# Increasing carbon burial over recent decades despite re-oligotrophication in deep peri-alpine Lake Aiguebelette, France

Jean-Philippe Jenny, Cécilia Barouillet, Isabelle Domaizon, David Etienne, Irene Gregory-Eaves, Olivia Itier-Desgué, Andrea Lami, Erwan Messager, Pierre Sabatier, Julie Rotschi

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Jean-Philippe Jenny, Cécilia Barouillet, Isabelle Domaizon, David Etienne, Irene Gregory-Eaves, et al.. Increasing carbon burial over recent decades despite re-oligotrophication in deep peri-alpine Lake Aiguebelette, France. Journal of Paleolimnology, 2025, 73 (4), pp.257-276. 10.1007/s10933-025-00358-y. hal-05328331

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Distributed under a Creative Commons Attribution 4.0 International License
2 Increasing carbon burial over recent decades

3 despite re-oligotrophication in deep peri-alpine Lake

4 Aiguebelette, France

5 Jean-Philippe Jenny · Cécilia Barouillet · Isabelle Domaizon · David Etienne · Irene Gregory-Eaves ·

6 Olivia Itier-Desgué · Andrea Lami · Erwan Messager · Pierre Sabatier · Julie Rotschi

7 Received: 21 September 2024 / Accepted: 10 April 2025

8 © The Author(s), under exclusive licence to Springer Nature B.V. 2025

9 **Abstract** Cultural eutrophication has contributed to enhanced phytoplankton biomass in lakes around the world, leading to an overall increase in sedimentary carbon (C) sequestration since the twentieth century. However, the effects of land use changes and restoration initiatives on sedimentary C sequestration are not yet well understood. To address this gap, we

A1 **Supplementary Information** The online version
A2 contains supplementary material available at https://doi.

A3 org/10.1007/s10933-025-00358-y.

A4 J.-P. Jenny (✉) · C. Barouillet · I. Domaizon · D. Etienne ·

A5 O. Itier-Desgué · J. Rotschi

A6 CARRTEL, UMR042, Institut National de Recherche pour

A7 l'Agriculture, l'Alimentation et l'Environnement (INRAE),

A8 Université Savoie Mont Blanc, 74203 Thonon-les-Bains,

A9 France

A10 e-mail: Jean-Philippe.Jenny@inrae.fr

A11 J.-P. Jenny · C. Barouillet · I. Domaizon · D. Etienne ·

A12 O. Itier-Desgué · J. Rotschi

A13 Pole ECLA, OFB, INRAE, USMB,

A14 74200 Thonon-les-Bains, France

A15 I. Gregory-Eaves

A16 Department of Biology, McGill University, Montréal,

A17 QC H3A 1B1, Canada

A18 A. Lami

A19 Unit Verbania, Water Research Institute (CRN-IRSA),

A20 Viale Tonolli 50, 28922 Verbania, Italy

A21 E. Messager · P. Sabatier

A22 EDYTEM, UMR 5204 CNRS, Université Savoie Mont

A23 Blanc, Pôle Montagne, 73376 Le Bourget du Lac, France

applied the paleolimnological approach to a temper-
ate, hardwater site, Lake Aiguebelette, to investigate
temporal changes in organic and inorganic C seques-
tration in relation to changes in C supplies, lake pri-
mary production, and preservation conditions over
the past 300 years. Our results show that C seques-
tration has constantly increased in Lake Aiguebelette
over the last 150 years due to local human distur-
bances. A first increase in lake carbon sequestration

A01 was recorded between 1850 and the 1860s, a period marked by deforestation, marsh drainage, and lake level regulation—activities that collectively accelerated carbonate supply and precipitation in the lake. Subsequently, lake organic biomass production exceeded inorganic supplies and became the main contributor to total C sequestration. A period of nutrient enrichment in lake waters spanned from the 1950s to the 1990s, which coincided with increased lake biomass and reduced hypolimnetic oxygen conditions. Over the last three decades, C sequestration has continued to increase, despite the reduction in nutrient loads following restoration measures. Our results suggest a long history of human disturbances, leading to a succession of perturbations in the carbon cycle in Lake Aiguebelette that favored C supplies and C fixation. In contrast, recent C burial appears to be controlled by decreased bottom water oxygen conditions, despite improvements in surface water quality. Overall, our study highlights a range of processes that can influence C burial over time, with recent reductions in nutrient loading during an era
of climate warming resulting in both improved water quality and enhanced C storage.

**Keywords** Lakes · Re-oligotrophication · Carbon sequestration · Primary production · Pigments · Algal assemblages · Lake trophic state · Thermal stratification · Hypoxia

## Introduction

Inland waters transfer, transform and stock substantial amounts of terrestrial carbon (C) along the land to ocean aquatic continuum (Wilkinson et al. 2018), of which approximately half reaches the sea (Cole et al. 2007; Tranvik et al. 2009). Carbon that is not transferred to the sea is partly emitted from inland waters to the atmosphere, acting as greenhouse gases or rapidly recycled and reintegrated into the biosphere (Mendonça et al. 2017). Another part of this C is stored in aquatic sediments and enters the long-term geological cycle, with annual accumulation estimates close to those measured for the global oceans (Downing et al. 2008; Tranvik et al. 2009; Wilkinson et al. 2018). Exchanges of carbon and the role of lakes along the land to ocean continuum have been investigated both for organic and inorganic forms, and have been placed for years now in the active pipe concept (Cole et al. 2007). Previous studies have examined lake carbon burial across various regions, including North America (Dietz et al. 2015; Heathcote et al. 2015), Europe (Anderson et al. 2014), the Tropics (Amora-Nogueira et al. 2022), and the Arctic (Anderson et al. 2009). Although lake sediments play an important role in the global carbon cycle (e.g., Anderson et al. 2020), the global drivers of lake C sequestration and its temporal variability have not been systematically assessed in the context of reoligotrophication, common in many European lakes.

Long-term accumulation of total C in lake sediments depends on 1) inputs of allochthonous and autochthonous carbon in the form of inorganic carbon (IC) and organic carbon (OC), 2) the decomposition of organic matter in lakes, and 3) its export via surface or groundwater outflows. Allochthonous inputs consist of externally derived C, including terrestrial dissolved and particulate forms of C from surface and groundwater inflows, litterfall, and direct-fall precipitation (McCullough et al. 2018). Autochthonous

inputs of OC originate from the lake itself through photosynthesis by primary producers and bacterial exudates (McCullough et al. 2018), whereas autochthonous inputs result from IC originate through internally derived bio-precipitation of carbonates caused by changes in the acid-base balance (CO₂ consumption) (Chen et al. 2021). Degradation of OC is primarily controlled by oxygen exposure time, the source and degradability of organic matter, and water temperature (Ferland et al. 2014). The long-term burial of total C in lake sediment is largely influenced by anthropogenic activities, such as agriculture, wastewater input, and urbanization (Heathcote et al. 2015; Anderson et al. 2013), which affect mass supplies, lake production and rates of organic matter degradation.

In the case of OC, human-induced increases in nutrient loading have enhanced the accumulation of C globally since 1900 (Anderson et al. 2020). Meanwhile, the application of restoration programs in a growing number of lakes around the world has successfully led to lower water phosphorus (P) levels toward pre-eutrophication concentrations in many of these lakes. However, less attention has been given to the effects of nutrient reduction on carbon sequestration, and the effects of the success of eutrophication mitigation practices on C trends in reoligotrophied lakes remains unclear. Moreover, an increase in soil erosion following land denudation accelerates both IC and OC exports to inland waters (Van Oost et al. 2007), with IC exports being particularly sensitive in carbonate-rich areas. However, the interplay between organic and inorganic carbon cycle in these reoligotrophied lakes remains elusive.

In this study, we investigate historical trends of carbon sequestration in Lake Aiguebelette, which has experienced reoligotrophication over the past few decades. More specifically, we evaluate the relative contribution of land exports, lake primary production and preservation conditions to carbon sequestration rates over the last 300 years. We hypothesize that pre-industrial C burial in Lake Aiguebelette was mainly controlled by the inorganic carbon cycle, carbonate saturation, and minor increases in lake primary production that may have resulted in accelerated carbonate precipitation periods without altering oxygen conditions (Hyp 1, Fig. 1). After 1850 CE, we propose that C burial was alternatively or simultaneously by lake primary production and oxygen conditions due

<figure><img src="image_2.png" /></figure>
<figure><img src="image_3.png" /><figcaption>**Fig. 1** Working hypothesis on the drivers of C burial in Lake Aiguebelette during the last 200 years. Hyp 1 assumes that carbonate precipitation in saturated waters is highly sensitive to first (small) increase in NPP. Hyp 2 and 4 assume that OC is driven by primary production or oxygen conditions, respectively. Hyp 3 assumes that productivity is the main driver of OC during eutrophication phase, whereas hypoxia creates a hysteresis and is the main driver of OC burial during reoligophotrophication phase. The grey shading in the right three panels represents the onset of low oxygen conditions as inferred by the varve records. Potential control factors on OC burial have been simplified and reduced to two opposed poles: lake organic production and low oxygen conditions, which effects on OC burial are generally difficult to disentangle. Asterisks denote the hypotheses supported by the study's findings (see results and discussion)</figcaption></figure>

to cultural eutrophication (Hyp 2, 3 & 4). We further hypothesize that lake primary production and OC burial should be positively and linearly correlated in sediment records, but that oxygen condition fluctuations may alter this linearity. This implies that analyzing linear correlation strength between lake primary production and OC burial in the case of Lake Aiguebelette should help disentangle the relative contribution of production and preservation condition on C burial in lake sediments (Fig. 1).

## 151 Study site

Lake Aiguebelette (45° 33' N, 5° 48' E, 374 m above sea level (a.s.l.))—Fig. 2, Table 1) is a deep hardwater lake, located on the north-western side of the French Alps. It is composed of three main basins, with a maximum depth of 71 m, a surface area of 5.4 km², and water residence time is approximately 3 years. The lake is fed by Leysse de Novalaise and Tuilerie rivers located on the north and south shores, respectively, and the outflow on the south-west is the canal du Thiers. Lake catchment is 58.9 km² with a mean altitude of 750 m a.s.l., and is mainly composed of marl, molasse and limestone. Aiguebelette is a warm

monomictic lake which is thermally stratified from March to November (Desgué-Itier et al. 2023). Therefore, mixing of the deepest layers is limited, and mean autumn bottom oxygen concentrations are as low as 0.8 $mgO_2 L^{-1}$ over the monitored period (Fig. S1c). Surrounding mountains prevents strong winds from blowing at its surface, making Lake Aiguebelette a good sentinel of air temperature changes.

Human activities have shaped the surroundings of the lake for a long time. In the 1850s, the lake level was regulated, and surrounding marshes were drained. In 1906, hydroelectric management at the outlet resulted in the raising of the level of the lake by 1 m. In the 1930s, signs of cultural eutrophication (i.e., high phosphorus levels, low oxygen concentrations and observations of filamentous algae) were first reported (Hubault 1947). Historical limnological surveys indicate that the lake was mesotrophic in the 1970s (Fig. S1b, ©OLA-IS, AnaEE-France, INRAE of Thonon-les-Bains, CCLA—Rimet et al. 2020). Restoration programs (wastewater diversion and treatment) were initiated in the 1970s and reduction in nutrient supplies brought phosphorus concentrations to below 10 $µgP L^{-1}$ since 2010, after peaking around 25 $µgP L^{-1}$ in the 1980s (Fig. S1).

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<figure><img src="image_4.png" /><figcaption>**Fig. 2** Lake Aiguebelette location and coring sites. Site location (a), bathymetric map, tributaries, coring sites in the deepest zone (red dot) and coring sites along an East–West tran-</figcaption></figure>

sect (white dots) in Lake Aiguebelette (b), elevation map and watershed area encompassing mountainous land to the East part (c)

<table><caption>**Table 1** Lake Aiguebelette geomorphological characteristics</caption><thead><tr><td>Lake parameter</td><td>Value</td></tr></thead><tbody><tr><td>Total lake area (km²)</td><td>5.4</td></tr><tr><td>Watershed area (km²)</td><td>58.9</td></tr><tr><td>Volume (m³)</td><td>166 10⁶</td></tr><tr><td>Maximum depth (m)</td><td>71</td></tr><tr><td>Mean depth (m)</td><td>30.7</td></tr><tr><td>Maximum length (km)</td><td>4</td></tr><tr><td>Maximum width (km)</td><td>3</td></tr><tr><td>Latitude</td><td>45° 33' N</td></tr><tr><td>Altitude (masl)</td><td>374</td></tr><tr><td>Water residence time (yr)</td><td>3</td></tr></tbody></table>

## 189 Material and methods

190 Sediment coring and dating

191 *Deep pelagic cores*—Three pelagic cores were retrieved from a depth of 69 m (Fig. 2) in November 2019 and January 2020 using a Uwitec gravity corer. Each core preserved an undisturbed sediment-water interface. Cores AIG19-02 (1.30 m in length), AIG20-01 (2.1 m in length) and AIG20-02

(2.16 m in length) were split lengthwise, and each half was photographed. One half was sectioned at every 1 cm for further analyses, while the other half was reserved for non-destructive analyses, including elemental X-Ray Fluorescence (XRF) measurements, spectro-colorimetry) prior to sediment sampling.

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<figure><img src="image_5.png" /></figure>
15 cm of the core were also counted, which in turn allowed us to validate the age model and to correlate cores. The varves were independently counted twice on high-resolution images by two operators, with a discrepancy of only one year between the counts. A summary of the analyses conducted for each core is provided in Table S1

*Transect of sediment cores*—In addition to pelagic cores, eleven short sediment cores were retrieved along an East–West transect at depths 0, 1, 3, 5, 13, 19, 25, 37, 46, 63 and 64 m. Analysis of the spatial distribution of laminated sediments was performed, and C,N,P measurements were performed at five depths (i.e. at 1, 3, 5, 10 cm and bottom) in each core. Note that the transect cores were used to study concentration variations within the sedimentary basin, not accumulation rates. No sediment dating was performed on the transect cores.

Sediment core analysis

*Geochemical analyses*

OC and IC contents were estimated using the loss on ignition (LOI) approach (Heiri et al. 2001). Since about half the organic matter in sediments is composed of OC, the weight loss after heating dried sediment samples at 550 °C was multiplied by 0.5 to estimate the OC content (Dean Jr. 1974; Vereš 2002). The IC content was calculated by multiplying the weight loss between loss-on-ignition at 550 °C and loss-on-ignition at 950 °C with 0.273, which is the ratio between the molar mass of C and CO₂ (Santisteban et al. 2004). Total C (TC) was estimated as the sum of IC and OC. Carbon accumulation rates (organic and inorganic forms) were calculated by multiplying the C content with a constant mass accumulation rate (MAR) obtained from the age model using $^{210}$Pb$_{ex}$.

High-resolution, semi-quantitative geochemical analyses were performed at the Environnements dynamiques et territoires de la montagne (EDYTEM) laboratory at University of Savoie Mont-Blanc, France, on a AVAATECH XRF Core Scanner 3. The open surface of the split half of the core was first covered with a 4-µm-thick Ultralene film to avoid contamination and desiccation of the sediment. All measurements were done at a 1 mm step resolution and element intensities expressed in counts per

second (cps). The parameters used for analysis were as follow: 10 kV, 0.25 mA and 20 s measurement time to detect light elements (Na to Ti), and 30 kV, 0.4 mA and 30 s measurement time for heavier elements (Mn to Pb—Pd-thin filter).

*Proxy-based environmental reconstructions* 273

Three environmental proxies were derived from the XRF measurements:

Hypolimnetic oxygen conditions were assessed using the Fe:Mn ratio, which provides information about changing redox conditions related to bottom water oxygen concentrations (Davison 1993). This signal must be interpreted with caution since it relies on biological- and chemical-dependent processes. However, the reliability of the signal was evaluated by conducting a correlation with bottom oxygen concentrations measured since 1974 and the presence of varves in perialpine lakes (i.e., annual layers of finely laminated sediment) (Jenny et al. 2014).

Internal nutrient loads were reconstructed with our estimates of phosphorus enrichment factor (EF-P) (Weiss et al. 1999), which can differentiate the increases of phosphorus above natural background value (detrital origin) and quantify the magnitude of anthropogenic enrichment levels. The enrichment factor is calculated as follows (Weiss et al. 1999):

$$EF - P = (P : Al)_{\text{sample}}/(P : Al)_{\text{background}} \quad 294$$

where $(P:Al)_{sample}$ is the phosphorus to aluminium ratio of the sample and $(P:Al)_{background}$ is the reference background value corresponding to the mean $P:Al$ value between 1800 and 1900. These values were interpreted with caution, acknowledging the higher level of uncertainty associated with phosphorus as a proxy due to its redox sensitivity, which can lead to migration within sediment records. We anticipate a higher level of uncertainty with this proxy, requiring prudent interpretation and tempered conclusions.

Trophic state variations were estimated using Daphnia spp. subfossil abundances following the method of DeMott and Gulati (1999) and Berthon et al. (2013). This method was tested in the case of the French perialpine lakes and shows good agreement with mean phosphorus concentrations in the water column during winter (Berthon et al. 2013).
Variations in terrigenous inputs were assessed using the titanium (Ti), which can be used as an indicator of weathering intensity and sedimentary discharge from the catchment area (Sheldon and Tabor 2009; Voigt et al. 2017).

Annual air temperatures have been obtained from the HISTALP database available since 1768 (Auer et al. 2007) to account for a potential climatic impact on the lake thermal regime.

### Pigment analysis

Sedimentary pigments were analysed following the method developed by Lami et al. (1994). The samples were preserved in the dark and frozen at -20 °C until analysis. A sub-sample of ~2 g of wet sediment was weighted and extracted overnight with ca 10 mL of 90:10 acetone:water. The sample was then centrifuged at 3000 rpm for 10 min in a glass centrifuge tube. The obtained pigment extract was then analysed spectrophotometrically to estimate total pigments (chlorophylls and their derivatives, CD; total carotenoids (i.e., carotenes and xanthophylls), TC) (Lami et al. 2000). The concentrations of specific carotenoids were estimated using an ion-pairing reverse-phase High-Pressure Liquid Chromatography (HPLC) with UV-VIS detector (DAD). A gradient pumping system and dual channel variable wavelength US-VIS detector (set at 460 nm and 656 nm for carotenoids and chloropigments, respectively). An auto-injector equipped with a Rheodyne valve was connected through a precolumn to a reverse-phase C18 ODS column (5 µm particle size; 250 mm x 4.6 mm i.d.). After sample injection (300 µl of acetone extract not dried in a rotary evaporator), a gradient program that ramped from 85% mobile phase A (80:20, by vol. methanol:aqueous solution of 0.001 M ion-pairing and 0.001 M propionic acid) to 100% mobile-phase B (60:40, acetone:methanol) in 30 min with a hold for 20 min provided sufficient resolution of all pigments of interest. Flow rate increased from 1 ml min -1 to 2 ml min. The column was re-equilibrated by linear ramping to 85% mobile-phase A for 5 min and maintenance for 10 min before sample injection. With this procedure, we were able to separate zeaxanthin from lutein and beta-carotene from phaeophytin a (Guilizzoni and Lami 1992). CD are expressed as spectrophotometric units per gram of dry sediment ($U \text{ g}^{-1}_{\text{dry sed}}$) (Guilizzoni et al. 1983).

The total and the single carotenoids are expressed as $mg \text{ g}^{-1}_{\text{dry sed}}$ and $nmoles \text{ g}^{-1}_{\text{dry sed}}$, respectively (Züllig and Rheineck 1985).

CD reflects total algal biomass and was thus used as a proxy of overall PP. Changes in phytoplankton communities were investigated by associating selected pigments to specific algal groups according to Leavitt and Hodgson (2002), Guilizzoni and Lami (2003) and Roy et al. (2011). Diatoxanthin and fucoxanthin were associated with Bacillariophyta, Dinophyta and Chrysophyta. Alloxanthin was associated to Cryptophyta, Echinenone and zeaxanthin were attributed to various Cyanobacteria, and lutein was associated to Chlorophyta. Pigment preservation was evaluated with the ratio 430:410, which reflects the different wavelength absorptions of chlorophyll-a related to its degradation products (Guilizzoni et al. 1992).

### Statistical analysis

Spearman rank correlations (Fig. S2) were computed to assess monotonic relationships between the response and potential forcing variables, accounting for the non-normal distribution of most of the variables (as revealed by Shapiro-Wilk's test; normal if $p > 0.05$). Explanatory variables (environmental proxies and pigments) showing a significant correlation to our response variables (C accumulation rates and CD) with a $p$-value < 0.05 were selected for further analysis.

Stratigraphical units of recent sediments in Lake Aiguebelette were assessed with constrained hierarchical clustering (CONISS—rioja R package, Grimm 1987; Juggins 2024) using a Bray-Curtis dissimilarity matrix (vegan R package, Oksanen et al. 2020) performed on the geochemical and pigment variables as well as other proxies (described above). This type of clustering was constrained by sample in order to respect the stratigraphy of the data. The number of zones was evaluated by comparing the dispersion of clusters with a broken stick distribution (Fig. S3) and represent the points of highest dissimilarity (Fig. S4).

Multivariate ordination techniques were conducted to explore relationships within and among matrices and extract synthetic variables. The general trends of the xanthophyll pigments were summarized in the first two principal component axes (PCs) of a principal component analysis (PCA). A Multiple Factor

<figure><img src="image_6.png" /></figure>
Analysis (MFA) was then applied on our dataset to evaluate the correlations among sets of variables that were structured into groups and evaluate the contribution of each group to the general variability. Both types of analyses were conducted using the Facto-MineR package (Le et al. 2008).

Generalized additive models (GAMs) were applied to estimate smooth functional relationship through time between 1) biogeochemical proxies and OC accumulation and 2) algal communities and CD using the mgcv package (version 1.8–33, Wood 2006), following the methods described in Simpson and Anderson (2009). In order to assess the relative influence and temporal contribution of primary production, hypolimnetic oxygen levels and inputs of terrigenous materials, the proxies CD, Fe:Mn and Ti, respectively, were used as the covariates in a first model. A second model was then applied to investigate the influence of changes in algal communities on the temporal variation of total primary production. For this purpose, the PC scores axis 1 and 2 were used as covariates. Each GAM was modelled using thin-plate

regression splines as the basis spline for all covariates, and the degree of smoothness or spline degree of freedom was determined via restricted maximum likelihood (REML).

## Results

### Sediment dating

Unsupported $^{210}$Pb activities ($^{210}$Pb$_{ex}$) plotted on a logarithmic scale revealed a well-constrained and constant mean SAR of 2.1 mm y$^{-1}$ (Fig. 3). The deepest samples, which show $^{210}$Pb$_{ex}$ activities close to the detection limit of secular equilibrium, were discarded to avoid an unrealistic change in SAR with lower rates in the upper part of the core. The $^{137}$Cs activity profile reveals two peaks at 55±5 mm and 110±10 mm, corresponding to the 1986 CE Chernobyl accident and the fallout from the 1963 CE nuclear weapon tests, respectively (Fig. 3). $^{137}$Cs activities are close to zero below 135±5 mm indicating that the deposition

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<figure><img src="image_7.png" /><figcaption>**Fig. 3** Chronology of the upper 26 cm of core AIG20-01, based on $^{210}$Pb$_{ex}$ activities, controlled by short-lived radionuclides ($^{137}$Cs and $^{241}$Am) and varve counting using a CFCS model, and showing a constant SAR of 2.1 mm</figcaption></figure>

y$^{-1}$. FF=nuclear weapon tests First Fallouts (i.e., 1955), NWT=Nuclear Weapon Tests peak (i.e., 1963) and Cher=Chernobyl accident (i.e. 1986)
occurred before 1955 CE (first nuclear fallouts). $^{210}$Pb$_{ex}$ derived ages and artificial radionuclides are in good agreement with the counting of laminated sediment, suggesting that sediments are annually laminated in Lake Aiguebelette, and can be considered as varved sediments. Dating of the remaining 46 cm of the core was obtained by applying a down-core extrapolation of the constant SAR of 2.1 mm y$^{-1}$.

Trophic state history of Lake Aiguebelette

The trophic state history of Lake Aiguebelette reconstructed from sedimentary *Daphnia* remains (see Berthon et al. 2013 for details) and water column chemistry data from 1989 show an increase in the nutrient concentrations since mid-twentieth century and then a reoligotrophication trend from 2000 onwards (Fig. S1). Three major time zones were identified (Fig. 4): zone I (1692–1902 CE), zone II (1911–1947 CE) and zone III (1956–2015 CE). Broken stick comparison also revealed two additional subdivisions around 1840 and 1975 CE. Zone I (1692–1902 CE) was characterized by low values of carbon accumulation and primary production, highlighted by CD values. Zone II (1911–1947 CE) shows an increase in OC accumulation and a slight increase in primary production. Zone III (1956–2015 CE) corresponds to an acceleration in carbon sequestration, along with an acceleration in primary production and a decrease in oxygen conditions.

Historical trends in C sequestration

Trend analysis of total carbon (TCAR) accumulation indicates baseline conditions of relatively low values of carbon until the 1840s, followed by an uninterrupted increasing trend from 1840s (Fig. 4). The rapid increase in TCAR accumulation resulted

<figure><img src="image_8.png" /><figcaption>**Fig. 4** Geochemical and environmental data from AIG19-02 and AIG20-01 cores: total, inorganic and organic carbon content (TCAR, IC and OC, respectively) and accumulation rates (red lines), chlorophyll derivatives concentration (CD), Ti:Al and Fe:Mn ratios and EF-P (black lines = 4-year averages). Sedimentary total phosphorus concentrations (orange points). Long-term air temperatures from the HISTALP project. Smoothed data trends from GAM in red. Limnological</figcaption></figure>

data from the OLA database in blue: mean interannual integrated chlorophyll-a concentrations, autumn bottom water oxygen, winter phosphorus and surface water temperature. High resolution core photograph showing varved sediment in the upper 15 cm. Cluster dendrogram defining three main temporal zones. The red lines indicate the boundaries of the major zones defined using the CONISS method, while the blue dashed lines represent the manually defined minor zones

<figure><img src="image_9.png" /></figure>
in concentrations of around 60 gC m⁻²y⁻¹ prior to the 1840s to 82 gC m⁻²y⁻¹ after the 1840s. Modern accumulation of total carbon in lake sediments corresponds to 102 gC m⁻²y⁻¹ in 2015.

Both inorganic and organic carbon contributed to C accumulation rates. Rapid increase in IC accumulation is observed in the sediment records at ca. 1840 (Fig. 4), with values increasing from 30 gC m⁻²y⁻¹ to 55 gC m⁻²y⁻¹. A Welch test highlighted significant trends between 1850 and 1900 for IC, with average accumulation rates of 33.8 gC m⁻² y⁻¹ before 1850 and 44.6 gC m⁻² y⁻¹ between 1850 and 1900 (p-value = 0.0026). This first increase is concomitant with the first increase in total carbon accumulation observed in sediments of Lake Aiguebelette. Thereafter, IC accumulation values plateaued with only minor increase. Increase in organic carbon accumulation started a century later than the increase in IC accumulation, approximately in the middle of the twentieth century. Accumulation of OC shows values of 30 gC m⁻²y⁻¹ until 1900 CE, and then gradually increases to reach 44 gC m⁻²y⁻¹ in 2015 CE. Moreover, a significant difference was observed when comparing the periods before and after 1900 CE, with

average Corg accumulation rates increasing from 32.1 gC m⁻²y⁻¹ to 37.8 gC m⁻²y⁻¹ (p-value = 2.5 × 10⁻⁶).

Variations in primary production and phytoplankton communities

Primary production remained low and stable until ca. 1840 (Fig. 5). Chlorophyll derivatives (CD) concentrations increased slightly until the 1900s. Mean CD values between 1850 and 1900 were 17.6 ± 3.6 U g⁻¹ dry sed whereas before 1850 mean values of CD were 12.2 ± 1.1 U g⁻¹ dry sed. (Welch Two Sample t-test, p-value = 0.1152). Then CD values significantly increased around 1950 CE (p-value = 4.487e-06) from values of 1.16 U g⁻¹ dry sed to 1.77 U g⁻¹ dry sed, with a maximum concentration of 9.94 U g⁻¹ dry sed occurring at ca. 1980. CD slightly decreased afterwards to a mean value of 6.36 U g⁻¹ dry sed between 1980 and 2015 CE but remain five times higher than during baseline conditions.

The pigment preservation (430:410 ratio) remains stable throughout the record and shows very limited degradation issues of our pigment records, which can thus be used as reliable indicators of PP and algal

<figure><img src="image_10.png" /><figcaption>**Fig. 5** Historical trends of chlorophyll derivatives (CD) and total carotenoids (TC) from spectrophotometric analysis, and xanthophyll concentrations from HPLC analysis. The 430:410</figcaption></figure>

ratio reflects overall pigment preservation (1 = 100%). Cluster dendrogram defining three main temporal zones (shown in red and subzones indicated by blue dashed lines)
communities (Fig. 5). Specific xanthophyll pigments follow a similar initial trend as CD concentrations with a general first increase from 1900 CE, followed by a second stronger increase around 1950 CE. While some pigment concentrations (i.e., diatoxanthin, echinenone and lutein) slightly declined to lower values, others remained high over the recent time period (fucoxanthin and zeaxanthin), or are still increasing (alloxanthin, zeaxanthin). Total carotenoids (TC) concentrations also increased over the studied period with persistent high values for the recent years (Fig. 5).

Temporal changes in oxygen conditions and terrigenous supplies

Low oxygen conditions were observed until around 1900 (Fig. 4, zone I). A first slight decrease in oxygen conditions, highlighted by the decrease in the Fe:Mn ratio, started around ca. 1900 CE (i.e. zone II). Then, a transition toward low oxygen conditions occurred in around 1950 CE highlighted by the onset of varve preservation in the upper 15 cm of the core (i.e. zone III). These results are in agreement with benthic autumnal oxygen concentrations recorded in limnological records (Fig. 4, blue dots).

The watershed soil alteration proxy, the Ti value, shows relatively constant mean values until around 1900 (i.e. zone I). Then, the proportion of terrigenous inputs from alteration started to decrease since the beginning of the twentieth century.

Principal component (PCA) and multiple factor analysis (MFA)

The PCA of our xanthophyll pigments explained 90.5% of the total variance, with PCA axis 1 and 2 explaining 81.2% and 9.3%, respectively (Fig. S5). While the variability explained by the first axis accounts for all the pigments, with a positive correlation and higher representation for fucoxanthin, alloxanthin and echinenone (cos²=0.93, 0.93 and 0.91, respectively), the second axis is mostly represented by diatoxanthin (cos²=0.33), and weakly by zeaxanthin and lutein (cos²=0.11 and 0.07, respectively), which are both negatively correlated.

The MFA analysis shows that 75.1% of the variability is explained by the first axis, the second axis accounting for 9.1%, for a total of 84.2% of

the variability explained (Fig. 6). The partial axes of the first dimensions show a good correlation between xanthophyll pigments and our response variables (terrigenous, organic and IC accumulation, CD and TC) on the first PC axis, which both contribute significantly to the variance (Fig. 6b). The geochemical proxies also have a strong negative contribution to our response variables on the first dimension. The second dimension is less explanatory, and both the pigments and geochemical variables are negatively correlated to our response variables. The correlations between the xanthophyll pigments and the response variables show that all pigments are strongly correlated to PP and C<sub>seq</sub> on the first dimension (Fig. 6b). The geochemical variables show a good correlation to C<sub>seq</sub> on the first dimension, with a positive contribution from temperature and the EF-P and a negative contribution from Ti and Fe:Mn. These results are summarized in the PCA biplot, showing the contribution of all the variables to the different temporal zones and their correlations (Fig. 6a). All the variables but Ti and Fe:Mn are positively correlated to the recent period (zone III).

Temporal contributions of environmental proxies to changes in primary production and C sequestration

The first General Additive Model (GAM) model was used to investigate the contribution of PP and environmental proxies to OC accumulation (Fig. 7 and Table 2). The model explained 93.1% of the deviance (R² adj.=0.911). PP reflected by CD concentrations (log-transformed) and Ti (terrigenous inputs) were the two statistically significant variable. No statistical significance was found for Fe:Mn (hypolimnetic oxygen levels).

The second model used the scores of the first PCA axes to evaluate the contribution of algal groups to PP, inferred by CD concentrations (Fig. 8 and Table 3). The PC scores were log-transformed (x=log₁₀[x+min(x)+1]) to minimize an overestimated contribution from isolated extreme values. The model explained 97.8% of the deviance (R² adj.=0.975). Both PC scores were statistically significant explanatory variables (p-value<2e⁻¹⁶ and 4.94⁻⁷ for PC1 and PC2, respectively) and showed a positive and monotonic relation to CD. However,

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<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 6** Results from the Multiple Factor Analysis. **a** Relation-ship between the first 2 dimensions of the MFA and those of each matrix based on independent PCA analyses. **b** Correla-tion circle describing the association between the variables, the quality of representation and the correlation between the</figcaption></figure>

variables and the dimensions. **c** PCA biplot showing the contri-bution of each variable and their correlations within the three temporal zones. **d** GAM fitted smooth functions between inor-ganic carbon (ICAR) and total pigment (CD) values as indica-tor of primary production

618 while the contribution from PC1 is strong and posi-tive from 1950 until present, PC2 only positively con-619 tributes to PP between 1970 and 1990.
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Transect—spatial distribution of varves and C, N, P values

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Transect analyses of the sediments showed that lami-nated sediments are present only in deep water cores

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<figure><img src="image_13.png" /><figcaption>**Fig. 7** Fitted smooth functions between organic carbon accumulation rates. **a** CD, **b** Fe:Mn and **c** Ti:Al resulting from our GAM analysis. Dashed lines represent the 95% uncertainty interval of the fitted function. Ticks on the axes show the distribution of observed values for variables. Numbers in brackets</figcaption></figure>

on the y-axis are the effective degrees of freedom (edf) of the smooth function. **d-f** temporal contributions of CD, Fe:Mn and Ti:Al to OC accumulation rates. Dashed lines represent the lower and upper boundaries of the standard error

<table><caption>**Table 2** Significance of the predictive variables from the first GAM on OC accumulation rates</caption><thead><tr><th></th><th>edf</th><th>Ref.df</th><th>F</th><th>p-value</th></tr></thead><tbody><tr><td>s(FeMn)</td><td>1.40</td><td>1.683</td><td>0.193</td><td>0.7593</td></tr><tr><td>s(log[CD])</td><td>1.00</td><td>1.000</td><td>48.939</td><td>6.54E-07 ***</td></tr><tr><td>s(Ti)</td><td>2.62</td><td>3.124</td><td>3.540</td><td>0.0292 *</td></tr></tbody></table>

$$R-sq.(adj)=0.911 \text{ Deviance explained}=93.1\%$$

$$-REML=45.583 \text{ Scale est.}=1.4699 n=28$$

$$\text{Signif. codes: } 0 \text{'***'} 0.001 \text{'**'} 0.01 \text{*} 0.05 \text{,.} 0.1 \text{''} 1$$

starting at 37 m water depth (Fig. 9), which suggests that low oxygen conditions required for preservation of laminae in sediments (Jenny et al. 2013) extend across at least half of the water depth in Lake Aiguebelette. Average $C_{org}$ and $C_{inorg}$ concentration along the E-W transect are 31.3 ± 12 and 62.1 ± 24 mg per gram of dry sediments respectively. $C_{org}$ concentrations seemed relatively homogeneously distributed in the pelagic zone and the littoral zone, at the exception of the shallowest core (Fig. 9). $C_{inorg}$ concentrations are higher in the littoral zone, probably due to crustacean remains and calcite precipitation induced

by macrophytes photosynthesis at that location. C:N 637 mass ratio with values of 11.7 ± 3.5 indicate a homogenized pelagic zone where sediments are composed mostly with autochthonous organic carbon. Sediments of the littoral zone have C:N ratio of 23.6 ± 26 indicating more allochthonous organic supplies. 638 639 640 641 642

## Discussion

643

Sensitivity of carbonate precipitation to primary production

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Our paleolimnological investigation indicates that carbon burial in the sediments of Lake Aiguebelette has been dynamic over the past few hundred years, driven mostly by human activities. Records of Lake Aiguebelette show a first alteration of the inorganic carbon cycle, highlighted by an increase in carbonate burial rates around the 1850s. According to palynological and SedaDNA analyses and historical archives, the forested area was reduced (Møsager et al. 2022), marshes surrounding the lake were drained and the lake level was regulated in

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<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 8** Fitted smooth functions between CD values. **a** PC1 scores and **b** PC2 scores (log transformed) resulting from the GAM. Dashed lines represent the 95% uncertainty interval of the fitted function. Ticks on the axes show the distribution of observed values for variables. Numbers in brackets on the</figcaption></figure>

y-axis are the effective degrees of freedom (edf) of the smooth function. **c** and **d** temporal contributions of the PC1 and PC2 scores to CD. Dashed lines represent the lower and upper boundaries of the standard error

<table><caption>**Table 3** Significance of the predictive variables from the second GAM on CD</caption><thead><tr><th></th><th>Edf</th><th>Ref.df</th><th>F</th><th>p-value</th><th></th></tr></thead><tbody><tr><td>s(log[PC1+|min(PC1)|+1])</td><td>1.989</td><td>2</td><td>367.4</td><td>&lt;2e-16</td><td>***</td></tr><tr><td>s(log[PC2+|min(PC2)|+1])</td><td>1.723</td><td>1.923</td><td>34.56</td><td>4.94E-07</td><td>***</td></tr></tbody></table>

R-sq.(adj) = 0.975 Deviance explained = 97.8%

-REML = 17.426 Scale est. = 0.12505 n = 28

Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘’ 1

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

1854 (Giguet-Covex et al. 2019). Such disturbances may have altered carbonated soils in the catchment. For instance, the newly exposed coastal zone may have resulted in an increase in carbonate and nutrient leakage to the lake. Although no change in nutrient

concentrations in the sediment archives was recorded, a small increase in primary productivity relative to baseline conditions was observed (from 12.2 ± 1.1 to 17.6 ± 3.6 U g⁻¹ dry sed). However, these changes were very subtle relative to the long-time frame provided

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>
<figure><img src="image_27.png" /><figcaption>**Fig. 9** C,N,P concentrations in sediment cores along a East-West transect in Lake Aiguebelte. **a** Location of sediment cores with laminated sediments (shown as black bands) along the transect from coastal to pelagic zone (c.f. white dots on Fig. 1b) **b** water depths at which sediment cores were retrieved</figcaption></figure>

c) Organic and inorganic carbon concentrations in sediments along the transect. Five intervals per sediment core were collected over the first meter **d** Variation in Corg:N ratio along the transect, and e) variation in Corg:P ratio

<figure><img src="image_28.png" /></figure>
by our study and not significant according to Welch
Two Sample t-test (p-value=0.1152). These results
validate our first hypothesis (Hyp 1) that pre-indus-
trial C burial in Lake Aiguebelette was mainly con-
trolled by inorganic carbon cycle and carbonate sat-
uration. Moreover, small increases in lake primary
production or the increase in carbonate supplies
induced by the human alteration of the landscape
have accelerated the precipitation of carbonates in
the lake (Fig. 1). Future work should explore more in
detail sensitivity of Lake Aiguebelette carbonate sys-
tem to small increases in lake primary production.

Linear response of organic carbon sequestration to
increased primary production

Post-industrial times are characterized by a signifi-
cant increase in organic carbon (OC) burial in Lake
Aiguebelette. Increase in OC burial in sediments
has recently been documented in lakes globally, and
was mainly attributed to cultural eutrophication and
land-use change during the first half of the twentieth
century (Anderson et al. 2013; Ferland et al. 2014;
Heathcote et al. 2015; Anderson et al. 2020). How-
ever, trends in OC burial at Lake Aiguebelette are not
so straightforward. Here, the rates of organic carbon
burial in sediments and primary production started
to increase at the end of the nineteenth century. Our
analyses suggest that both OC burial and primary
production (as inferred by the sedimentary pigments)
increases were synchronous, suggesting that primary
production was the main control of carbon burial
during the transition from oligotrophic toward meso-
trophic conditions (Hyp2, Fig. 1 and S1). However,
other drivers appear to have contributed to C burial
over the past three decades.

The onset of low oxygen conditions is observed at
around 1950 CE. Transition toward low hypolimnetic
oxygen conditions were also observed in three other
French perialpine lakes as a response to eutrophica-
tion (Jenny et al. 2013, 2014). These other perialpine
lakes were well oxygenated over a millennia-long
period, but then experienced a shift to hypoxic con-
ditions in the 1950s in response to an unprecedented
rise in total phosphorus concentrations above 10–15
µg PL⁻¹. Following that shift, later fluctuations in
oxygen conditions in the three other French perial-
pine lakes were essentially driven by climatic fac-
tors and not by nutrient supplies, which is congruent

with global observations spanning the last four dec-
ades (Jane et al. 2021). Interestingly, these low oxy-
gen conditions did not affect the linear response of
organic carbon to increase in primary production dur-
ing the eutrophication phase in Lake Aiguebelette.

The paradox of increasing carbon sequestration
despite reoligotrophication

The rates of organic carbon burial in sediments started to increase at the end of the nineteenth century and showed no sign of decline despite reduction in nutrient supplies following restoration programs launched in the 1970s and 1990s. Changes in Cseq following re-oligotrophication have discussed in a limited way so far (e.g., see Fiskal et al. 2019). In this study, GAM results revealed that the persistent high primary production was the dominant factor explaining the increasing OC accumulation over the middle of the twentieth century, but not as much during the latest reoligotrophication phase since year 2000 (Figs. 4 and 7). We suggest that the decoupling between organic carbon sequestration and primary production since year 2000 is due to 1) the lower oxygen regime and associated changes in C preservation conditions; 2) delayed response of lake primary production to nutrient abatement; 3) changes in the composition of the plankton community which has altered the efficiency of C burial (e.g., more picoplankton that act as nuclei for C sequestration and deposition); and/or 4) a different efficiency of nutrient use, due to changes in the structure and functioning of foodwebs.

Furthermore, diagenesis could influence the accu-
mulation of organic carbon in the uppermost sediment
layers, particularly within the first ten years following
deposition (e.g., Gälman et al. 2008). However, pri-
mary production has been declining since the 1980s,
providing a 40-year window to evaluate the effects of
reduced production on C<sub>org</sub> accumulation. This time-
frame exceeds the typical period during which incom-
plete mineralization could significantly impact C<sub>org</sub>
burial, suggesting that the observed trends are not
solely attributable to diagenetic processes.

The analysis of temporal variations in algal groups
showed a relative change in communities over the
last decades, with a recent pre-dominance of Cyano-
bacteria, Chrysophyta and Cryptophyta. Previous
studies have reported similar changes in phyto-
plankton assemblages in lakes that have undergone

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re-oligotrophication (Anneville et al. 2002, 2019; Jeppesen et al. 2005; Pomati et al. 2012; Horn et al. 2015), mostly correlated to higher thermal stability and temperature of the epilimnion, and lower light and nutrient concentrations. Based on their general ecological properties, we can hypothesize on the role of these groups in contributing to the recent increase in OC accumulation. Whereas filamentous and colonial Cyanobacteria have been widely identified as dominant in eutrophicated waters (Reynolds 1987; Scheffer et al. 1997), they may be partly replaced following re-oligotrophication by picocyanobacteria (e.g., Synechococcus spp.). Indeed, picocyanobacteria tend to dominate the picophytoplankton under low-nutrient concentrations, and increased thermal stability have been found to provide them with a competitive advantage (Morán et al. 2010; Domaizon 2013). Recent studies have highlighted the importance of picophytoplankton, including picocyanobacteria, in carbon export, especially in oligotrophic aquatic environments (Richardson and Jackson 2007; Cruz and Neuer 2019). Additionally, due to their high abundance and high surface area to volume ratio, picocyanobacteria tend to enhance adsorption and heterogeneous nucleation, which can induce and amplify authigenic carbonate bioprecipitation in the pelagic zone of oligotrophic waters. Such processes can cause annual whiting events in hard-water lakes (Thompson et al. 1997; Dittrich et al. 2004), as previously observed in Lake Aiguebelette (Blake and Lascombe 1978; Gaillard et al. 1987). Furthermore, this adsorption may increase their burial efficiency through more rapid deposition and reduced remineralization in the water column (Iversen and Ploug 2010), thus potentially increasing their contribution to OC and IC accumulation.

Mixotrophic algae, including Cryptophyta and Chrysophyta species, can also thrive in nutrient-poor waters and dominate the deep chlorophyll maximum below the thermocline during summer stratification (Ilmavirta 1988; Gervais 1998). Recent studies show that climate and anthropogenic changes are likely to favour the growth of mixotrophic plankton in aquatic systems (e.g. Keck et al. 2020). Along with this, it is reasonable to expect trophodynamics changes and modification in the so-called biological pump (Mitra et al. 2014). The mixotroph-dominated food-web generally exhibits a shortened and potentially more efficient chain/transfer from nutrient regeneration to

primary production (C fixation in particulate organic matter), and allows a more direct transfer from microbial/bacterial production to primary producers (and to secondary producers, if those mixotrophs are edible by zooplankton) (Mitra et al. 2014). Those mixotrophic taxa are able to exploit meta- or hypolimnion zones (i.e., zones that remain rather rich in organic and mineral nutrients in comparison to the epilimnion where the level of nutrients decrease rapidly during the productive period), which can directly influence the global capacity of C fixation in the lake and potentially C sequestration at the end. In Lake Aiguebelette, these planktonic flagellates generally present relatively high C:N ratios (Quigg et al. 2011) that could disproportionally contribute to OC accumulation, as revealed by their strong correlation in our MFA.

Lakes worldwide show a decline in oxygen conditions due to the combined effects of nutrient loading and climate change, and Lake Aiguebelette is no exception to this trend (Jane et al. 2021; Jenny et al. 2016). Despite the observed decrease in oxygen conditions in Lake Aiguebelette since the 1950s, oxygen levels inferred by the Fe:Mn ratio were not a statistically significant predictor of OC accumulation based on our GAM results. However, the varve preservation in the sediment records over that period confirm low oxygen conditions in the hypolimnetic waters of Lake Aiguebelette from 1950 CE. According to varve preservation (Core image in Fig. 4), there is no evidence for a post-1950s return to well-oxygenated lacustrine conditions in Lake Aiguebelette despite the implementation of restoration programs. This pattern is also observed in other perialpine lakes (Jenny et al. 2014), and may results in the hysteresis response of carbon during the re-oligotrophication period. Hence, although re-oligotrophication could theoretically increase hypolimnetic oxygen conditions by reducing primary production and organic matter remineralization, the observed rising temperatures are more likely to support prolonged periods of hypolimnetic hypoxia due to stronger and longer thermal stratification and reduced winter mixing, (Jenny et al. 2014; Jane et al. 2021; Soares et al. 2025, 2024) and decreasing oxygen solubility, therefore sustaining better OC preservation (Hyp 3).

Variations in terrigenous inputs to the lake were also proposed as a potential controlling factor on IC and OC accumulation. However, in the case of Lake

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<figure><img src="image_29.png" /></figure>
Aiguebelette, OC accumulation in sediments of the pelagic zone has a relatively low C:N ratio signature, compared to OC accumulation of the littoral zone, and hence seems mainly composed of autochthonous material (Fig. 9). Although terrigenous inputs, as inferred by the Ti value, were identified as a less significant factor influencing the temporal sequestration of OC, they remained high until the early 1900s, thus suggesting inputs of allochthonous OC were slightly higher in the past. It is possible that the expansive littoral zone of Lake Aiguebelette which today supports substantial macrophyte growth has acted as a trap for allochthonous inputs of C and other elements, as has been observed in other systems (Rooney et al. 2003, Fig. 9). In contrast, high OC accumulation after 1950 CE were attributed to the increasing autochthonous OC fraction from sustained high PP.

Based on our study, we can assume that past increases in water column phosphorus, even at modest levels, seem to have modified the ecological status of Lake Aiguebelette and affected its sensitivity to recent climatic pressures. Similar observations have been made for deep peri-alpine lakes following re-oligotrophication, highlighting the role of climate warming to the persistence of ecological changes (Perga et al. 2015) and the complexity of multiple stressors which are introduced asynchronously but have lingering effects (Bruel et al. 2021). Whereas climate change has a direct impact on lakes physical properties by affecting their thermal stability, little is still known about its effect on the interactions between biological and geochemical compartments, which are both sensitive to rising temperatures, and have a resulting effect on $C_{seq}$. Therefore, our study provides an analysis of such interactions and their impact on lacustrine carbon dynamics in the context of global changes. Future developments related to more detailed algal communities change should provide better understanding of their role on $C_{seq}$.

**Acknowledgements** We gratefully acknowledge the Conservatoire des Espaces Naturels de Savoie, the Communauté de Communes du Lac d'Aiguebelette (CCLA), and the Réserve Naturelle Régionale du Lac d'Aiguebelette for providing the bathymetric data used in this study. Their contributions were crucial to the successful completion of this research. This work is part of project DEEP-C of the exploratory research program FairCarboN and received government funding managed by the Agence Nationale de la Recherche under the France 2030 program, reference ANR-22-PEXF-0009 – projet DEEP-C.

**Author contributions** J.-P.J. and J.R. wrote the main manuscript text. J.-P.J., C.B., I.D., D.E., and I.G.-E. contributed to the study design and data interpretation. O.I.-D., J.-P.J. and J.R. prepared figures and analyzed the data. E.M. and P.S. provided paleolimnological insights and contributed to sediment dating analysis. J.R. assisted with data collection and laboratory work. All authors reviewed and approved the final manuscript.

**Funding** This work is part of project DEEP-C of the exploratory research program FairCarboN and received government funding managed by the Agence Nationale de la Recherche under the France 2030 program, reference ANR-22-PEXF-0009.

**Data availability** No datasets were generated or analysed during the current study.

**Declarations**

**Conflict of interests** The authors declare no competing interests.

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