J Canal

P Laffaille

F Gilbert

C Lauzeral

L Buisson

Influence of temperature on surface sediment disturbance by freshwater fish: a microcosm experiment

Keywords: bioturbation, Barbatula barbatula, Parachondrostoma toxostoma, Squalius cephalus, zoogeomorphology

Water temperature is a key parameter that regulates activities of ectotherms (e.g., fish) and has been projected to rise in the future in the context of climate change. Surface sediment disturbance (SSD) is an important function performed by fish that modifies benthic habitat properties and may thus influence ecosystem functioning. However, the link between SSD by fish and temperature remains unexplored. In this study, we experimentally assessed the influence of two contrasted temperatures (10 versus 20 xC) on SSD by three freshwater fish species in aquaria. After 10 days, the total surface disturbed by the species was approximately 2-3 times higher at warm than at cold temperature but the number and area of patches disturbed by the species were not significantly different. The stone loach Barbatula barbatula, a benthic fish species, had a higher SSD activity (i.e., total surface disturbed, number and size of patches disturbed) both at cold and warm temperatures than two cyprinid species, chub Squalius cephalus and sofie Parachondrostoma toxostoma. On average, SSD by B. barbatula resulted in approximately two and three more patches than the cyprinid species at 10 and 20 xC, respectively, and the mean patch area disturbed by B. barbatula was about two times larger than the ones disturbed by the cyprinid species. The total surface disturbed by the cyprinids at warm temperature was roughly equivalent to the surface disturbed by B. barbatula at cold temperature. Our results thus suggest that SSD by fish could increase in future warmed conditions.

Introduction

Given the growing interest in understanding the changes of physical environment induced by organisms [START_REF] Statzner | Geomorphological implications of engineering bed sediments by lotic animals[END_REF], bed sediment reworking by a variety of animal species acting as zoogeomorphic agents has been extensively studied in the bioturbation research field both for marine and freshwater environments (e.g., macroinvertebrates: [START_REF] Cade´e | Sediment reworking by the polychaete Heteromastus filiformis on a tidal flat in the Dutch Wadden Sea[END_REF][START_REF] Caradec | Macrofaunal reworking activities and hydrocarbon redistribution in an experimental sediment system[END_REF][START_REF] Heldman | Particle reworking and solute transport by the sediment-living polychaetes Marenzelleria neglecta and Hediste diversicolor[END_REF][START_REF] Marmonier | The role of organisms in hyporheic processes: gaps in current knowledge, needs for future research and applications[END_REF][START_REF] Zhang | The role of tubificid worms (Limnodrilus hoffmeisteri) in sediment resuspension: a microcosm study[END_REF]molluscs: Orvain and Sauriau, 2002;[START_REF] Gilbert | Sediment reworking by marine benthic species froom Gullmar Fjord (Western Sweden): importance of faunal biovolume[END_REF]crayfish: Statzner et al., 2003a;[START_REF] Statzner | Assessing potential abiotic and biotic complications of crayfish-induced gravel transport in experimental streams[END_REF][START_REF] Johnson | Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus)[END_REF]mammals: Jones et al., 1994). Among this fauna, many studies have focused on fish as actors of bed surface disturbance in streams (e.g., [START_REF] Flecker | Ecosystem engineering by a dominant detritivore in a diverse tropical stream[END_REF][START_REF] Flecker | Tropical fishes as biological bulldozers: density effects on resource heterogeneity and species diversity[END_REF][START_REF] Holtgrieve | Marine-derived nutrients, bioturbation, and ecosystem metabolism: reconsidering the role of salmon in streams[END_REF], given their key role in the flux and transfer of particles and solutes in aquatic ecosystems. The influence of spawning behaviour of salmonid species on streambed has been extensively documented (e.g., [START_REF] Montgomery | Stream-bed scour, egg burial depth, and the influence of salmonid spawning on bed surface mobility and embryo survival[END_REF][START_REF] Peterson | Disturbance of small-stream habitat by spawning sockeye salmon in Alaska[END_REF][START_REF] Gottesfeld | Sediment dispersion in salmon spawning streams: the influence of floods and salmon redd construction[END_REF][START_REF] Moore | Biotic control of stream ecosystem fluxes: spawning salmon drive nutrient and matter export[END_REF][START_REF] Vries | Salmonid influence on rivers: a geomorphic fish tail[END_REF] but knowledge of other behaviours as zoogeomorphic activity (e.g., foraging, swimming and burrowing) remains scarce, especially for non-salmonid species (but see [START_REF] Statzner | Crayfish and fish as bioturbators of streambed sediments: assessing joint effects of species with different mechanistic abilities[END_REF][START_REF] Shirakawa | Lamprey larvae as ecosystem engineers: physical and geochemical impact on the streambed by their burrowing behavior[END_REF][START_REF] Pledger | Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)[END_REF].

Bed sediment reworking by fish can result from a wide range of activities and be a substantial source of disturbance to benthic habitat [START_REF] Moore | Animal ecosystem engineers in streams[END_REF] and ecosystem functioning. For example, De [START_REF] Vries | Salmonid influence on rivers: a geomorphic fish tail[END_REF] showed that salmonids, by digging nests during the reproduction season, cause a strong local reworking of the streambed sediment. Spawning runs of sockeye salmon have also been reported to increase temporarily surface gravel size and invertebrate drift while decreasing periphyton biomass [START_REF] Peterson | Disturbance of small-stream habitat by spawning sockeye salmon in Alaska[END_REF]. [START_REF] Shirakawa | Lamprey larvae as ecosystem engineers: physical and geochemical impact on the streambed by their burrowing behavior[END_REF] also recently highlighted that the burrowing and feeding behaviour of larval lampreys caused increased oxygen levels and bioturbation in the streambed. Fish swimming movement close to, or directed at the streambed can also cause significant modifications of bed sediment structure [START_REF] Statzner | Crayfish and fish as bioturbators of streambed sediments: assessing joint effects of species with different mechanistic abilities[END_REF], as can the excavation of shelters [START_REF] Reise | Sediment mediated species interactions in coastal waters[END_REF].

Temperature plays a fundamental role in regulating the physiological and biochemical processes of fishes [START_REF] Fry | Effects of the environment on animal activity[END_REF][START_REF] Brett | Energetic responses of salmon to temperature. A study of some thermal relations in the physiology and freshwater ecology of sockeye salmon (Oncorhynchus nerka)[END_REF], acting on physiology as a lethal factor (below or beyond the thermal tolerances of species) or as a stress factor [START_REF] Elliott | Some aspects of thermal stress on freshwater teleosts[END_REF]. Consequently, temperature controls the fish activities (e.g., reproduction, migration and feeding).

The assessment of the effects of temperature on fish activity is a significant challenge in the context of climate warming [START_REF] Harborne | The ecology, behaviour, and physiology of fishes on coral reef flats, and the potential impacts of climate change[END_REF], given that fish may influence habitat features and ecosystem functioning through both trophic and non-trophic activities [START_REF] Shirakawa | Lamprey larvae as ecosystem engineers: physical and geochemical impact on the streambed by their burrowing behavior[END_REF]. Although some studies have assessed the role of temperature in regulating the activity of sediment reworking (e.g., [START_REF] White | Effects of temperature and density on sediment reworking by Stylodrilus heringianus (Oligochaeta: Lumbriculidae)[END_REF][START_REF] Ouellette | Effects of temperature on in vitro sediment reworking processes by a gallery biodiffusor, the polychaete Neanthes virens[END_REF], these have not yet assessed this for freshwater fish. The studies completed on other biota suggested a strong relationship between sediment reworking and water temperature, both in marine and freshwater systems. For instance, [START_REF] White | Effects of temperature and density on sediment reworking by Stylodrilus heringianus (Oligochaeta: Lumbriculidae)[END_REF] reported a 90% decline in sediment reworking rate by oligochaetes from the Laurentian Great Lakes when temperature was experimentally lowered from 10 to 4 xC. [START_REF] Ouellette | Effects of temperature on in vitro sediment reworking processes by a gallery biodiffusor, the polychaete Neanthes virens[END_REF] documented a higher sediment transport by a marine polychaete species at warm (13 and 18 xC) than at cold (1 and 6 xC) temperatures. Similar trends were found for the bioturbation activity by a heart urchin [START_REF] Hollertz | Burrowing behavior and sediment reworking in the heart urchin Brissopsis lyrifera Forbes (Spatangoida)[END_REF] or the reworking rate by a surface deposit-feeding bivalve [START_REF] Ducheˆne | A comparison of sediment reworking rates by the surface deposit-feeding bivalve Abra ovata during summertime and wintertime, with a comparison between two models of sediment reworking[END_REF]. In another study, [START_REF] Berkenbusch | Factors influencing sediment turnover by the burrowing ghost shrimp Callianassa filholi (Decapoda: Thalassinidea)[END_REF] have followed the rate of sediment expulsion by the burrowing shrimp Callianassa filholi at an intertidal sandflat in New Zealand over a 12-month period. They found that sediment expulsion was significantly related to seawater temperature, being maximal in summer and dropping to a minimum at the end of winter.

In the context of climate change, all these findings thus suggest that surface sediment reworking by biological organisms could increase with the rapid on-going warming, as long as temperature does not exceed their upper thermal tolerance. Fish, especially warm-water species, are acknowledged to be more active at warm than at cold temperatures [START_REF] Lo´pez-Olmeda | Thermal biology of zebrafish (Danio rerio)[END_REF]. Consequently, we assume that benthic foraging fishes preferring cool and warm waters could increase sediment displacement and transport due to their feeding and locomotor activity in response to water warming. Given that cool-and warm-water fish species are also expected to expand their spatial distribution [START_REF] Buisson | Climate change hastens the turnover of stream fish assemblages[END_REF][START_REF] Lyons | Predicted effects of climate warming on the distribution of 50 stream fishes in Wisconsin[END_REF], the increase in surface sediment disturbance (SSD) by benthic species might have important implications for bedload sediment flux at local and larger scales [START_REF] Statzner | Crayfish and fish as bioturbators of streambed sediments: assessing joint effects of species with different mechanistic abilities[END_REF], and subsequently on biogeochemical processes and ecosystem functioning (e.g., Persson and Svensson, 2006a). For instance, the micro-topography of the surface of sediment could change and its erodibility by water rise due to more intensive disturbance by fish [START_REF] Pledger | Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)[END_REF]. Benthivorous fish species (e.g., Cyprinus carpio, Abramis brama) are also known to influence directly and indirectly water quality (i.e., nutrients, suspended solids and organic matter) and biological communities (e.g., macrophytes, phytoplankton, zooplankton and macro-invertebrates) through nutrients and organic matter release from the resuspension of sediments to the water column due to bioturbation (i.e., by feeding) but also excretion [START_REF] Breukelaar | Effects of benthivorous bream (Abramis brama) and carp (Cyprinus carpio) on sediment resuspension and concentrations of nutrients and chlorophyll a[END_REF]Persson and Svensson, 2006a;[START_REF] Roozen | Resuspension of algal cells by benthivorous fish boosts phytoplankton biomass and alters community structure in shallow lakes[END_REF][START_REF] Matsuzaki | Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis[END_REF]. Hence, changes in SSD by benthic fish in response to water warming could lead to a wide range of disturbances in ecosystem functioning, among which changes in primary productivity by a spatial redistribution of the micro-phytobenthos [START_REF] Orvain | Interacting effects of Hydrobia ulvae bioturbation and microphytobenthos on the erodibility of mudflat sediments[END_REF], increases of the water turbidity [START_REF] Breukelaar | Effects of benthivorous bream (Abramis brama) and carp (Cyprinus carpio) on sediment resuspension and concentrations of nutrients and chlorophyll a[END_REF][START_REF] Matsuzaki | Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis[END_REF], changes in nutrient recycling [START_REF] Biles | Bioturbation, ecosystem functioning and community structure[END_REF][START_REF] Ieno | How biodiversity affects ecosystem functioning: roles of infaunal species richness, identity and density in the marine benthos[END_REF], modifications in the texture of the sediment [START_REF] Montserrat | Sediment segregation by biodiffusing bivalves[END_REF] or redistribution of macro-invertebrates in the first centimetres of the substrate (Persson and Svensson, 2006b;[START_REF] Merchant | Indirect effects of non-lethal predation on bivalve activity and sediment reworking[END_REF].

In that context, the aim of our study was to experimentally assess the influence of contrasted temperatures on SSD by three benthic freshwater fish species with different temperature tolerance limits. Following the results of earlier studies, we hypothesized that SSD by cool-and warm-water fishes could rise with temperature warming due to increased foraging and swimming activity. Coldwater benthic species may also become more active due to thermal stress but changes in SSD may also depend on the intensity of warming. These results may help to better understand how temperature warming may modify the role of fish in particles movement.

Material and methods

Fish species

Three species of freshwater fish were utilized during the experiment: the chub Squalius cephalus L. 1758, the sofie Parachondrostoma toxostoma (Vallot 1837), and the stone loach Barbatula barbatula L. 1758. Due to various activities, they are assumed to disturb surface sediment (e.g.,De [START_REF] Nadaı¨-Monoury | A cost-effective method to quantify biological surface sediment reworking in streams[END_REF]. B. barbatula buries in sediment for refuge and forages by sucking in large amounts of sediment, extracting a wide variety of small benthic invertebrates and plants, and releasing the sediment [START_REF] Keith | Les poissons d'eau douce de France[END_REF]. P. toxostoma feeds mainly by grazing on benthic algae, diatoms and invertebrates [START_REF] Chappaz | Donne´es nouvelles sur la biologie et l'e´cologie d'un poisson Cyprinide´peu e´tudieĆ hondrostoma toxostoma[END_REF][START_REF] Keith | Les poissons d'eau douce de France[END_REF], whereas S. cephalus is an opportunistic feeder eating both pelagic and benthic material [START_REF] Hellawell | The autecology of the chub, Squalius cephalus (L.), of the River Lugg and the Afon Llynfi[END_REF][START_REF] Keith | Les poissons d'eau douce de France[END_REF]. Both P. toxostoma and S. cephalus are also able to displace sediments when moving close to the streambed (see [START_REF] Statzner | Crayfish and fish as bioturbators of streambed sediments: assessing joint effects of species with different mechanistic abilities[END_REF] for other cyprinid species; personal observation). In France, these three species are abundant in small rivers and large streams of trout and barbel zones [START_REF] Huet | Profiles and biology of western European streams as related to fish management[END_REF] with riffles and pools and mainly covered by pebble-gravel and sandy substrates [START_REF] Keith | Les poissons d'eau douce de France[END_REF]. These species were selected according to their contrasted thermal requirements. B. barbatula is indeed a cold-to cool-water species whose upper tolerance limit does not exceed 19 xC [START_REF] Elliott | A functional model for maximum growth of immature stone loach, Barbatula barbatula, from three populations in north-west England[END_REF], while the two other cyprinid species prefer warmer waters with an optimum temperature ranging between 12 and 25 xC depending on species and regions [START_REF] Horoszewicz | Lethal and "disturbing" temperatures in some fish species from lakes with normal and artificially elevated temperature[END_REF][START_REF] Mann | Observations on the age, growth, reproduction and food of the chub Squalius cephalus (L.) in the River Stour, Dorset[END_REF][START_REF] Elliott | Some aspects of thermal stress on freshwater teleosts[END_REF][START_REF] Bouchard | Influence of environmental variables on the growth of Leuciscus cephalus (Linnaeus 1766), in the River Durance, South-east France[END_REF][START_REF] Gozlan | Environmental biology and morphodynamics of the sofie Chondrostoma toxostoma (Cyprinidae), with emphasis on early development[END_REF][START_REF] Souchon | Synthesis of thermal tolerances of the common freshwater fish species in large Western Europe rivers[END_REF]. When modelling the distribution of stream fish species at the European scale, [START_REF] Logez | Modelling the habitat requirement of riverine fish species at the European scale: sensitivity to temperature and precipitation and associated uncertainty[END_REF] found a thermal range between 14.1 and 22.6 xC for the stone loach and between 14.7 and 25.8 xC for the chub. These three species were also shown to have contrasted response to future climate change in terms of spatial distribution [START_REF] Buisson | Climate change hastens the turnover of stream fish assemblages[END_REF]. Consequently, warmer temperature might have a different effect on SSD by these three fish species.

Fish sampling and acclimatization

Sixteen individuals of each three fish species were collected by electrofishing in spring 2011. S. cephalus and P. toxostoma were sampled in the Volp River, whereas B. barbatula was sampled in the Saudrune River. These two streams are located in the Garonne River catchment (Southwestern France). Water temperature in the two streams during the sampling was approximately 14 xC. The mean total length and mass of the fish caught was 81.6 ¡ 8.1 mm and 5.2¡ 0.9 g, 108.7 ¡ 18.6 mm and 12.7 ¡ 2.6 g and 112.6 ¡ 13.3 mm and 10.2 ¡ 2.5 g, for B. barbatula, S. cephalus and P. toxostoma, respectively.

At the laboratory, fish species were separated in three acclimatization opaque tanks (500 cm long, 200 cm wide, 100 cm high) filled with water extracted from the river where fish were sampled and placed in a temperaturecontrolled room (15 xC for the first 2 days of acclimatization) under a 12:12 h light regime. Two days after the introduction of fishes into the acclimatization tanks, temperature was increased or decreased by 0.5 xC every 12 h until reaching the experimental temperature. The experiments were conducted at 10 and 20 xC thus representing on average colder and warmer conditions than the temperature experienced by fish in the days before sampling, but going no further than the lethal temperatures of the three fish species [START_REF] Keith | Les poissons d'eau douce de France[END_REF][START_REF] Souchon | Synthesis of thermal tolerances of the common freshwater fish species in large Western Europe rivers[END_REF]. In addition, these two temperatures were quite realistic according to water temperature measured at present in the small rivers and large streams of trout and barbel zones in the Garonne catchment (ALADIN model: Bubnova´et al., 1995;[START_REF] Sevault | A fully coupled Mediterranean regional climate system model: design and evaluation of the ocean component for the 1980-2012 period[END_REF]. They were also relevant in the context of climate change given the air temperature warming forecasted for the end of 21st century in an area of 1536 km 2 including the two streams where fish were sampled [START_REF] Sevault | A fully coupled Mediterranean regional climate system model: design and evaluation of the ocean component for the 1980-2012 period[END_REF]. Indeed, under the most severe climate change scenario (RCP 8.5), the mean monthly temperature is forecasted to become higher than 10 xC from March to November (i.e., 9 months) and exceed 20 xC from June to September in the future (mean over 2076-2100), whereas it only exceeds 20 xC in August and is higher than 10 xC from April to October (i.e., 7 months) at present . Hence, comparing these two contrasted and realistic temperatures enabled to test for the effect of future water warming on SSD by the three fish species and better understand potential implications for physical processes and ecosystem functioning.

During the acclimatization period, fishes were fed with dead chironomid larvae, and water in the tanks was gradually diluted with dechlorinated tap-water. Two days before the experiment, fishes were starved to optimize their foraging behaviour during the experimental period. No damage to individual fish was observed at the end of this starvation period.

Experimental design

The cost-effective method proposed by De Nadaı¨-Monoury et al. (2013) was used to experimentally quantify SSD by fish. For technical reasons, the experiments were conducted in aquaria without any flow, hence rather mimicking lentic than lotic environmental conditions. Opaque glass aquaria (40 cm long, 25 cm wide, 25 cm high) were placed in temperature-controlled rooms and filled with a 5-cm layer of commercial white quartz sand (1-2.5 mm in diameter). A thin layer of dark blue sand (same diameter as the white sand) was used as tracers and then meticulously sprinkled on the white sand surface. Finally, tap water (about 18 cm high) was added in the aquaria, paying attention not to disturb the sediment layer, and then dechlorinated for 2 days. Aquaria were placed on polystyrene plates (20-mm thick) to isolate them from vibrations and lighted by an indirect light source (12:12 h light regime), to reduce fish stress.

Given the dimensions of the aquaria used, a single fish was placed in each aquarium resulting in a density of ten individuals per square meter. This density is higher than natural densities observed in streams but is consistent with patch-scale densities often observed during low-flow periods. It is worth noting that the behaviour of single fish placed in an aquarium is likely different from what is observed in the wild, especially for cyprinid species that naturally shoal. However, this experimental design enabled us to avoid potential interactions between fishes and additive surface disturbance due to multiple interacting fishes. For each temperature, experiments included eight replicates of each species, and three control microcosms to quantify sediment displacements due to handling or gravity in fish absence. At the end of the experiment, fishes were released back into the streams from where they had been collected.

Assessment of the SSD

Using a digital camera (Canon EOS 20D) mounted on a tripod, a picture of the initial sediment layer was taken directly above each aquarium before introducing the fish (Fig. 1(a)). Then, pictures of the sediment layer were taken daily to follow the changes in the sediment over the course of the experiment. The pictures were taken quietly with the fish within the aquarium to limit sudden and abrupt movements due to stress that could result in non-natural SSD. Appearance of white sand within the blue sand surface layer reflected the intensity of SSD by fish and spatial patterns of bed disturbed were also analysed. We stopped the experiment after 10 days (Fig. 1 SSD was quantified using several metrics. First, the proportion of surface disturbed was measured. Automatic pixel counts estimating the surface occupied by appearing white sand relative to the total surface of the aquarium were performed using image analysis software Image-Pro Plus (MediaCybernetics, Inc; see De Nadaı¨-Monoury et al., 2013 for a detailed description of pictures processing). Second, spatial patterns in the disturbed surface were analysed using the packages raster and SDMTools in the R environment software (R Development Core Team, 2011). They enabled calculation of the number and size of patches of appearing white sand (Fig. 1(c)). A patch was defined as adjacent grains of white sand, i.e. white sand grains sharing a border or at least a corner (Queen's method; [START_REF] Hijmans | Raster: Geographic data analysis and modeling[END_REF]. To avoid overestimating the number of patches due to isolated sand grains, only patches made up of more than five grains of white sand were counted. The proportion of disturbed surface as well as both the number and mean size of the patches were divided by the mass of the fish to ensure robust comparisons between individuals and species. For all three metrics, the sediment surface under the fish was not accounted for in the computation of SSD.

Statistical analyses

Given small sample sizes, a non-parametric equivalent of a two-way analysis of variance with replication (Scheirer-Ray-Hare test; [START_REF] Scheirer | Analysis of ranked data derived from completely randomized factorial designs[END_REF] was performed to determine whether temperature (here, considered as a factor) and/or species significantly influenced the proportion of disturbed surface at the end of the experiment, as well as the number and size of the patches disturbed. Then, comparisons between pairs of species and temperature were performed by Mann-Whitney tests and P-values were adjusted with the Bonferroni correction.

To determine whether SSD by fish was clustered or not, the spatial randomness of the point patterns derived from the sediment disturbance activity of fish was tested using the simple quadrat method [START_REF] Greig-Smith | The use of random and contiguous quadrats in the study of the structure of plant communities[END_REF][START_REF] Cressie | Statistics for Spatial Data: Revised Edition[END_REF]. Complete Spatial Randomness (CSR) assumes that points (here, white sand grains) follow a homogenous Poisson process over the study area and thus, that the density of white sand grains is constant over the whole aquarium surface. The quadrat method estimated how intensity of white sand grain patterns varied over the aquarium surface by counting points in delimited quadrats. Given that the result of the test depends on the size of the quadrats, the aquarium surface was partitioned in quadrats ranging from 1 to 250 cm 2 . Quadrat tests were performed using the package spatstat of the R environment software (R Development Core Team, 2011).

Results

In absence of fish (controls), no changes in the sediment surface were measured. In aquaria with fish, SSD was detected shortly after the fishes were introduced in aquaria (day 1) and the proportion of surface disturbed by the three fish species steadily increased for the 10 days of experiment, at both temperatures (Fig. 2). Both cyprinid species P. toxostoma and S. cephalus had comparable kinetics at 10 xC (Fig. 2(a)) but differed at 20 xC with higher disturbance by S. cephalus over the experiment (Fig. 2(b)). Throughout the 10 days of experiment, B. barbatula had a higher sediment disturbance activity than both cyprinid species at both temperatures (surface disturbed approximately 3-4 and 2-4 times higher for B. barbatula at 10 and 20 xC, respectively; Fig. 2).

After 10 days of experiment, sediment surface disturbed appeared significantly clustered (i.e., patches of appearing white sand) for all three species and temperatures (CSR; P< 0.001) whatever the size of the quadrats chosen except for two individuals of S. cephalus at 20 xC for the largest quadrat size.

When comparing the sediment surface after 10 days between species and temperatures, the proportion of white sand disturbed (Fig. 3(a)) varied depending on species (Scheirer-Ray-Hare; H 2,42 = 23.4, P < 0.001) and temperature (H 1,42 = 14.7, P< 0.001), while the number (Fig. 3(b)) and size (Fig. 3(c)) of white sand patches significantly differed between species (number of patches: H 2,42 = 33.4, P < 0.001; size of patches: H 2,42 = 30.8, P< 0.001) but not between temperatures (all P > 0.05). For the three species, the proportion of sediment surface disturbed was significantly higher (approximately 2-3 times higher depending on species; Table 1) at 20 xC than at 10 xC (Mann-Whitney; all P < 0.05; Fig. 3(a)), with an average of 10.5, 5.0 and 3.2%.g x1 of aquarium surface for B. barbatula, S. cephalus and P. toxostoma, respectively, at 20 xC. At both temperatures, B. barbatula significantly disturbed a larger surface with more and larger patches than S. cephalus (Mann-Whitney; P < 0.05) and P. toxostoma (Mann-Whitney; P < 0.05) (Fig. 3). On average, SSD by B. barbatula after 10 days resulted in 2.6 and 1.9 more patches at 10 xC than S. cephalus and P. toxostoma, respectively, and 3.3 and 2.5 more patches at 20 xC (Table 1). The mean patch area disturbed by B. barbatula was about two times larger than the patches disturbed by the two cyprinid species at both temperatures (Table 1). There was no significant difference in the proportion of surface disturbed and patterns of SSD (i.e., number and size of patches) between S. cephalus and P. toxostoma at both 10 and 20 xC (Mann-Whitney; all P> 0.05; Fig. 3; Table 1).

Discussion

This study is one of the first to investigate the potential influence of temperature on the intensity of SSD by freshwater fishes. So far, the only studies dealing with the effect of temperature on sediment disturbance were conducted for marine invertebrates (e.g., [START_REF] Hollertz | Burrowing behavior and sediment reworking in the heart urchin Brissopsis lyrifera Forbes (Spatangoida)[END_REF][START_REF] Ouellette | Effects of temperature on in vitro sediment reworking processes by a gallery biodiffusor, the polychaete Neanthes virens[END_REF][START_REF] Ducheˆne | A comparison of sediment reworking rates by the surface deposit-feeding bivalve Abra ovata during summertime and wintertime, with a comparison between two models of sediment reworking[END_REF]. They all reported an increase in sediment reworking with rising temperature. Our results highlighted that the three freshwater fish species considered were more active in SSD at warm than at cold temperature confirming these previous findings. It is well known that temperature is one of the major drivers of the biology and activities of ectothermic animals. Several studies have demonstrated that temperature controls many biochemical processes of metabolic rate [START_REF] Gillooly | Effects of size and temperature on metabolic rate[END_REF][START_REF] Brown | Toward a metabolic theory of ecology[END_REF] and that a warmed temperature leads to an increase in basal and active metabolic rate as well as rates at which energy reserves are used by ectothermic organisms [START_REF] Kassahn | Animal performance and stress: responses and tolerance limits at different levels of biological organisation[END_REF]. Increases in the rate of physiological processes enable to raise fish activity among which locomotor [START_REF] Lo´pez-Olmeda | Thermal biology of zebrafish (Danio rerio)[END_REF] and foraging [START_REF] Hathaway | The relation of temperature to the quantity of food consumed by fishes[END_REF][START_REF] Hellawell | The autecology of the chub, Squalius cephalus (L.), of the River Lugg and the Afon Llynfi[END_REF] activities.

The inter-specific difference observed during this experiment is likely due to the contrasted thermal requirements and tolerances of the three species studied. Indeed, the optimum temperature for S. cephalus ranges between 12 and 25 xC [START_REF] Souchon | Synthesis of thermal tolerances of the common freshwater fish species in large Western Europe rivers[END_REF], the one of P. toxostoma between 16 and 25 xC [START_REF] Gozlan | Environmental biology and morphodynamics of the sofie Chondrostoma toxostoma (Cyprinidae), with emphasis on early development[END_REF], whereas the upper tolerance limit of B. barbatula is colder, around 19 xC [START_REF] Elliott | A functional model for maximum growth of immature stone loach, Barbatula barbatula, from three populations in north-west England[END_REF]. Outside their thermal range, species are under thermal stress that is generally characterized by lethargy at cooler temperatures, and sudden activity sequences and rapid breathing movements at warmer temperatures [START_REF] Elliott | Some aspects of thermal stress on freshwater teleosts[END_REF]. The temperatures selected for our experiment made it possible to assess such behavioural changes while keeping temperature within tolerance limits. At 10 xC, S. cephalus and P. toxostoma were indeed under "hypo-thermal" stress: their metabolism was likely slowed down to avoid energy consumption, probably resulting in the observed reduced SSD. Both species of cyprinids were within their optimal thermal range at 20 xC, explaining the increased SSD behaviour. By contrast, B. barbatula was within its optimal thermal range at 10 xC, and its proportion of sediment surface disturbed was roughly equivalent to the one of both cyprinid species at 20 xC. B. barbatula was however under "hyper-thermal" stress at 20 xC resulting in an accelerated metabolism and subsequently an increase in SSD activity.

Differences between species were also found in terms of patterns in the surface disturbed. Overall, the three species disturbed surface sediment particles in a non-random manner with patches of white sand appearing at the surface. However, it seems that B. barbatula tended to dig more patches that were larger on average than the other two species for which the small number and size of patches disturbed were similar. Several activities may generate SSD by fish such as: (i) foraging activity with the ingestion, digestion and excretion of sediment into the water column [START_REF] Flecker | Habitat modification by tropical fishes: environmental heterogeneity and the variability of interaction strength[END_REF]Statzner et al., 2003b;[START_REF] Pledger | Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)[END_REF]; (ii) displacement of fine particulate matter [START_REF] Zhang | Detritus processing, ecosystem engineering and benthic diversity: a test of predator-omnivore interference[END_REF]; (iii) burrowing activity [START_REF] Shirakawa | Lamprey larvae as ecosystem engineers: physical and geochemical impact on the streambed by their burrowing behavior[END_REF]; (iv) nest digging [START_REF] Moore | Animal ecosystem engineers in streams[END_REF]; or (v) swimming activity near the substratum (personal observation). In our study, fish observations conducted during the experiments have enabled us to highlight that SSD was mainly due both to foraging and swimming near the sediment surface. Being a benthic fish species, B. barbatula was also observed to excavate sand grains to bury and probably to take refuge, resulting in moderate-sized patches of sediment disturbed. By contrast, the two cyprinid species rather tended to produce patches more evenly distributed across the bed. The warmer temperature resulted in a larger surface disturbed for all species, but did not result in significantly more and larger patches being disturbed when compared with 10 xC. This counter-intuitive outcome is likely an artefact of the experimental design. Indeed, experimental units are enclosed spaces, where the number and size of patches cannot increase indefinitely. However, to limit this experimental bias, the experiment concluded after 10 days, since it appeared that the proportion of surface disturbed reached a plateau. Conducting a finer analysis of the position and size of appearing white sand patches throughout the 10 days of experiment would likely strengthen our results.

Other technical constraints induced by our experimental design in small-size aquaria hinder to extrapolate these findings to more natural lotic conditions. First, despite the innovative nature of this experiment in microcosms, it is a preliminary approach that deserves to be applied ex situ with more realistic stream flow conditions (e.g., flumes, larger sediment diameters; see such experimental designs in: [START_REF] Statzner | Crayfish as geomorphic agents and ecosystem engineers: biological behavior affects sand and gravel erosion in experimental streams[END_REF][START_REF] Pledger | Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)[END_REF] or in situ, in both lentic and lotic systems. In streams, water flow can indeed hide the movement of particles due to bioturbation by invertebrates and small fish that have thus limited direct geomorphological implications [START_REF] Statzner | Geomorphological implications of engineering bed sediments by lotic animals[END_REF]. Fine sediment displacements by foraging can also be harder due to water-working by the flow that consolidate and structure the bed surface. However, benthic foraging fishes have also been reported to undo the water-worked imbricate structures [START_REF] Pledger | Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)[END_REF], thereby potentially counteracting the sediment consolidation by flow or other lotic organisms (e.g., plants, aquatic insects; [START_REF] Statzner | Geomorphological implications of engineering bed sediments by lotic animals[END_REF]. Stream flow might also influence foraging and swimming activities as fish often feed whilst facing upstream into the flow [START_REF] Pledger | Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)[END_REF], and consequently, the nature of SSD. Accounting for the close tangling between stream flow and SSD by fish activities would likely help better understanding their relative effect in natural systems.

Second, our experimental design limited to the comparison of two contrasted temperatures prevented us to identify potential threshold temperatures above which SSD started to increase or decrease. Such thermal thresholds in sediment reworking have been reported in [START_REF] White | Effects of temperature and density on sediment reworking by Stylodrilus heringianus (Oligochaeta: Lumbriculidae)[END_REF] for oligochaetes experiencing cycles of fall and rise temperatures between 4 and 20 xC, and in [START_REF] Berkenbusch | Factors influencing sediment turnover by the burrowing ghost shrimp Callianassa filholi (Decapoda: Thalassinidea)[END_REF] who followed the rate of sediment expulsion by a burrowing shrimp over a 12-month period. Nevertheless, most other previous studies that have focused on the relationship between temperature and sediment reworking have been limited to two or three temperature treatments [START_REF] Ouellette | Effects of temperature on in vitro sediment reworking processes by a gallery biodiffusor, the polychaete Neanthes virens[END_REF][START_REF] Ducheˆne | A comparison of sediment reworking rates by the surface deposit-feeding bivalve Abra ovata during summertime and wintertime, with a comparison between two models of sediment reworking[END_REF], probably for practical and technical constraints. Further experimental work conducted in a controlled temperature room where a continuous warming would be simulated would make it possible to precisely measure the fine relationship between temperature and SSD and to identify a potential peak activity in fish sediment disturbance activity.

Third, it should be noted that our experiment was limited to three fish species, thus neglecting other fish that are known to be active streambed sediments bioturbators either by feeding or swimming (e.g., the barbel Barbus barbus L.1758: Pledger et al., 2014; the gudgeon Gobio gobio L.1758: Statzner et al., 2003b;[START_REF] Statzner | Crayfish and fish as bioturbators of streambed sediments: assessing joint effects of species with different mechanistic abilities[END_REF]larval lamprey: Shirakawa et al., 2013) or nest digging (e.g., salmonids: De Vries, 2012), but also other zoogeomorphic animals such as crayfish (e.g., [START_REF] Statzner | Crayfish as geomorphic agents and ecosystem engineers: biological behavior affects sand and gravel erosion in experimental streams[END_REF] or benthic macro-invertebrates (e.g., De [START_REF] Nadaı¨-Monoury | A cost-effective method to quantify biological surface sediment reworking in streams[END_REF]. Moreover, due to limited space, a single fish was placed into each aquarium preventing all complex biotic interactions known to affect bioturbating activities. In addition, using single fishes has limited our ability to study the joint effects of several bioturbators on physical interactions among sediment surface modifications [START_REF] Statzner | Crayfish and fish as bioturbators of streambed sediments: assessing joint effects of species with different mechanistic abilities[END_REF]. All these experimental constraints suggest that a great caution should be exercised if extrapolating these preliminary results to the whole fish communities and more natural conditions.

In the context of the ongoing climate change, our finding that water warming could potentially increase SSD by fish may have important consequences for both physical and geomorphological processes and aquatic ecosystem functioning. This warming, simulated by a temperature of 20 xC, seems realistic in the streams of the Garonne catchment for the end of the 21st century according to the regional climate models (ALADIN climate model; [START_REF] Sevault | A fully coupled Mediterranean regional climate system model: design and evaluation of the ocean component for the 1980-2012 period[END_REF]. Among the potential impacts of an increase in SSD by fish in the future are the increased erodibility of sediment surface and fine particles transport in rivers [START_REF] Pledger | Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)[END_REF]. They could result simultaneously in waters with higher suspended solids and fine elements (e.g., organic detrital matter) concentrations, but lower fine sediment volumes [START_REF] Statzner | Geomorphological implications of engineering bed sediments by lotic animals[END_REF] in local places such as deposition areas. Fine sediment removal may subsequently affect negatively biological communities by reducing the establishment and presence of sessile or movement-limited insects [START_REF] Pringle | Effects of omnivorous shrimp in a montane tropical stream: sediment removal, disturbance of sessile invertebrates and enhancement of understory algal biomass[END_REF] and reducing bed biofilms [START_REF] Statzner | Crayfish as geomorphic agents and ecosystem engineers: biological behavior affects sand and gravel erosion in experimental streams[END_REF] that indirectly affect leaf decomposition processing. In lentic waters, turbidity could be hastened and concentrations in nutrients and organic material higher due to the resuspension of sediments to the water column through bioturbation by fish (e.g., [START_REF] Breukelaar | Effects of benthivorous bream (Abramis brama) and carp (Cyprinus carpio) on sediment resuspension and concentrations of nutrients and chlorophyll a[END_REF][START_REF] Matsuzaki | Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis[END_REF], resulting subsequently in phytoplankton blooming and water eutrophication, already enhanced by warming [START_REF] Jeppesen | Impacts of climate warming on lake fish community structure and potential effects on ecosystem function[END_REF]. Changes in community composition [START_REF] Matsuzaki | Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis[END_REF] and vertical distribution of benthic fauna (Persson and Svensson, 2006b) due to predation by benthivorous fish may also impact nutrients fluxes as well as the incorporation of organic matter into the benthic food web (Maire et al., 2010), especially at higher temperatures [START_REF] Zhang | The effects of temperature on oxygen uptake and nutrient flux in sediment inhabited by molluscs[END_REF].

To conclude, given that benthic foraging is a common feeding behaviour in freshwater environments observed all year round and that it appears a substantial zoogeomorphic mechanism [START_REF] Pledger | Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)[END_REF], SSD by fish should not be overlooked in future research assessing the potential impacts of climate change on zoogeomorphology. In addition, many cool-and warm-water benthic fish species are forecasted to expand their distribution in the coming decades while some others are expected to contract [START_REF] Buisson | Climate change hastens the turnover of stream fish assemblages[END_REF][START_REF] Lyons | Predicted effects of climate warming on the distribution of 50 stream fishes in Wisconsin[END_REF]. As climate change impacts both habitats and food sources, the distribution of some geomorphologically active animals may change dramatically, affecting subsequently the zoogeomorphic patterns of the influences and intensities of these impacts [START_REF] Butler | The impact of climate change on patterns of zoogeomorphological influence: examples from the Rocky Mountains of the Western U[END_REF]. Hence, these issues should be urgently addressed.

  (b)) as most of the surface appeared to have been disturbed and the disturbed surface stopped increasing (see De Nadaı¨-Monoury et al., 2013 for more details).

Fig. 1 .

 1 Fig. 1. Picture of the surface of an aquarium at (a) the beginning of the experiment, (b) after 10 days of sediment disturbance, and (c) after image treatment. An uniform blue sand layer had been sprinkled above a surface of white sand before one fish has been placed in the aquarium and sediment disturbance activity daily measured by picture analyses.

Fig. 2 .

 2 Fig. 2. Mean percentage of surface disturbed per gram of fish at (a) 10 xC and (b) 20 xC for the three fish species studied for the 10-day experiment period. The bars shown are standard deviations calculated across eight individuals for each species and temperature.

Fig. 3 .

 3 Fig. 3. SSD by fish after 10 days of experiment: (a) percentage of sediment surface disturbed per gram of fish; (b) number of white sand patches per gram of fish; (c) mean patch area (log transformed) per gram of fish. Different letters indicate significant differences (P < 0.05) between factors according to multiple pairwise Mann-Whitney tests and adjustments of P-values by a Bonferroni correction.

Table 1 .

 1 Summary statistics of the surface sediment disturbance metrics derived from the pictures analysis. Mean and standard deviation across the eight replicates are given.

		Barbatula barbatula	Squalius cephalus	Parachondrostoma toxostoma
	Surface disturbed (%.g x1 )
	10 xC	4.90 ¡ 1.56	1.70 ¡ 0.79	1.13 ¡ 0.66
	20 xC	10.52 ¡ 1.74	4.98 ¡ 1.30	3.16 ¡ 1.41
	Number of patches after 10 days per gram
	10 xC	16.17 ¡ 2.16	6.27 ¡ 1.85	8.62 ¡ 2.09
	20 xC	23.67 ¡ 6.20	7.09 ¡ 4.22	9.58 ¡ 2.15
	Mean patch area (log) per gram
	10 xC	0.74 ¡ 0.15	0.33 ¡ 0.06	0.38 ¡ 0.09
	20 xC	0.94 ¡ 0.13	0.42 ¡ 0.10	0.42 ¡ 0.10

Acknowledgements. We are grateful for the help provided by S. Jean and F. Dauba during the experiment and fish sampling. We thank Me´te´o-France for providing present and future climate data extracted from the ALADIN model. This work has been carried out as a part of "ADAPT'EAU" (ANR-11-CEPL-008), a project supported by the French National Research Agency (ANR) within the framework of "The Global Environmental Changes and Societies (GEC&S) programme".


Romain Mailler
email: romain.mailler@leesu.enpc.fr

Johnny Gasperi

Yves Coquet

Steven Deshayes

Sifax Zedek

Audrey Buleté

Emmanuelle Vulliet

Véronique Eudes

Adèle Bressy

Emilie Caupos

Removal of emerging micropollutants from wastewater discharges by powdered and micro-grain activated carbon -Progress of the CarboPlus® project

Removal of emerging micropollutants from wastewater discharges by powdered and micro-grain activated carbon -

Progress of the CarboPlus® project R. Mailler 1 , J. Gasperi 1 , Y. Coquet 2 , S. Deshayes 1,3 , S. Zedek 1 , A. Buleté 4 , E. Vulliet 4 , V. Eudes 3 , A. Bressy 1 , E. Caupos 1 , R. Moilleron 1 , G. Chebbo 1 and V. Rocher 5

Introduction

Among the solutions to reduce the emerging contaminant discharges into the environment, the implementation of a tertiary treatment in actual wastewater treatment plants (WWTP) is more and more considered. Besides, emerging micropollutants, particularly pharmaceuticals and hormones (PhPHs), may be included in modifications of existing regulations.

In this context, a collaboration between the SIAAP and the OPUR research program has been initiated since 2013 to study at large scale a tertiary treatment pilot (CarboPlus® -SAUR/STEREAU) based on a fluidized bed activated carbon reactor. The objectives of this project are to i) characterize the efficiency of the process for priority and emerging pollutants removal, ii) characterize the sorption mechanism and the different parameters influencing it (organic matter competition, operating parameters, activated carbon structure and properties, etc.) and iii) determine if any improvement of the conventional quality parameters is achieved thanks to the process.

The process was initially tested with powdered activated carbon (PAC) and its performances in this configuration are already published [START_REF] Mailler | Study of a large scale powdered activated carbon pilot: removals of a wide range of emerging and priority micropollutants from wastewater treatment plant effluents[END_REF]. Then, a micro-grain activated carbon (µGAC) is studied instead of PAC since January 2014. In addition to being reactivable, its intermediary size (300-500 µm) between powder (<50 µm) and grain (>1 mm) results in a simplification of the process operation, as no coagulant and flocculant are needed anymore to stabilize the bed.

To evaluate the treatment in the µGAC configuration, the fate of 131 micropollutants has been studied, including 61 PhPHs and 70 other emerging pollutants, in addition to various conventionnal wastewater parameters, such as DOC, TSS, UV-254 or nitrogen species.

This presentation aims at displaying the efficiency of this innovative process in a large WWTP application. Thus, the concentrations and the removals obtained with µGAC for a wide range of emerging micropollutants are presented. Then, the comparison of the two configurations (PAC versus µGAC) and with the scarce studies available in the literature is given [START_REF] Boehler | Removal of micropollutants in municipal wastewater treatment plants by powder-activated carbon[END_REF][3] [START_REF] Altmann | Direct comparison of ozonation and adsorption onto powdered activated carbon for micropollutant removal in advanced wastewater treatment[END_REF]. Finally, the influence of the various parameters that affect the adsorption of micropollutants in the process is discussed, as well as the results of complementary labscale experiments performed to better understand adsorption mechanisms.

Materials and methods

The process studied is the CarboPlus® process (SAUR -STEREAU). This technology is based on the flowing of water through a high concentration fluidized activated carbon bed (>100 kg/m The project was divided in two configurations: 1) PAC in 2013 and 2) µGAC in 2014. A total of 28 (14 + 14) sampling campaigns were planned, using refrigerated automatic samplers performing 24 h flow weighted composite samples from inlet and outlet waters of the pilot.

Various general water quality parameters were analysed by the SIAAP laboratory (French accreditation) to characterize the effluents and determine if the process allows a general improvement of the water quality. Thus, dissolved organic carbon, chemical and biological oxygen demand, UV absorbance at 254 nm, total Kjeldahl nitrogen, NH 4 + , NO 2 -, NO 3 -, PO 4 3-, total phosphor and total suspended solids (TSS) were monitored.

A total of 131 organic micropollutants were monitored including 61 pharmaceuticals (analgesics, antibiotics, anxiolytics, X-ray contrast agents) and hormones, and 70 various emerging pollutants such as pesticides (N=28), alkylphenols (N=7), phthalates (N=4), personal care products (N=7), artificial sweeteners (N=4), polycyclic aromatic hydrocarbons (N=13), chlorinated solvents (N=3), perfluorinated acids (N=2) or bisphenol A and benzotriazole. The analyses are performed by 5 laboratories: the Institute of Analytical Sciences (ISA), the Water Environment and Urban Systems laboratory (LEESU), the Central laboratory of the Police Prefecture (LCPP), the CARSO laboratory and the Water Technology Center (TZW -Germany).

Results and discussion

After a period of stabilization, high removals of the emerging micropollutants are reached thanks to the CarboPlus® -µGAC process, with a fresh µGAC dose of 20 g/m

3

. Indeed, the 20 PhPHs quantified in influents are all removed at more than 70%. In particular, ketoprofen, paracetamol, ibuprofen, ofloxacin, ciprofloxacin, trimethoprim, roxithromycin, atenolol, propranolol, carbamazepine, oxazepam, lorazepam and estrone have removals higher than 80% in average. In addition, first results on the other emerging micropollutants show high or very high removals for diuron, terbutryn and mecoprop (60-95%), bisphenol A (60-70%), alkylphenols (60-95%), PFOS (75-95%) and X-ray contrast agents (50-70%). Finally, the general quality of the water is significantly improved thanks to the pilot. In particular, the µGAC bed seems to trap a part of the TSS (36% in average) and to remove a high quantity of organic matter (DOC -35%; COD -43%). Moreover, the high solid retention time (60-90 days) of the µGAC results in a biological activity within the reactor, as displayed by NO 2 -(91%) and NH 4 + (44%) average removals.

Although these results are preliminary (not all campaigns and molecules), they tend to highlight a comparable efficiency of the CarboPlus® process in both PAC and µGAC configuration. In addition, they are in accordance with the scarce recent studies available in the literature [START_REF] Boehler | Removal of micropollutants in municipal wastewater treatment plants by powder-activated carbon[END_REF][3][4], despite differences in processes and quality of waters. If this is confirmed with the remaining campaigns (µGAC dose of 10 g/m 3

), this would mean that the µGAC allows similar performances with an operation simplification and a reactivability of the carbon.

The BET surface was confirmed as the most relevant activated carbon structural property to predict the efficiency of adsorption, but comparison of different activated carbons showed that bulk density was correlated to it and could be used as a tool to select the most efficient activated carbons.

Conclusions

The use of micro-grain activated carbon instead of PAC has several operational and economical advantages, but this type of carbon was never tested with wastewater. The first results of the study tend to indicate that the CarboPlus® process is as efficient with µGAC than with PAC despite the higher granulometry of the first one. In particular, most of the studied micropollutants have removals higher than 80% within this process, particularly the PhPHs. In addition, this operation configuration allows to improve slightly the treatment of nutrients, in particular carbon and nitrogen. However, the results are still partial at this stage of the project, another µGAC dose is currently tested on the pilot and complementary lab-scale tests are still performed to better understand the adsorption mechanisms.

3 ) 3 / 3 /

 333 with a continuous injection of a moderate fresh dose of activated carbon. The pilot studied (5 m high -surface area of 4 m2) is fed with the discharges of the Seine Centre WWTP (Colombes, France -240 000 m day) and can treat between 700 and 1900 m day, corresponding to hydraulic velocities of 6 to 20 m/h.

Acknowledgement -The authors thank the OPUR research program, the SIAAP, LEESU and SAUR teams for their active participation to the project, as well as Yen Pham (internship).