N Milaniak

P Aude

P R Griffiths

F Massines

G Laroche
email: gaetan.laroche@gmn.ulaval.ca

Interpretation of artifacts in Fourier transform infrared spectra of atmospheric pressure dielectric barrier discharges: relationship with the plasma frequency between 300 Hz and 15 kHz

Keywords: dielectric barrier discharge, Fourier transform infrared spectroscopy, atmospheric pressure plasma, highvoltage plasma excitation frequency, interferogram sampling frequency

This article describes the occurrence of a phenomenon that is observed while recording midinfrared (4000-700 cm -1 ) absorption spectra of dielectric barrier discharges sustained at frequencies ranging from 300 Hz to 15 kHz. This phenomenon is observed as the presence of very sharp spikes in the spectrum, for which the wavenumber depends on both the high voltage frequency used to generate the discharge and the velocity of the moving mirror of the interferometer (which in turn determines the interferogram sampling frequency). While it is well known that the consumption of gas precursor within plasmas can be followed, we demonstrate that Fourier transform infrared spectroscopy also makes it possible to monitor frequencies and coupling of excitation mechanisms occurring in the plasma.

Introduction

The dielectric barrier discharge (DBD) configuration [START_REF] Kogelschatz | Dielectricbarrier discharges[END_REF][START_REF] Brandenburg | Dielectric barrier discharges: progress on plasma sources and on the understanding of regimes and single filaments Plasma Sources[END_REF] that enables the creation of nonthermal plasmas at atmo spheric pressure has gained increasing interest in the past years, with applications in surface functionalization [START_REF] Klages | Surface functionalization at atmospheric pressure by DBDbased pulsed plasma polymerization Plasma Process[END_REF], synthesis of homogeneous or nanostructured thin films [START_REF] Fricke | Plasma polymers deposited in atmospheric pressure dielectric barrier discharges: Influence of process parameters on film properties[END_REF][START_REF] Vallade | Effect of glow DBD modulation on gas and thin film chemical composition: case of Ar/SiH 4 /NH 3 mixture[END_REF][START_REF] Brunet | Control of composite thin film made in an Ar/isopropanol/TiO 2 nanoparticles dielectric barrier discharge by the excitation frequency Plasma Process[END_REF][START_REF] Profili | Nebulization of nanocolloidal suspensions for the growth of nanocomposite coatings in dielectric barrier discharges Plasma Process[END_REF][START_REF] Profili | Deposition of nanocomposite coatings on wood using cold discharges at atmospheric pressure Surf[END_REF], removal UPR 8521, of pollutants [START_REF] Mustafa | Volatile organic compounds (VOCs) removal in nonthermal plasma double dielectric barrier discharge reactor[END_REF], disinfection of medical tools [START_REF] Scholtz | Nonthermal plasma-a tool for decontamination and disinfection[END_REF], as well as tissue sterilization [START_REF] Wu | Porcine intact and wounded skin responses to atmospheric nonthermal plasma[END_REF] and solar cells production [START_REF] Dhere | Research on the nonthermal DBD surface modification and the humidityheat aging resistant performance in solar cell modules of FFC backsheet[END_REF] to name only a few. Despite the fact that DBDs are nowadays part of several industrial processes, the nature of the excited species that they contain and the mech anisms by which they interact with material surfaces still has to be understood. In this context, the chemical transformation of the active species in DBDs is a key point that requires new characterization methods to be developed.

Fourier transform infrared (FTIR) spectroscopy is a pow erful tool to identify the chemical structure of molecules through the assignment of the vibrational frequencies of their chemical groups, either in the gas [START_REF] Coronado | Dynamic phenomena during the photocatalytic oxidation of ethanol and acetone over nanocrystalline TiO 2 : simultaneous FTIR analysis of gas and surface species[END_REF], liquid [START_REF] Kazarian | Applications of ATRFTIR spectroscopic imaging to biomedical samples[END_REF], or solid phases including thin layers [START_REF] León | FTIR and Raman characterization of TiO 2 nanoparticles coated with polyethylene glycol as carrier for 2methoxyestradiol[END_REF][START_REF] Liu | Quantitative characterization of the solid acidity of montmorillonite using combined FTIR and TPD based on the NH3 adsorption system[END_REF] and powders [START_REF] Planinsek | Surface analysis of powder binary mixtures with ATR FTIR spectroscopy[END_REF]. An interest has grown in recent years to extend the utility of FTIR spectrometry to analyze gaseous precursors in the plasma phase, while working in a DBD. This approach was shown to enable the analysis of the precursor consumption and dissoci ation in the plasma phase and the correlation of this informa tion with the chemical composition of the plasma deposited thin film [START_REF] Vallade | Fouriertransformed infrared absorption spectroscopy: a tool to characterize the chemical composition of Ar-NH 3 -SiH 4 dielectric barrier discharge[END_REF][START_REF] Christensen | A direct fourier transform infrared spectroscopic comparison of the plasma and thermally driven reaction of CO 2 at macor Plasma[END_REF][START_REF] Alabduly | An in situ and downstream study of nonthermal plasma chemistry in an air fed dielectric barrier discharge (DBD)[END_REF][START_REF] Nair | In situ Fourier transform infrared (FTIR) study of nonthermalplasma assisted methane oxidative conversion[END_REF].

In the most common sampling configuration, the infrared beam is directed through the sample. In this situation, the infrared spectrum is calculated by ratioing the intensity of the beam that passes through the sample to that of the beam trave ling through the entire spectrometer optical path in the absence of the sample [START_REF] Griffiths | Fourier Transform Infrared Spectrometry 2nd edn[END_REF]. In the present work, the aforementioned sampling configuration was used to characterize atmospheric plasmas, using an experimental setup previously described by Laroche et al [START_REF] Laroche | Fourier transform infrared absorption spectroscopy characterization of gaseous atmospheric pressure plasmas with 2 mm spatial resolution[END_REF]. For plasmas generated at highvoltage frequencies lower than 15 kHz, sharp spikes were observed in the infrared spectra at frequencies that depended on both the frequency of the high voltage used to sustain the plasma discharge and the interferogram sampling rate. Despite the fact that such spectral artifacts have already been observed by other groups working with similar plasma frequency con ditions, their cause has not been assigned [START_REF] Nair | In situ Fourier transform infrared (FTIR) study of nonthermalplasma assisted methane oxidative conversion[END_REF] or their pres ence was identified as a peak absorbance [START_REF] Ligot | Experimental and theoretical study of the plasma chemistry of ethyl lactate plasma polymerization discharges Plasma Process[END_REF]. Knowing for which conditions and at which wavenumber the spike appears would help in experiment preparation to avoid overlap with infrared features of interest. In addition, understanding this peak origin would enable to use it as a tool to characterize the plasma as its wavenumber is shown to be determined by the plasma frequency. Therefore, it is essential to shed more light on this phenomenon and bring it to the attention of the plasma scientific community, as a limitation of FTIR for the study of plasmas, but also as a new method of plasma characterization. This article aims to understand and interpret the origin of this measurement artifact.

Methods

The plasma reactor in which the infrared studies were per formed has been previously described by Brunet et al [START_REF] Brunet | Control of composite thin film made in an Ar/isopropanol/TiO 2 nanoparticles dielectric barrier discharge by the excitation frequency Plasma Process[END_REF]. Briefly, the plasma reactor upper electrode was connected to the high voltage (HV) while the bottom electrode was grounded. Both electrodes were covered with an alumina plate acting as the dielectric barrier in the plasma reactor configu ration. The HV was issued from a low frequency waveform generator (Agilent 33210) connected to an audio amplifier (Crest CC4000) linked to a voltage transformer (Boige et Vignal). The transformer frequencies ranged from 1 to 10 kHz with a voltage ranging between 1 and 15 kV. The gap between the electrodes was kept at 2 mm. The pressure within the reactor chamber was first reduced to 2 × 10 -2 Torr to remove impurities. Nitrogen was then introduced within the chamber to bring the reactor pressure back to 760 Torr. At this time, argon was introduced between the electrodes. The filling of the chamber with nitrogen followed by the injection of argon between the electrodes was done because nitrogen has 10 times higher breakdown voltage than argon. Hence an easy ignition is induced in between the electrodes, whilst nitrogen prohibits arc creation around the electrode [START_REF] Massines | Atmospheric pressure low temperature direct plasma technology: status and challenges for thin film deposition Plasma Process[END_REF].

The optical layout used to record midinfrared absorption spectra in atmospheric plasmas has been described in detail elsewhere [START_REF] Laroche | Fourier transform infrared absorption spectroscopy characterization of gaseous atmospheric pressure plasmas with 2 mm spatial resolution[END_REF]. The infrared spectrometer was an Agilent Cary 670 equipped with a liquid nitrogencooled narrow band mercury cadmium telluride (MCT) detector. Depending on the nature of the information that was targeted, between 1 and 256 scans were coadded, apodized using a medium NortonBeer function, and Fourier transformed. The spectra were recorded between 700 and 4000 cm -1 at a resolution of 4 cm -1 . The background spectrum was recorded without any applied voltage between the electrodes. To help understanding the cause of the phenomenon of interest, the interferogram sampling rate was set either to 12.5 kHz, 25 kHz, or 37.5 kHz as determined by the optical velocity of the interferometer moving mirror and the HeNe laser beam wavenumber (ca. 15 800 cm -1 ).

Results and discussion

Before trying to explain the appearance of spikes in the plasma spectra, it is worth mentioning some key observations. First, this observation is true whatever the breakdown voltage as observed by comparing different gases, which have break down voltage varying from 1.4 kV -Ar + NH 3 , 2.7 kV -Ar to 9 kV for He 25 ). Thus, the presence of the spike is neither related to the voltage of the power supply nor to the plasma regime. Second, there is no observance of this peak when the plasma is on, but the infrared beam path directed aside from the plasma region. These two observations therefore elimi nate any possibility of an electronically induced phenom enon. Third, this peak artifact could be observed without the infrared beam crossing the plasma region. Indeed, blocking the spectro meter infrared beam before its entrance in the plasma and only recording the signal from the emitted plasma light led to the recording of noise when a high number of scans (typically 128 or 256, which is usually used to record the FTIR spectra of plasmas) are coadded, while recording only one scan led to spectra with one or more discrete spikes. Of note, these spikes were observed at identical wavenumbers in various gas environments and plasma regimes (e.g. in argon, helium, argon + ammonia; results not shown), therefore indi cating that their position is related to the plasma excitation frequency as opposed to the gas environment. Along with the other experimental observations, this points toward the fact that plasma infrared spectra are the result of the sum of the modulated signal originating from the spectrometer infrared source and the infrared signal coming from the plasma which is, in turn, modulated at the frequency of the HV that is used to sustain the discharge and not synchronized with the inter ferometer sampling process. Finally, the absorbance of these spikes can be either positive (as exemplified by the spikes seen in figures 1(a) and (c)) or negative, as seen in figure 1(b). They can also have the shape of the first derivative of a spike going in the positive or negative direction. This observation leads to the deduction that the cause of the spike is outofphase with the corresponding Fourier frequency in the interferogram.

Figure 1 shows infrared spectra of argon plasmas at applied frequencies ranging from 4 to 6 kHz at a sampling rate of 25 kHz. As may be seen in this figure, a single spike can be observed in each of these spectra. The position of these peaks clearly depends on the applied voltage frequency used to sus tain the plasma discharge and the ratio of their wavenumbers is identical to the ratio of the applied voltage frequencies.

The cause of these spikes must be some type of sinusoidal interference. The varying shape and direction of the spikes indicates that the phase of the interfering signal must not be correlated to the phase of the corresponding frequency in the interferogram.

As seen in figure 2, the frequency at which the artifact peak is observed also depends on the optical velocity of the inter ferometer and hence the interferogram sampling frequency, f s . For plasmas generated with a voltage frequency of 2 kHz, the peak position is 840, 1263, and 2523 cm -1 for sampling fre quencies of 37.5, 25, and 12.5 kHz, respectively. In this case, the ratio of the wavenumbers of the observed peak is propor tional to the inverse of the high voltage frequency ratio. Taken together, the data presented in figures 1 and 2 enable a very simple relationship to predict the wavenumber, w, where the spike will be observed to be established:

w = 15 800 cm -1 × f m (Hz) f s (Hz) (1)
where f m is the frequency of the high voltage used to sustain the plasma discharge, and f s is the interferogram sampling frequency. Table 1 reports the calculated and the measured wavenum bers of the artifact peak for different interferogram sampling frequencies and applied high voltage frequencies to sustain the plasma discharge. As may be seen, the peak artifact may be accurately predicted from equation [START_REF] Kogelschatz | Dielectricbarrier discharges[END_REF].

Interestingly, some of the recorded spectra display two peak artifacts, with the higher wavenumber peak value being twice that of the lowest wavenumber feature, as seen in figure 3. It is therefore likely that the high wavenumber component is a harmonic of the fundamental observed at lower wavenumber. In these circumstances, it is also possible to calculate the wavenumber at which this harmonic contrib ution should be observed by simply multiplying the right term of equation ( 1) by two.

At this point, one could raise the question whether the pres ence of a single peak artifact in the recorded infrared spec trum necessarily means that the frequency of the associated phenomenon corresponds to the fundamental transition. The answer to this question is brought in figure 4, which correlates the spectrometer effective optical range (approx. between 700 and 4000 cm -1 ) with the plasma HV frequency at a sampling frequency of 25 kHz. As may be seen in this figure, it is actu ally the harmonics that are seen up to a HV frequency of 1 kHz. On the other hand, both the fundamental and the harmonic component wavenumbers fall within the spectrometer effec tive optical range for HV frequencies ranging between ≈1 and 3.2 kHz, therefore leading to the observation of both the fundamental and its harmonic. Finally, only the fundamental feature is observed at HV frequencies higher than 3.2 kHz.

All in all, this means that care should be taken while attempting to ascertain the frequency of the plasma HV mod ulation frequency from the wavenumber of the artifact peak in the case where only one peak is observed in the spectrum. Indeed, several FTIR measurements have to be performed at different sampling rate and plasma frequencies to correctly assign whether the artifact feature is a fundamental or a har monic contribution. This is exemplified in table 2 where the calculated wavenumbers of both the fundamental and har monic peak artifacts are reported as a function of the HV fre quency for sampling rates of 12.5, 25, and 37.5 kHz.

Even though the wavenumbers at which the spikes should be observed can be precisely calculated, their origin still has to be identified. The description of the phenomenon illustrated above points toward the fact that the infrared light emitted by the plasma acts as a type of sinusoidal interference that com bines with the sinusoidal components of the interferogram. Accordingly, if this hypothesis holds, the intensity of the bands related to the plasma light fluctuation should decrease with time as this signal would vary randomly from scan to scan whereas the amplitude of the interferogram increases lin early with the number of scans that are coadded. Thus, as noted above, the phase of the sinusoidal interference arising from the plasma should be independent of the corresponding phase of the sinusoidal components of the interferogram cre ated by the scanning mirror of the interferometer. In terms of the origin of the phenomenon, the behavior can be explained since each half cycle of the voltage, the plasma switches on when the applied voltage reaches a certain threshold and switches off when the voltage applied to the gas decreases too much [START_REF] Massines | Glow and Townsend dielectric barrier discharge in various atmosphere Plasma[END_REF]. The spectrometer highpass filter removes the low frequency components, while the lowpass filter eliminates any highfrequency signal. Therefore, even if the plasma itself is a signal longer than a spike, which repeats every halfcycle of the applied frequency, the detector signal looks like a series of spikes that appear each time the applied voltage reaches the plasma breakdown threshold.

Figure 5 shows the interferograms recorded after 1 scan and 64 scans at a sampling frequency of 25 kHz of an argon DBD generated at a high voltage frequency of 1 kHz. As may be seen in this figure, the interferogram recorded after a single scan nicely shows the superimposition of a series of spikes with the interferogram of the infrared source. The optical path dif ference between two successive interference peaks is approx. 0.8 × 10 -3 cm. Considering that a maximal interference peak intensity should be observed at every mirror displacement of λ/2, the resulting wavelength value precisely matches with the prediction of equation (1) (with λ = 1/w). In addition, the interferogram recorded after coadding 64 scans shows a clear decrease of the interference fringes observed after a single scan, therefore supporting the aforementioned hypothesis of the independence between the sinusoidal components of the phase generated by the pulsed infrared emission of the plasma and the phase of the sinusoidal components of the interfero gram originating from the infrared source of the interferometer.

Figure 5 shows that the interference is neither a sine wave (in which case only the fundamental would be observed in the spectrum) or a square wave (in which case the funda mental and its odd harmonics would be seen). Instead it is better described as a shah function, Ш. The Fourier transform of the shah function is another shah function, with spikes at the fundamental frequency and both even and odd harmonics. The high harmonics are attenuated by the spectrometer's low pass filter so only the loworder harmonics are observed in the spectrum as shown in figure 3.

Figure 6 shows an interesting infrared spectrum recorded for a plasma generated at a HV frequency of 2 kHz with an amplitude modulation (AM) set at a 100 Hz (10 ms) repetition frequency, during which the plasma is sustained for 5 ms, and then turned off for the following for 5 ms. As can be seen, the expected fundamental and 1st harmonic are observed at 1263 and 2525 cm -1 , respectively, (shown in black in figure 6) as predicted in table 2. However, these two peaks are surrounded by a series of two sidebands, for which the wavenumbers are indicated in red and blue. The differences of wavenumber between both main spectral artifacts and the peaks identified in red range between 60 and 71 cm -1 , which is comparable to the calculated value of the fundamental artifact component wavenumber that should be observed for a plasma with applied HV frequency of 100 Hz, shown in table 2. Furthermore, the wavenumber differences between the spectral components identified in red and blue from each side of the main artifact components range from 121 to 130 cm -1 , in close agreement with the expected value of the 1st harmonic artifact wave number when the plasma HV frequency is set as 100 Hz. These observations clearly put in evidence that FTIR spectroscopy can not only be used to observe plasma frequency modulation, but also to monitor the coupling between the principal plasma frequency and the plasma modulation frequency.

Conclusions

This study has shed more light on the origin of spectral arti fact components which are observed while recording the FT midinfrared spectra of dielectric barrier discharges generated at high voltage frequencies ranging from 300 Hz to 15 kHz. The results presented therein clearly demonstrated that the wavenumber of these spikes depend on both the high voltage frequency used to generate the plasma and the interferogram sampling frequency that is selected to record the FTIR spectra. Occurrence of the infrared artifacts recorded at a sampling frequency of 25 kHz from a plasma generated at a HV frequency of 2 kHz, with an amplitude modulation set at a 100 Hz (10 ms) repetition frequency, during which the plasma is sustained for 5 ms, and then turned off for the following for 5 ms.

In addition, the FTIR spectra of plasmas submitted to a fre quency modulation also display peak artifacts which originate from the interaction of this modulation with the plasma high voltage frequency. Therefore, in addition to enabling following gas precursor consumption within plasmas, FTIR spectrom etry also makes it possible to monitor frequencies of excitation mechanisms occurring in the plasma. Future works will focus on the use of these infrared artifacts to quantify the coupling in plasmas generated using multiple frequency conditions.

Figure 1 .

 1 Figure 1. Occurrence of the infrared artifact observed for plasma high voltage excitation frequencies of 4 (a), 5 (b), and 6 (c) kHz at a sampling rate of 25 kHz. The spectra are offset for clarity.

Figure 2 .

 2 Figure 2. Occurrence of the infrared artifact observed at a plasma high voltage frequency of 2 kHz and interferogram sampling rates of 37.5 (a), 25 (b), and 12.5 (c) kHz. The spectra are offset for clarity.

Figure 3 .

 3 Figure 3. Observation of two peak artifacts at a plasma high voltage frequency of 800 Hz and interferogram sampling frequency of 12.5 kHz.

Figure 4 .

 4 Figure 4. Calculations of the fundamental (black and first harmonic (red curve) artifact wavenumber at an interferogram sampling frequency of 25 kHz, showing the plasma high voltage frequencies at which the infrared artifacts can be observed in the midinfrared region corresponding to the FTIR spectrometer effective optical range. (Blue) Only the fundamental component of the peak artifact is observed; (pink) only the first harmonic component of the peak artifact is observed; (green) both the fundamental and the first harmonic components of the peak artifacts are observed.

Figure

  Figure Interferograms recorded after 1 (black) and 64 (red) scans for plasma excitation frequency set at 1 kHz and interferogram sampling frequency of 25 kHz.

Figure 6 .

 6 Figure 6. Occurrence of the infrared artifacts recorded at a sampling frequency of 25 kHz from a plasma generated at a HV frequency of 2 kHz, with an amplitude modulation set at a 100 Hz (10 ms) repetition frequency, during which the plasma is sustained for 5 ms, and then turned off for the following for 5 ms.

Table 1 .

 1 Comparison between the calculated and measured artifact wavenumbers.

		Interferogram sampling	Interferogram sampling	Interferogram sampling
		frequency of 12.5 kHz	frequency of 25 kHz	frequency of 37.5 kHz
	Plasma excitation	Calculated	Measured	Calculated	Measured	Calculated	Measured
	frequency (Hz)	(cm -1 )	(cm -1 )	(cm -1 )	(cm -1 )	(cm -1 )	(cm -1 )
	100	126		63		42
	200	253		126		84
	300	379		190		126
	400	506		253		169
	500	632		316		211
	600	758		379		253
	700	885		442		295
	800	1011		506		337
	900	1138		569		379
	1000	1264		632		421
	2000	2528	2526	1264	1263	843	842
	3000	3792	3793	1896	1895	1264	1263
	4000	5057		2528	2528	1686	1685
	5000	6321		3160	3160	2107	2105
	6000	7585		3792	3792	2528	2528
	7000	8849		4425	4422	2950	2949
	8000	10 113		5057	5056	3371	3371
	9000	11 377		5689	5686	3792	3789
	10 000	12 642		6321	6319	4214	4212
	11 000	13 906		6953	6950	4635	4634
	12 000	15 170		7585		5057	5056
	13 000	16 434		8217		5478	5476
	14 000	17 698		8849		5899	5897
	15 000	18 962		9481		6321	6320

Table 2 .

 2 Calculated values for peak artifact position (fundamental and 1st harmonic) for interferogram sampling frequencies.

		Interferogram sampling	Interferogram sampling	Interferogram sampling
		frequency of 12.5 kHz	frequency of 25 kHz	frequency of 37.5 kHz
		Calculated	Calculated	Calculated	Calculated	Calculated	Calculated
	Plasma excitation	fundamental	harmonic	fundamental	harmonic	fundamental	harmonic
	frequency (Hz)	(cm -1 )	(cm -1 )	(cm -1 )	(cm -1 )	(cm -1 )	(cm -1 )
	100	126	253	63	126	42	84
	200	253	506	126	253	84	169
	300	379	758	190	379	126	253
	400	506	1011	253	506	169	337
	500	632	1264	316	632	211	421
	600	758	1517	379	758	253	506
	700	885	1770	442	885	295	590
	800	1011	2023	506	1011	337	674
	900	1138	2275	569	1138	379	758
	1000	1264	2528	632	1264	421	843
	2000	2528	5057	1264	2528	843	1686
	3000	3792	7585	1896	3792	1264	2528
	4000	5057	10 113	2528	5057	1686	3371
	5000	6321	12 642	3160	6321	2107	4214
	6000	7585	15 170	3792	7585	2528	5057
	7000	8849	17 698	4425	8849	2950	5899
	8000	10 113	20 227	5057	10 113	3371	6742
	9000	11 377	22 755	5689	11 377	3792	7585
	10 000	12 642	25 283	6321	12 642	4214	8428
	11 000	13 906	27 812	6953	13 906	4635	9271
	12 000	15 170	30 340	7585	15 170	5057	10 113
	13 000	16 434	32 868	8217	16 434	5478	10 956
	14 000	17 698	35 396	8849	17 698	5899	11 799
	15 000	18 962	37 925	9481	18 962	6321	12 642

Acknowledgments

This work was supported by French 'Investments for the future' ('Investissements d'Avenir') programme managed by the National Agency for Research (ANR) under contract ANR10LABX2201 (labex SOLSTICE). The financial support of the National Science and Engineering Research Council (NSERC) of Canada is also acknowledged. N Mila niak is the recipient of a scholarship from the Fondation du Centre de Recherche du CHU de QuébecUniversité Laval.

ORCID iDs

N Milaniak https://orcid.org/0000000207580955 F Massines https://orcid.org/000000029985342X G Laroche https://orcid.org/000000030661628X


Caio V Z Cipro
email: caiovzc@usp.br

Y Cherel

P Bocher

F Caurant

P Miramand

P Bustamante

Trace elements in invertebrates and fish from Kerguelen waters, southern Indian Ocean

Keywords: plankton, myctophid, food web, metals, trace elements, Kerguelen

Given the lack of background data on essential and non-essential trace elements in invertebrates and fish known to be the predominant prey of marine mammals and seabirds breeding at the Kerguelen Islands, this study intends to provide these results of great influence for predators in higher trophic levels. To this end, plankton organisms (9 species/4 phyla), mollusks (2 bivalves and 2 squid species) and fishes (8 benthic and 10 pelagic species) from Kerguelen waters were analysed for cadmium (Cd), copper (Cu), mercury (Hg), and zinc (Zn). Individual concentrations of non-essential elements (particularly Cd) showed larger variation in comparison with essential ones due to homeostasis. Cd ranged over 4 orders of magnitude; however, Hg ranged only 1, without significant correlation to trophic level. Instead, ecological parameters (benthic/mesopelagic habitat and feeding ecology) showed a more important influence on the results. Concerning seashore organisms, bivalves collected inside the Gulf of Morbihan had higher Cd concentrations compared to those from the Kerguelen shelf, suggesting a local source of Cd, such as runoff water from bird colonies. Comparison with literature showed metal concentrations in invertebrates and fishes from Kerguelen Islands somewhat lower than in the Antarctic area, with Hg prevailing in benthic species and Cd in pelagic ones. In contrast to Hg, Cd values of squids, jellyfish and the amphipod Themisto gaudichaudii were significantly higher than all other species. Finally, top predators foraging in this area that can be subject to potentially high Hg and Cd exposure through their diet at Kerguelen are reviewed.

INTRODUCTION

The Southern Ocean constitutes a particular environment for marine biota where human inputs of metals are supposed to be very low. Several essential elements such as iron (Fe) or copper (Cu) are poorly concentrated and/or bioavailable in these waters and could therefore act as limiting factors for phytoplankton (e.g. [START_REF] Coale | Effects of iron, manganese, copper, and zinc enrichments on productivity and biomass in the subarctic Pacific[END_REF]. In higher trophic level organisms, low Cu and zinc (Zn) concentrations in regard to organisms from temperate regions have been found in crustaceans and mollusks (e.g. Bustamante et al. 1998a[START_REF] Bustamante | Distribution of trace elements in the tissues of benthic and pelagic fish from the Kerguelen Islands[END_REF][START_REF] Petri | Trace metal in the crustaceans in the Antarctic Ocean[END_REF][START_REF] Rainbow | Copper, cadmium and zinc concentrations in oceanic amphipod and euphausiid crustaceans, as a source of heavy metals to pelagic seabirds[END_REF], raising the question of how can these organisms cope with essential element supposed deficiency. Several hypotheses have been suggested to answer this question such as a relatively higher efficiency of mechanisms of element uptake compared to similar organisms from nondeficient areas and/or the replacement of essential elements by non-essential ones in biochemical reactions or in enzymes. To date, such replacement has only been evidenced for Zn substitution by Cd in carbonic anhydrase from marine diatoms [START_REF] Lane | A biological function for cadmium in marine diatoms[END_REF][START_REF] Lane | A cadmium enzyme from a marine diatom[END_REF][START_REF] Xu | Structure and metal exchange in the cadmium carbonic anhydrase of marine diatoms[END_REF]. In this context, the interactions between essential and non-essential elements in invertebrates and fish from the Southern Ocean appear poorly documented and, moreover, baseline values necessary for the study of such interactions are even scarcer, if not inexistent in the literature.

Despite the comparatively low inputs of non-essential elements, very high concentrations of Cd and Hg have been reported in several organisms from distinct trophic levels in Antarctic and subantarctic environments (Sanchez-Hernandez 2000; [START_REF] Bustamante | Distribution of trace elements in the tissues of benthic and pelagic fish from the Kerguelen Islands[END_REF][START_REF] Dos Santos | Baseline mercury and zinc concentrations in terrestrial and coastal organisms of Admiralty Bay, Antarctica[END_REF][START_REF] Bargagli | Environmental contamination in Antarctic ecosystems[END_REF]. Population growth and industrial development in several countries of the Southern Hemisphere are changing the global pattern of persistent anthropogenic contaminants and new classes of chemicals have already been detected in the Southern Ocean (e.g. [START_REF] Bargagli | Environmental contamination in Antarctic ecosystems[END_REF][START_REF] Von Waldow | Quantifying remoteness from emission sources of persistent organic pollutants on a global scale[END_REF]Carravieri et al. 2014a). Very high concentrations of both Cd and Hg were also found in the tissues of top predators such as seabirds and marine mammals from the Southern Ocean (see the review of [START_REF] Sanchez-Hernandez | Trace element contamination in Antarctic ecosystems[END_REF].

These high trophic level vertebrates are mainly exposed to trace elements through their food (Aguilar et al. 1999;[START_REF] Muirhead | Heavy metal concentrations in the tissues of seabirds from Gough Island, South Atlantic Ocean[END_REF] and some specific prey highly contribute to the exposure to a given element. For example, cephalopod consumption is well-known to provide elevated concentrations of Cd under a bioavailable form (Bustamante et al. 1998b[START_REF] Bustamante | Cadmium detoxification processes in the digestive gland of cephalopods in relation to accumulated cadmium concentrations[END_REF] and mesopelagic fish contain high amounts of methyl-Hg (e.g. [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][START_REF] Monteiro | Mercury concentrations in prey fish indicate enhanced bioaccumulation in mesopelagic environments[END_REF]. However, there is only a few data on invertebrates and fish to provide background for explaining the high Cd and Hg concentrations in the top predators from the Southern Ocean. Therefore, it is of major concern to provide data about lower trophic level organisms they feed on to give a more comprehensive and evidence supported basis to their contamination pattern. In addition to that, age, trophic position, sex, size among other ecological parameters likely play a role in trace elements concentration in these prey organisms (e.g. [START_REF] Dehn | Trophic relationships in an Arctic food web and implications for trace metal transfer[END_REF][START_REF] Locarnini | Trace element concentrations in Antarctic krill, Euphausia superba[END_REF][START_REF] Mcintyre | Age and trophic position dominate bioaccumulation of mercury and organochlorines in the food web of Lake Washington[END_REF].

Situated near the Polar Front, the Kerguelen Islands are a particularly important area for breeding seabirds and for mammals (see [START_REF] Guinet | Consumption of marine resources by seabirds and seals in Crozet and Kerguelen waters: changes in relation to consumer biomass 1962-85[END_REF]. Specifically, this archipelago hosts a large and highly diverse avian assemblage (35 different breeding species according to [START_REF] Weimerskirch | The Avifauna of the Kerguelen Islands[END_REF]) and 13 species of marine mammals (3 mysticetes, 7 odontocetes and 3 pinnipeds; [START_REF] Borsa | Seasonal trends in the occurrence of marine mammals in the golfe du Morbihan, Kerguelen Islands[END_REF]). According to their reproduction strategies, many seabird species catch their prey in the highly productive waters around the Archipelago and feed on a few key species of marine organisms, including some crustaceans (e.g. euphausiids, hyperiids, copepods), fish (e.g. myctophids, notothenioids) and cephalopods (e.g. oceanic squids) [START_REF] Bocher | Amphipod-based food web: Themisto gaudichaudii caught in nets and by seabirds in Kerguelen waters, southern Indian Ocean[END_REF][START_REF] Cherel | Stable isotopes, beaks and predators: a new tool to study the trophic ecology of cephalopods, including giant and colossal squids[END_REF][START_REF] Cherel | Isotopic niches and trophic levels of myctophid fishes and their predators in the Southern Ocean[END_REF][START_REF] Guinet | Consumption of marine resources by seabirds and seals in Crozet and Kerguelen waters: changes in relation to consumer biomass 1962-85[END_REF]. Their exposure to contaminants and that of their offspring is therefore determined by the concentrations in these lower trophic level organisms consumed specifically in this area, at least during the breeding period.

In this context, the present study was conducted to document selected trace element concentrations in zooplankton, mollusks and fishes around the Kerguelen Islands to provide understanding on the degree of metal contamination in low trophic level organisms being the prey of seabirds and marine mammals. To this end, Cd, Cu, Hg and Zn have been analysed in organisms belonging to different phyla of pelagic invertebrates and in 17 coastal and oceanic fish species to cover the main categories of prey species of the Kerguelen seabird community and of the marine mammals foraging in these waters. The non-essential elements Cd and Hg constitute the main metals of concern for wild vertebrates because of their known toxicity [START_REF] Scheuhammer | The chronic toxicity of aluminium, cadmium, mercury, and lead in birds: a review[END_REF][START_REF] Tan | The endocrine effects of mercury in humans and wildlife[END_REF][START_REF] Tartu | To breed or not to breed: endocrine response to mercury contamination by an Arctic seabird[END_REF]. In turn, Cu and Zn can be disturbed by the interaction of Cd and Hg on their regulation proteins such as the metallothioneins (e.g. Øverjordet et al. 2015). The levels of these trace elements were compared within the benthic and pelagic food webs and were globally compared with similar organisms from other marine ecosystems, when available. Finally, the significance of key species in contaminants transfer towards top predators was examined.

MATERIALS AND METHODS

Sampling of organisms

Pelagic and benthic organisms were successively collected in the waters surrounding the Kerguelen Island Archipelago (Figure 1) during the austral summers from 1997 to 1999.

Information on these organisms are summarised in Table 1. In coastal waters, pelagic zooplankton mainly constituted by hyperiid amphipods (Themisto gaudichaudii) and copepods (Paraeuchaeta antarctica), was sampled inside the Morbihan Gulf in March 1997 with an ORInet (2 m², 1 mm mesh aperture). In this area, several benthic fish species (mostly Notothenidae) were also collected by net fishing overnight (Table 1). Outside the gulf, hyperiid amphipods (T. gaudichaudii), euphausiids (Euphausia vallentini, E. frigida and E. triacantha) and six myctophid species were collected in the eastern part of the peri-insular shelf in February 1998, using a IYGPT trawl (International Young Gadoid Pelagic Trawl; opening: 12 x 7 m) with 10mm mesh size in the cod-end. Other pelagic material (gelatinous plankton, planktonic crustaceans, squid and fish) were sampled during February 1999 on cruises of the RV "La Curieuse". Benthic fish were collected by bottom trawl used for commercial fishery in the Southern Kerguelen shelf. The samples of the copepod Thysanoessa sp. were obtained from stomach contents of the South Georgian diving petrel collected during investigations on the diet of this species [START_REF] Bocher | Influence of the diet on the bioaccumulation of heavy metals in zooplankton-eating petrels at Kerguelen archipelago, Southern Indian Ocean[END_REF] and potential effects of partial digestion must be taken into account.Mussels were collected by hand on the shore during low tides in January 1999.

Immediately after collection, the organisms were separated by species, then sex and size or age classes whenever possible (Euphausia vallentini above and below 25mm, and Champsocephalus gunnari adults and juveniles respectively), and frozen in plastic bags or vials. Then, samples were stored at -20°C until analysis. All organisms were analysed whole, except the mussels, which had their shells removed.

Sample preparation and analysis

Length (mm) and mass (g) of fish and squids were thoroughly determined, as well as the sex, whenever possible (i.e., when size and maturity allowed so), and their gut content was removed.

In the case of myctophids, otoliths were taken out to ensure identification of the species. Fish and squids were systematically treated individually, except Harpagifer sp. (12 individuals resulted in 4 samples). In contrast, all other invertebrates were pooled, except the jellyfish, which were treated individually. Pooling was primarily made because of analytical reasons, in order to gather enough mass to attend protocol specifications. Sample characteristics, i.e. family, species, length, weight and sex (for squid, fish and some crustacean species), are given in Table 1. Trophic level is assessed based on the personal data of the co-authors, mostly based on stomach contents. Six of the seventeen fish species had also their trophic levels determined by stable isotope ratio of nitrogen published in other studies [START_REF] Cherel | Isotopic niches and trophic levels of myctophid fishes and their predators in the Southern Ocean[END_REF]. Since the difference between these two methods was lower than one trophic level, only the former will be further considered during data interpretation.

Samples were dried for two to three days at 50°C to a constant weight and then homogenised. Next, 2 aliquots of approx. 100 to 300 mg each (according to availability) of homogenised dry sample were digested with 5 ml of 65% HNO3 and 0.3 ml of 70% HClO4 during 72 h at 80°C. When the solution was clear, acids were evaporated and the obtained residues were dissolved in 10 ml 0.3 N nitric acid. Cd, Cu and Zn were analysed using a flame and graphite furnace atomic absorption spectrophotometer Varian 250 Plus with deuterium background correction. For Hg, aliquots ranging from 5 to 20 mg of dried material were analysed directly in an Advanced Mercury Analyser spectrophotometer (Altec AMA 254). Hg determination involved evaporation of Hg by progressive heating until 700°C were reached and then held under oxygen atmosphere for 3 min, followed by an amalgamation on a gold-net.

Afterwards, the net was heated to liberate the collected mercury, subsequently measured by UV atomic absorption spectrophotometry.

Quality assurance was assessed using dogfish liver DOLT-2 (NRCC) and dogfish muscle DORM-2 (NRCC) as reference materials. Such standards were analysed and treated under the same conditions as the samples, with errors in regard to the certified values remaining below 5% in both cases. Detection limits were 0.004 for Cd, 0.5 for Cu, 0.005 for Hg, and 3 µg g -1 dry weight (dw) for Zn. All trace element concentrations in Kerguelen Islands marine organisms are reported in µgg -1 dw unless stated otherwise. Water contents allowing recalculations of the metal concentrations from dw to wet weight (ww) are given in Table 1.

Statistical analyses

Statistical analyses were performed in Microsoft Excel 2007 and Statsoft Statistica 11 and 12.

Before analyses, data were checked for normality of distribution and homogeneity of variances using Shapiro-Wilk and Brown-Forsythe tests, respectively, followed by ANOVA and Posthoc Tukey HSD. Spearman's or Pearson's correlations are used to assess the degree of monotonic or linear dependence, respectively, between two variables. At-test was used to assess whether female and male datasets differed when available. All statistically significant results were set at =0.05 and all presented correlations should be assumed significant unless stated otherwise.

RESULTS

Metal concentrations in invertebrates and fishes from the Kerguelen Island waters are presented in Table 2. Cu, Hg, and Zn concentrations showed, in a general way, a lesser degree of variability when compared to Cd. Indeed, Cu, Hg and Zn average concentrations vary one order of magnitude each whereas Cd ranged over three orders of magnitude.

Regarding the influence of biological factors in the results, the only crustacean species for which gender comparison was possible was Paraeuchaeta antarctica. All four trace elements presented significant differences between males and females, these later showing significantly higher Cd (p<0.001), Cu (p=0.004) and Zn (p=0.015) levels than males. This difference can be attributed to their considerably higher length and weight (Table 1).

Concerning Hg, significantly higher (p=0.037) concentrations were found in males than in females.

In cephalopods, there were no differences on metal concentrations between males and females (for Todarodes angolensis Cd, pt-test = 0.149 and Hg, pt-test = 0.867; for Moroteuthis ingens Cd, pt-test = 0.206 and Hg, pt-test = 0.923); moreover, no correlation (neither Pearson's nor Spearman's) between biometrics and metal concentrations were found. In fish, metal concentrations showed no significant difference between the sexes nor correlation with the biometric parameters (total length and total weight) with a few exceptions concerning Cd and Hg. Cd correlated significantly with total length in L. squamifrons (r=0.723, p=0.018) and with both length and weight in G. fraseri (r=0.704, p=0.005 and 0.862, p<0.001, respectively).

Interestingly, a significant negative correlation of Cd with both biometric parameters was found for juvenile C. gunnari (r=-0.702, p=0.023 and -0.636, p=0.048) and Protomyctophum tenisoni (r=-0.549, p=0.034 and -0.627, p=0.012). In turn, Hg correlated (Spearman's rank) significantly with total length in Gymnoscopelus piabilis (r=0.591, p=0.033) and Lepidonotothen squamifrons (r=0.670, p=0.034); with weight in Protomyctophum bolini (r=0.697, p=0.004) and with both length and weight (shown respectively) in Channichthys rhinoceratus (r=0.843, p=0.02 and r=0.802, p=0.07), G. fraseri (r=0.771, p=0.001 for both), Notothenia rossii (r=0.554,p=0.049 and r=0.577,p=0.038) and Zanclorhynchus spinifer (r=0.959,p=0.002 and r=0.965,p=0.002). Finally, in regard to between-elements relationships, Cd and Hg levels correlated with one another significantly in Champsocephalus gunnari (total, r=0.947, p<0.001) and G. fraseri (r=0.843, p<0.001).

Concerning Bivalves, two mussel species (the blue mussel, Mytilus edulis desolationis and the Magellan mussel, Aulacomya atra) sampled in four different locations: (Foch, Cap Noir, Port-aux-Français and Mayes Island) were analysed in the present study. A significant negative correlation (Spearman's rank) between Cd and Cu (r=-0.725, p=0.039) was found in the blue mussel, and also in the Magellan mussel (r=-0.798, p=0.004). This latter species presented yet significant correlation between Cu and total weight (r=-0.682, p=0.042), Cd and total weight (r=0.673, p=0.048) and Cd and Hg (r=-0.702, p=0.034). When all samples (both species) are considered together, there is a significant correlation between Cd and total weight (r=0.547, p=0.007) and also a significant negative correlation between Cd and Cu (r=-0.537, p=0.009).

DISCUSSION

The main objective of this study was to provide baseline levels in a wide range of phyla which constitute the prey for high trophic level organisms such as large fish, seabirds, and marine mammals. The species collected in the present study represent a wide range of ecological groups from the pelagic/benthic and neritic/oceanic communities from the Kerguelen Island waters. The size of the collected organisms falls within the range of the size preyed by top predators from this area and more specifically, from the large seabird community that includes 35 species [START_REF] Weimerskirch | The Avifauna of the Kerguelen Islands[END_REF].

Influence of biological factors on metal concentrations

Among factors influencing metal concentrations, sex differences were found for the crustacean Paraeuchaeta antarctica. Higher concentrations of Cd, Cu and Zn in female P.

antarctica might be due to the fact that metals are highly retained by copepods and Cd, Cu and Zn bioaccumulation with the size is likely to have played a major role in this difference, not necessarily via diet, but also possibly via the dissolved phase, in a passive way [START_REF] Wang | Assimilation and regeneration of trace elements by marine copepods[END_REF][START_REF] Wang | Accumulation of trace elements in a marine copepod[END_REF] or else via a possible remobilisation due to sexual maturation, as further discussed. In contrast to Cd, Cu and Zn, higher Hg concentrations in males is likely due to the fact that the sampled females could have already reached sexual maturity size and have excreted Hg in the laid eggs. Indeed, maternal transfer may act as a major pathway for Hg (II) and MeHg elimination in crustaceans [START_REF] Tsui | Uptake and Elimination Routes of Inorganic Mercury and Methylmercury in Daphnia magna[END_REF]. The size of the specimens analysed here ranged from 5 to 10 mm, whereas this species presents CV (last copepodite stage) at 6.5±0.3 mm and CVI (adults) at 8.7±0.4 mm [START_REF] Bocher | Importance of the large copepod Paraeuchaeta antarctica (Giesbrecht, 1902) in coastal waters and the diet of seabirds at Kerguelen, Southern Ocean[END_REF], therefore the sampleset likely contained sexually mature individuals.

In cephalopods, the lack of difference in Cd and Hg concentrations between males and females is surprising considering the sexual dimorphism in both species, females reaching larger sizes than males. Indeed, sexual dimorphism and ontogenic effects can influence in metal concentrations in cephalopods (e.g. [START_REF] Pierce | Geographic, seasonal and ontogenetic variation in cadmium and mercury concentrations in squid (Cephalopoda: Teuthoidea) from UK waters[END_REF][START_REF] Chouvelon | Inter-specific and ontogenic differences in δ13C and δ15N values and Hg and Cd concentrations in cephalopods[END_REF]. The absence of variation of metal concentrations in both cephalopod species is likely due to the limited number of specimens of each sex and limited size range for both T. angolensis and M. ingens.

Nevertheless, metal concentrations found in both squid species were among the highest for all the species from the present study, especially for Cd. Cephalopods are considered as superbioaccumulators of many trace elements and their capacity to bioaccumulate remarkable Cd levels was already shown in several environments [START_REF] Dorneles | Cephalopods and cetaceans as indicators of offshore bioavailability of cadmium off Central South Brazil Bight[END_REF][START_REF] Martin | High copper concentrations in squid livers in association with elevated levels of silver, cadmium, and zinc[END_REF][START_REF] Miramand | Concentration and distribution of heavy metals in tissues of two cephalopods, Eledone cirrhosa and Sepia officinalis, from the French coast of the English Channel[END_REF][START_REF] Miramand | High concentrations of some heavy metals in tissues of the mediterranean Octopus[END_REF], including Kerguelen waters (Bustamante et al. 1998b). Even though the specific proteins involved in Cd bioaccumulation are not fully known, its significant correlation (Spearman's rank) with Zn (r=0.902) in T.

angolensis (the highest Cd level) suggests the involvement of MTLP in Cd detoxification. For Hg as well, cephalopods were among the most contaminated species. Within this group, Hg is mainly under the highly bioavailable organic form [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]) and therefore they represent a significant source of this element for their predators.

In fish, the significant negative correlation of Cd with both biometric parameters (size and weight) for C. gunnari and P. tenisoni could be due to a higher Cd exposure in earlier life stages (due to diet, habitat or maternal transfer), with a growth dilution effect taking place and concentrations decreasing thereafter. Since benthic organisms present, in a general way, overall lower Cd values when compared to pelagic ones (Fig. 2), this is in accordance with previous studies describing C. gunnari as bentho-pelagic, but with a shift from the pelagic to the benthic environment as the fish grow (Kock 2005a, b). Concerning Hg, the correlations found between this element and biometrics are likely related to the bioaccumulation of Hg as the fish grow.

Moreover, benthic species presented mostly higher Hg levels than pelagic ones (Fig. 2). In sediments, organic carbon and microbial activity play an important role on Hg bioavailability as microorganisms highly contributes to the methylation of inorganic Hg [START_REF] Andersson | The role of sediments as sink or source for environmental contaminants--a case study of mercury and chlorinated organic compounds[END_REF]), enhancing the exposure of benthic species to Hg. In regard to Cd and Hg positive significant correlations with size and weight in C. gunnari and G. fraseri, it would mean that these species are simultaneously exposed to sources of both contaminants. This could be due to the consumption 1) of one specific prey that presents high concentrations of Cd and Hg or 2) of several prey that present conversely high concentrations of Cd or Hg. The second hypothesis seem more likely in regard to C. gunnari, since its diet in a subantarctic environment (South Georgia Archipelago) comprised a large proportion of Antarctic krill Euphausia superba, knowing that Euphausids had comparatively high Hg levels (See Table 2) and, more importantly, the hyperiid amphipod Themisto gaudichaudii (with very high Cd levels) as the most frequent prey [START_REF] Kock | Variations in the diet composition and feeding intensity of mackerel icefish Champsocephalus gunnari at South Georgia (Antarctic)[END_REF]. In regard to G. fraseri, it feeds mainly on copepods and to a lesser extent, on the Euphausid Thysanoessa spp. [START_REF] Saunders | Distribution, population structure and trophodynamics of Southern Ocean Gymnoscopelus (Myctophidae) in the Scotia Sea[END_REF], both presenting high Cd and Hg concentrations (Table 2). Therefore it seems reasonable to state that the first one of the previous hypotheses would be more likely in this case.

The influence of the trophic position on metal concentrations is examined in Figure 2, which presents the concentrations of Cd and Hg stratified by trophic level. In this regard, stable isotopes ratios of nitrogen bring slightly superior data for trophic levels (TLs) of six fish species included in the present study [START_REF] Cherel | Isotopic niches and trophic levels of myctophid fishes and their predators in the Southern Ocean[END_REF]), all of them inferior to one trophic level: 0.9 TL for Protomyctophum bolini, 0.7 TL for Gymnoscopelus nicholsi, 0.5 TL for P. tenisoni, 0.3 TL for Electrona antarctica and G. fraseri and finally 0.2 TL for G. piabilis. The difference between these TLs obtained by different methods was negligible and did not change the stratification of concentrations along the TLs.

Cd concentrations ranged over 4 orders of magnitude, i.e. from 0.063 µg g -1 in Gobionotothen acuta to 79.4 µg g -1 in T. angolensis across the different trophic levels, interestingly, a relatively similar pattern to the one reported for an Arctic marine food web [START_REF] Macdonald | Cadmium in marine invertebrates and arctic cod in the Canadian Arctic. Distribution and ecological implications[END_REF]. Moreover, a large intraspecific variation is shown by different taxa as well, such as in salps and specially crustaceans (Table 2). However, no significant correlation between Cd concentrations and the trophic level of the species was found in our sampling, suggesting that Cd is not biomagnified within our sampling set. However, it is important to remark that the increase in trophic level within our sample set does not always infer a direct food-consumer link between the sampled organisms, so biomagnification strictu sensu must be regarded with caution. Moreover, samples were taken in different years and interannual and local differences are possible as well. Also, the transfer of Cd appears to be more related to the species rather than to the trophic level itself, as previously shown in other environments [START_REF] Miramand | Ecotoxicologie intégrée de l'espèce à l'écosystème[END_REF][START_REF] Pigeot | Cadmium pathways in an exploited intertidal ecosystem with chronic cadmium inputs (Marennes-Oléron, Atlantic coast, France)[END_REF]). This specificity is likely related to Cd bioaccumulation capacities which are particularly elevated for some taxa such as the hyperiid amphipods Themisto sp. (e.g., [START_REF] Ritterhoff | Influence of body length, life-history status and sex on trace metal concentrations in selected zooplankton collectives from the Greenland Sea[END_REF] and cephalopods (e.g., Bustamante et al.1998a[START_REF] Bustamante | Cadmium detoxification processes in the digestive gland of cephalopods in relation to accumulated cadmium concentrations[END_REF][START_REF] Dorneles | Cephalopods and cetaceans as indicators of offshore bioavailability of cadmium off Central South Brazil Bight[END_REF][START_REF] Penicaud | Metal bioaccumulation and detoxification processes in cephalopods: a review[END_REF], or particularly weak as in most marine fish species [START_REF] Kojadinovic | Bioaccumulation of trace elements in pelagic fish from the Western Indian Ocean[END_REF][START_REF] Wang | Interactions of trace metals and different marine food chains[END_REF].

The lack of biomagnification is apparently repeated for Hg in our set of samples, contrary to what is previously reported for local avifauna [START_REF] Blévin | Wide Range of Mercury Contamination in Chicks of Southern Ocean Seabirds[END_REF]Carravieri et al. 2014b). It is important to remark that Hg biomagnification concerns its main organic form, i.e. methyl-Hg, due to its high bioavailability (e.g. [START_REF] Kannan | Distribution of Total Mercury and Methyl Mercury in Water, Sediment, and Fish from South Florida Estuaries[END_REF]) whereas our analyses were made only for total Hg. A much higher assimilation efficiency is displayed for methyl-Hg than for inorganic Hg and a slight variation in this property may determine whether or not some of the element is biomagnified [START_REF] Reinfelder | Trace element trophic transfer in aquatic organisms: a critique of the kinetic model approach[END_REF]). Yet, the proportion of methyl-Hg is poorly documented for low trophic level prey in the Southern Ocean. This issue clearly deserves further research in this ecologically important and representative oceanographic area.

Sessile organisms as indicators of possible secondary metal sources

In mussels, the positive correlation between Cd and weight and the negative correlation between Cd and Cu suggest that as they grow, the homeostatically controlled Cu is proportionally surpassed by the bioaccumulative Cd in their organisms, likely bound to metallothioneins [START_REF] Klaassen | Metallothionein: an intracellular protein to protect against cadmium toxicity[END_REF]. Indeed, mussels produce Cd-induced metallothioneins [START_REF] Mackay | Complete amino acid sequences of five dimeric and four monomeric forms of metallothionein from the edible mussel Mytilus edulis[END_REF] and Cd can displace essential metals such as Cu and Zn normally associated to these proteins [START_REF] Ahn | Baseline heavy metal concentrations in the Antarctic clam, Laternula elliptica in Maxwell Bay, King George Island, Antarctica[END_REF]. Tukey post-hoc tests resulted in separated groups for Cd (raw data for average lot length and mass and TE concentrations are available as Online Resource 1). One group containing samples from Foch, Cap Noir and Port-aux-Français; and the second one, from Port-aux-Français and Mayes (always in crescent order). This result suggests some local Cd source enrichment inside the Gulf of Morbihan, where Port-aux-Français, the largest human settlement in the area (between 45 in austral winter and 120 people in austral summer) and Mayes are both located. Conversely, both other locations (Foch and Cap Noir) are exposed to the open ocean, in the northern shore. This is apparently contrary to the T. gaudichaudii data (Table 2), which presented levels for continental shelf samples three times higher than coastal ones. However, the average individual weight from the continental shelf samples was around three times the one from coastal samples, therefore there is likely an influence from age and, thus, from bioaccumulation. A strong effect of age is confirmed by a previous study on the closely related species T. libellula from the Greenland Sea [START_REF] Ritterhoff | Influence of body length, life-history status and sex on trace metal concentrations in selected zooplankton collectives from the Greenland Sea[END_REF]. Nevertheless, a growth dilution effect should be expected for the continental shelf samples, which leaves two hypotheses: whether exposure is increased for larger individuals, or the influence of local Cd sources within the Gulf of Morbihan is not as important to T. gaudichaudii as it is for mussels. Since literature presents related species (T. japonica and probably T. gaudichaudii as well) switching from herbivory to carnivory during their life cycles [START_REF] Pakhomov | Trophodynamics of the hyperiid amphipod Themisto gaudichaudi in the South Georgia region during late austral summer[END_REF], the first hypothesis remains more likely, corroborating the existence of a local Cd secondary source.

Using fish for comparison did not prove to be helpful, since the only two species collected inside the Gulf were benthic (N. rossii and P. magellanica) and there was an evident bias in Cd levels towards pelagic species, as stated above, possibly because of Cd enriched upwelled waters around the archipelago (Bustamante et al. 1998b[START_REF] Bustamante | Distribution of trace elements in the tissues of benthic and pelagic fish from the Kerguelen Islands[END_REF]. For coastal sites inside the Morbihan Bay, another source can be suspected. Indeed, Mayes provides diverse and highly suitable breeding sites for large colonies of burrowing petrels, which breed there in very high densities [START_REF] Weimerskirch | The Avifauna of the Kerguelen Islands[END_REF], with up to 6 burrows per square metre [START_REF] Mougeot | Predation on burrowing petrels by the brown skua (Catharacta skua lönnbergi) at Mayes Island, Kerguelen[END_REF]. Seabird faeces can alter the trace metal composition in soils [START_REF] Headley | Heavy metal concentrations in peat profiles from the high Arctic[END_REF] and therefore, colonies might work as secondary trace metal sources (see [START_REF] Choy | An isotopic investigation of mercury accumulation in terrestrial food webs adjacent to an Arctic seabird colony[END_REF][START_REF] Espejo | Concentration of trace metals in excrements of two species of penguins from different locations of the Antarctic Peninsula[END_REF]. Water percolating bird colonies has already been shown to have an important role in the exposure of terrestrial flora to contaminants in Antarctic lands [START_REF] Cipro | Organic pollutants and their correlation with stable isotopes in vegetation from King George Island, Antarctica[END_REF]. It is therefore plausible to assume an analogous effect for the mussels when this water reaches the sea. Nevertheless, this subject deserves further investigation.

Comparison with similar ecosystems in other regions

Table 3 presents a comparison of the obtained results with those from the literature despite the scarcity of data for equivalent organisms in similar environments, which reinforces the need for studies such as this one. Therefore taxonomic, geographic and ecological differences might present some bias that has to be taken into account when interpreting the data.

Having said that, the jellyfish in our work showed Cd values at least one order of magnitude higher than its counterparts from North-eastern Atlantic waters [START_REF] Caurant | Bioaccumulation of Cadmium, Copper and Zinc in some Tissues of Three Species of Marine Turtles Stranded Along the French Atlantic Coasts[END_REF].

Its Zn concentrations were around three times higher as well, not characterising the coaccumulation of Cd and Zn seen in other cases. To the best of our knowledge, the presence of metallothioneins in jellyfish has not been reported yet.

For Ctenophora, our results are somewhat closer to the ones for the North Atlantic Ocean than the ones for the Mediterranean, suggesting an influence from local conditions.

Tunicates presented results for Cd and Cu in a reasonable agreement with the ones from the literature.

Crustaceans, in turn, showed some differences: in a general way, Euphausia spp. values of Cd, Cu and Zn are lower than in organisms from Antarctica and Thysanoessa spp. values for Cd were higher than in organisms from the Bering Sea, closer to pollution sources than Kerguelen Islands [START_REF] Kahle | Trace metals in Antarctic copepods from the Weddell Sea (Antarctica)[END_REF]. So, the difference might be due to a species-specific reason or in a lesser extent, to the influence of water masses dynamics As for mollusks, bivalves presented lower values when compared to Antarctic filter feeders, even if the previously discussed influence of a local source is considered [START_REF] Ahn | Baseline heavy metal concentrations in the Antarctic clam, Laternula elliptica in Maxwell Bay, King George Island, Antarctica[END_REF][START_REF] Bargagli | Trace metals in Antarctic organisms and the development of circumpolar biomonitoring networks[END_REF]. Cephalopods, on the other hand, presented similar Hg levels when compared to another subantarctic environment, Macquarie Island [START_REF] Mcarthur | Mercury in the marine food chain in the Southern Ocean at Macquarie Island: an analysis of a top predator, Patagonian toothfish (Dissostichus eleginoides) and a mid-trophic species, the warty squid (Moroteuthis ingens)[END_REF] or slightly superior when compared to branchial hearts concentrations from samples collected off Amsterdam Islands [START_REF] Kojadinovic | Multi-elemental concentrations in the tissues of the oceanic squid Todarodes filippovae from Tasmania and the southern Indian Ocean[END_REF], which is located north of the Polar Front. Care must be taken when comparing digestive gland data [START_REF] Kojadinovic | Multi-elemental concentrations in the tissues of the oceanic squid Todarodes filippovae from Tasmania and the southern Indian Ocean[END_REF] to muscle or even whole organism data, since this organ might present higher concentrations, even if it does not greatly contribute to the total Hg burden in some oceanic squids [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].

For benthic fish species, Antarctic organisms (Table 3) presented levels for both Cd and Hg one order of magnitude higher than in the present study. For pelagic ones, the only truly pelagic neritic fish present in all sizes throughout the water column is Pleuragramma antarctica [START_REF] Wöhrmann | Adaptations of the Antarctic silverfish Pleuragramma antarcticum (Pisces: Nototheniidae) to pelagic life in high-Antarctic waters[END_REF]. For this species, [START_REF] Brasso | Ontogenetic niche expansion influences mercury exposure in the Antarctic silverfish Pleuragramma antarcticum[END_REF] present Hg data averaging 0.014 µg g -1 for juveniles and 0.021 µg g -1 for adults (for whole fish, after conversion to dry weight). This is an apparent contradiction to the trend previously discussed (Antarctic organisms with higher levels than subantarctic ones), however, this species feeds on a lower trophic level than the ones in Table 3 and also lives in shallower shelf waters [START_REF] Giraldo | Ontogenic changes in the feeding ecology of the early life stages of the Antarctic silverfish (Pleuragramma antarcticum) documented by stable isotopes and diet analysis in the Dumont d'Urville Sea (East Antarctica)[END_REF][START_REF] Pinkerton | Diet and trophic niche of Antarctic silverfish Pleuragramma antarcticum in the Ross Sea, Antarctica[END_REF]), which makes it less exposed to these contaminants. Nevertheless, [START_REF] Goutte | Trace elements in Antarctic fish species and the influence of foraging habitats and dietary habits on mercury levels[END_REF] present an average of 0.065 µg g -1 for P. antarcticum collected in Adelie Land, Antarctica, much similar to the concentration we found for E. antarctica.

Taking all the previous information into account, Kerguelen marine invertebrates and fish seemed to present lower metal concentrations when compared to other sub-Antarctic and especially to Antarctic environments, with the exception of some particular species, notably Themisto gaudichaudii (see also [START_REF] Beltcheva | Heavy metals in Antarctic notothenioid fish from South Bay, Livingston Island, South Shetlands (Antarctica)[END_REF][START_REF] Guynn | Mercury concentrations in the Patagonian toothfish, Dissostichus eleginoides Smitt 1898, among three distinct stocks[END_REF][START_REF] Hennig | Metal Concentrations in Antarctic Zooplankton Species[END_REF][START_REF] Kahle | Trace metals in Antarctic copepods from the Weddell Sea (Antarctica)[END_REF][START_REF] Mcarthur | Mercury in the marine food chain in the Southern Ocean at Macquarie Island: an analysis of a top predator, Patagonian toothfish (Dissostichus eleginoides) and a mid-trophic species, the warty squid (Moroteuthis ingens)[END_REF][START_REF] Rainbow | Copper, cadmium and zinc concentrations in oceanic amphipod and euphausiid crustaceans, as a source of heavy metals to pelagic seabirds[END_REF][START_REF] Stoeppler | Comparative studies on trace metal levels in marine biota[END_REF] and the comparison in Table 3).

Implications concerning the transfer to predators

Because all of the species investigated here constitute to a wide extent, primary or secondary prey for seabirds breeding on the Kerguelen Islands, it is important to provide information on their exposure to trace elements. This is especially important for the non-essential Cd and Hg which can have toxic effects at different levels on wild vertebrates (e.g. [START_REF] Gallien | Cadmium-containing granules in kidney tissue of the Atlantic white-sided dolphin (Lagenorhyncus acutus) off the Faroe Islands[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][START_REF] Tartu | To breed or not to breed: endocrine response to mercury contamination by an Arctic seabird[END_REF]). In addition to linking prey and predator, this section is intended to highlight the fact that some of the accumulation/magnification occur not only in high trophic levels.

In regard to Cd, the main homogenous group after Tukey HSD was composed by all species but T. gaudichaudii, jellyfish and both the cephalopods. T. gaudichaudii is an important part of local macrozooplankton and the main prey for local planktivorous seabirds [START_REF] Bocher | Amphipod-based food web: Themisto gaudichaudii caught in nets and by seabirds in Kerguelen waters, southern Indian Ocean[END_REF], therefore their main Cd source [START_REF] Bocher | Influence of the diet on the bioaccumulation of heavy metals in zooplankton-eating petrels at Kerguelen archipelago, Southern Indian Ocean[END_REF]. Jellyfish showed a Cd concentration one to two orders of magnitude higher than the fish, similar to the findings of [START_REF] Caurant | Bioaccumulation of Cadmium, Copper and Zinc in some Tissues of Three Species of Marine Turtles Stranded Along the French Atlantic Coasts[END_REF]. Jellyfish could therefore represent a vector for Cd transfer, since their energetic value is likely very low [START_REF] Caurant | Bioaccumulation of Cadmium, Copper and Zinc in some Tissues of Three Species of Marine Turtles Stranded Along the French Atlantic Coasts[END_REF]) so their predators should ingest a large amount of them to satisfy their energetic needs. Cephalopods, in turn, may also function as vectors for Cd transfer to top predators (e.g., Bustamante et al. 1998a;[START_REF] Lahaye | Long-term dietary segregation of common dolphins Delphinus delphis in the Bay of Biscay, determined using cadmium as an ecological tracer[END_REF]). This role is even more evident at higher latitudes as they showed somewhat higher Cd concentrations in Antarctic and subantarctic areas when compared to temperate and tropical waters (e.g. [START_REF] Dorneles | Cephalopods and cetaceans as indicators of offshore bioavailability of cadmium off Central South Brazil Bight[END_REF][START_REF] Kojadinovic | Multi-elemental concentrations in the tissues of the oceanic squid Todarodes filippovae from Tasmania and the southern Indian Ocean[END_REF][START_REF] Pierce | Geographic, seasonal and ontogenetic variation in cadmium and mercury concentrations in squid (Cephalopoda: Teuthoidea) from UK waters[END_REF]. Since cephalopods are present in the diet of several predators from Kerguelen Islands such as albatrosses or elephant seals [START_REF] Cherel | Food and feeding ecology of the neritic-slope forager black-browed albatross and its relationships with commercial fisheries in Kerguelen waters[END_REF][START_REF] Cherel | Cephalopod fauna of subantarctic islands: new information from predators[END_REF][START_REF] Guinet | Consumption of marine resources by seabirds and seals in Crozet and Kerguelen waters: changes in relation to consumer biomass 1962-85[END_REF][START_REF] Lescroël | Spatial and temporal variation in the diet of the gentoo penguin (Pygoscelis papua) at Kerguelen Islands[END_REF], their role as Cd vectors is evident. Reported concentrations in the kidney of seabirds from other subantarctic areas such as Gough Island in the South Atlantic Ocean clearly highlighted that seabirds feeding on squids displayed higher Cd concentrations that crustacean-and fish-feeder species [START_REF] Muirhead | Heavy metal concentrations in the tissues of seabirds from Gough Island, South Atlantic Ocean[END_REF].

Hg, as previously presented, had a much less species-specific distribution, with less interspecific variation when compared to Cd, even though some stratification within the trophic levels could be detected. No statistical difference was found among all invertebrates taken together (Tukey HSD) and, concerning the fish, only Notothenia rossii, Gymnoscopelus piabilis, G. fraseri and C. rhinoceratus (in crescent Hg concentration) departed from a homogenous group that contained all the other species. The unicorn icefish C. rhinoceratus presented elevated Hg concentrations, which is consistent with its feeding ecology (Kock 2005a,b): indeed, the diet of juveniles is composed by crustaceans and then adults shift to forage on various notothenioids (mostly benthic) and mesopelagic fish in a lesser extent. In turn, mesopelagic fish, notably the myctophids, are among the most Hg contaminated. Therefore, predators relying on benthic or benthopelagic prey, as the gentoo penguin Pygoscelis papua [START_REF] Lescroël | Foraging under contrasting oceanographic conditions: the gentoo penguin at Kerguelen Archipelago[END_REF] or mesopelagic fish, such as elephant seals Mirounga leonina [START_REF] Cherel | Stable isotopes reveal the trophic position and mesopelagic fish diet of female southern elephant seals breeding on the Kerguelen Islands[END_REF]) and the white-chinned petrel Procellaria aequinoctialis [START_REF] Delord | At-sea distribution and diet of an endangered top predator: relationship between white-chinned petrels and commercial longline fisheries[END_REF], would be highly exposed to Hg.

The present study highlights that not only top predators will be exposed to elevated amounts of Cd and Hg through their diet when consuming specific types of prey, but also lower trophic level organisms can be subject to the same effect. Zooplankton eating predators are exposed to Cd especially if they consume the amphipod T. gaudichaudii (i.e., Halobaena caerulea, Pachyptila desolata, P. belcheri, Pelecanoides georgicus and P. urinatrix, according to [START_REF] Bocher | Influence of the diet on the bioaccumulation of heavy metals in zooplankton-eating petrels at Kerguelen archipelago, Southern Indian Ocean[END_REF]. Cephalopod eating species as the wandering albatross (Diomedea exulans) and the great-winged-petrel (Pterodroma macroptera) are also highly exposed to Cd but also to Hg which is consistent with the concentrations recorded in their internal tissues and their feathers [START_REF] Anderson | Influence of trophic position and foraging range on mercury levels within a seabird community[END_REF][START_REF] Bustamante | High feather mercury concentrations in the wandering albatross are related to sex, breeding status and trophic ecology with no demographic consequences[END_REF][START_REF] Muirhead | Heavy metal concentrations in the tissues of seabirds from Gough Island, South Atlantic Ocean[END_REF][START_REF] Stewart | Influence of phylogeny, diet, moult schedule and sex on heavy metal concentrations in New Zealand Procellariiformes[END_REF][START_REF] Tavares | Influence of age, sex and breeding status on mercury accumulation patterns in the wandering albatross Diomedea exulans[END_REF]) and also with blood [START_REF] Anderson | Element patterns in albatrosses and petrels: influence of trophic position, foraging range, and prey type[END_REF]Carravieri et al. 2014a). Finally, fish eating species, as the grey (Procellaria cinerea) or the white-chinnedpetrel (Procellaria aequinoctialis) are highly exposed (see [START_REF] Stewart | Influence of phylogeny, diet, moult schedule and sex on heavy metal concentrations in New Zealand Procellariiformes[END_REF][START_REF] Anderson | Influence of trophic position and foraging range on mercury levels within a seabird community[END_REF][START_REF] Cipro | Trace elements in tissues of white-chinned petrels (Procellaria aequinoctialis) from Kerguelen waters, Southern Indian Ocean[END_REF] to Hg through the consumption of mesopelagic and benthic fish [START_REF] Delord | At-sea distribution and diet of an endangered top predator: relationship between white-chinned petrels and commercial longline fisheries[END_REF].

Zauke G-P, Clason B, Savinov VM, Savinova T (2003) Heavy metals of inshore benthic invertebrates from the Barents Sea. Sci Total Environ 306:99-110. doi: 10.1016/S0048-9697(02)00487-4

Figure 1 -

 1 Figure 1 -Kerguelen location in the Southern Indian Ocean among the French Southern Lands (left) and detailed map (right).STZ, SAZ and AZ stand for, respectively, the subtropical, sub-Antarctic and Antarctic zones; whereas STF and PF stand for the subtropical and polar fronts. Taken from Cipro et al (2014).

Figure 2 -

 2 Figure 2 -Hg (left, linear scale) and Cd (right, log scale) concentrations (µg -1 dw) stratified by trophic level. Benthic species in black, pelagic ones in white.

Table 2 -

 2 Metal concentrations (µgg -1 dry wt) of organisms from the Kerguelen Islands. Pooled samples are marked with a p Comparison of heavy metal concentrations in organisms from the Kerguelen Islands with those from the literature (range or mean ±SD, in g g -1 dw).

	Group		N	Cd		Cu		Hg		Zn
	Species	Family		Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range
	Ctenophors
	Beroe sp.	Beroidae	3 p	0.077 ± 0.009	0.070-0.088	6.7 ± 0.2	6.4-6.9	0.054 ± 0.010	0.045-0.065	642 ± 58	582-697
	Euphausids
	Euphausia frigida	Euphausiidae	4 p	0.137 ± 0.047	0.109-0.207	5.8 ± 0.1	5.7-5.8	0.023 ± 0.002	0.021-0.025	43 ± 3	41-43
	E. triacantha	"	3 p	0.289 ± 0.025	0.260-0.306	14.9 ± 6.7	4.3-19.8	0.036 ± 0.006	0.029-0.041	55 ± 2	53-57
	E. vallentini (small)	"	3 p	0.500 ± 0.217	0.278-0.711	17.8 ± 2.1	15.4-19.5	0.042 ± 0.003	0.039-0.045	58 ± 9	52-68
	E. vallentini (large)	"	3 p	0.659 ± 0.093	0.589-0.765	30.0 ± 1.0	29.0-30.9	0.017 ± 0.001	0.016-0.018	39 ± 3	37-43
	Thysanoessa sp.	"	8 p	3.66 ± 1.80	1.38-6.26	33.7 ± 12.8	12.8-45.4	0.067 ± 0.031	0.024-0.125	30 ± 11	10-47
	Copepods
	Paraeuchaeta antarctica (♀)	Euchaetidae	3 p	2.11 ± 0.05	2.06-2.16	8.8 ± 1.1	7.7-9.8	0.038 ± 0.019	0.023-0.060	245 ± 66	197-321
	P. antarctica (♂)	"	3 p	0.81 ± 0.05	0.78-0.87	4.9 ± 0.4	4.4-5.2	0.072 ± 0.002	0.071-0.074	87 ± 9	77-92
	Amphipods
	Themisto gaudichaudii(coast)	Hyperridae	4 p	28.5 ± 7.4	21.2-38.8	12.5 ± 0.7	11.9-13.4	0.026 ± 0.005	0.022-0.034	52 ± 3	48-55
	T. gaudichaudii (shelf)	Hyperridae	4 p	73.4 ± 7.2	70.4-81.7	16.0 ± 7.1	11.8-24.2	0.024 ± 0.002	0.023-0.027	82 ± 5	80-88
	Tunicates
	Salpa thompsoni	Salpidae	3 p	0.624 ± 0.066	0.583-0.700	19.9 ± 5.5	14.9-25.7	0.033 ± 0.003	0.030-0.035	969 ± 142	821-1104
	"	"	3 p	2.154 ± 0.135	2.066-2.310	7.7 ± 1.8	5.6-9.0	0.015 ± 0.001	0.013-0.016	367 ± 82	302-458
	Cnidarians
	Jellyfish	Not determined	3	29.0 ± 30.8	9.98-64.53	7.4 ± 0.9	6.8-9.4	0.081 ± 0.015	0.067-0.097	353 ± 305	142-703
	Mollusks
	Moroteuthis ingens	Onychoteuthidae	8	29.6 ± 20.0	6.0-60.0	39 ± 14	23-61	0.099 ± 0.063	0.034-0.215	66 ± 20	19-81
	Todarodes angolensis	Ommastrephidae	11	79.4 ± 41.8	17.5-172.3	80 ± 39	50-194	0.100 ± 0.058	0.057-0.270	107 ± 27	59-158
	Mytilus edulis desolationis	Mytilidae	12 p	6.50 ± 5.83	2.27-27.71	6.23 ± 1.95	3.57 -9.08	0.273 ± 0.187	0.113-0.647	83 ± 25	48.7-125
	Aulacomya atra	"	9 p	9.11 ± 4.79	4.05-18.49	8.49 ± 2.08	5.87-12.71	0.208 ± 0.116	0.105-0.417	135 ± 39	84.2-221
	Benthic fish
	Channichthys rhinoceratus	Channichthyidae	13	0.173 ± 0.125	0.022-0.484	1.8 ± 0.4	1.3-2.7	0.345 ± 0.296	0.058-0.870	79 ± 26	50-128
	Gobiotothen acuta	Notothenidae	1	-	0.063	-	2.9	-	0.075	-	88
	Harpagifer sp.	Harpagiferidae	4 p	0.404 ± 0.091	0.330-0.536	3.5 ± 0.45	3.00-3.93	0.230 ± 0.084	0.128-0.334	107 ± 19	80-123
	Lepidonotothen squamifrons	Notothenidae	10	0.475 ± 0.198	0.232-0.808	1.5 ± 0.4	1.1-2.5	0.150 ± 0.089	0.063-0.271	43 ± 9	31-58
	Notothenia cyanobrancha	"	1	-	0.323	-	2.1	-	0.216	-	64

Acknowledgments

We thank G. Duhamel and the crew of the RV 'La Curieuse' for their help in the collection of the fish specimens. This work was financially supported by the Agence Nationale de la Recherche (program POLARTOP, O. Chastel), the Institut Paul Emile Victor(IPEV, Programme No. 109, H. Weimerskirch), the Terres Australes et Antarctiques Françaises (TAAF) and the CPER (Contrat de Projet Etat-Région) for funding the AMA of the Plateforme Analyses Elémentaires of LIENSs. C.V.Z. Cipro was supported financially through a postdoctoral grant from the University of La Rochelle and the CNRS (French acronym for National Council of Scientific Research), and also scholarships from CAPES (Coordination for the Improvement of Higher Education Personnel via the Science without Borders programme, Brazil) and FAPESP (São Paulo Research Foundation, Brazil) during the course of this work and its publication.