Chouvelon Tiphaine
email: tiphaine.chouvelon@ifremer.fr

Cresson Pierre

Bouchoucha Marc

Brach-Papa Christophe

Bustamante Paco

Fabri Marie-Claire

Marco-Miralles Françoise

Thomas Bastien

Knoery Joël

Tiphaine Chouvelon

Oligotrophy as a major driver of

Keywords: mercury, Biomagnifying metal, Bioaccumulation, Trophic status, Ecosystem productivity, Temperate systems Highlights

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Mercury (Hg) is released into the environment from both natural and anthropogenic sources (e.g., volcanism, fuel and waste combustion). It reaches marine waters through atmospheric depositions and riverine inputs, the former being the main source of Hg for the surface ocean [START_REF] Fitzgerald | Marine biogeochemical cycling of mercury[END_REF].

Also, Hg is volatile and as such it can be transported through the atmosphere over long distances and deposited in areas away from its point sources. In the ocean, trophic transfer represents the main pathway for the intake and bioaccumulation of Hg by marine organisms. Hg exhibits a specific behaviour compared to other trace metals, as it biomagnifies through food webs in its organic methyl-Hg (Me-Hg)

forms [START_REF] Chen | Methylmercury in marine ecosytems: Spatial patterns and processes of production, bioaccumulation, and biomagnification[END_REF]. Finally, Hg is of particular environmental concern because of its toxicity on the nervous, reproductive, immunological and hormonal systems, sometimes leading to harmful effects at the population level [START_REF] Boening | Ecological effects, transport, and fate of mercury: A general review[END_REF][START_REF] Tan | The endocrine effects of mercury in humans and wildlife[END_REF][START_REF] Goutte | Demographic responses to mercury exposure in two closely-related Antarctic top predators[END_REF].

The methylation of Hg by microorganisms into Me-Hg is the fundamental process giving this contaminant its bioaccumulation and biomagnification properties. In the marine environment, Hg methylation occurs both in coastal and shelf sediments, and in subthermocline low-oxygen oceanic waters, at depths where organic matter is intensively remineralized [START_REF] Mason | Methylated and elemental mercury cycling in surface and deep ocean waters of the North Atlantic[END_REF][START_REF] Fitzgerald | Marine biogeochemical cycling of mercury[END_REF][START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF][START_REF] Cossa | The open sea as the main source of methylmercury in the water column of the Gulf of Lions (Northwestern Mediterranean margin)[END_REF]. In its methylated forms, Hg passes through biological membranes easily and is incorporated into cellular cytoplasm (e.g., in phytoplankton), from which it is assimilated very efficiently by higher trophic levels [START_REF] Faucheur | Interactions between mercury and phytoplankton: Speciation, bioavailability, and internal handling[END_REF]. Once incorporated in organisms, Hg strongly binds with protein sulfhydryl groups (-SH). Due this affinity, the elimination or the excretion of the bioaccumulated Hg is very slow over time, sometimes equal to zero, like in the muscle tissues [START_REF] Wang | Bioaccumulation kinetics and exposure pathways of inorganic mercury and methylmercury in a marine fish, the sweetlips Plectorhinchus gibbosus[END_REF][START_REF] Maulvault | Bioaccumulation and elimination of mercury in juvenile seabass (Dicentrarchus labrax) in a warmer environment[END_REF]. As a consequence, Hg has been widely documented to bioaccumulate with size or age in marine organisms (e.g., [START_REF] Monteiro | Mercury content of swordfish, Xiphias gladius, in relation to length, weight, age, and sex[END_REF][START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF]Chouvelon et al., 2014a).

Several factors linked to Hg marine biogeochemical cycling and its chemical properties are known to influence Hg bioaccumulation in marine fauna. The concentrations of dissolved Me-Hg in ambient waters or in the different habitats of species is a first factor. As an example, mesopelagic species were shown to present higher Hg concentrations than epipelagic ones [START_REF] Monteiro | Mercury concentrations in prey fish indicate enhanced bioaccumulation in mesopelagic environments[END_REF][START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF][START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (North-East Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF], due to the probably higher exposition of mesopelagic organisms to Me-Hg in these deep pelagic layers of organic matter remineralisation (e.g., [START_REF] Cossa | The origin of methylmercury in open Mediterranean waters[END_REF][START_REF] Heimbürger | Methyl mercury distributions in relation to the presence of nano-and picophytoplankton in an oceanic water column (Ligurian Sea, North-western Mediterranean)[END_REF]. Time of exposure to Hg is a second factor that directly influences its bioaccumulation. It is generally inferred by the size or the age of the organisms considered, and usually higher Hg concentrations are measured in older organisms (due to low elimination rates; see above). The trophic status and/or trophic functioning of ecosystems is a third factor that may be put forward to explain different bioaccumulation rates (e.g., [START_REF] Chen | High plankton densities reduce mercury biomagnification[END_REF]. This "trophic factor" itself includes several aspects, some being intimately linked to the first two described above. The first aspect of this trophic factor directly concerns system productivity, through the "bio-dilution effect" [START_REF] Pickhardt | Algal blooms reduce the uptake of toxic methylmercury in freshwater food webs[END_REF].

The bio-dilution effect suggests lower Hg bioaccumulation at all trophic levels in mesotrophic areas compared to oligotrophic areas [START_REF] Harmelin-Vivien | Difference of mercury bioaccumulation in red mullets from the north-western Mediterranean and Black seas[END_REF]Cresson et al., 2014a). It is due to the higher number and higher surface area/volume ratio (i.e. size) of cells at the base of mesotrophic systems. This configuration is less favourable to an efficient uptake of Hg by cells, whose the lower Hg burden (in comparison with cells in oligotrophic area) is then transferred to consumers. Moreover in oligotrophic areas, where cells are thus less abundant and potentially contain higher Hg burden, primary consumers probably consume virtually all of them. A second aspect of the trophic factor is directly related to the occurrence of specific planktonic communities at the base of food webs. Indeed, in oligotrophic environments, the higher proportions of slowly sinking pico-and nanophytoplanctonic cells that are readily degraded by bacteria within the water column promote Me-Hg formation in the heterotrophic active layers of organic matter remineralisation (e.g., [START_REF] Cossa | The origin of methylmercury in open Mediterranean waters[END_REF][START_REF] Heimbürger | Methyl mercury distributions in relation to the presence of nano-and picophytoplankton in an oceanic water column (Ligurian Sea, North-western Mediterranean)[END_REF]. This aspect is thus intimately derived from the first factor concerning the level of bioavailable Hg in ambient waters. A third aspect of the trophic factor concerns the "trophic chain length", also corresponding to the number of "nodes" between the considered organisms and the primary producers.

The trophic chains are sometimes considered to be longer in less productive environments (i.e. due to the higher recycling of organic matter and bacterial activity; e.g., Biddanda et al., 2001). In the case of biomagnifying contaminants such as Hg, it may therefore lead to higher Hg concentrations measured in apex predators of oligotrophic systems. Finally, we may hypothesize that the trophic status and associated primary productivity of marine systems influence the growth rate of organisms as well (i.e. lower grow rates in less productive or oligotrophic environments may be expected), hence also influencing the rate of Hg bioaccumulation (e.g., [START_REF] Simoneau | Fish growth rates modulate mercury concentrations in walleye (Sander vitreus) from eastern Canadian lakes[END_REF][START_REF] Trudel | Bioenergetics and mercury dynamics in fish: a modelling perspective[END_REF]Ward et al., 2010).

Following these three fundamental factors (i.e. environmental concentrations of Me-Hg in ambient waters; age or growth rate of the organisms considered in the different systems; trophic status of systems), which can be related to each other, the bioaccumulation and the biomagnification of Hg were proved to vary sometimes greatly between marine ecosystems (e.g., [START_REF] Harmelin-Vivien | Difference of mercury bioaccumulation in red mullets from the north-western Mediterranean and Black seas[END_REF][START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF], or even between environments of a same marine ecosystem (e.g., between epipelagic and mesopelagic environments; [START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF][START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (North-East Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF]Cresson et al., 2014a). For instance, the Mediterranean Sea was found to be a peculiar marine ecosystem, characterized by several factors favourable to Hg contamination yielding to the so-called "Mediterranean Hg anomaly", whereby organisms' Hg concentrations reported are often higher in this sea than in others while seawater concentrations are comparable [START_REF] Cossa | The Mediterranean mercury anomaly, a geochemical or a biological issue[END_REF][START_REF] Harmelin-Vivien | Difference of mercury bioaccumulation in red mullets from the north-western Mediterranean and Black seas[END_REF][START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF]. However, relatively few ecosystem-comparative or meta-analysis studies exist for assessing the relative influence of the different factors described above (e.g, [START_REF] Lavoie | Biomagnification of mercury in aquatic food webs: A worldwide meta-analysis[END_REF], and the probable primary role of oligotrophy on Hg bioaccumulation; especially considering offshore species (i.e.

expected to be more affected by oligotrophic conditions than neritic species in any system), and/or considering several species at a time (and not only a single species). Moreover, lake ecosystems are more documented (e.g., [START_REF] Chen | High plankton densities reduce mercury biomagnification[END_REF][START_REF] Kidd | Biomagnification of mercury through lake trout (Salvelinus namaycush) food webs of lakes with different physical, chemical and biological characteristics[END_REF][START_REF] Lavoie | Biomagnification of mercury in aquatic food webs: A worldwide meta-analysis[END_REF] than the marine environment (but see [START_REF] Harmelin-Vivien | Difference of mercury bioaccumulation in red mullets from the north-western Mediterranean and Black seas[END_REF][START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF]. Finally, the reasons for amongsystems differences in Hg bioaccumulation and biomagnification have been recently described as being yet not well understood, especially those relative to ecosystems' characteristics [START_REF] Kidd | Biomagnification of mercury through lake trout (Salvelinus namaycush) food webs of lakes with different physical, chemical and biological characteristics[END_REF][START_REF] Lavoie | Biomagnification of mercury in aquatic food webs: A worldwide meta-analysis[END_REF].

In this context, the general objective of this study was to compare and evaluate the influence of the trophic status of systems and of biological processes (i.e. over geochemical ones; e.g., [START_REF] Cossa | The Mediterranean mercury anomaly, a geochemical or a biological issue[END_REF] on Hg bioaccumulation and biomagnification in offshore species and food webs from the Bay of Biscay (BoB) in the north-eastern (NE) Atlantic, and from the Gulf of Lions (GoL) in the north-western (NW) Mediterranean. The species included benthopelagic crustaceans and fish (both teleost and cartilagenous) that are characteristic of the shelf-edge in both ecosystems. They were selected to avoid potential bias linked to direct water discharge inputs of Hg and/or coastal processes on Hg cycling and bioaccumulation. Besides, the BoB in the NE Atlantic and the GoL in the NW Mediterranean represent good candidates for such comparative study, firstly due their expected difference in terms of trophic status, even offshore (one being open onto the Ocean, the other being a semi-enclosed Sea). Indeed, the NE Atlantic shelves (such as the Bay of Biscay) and the Mediterranean Sea are considered distinct biogeochemical provinces for a long time (e.g., Longhurst, 1998Longhurst, , 2007;;Reygondeau et al., 2013). Moreover, the more oligotrophic character of the Mediterranean system compared to the Atlantic one has been previously documented, especially from the composition of the organic matter (e.g., presence of diazotrophic organisms) sustaining food webs (Kerhervé et al., 2001;Liénart et al., 2017). Nonetheless, a number of species can be found in both ecosystems, allowing direct comparisons. Finally, both areas are major areas for fisheries (for which Hg bioaccumulation in commercial species may be of concern), and they constitute two out the three French façades considered and monitored by the European Marine Strategy Framework Directive.

To fulfil its general objective, this ecosystem-comparative study used comparable and analytically coherent data on total Hg (T-Hg) concentrations and carbon (C) and nitrogen (N) stable isotope ratios (as markers of food sources, trophic positions, and ecosystems' properties in terms of trophic status) analysed on the selected species, as described separately by [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (North-East Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF] for the BoB and by Cresson et al. (2014b) for the GoL. In addition, for the present study, analyses of Me-Hg were performed on the samples, to investigate whether the percentage of Me-Hg measured in the selected species may differ between systems. Indeed, while Me-Hg represents the bioaccumulated form of Hg, it is rarely measured and is often considered to be nearly equal to 100% of total Hg (THg), although this can vary between taxa and species [START_REF] Bustamante | Total and organic Hg concentrations in cephalopods from the North Eastern Atlantic waters: influence of geographical origin and feeding ecology[END_REF][START_REF] Kehrig | Inorganic and methylmercury: Do they transfer along a tropical coastal food web?[END_REF][START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF][START_REF] Briant | Spatial and temporal distribution of mercury and methylmercury in bivalves from the French coastline[END_REF]. Finally, the present additional analysis of Me-Hg allowed comparing the biomagnification rates of T-Hg and Me-Hg within each system, and between the two contrasted systems for each Hg form.

Overall, we hypothesized that the trophic status and especially the oligotrophic character of Mediterranean waters should strongly influence the bioaccumulation and/or biomagnification of Hg observed in medium-to high-trophic level consumers, due to the lower productivity and consequently the lower growth rate and lower "bio-dilution effect" most probably generated at all trophic levels in oligotrophic systems.

Material and Methods

Sampling and sample preparation

The offshore (shelf-edge) benthopelagic species considered in this study included the lesser-spotted dogfish Scyliorhinus canicula (Linnaeus, 1758) and the blackmouth catshark Galeus melastomus (Rafinesque, 1810) as cartilaginous fish; the blackbelly rosefish Helicolenus dactylopterus (Delaroche, 1809), the four-spot megrim Lepidorhombus boscii (Risso, 1810), the greater forkbeard Phycis blennoides (Brünnich, 1768) and the blue whiting Micromesistius poutassou (Risso, 1827) as teleost fish; and finally the Norway lobster Nephrops norvegica (Linnaeus, 1758) as crustacean (Table 1).

Organisms were collected during bottom-trawling groundfish surveys conducted by the French Institute for the Exploration of the Sea (IFREMER), more specifically during the EVHOE 2008 survey for the BoB, and the MEDITS 2012 survey for the GoL (Fig. 1; [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (North-East Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF]Cresson et al., 2014b).

After collection (N=134 fishes in total considered in the present meta-analysis and comparative study), each individual was measured (total length for fish, cephalothorax length for crustaceans), at least to nearest centimetre for fish or millimetre for crustaceans. A piece of white muscle (without skin nor carapace) was taken for both further Hg analyses and C and N stable isotope analysis (SIA), performed individually. All muscle samples were finally frozen at -20° C, freeze-dried and ground into a fine powder until further chemical analyses [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (North-East Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF]Cresson et al., 2014b).

The determination of the age of organisms could not be performed due to non-uniform sampling of otoliths for age lecture, although in any case, this age determination would have been only potentially possible for teleost fish (that is, for four of the seven species considered here), if the otolith sampling had been done uniformly. Thus, in the present study, only the individual sizes were used as an indicator of the time of exposure to contaminant (i.e. as a proxy of organisms' age within a species), and/or related to potential differences in the growth rate of all organisms between systems.

Stable isotope analyses

Muscle sub-samples for SIA (N=134) were prepared as described by [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (North-East Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF] and Cresson et al. (2014b). 0.40 ± 0.05 mg of powder were finally weighed in tin cups. Analyses were performed were performed with a Thermo Scientific Delta V Advantage mass spectrometer coupled to a Thermo Scientific Flash EA1112 elemental analyser. The results are presented in the usual d notation relative to the deviation from international standards (Pee Dee Belemnite for  13 C values, and atmospheric nitrogen for  15 N values), in parts per thousand (‰). Based on replicate measurements of internal laboratory standards (acetanilide), the experimental precision was <0.2‰ for both  13 C and  15 N. Moreover, a previous study using the same instruments as in the present study indicated no statistically significant difference for both  13 C and  15 N on duplicate measurements of a same sample (Chouvelon et al., 2014b), indicating highly reliable and precise values for the unique analysis of wellhomogenized material such as the muscle samples used here.

Total and methyl-Hg analyses

As in the case of SIA, muscle sub-samples for T-Hg determination (N=134) were prepared and analysed as described by [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (North-East Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF] and Cresson et al. (2014b). T-Hg analyses were carried out on aliquots of powder (10-50 mg) with Advanced Mercury Analysers (ALTEC AMA-254, Altec Ltd).

Through this mean, the determination of Hg is done in three steps: (1) combustion of the dedicated subsample (dried powder) and volatilization of Hg;

(2) amalgamation of elemental Hg on a gold trap; and

(3) spectrophotometric atomic absorption measurement of the Hg swept into the flow cell following heating (800°C) of the gold trap. All T-Hg analyses were run according to thorough quality control programmes including the analysis of certified reference materials (CRMs): TORT-2 (lobster hepatopancreas, National Research Council Canada/NRCC), IAEA-142 (mussel homogenate, International Atomic Energy Agency/IAEA), DORM-2 and/or DORM-4 (fish protein, NRCC). CRM results were in good agreement with the certified values in all cases, with recovery rates varying between 95% and 102% [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (North-East Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF]Cresson et al., 2014b). The limit of quantification was ≤0.02 µg g -1 dry mass (dm).

Me-Hg determination was performed on a subsample of individuals (n=75) according to the method described by [START_REF] Azemard | Determination of methylmercury in marine biota samples with advanced mercury analyser: Method validation[END_REF], which uses liquid-liquid extractions of Me-Hg in samples before analysis by automated Hg analysers such as AMA-254. Briefly, aliquots of powder (20-800 mg, depending on T-Hg concentrations) were acidified with 5 mL of HCl (25%, v/v, prepared with HCL 30% Suprapur) to solubilize Hg. A volume of toluene (10 mL) was added and both phases were homogenized. After centrifugation, a fraction (5 mL) of the upper organic phase (i.e. toluene containing extracted Me-Hg) was transferred to a second tube containing the same volume of a 0.002 M sodium thiosulfate solution. This second tube was vigorously shaken and centrifuged. Finally, an aliquot of a known volume (100 or 200 µL) of the lower phase, containing the back-extracted Me-Hg, was directly analysed with AMA 254. As for T-Hg determination, Me-Hg analyses were run according to a thorough quality control programme including the analysis of the CRM IAEA-436 (tuna fish flesh homogenate, IAEA). The average recovery rates for the CRM varied from 81 to 90% between the series of analyses (i.e. between the days of analyses), against 92% on average expected by [START_REF] Azemard | Determination of methylmercury in marine biota samples with advanced mercury analyser: Method validation[END_REF] for this CRM. However, within a same series or day of analyses, the variation in the recovery rates of CRMs (2 to 3 CRMs analysed at each series/day of Me-Hg analyses) was very low (i.e. average standard deviation of 2%). Therefore, the repeatability of the CRM results was ensured within a same series, and Me-Hg results for the samples could be corrected from the average recovery rate of the CRMs analysed during the same series/day of analyses. Finally, a subsample of individuals was analysed twice (in two different series/day of analyses). The mean difference observed between the two measures, for the calculated percentage of Me-Hg relative to T-Hg (i.e. ratio of Me-Hg concentration/T-Hg concentration, in %), was of 4 ± 2%.

Data treatment

All data submitted to statistical tests (and/or model residuals) were checked for normality, and for homogeneity of variances and no violation of independence when appropriate [START_REF] Zuur | Analysing Ecological Data[END_REF].

For each species considered, differences between systems in individual sizes,  13 C and  15 N values, T-Hg concentrations and percentages of Me-Hg were thus tested by the parametric Student t-test or the non-parametric Mann-Whitney-Wilcoxon test, depending on whether the data satisfied the conditions for parametric statistics or not.

The correlation between T-Hg and Me-Hg concentrations (for the subsample of individuals analysed for both parameters) was tested through the non-parametric Spearman correlation coefficient test.

The relationship between T-Hg or Me-Hg concentrations and  15 N values (i.e. as proxy of the individual trophic positions within a given ecosystem) was investigated through Generalized Linear Models (GLMs). Data effectively showed a marked departure from normality, preventing the application of classical multiple linear regressions. Besides, thorough data exploration suggested the linearity of trends to be modelled, making the application of Generalized Additive Modelsthan can capture and model complex non-linear relationshipsunnecessary. GLMs were thus fitted to log-transformed Hg concentrations with an identity link function, as it is generally the case when dealing with contaminant data such as trace metals (e.g., [START_REF] Pierce | Geographic, seasonal and ontogenetic variation in cadmium and mercury concentrations in squid (Cephalopoda: Teuthoidea) from UK waters[END_REF]Mendez-Fernandez et al., 2013;[START_REF] Chouvelon | Species-and sizerelated patterns in stable isotopes and mercury concentrations in fish help refine marine ecosystem indicators and provide evidence for distinct management units for hake in the Northeast Atlantic[END_REF][START_REF] Briant | Spatial and temporal distribution of mercury and methylmercury in bivalves from the French coastline[END_REF]. This allowed estimating the effect of  15 N values in explaining Hg concentration variability. Moreover, such relationships allow determining the biomagnification power (BP) of T-Hg and Me-Hg to be considered and to be compared between ecosystems [START_REF] Chen | Methylmercury in marine ecosytems: Spatial patterns and processes of production, bioaccumulation, and biomagnification[END_REF][START_REF] Borgå | Trophic magnification factors: considerations of ecology, ecosystems, and study design[END_REF][START_REF] Lavoie | Biomagnification of mercury in aquatic food webs: A worldwide meta-analysis[END_REF].  15 N values were thus treated as the single continuous explanatory variable in the models, while the factor "Ecosystem" was treated as a categorical explanatory variable and added as a potential interaction term. The general form of the original models performed was:

Log10 [T-Hg] or Log10 [Me-Hg] ~  15 N + Ecosystem [+ interaction term  15 N:Ecosystem].
Results (fitted values) of the models were plotted on observed (log-transformed) data. Final models' parameters (estimates, p-values, etc.) and details on their interpretation are given in Table 2. For each model, we retained the variables that improved the relative goodness of fit in the GLMs (i.e. most parsimonious models) based on the Akaike Information Criterion (AIC). When the AIC was not significantly different between the last two nested models, the simplest model was preferred. Finally, a model validation was systematically applied [START_REF] Zuur | Analysing Ecological Data[END_REF], and the percentage of total deviance The level of significance for statistical analyses was always set at  =0.05.

Results

In both systems, species-dependent patterns in Hg concentrations (either T-Hg or Me-Hg) were similar, with the two shark species S. canicula and G. melastomus and the teleost fish H. dactylopterus presenting the highest Hg concentrations, the two teleost fish P. blennoides and L. boscii presenting intermediate concentrations, and the teleost fish M. poutassou and the crustacean N. norvegica presenting the lowest Hg concentrations (Table 1).

Mediterranean individuals had significantly lower sizes than those from the NE Atlantic (BoB) for five out of the seven species considered (Table 1, Fig. 2a), along with higher T-Hg concentrations for all species except L. boscii (Table 1, Fig. 2b). However, although non-significant, higher average values were found in Mediterranean individuals of this species as well (Table 1). Concomitantly, for all species considered, significantly lower δ 13 C and δ 15 N values were measured in Mediterranean organisms relative to Atlantic ones (Table 1, Fig. 2c).

Me-Hg concentrations were strongly correlated with T-Hg concentrations (Fig. 3), and the average percentage of Me-Hg (relative to T-Hg) was over 85% for all the considered species and in both systems (Table 1, Fig. 2d). This percentage of Me-Hg was significantly higher in Atlantic individuals relative to Mediterranean ones for only three out of the seven species considered (i.e. for H. dactylopterus, P.

blennoides and M. poutassou), although slightly, and the difference was not significant for the four other species (Fig. 2d). Moreover, the standard deviations around average Me-Hg/T-Hg ratios were quite high (Table 1), and the recovery of values between individuals from the different systems was quite important (Fig. 2d).

The effect of δ 15 N values for explaining T-Hg or Me-Hg concentrations' variability was significant in all cases (i.e. significant relationships between parameters), as well as the factor "Ecosystem" (i.e.

significant difference between Atlantic and Mediterranean). The intercept was systematically higher for Mediterranean organisms than for Atlantic ones in all relationships (Table 2, Fig. 4). For Me-Hg, contrary to T-Hg, the interaction term was not significant. As such, the model predicted similar slope/similar BP for Me-Hg in both ecosystems, at least at the scale of the species and food webs considered in the present study. Finally, when considering each system separately, the difference of slope/of BP between T-Hg and Me-Hgwas not significant (i.e. no effect of the factor "Hg form" on the relationships with δ 15 N values within each system), probably due to the high correlation between T-Hg and Me-Hg concentrations (Fig. 3).

Discussion

General trends and differences between systems

Results clearly showed a significantly higher Hg bioaccumulation by Mediterranean organisms, along with significantly lower C and N stable isotope ratios measured for all species, and lower individual sizes for most of them. Conversely, the percentage of MeHg was similar between both systems for most of the species examined, although significantly slightly higher in Atlantic organisms than in Mediterranean ones for some species. When considering Me-Hg, the BP was not significantly different between systems, but was significantly higher in the Atlantic food web than in the Mediterranean one when considering T-Hg. Besides, within each system, T-Hg and Me-Hg BPs appeared to not differ significantly.

Firstly, in terms of T-Hg concentrations, the species-dependent pattern observed was similar between systems. Moreover, it was consistent with the general diet and ecology documented for the considered species (www.sealifebase.org and associated references) and with the well-documented biomagnifying property of Hg in food webs [START_REF] Boening | Ecological effects, transport, and fate of mercury: A general review[END_REF]. Indeed, the two shark species S. canicula and G.

melastomus and the teleost fish H. dactylopterus (i.e. species presenting the highest Hg concentrations) are considered high-trophic level consumers mainly feeding on a wide variety of prey, including both benthic and pelagic fish, cephalopods and crustaceans. Compared with the others, these three species are also morphologically able to capture relatively large prey (i.e. of potentially high trophic level as well). Alternatively, P. blennoides is documented to feed mostly on crustacean and fish prey, and L.

boscii on small bottom-living crustaceans, fish and squids. The diet of the individuals of M. poutassou corresponding to the sizes sampled here is mainly composed of small crustaceans such as zooplankton.

Finally, the crustacean N. norvegica is considered mainly scavenger, feeding on detritus, benthic crustaceans and worms (www.sealifebase.org and associated references).

Secondly, the different parameters considered in this ecosystem-comparative study (i.e. sizes of the organisms compared, δ 13 C and δ 15 N values, T-Hg and Me-Hg concentrations) showed the crucial role of the trophic status at the base of systems -especially oligotrophy -in the trophic transfer and in the bioaccumulation of Hg by high-trophic level consumers. Similarly to our study, higher concentrations in Mediterranean organisms than in their counterparts in the Atlantic were recently described for the neritic food web of the European hake Merluccicus merluccius between the GoL and the BoB [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF], being coherent with the "Mediterranean Hg anomaly" (i.e. higher Hg bioaccumulation observed in organisms from this system relative to other systems, despite comparable concentrations in seawaters from both areas; [START_REF] Aston | Mercury in the open Mediterranean: evidence of contamination?[END_REF][START_REF] Cossa | The Mediterranean mercury anomaly, a geochemical or a biological issue[END_REF]. However, to the best of our knowledge, such comparison of systems as done in the present study was relatively rarely performed, especially on several medium-to high-trophic level species at a time, and on offshore species, which are potentially submitted to oligotrophic conditions e at least in part e in both systems (compared to neritic species).

The C and N stable isotope compositions (especially δ 15 N values) of the species examined, although characteristic offshore species of the shelf-edge in both systems, revealed the significant differences between the two systems in terms of trophic status and consequently in terms of sources of organic matter and primary producers sustaining the respective food webs. Unfortunately, no dedicated sampling of plankton could be carried out for the present study to get the δ 15 N values (and Hg concentrations) of the planktonic compartment, which could be directly related to those of the medium to high-trophic level consumers here of concern. However, the δ 15 N values of the consumers considered here clearly showed the more oligotrophic character of the Mediterranean waters in general. Indeed, the fixation of atmospheric N2 by diazotrophic organisms (e.g., cyanobacteria) in offshore and oligotrophic waters is well documented to lower the δ 15 N values of the residual NO3 -pool available for primary producers in these areas (see review by Montoya (2007). Diazotrophic organisms were demonstrated to be preponderant in the GoL (Kerhervé et al., 2001;[START_REF] Moal | Diazotrophic unicellular cyanobacteria in the northwestern Mediterranean Sea: a seasonal cycle[END_REF]Liénart et al., 2017), thus resulting in low δ 15 N values in consumers at all trophic levels (especially when compared to their counterparts in the BoB waters, for instance), from mesozooplankton [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (North-East Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF](Chouvelon et al., , 2014b;;[START_REF] Espinasse | Patterns of variations in C and N stable isotope ratios in size-fractionated zooplankton in the Gulf of Lion, NW Mediterranean Sea[END_REF]Banaru et al., 2014) and suspension-feeders like bivalves [START_REF] Cresson | Feeding strategies of co-occurring suspension feeders in an oligotrophic environment[END_REF]Briant et al., 2018) to deep-sea fish and crustaceans (present study; Cresson et al., 2014b). Finally, the results of the present study highlighted the higher primary productivity of the BoB offshore waters (i.e.

higher δ 15 N values measured in individuals from this ecosystem) compared to offshore GoL waters. In the BoB, in the Cap-Ferret canyon's head area (i.e. offshore area) for instance, the annual primary productivity was effectively estimated to be 145-170 gC/m² [START_REF] Laborde | Seasonal variability of primary production in the Cap-Ferret Canyon area (Bay of Biscay) during the ECOFER cruises[END_REF], while it was estimated to be 78-142 gC/m² in the GoL [START_REF] Lefevre | Review of gross community production, primary production, net community production and dark community respiration in the Gulf of Lions[END_REF].

The significant difference of sizes observed for most of the species considered here, despite comparable sampling methodology and effort, were also indicative of a difference in individual size distribution between the two systems with contrasted trophic status. This difference is likely due to the lower growth rate of Mediterranean organisms compared their Atlantic counterparts, like observed for the hake M.

merluccius, for instance [START_REF] De Pontual | Evidence of underestimation of European hake growth in the Bay of Biscay, and its relationship with bias in the agreed method of age estimation[END_REF][START_REF] Mellon-Duval | Growth of European hake (Merluccius merluccius) in the Gulf of Lions based on conventional tagging[END_REF]. As such, for similar-sized individuals, organisms from the Mediterranean are surely much older than their Atlantic counterparts.

In the previous study by [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF], this partly explained the higher concentrations of Hg observed in Mediterranean hakes compared to Atlantic ones due to age-related bioaccumulation of the metal, in addition to the fact that Hg burden is also more bio-diluted in larger organisms.

4.2. Me-Hg/T-Hg ratios, bioaccumulation and biomagnification of Hg

The relatively high percentage of Me-Hg measured (i.e. >85% on average for all species and in both ecosystems) was expected, since muscle was the tissue examined, and since the trophic position of the species considered was quite elevated. Indeed, this percentage has long been considered virtually equal to 100% in the muscle of high-trophic level consumers such as fish and cephalopod molluscs (e.g., [START_REF] Bloom | On the chemical form of mercury in edible fish and marine invertebrate tissue[END_REF], due to the high affinity of Hg for muscular proteins' sulfhydryl groups, although it generally varies between 60 and 90% depending on species and on their trophic level (e.g., [START_REF] Bustamante | Total and organic Hg concentrations in cephalopods from the North Eastern Atlantic waters: influence of geographical origin and feeding ecology[END_REF][START_REF] Kehrig | Inorganic and methylmercury: Do they transfer along a tropical coastal food web?[END_REF][START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF]. It may be also elevated in some crustacean species, but it is clearly lower (generally largely <50%) in low-trophic level consumers such as bivalve molluscs (e.g., [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF][START_REF] Briant | Spatial and temporal distribution of mercury and methylmercury in bivalves from the French coastline[END_REF]. Opposite to the other parameters regarded (i.e. individual sizes, δ 13 C and δ 15 N values, raw T-Hg and Me-Hg concentrations), the percentage of Me-Hg in the muscle of the selected organisms generally did not differ a lot between the BoB and the GoL organisms.

These results are in agreement with those of [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF] that observed no significant differences in the percentage of Me-Hg in Mediterranean vs. Atlantic hakes. The fact that only medium-to hightrophic levels were considered here may partly explain this absence of difference in the Me-Hg/T-Hg ratios between organisms from the two systems. Indeed, when considering organisms at the top of food webs, potential differences that could exist in the Me-Hg/T-Hg proportions at the basal levels (in seawater, and/or in low-trophic level organisms) may no longer be visible.

In fact, water Me-Hg concentrations were shown to be comparable between the BoB and the GoL shelfedge ecosystems [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF]. However, the BoB has probably a deeper thermohalocline than the GoL and consequently a deeper organic matter regeneration zone, where Hg is likely methylated [START_REF] Heimbürger | Methyl mercury distributions in relation to the presence of nano-and picophytoplankton in an oceanic water column (Ligurian Sea, North-western Mediterranean)[END_REF][START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF][START_REF] Cossa | The open sea as the main source of methylmercury in the water column of the Gulf of Lions (Northwestern Mediterranean margin)[END_REF]. As the species considered here are endemic of the shelf-edge and heads of canyons, they may be therefore more exposed to Me-Hg in Mediterranean waters than in Atlantic ones (through the higher content of Me-Hg in the lower trophic levels themselves). This hypothesis is also supported by the lower percentage of Me-Hg measured for M. poutassou in the GoL in particular, relative to the other fish species examined. Indeed, the shallower foraging zone of this species was previously proposed as an explanation of its lower T-Hg burdens (Cresson et al., 2014b), but this may also explain the lower proportion of Me-Hg in the muscle of this species, in addition to its lower trophic level in comparison with other species.

The significant relationships between log-transformed Hg concentrations and δ 15 N values were in agreement with the species dependent patterns previously observed (i.e. increasing concentrations with increasing trophic levels), and were thus also consistent with the well-documented property of Hg to biomagnify in food webs [START_REF] Boening | Ecological effects, transport, and fate of mercury: A general review[END_REF]. Furthermore, the equations of these relationships allow to compare the BP of this metal across food webs, using the slope of the relationships as a measure of the biomagnification rate, and the intercept as the baseline value for primary producers in the different systems [START_REF] Chen | Methylmercury in marine ecosytems: Spatial patterns and processes of production, bioaccumulation, and biomagnification[END_REF][START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF][START_REF] Lavoie | Biomagnification of mercury in aquatic food webs: A worldwide meta-analysis[END_REF]. Then, the biomagnification power (BP) of Hg may be expected to differ between systems when the δ 15 N values of plankton (i.e. low trophic levels) differ between systems, and/or when Hg concentrations in plankton differ between systems.

Besides, variations in the δ N values or Hg concentrations of plankton are linked, at least part, to the composition of plankton (some planktonic species being able to fix atmospheric nitrogen with peculiar N signature, for instance; Kerhervé et al., 2001). As a consequence, the BP may be expected to differ between systems with different trophic status, i.e. between systems with different planktonic communities at the base of food webs. Here, higher intercepts were found for the Mediterranean ecosystem, indicating a probably higher Hg burden from the lowest trophic levels in the GoL compared to BoB (i.e. lower "bio-dilution" effect in the GoL's low trophic levels than in those from the BoB).

However, the intercept is intimately linked to the slope, and such direct link with Hg baseline values should be done with caution when the lowest trophic levels (plankton) are not available [START_REF] Borgå | Trophic magnification factors: considerations of ecology, ecosystems, and study design[END_REF], as it is the case for the present study. Biomagnification slopes (i.e. BPs), were previously reported to range between 0.09 and 0.22 for T-Hg (average 0.20 ± 0.10) and between 0.14 and 0.26 for Me-Hg (average 0.22 ± 0.09) in marine ecosystems [START_REF] Lavoie | Biomagnification of mercury in aquatic food webs: A worldwide meta-analysis[END_REF]. When considering Me-Hg concentrations, the BPs measured here (0.21 for both BoB and GoL ecosystems, not significantly different) are therefore within this range of values. Alternatively, when calculated from T-Hg concentrations, the BPs significantly differed between the BoB (0.35) and the GoL (0.11), with a lower BP estimated for the Mediterranean system. This differs from the results of [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF], for instance, which found a higher BP in the hake food web from the GoL (Mediterranean) than in those from the BoB (Atlantic). The consideration of different food webs and organisms may be at the origin of the discrepancy between our study and those of [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF] (i.e. consideration of the neritic hake food web by [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF], including lower trophic levels, vs. shelf-edge/offshore organisms and medium-to high trophic levels only here). Moreover, the lower BP found here for the Mediterranean ecosystem may favour the hypothesis of potentially higher basal Hg levels (i.e. in plankton) than in Atlantic for the shelf-edge food webs considered here (as suggested by the different intercept as well),

although the biomagnification potential of Hg may be then not especially higher in the rest of the food web (as suggested by similar BPs when considering the bioavailable and bioaccumulable form Me-Hg).

Finally, if our estimated BPs probably remain comparable at the scale of our study between GoL and BoB, with the same species considered in both systems, they are not necessarily comparable to other studies that would include other (non-fish) species and especially lower trophic levels (e.g., [START_REF] Signa | Diet and habitat use influence Hg and Cd transfer to fish and consequent biomagnification in a highly contaminated area: Augusta Bay (Mediterranean Sea)[END_REF].

Overall, the present study thus showed that the higher Hg bioaccumulation rate by Mediterranean organisms is likely linked to the oligotrophic character of waters and associated lower productivity in this system. Indeed, compared to mesotrophic environments, oligotrophic conditions are likely associated with: i) increased formation of the bioavailable Me-Hg at the depths of organic matter regeneration (i.e. where the shelf-edge/offshore species considered here likely live), due to greater proportions of slowly sinking pico-and nanophytoplanctonic cells that are readily degraded by bacteria there [START_REF] Cossa | The origin of methylmercury in open Mediterranean waters[END_REF][START_REF] Heimbürger | Methyl mercury distributions in relation to the presence of nano-and picophytoplankton in an oceanic water column (Ligurian Sea, North-western Mediterranean)[END_REF]; ii) lower bio-dilution of Hg from the lower trophic levels (i.e. phytoplankton; [START_REF] Pickhardt | Algal blooms reduce the uptake of toxic methylmercury in freshwater food webs[END_REF], with the presence of smaller and less abundant cells that are then also much more consumed -including their high Hg burden -by higher trophic level organisms; iii) lower growth rate of consumer organisms and consequently, lower bio-dilution of Hg burden in their tissues as well and high Hg retention with the age of organisms, the elimination rate of the bioaccumulated Hg being very low [START_REF] Wang | Bioaccumulation kinetics and exposure pathways of inorganic mercury and methylmercury in a marine fish, the sweetlips Plectorhinchus gibbosus[END_REF][START_REF] Maulvault | Bioaccumulation and elimination of mercury in juvenile seabass (Dicentrarchus labrax) in a warmer environment[END_REF].

Finally, the potential impact of temperature differences between GoL and BoB on Hg bioaccumulation is also interesting to address in the context of global change. Higher temperatures are likely to enhance bacterial activity and consequently Me-Hg formation, which in our study case may also partly explain the higher Hg bioaccumulation observed in Mediterranean organisms. Indeed in the GoL, the sea surface temperatures were shown to vary between 14 and 20 °C and to remain relatively constant around 13 °C below 100-200 m depth [START_REF] Conan | Variabilité hydrologique et biologique du golfe du Lion. II. Productivité sur le bord interne du courant[END_REF], while in the BoB, the sea surface temperatures are slightly colder. They vary between 11 and 20 °C (for the southern part, less in the northern one) and remain below 12 °C in depth [START_REF] Koutsikopoulos | Physical processes and hydrological structures related to the Bay of Biscay anchovy[END_REF]. In addition, higher temperatures such as it is likely the case for Mediterranean waters were proved to promote Me-Hg bioaccumulation and to hamper its elimination in fish tissues, for instance [START_REF] Maulvault | Bioaccumulation and elimination of mercury in juvenile seabass (Dicentrarchus labrax) in a warmer environment[END_REF], which is also in favour of higher potential for Hg bioaccumulation in Mediterranean organisms.

Conclusion and perspectives

The effect of ecosystems' characteristics on Hg bioaccumulation and biomagnification was already demonstrated in freshwater ecosystems (e.g., lakes) through comparative studies (e.g., [START_REF] Kidd | Biomagnification of mercury through lake trout (Salvelinus namaycush) food webs of lakes with different physical, chemical and biological characteristics[END_REF].

Such ecosystem-comparative or meta-analysis studies remained rare for marine ecosystems. They were therefore recently encouraged to be done for generating new testable hypotheses concerning Hg bioaccumulation and biomagnification [START_REF] Lavoie | Biomagnification of mercury in aquatic food webs: A worldwide meta-analysis[END_REF].

Benefiting from archived samples from the GoL and the BoB shelf-edge/offshore ecosystems, our study showed the strong influence of the trophic status and/or functioning of systems on Hg bioaccumulation in marine medium-to high-trophic level organisms, which are generally important commercial species.

It also reinforced and confirmed the hypothesis made by [START_REF] Cossa | The Mediterranean mercury anomaly, a geochemical or a biological issue[END_REF] that biological processes dominate the geochemical ones in explaining the "Mediterranean mercury anomaly".

Furthering some previous studies focusing on the coastal or neritic species that are red mullets or hakes [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF]Cresson et al., 2014a[START_REF] Cresson | A multitracer approach to assess the spatial contamination pattern of hake (Merluccius merluccius) in the French Mediterranean[END_REF], the present marine ecosystem-comparative study demonstrated the crucial role of oligotrophy and associated lower productivity on Hg bioaccumulation, at both multi-species and multi-systems scales.

As the processes of bio-dilution and low elimination rates may also apply to other metals, we propose that the bioaccumulation of other metals may be also determined by the trophic status of the considered ecosystems in a non-negligible part. Thus, it would be interesting to examine other trace metals on the samples used in the present study, to investigate whether oligotrophy may also influence their bioavailability, transfer and bioaccumulation, and whether the patterns observed for Hg are also applicable or not for other trace elements (e.g., species-dependant patterns kept or not between the systems). Also, investigating the potential differences in the energy content of prey species/lower trophic levels between systems (i.e. expected to be affected by oligotrophy in Mediterranean waters, for instance) may be interesting. This would enable to more globally apprehend the mechanisms involved in the transfer of contaminants (especially the lipophilic ones) to higher trophic levels, and their transfer in food webs in general. Finally, with regard to the high significance of the trophic status of systems on Hg bioaccumulation shown in the present study, further work on very low trophic levels (i.e. plankton) would be also highly relevant. (lines) are plotted on observed log-transformed data. When the interaction term was not significant (i.e. for MeHg, the model assumes that the form of the relationship is not different between the ecosystems), lines are parallel (i.e. same slope, although different intercept). This can also be interpreted (i.e. slope of the lines) as the biomagnification power of T-Hg or Me-Hg within each ecosystem considered (as performed by [START_REF] Cossa | Influences of bioavailability, trophic position, and growth on methylmercury in hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic[END_REF] for Me-Hg). Finally, the equations of lines (derived from model outputs; Table 2) are indicated.

  Oligotrophy as a major driver of mercury bioaccumulation in marine medium-to high-trophic level consumers: A marine ecosystem-comparative study Tiphaine Chouvelon, Pierre Cresson, Marc Bouchoucha, Christophe Brach-Papa, Paco Bustamante, Sylvette Crochet, Marie-Claire Fabri, Francoise Marco-Miralles, Bastien Thomas, Joel Knoery To cite this version: Tiphaine Chouvelon, Pierre Cresson, Marc Bouchoucha, Christophe Brach-Papa, Paco Bustamante, et al.. Oligotrophy as a major driver of mercury bioaccumulation in marine medium-to high-trophic level consumers: A marine ecosystem-comparative study. Environmental Pollution, 2018, 233, pp.844-854. 10.1016/j.envpol.2017.11.015. hal-02342820

  explained (DE) by each final model was calculated as follows: Explained deviance = ((Null model deviance -final model residual deviance)/Null model deviance)*100, with the null model that only contained the intercept terms(Mendez-Fernandez et al., 2013;[START_REF] Briant | Spatial and temporal distribution of mercury and methylmercury in bivalves from the French coastline[END_REF].The same type of models (GLMs) was applied to test potential significant difference in the slope of the relationships/in the BP between T-Hg and Me-Hg (i.e. between the different forms of Hg) within each system.  15 N values were then treated as the single continuous explanatory variable in the models, while the factor "Hg form" was treated as a categorical explanatory variable and added as a potential interaction term. The general form of the original models performed was then: Log10 [Hg] in the BoB or in the GoL ~  15 N + Hg form [+ interaction term  15 N:Hg form].

Figure 1 .

 1 Figure 1. Maps of the sampling areas and of trawling stations in the NW Mediterranean (MED -Gulf of Lion) and in the NE Atlantic (ATL -Bay of Biscay).

Figure 2 .

 2 Figure 2. a) Boxplots of sizes (total length for fish, cephalothorax length for crustaceans) for the seven species analysed in the NW Mediterranean e Gulf of Lion (MED -GoL) and in the NE Atlantic -Bay of Biscay (ATL -BoB) (N = 134); b) Boxplots of muscle total Hg (T-Hg) concentrations (N = 134); c) Biplot of δ 13 C and δ 15 N values (N = 134); d) Boxplots of percentages of Me-Hg (i.e. ratios Me-Hg/T-Hg, in %) in the muscle of the subsample of individuals analysed for Me-Hg concentrations (n = 75).For boxplots (a, b, d), the box length represents the interquartile, the bar length represents the range, and the horizontal lines in bold are median values. For each species, the significant difference between systems is indicated (tested by Student t-test or Mann-Whitney-Wilcoxon test, depending on data satisfying conditions for parametric statistics or not). *p < 0.05; **p < 0.01; ***p < 0.001; NS = non significant. For the biplot (c), values are mean ± standard deviation per species. Abbreviations for species are specified in Table1.

Figure 3 .

 3 Figure 3. Relationship between muscle methyl-Hg (Me-Hg) and total Hg (T-Hg) concentrations for the subsample of individuals analysed for Me-Hg concentrations (n = 75), and for the seven species analysed in the NW Mediterranean e Gulf of Lion (MED -GoL) and in the NE Atlantic e Bay of Biscay (ATL -BoB). The correspondence line 1:1 is indicated, as well as the results of the non-parametric Spearman correlation coefficient test. Symbols used for species are the same than in Fig. 2c.

Figure 4 .

 4 Figure 4. Relationships between individual log-transformed total Hg (T-Hg) concentrations (left panel, N = 134) or methyl-Hg (Me-Hg) concentrations (right panel, n = 75), and δ 15 N values (as a proxy of the trophic level within a given systems), for the seven species analysed in the NW Mediterranean -Gulf of Lion (MED -GoL) and in the NE Atlantic -Bay of Biscay (ATL -BoB). The symbology used for each species is the same as in Fig. 2c. Results from the GLMs

Acknowledgments

Authors are particularly grateful to the "Agence de l'Eau Rhône Méditerranée Corse" (AERMC) for financial support of the Mediterranean analyses and of the associated RETROMED project, as well as to the "Conseil Régional de Poitou-Charentes" and the "Contrat de projet Etat-Région" (CPER 2013) that supported the analyses relative to the Bay of Biscay area during the PhD of T. Chouvelon. We are grateful to C. Churlaud from the "Plateforme Analyses Elémentaires" of LIENSs for her assistance during Hg analyses and to G. Guillou, P. Richard and B. Lebreton from the "Plateforme Analyses Isotopiques" of the LIENSs laboratory for their assistance during SIA analyses. Thanks are also due to M.C. Fabri for its implication in the RETROMED project, and to F. Chavanon and C. Ravel for their help during sampling and laboratory work. Finally, authors want to thank the chief scientists and the crew of the R/V "Thalassa" and "L'Europe" for facilitating the sampling during EVHOE 2008 (https://doi.org/10.17600/8040120) and MEDITS 2012 (https://doi.org/10.17600/12060100) surveys.


O Spitz

) J Wu

) M Carras

C W Wong

F Grillot

Low-frequency fluctuations of a mid-infrared quantum cascade laser operating at cryogenic temperatures

Keywords: Quantum cascade laser, mid-infrared, optical feedback, non-linear dynamics

This work demonstrates that mid-infrared quantum cascade lasers operating under external optical feedback can output a chaotic dynamics through low-frequency fluctuations close to 77 K. Results also show that the birth of chaotic dynamics is not limited to near-threshold pumping levels. In addition, when the semiconductor material is cooled down from room temperature to 77 K, it is found that the laser destabilization takes place at a lower feedback ratio which proves that quantum cascade lasers are sensitive to temperatures, likely due to changes in the upper state lifetime. These examinations are meaningful for chaotic operation of quantum cascade lasers in secure atmospheric transmission lines and optical countermeasure systems.

I. INTRODUCTION

Quantum cascade lasers (QCLs), theoretically demonstrated for the first time in the early 1970s 1 and then experimentally produced since 1994, 2 are unipolar semiconductor laser sources based on intersubband transitions and have proved to be powerful, tunable and versatile mid-infrared light sources operating at room temperature. 3 The wide range of achievable wavelengths from mid-infrared to terahertz domain paves the way for multiple applications 4 such as optical countermeasures for defense purposes, particle detection below one per million, jamming-resistant free-space communications and LIDAR remote sensing, 5 all demanding stable single-mode operation with a narrow linewidth, high output power and high modulation bandwidth. External optical feedback consisting of re-injecting part of the light of a semiconductor laser in order to modify its emission properties has been studied in a wide range of purposes. [START_REF] Kane | Unlocking dynamical diversity: optical feedback effects on semiconductor lasers[END_REF] It has strong influence on the QCL dynamics and several feedback regimes have been analogously identified to interband diode lasers, [START_REF] Jumpertz | [END_REF] including noise reduction 8 or mode selection in widely tunable sources. 9 However, contrary to interband lasers in which the carrier-to-photon lifetime is around 10 3 , QCLs exhibit a sub-picosecond intersubband carrier dynamics hence leading to a very small carrier-to-photon lifetime ratio around 0.1. 10 Recent work showed that mid-infrared QCLs under external a) Electronic mail: olivier.spitz@telecom-paristech.fr b) O. Spitz and J. Wu contributed equally to this work.

optical feedback can experience a route to chaos when pumped close to threshold. 11 The latter was first observed through a Hopf bifurcation to periodic dynamics at the external cavity frequency and then through low frequency fluctuations (LFF) which is a signature of deterministic chaos. The LFF can be described as a competition between modes which are located on a feedback ellipse. The trajectory wonders around an external cavity mode for a few revolutions and then hops to the next mode with a higher intensity, repeating it until it reaches the highest order mode where the collision with its antimode produces the drop off [START_REF] Kane | Unlocking dynamical diversity: optical feedback effects on semiconductor lasers[END_REF] . In addition, experiments proved that the route to chaos was not associated to the undamping of the relaxation oscillations, which strongly differs from what is commonly observed in interband semiconductor lasers. [START_REF] Kane | Unlocking dynamical diversity: optical feedback effects on semiconductor lasers[END_REF] This work goes a step further by investigating the impact of the temperature on the nonlinear dynamics properties of a mid-infrared QCL operating under external optical feedback. In particular, it is shown that the LFF regime remains highly sustained in liquid nitrogen (77 K) and that its occurrence is not limited to near-threshold operation. Overall, when the QCL is cooled down from room temperature (290 K) to 77 K, experiments reveal that the laser destabilization appears at a lower feedback ratio owing to a 100% increase of the upper state lifetime between 290 K and 77 K which is also confirmed by a numerical analysis. These novel insights are meaningful for understanding and controlling the intersubband dynamics as well as for developing secure atmospheric transmission lines and optical countermeasure systems.

II. METHODS

The QCL from mirSense under study is 2 mm long and 14 µm wide, with emission at 5.6 µm wavelength. The device consists of a distributed feedback (DFB) laser working in single mode operation, achieved via a metal grating cladding at the top of the laser ridge. One of the facets of the laser is highly reflective and the other one is cleaved to have a 70% transmission coefficient, allowing the light to be emitted from the QCL but also the back-reflected wave to couple inside the laser cavity. The laser ridge is made of two InP cladding layers surrounding the 1.5 µm active region, which consists of 30 periods of strained AlInAs/GaInAs grown by molecular beam epitaxy, inspired by a design from Ref. 12. The laser die is epi-side down mounted using AuSn to an AlN submount to ensure good thermal dissipation as shown on Fig. 1. Indeed, the QCL is pumped with a quasicontinuous wave and this induces a strong heating of the whole structure. This QCL exhibits a threshold current I th of 590 mA when pumped at 290 K with a 300 ns pulse at a repetition rate of 100 kHz (i.e. a 3% duty cycle). At 77 K, the threshold current is 331 mA and the current leading to the maximum emitted power is 950 mA. The QCL emits single-mode at 1775 cm -1 (corresponding to an optical wavelength of 5.63 µm), as shown on Fig. 3.

Fig. 2 shows that the external optical feedback set up is composed of two parts. On the one hand, there is a back-reflection path with a polarizer designed for mid-infrared light and a gold plated mirror placed on a accurately moving cart. This mirror defines the external cavity length which is one of the main parameters of external optical feedback. The polarizer is the key optical device for varying the amount of optical feedback knowing that the QCL wave is indeed TM polarized. This defines the feedback ratio f , standing for the ratio between the back-reflected power that couples inside the laser cavity and the total power emitted by the laser. In what follows, the optical feedback f is controlled by rotating the polarizer. In the detection path, we use a high bandwidth mid-infrared detector (Vigo PEM Mercury-Cadmium-Telluride; MCT) operating at room temperature. The signal retrieved from the MCT detector is amplified with a low noise amplifier (Mini-Circuits ZFLN-1000) with 1000 MHz bandwidth, in order to overcome the background noise. This is subsequently analyzed with a real time 500 MHz oscilloscope. For an external cavity length between 30 and 60 cm, the related frequency at which the destabilization takes place in the laser, is between 500 and 250 MHz, within the oscilloscope bandwidth. Indeed, no relaxation oscillations appear in a QCL, contrary to what is usually found in diode lasers. A 60/40 mid-infrared beam splitter then splits the focused laser beam into both paths. Focusing is achieved with a lens in front of the laser. Two different setups are implemented, based on the measurement temperature. When the laser is studied at room temperature, the QCL package is horizontally clamped over an indium foil and a copper mount with a Peltier module for temperature control. In that configuration, the wave hitting the beam splitter is P-polarized and the transmission of the beam splitter at this wavelength is about 60%. Low temperature measurements down to 77 K are implemented in a cryostat. The QCL is vertically clamped over a copper mount, the latter being placed inside a vacuum chamber in order to insulate from the outside. A heater and a temperature controller inside the vacuum chamber allow a better control of this key parameter. The cryostat has an output covered with a ZnSe window made for mid-infrared light and a focusing lens is placed between this window and the beam splitter. The wave hitting the beam splitter is S-polarized and the maximum achievable transmission is 35% because the QCL is vertically inserted inside the vacuum chamber. In order to minimize the environmental perturbations such as acoustic and mechanical noises, the laser is mounted on a suspended optic table. Consequently, the feedback mirror remains immobile and the observed pattern cannot be related to self-mixing effects. Furthermore, the applied feedback ratios are well above the ones required for self-mixing interferometry. 13

III. RESULTS

Initial experiments showed that the LFF chaotic regime was observed when driving the QCL very close to threshold at I = 1.005 ×I th 11 whereas in our case, the laser operates at high bias current, i.e I = 2.2 ×I th at 290 K and at 77 K. In what follows, the laser is biased with a quasi-continuous source which emits a 2 µs pulse and the external cavity length is set to 35 cm which leads to an external cavity frequency of 430 MHz. Fig. 4 displays the temporal waveforms recorded at 290 K (a) with f = 0.246 and at 77 K (b) with f = 0.089. These measurements show that the LFF dynamics is still highly sustained at 77 K and, in addition, the operating feedback can be much smaller than that used at room temperature. In both cases, the chaotic pattern is composed of slow oscillations modulated by faster ones. The fast oscillations occur at a frequency close to the external cavity roundtrip frequency, which is 320 MHz as illustrated in the signal FFT (inset of Fig. 4). The 320 MHz peak is in the same order-of-magnitude as our expectations, though slightly lower than our theoretical estimates. The slight deviation is attributed to the transient regime that still appears in the pulse due to the internal heating of the structure. The slow oscillation is about 20 MHz and corresponds to the contribution of the low-frequency chaotic dynamics as already reported elsewhere. [START_REF] Kane | Unlocking dynamical diversity: optical feedback effects on semiconductor lasers[END_REF] However, as compared to the LFF pattern reported in Ref. 11, our measurements illustrate another LFF regime taking place at higher injected currents, in a similar way to interband diode lasers 14 . As the QCL is pumped well above the threshold and because the bias current is fixed during the whole experiment, the retrieved LFF pattern is purely delay-induced and does not link to or is not supported by a possible noise current source dependence. Fig. 5 exhibits the whole evolution of the temporal waveforms of the QCL for different amount of external optical feedback f at 77 K. For each trace, the transient regime lasts between 0.8 and 1.2 µs. It can be clearly seen for Fig. 5 f) and Fig. 5 g) until approximately 1.2 µs. Under free-running operation (f = 0), the QCL operates in a steady-state hence the time trace is unchanged. Let us note that the latter is not completely flat because the amplifier used for the experiments had a low-frequency cutoff below 100 kHz and therefore, part of the square signal harmonics are lost. When the feedback is slightly increased to f = 0.0003, oscillations start to appear and hence the QCL is likely operating in a limit cycle with oscillation frequency slightly smaller than the external cavity as aforementioned. At f = 0.0003 and f = 0.0008, the traces do not exhibit LFF pattern. The later is observed for feedback ratios above 0.0046. At f = 0.0298, f = 0.0393 and f = 0.089, an increase of the back-reflected light gives rise to a pattern composed of pure LFF with an intensity modulation, as described on Fig. 5 g). At the maximum reachable feedback, the time trace at 77 K also exhibits another stage with lower frequencies as shown on Fig. 5 i). This step was never reached at 290 K even when the feedback ratio was higher than in our cryogenic measurements here. The coexistence of LFF with more complex dynamics is a good precursor for generating chaos with a higher dimensionality. To confirm the chaotic behavior, we calculated the Lyapunov exponents (LEs) from the time traces, which describe the divergence rate of nearby attractor trajectories and are a widely used criterion in defining chaos. [START_REF] Sprott | Chaos and Time-Series Analysis[END_REF][START_REF] Ott | Chaos in dynamical systems[END_REF] Positive LEs indicate the ultra-sensitivity to initial conditions of chaotic state. [START_REF] Sprott | Chaos and Time-Series Analysis[END_REF][START_REF] Ott | Chaos in dynamical systems[END_REF][START_REF] Grassberger | [END_REF][START_REF] Schuster | Deterministic chaos: an introduction[END_REF][START_REF] Grassberger | [END_REF][20] The phase portraits (Fig. 6 calculated LLEs converge to values λ 1 = 0.485 µs -1 , λ 2 = 0.284 µs -1 , λ 3 = -0.021 µs -1 and λ 4 = -0.656 µs -1 at 290 K condition. The two maximum LEs with are positive, illustrating a fast divergence rate between adjacent orbits and indicating that the system is in a chaotic state. The third LE has a value of nearly zero, which is related to the periodic part in the chaotic temporal evolution of QCL. The LEs of the QCL operating at cryogenic temperatures (77 K) are also calculated. The LEs curves converge to the values λ 1 = 0.444 µs -1 , λ 2 = 0.238 µs -1 , λ 3 = -0.002 µs -1 and λ 4 = -0.582 µs -1 , respectively. The positive LEs values clearly illustrate that the QCL system is under a chaotic state and even an upper chaos state, since there are two positive LEs. [START_REF] Sprott | Chaos and Time-Series Analysis[END_REF][START_REF] Ott | Chaos in dynamical systems[END_REF] Different LFF regimes were originally pointed out in interband diode lasers 14 while a recent paper has even analyzed the dynamical transitions between them showing the birth of well-defined dropouts near the threshold, followed by faster and irregular fluctuations at higher pumps. 21 Although a route to chaos involving periodic oscillations at the external cavity frequency followed by deterministic LFF is also observed at 77 K in agreement with Ref. 11, this work shows that decreasing the temperature makes the QCL more sensitive to the optical feedback with LFF dynamics taking place over a wide range of feedback levels. Indeed, at 290 K, a feedback ratio as high as 0.0199 is required to observe the birth of the LFF dynamics while it does not exceed 0.0046 here at 77 K (0.0085 at 170 K as shown in Fig. 5 left axis). Such a difference can be attributed to the increase of the carrier-to-photon lifetime ratio. At room temperature, the latter is typically of the order of 0.1 in a QCL whereas it is four orders of magnitude larger in interband lasers [START_REF] Jumpertz | Nonlinear Photonics in Mid-infrared Quantum Cascade Lasers[END_REF] .

The carrier lifetime can be approximated via the upper-state lifetime τ c , 23 defined as follows :

1 τ c ≈ 1 τ 31 + 1 τ 32 (1)
with τ 32 the time constant related to the carrier scat-

) )

) ) tering into the lower laser level and τ 31 the one into the bottom level with a time constant through longitudinaloptical phonon emissions (the energy levels are shown in the inset of Fig. 7). In this work, the upper-state lifetime is investigated with a custom heterostructure simulation software named METIS based on semi-classical Boltzmann equations with thermalized subbands. 24 Fig. 7 shows the evolution of τ c from 373 K down to 73 K with steps of 20 K. Other numerical studies using a density matrix transport model showed similar results for the carrier lifetime of mid-infrared QCLs. 25 For each temperature, the upper bound represents the value of τ c just above threshold and the lower bound represents the value of τ c close to the current value leading to the maximum output power. Numerical simulations show that the mean upper state lifetime increases from 0.47 ps to 1.26 ps when cooling down the device which corresponds to a total variation of about 170%. The latter directly transforms into a larger carrier-to-photon lifetime ratio. Assuming the photon lifetime is slowly varying compared to the upper state lifetime between 290 K and 77 K, 26 the carrier-to-photon lifetime is thus expected to increase from 0.13 to 0.26 on average. This largely ex-plains the increased sensitivity to external optical feedback of the QCL in this study, as underlined in the Lang and Kobayashi model for semiconductor lasers under external optical feedback 27 in which the carrier-to-photon lifetime ratio plays a predominant role. However, in comparison with diode lasers in which the threshold leading to instabilities usually decreases at higher temperature due to a reduced output power and a larger linewidth enhancement factor 28 , this work shows that the QCL becomes more sensitive to optical feedback at low temperature, despite a larger output power. As a consequence of that, other parameters such as the linewidth enhancement factor and the damping rate which were not studied in that experiment, may also vary with temperature and further influence the QCL sensitivity to optical feedback.

IV. CONCLUSION

To summarize, we qualitatively retrieve experimental evidence showing the main feedback regimes and a bifurcation process from steady state to LFF dynamics through a limit cycle, the deterministic chaotic behavior being confirmed by a Lyapunov exponents analysis. We show that the LFF regime is stronger at 77 K owing to a 100% increase of the upper state lifetime between 290 K and 77 K. As a result, the laser destabilization takes place at a lower feedback ratio through an increase of the carrier-to-photon lifetime ratio which is an important feature for understanding the underlying physics of QCLs. This work paves the way towards the development of possible secure atmospheric transmission lines and unpredictable optical countermeasures operating in the mid-infrared wavelengths.

FIG. 1 .

 1 FIG. 1. Scanning electron micrographe of a buried heterostructure quantum cascade laser.

FIG. 2 .

 2 FIG. 2. Experimental setup with the feedback path allowing controlling the back-reflected light and the detection path.

FIG. 3 .

 3 FIG.3. LIV and spectral (inset) characteristics of the free-running QCL operating at 290 K and under pulsed wave with 3% duty cycle.

FIG. 4 .FIG. 5 .

 45 Fig.5exhibits the whole evolution of the temporal waveforms of the QCL for different amount of external optical feedback f at 77 K. For each trace, the transient regime lasts between 0.8 and 1.2 µs. It can be clearly seen for Fig.5f) and Fig.5 g) until approximately 1.2 µs. Under free-running operation (f = 0), the QCL operates in a steady-state hence the time trace is unchanged. Let us note that the latter is not completely flat because the amplifier used for the experiments had a low-frequency cutoff below 100 kHz and therefore, part of the square signal harmonics are lost. When the feedback is slightly increased to f = 0.0003, oscillations start to appear and hence the QCL is likely operating in a limit cycle with oscillation frequency slightly smaller than the external cavity as aforementioned. At f = 0.0003 and f = 0.0008, the traces do not exhibit LFF pattern. The later is observed for feedback ratios above 0.0046. At f = 0.0298, f = 0.0393 and f = 0.089, an increase of the back-reflected light gives rise to a pattern composed of pure LFF with an intensity modulation, as described on Fig.5 g). At the maximum reachable feedback, the time trace at 77 K also exhibits another stage with lower frequencies as shown on Fig.5 i). This step was never reached at 290 K even when the feedback ratio was higher than in our cryogenic measurements here. The coexistence of LFF with more complex dynamics is a good precursor for generating chaos with a higher dimensionality. To confirm the chaotic behavior, we calculated the Lyapunov exponents (LEs) from the time traces, which describe the divergence rate of nearby attractor trajectories and are a widely used criterion in defining chaos.[START_REF] Sprott | Chaos and Time-Series Analysis[END_REF][START_REF] Ott | Chaos in dynamical systems[END_REF] Positive LEs indicate the ultra-sensitivity to initial conditions of chaotic state.[START_REF] Sprott | Chaos and Time-Series Analysis[END_REF][START_REF] Ott | Chaos in dynamical systems[END_REF][START_REF] Grassberger | [END_REF][START_REF] Schuster | Deterministic chaos: an introduction[END_REF][START_REF] Grassberger | [END_REF][20] The phase portraits (Fig.6a) for 290 K, Fig. 6 c) for 77 K) of the temporal waveforms are plotted and show the rich attractor structure. The four Largest Lyapunov Exponents (LLEs) are also given in Fig. 6 b) and Fig. 6 d), for 290 K and 77 K respectively. First, in Fig. 6 b), the

FIG. 6 .

 6 FIG. 6. System dynamics analysis through Lyapunov Exponents (LEs). The upper row is the combination for the 290 K case. The lower row is the combination for the 77 K case. Each combination includes the phase portrait (a, c) of temporal waveform and the calculated spectrum of Lyapunov exponents (b, d). As the time evolves, curves in each panel converge to the values of such exponents. Only the largest four LEs are plotted in each diagram. For the calculated spectra, the curves converge to values (b) λ1 = 0.485 µs -1 , λ2 = 0.284 µs -1 , λ3 = -0.021 µs -1 and λ4 = -0.656 µs -1 under 290 K, and values (d) λ1 = 0.444 µs -1 , λ2 = 0.238 µs -1 , λ3 = -0.002 µs -1 and λ4 = -0.582 µs -1 under 77 K. Blue dots on the left diagrams are the retrieved values of the derivative of the laser's intensity I' as a function of the laser's intensity I and the red curve represents the phase diagram after noise filtering.

ACKNOWLEDGMENTS

This work is supported by the French Defense Agency (DGA), the French ANR program under grant ANR-17-ASMA-0006, the Office of Naval Research (N00014-16-1-2094) and the National Science Foundation (DMR-1611598). Authors acknowledge Dr. Virginie Trinité for helping with METIS, Zhangji Zhao for assistance in the cryogenic measurements, Prof. Benjamin S. Williams and Dr. Sudeep Khanal for the fruitful discussions.