Seminar Louis-Marie Le Fer

Abstract

Hydrology plays a fundamental role in peatland functioning and controls the silicon (Si) cycle in these ecosystems. Most  eatlands are groundwater-dependent systems, where interactions between aquifers, peat layers, and surface waters regulate solute transport and nutrient cycling, including dissolved silicon (DSi). DSi is a key nutrient for primary producers in freshwater and marine ecosystems and contributes critically in regulating carbon cycle both at local (watershed) and global scale (land-ocean-atmosphere continuum). Hydrological processes, including groundwater fluxes and water circulation between surface and deep peat layers strongly influence silicon dynamics. Understanding local peatland hydrology is therefore essential to assess the role of peatlands as filters controling continental Si exports. Despite this central function, groundwater-peatland interactions remain poorly understood, limiting our ability to quantify peatlands’ contribution to global Si fluxes.

In this context, this study investigates how peatland hydrology, and particularly aquifer-peatland connectivity, shapes the dynamics of dissolved silicon within a temperate peatland system.

We implemented an integrated field-based approach, relying on long-term hydrological data collected hourly since 2008 and monthly hydrochemical measurements acquired since 2014, providing a robust framework to investigate peatland control on dissolved silicon fluxes. This was conducted at La Guette peatland, a lowland temperate peatland in the Sologne region (France) and part of the SNO Tourbières long-term observatory network. Paired surface and deep piezometers were installed both upstream and downstream of the peatland allowing for investigation of vertical, lateral and temporal variability in water and solute dynamics. Hydrological analyses based on water balance calculations are combined with a multi-tracer geochemical approach to assess groundwater contribution. Silicon dynamics are further examined using concentration-isotope ratio (DSi-δ³⁰Si) relationships to disentangle biological and hydrological controls.

Preliminary results indicate the presence of groundwater inputs originating from the surrounding sandy aquifer, supporting the characterization of the La Guette peatland as a groundwater-dependent ecosystem. The data reveal vertical and lateral gradients, as well as temporal variability in both DSi concentrations and δ³⁰Si signatures. Concentration-isotope ratio relationships suggest a seasonal shift in dominant controls, with biologically influenced silicon dynamics during spring and summer and hydrologically driven processes during autumn and winter. Together, these observations provide new insights into the links between peatland hydrology and silicon dynamics and highlight the need to further investigate groundwater-peatland interactions when assessing peatland contributions to continental biogeochemical fluxes.