Ricardo M Ribeiro

Frédéric Lucarz
email: frederic.lucarz@telecom-bretagne.eu

Bruno Fracasso

An All-Optical Sampler for Digitising Radio-over-Fibre Transceivers

Keywords: analogue-digital conversion, sampling, microwavephotonics, optical signal processing, semiconductor optical amplifier I

In this paper it is proposed and designed an alloptical Sampler (o-Sampler) intended to be part of future optical Digitising Radio-over-Fibre (o-DRoF) transceivers. It is based on the Semiconductor Laser Amplifier Loop Mirror (SLALOM) configuration thus using the nonlinearities of a semiconductor optical amplifier (SOA) in cross-switching regime. The proof-ofprinciple is established by computer simulations using 500 MHz analogue signal and 2.5 GS/s sampling pulses both in the C-band. The proposed o-Sampler outputs real-time generated optical samples from an analogue RoF signal. The simulated device is rather compact, polarisation independent, potentially optically integrable and requires few mW of sampling peak power.

INTRODUCTION

Analogue Radio-over-Fibre (ARoF) allows a relatively simple distribution of RF signals over optical fibres being a key-technology for the convergence of fibre-based wireline and wireless networks [START_REF] Williamson | RF Photonics[END_REF].

However, ARoF systems are constrained by two basics requirements, which are inherent to analogue transmissions [START_REF] Williamson | RF Photonics[END_REF][START_REF] Nirmalathas | Digitized Radio-Over-Fiber Technologies for Converged Optical Wireless Access Network[END_REF][START_REF] Abdollahi | Fully-Photonic Analogue-to-Digital Conversion Technique for Super-Broadband Digitized-Radio over Fibre Link[END_REF][START_REF] Abdollahi | Digital Radio over Fibre for Future Broadband Wireless Access Network Solution[END_REF][START_REF] Abdollahi | Non-Uniform Wavelength Allocation in All-Photonic Digitized-Radio over Fibre Access Network[END_REF]: all used devices should be highly linear and the received signal should have higher SNR (Signal to Noise Ratio) than those obtained with digital systems.

In order to circumvent the known limitations of ARoF networks, the use of digitised radio-over-fibre (DRoF) technologies [START_REF] Williamson | RF Photonics[END_REF][START_REF] Nirmalathas | Digitized Radio-Over-Fiber Technologies for Converged Optical Wireless Access Network[END_REF][START_REF] Abdollahi | Fully-Photonic Analogue-to-Digital Conversion Technique for Super-Broadband Digitized-Radio over Fibre Link[END_REF][START_REF] Abdollahi | Digital Radio over Fibre for Future Broadband Wireless Access Network Solution[END_REF][START_REF] Abdollahi | Non-Uniform Wavelength Allocation in All-Photonic Digitized-Radio over Fibre Access Network[END_REF][START_REF] Viglienzoni | Net! Works -Next Generation Network: Wireless-Optics Technology[END_REF] is an interesting alternative. Due to the digital nature of the optical link, the dynamic range is independent of the fibre length as long as the power of the received signal is lower than the link sensitivity.

In electronic DRoF (e-DRoF) links the analogue RF input signal is first digitised by an analogue-to-digital converter (ADC), the generated electrical bits drive a light source thus generating optical bits then transported over an optical fibre and finally are reconstructed by a digital-to-analogue converter (DAC) at the output of a photo-detector. e-DRoF technology is relatively more complex than ARoF [START_REF] Nirmalathas | Digitized Radio-Over-Fiber Technologies for Converged Optical Wireless Access Network[END_REF][START_REF] Abdollahi | Fully-Photonic Analogue-to-Digital Conversion Technique for Super-Broadband Digitized-Radio over Fibre Link[END_REF], but exhibits many useful characteristics: centralized digital signal processing such as digitisation itself, enhanced sharing capacity and better compatibility of digital equipments and communication channels, significant increase in transmission distance using optical fibre (larger dynamic range), larger coverage capacity, possible cost reduction and energy saving.

The signal conversion and high-speed processing (including switching and routing) in the all-photonic domain are required for next-generation networks due to the accelerated growth of demanded traffic. All these processes when performed in the electronic domain require optical/electrical (O/E) conversion that introduces complexities and delays. Therefore, by using all-photonic DRoF more operations and signal processing centralization at the Central Station would be achievable. Although the e-DRoF technology is very useful and powerful, the main challenges are: ADC and DAC boards seem to exhibit an intrinsic limitation in their stability, limitations on "bit resolution x input frequency" product and data conversion rates of up to a few tenths of GSa/s, dispersions, nonlinearities (in electrical and optical domains), additive noise, integration of system and devices, size and system cost [START_REF] Abdollahi | Fully-Photonic Analogue-to-Digital Conversion Technique for Super-Broadband Digitized-Radio over Fibre Link[END_REF][START_REF] Abdollahi | Digital Radio over Fibre for Future Broadband Wireless Access Network Solution[END_REF][START_REF] Abdollahi | Non-Uniform Wavelength Allocation in All-Photonic Digitized-Radio over Fibre Access Network[END_REF][START_REF] Viglienzoni | Net! Works -Next Generation Network: Wireless-Optics Technology[END_REF][START_REF] Miyoshi | All-Optical Analog-to-Digital Conversion Using Splitand-Delay Technique[END_REF].

An o-DRoF transmitter (o-DRoF-T) may comprise an RF interface circuit connected to an optical sampler and then to an all-optical encoder and quantizer. Alternatively, if an analogue RoF signal is already available, the o-DRoF-T comprises the optical sampler, the all-optical encoder and quantizer, only. After the encoder an additional circuit (thresholder or quantizer) is generally needed in order to enhance the contrast between the optical bits [START_REF] Miyoshi | All-Optical Analog-to-Digital Conversion Using Splitand-Delay Technique[END_REF][START_REF] Ribeiro | A Thresholder for All-Optical Digitising Radio-over-Fibre Transceivers[END_REF][START_REF] Kitayama | Optical Signal Processing Device for A/D Converter Including Optical Encoders with Nonlinear Loop Mirrors[END_REF].

To the best of our knowledge, no complete experimental o-DRoF communication system has been reported in the literature. However, some recently published papers [START_REF] Abdollahi | Fully-Photonic Analogue-to-Digital Conversion Technique for Super-Broadband Digitized-Radio over Fibre Link[END_REF][START_REF] Abdollahi | Digital Radio over Fibre for Future Broadband Wireless Access Network Solution[END_REF][START_REF] Abdollahi | Non-Uniform Wavelength Allocation in All-Photonic Digitized-Radio over Fibre Access Network[END_REF] from Brunel University (UK) report valuable theoretical simulations of an o-DRoF system but without any detailed description of the operational principles of both all-optical ADC and DAC.

An optical sampling oscilloscope for real-time analyses, but using a scan-sampling technique based on Nonlinear Optical Loop Mirror (NOLM) was already demonstrated [START_REF] Nelson | Optical Sampling Oscilloscope Using Nonlinear Fibre Loop Mirror[END_REF]. The Ricardo M. Ribeiro thanks Foundation Capes/MEC-Brasil for the financial support through a Post-Doctoral fellowship under the grant BEX 9096/11-6.

described NOLM was 100m long and uses an Nd-YAG laser (1.3 µm) as the sampling pulses source at 10 W peak power.

More recently published papers use NOLM [START_REF] Kwok | All-Optical Sampling System Using Nonlinear Optical Loop Mirror with Soliton Self-Frequency Shifted Control Pulses[END_REF] or the Four Wave Mixing (FWM) effect [12 and references therein] on optical sampling to build a fast optical oscilloscope, but intended for signal analyses not in real-time, i.e. by using scan sampling and post-processing.

The paper [START_REF] Bogoni | New Optical Sampler Based on TOAD and Data Postprocessing for Sub-Picosecond Pulse Resolution[END_REF] was the only found that describes a Semiconductor Optical Amplifier (SOA) inserted in a Sagnac loop to demonstrate an optical scan-sampler for pulse analyses. The paper [START_REF] Rampone | Conversion gain characterisation of optical radiofrequency signal mixing by all-optical sampling on SOA-MZI[END_REF] describes the optical sampling and the generated sampling replicas to obtain photonic mixing for frequency up conversions.

Computer simulations [START_REF] Zhang | All-optical sampling based on nonlinear polarization rotation in Semiconductor Optical Amplifiers[END_REF] and experiments [START_REF] Zhang | All-optical sampling exploiting nonlinear polarization rotation in a single semiconductor optical amplifier[END_REF] on realtime sampling technique up to 40 GS/s based on the use of the nonlinear polarisation rotation in SOAs were demonstrated using 5 mW pump (control) power.

At least, an optical ADC (o-ADC) should be able to optically sample the incoming analogue signal in real time, to encode, to quantize and eventually to automatically amplify the output optical bits [START_REF] Abdollahi | Digital Radio over Fibre for Future Broadband Wireless Access Network Solution[END_REF][START_REF] Ribeiro | A Thresholder for All-Optical Digitising Radio-over-Fibre Transceivers[END_REF]. In this paper, we propose for the first time, in the best of our knowledge, the use of an optical circuit based on the SLALOM configuration in the cross-switched regime to comprise an o-Sampler for real-time operations. Computer simulation results on VPI Transmission Maker TM software are presented for 1550 nm wavelength window, 2.5 GHz repetition rate with 40-ps RZ sampling pulses. The o-DRoF systems may well integrate the next generation of allphotonic telecommunication networks [START_REF] Abdollahi | Fully-Photonic Analogue-to-Digital Conversion Technique for Super-Broadband Digitized-Radio over Fibre Link[END_REF][START_REF] Abdollahi | Digital Radio over Fibre for Future Broadband Wireless Access Network Solution[END_REF][START_REF] Abdollahi | Non-Uniform Wavelength Allocation in All-Photonic Digitized-Radio over Fibre Access Network[END_REF][START_REF] Viglienzoni | Net! Works -Next Generation Network: Wireless-Optics Technology[END_REF] and provides the connection between the access with the metropolitan networks and even with the high-capacity transport networks.

II. ALL-OPTICAL DIGITISING RADIO-OVER-FIBRE TRANSMITTER (O-DROF-T)

This section proposes an o-DRoF-T able to simultaneously perform wavelength conversion in the optical domain. It essentially comprises an o-ADC assuming that the analogue RoF signal is already available.

Since the end of 1970's the vast majority of all-optical ADC and DAC systems have always been separately reported and for various types of applications, but excluding any communication link application [START_REF] Hou | GSPS All-Optical ADC for Future Communication and Imaging[END_REF][START_REF] Dayoush | All-Optical ADC and its Applications in Future Communication Satellites[END_REF]. [START_REF] Ribeiro | Proposal and Design of an All-Optical Encoder for Digitising Radio-over-Fibre Transceivers[END_REF] and (5) optical thresholder or o-Thresholder [START_REF] Ribeiro | A Thresholder for All-Optical Digitising Radio-over-Fibre Transceivers[END_REF]. The o-Sampler carries out the real-time optical sampling of an input analogue RoF signal and is driven by an optical pulse train that in turn is generated by the o-Generator. This pulse train is also used as a reference clock signal by the o-Encoder that optically encodes the sampled signal, thereby accomplishing the photonic AD-conversion itself [START_REF] Ribeiro | Proposal and Design of an All-Optical Encoder for Digitising Radio-over-Fibre Transceivers[END_REF]. The o-Thresholder is provided mainly to significantly enhance the contrast between the "0" and "1" optical bits at the output of the o-Encoder and automatically providing amplification [START_REF] Ribeiro | A Thresholder for All-Optical Digitising Radio-over-Fibre Transceivers[END_REF]. The o-Generator is rather implemented by a mode-locked laser, a key component used to generate a series of picoseconds pulses at GHz rates with high temporal stability, i.e. with very low jitter in the femtoseconds time-scale [START_REF] Miyoshi | All-Optical Analog-to-Digital Conversion Using Splitand-Delay Technique[END_REF][START_REF] Currie | High-Performance Photonic Analog-to-Digital Converter and Low-Noise Mode-Locked Fiber Laser[END_REF]. Therefore, these pulses can be used as the clock signal for the optical sampling and encoding steps. The sub-modules (3') and (3'') essentially multiplies the repetition rate of the clock signal generated by ( 1) and ( 2) and is better explained in the reference [START_REF] Ribeiro | Proposal and Design of an All-Optical Encoder for Digitising Radio-over-Fibre Transceivers[END_REF].

III. THE OPERATIONAL PRINCIPLE OF THE O-SAMPLER

A. General

Sagnac interferometric fibre rings as NOLMs have been used as a nonlinear device based on self-phase modulation (SPM) or cross-phase modulation (XPM) that are very useful for many ultrafast signal-processing applications [START_REF] Miyoshi | All-Optical Analog-to-Digital Conversion Using Splitand-Delay Technique[END_REF][START_REF] Kitayama | Optical Signal Processing Device for A/D Converter Including Optical Encoders with Nonlinear Loop Mirrors[END_REF][START_REF] Nelson | Optical Sampling Oscilloscope Using Nonlinear Fibre Loop Mirror[END_REF][START_REF] Kwok | All-Optical Sampling System Using Nonlinear Optical Loop Mirror with Soliton Self-Frequency Shifted Control Pulses[END_REF][START_REF] Huang | All-Optical OOK to 16-QAM Modulation Format Conversion Employing Nonlinear Optical Loop Mirror[END_REF]. However, for fibre lengths greater than a few hundred meters, the stability of the NOLM is significantly degraded. Furthermore, the long fibre introduces excessive latency for some optical processing applications [START_REF] Cotter | Nonlinear Optics for High-Speed Digital Information Processing[END_REF].

The use of a laser amplifier as a nonlinear medium instead of the optical fibre leads to an interesting class of devices usually named as SLALOM [START_REF] Eiselt | Optical Loop Mirror with Semiconductor Laser Amplifier[END_REF] or Terahertz Optical Asymmetric Demultiplexer (TOAD) [START_REF] Bogoni | New Optical Sampler Based on TOAD and Data Postprocessing for Sub-Picosecond Pulse Resolution[END_REF][START_REF] Sokoloff | A Terahertz Optical Asymmetric Demultiplexer (TOAD)[END_REF] depending on application.

The optical circuits of the o-Sampler, o-Interleaver, o-Encoder [START_REF] Ribeiro | Proposal and Design of an All-Optical Encoder for Digitising Radio-over-Fibre Transceivers[END_REF] and o-Thresholder [START_REF] Ribeiro | A Thresholder for All-Optical Digitising Radio-over-Fibre Transceivers[END_REF] were designed and simulated using the VPI Transmission Maker TM software. However, in this paper only the results for the o-Sampler are presented. The circuit was probed at 1550 nm wavelength window with 2.5 GHz repetition rate 40-ps timewidth sampling pulses and 500 MHz carrier frequency of the RoF signal. The peak power (P C ) of the sampling (control) pulses could be varied from µW to few mW in order to obtain the sampled pulses envelope in the time domain.

The input analogue signal at 1562.1 nm wavelength is generated from a continuous-wave DFB laser diode after traversing an amplitude modulator driven by a function generator delivering 500 MHz sinusoidal waveform. The sampling pulses at 1546.0 nm wavelength were generated from another continuous-wave DFB laser diode after traversing a Mach-Zehnder modulator (MZM). The MZM is biased at the quadrature point and is driven by 5.0 Gb/s RZ electrical pulses in an alternate pattern.

The optical circuits of the o-Sampler, o-Encoder [START_REF] Ribeiro | Proposal and Design of an All-Optical Encoder for Digitising Radio-over-Fibre Transceivers[END_REF] and o-Thresholder [START_REF] Ribeiro | A Thresholder for All-Optical Digitising Radio-over-Fibre Transceivers[END_REF] are based on Sagnac interferometers using fibre-couplers with coupled-ratio equal to 50:50 thus allowing a maximum interference visibility contrast. The SOA is modelled as to be isotropic. The polarisation of the clockwise (CW) and counter-clockwise (CCW) signals is assumed to be always aligned. Some commercially available SOAs indeed present polarisation dependence as low as 1 dB [START_REF]Nonlinear SOA from Amphotonix[END_REF]. The polarisation alignment can be easily achieved in practice by using polarisation controllers (PC) in the fibre-optic circuits or by using polarisation-maintaining fibres and components.

B. The design of the o-Sampler

Figure 2 depicts the proposed fibre-optic circuit design of the o-Sampler. The circuit uses two 3-dB fibre-couplers (FC) and three optical delay lines. The "Delay 0" is used to perform the temporal alignment between the analogue and the sampling pulses (see Figures 7 and8). Each single analogue cycle can be adjusted to symmetrically encompass 5 sampling pulses (as is shown in Figure 8). The perimeter of the Sagnac fibre-ring must be kept constant and equal of an integer number of periods of the analogue signal. Therefore, the "Delay 1" and "Delay 2" allows fine and independent adjustments of the asymmetric placement "x" of the SOA in reference of the midpoint of the ring. In other words, the "Delay 1" and "Delay 2" allow independent adjustment of temporal coincidence of the CW and CCW analogue signal when they propagate along the SOA excited by the sampling pulses. The polarisation controller PC allows initial adjustment of the ring to be 100% reflective when the control power (sampling pulses) is turnedoff, i.e. P C = 0 mW. The sampled pulses are collected from the transmission arm (P T ) of the interferometer. The band-pass filters BPF centred in λ 1 reduce the ASE noise from the SOA and can block the sampling pulses light (λ 2 ). As a result, the transmitted P T and the reflected P R powers and waveforms are both measured at the same wavelength λ 1 of the input analogue signal P IN .

The power of the transmitted (P T ) sampled pulses of an ideal Sagnac interferometer is given by the transfer function (1) [START_REF] Miyoshi | All-Optical Analog-to-Digital Conversion Using Splitand-Delay Technique[END_REF][START_REF]Applications in Nonlinear Fiber Optics[END_REF].

P T = ½ P IN [1-cos(2γL SOA P C )] (1)
In ( 1), Δφ = 2γL SOA P C means the XPM-generated differential phase-shift between the CW and CCW [START_REF] Miyoshi | All-Optical Analog-to-Digital Conversion Using Splitand-Delay Technique[END_REF] analogue signals, respectively. The Δφ phase-shift depends on the optical nonlinear strength γ and the length L SOA of the SOA inserted in the ring, and the peak power P C of the launched sampling pulses. The SOA inserted in the Sagnac ring is an active element and the output signal will experiment an optical gain, i.e. P T > P IN .

In the absence of sampling pulses, Δφ = 0 rad, the SLALOM acts as a perfect mirror (after a proper adjustment of the PC) for the analogue signal that is totally reflected back, i.e. P T = 0 mW according [START_REF] Williamson | RF Photonics[END_REF]. The sampling pulses are launched by means of a fibre-coupler (3 dB-FC) and propagate along the SOA thus imprinting a phase-shift in a very small portion of the analogue signal waveform. Such phase-shift occurs due to the XPM effect that takes place into the SOA. Since the SOA is asymmetrically placed in the Sagnac ring, the Δφ phase-shift is imprinted in slightly different parts of the CW and CCW analogue waveforms. This change in phase means that during the timewidth of the sampling pulses the Sagnac loop turn to be transmitting. However, the SOA response should not be necessarily fast. As is better explained in the next section, by adjusting the "Delay 1" (and also the "Delay 2") at few picoseconds, say 10-20 ps, it opens a switching window and the switching efficiency may be controlled at some extent. It should be observed that the real peak power P C of the sampling pulses is the half of the launched into the circuit (see Figure 2). The reason is because a conventional 3 dB fibreoptic coupler is used instead of a WDM-coupler to launch the control power into the SOA. Before the simulations of the optical sampling itself, the PC (or the initial relative optical phase between CW and CCW) is adjusted to set the Sagnac ring to be 100% reflective without sampling pulses. Figure 5 shows the plot of the reflected (R) and transmitted (T) analogue signal for P C = 0 mW. The reflected analogue signal is amplified by 20.8 dB thus reaching 18 mW. Hereafter, the delay adjustments were set to be Delay 0 = 300-ps, Delay 1 = 2.000-ns, Delay 2 = 6.000-ns and the sampling pulses were turned-on at P C = 15 mW (or effectively at 7.5 mW). It must be observed that Delay 1 + Delay 2 = 8.000-ns, i.e. 8-ns/2-ns = 4 analogue cycles fitted along the Sagnac ring perimeter. Therefore, the sampling pulses could imprint phase-shifts in the same temporal position of the analogue CW and CCW signals. As a result, almost none transmitted sampled pulses were achieved, i.e. the central sampled pulse at < 0.2 mW peak power with the envelope buried in some noise were calculated.

By slightly detuning the optical delays as to be Delay #1, i.e. Delay 1 = 1.995-ns and Delay 2 = 6.005-ns, the switching efficiency dramatically increases and the central sampled pulse reaches 2 mW peak power, an increase of > 10 dB when compared with previous setting. A further slightly detuning as to be now Delay #2, i.e. Delay 1 = 1.990-ns and Delay 2 = 6.010-ns, enables an even larger switching efficiency now reaching 4.5 mW peak power, i.e. an increase of 13.5 dB.

Figure 6 comparatively shows the central sampled pulses in a higher temporal resolution for both Delay #1 and Delay #2 under P C = 15 mW, reaching 2.0 mW and 4.5 mW peak power, respectively. Both pulses are also compressed to reach 20-ps timewidth, i.e. the half in comparison with the sampling pulses. A small temporal shift of ~ 8-ps between the pulses is observed. However, the ratio of 8-ps/2-ns = 0.4% is in principle regarded to be negligible when compared with the period of the analogue waveform. From few µW to few mW peak power range of sampling control pulses, the calculated sampled pulses were unstable. This is because the SOA is far to exhibits the gain saturation and the nonlinearity strength vary from pulse-to-pulse. However, starting from P C = 15 mW, stable sampled pulses could be calculated to exit the o-Sampler when the P C is increased. This is because at 15 mW, a steady state is already reached after the partial carrier depletion in SOA due the relatively high power of the sampling pulses. Figure 7 also shows that the dip of the "sliced" reflected signal has not the same value of the amplitude of the corresponding sampled pulses. The carrier depletion in the semiconductor under light pump is a very fast process whereas the generation of the sampled pulses follows the switching window after setting the Delays #1 and #2. Therefore the energy that is switched from the each "slice" of the reflected signal is essentially the same of that is contained in the corresponding sampled pulse. When the switching window closes, the sampled pulse is already shaped and the SOA gain begins to slowly recovering until the arrival of the next sampling pulse in a steady-state regime. The optical components required to set-up the circuit shown in Figure 2 are similar of those used in an optical sampler based on the nonlinear polarisation rotation [START_REF] Zhang | All-optical sampling based on nonlinear polarization rotation in Semiconductor Optical Amplifiers[END_REF][START_REF] Zhang | All-optical sampling exploiting nonlinear polarization rotation in a single semiconductor optical amplifier[END_REF]. In the present fibre-based scheme, none polarisationdiscriminator device is needed and only a single PC is required to set P T = 0 mW for P C = 0 mW. Another interesting feature of the present o-Sampler is the observation that the sampled pulses are compressed from 40-ps of the sampling pulses to the 20-ps timewidth. The temporal compression of pulses launched in Sagnac rings is expected because when a sampling pulse starts and ends (low pump power), the differential phase-shift is very low and then P T ≅ 0 mW. When the pump power increases (and decreases), the increase (decrease) in the transmitted power is not linear thus leading the compression of sampled pulses. However, such compressed pulses are useful for the next sub-module of the o-DRoF-T, i.e. the 3' and 3'' o-Interleavers [START_REF] Ribeiro | Proposal and Design of an All-Optical Encoder for Digitising Radio-over-Fibre Transceivers[END_REF]. The reduced timewidth allows more densely optical multiplexing in time and then an increase in the serialised bit resolution of the o-DRoF-T [START_REF] Ribeiro | Proposal and Design of an All-Optical Encoder for Digitising Radio-over-Fibre Transceivers[END_REF].

The SOAs itself are relatively complex devices to be designed. It should be treated not only as an optical amplifier but also as a nonlinear waveguide where analogue signal and sampling pulses are propagating and interacting with each other. The dynamics of operation are not trivial. Besides those parameters here adjusted, many others can or should be simultaneously taken into account: wavelengths of the analogue signal and sampling pulses, the use of a continuous wave holding beam [START_REF] Manning | Three-wavelength device for alloptical signal processing[END_REF] to obtain gain saturation and faster nonlinearities, semiconductor structure and length of the SOA, etc.

V. CONCLUSIONS

To the best of our knowledge, neither experimental nor commercially available o-DRoF links or networks have been reported as they are still under R&D stage. This paper proposes a transmitter module comprising at least five optical sub-modules for use in an o-DRoF link. The o-DRoF-T is potentially stable in hundreds of GSa/s due the use of a modelocked laser as the clock signal and is able to deals with RoF (optical) signal, i.e. none optoelectronic conversion is needed. It was proposed, designed and simulated here only the o-Sampler sub-module under control pulses launched in a single direction. The circuit operate at 1550 nm wavelength window. It provides some useful characteristics: compactness, prone to optical integration, requirement of few mW optical power for sampling pulses, polarisation independence, linearity and amplified output. D. Cotter et al also describe the realization of a SLALOM circuit on a single 1x2 mm InP semiconductor chip [START_REF] Cotter | Nonlinear Optics for High-Speed Digital Information Processing[END_REF] that supports the possibility of optical integration of the proposed o-DRoF transmitter.

In this paper, the simulations were constrained to only 2.5 GHz & 40-ps RZ input sampling pulses mainly to demonstrate the proof-of-principle of the proposed real-time o-Sampler able to produce 2.5 GS/s sampled pulses. However, SOAs operating at 80 Gb/s are already commercially available [START_REF]Nonlinear SOA from Amphotonix[END_REF]. Furthermore, SOAs incorporated in fibre circuits in R&D stage able to operate up to 160 Gb/s [START_REF] Szuki | Error-free demultiplexing of 160 Gbit/s pulse signal using optical loop mirror including semiconductor laser amplifier[END_REF] and 320 Gb/s [START_REF] Liu | Error-Free 320-Gb/s All-Optical Wavelength Conversion Using a Single Semiconductor Optical Amplifier[END_REF] were already reported.

Simulations and experiments of the o-Sampler operating at higher sampling pulse rate with shorter timewidth, other independents sub-modules of the proposed o-DRoF transmitter as well such transmitter as a whole are currently planned or in progress and will be presented in a near future.

Figure 1

 1 Figure 1 depicts a flowchart of the basic transmitter module architecture of an o-DRoF link [7,8,19,20] able to digitise RoF signals. The o-DRoF-T comprises at least five main optical submodules: (1) optical pulse generator or o-Generator, (2) optical sampler or o-Sampler, (3') and (3'') temporal interleavers or o-Interleaver, (4) optical encoder or o-Encoder[START_REF] Ribeiro | Proposal and Design of an All-Optical Encoder for Digitising Radio-over-Fibre Transceivers[END_REF] and (5) optical thresholder or o-Thresholder[START_REF] Ribeiro | A Thresholder for All-Optical Digitising Radio-over-Fibre Transceivers[END_REF]. The o-Sampler carries out the real-time optical sampling of an input analogue RoF signal and is driven by an optical pulse train that in turn is generated by the o-Generator. This pulse train is also used as a reference clock signal by the o-Encoder that optically encodes the sampled signal, thereby accomplishing the photonic AD-

Fig. 1 .

 1 Fig. 1. The flowchart of the basic o-DRoF-T. The o-Sampler sub-module treated is this paper is highlighted and is numbered as (2).

Fig. 2 .

 2 Fig. 2. The fibre-optic circuit design of the o-Sampler.

Figure 3

 3 Figure 3 shows the plot of the sinusoidal analogue signal P IN with 150 µW amplitude at carrier frequency of 500 MHz or 2-ns period that is launched into the o-Sampler circuit. The P IN is assumed to be a RoF signal not carrying modulation on the RF carrier. The aim here is only to show the possibility of sampling with the circuit design shown in Figure 2.

Fig. 3 .

 3 Fig. 3. The analogue RoF sinusoidal waveform at 500 MHz frequency as to be the input signal into the o-Sampler circuit.

Figure 4

 4 Figure 4 shows the plot of the sampling pulses at P C = 80 µW peak power at 2.5 GHz repetition rate and 40-ps timewidth.

Fig. 4 .

 4 Fig. 4. The control signal at 2.5 GHz & 40-ps as the sampling pulses launched into the o-Sampler circuit.

Fig. 5 .

 5 Fig. 5. The reflected (R) and transmitted (T) analogue signal from the o-Sampler for PC = 0 mW after polarisation adjustment.

Fig. 6 .

 6 Fig. 6. The central sampled pulses for the Delay #1 and Delay #2 setting at PC = 15 mW.

Figure 7

 7 Figure7comparatively shows the transmitted "T" sampled pulses enveloped by the analogue signal waveform that coincides in time with the "sliced" reflected "R" signal.

Fig. 7 .

 7 Fig. 7. The transmitted sampled (T) pulses and the "sliced" analogue reflected (R) signal at PC = 15 mW.

Figure 8

 8 Figure 8 shows the normalised plots of the five sampled pulses that are contained and enveloped by a single cycle of the analogue signal using P C = 15 mW. It demonstrates the reasonably linearity of the proposed o-Sampler to carry out the sampling process.

Fig. 8 .

 8 Fig. 8. The normalised plots of the sampled pulses enveloped by the analogue signal at PC = 15 mW.

ACKNOWLEDGMENT Ricardo M. Ribeiro thanks the team at Département d'Optique of Telecom Bretagne and the CapilR TM platform for hosting his Post-Doctoral study during which the present work was carried out. The authors also thank INCT-Fotonicom/CNPq.


Julien Cornut
email: cornut@igb-berlin.de

Arnaud Elger

Axel Greugny

Maelys Bonnet

Eric Chauvet

Coarse particulate organic matter in the interstitial zone of three French headwater streams

Keywords: Organic matter, hyporheic zone, river sediment, headwater stream, litter breakdown

Headwater woodland streams are primarily heterotrophic: they receive substantial inputs of organic matter from the riparian vegetation, while autochthonous primary production is generally low. A substantial part of leaf litter entering running waters may be buried in the streambed because of flooding and sediment movement. Although the general significance of the hyporheic zone for stream metabolism has been reported early, organic matter storage within the sediment of streams has received less attention, with most studies only quantifying accumulations at the streambed surface and ignoring other stream compartments. In the present study, the amounts of three fractions of coarse particulate organic matter (CPOM; > 16, 4-16 and 1-4 mm) were determined in late autumn and early spring in the interstitial and benthic zones of three headwater streams of the Montagne Noire (South-Western France) differing in their substratum grain size. Our findings demonstrated that the total CPOM content in the interstitial zone can be much (up to one order of magnitude) higher than at the sediment surface. The sandy bottomed stream exhibited a higher amount of CPOM (whatever the size fraction) than the two other streams, suggesting that the sediment particle size may be a major determinant of CPOM storage. Given the large amount of organic matter stored in the interstitial zone, this compartment may play an important role for the carbon turnover and associated trophic dynamics in the stream ecosystem.

Introduction

Low-order forested streams, where light limitation restricts primary production, rely upon the input of organic matter from the riparian zone to fuel in-stream processes [START_REF] Vannote | The river continuum concept[END_REF]. Thus, benthic coarse particulate organic matter (CPOM; e.g. leaves, wood and twigs) is a pivotal component of the functioning of these streams. It often forms the basis of the trophic structure of streams, being the major source of organic matter and energy in woodland stream ecosystems [START_REF] Cummins | Shredders and riparian vegetation[END_REF]. The decomposition of this organic matter is a key ecosystem-level process integrating the activities of both microbial and invertebrate decomposers [START_REF] Gessner | Importance of microfungi in controlling breakdown rates of leaf litter[END_REF][START_REF] Suberkropp | Microorganisms and organic matter processing[END_REF][START_REF] Grac¸a | The role of invertebrates on leaf litter decomposition in streams -a review[END_REF][START_REF] Hieber | Contribution of stream detritivores, fungi, and bacteria to leaf breakdown based on biomass estimates[END_REF]. The accumulations of litter also provide physical structures, affecting geomorphology [START_REF] Keller | Effects of large organic material on channel form and fluvial processes[END_REF] and serving as a habitat for organisms [START_REF] Malmqvist | Dynamics of detritus in a small stream in southern Sweden and its influence on the distribution of the bottom animal communities[END_REF][START_REF] Short | Benthic detritus dynamics in a mountain stream[END_REF].

During the last four decades, studies on the dynamics of organic matter in streams have mostly been limited to benthic habitats, i.e., with processes occurring above the streambed surfaces [START_REF] Kaushik | The fate of the dead leaves that fall into streams[END_REF][START_REF] Webster | Vascular plant breakdown in freshwater ecosystems[END_REF][START_REF] Jones | Benthic organic matter storage in streams: influence of detrital import and export, retention mechanisms, and climate[END_REF]. In parallel, the general significance of a second compartment, the hyporheic zone, was reported [START_REF] Orghidan | Ein neuer Lebensraum des unterirdischen Wassers: der hyporheische Biotop[END_REF][START_REF] Schwoerbel | U ¨ber die Lebensbedingungen und die Besiedlung des hyporheischen Lebensraumes[END_REF][START_REF] Schwoerbel | Die Wassermilben (Hydrachnellae und Limnohalacaridae) als Indikatoren einer biozonotischen Gliederung von Breg und Brigach sowie der obersten Donau[END_REF][START_REF] Williams | The occurrence of benthos deep in the substratum of a stream[END_REF][START_REF] Triska | Retention and transport of nutrients in a third-order stream in northwestern California: hyporheic processes[END_REF][START_REF] Gibert | Surface water-groundwater ecotones[END_REF][START_REF] Vervier | A perspective on the permeability of the surface freshwater-groundwater ecotone[END_REF][START_REF] White | Perspectives on defining and delineating hyporheic zones[END_REF] and the major contribution of the hyporheic zone to stream metabolism was also reported by [START_REF] Grimm | Exchange between interstitial and surface waters: implications for stream metabolism and nutrient cycling[END_REF]. In the same time, authors such as [START_REF] Webster | Analysis of potassium and calcium dynamics in stream ecosystems on three southern Appalachian watersheds of contrasting vegetation[END_REF], [START_REF] Webster | Effects of watershed perturbation on stream potassium and calcium dynamics[END_REF] and [START_REF] Newbold | Measuring nutrient spiralling in streams[END_REF][START_REF] Newbold | Organic carbon spiralling in stream ecosystems[END_REF] introduced the concepts of nutrient and carbon spiralling describing the simultaneous processes of downstream movement with patterns of immobilization and mineralization. This spiralling concept, stressing that streams are prime sites of matter retention and processing, strongly contrasted with the reductionist view of other researchers as reported by [START_REF] Boulton | River ecosystem health down under: assessing ecological condition in riverine groundwater zones in Australia[END_REF], considering these lotic ecosystems only as "pipes" conducting water, solutes and organic matter from the catchment downstream. Organic matter storage within stream sediments has received less attention than at the surface of sediments. However large standing stocks of particulate organic matter (POM) may occasionally be buried in sediments and significantly contribute to hyporheic metabolism [START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF][START_REF] Sobczak | Relationships between bacterial productivity and organic carbon at a soilstream interface[END_REF], and therefore to the overall ecological functioning of headwater streams.

Relatively little work has attempted to quantify POM buried in the sediments of headwater streams as usually it occurs following spates or other depositional events (but see [START_REF] Leichtfried | Organic matter in gravel streams (Project Ritrodat-Lunz)[END_REF][START_REF] Leichtfried | Bacterial substrates in gravel beds of a second order alpine stream (Project Ritrodat-Lunz)[END_REF][START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF][START_REF] Smock | Spatial and temporal variation in organic matter storage in low-gradient, headwater streams[END_REF][START_REF] Bretschko | The limnology of a low order alpine gravel stream (Ritrodat-Lunz study area, Austria)[END_REF][START_REF] Wagner | The hyporheic habitat of the Breitenbach, spatial structure and physicochemical conditions as a basis for benthic life[END_REF][START_REF] Jones | Vertical hydrologic exchange and ecosystem metabolism in a Sonoran Desert stream[END_REF][START_REF] Naegeli | POM-dynamics and community respiration in the sediments of a floodprone prealpine river (Necker, Switzerland)[END_REF]. In some of these studies, authors have evaluated POM in the sediments by analysing the total organic carbon (TOC) without really paying attention to the relative contribution of the different size fractions [START_REF] Leichtfried | Organic matter in gravel streams (Project Ritrodat-Lunz)[END_REF][START_REF] Leichtfried | Bacterial substrates in gravel beds of a second order alpine stream (Project Ritrodat-Lunz)[END_REF][START_REF] Bretschko | The limnology of a low order alpine gravel stream (Ritrodat-Lunz study area, Austria)[END_REF]. [START_REF] Wagner | The hyporheic habitat of the Breitenbach, spatial structure and physicochemical conditions as a basis for benthic life[END_REF] have investigated the deposition of POM over a period of 9 months, but only from the upper 3 cm of streambed, while [START_REF] Jones | Vertical hydrologic exchange and ecosystem metabolism in a Sonoran Desert stream[END_REF] have measured deep storage of FPOM ( > 2 cm depth) by digesting sediments with persulphate and analysing the resulting release of CO 2 . [START_REF] Naegeli | POM-dynamics and community respiration in the sediments of a floodprone prealpine river (Necker, Switzerland)[END_REF] have quantified POM within the sediment, but only considered the relative contribution of POM size fractions from < 63 mm to 4 mm (their sampling device was equipped with 4 mm mesh size, thereby limiting their investigations). The studies by [START_REF] Smock | Spatial and temporal variation in organic matter storage in low-gradient, headwater streams[END_REF] and [START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF] appear as the most complete in this regard, as they were performed to measure CPOM inputs and storage both in the benthic and hyporheic compartments of headwater streams. However, they encompassed a reduced range of sediment grain-size characteristics (i.e., sandy and silt-clay bottomed streams).

Usually, fine-grained stream sediments contain larger amounts of stored detritus than streams with coarse sediments [START_REF] Brunke | The ecological significance of exchange processes between rivers and groundwater[END_REF]. However, results from the few available studies also report contradictory or less clear patterns. Therefore, conclusions that the retention of POM in sandy-bottomed streams is more efficient than in gravel bed streams must be made cautiously [START_REF] Brunke | The ecological significance of exchange processes between rivers and groundwater[END_REF]. Moreover, because of the heterogeneous nature of the sediment matrix and of discharge variability, the characteristics of the interstitial zone tend to vary widely following temporal and spatial patterns, as well as from one system to another [START_REF] Brunke | The ecological significance of exchange processes between rivers and groundwater[END_REF].

Some studies suggest that organic matter content within the interstitial zone may be considerably higher compared with the one at the sediment surface, partly because buried organic matter has a lower decomposition rate [START_REF] Herbst | Effects of burial on food value and consumption of leaf detritus by aquatic invertebrates in a lowland forest stream[END_REF][START_REF] Rounick | Leaf processing in 2 contrasting beech forest streams -effects of physical and biotic factors on litter breakdown[END_REF][START_REF] Cornut | Early stages of leaf decomposition are mediated by aquatic fungi in the hyporheic zone of woodland streams[END_REF] and is less subject to downstream transport during storms and floods. For the streams in which both benthic and hyporheic CPOM amounts have been measured, the latter account for 25-90% of the total stored organic matter [START_REF] Cummins | Organic matter budgets for stream ecosystems: problems in their evaluation[END_REF][START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF]. Therefore, the hyporheic zone is potentially a major site for organic matter storage, and [START_REF] Jones | Benthic organic matter storage in streams: influence of detrital import and export, retention mechanisms, and climate[END_REF] recommended considering subsurface storage in future studies to determine its importance in a range of streams. Accounting for spatial and temporal variability in organic matter storage is also needed to better understand the mechanisms that control the structure and functioning of these ecosystems.

The present study was performed to measure CPOM (>1 mm) content in three low-order headwater streams of the Montagne Noire (South-Western France) varying in grain-size characteristics. The spatial variability of CPOM contents, above and within stream sediments, was examined at two different sampling dates (late autumn and early spring). The following hypotheses have been tested: (1) a substantial part of CPOM should be stored within streambed sediments and (2) the relative importance of allochthonous organic matter stored in interstitial versus benthic habitats should differ among streams, notably due to contrasting grain-size characteristics of their streambed.

Methods

The study was carried out in the Montagne Noire, South-Western France, a 1450-km 2 region covered by a mixed broadleaf forest with an altitudinal range of 250-1211 m a.s.l. Climatic conditions are marked by high rainfall (average 1500 mm.year x1 ). Three first-or secondorder permanent streams with similar physical and chemical characteristics were selected in forested areas, situated between 02x09'25''E and 02x19'51''E longitude and 43x21'15''N and 43x27'03''N latitude. The riparian zones are covered by broadleaf forests dominated by oak (Quercus petraea (Mattus.) Liebl.), hazel (Corylus avellana L.), alder (Alnus glutinosa (L.) Gaertn.), ash (Fraxinus excelsior L.), sweet chestnut (Castanea sativa Mill.) and beech (Fagus silvatica L.). Tree density along the three stream reaches (0.42-0.44 m x1 , in the Be´al and the Bergnassonne, respectively; > 0.15 m diameter at breast height, 5 m stream distance) and canopy cover of the streambed (85-95%, in the Be´al and the Bergnassonne, respectively) were similar.

Water chemistry was determined at two sampling dates (i.e., late autumn: 25 November 2009 and early spring: 21 March 2010), in three representative riffles per stream, both at the surface of the sediment, and in the interstitial zone (i.e., at 15 cm below the sediment surface) with a specific sampling device using a hand-held vacuum pump. PVC cylinders (10 cm length and 12 mm inner diameter) were pierced (2.5 mm diameter hole r 50) and capped at bottom with a plastic stopper. A length of plastic tubing (Tygon, 5 mm inner diameter) was hermetically sealed at the top of each cylinder to allow sampling of interstitial water, and kept isolated from surface water with a clamp. Two cylinders were carefully introduced in the sediment of each of three representative riffles per stream, by using a piezometer pushed to 20 cm depth into streambed using an internal metallic rod, as described in [START_REF] Baxter | Measuring groundwater-stream water exchange: new techniques for installing minipiezometers and estimating hydraulic conductivity[END_REF]. The middle of each cylinder was approximately 15 cm below the streambed surface. The cylinders were installed several months before water sampling to allow the initial hydraulic properties to stabilize, thereby avoiding any bias due to exchanges between surface and interstitial waters. At each sampling date, 250 mL of interstitial water per riffle was slowly collected from cylinders with the hand vacuum pump placed at the end of the Tygon tubing. Temperature, pH, conductivity and dissolved oxygen concentration were measured in the field using portable instruments (pH-meter 320i and Oxi 330i, WTW, Weilheim, Germany; Conductimeter Dist5, HANNA, Woonsocket, Rhode Island, USA). Water samples were filtered in the field with 0.7 mm glass fibre filters (Glass fibre GF/F, Whatman, Clifton, New Jersey, USA), stored in pre-rinsed polyethylene bottles and placed in an icebox until they were returned to the laboratory. Concentrations of PO 4 measured as soluble reactive phosphorus (SRP), NO 3 , NO 2 and NH 4 , were determined using standard colorimetric methods by flow injection analysis with an Alpkem Flow Solution IV system (OI Analytical, College Station, Texas, USA). Alkalinity was determined using potentiometric titration.

In the rivers studied, the channel consists of a sparsely extended sediment layer (ca. 15 cm on average) above an impermeable layer (bedrock). Also, the interstitial zone only occurs below the streambed, but not laterally into the parafluvial zone below the banks and floodplain. Thus, the lateral expansion of the interstitial zone into the stream banks was not considered. Therefore, the interstitial zone is influenced and exclusively maintained by advected surface water.

CPOM at the channel surface and subsurface was quantified using a Surber net (30 cm r 30 cm; 500 mm mesh size) and a coring device similar to those described previously by [START_REF] Cummins | An evaluation of some techniques for the collection and analysis of benthic samples with special emphasis on lotic waters[END_REF], [START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF] and [START_REF] Smock | Spatial and temporal variation in organic matter storage in low-gradient, headwater streams[END_REF]. The corer used in our study (80 cm lengthr 16 cm inner diameter) was specially designed to sample sediments to a depth of 15 cm below the streambed surface (i.e., the average depth of the whole sediment layer before bedrock is encountered). This allowed us to obtain a complete estimation of the organic matter stored in the interstitial zone.

The amount of CPOM stored in the sediment and that accumulated at the sediment surface were referred to the same surface area basis (1 m 2 ). The number of samples has been doubled for the subsurface zone compared with those made at the surface of the sediment to partly compensate for the difference in surface areas sampled by both devices (i.e., Surber net: 0.09 m 2 and corer: 0.02 m 2 ). Thus, three samples at the sediment surface and six in the interstitial zone have been collected at each of the three selected riffles in the three streams. Each riffle has been virtually divided into nine zones (i.e., 3 sections in the length r 3 zones in the width). Random selection of one sampling zone at each of the three streams was previously achieved at the laboratory by using computer software, before to be sampled. Before each sediment sampling, all CPOM at the surface of the sediment was carefully removed and discarded.

Sediments and detritus including fine and coarse POM were cautiously excavated to a depth of 15 cm, using the PVC corer to prop up the edges of the hole and divert the water current. In addition, a hand vacuum pump with silicon tubing (10 mm inner diameter) was used to collect remaining organic matter in suspension and at the bottom of the hole before removing the coring device. Samples from both benthic and interstitial zones were stored individually in plastic zip-lock bags and transported to the laboratory in an icebox.

At the laboratory, organic matter was elutriated from the sediment sample by repeated washes and all material retained on a 1 mm sieve (CPOM) was sorted and divided into three categories as proposed by [START_REF] Minshall | Interbiome Comparison of Stream Ecosystem Dynamics[END_REF]: > 16, 4-16 and 1-4 mm. Once the organic matter had been removed from each grain size fraction, each sediment core was meticulously washed and wet-sieved through a series of mesh sizes (20, 10, 5, 2, 1, 0.5 and 0.25 mm).

The CPOM of each category and the remaining sediment from each grain size were placed into an oven at 105 xC until constant mass and weighed to the nearest 0.01 g. CPOM contents from both surface samples (benthic zone) and sediment cores (interstitial zone) were expressed in grams of dry mass per square meter of streambed (taking into account the uppermost 15 cm of streambed sediment for the latter [START_REF] Naegeli | POM-dynamics and community respiration in the sediments of a floodprone prealpine river (Necker, Switzerland)[END_REF])) (Fig. 1). A non-negligible amount of POM could potentially remain within the sediment during the sieving and washing steps, especially in the finest grain size. To estimate this potential bias, the remaining organic matter has been determined by ignition (three measures per grain size fraction and per stream). This indicated that the organic matter remaining within the sediment represented less than 1% of the organic matter content, even for the finest grain size fraction.

Cumulative grain-size distribution curves (phi units) of sediments in the three streams (Fig. 1) and statistics were calculated with GRADISTAT [START_REF] Blott | Gradistat: A grain size distribution and statistics package for the analysis of unconsolidated sediments[END_REF], including the 10th, 50th and 90th percentiles, and the sorting index (Si) following the Folk and Ward method as suggested by [START_REF] Folk | Brazos River bar: a study in the significance of grain size parameters[END_REF] (Table 1). This index reveals the level of sediment transport: the more the sediment is submitted to a high disturbance frequency, the higher is the value of the Si [START_REF] Folk | Brazos River bar: a study in the significance of grain size parameters[END_REF]. A three-way factorial analysis of variance (ANOVA) was used to assess differences in CPOM content and physicochemical variables among streams, compartment (i.e., benthic or interstitial zones) and sampling date. When significant differences were detected, Tukey HSD tests were then carried out for post-hoc pairwise comparisons. Data were log-transformed to improve homoscedasticity when necessary to meet the assumptions of ANOVA. STATISTICA 6.0 (StatSoft Inc., 2001) was used for all statistical analyses. Differences were considered significant when P < 0.05.

Results

The streams had very similar geomorphologic and hydrologic characteristics, especially in terms of water depth (j 0.4 m), width (j 2.3 m) and discharge (75-166 L.s x1 ). The streambed substrata were unconsolidated and mostly made of sandy coarse gravel to coarse gravel sediments (as determined from grain size fractionation of sediment cores) for the Bergnassonne and the Orbiel, while the Be´al had a streambed substratum mainly made of sand and very fine gravel (Table 1). In addition, the Si showed a narrower range of grain size in the sediments from the Orbiel (Si: 0.097) compared with the Bergnassonne (Si: 0.611) and, in a more pronounced manner, to the Be´al (Si: 1.397; Fig. 1). The streambed sediment of the latter is characterized by a high frequency of transport and thus disturbance. In contrast, the Orbiel sediments showed the largest range of particle size distribution, denoting their high stability. The Bergnassonne exhibited an intermediate value of Si suggesting a frequency of streambed sediment disturbance intermediary between the two others. The three stream waters were slightly acidic (pH 6.0-7.0) with low conductivity (28-58 mS.cm x1 ) and relatively low alkalinity (3.1-9.6 mg CaCO 3 .L x1 ). Concentrations of SRP (2.0-4.2 mg.L x1 ) were low, while concentrations of NO 3 (0.34-1.75 mg.L x1 ) were relatively high, with significant differences among streams (ANOVA, F 2,24 = 40.88, P < 10 x6 ). SRP content of interstitial waters from the three streams were systematically higher than those of surface waters (ANOVA, F 1,24 = 6.57, P = 0.02). In contrast, NO 3 concentration was lower in the interstitial compartment (ANOVA, F 1,24 = 19.15, P < 10 x3 ). NO 3 concentration in both surface and interstitial waters followed the same pattern among streams, with Be´al and Bergnassonne exhibiting quite similar NO 3 content, contrasting with the lower content found for Orbiel. Finally, NO 3 concentrations for the three streams at the second sampling date were more than twice as high as at the first one (ANOVA, F 1,24 = 301.81, P< 10 x6 ), while concentrations of SRP remained quite similar (ANOVA, F 1,24 = 1.59, P = 0.22). Conductivity and alkalinity of interstitial waters from the three streams were consistently higher than those of surface waters (ANOVA, F 1,24 = 16.78, P < 10 x4 ; ANOVA, F 1,24 = 15.28, P< 10 x4 , respectively). At both dates, the pH of interstitial waters from the three streams were lower than that of surface waters (ANOVA, F 1,24 = 54.90, P < 10 x6 ). Moreover, between streams, differences in terms of pH were more pronounced in the interstitial compared with the surface compartment, with Orbiel exhibiting the most acidic water (ANOVA, F 1,24 = 54.90, P < 10 x3 ). The surface waters of the three streams were always well oxygenated (Table 1) and exhibited very similar values. Dissolved oxygen in interstitial waters was significantly lower (51-79% of the saturation; ANOVA, F 2,24 = 109.58, P< 10 x6 ), but still relatively similar among streams. The three streams differed in terms of average temperature (ANOVA, F 2,24 = 140.51, P < 10 x6 ), with 9.3, 7.9 and 6.9 xC for the Be´al, the Bergnassonne and the Orbiel, respectively. Temperature was also higher in the interstitial than in the benthic zone. However, this difference (i.e., nearly 1.5 xC) was on average only significant for the Orbiel (ANOVA, F 2,24 = 7.93, P < 10 x3 and HSD test).

CPOM content in the interstitial zone was higher than at the sediment surface (ANOVA, F 1,96 = 7.07, P< 10 x2 ; Fig. 1 andTable 2). Indeed, it was up to one order of magnitude higher (e.g., Be´al and Bergnassonne in late autumn). The total amount of CPOM markedly differed among streams (ANOVA, F 2,96 = 6.85, P < 10 x2 and HSD test; Fig. 1), with the Be´al followed by the Orbiel exhibiting the highest amounts of the total CPOM. The proportion of the different CPOM size fractions also strongly varied among streams. Amounts of large POM (i.e., > 16 mm) in the Be´al were significantly higher than in the Bergnassonne, but did not differ from the Orbiel (ANOVA, F 2,96 = 4.26, P = 0.017 and HSD test; Table 2). The same patterns were found for both compartments and seasons. In contrast, the amount of CPOM between 4 and 16 mm was markedly higher in the interstitial zone than at the surface of sediment (ANOVA, F 1,96 = 28.44, P< 10 x6 ). Moreover, our results showed differences among streams, with the Be´al having higher amounts of this type of particles compared with the other streams (ANOVA, F 2,96 = 4.27, P= 0.016 et HSD test). Finally, the same although more contrasted patterns were observed with the finest POM (1-4 mm). As previously, results showed marked differences among streams with once again the Be´al having the higher content for this type of POM (ANOVA, F 2,96 = 6.87, P < 10 x2 ), while the two other streams exhibited very similar fine POM contents. Moreover, differences in terms of fine POM contents were also found between compartments (ANOVA, F 1,96 = 5.98, P= 0.016) and seasons (ANOVA, F 1,96 = 15.88, P< 10 x5 ), with an higher amount in the interstitial zone in late autumn.

Table 3 shows the mean relative amount of the three CPOM size-classes, measured at the sediment surface and in the interstitial zone of the three streams. Patterns were comparable between the two sampling dates for the Be´al and Orbiel taken separately, whereas the one for Bergnassonne was much less well defined. The interstitial zone of the Be´al exhibited a higher amount of coarse and intermediate POM size classes (i.e., > 16 mm and 4-16 mm, respectively) than the surface one, but also on average when compared with the two other streams. This pattern was much more contrasted with regard to the finest POM size class. Interestingly, the amounts of POM for intermediate and fine size-classes were relatively higher in the interstitial than in the benthic zone of Orbiel, while the coarsest POM showed the opposite pattern. Finally, it was difficult to identify any trend for the Bergnassonne, Table 1. Physicochemical characteristics of water from the benthic and interstitial zones of the three streams, measured at each date (late autumn and early spring, values are means ¡ SE, n = 6 per stream and per compartment) and sediment physical descriptors. The d50 value represents the median grain diameter, whereas d10 and d90 represent the grain diameters at the 10th and 90th percentile values. Si is calculated as a measure of standard deviation. partly due to the contrasted patterns between the two sampling dates.

Be´al

Discussion

CPOM in benthic versus interstitial zones

To our knowledge, this is the first study in France that attempted to evaluate the CPOM content both in the benthic and interstitial zones of headwater streams. More generally, as reported by [START_REF] Jones | Benthic organic matter storage in streams: influence of detrital import and export, retention mechanisms, and climate[END_REF], most studies of organic matter storage remain limited to the benthic layer, whereas the interstitial zone potentially constitutes a significant site for organic matter storage [START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF][START_REF] Smock | Spatial and temporal variation in organic matter storage in low-gradient, headwater streams[END_REF][START_REF] Naegeli | POM-dynamics and community respiration in the sediments of a floodprone prealpine river (Necker, Switzerland)[END_REF]. Despite a high variability between streams, the present study demonstrated that the amounts of CPOM buried in the sediment were far to be negligible. Generally, these amounts for the three streams were even higher or equivalent to those at the surface of the sediment (Fig. 1). For instance, the CPOM stored in the Be´al and the Bergnassonne in late autumn was up to one order of magnitude higher in the interstitial zone than in the benthic one. The highest amounts were found in the Be´al, a sandy bottomed stream, where almost 1.2 kg DM.m x2 was retained in the subsurface. This value is lower than those reported by [START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF] and [START_REF] Smock | Spatial and temporal variation in organic matter storage in low-gradient, headwater streams[END_REF], with 4.8 and almost 2.9 kg AFDM.m x2 , respectively. However, our value derived from punctual assessments of CPOM contents, in contrast to the cited studies reporting cumulated amounts of CPOM buried in the sediment over 1 year.

Dynamics of CPOM in the interstitial zone

The spatio-temporal distribution and the total amount of detritus stored in streams are not only related to terrestrial organic matter production, but are also dependent on the detritus retention mechanisms [START_REF] Jones | Benthic organic matter storage in streams: influence of detrital import and export, retention mechanisms, and climate[END_REF]. General characteristics of streams in terms of grain-size substrate, hydrology, watershed morphology and slope are likely to affect detritus retention in streams. Our streams were fairly similar in terms of riparian cover density, slope and water discharge, suggesting a prime role of sediment grain-size in controlling the amount of stored CPOM. Leaf retention and organic matter storage at the surface of sediments in streams generally increase with sediment grain size and bed roughness [START_REF] Webster | Experimental studies of physical factors affecting seston transport in streams[END_REF]. Due to the lack of obstruction at the surface of sediments, sandybottomed streams usually retain and store POM much less efficiently. In the present study, based solely on two snapshot samplings, the sandy-bottomed stream (i.e., Be´al) did not exhibit a different retention of CPOM in the benthic zone when compared with the two other streams, which yet showed contrasted substrate grain sizes. An examination of the Si in Table 1 reveals a narrower range of grain size in the sediments from the Be´al than from the two other streams. Figure 1 shows that the subsurface CPOM content is positively related to the Si and consequently to the disturbance frequency of the superficial layer of streambed sediment. Although POM retention in the interstitial zone may be related to factors other than sediment grain size [START_REF] Brunke | The ecological significance of exchange processes between rivers and groundwater[END_REF], our results and several other studies confirm that finegrained stream sediments generally contain larger amounts of stored detritus than streams with coarser sediments [START_REF] Leichtfried | Organic matter in gravel streams (Project Ritrodat-Lunz)[END_REF][START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF][START_REF] Wagner | The hyporheic habitat of the Breitenbach, spatial structure and physicochemical conditions as a basis for benthic life[END_REF]. However, the potentially lower decomposition rate of CPOM in the sandy sediments of the Be´al stream compared with other streams might also explain, at least partially, the higher storage of POM in the first. Indeed, burial within the substratum might impede shredders' access to leaves with the small interstitial pore spaces of the sandy sediment acting as a physical barrier, especially for the large-bodied shredders, often identified in temperate streams as being the element of decomposers consortium playing the most critical role in leaf litter decomposition [START_REF] Hieber | Contribution of stream detritivores, fungi, and bacteria to leaf breakdown based on biomass estimates[END_REF]. Conversely, large grain size is likely to permit interstitial spaces large enough to be colonized by large-bodied shredders able to drive the decomposition process of CPOM.

An examination of Figure 1 reveals that the CPOM stock was temporally variable. Indeed, CPOM content in subsurface at our second sampling date (early spring) was markedly lower when compared with the previous one, however, these differences were not statistically significant, probably because of the high variability in the Be´al. This high variability in the sandy-bottomed stream is not surprising and may be partly explained by sediment instability, which is the function of particle size. The interstitial zone of sandy-bottomed streams is generally characterized by a higher disturbance frequency, increasing its potential to store large quantities of organic matter, and a higher spatial and temporal variability of organic matter stored.

These changes in the pool of interstitial detritus may be explained both by their processing, even though rates remain generally much lower than at the surface of the sediment [START_REF] Herbst | Effects of burial on food value and consumption of leaf detritus by aquatic invertebrates in a lowland forest stream[END_REF][START_REF] Rounick | Leaf processing in 2 contrasting beech forest streams -effects of physical and biotic factors on litter breakdown[END_REF][START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF][START_REF] Cornut | Early stages of leaf decomposition are mediated by aquatic fungi in the hyporheic zone of woodland streams[END_REF], and by their transport after they have been released from subsurface as demonstrated in the study by [START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF]. These authors reported that high discharge and associated bedload movement due to a flood in late summer had dramatically reduced the amount of stored organic matter by scour removing 612 g.m x2 of detritus from the sediment at that time. Thus, and in line with the few studies that have quantified CPOM amounts in the interstitial habitat, our findings demonstrate that burial can be an important storage mechanism, particularly in streams with loose sediment and potentially high sediment disturbance frequency [START_REF] Herbst | Detrital leaf dynamics in a lowland forest stream[END_REF][START_REF] Godbout | The three-dimensional distribution of the fauna in a single riffle in a stream in Ontario[END_REF][START_REF] Cummins | Organic matter budgets for stream ecosystems: problems in their evaluation[END_REF][START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF].

In temperate forested streams, most litter inputs occur in autumn after leaf abscission, i.e., during high discharge and surface runoff following high rainfall in the autumn and winter, even though a substantial part of detritus from the stream banks and the adjacent zones may also be subject to lateral movement and therefore continue entering streams over the whole year [START_REF] Benfield | Comparisons of litterfall input to streams[END_REF]. In parallel, as reported by [START_REF] Brunke | The ecological significance of exchange processes between rivers and groundwater[END_REF] and [START_REF] Naegeli | POM-dynamics and community respiration in the sediments of a floodprone prealpine river (Necker, Switzerland)[END_REF], even though, usually most of the fine and coarse POM are carried in the sediments during floods (i.e., late autumn and winter), [START_REF] Bretschko | Transport and retention of matter in riparian ecotones[END_REF] demonstrated that in a porous gravel stream more than 80% of the annual POM input was transported into the sediment in spring and summer during high discharge fluctuations, while a small proportion (i.e., < 10%) entered after litter fall. In the same line, [START_REF] Wagner | The hyporheic habitat of the Breitenbach, spatial structure and physicochemical conditions as a basis for benthic life[END_REF] reported that organic matter content of the sediment in a small forested sandy-bottomed stream in Germany was inversely related to the discharge. Thus, as reported in the present study and previous works, the peak amount of POM in the interstitial zone of headwater streams is closely related to the nature of the streambed substrate, stream hydrology (e.g., high discharge fluctuation and current velocity) and meteorological events (Fig. 3), and is likely to occur more or less later, after litter fall and thus the maximum availability of POM in the benthic compartment.

As demonstrated by [START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF], the spatio-temporal distribution of detritus stored in the interstitial zone is also likely subject to the vertical dimension. Consequently, the total interstitial POM storage can be divided into two components, i.e., the shallow and deep ones. Shallow storage only refers to the upper layer of sediment (i.e., several top centimetres) involving POM frequently buried and released likely due to sediment instability and high disturbance frequency. Conversely, deep POM storage encompasses detritus deposited below the shallow layer occasionally scoured during rare high-flow events related to storms [START_REF] Herbst | Effects of burial on food value and consumption of leaf detritus by aquatic invertebrates in a lowland forest stream[END_REF] or as a consequence of prolonged heavy rainfalls, which are both expected to increase over the next few decades according to the simulations made by the IPCC (Intergovernmental Panel on Climate Change) (2007) assuming a doubling of atmospheric CO 2 concentrations by the year 2100. Low-gradient streams exhibiting loose, shifting streambed sediment and deeper hyporheic sediments are likely subject to deep POM storage and therefore much higher interstitial POM storage than surface storage when compared with high-gradient streams with coarser sediments and much less extended hyporheic zone [START_REF] Godbout | The three-dimensional distribution of the fauna in a single riffle in a stream in Ontario[END_REF][START_REF] Cummins | Organic matter budgets for stream ecosystems: problems in their evaluation[END_REF][START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF]. Therefore, POM deeply stored in the interstitial zone may not be released for years due to both physical immobilization and the much slower decomposition occurring in the interstitial sediment partially explained by the often anoxic conditions [START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF], thus resulting in decreased carbon spiralling lengths [START_REF] Newbold | Organic carbon spiralling in stream ecosystems[END_REF]. In higher-gradient streams, the predominance of POM shallow storage may, on the contrary, increase the carbon spiralling lengths through the faster release of total immobilized carbon sources, potentially leading to a lower in-stream stabilization of energy flow. The vertical dimension, depending on its extent, by releasing new carbon sources plays a potentially significant role in the overall stream metabolism.

Functional significance of buried CPOM

Organic matter provides the basis of the energy driving nutrient cycles within lotic aquatic ecosystems. The decomposition of this organic matter involves the activities of both microbial and invertebrate decomposers [START_REF] Gessner | Importance of microfungi in controlling breakdown rates of leaf litter[END_REF][START_REF] Suberkropp | Microorganisms and organic matter processing[END_REF][START_REF] Grac¸a | The role of invertebrates on leaf litter decomposition in streams -a review[END_REF][START_REF] Hieber | Contribution of stream detritivores, fungi, and bacteria to leaf breakdown based on biomass estimates[END_REF]. With regard to our results and previous studies [START_REF] Maridet | Vertical distribution of fauna in the bed sediment of three running water sites: influence of physical and trophic factors[END_REF][START_REF] Maridet | Spatial and temporal distribution of macroinvertebrates and trophic variables within the bed sediment of three streams differing by their morphology and riparian vegetation[END_REF][START_REF] Strayer | Oxygen, organic matter, and sediment granulometry as controls on hyporheic animal communities[END_REF], due to the temporal disconnection that may occur between the maximum of stored POM in the benthic and interstitial compartments, we hypothesized that vertical and active migration of aquatic invertebrates between the channel and the interstitial zone should occur, especially at times when supplies from the surface are scarce and accumulated detritus represent large quantities of new carbon sources. Organic matter storage within the interstitial zone of streams leads to carbon retention and reduced transport downstream. This process seems to be particularly significant in sandy-bottomed streams where organic matter storage on the streambed surface is relatively inefficient, a feature that is exacerbated during floods [START_REF] Smock | Role of debris dams in the structure and functioning of low-gradient headwater streams[END_REF]. The interstitial storage thus affects carbon spiralling lengths [START_REF] Newbold | Organic carbon spiralling in stream ecosystems[END_REF], both the transport and biological utilization components of spiralling being decreased when compared with situations where processes are limited to surface waters. The lower microbial and invertebrate utilization of buried organic matter compared with the surface is, however, potentially important for the overall stream metabolism [START_REF] Grimm | Exchange between interstitial and surface waters: implications for stream metabolism and nutrient cycling[END_REF][START_REF] Cornut | Early stages of leaf decomposition are mediated by aquatic fungi in the hyporheic zone of woodland streams[END_REF], in particular through the delayed release of dissolved organic carbon and nutrients [START_REF] Crocker | Interstitial dissolved organic carbon in sediments of a southern Appalachian headwater stream[END_REF]. More generally, this non-negligible amount of decomposing organic matter buried in the sediments, together with a more or less reduced water's travel time in the interstitial zone depending on numerous factors such as geomorphology, sediment structure and hydraulic conductivity (Fig. 3), are likely to affect the physicochemical characteristics of interstitial water (Table 1). The low levels of NH 4 in the three studied streams both in interstitial and benthic compartments can reflect the low level of mineralization of POM through the microbial activities and/or the availability of dissolved oxygen due to adequate water exchange, providing the required conditions for nitrifying bacteria to oxidize ammonium to nitrate. In parallel, leaf litter decomposition buried in the sediment occurs with a lower rate when compared with the surface environment [START_REF] Herbst | Effects of burial on food value and consumption of leaf detritus by aquatic invertebrates in a lowland forest stream[END_REF][START_REF] Rounick | Leaf processing in 2 contrasting beech forest streams -effects of physical and biotic factors on litter breakdown[END_REF][START_REF] Metzler | Storage and dynamics of subsurface detritus in a sandbottomed stream[END_REF][START_REF] Cornut | Early stages of leaf decomposition are mediated by aquatic fungi in the hyporheic zone of woodland streams[END_REF]. Then, CPOM once buried in the sediment is temporarily removed from the surface system. However, because of biogeochemical activities in the interstitial zone, transformed nutrients and fine POM are released into the groundwater and stream water depending on the direction and magnitude of vertical hydrological exchanges [START_REF] Vervier | A perspective on the permeability of the surface freshwater-groundwater ecotone[END_REF]. Thus, the interstitial zone may serve as a temporary reservoir for organic matter in streams and affect the overall pattern of stream metabolism.

Future prospects

As suggested by [START_REF] Jones | Benthic organic matter storage in streams: influence of detrital import and export, retention mechanisms, and climate[END_REF] more than a decade ago, our understanding of CPOM storage in the interstitial zone of streams still remains limited in several aspects. First, while hydrology at the stream scale has been shown to rule POM storage, local vertical hydraulic gradients at a smaller scale (i.e., riffle-scale) can also be determinant. Research in this area is needed to compare patterns and processes in upwelling and downwelling zones; while exfiltration is likely to prevent deposition of fine particle, infiltration may result in interstitial pores being filled [START_REF] Brunke | The ecological significance of exchange processes between rivers and groundwater[END_REF]. Second, storms and associated spates represent brief but ecologically relevant events. In line with [START_REF] Cummins | Organic matter budgets for stream ecosystems: problems in their evaluation[END_REF], [START_REF] Webster | Organic matter budgets for streams: a synthesis[END_REF] and [START_REF] Tank | A review of allochthonous organic matter dynamics and metabolism in streams[END_REF], who suggest to include the rare -but determinant -high-discharge events in measurements of organic matter transport, we suggest to consider subsurface storage (i.e., uppermost and deep sediment layers) in the construction of organic matter budgets. Third, inputs of CPOM, such as leaf litter, are mainly influenced by the occurrence and the type of riparian vegetation [START_REF] Maridet | Spatial and temporal distribution of macroinvertebrates and trophic variables within the bed sediment of three streams differing by their morphology and riparian vegetation[END_REF][START_REF] Maridet | Seasonal dynamics and storage of particulate organic matter within bed sediment of three streams with contrasted riparian vegetation and morphology[END_REF], with streams draining forested watershed receiving higher POM inputs than streams from non-forested areas [START_REF] Tank | A review of allochthonous organic matter dynamics and metabolism in streams[END_REF]. Similarly, forestry practices tend to alter the quantity, quality and timing of POM inputs in streams. Consequences of such practices, as well as other anthropogenic alterations, must now also be evaluated in the subsurface compartment of streams, which may be more sensitive than the surface one and contribute to the whole stream ecosystem's resilience. Finally, being aware that the generalization of our findings is limited by our approach relying on two sampling dates, we suggest to extend study periods beyond 1 year. As organic matter production and input vary across years due to extreme events such as flooding or droughts [START_REF] Smock | Effects of drought and a hurricane on a coastal headwater stream[END_REF], stored detritus in streams are also influenced by this variability [START_REF] Jones | Benthic organic matter storage in streams: influence of detrital import and export, retention mechanisms, and climate[END_REF]. A multi-year budget-based approach would bring interesting insights into the effects of plant phenology and meteorological conditions that control the timing of leaf fall and thus the dynamics of allochthonous organic matter entering surface and subsurface compartments of streams.

Fig. 1 .

 1 Fig. 1. Cumulative grain-size distribution curves (mean, n = 9 per stream, phi units) of sediments in the three streams.

Fig. 2 .

 2 Fig. 2. Total CPOM contents ( + SE, n = 9 per stream) at the sediment surface and in the interstitial zone of the three streams.

Fig. 3 .

 3 Fig. 3. Factors affecting the CPOM storage in the hyporheic zone (HZ) of headwater streams (adapted from Boulton et al., 2010; Brunke and Gonser, 1997).

Table 2 .

 2 Summary of the results from three-way ANOVAs performed on the total amount of CPOM and the three CPOM size classes, measured at each date (late autumn and early spring) at the sediment surface and in the interstitial zone of the three streams.

			CPOM > 16 mm	CPOM = 4-16 mm	CPOM = 1-4 mm		Total CPOM
	Analysis	df	F	P	df	F	P	df	F	P	df	F	P
	Three-way ANOVA
	Intercept	1 229.9 < 0.001	1	409.0	< 0.001	1 499.8	< 0.001	1 873.1 < 0.001
	Stream	2	4.3	0.017	2	4.3	0.016	2	6.9	0.002	2	6.8	0.002
	Date	1	1.1	0.290	1	4.1	0.046	1	15.9	< 0.001	1	1.4	0.234
	Compartment	1	0.2	0.651	1	28.4	< 0.001	1	6.0	0.016	1	7.1	0.009
	Stream r date	2	1.3	0.276	2	2.6	0.083	2	7.6	< 0.001	2	3.7	0.029
	Stream r compartment	2	2.0	0.138	2	1.3	0.230	2	2.0	0.140	2	3.4	0.037
	Date r compartment	1	0.3	0.617	1	1.7	0.190	1	63.8	< 0.001	1	6.1	0.015
	Stream r date r compartment	2	0.3	0.759	2	0.8	0.444	2	2.0	0.147	2	1.2	0.305
	Error	96			96			96			96

Table 3 .

 3 Relative contribution (%) of the benthic and interstitial zones to the amount of three size-classes of CPOM stored in the three streams (mean of 9 replicates per zone, date and stream).

		Be´al		Bergnassonne	Orbiel
	CPOM size-classes	Benthic	Interstitial	Benthic	Interstitial	Benthic	Interstitial
	Late autumn
	> 16 mm	15.3	84.7	10.1	89.9	86.3	13.7
	4-16 mm	11.3	88.7	7.1	92.9	10.8	89.2
	1-4 mm	5.0	95.0	8.2	91.8	14.1	85.9
	Early spring
	> 16 mm	27.8	72.2	66.5	33.5	74.4	25.6
	4-16 mm	9.2	90.8	38.9	61.1	35.1	64.9
	1-4 mm	79.4	20.6	84.2	15.8	42.0	58.0

Acknowledgements. We thank Felix Ba¨rlocher, Manuel Grac¸a, Franc¸ois Gue´rold, Florian Mermillod-Blondin and two anonymous reviewers whose comments led to significant improvements of the manuscript. We are grateful to Virginie Payre and Fre´deric Julien for their expert technical assistance. This study was funded through the InBioProcess project of the ANR Biodiversity programme (ANR-06-BDIV-007).