, even though HCO 3 -can compete with PO 4 3-for binding with La 3+ 97, 98 . Alkalinity has also been shown to negatively impact dissolved La, due to low dispersion of LMB aggregates [START_REF] Reitzel | The influence of lake water alkalinity and humic substances on particle dispersion and lanthanum desorption from a lanthanum modified bentonite[END_REF] and to it further reducing the efficacy of PO 4 3-removal in the presence of DOC [START_REF] Zhi | Influence of natural organic matter and pH on phosphate removal by and filterable lanthanum release from lanthanum-modified bentonite[END_REF] . Together, these effects are detrimental to LMB's long-term efficacy.

Conclusions.

An application of 174 ton of La-modified bentonite to the eutrophic Lake Bromont (Canada)

resulted in an abatement of harmful algae blooms. We show that this treatment induced a shift in P partitioning from the sediment to more recalcitrant phases, away from pH-sensitive and organic P. However, porewater systematics revealed that P remained mobile, in close association with the redox-sensitive Fe cycle. Thermodynamic modelling suggested that DOC plays a key role in mediating this interplay, as it binds La 3+ , lowering the saturation of the porewater with respect to the key mineral phase, LaPO 4(s) , which the treatment seeks to promote. Our results

show that Fe 2+ indirectly promotes the formation of LaPO 3(s) suggesting that anoxic conditions may improve the longevity of the LMB treatment in the presence of DOC. Fig. 1 Location and bathymetry of Lake Bromont, along with sampling site locations (circles) and lake outlet (triangle). House (e-h) and Inflow (i-l) sites. with (solid line) and without (dashed line) accounting for the effect of DOC on aqueous La speciation (panel a,f,i).

Fig. 7 Model-predicted speciation (%) of La as function of both dissolved Fe 2+ and DOC under geochemical conditions prevailing at lake Bromont. The upper left zone represents a system where LaPO 4(s) dominates, while the lower right zone represents a system where DOC binds La 3+

and hinders LaPO 4(s) formation.

  Environmental Science: Processes & Impacts Accepted Manuscript Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. View Article Online DOI: 10.1039/D1EM00544H

  Environmental Science: Processes & Impacts Accepted ManuscriptPublished on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM.

  Environmental Science: Processes & Impacts Accepted Manuscript Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. View Article Online DOI: 10.1039/D1EM00544H

  Environmental Science: Processes & Impacts Accepted Manuscript Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. View Article Online DOI: 10.1039/D1EM00544H

  Environmental Science: Processes & Impacts Accepted Manuscript Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. View Article Online DOI: 10.1039/D1EM00544H

  Environmental Science: Processes & Impacts Accepted Manuscript Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. View Article Online DOI: 10.1039/D1EM00544H

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[

  Fe 2+ ] and [DOC] on the percentage of La precipitated as LaPO 4(s) was investigated using the software Phreeplot 64 , which can draw contour plots from PHREEQC calculations. The porewater composition used for the latter calculations was set to the mean concentrations of all samples analyzed in the present study ([P] = 22.85 µM, [Mn] = 54.50 µM, [Ca] = 507.62 µM, [S] = 24.97 µM, [La] = 29.6 nM). Here, we assumed that the redox potential was controlled by the redox couples O 2 /H 2 O in the oxygenated overlying waters of the Inflow site, and Fe(II)/Fe(OH) 3(s) in porewater samples with HS -concentrations <1 M. In all sulfidic samples, hereafter defined as those with HS -concentrations greater than 1 M, the redox potential was fixed by the redox couples SO 4 2-/HS -65 .

  Environmental Science: Processes & Impacts Accepted Manuscript Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. View Article Online DOI: 10.1039/D1EM00544Hand House sites, while that associated with the HCl step dominates at the Inflow site. HClextracted P does not dominate at the House site but demonstrates a sharp peak at the 4-6 cm depth interval.La and Fe concentrations were analyzed in the same fractions (Fig.3). Maximum values are found in the HCl fraction, in contrast to P which was highest in the NaOH fraction at both the Deep site and House sites. Sharp peaks of solid-phase HCl-extractable La are found at 3 and 5 cm depth at the Deep and House side, respectively, and to a lesser extent at the Inflow site. La concentrations were below the detection limit (2 g kg -1 ) at all depths below 8 cm at the three sites. Finally, P was correlated with La in the HCl-extracts at the Deep (R 2 = 0.86) and House (R 2 = 0.93) sites, but not at the Inflow site (R 2 = 0.005). TOC and C:N ratios, measured in decarbonated sediments, indicate values centred around 5% C org and a C:N ratio of 10 at the Deep site. While surface values are similar at the House site, a strong increase in TOC is observed below 15 cm depth, reaching 12% at depth and a C:N ratio of 15. Values at the Inflow site are the most variable, with peaks reaching 14% at 9 cm depth and 19% at 13 cm depth along with a minimum at 2% at 15 cm depth. The C:N ratio fluctuations parallel those of TOC.

3 . 4 .

 34 Environmental Science: Processes & Impacts Accepted ManuscriptPublished on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. Vertical distribution of dissolved P, La, Fe and C.The deep site was below the thermocline at the time of sampling (water temperature = 10.8 o C), while the two other sites were above the thermocline (19 o C). As for oxygen concentrations, the deep site hypolimnion was anoxic (DO <0.1mg/L), while the two other sites were oxygenated during sampling (DO of 7.4 and 6.2 mg/L at the Inflow and House sites, respectively.).The minimum concentration of porewater PO 4 3-is located just above the SWI, ranging between 1 and 1.5 µM at all sites (Fig.5 a,f,k). The PO 4 3-concentration then sharply increases to just below the SWI, where it either remains at peak value at the Deep and Inflow sites, or continually increases with depth at the House site. The highest PO 4 3-concentrations in porewater are 39 µM at 12 cm, 45 µM at 12 cm, and 23 µM at 6 cm for the Deep, House, and Inflow sites, respectively (Fig.5 a,f,k). The measured dissolved Fe profiles parallel those of P with a minimum concentration above the SWI and increases below it. Dissolved Fe reaches 500 µM at the Deep site, two orders of magnitude higher than at the other two sites where it remains between 5-10 µM. Maximum Fe concentrations are around 1219 µM at 4 cm, 813 µM 12 cm, and 322 µM at 12 cm for the Deep, House, and Inflow sites, respectively (Fig.5 b,g,i). Dissolved La concentrations, assumed to comprise truly dissolved and colloidal fractions passing through the 0.2 mm membrane, were below the detection limit of 0.004 µM above the SWI at the three sites (Fig.5 c,h,m), increased sharply at or below the SWI and remained at 0.06-0.07 µM between 2-6 cm depth, then decreased further. SO 4 2-shows maximum concentrations above the SWI, around 11 µM at the Deep site and about an order of magnitude higher (80-85 µM) at the other two sites (Fig.5 d,i,n). SO 4 2-sharply decreases below the SWI, at the Deep and House sites, while it decreases linearly at the Inflow site. S -II were below the detection limit of 0.7 M at the Deep site, while it increased markedly, Environmental Science: Processes & Impacts Accepted Manuscript Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. View Article Online DOI: 10.1039/D1EM00544H

  Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM.

  Environmental Science: Processes & Impacts Accepted Manuscript Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. View Article Online DOI: 10.1039/D1EM00544H

Fig. 2

 2 Fig. 2 Phosphorus partitioning determined by SEQ within the first 0-4 cm of the sediment

Fig. 3

 3 Fig. 3 Vertical distribution of total P (shaded symbols), soluble reactive P (open symbols (panels

Fig. 4

 4 Fig. 4 Upper panel: Distribution of sedimentary P species identified by 31 P NMR at depths

Fig. 5

 5 Fig. 5 Porewater concentration profiles of P (circles), La (squares), Fe (triangles), SO 4 2-(solid inverted triangles), S -II (open inverted triangle) and DOC (diamonds) at the Deep (a-e), House (fj), and Inflow (k-o) sites. The horizontal dashed line indicates the sediment-water interface. S -II were below detection limit of 0.7 mol L -1 at the Deep site.

Fig. 6

 6 Fig. 6 Measured DOC:P and DOC:La ratios as a function of depth at the Deep, House and Inflow

Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6

 123456 Fig. 1 Location and bathymetry of Lake Bromont, along with sampling site locations (circles) and lake outlet (triangle). 83x133mm (300 x 300 DPI)

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  Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. Michelle Cormier for help in the laboratory. We acknowledge funding from the Town of Bromont, and Quebec's Ministère de l'Économie et de l'Innovation. RMC acknowledges funding from the Sentinel North program of Université Laval and from the Global Water Futures project FORMBLOOM (made possible in part thanks to funding from the Canada First Research Excellence Funds), as well as from the National Science and Natural Sciences and Engineering Research Council of Canada thought the Discovery Grant program. Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM.

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	DOI: 10.1039/D1EM00544H DOI: 10.1039/D1EM00544H DOI: 10.1039/D1EM00544H DOI: 10.1039/D1EM00544H DOI: 10.1039/D1EM00544H DOI: 10.1039/D1EM00544H DOI: 10.1039/D1EM00544H DOI: 10.1039/D1EM00544H
	1 2 3 4 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 during sampling and Sarah-1 4 4 4 4 4 4 4
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Table 1 .

 1 Summary of the phosphorus sequential extraction scheme.

	Step Extractant	Duration	Target P phase
			(hrs)
	1	Oxygen-free H 2 O	2	Loosely bound P
	2	0.11 M Bicarbonate/Dithionite, pH 7 1	Bound to redox sensitive Fe and/or
				Mn oxides and P org
	3	0.1 M NaOH	16	Bound to Fe/Al oxides and
				comprising microorganisms,
				detritus and humic substances (P org )
	4	0.5 M HCl	16	Bound to apatite and La

Table 2 .

 2 Reactions for the formation of the various La species and their corresponding equilibrium constants added to PHREEQC. Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM.

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	6 7 8 9	Reaction Ln 3+ + H 2 O LnOH 2+ + H + ⇌ Ln 3+ + 2 H 2 O Ln(OH) 2 + + 2H + ⇌ Ln 3+ + 3 H 2 O Ln(OH) 3(aq) + 3H + ⇌ Ln 3+ + HCO 3 -LnHCO 3 2+ ⇌ Ln 3+ + CO 3 2-LnCO 3 + ⇌ Ln 3+ + 2 CO 3 2-Ln(CO 3 ) 2 -⇌ Ln 3+ + Cl -LnCl 2+ ⇌ Ln 3+ + NO 3 -LnNO 3 2+ ⇌ Ln 3+ + F -LnF 2+ ⇌ Ln 3+ + H 2 PO 4 -LnH 2 PO 4 2+ ⇌ Ln 3+ + HPO 4 2-LnHPO 4 + ⇌ Ln 3+ + 2 HPO 4 2-Ln(HPO 4 ) 2 -⇌ Ln 3+ + PO 4 3-LnPO 4(aq) ⇌ Ln 3+ + 2 PO 4 3-Ln(PO 4 ) 2 -3 ⇌ Ln 3+ + SO 4 2-LnSO 4 + ⇌ Ln 3+ + 2 SO 4 2-Ln(SO 4 ) 2 -⇌ Ln 2 (CO 3 ) 3(s) 2Ln 3+ (aq) + 3CO 3 2-(aq) ⇌ Ln(OH) 3(s) + 3H + Ln 3+ + 3H 2 O ⇌ Ln(OH) 3(am,fresh) + 3H + Ln 3+ + 3H 2 O ⇌ LnPO 4(s) Ln 3+ + PO 4 3-⇌	Log K eq -8.81 -18.14 -27.90 2.34 6.73 11.3 0.65 0.58 3.11 2.5 5.1 8.4 10.96 17.6 3.4 5.1 -29.91 19.72 21.94 -25.75	Ref. 99, 100 99, 100 99, 100 101 101 101 101 102 103 104 104 104 104 104 104 104 98 105 106 107	Environmental Science: Processes & Impacts Accepted Manuscript

Table 3 .

 3 Fluxes of P, La and Fe across the sediment-water interface (SWI) calculated via inverse diagenetic modelling using PROFILE. Positive values indicate transport from the sediment to the water column. Units of mass are used instead of molar units for comparisons with previous estimates and other sites.

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	9	Flux across the SWI (mg m -2 d -1 )
		La	P	Fe
	Deep site	+2.1 × 10 -7	+ 2.5	+40
	House site +1.1 × 10 -7	-0.2	+7.2
	Inflow site +1.3 × 10 -7	+ 1.0	+8.3

Environmental Science: Processes & Impacts Accepted Manuscript Published

  on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM.

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	DOI: 10.1039/D1EM00544H

Published on 23 May 2022. Downloaded by Bibliotheque de L'Universite de Rennes I on 5/29/2022 9:59:47 AM. View Article Online DOI: 10.1039/D1EM00544H

Acknowledgements:

We acknowledge Pierre Audet for help with NMR, Serge Groleau for help with ICP-QQQ-MS, Marie-Pierre Varin, Anne-Sophie T. Carrier and Michelle Champagne from ACBVLB for help