Gérard Gruau

University of Rennes | UR1 · UMR CNRS 6118 - Géosciences Rennes

High flow events in headwater catchments are associated with changes in the dissolved organic matter (DOM) composition. The aim of this work was to determine whether these changes are characteristic of headwater catchments and similar to what has been observed in other catchments. The evolution of theDOMcomposition during flood events was studied f...

Despite recent studies of researchers’ roles in sustainability science, understanding the factors that influence them is a complex challenge. To address this lack of knowledge, we conducted a self-reflexive analysis involving 11 researchers from Rennes, France, who self-reflected on 12 projects conducted in north-western France over the past 15 yea...

The origin of sedimentary organic matter (SOM) has often been mentioned as a driver of SOM reactivity. This was quantified by statistically relating the isotopic and lipid composition of SOM to benthic nutrient fluxes in 200 intertidal mudflats sampled along the Brittany coast (France). The origin of SOM explained 24% and 31% of the variance of NH4...

Researches have proved that agricultural phosphorus (P) loss contributes significantly to surface water eutrophication. Various soil test P (STP) methods have been developed to assess the P loss risk from agricultural soils. In the intensively-cultivated Brittany region of Western France, hydromorphic soils in wetland domains exhibithigh risks of l...

Increasing concerns over water eutrophication due to agricultural phosphorus (P) loss have led to the development of indicators to assess the risk of P release from agricultural soils. Recently, a logarithmic equation linking the degree of phosphorus saturation (DPS) to the simple water‐soluble P (WSP) content of soils has been proposed as a univer...

It is well established that shallow tillage (ST) and no-tillage (NT) of cultivated soils can have higher risk of dissolved phosphorus (P) loss during runoff than conventional tillage (CT). However, quantitative descriptors of how differences in tillage practices influence soil P sorption properties and how these properties in turn influence dissolv...

Context Nitrogen (N) and phosphorus (P) exports from rural landscapes can cause eutrophication of inland and coastal waters. Few studies have investigated the influence of the spatial configuration of nutrient sources—i.e. the spatial arrangement of agricultural fields in headwater catchments—on N and P exports. Objectives This study aimed to (1)...

In coastal areas and estuaries, such as those encountered in the western part of France (Brittany region), the recycling of carbon and nutrients from sediments can participate in the development of micro and macro-algal blooms with harmful consequences for these ecosystems. One of the main processes controlling this recycling is the microbial miner...

Protecting water quality at catchment scales is complicated by the high spatiotemporal variability in water chemistry. Consequently, determining pollutant sources requires costly monitoring strategies to diagnose causes and guide management solutions. However, recent studies have shown that spatial patterns in water chemistry can be persistent a...

A broad sampling program was carried out in the spring of 2019 on the Brittany coast to assess how the surface sediment characteristics drive the benthic effluxes of ammonium (NH + 4) and phosphate (PO 4) from intertidal mudflats. A total of 200 sediment samples were characterized by their porosity, grain-size, elemental composition and pigment con...

Characterizing and understanding spatial variability in water quality for a variety of chemical elements is an issue for present and future water resource management. However, most studies of spatial variability in water quality focus on a single element and rarely consider headwater catchments. Moreover, they assess few catchments and focus on ann...

This study investigates the combined effects of land management and hydrology on the temporal dynamics of dissolved organic matter (DOM) quantity and composition in stream water and groundwaters in an agricultural watershed. We assessed dissolved organic carbon (DOC) concentrations, DOM UV–Vis absorbance, and DOM fluorescence in groundwater under c...

The majority of freshwater ecosystems worldwide suffer from eutrophication, particularly because of agriculture-derived nutrient sources. In the European Union, a discrepancy exists between the scale of regulatory assessment and the size of research catchments. The Water Framework Directive sets water quality objectives at the mesoscale (50–500 km²...

Agriculture affects the biogeochemical cycles of carbon, nitrogen, and phosphorus, leading to a deterioration of surface water quality. The increasing magnitude of climate change raises questions about potential additional or mitigating effects of climate change on this deterioration. One way to understand these potential effects is to cross‐analyz...

Characterizing and understanding spatial variability in water quality for a variety of chemical elements is an issue for present and future water resource management. However, most studies of spatial variability in water quality focus on a single element and rarely consider headwater catchments. Moreover, they assess few catchments and focus on ann...

Colloids (1–1,000 nm) are important phosphorus (P) carriers in agricultural soils. However, most studies are based on colloids from soil waters extracted in the laboratory, thus limiting the understanding of the natural transfer of colloidal P along the soil‐to‐stream continuum. Here, we conducted a field study on the colloidal P in both natural so...

Quantifying nutrient attenuation at watershed scales requires long-term water chemistry data, water discharge, and detailed nutrient input chronicles. Consequently, nutrient attenuation estimates are largely limited to long-term research areas or modeling studies, constraining understanding of the ecological characteristics controlling nutrient att...

Abstract: Carbon and nutrient (N, P) exports from lands to rivers are of major importance in managing water quality issues such as eutrophication of inland and coastal waters. Studying the spatial and temporal variation of nutrients concentrations in rivers helps to better understand how catchment geomorphology, land use and agriculture practices c...

Studies of spatial and temporal variation of Nitrate (NO3) and Dissolved Organic Carbon (DOC) concentrations in rivers can inform on the control of their exports by hydrological processes, soil biogeochemical processes and catchment characteristics (geomorphology, land use, etc…). However, such studies are often conducted on large river basins, in...

The development of anoxic conditions in riparian wetland (RW) soils is widely known to release dissolved phosphorus (DP), but the respective roles of reductive dissolution of Fe-oxyhydroxide, pH changes and sediment inputs in this release remain debated. This study aimed to identify and quantify these respective roles via laboratory anaerobic/aerob...

Core Ideas AgrHyS is a long‐term observatory of the agroecosystem. AgrHyS supports strongly interdisciplinary environmental research. AgrHyS offers an original experimental setup to explore the soil–groundwater–water–plants–atmosphere continuum. AgrHyS supports original and innovative techniques for environmental monitoring. The AgrHyS is a long‐t...

In this paper, we outline several recent insights for the priorities and challenges for future research for reducing phosphorus (P) based water eutrophication in the agricultural landscapes of Northwest Europe. We highlight that new research efforts best be focused on headwater catchments as they are a key influence on the initial chemistry of the...

The long-term evolution of nutrient dynamics in rivers under changing external forcings, termed hereafter trajectory, is influenced by local human activities and regional climatic variations. Here we investigate nitrogen (N) and phosphorus (P) dynamics in seven mesoscale agricultural catchments (median size 800 km²) of western France from seasonal...

Understanding how water and solutes enter and propagate through freshwater landscapes in the Anthropocene is critical to protecting and restoring aquatic ecosystems and ensuring human water security. However, high hydrochemical variability in headwater streams, where most carbon and nutrients enter river networks, has hindered effective modelling a...

The response of stream chemistry to storm is of major interest for understanding the export of dissolved and particulate species from catchments. The related challenge is the identification of active hydrological flow paths during these events and of the sources of chemical elements for which these events are hot moments of exports. An original fou...

Catchment studies on water quality started in the 1960s, when scientists identified long-term environmental impacts and realized the complexity involved in environmental degradation and recovery. Since scientists need to understand biogeochemical and hydrological processes in detail, long-term observatories have been established over time, particul...

Drying and rewetting (DRW) events in soils cause the release of molybdate-unreactive dissolved phosphorus (MUP) into soil solutions, which has been historically considered biologically-derived phosphorus (P) from microbial cell lysis. This unreactive P, however, could also represent P bound to soil colloids/nanoparticles, whose releases are also kn...

Water chemistry was collected for 56 subcatchments of the Couesnon and Rance Rivers in western France. Sites were sampled periodically from 2003 to 2016 and subcatchment characteristics were extracted for the area draining to each sampling site.

In agricultural landscapes, establishment of vegetated buffer zones in riparian wetlands (RWs) is promoted to decrease phosphorus (P)emissions becauseRWs can trap particulate P fromupslope fields. However, long-term accumulation of P risks the release of dissolved P, since the unstable hydrological conditions in these zones may mobilize accumulated...

Storm events are responsible for more than 60 % of the export of dissolved organic matter (DOM) from headwater catchments due to an increase in both the discharge and concentration. The latter was attributed to changing water pathways inducing the mobilization of DOM from the surface soil horizons. Recent molecular investigations have challenged th...

Les dispositifs enherbés placés dans les zones humides ripariennes permettent l’interception et la rétention de polluants agricoles, comme le phosphore apporté par l’érosion des parcelles agricoles situées en amont. Cependant, l’accumulation de phosphore dans ces zones présente un risque pour les masses d’eaux superficielles, puisque, comme le mont...

Storm events are responsible for more than 60 % of the export of dissolved organic matter (DOM) from headwater catchments due to an increase in both the discharge and concentration. The latter was attributed to changing water pathways inducing the mobilization of DOM from the surface soil horizons. Recent molecular investigations have challenged th...

Biogeochemistry of dissolved organic matter (DOM) is important to ecology, ecotoxicology and the carbon cycle. DOM is operationally defined as the OM fraction that passes through filters. Since different filter pore-sizes, ranging from 0.2 to 0.7 μm, are commonly used, it is necessary to test if this choice has an effect on the concentration and co...

Riparian vegetated buffer strip (RVBS) are currently used to protect surface waters from phosphorus (P) emissions because of their ability to retain P-enriched soil particles. However, this protection role may be counterbalanced by the development in these zones of conditions able to trigger the release of highly mobile dissolved or colloidal P for...

Riparian vegetated buffer strips (RVBS) have been promoted worldwide as a tool to reduce diffused phosphorus (P) emission from agriculture lands, mainly through their ability to retain particulate P. However, RVBSs are zones of periodic water table fluctuations, which may stimulate the transformation and release of particulate P into mobile dissolv...

Colloids (1~1000 nm) are major phosphorus (P) carrier phases in soil solutions. Their size and composition is therefore essential to understand the transport of P in soils, as is important to unravel what are the factors and processes that control the composition and spatio-temporal distribution of colloids in soils at catchment scale. Most studies...

Accurate identification of phosphorus (P) forms is crucially important for understanding the geochemical cycle of P; however, until now the role of ferrous iron P (Fe(II)-P) buried in sediments has been completely ignored in nearly all sequential extraction procedures developed. Using sediment cores sampled from Donghu Lake in Wuhan, China, this st...

This study analyses 15 years of daily nitrate concentration data at the outlet of an intensively farmed catchment in Western France (Kervidy-Naizin, 5 km²) and quarterly nitrate concentration data in the groundwater of two hillslopes equipped with piezometers(Kerroland and Gueriniec) within the same catchment. In this catchment groundwater contribu...

Storm events are critical hot moments of emission for several dissolved and particulate chemical species at major stake for water quality (e.g. dissolved organic carbon DOC, suspended sediments, phosphorus). During such events, the solutes or particles are exported from heterogeneous sources through various pathways to stream leading to specific in...

Although it has been suggested that several mechanisms can describe the direct binding of As(III) to organic matter (OM), more recently, the thiol functional group of humic acid (HA) was shown to be an important potential binding site for As(III). Isotherm experiments on As(III) sorption to HAs, that have either been grafted with thiol or not, were...

Colloids (1~1000 nm) are major phosphorus (P) carrier phases in agricultural soils. Most studies developed so far on the role of colloids in P transport have however focused on laboratory extracted colloids with only little attention being paid to natural soil solutions. Here, we monitored P speciation in natural soil solutions along two transects...

As a phenomenon integrating climate conditions and hydrological control of the connection between streams and terrestrial dissolved organic carbon (DOC) sources, groundwater dynamics control patterns of stream DOC characteristics (concentrations and fluxes). Influence of intra-annual variations in groundwater level, discharge and climatic factors o...

Because of the high sorption affinity of phosphorus (P) for the soil solid phase, mitigation options to reduce diffuse P transfer usually focus on trapping particulate P delivered via surface flow paths. Therefore, placing riparian buffers between croplands and watercourses has been promoted worldwide, sometimes in wetland areas. To investigate the...

The transfer of dissolved organic matter (DOM) at soil–river interfaces controls the biogeochemistry of micropollutants and the equilibrium between continental and oceanic C reservoirs. Understanding the mechanisms controlling this transfer is fundamental to ecology and geochemistry. DOM delivery to streams during storms is assumed to come from the...

Wetlands are specific areas able to regulate metals mobility in the environment. Among metals, rare earth elements (REE) appear to be particularly interesting because of the information that could be provided by the REE patterns. Moreover, as REE are becoming a matter of great economic interest, their significant release into the environment may be...

Arsenic (As) is a toxic and ubiquitous element which can be responsible for severe health problems. Recently, Nano-scale Secondary Ions Mass Spectrometry (nanoSIMS) analysis has been used to map organomineral assemblages. Here, we present a method adapted from Belzile et al. (1989) to collect freshly precipitated compounds of the re-oxidation perio...

The presence of arsenic(As)-bearing Fe(III) oxyhydroxides in wetland zones may threat water quality due to the reduction processes that affect these zones. These processes have indeed the potential of releasing As into the soil solutions, and ultimately into the nearby river network, being given the hydrological connectivity that exists between wet...

Understanding the processes involved in the control of arsenic (As) dynamics within soils has become a challenging issue for soil and water quality preservation. Interactions between mineralogical phases, organic ligands and bacterial communities - closely linked to the chemical conditions of the medium - were thus investigated through a geochemica...

The transfer of dissolved organic matter (DOM) at soil–river interfaces controls the biogeochemistry of micropollutants and the equilibrium between continental and oceanic C reservoirs. Then determining the transfer mechanisms of DOM is of main importance for ecological and geochemical reasons. Is stream DOM the result of the flushing of pre-existi...

This paper investigates particulate phosphorus (PP) and soluble reactive phosphorus (SRP) concentrations at the outlet of a small (5 km2) intensively farmed catchment to identify seasonal variability of sources and transport pathways for these two phosphorus forms. The shape and direction of discharge-concentration hystereses during floods were rel...

Humic substances are major natural complexing agents that strongly influence the cycling of metal cations in the environment. The metal to humic acid (HA) concentration ratio (metal loading) can significantly impact the overall HA complexation properties in natural soils and waters. At low metal loading, cations bind to strong multi-carboxylate or...

We investigated soluble reactive phosphorus (SRP) at the outlet of a lowland agricultural catchment (Kervidy-Naizin, France) to identify the hydroclimatic controls on the seasonal and inter-annual variability in concentrations. Six years of stream data have been used, including a regular 6-daily sampling and high-frequency monitoring of 52 floods....

L’empoisonnement à l’arsenic (As), via la contamination des eaux souterraines utilisées comme eaux de boisson est responsable de l’une des plus grandes mortalités et morbidités. En conditions oxydantes, l’As est majoritairement présent à l’état V et le fer (Fe) sous forme d’oxydes, connus pour adsorber l’As, alors qu’en conditions réduites l’As(III...

Spatio-temporal modifications of the composition of dissolved organic matter (DOM) from a wetland in an agricultural catchment were investigated using thermally assisted hydrolysis and methylation with tetramethylammonium hydroxide coupled to gas chromatography and mass spectrometry (THM-GC-MS). The results were compared with previous data acquired...

Monitoring the isotopic composition (δ13CDOC) of dissolved organic carbon (DOC) during flood events can be helpful for locating DOC sources in catchments and quantifying their relative contribution to stream DOC flux. High-resolution (< hourly basis) δ13CDOC data were obtained during six successive storm events occurring during the high-flow period...

Arsenic (As) is a toxic and ubiquitous element found in the atmosphere, soils, rocks, natural waters and organisms, which can be responsible for severe health problems. The As behaviour in soil is controlled by both pH and redox potential of the medium, as well as the metals and the organic matter concentration. However, which kind of interaction b...