Ms Federica Ferraro

Mr Raffaele D'

Elia Ss Cosma

Mr Simone Paternostro

Ms Antonella Simonetti

Prof Claudio Bruno

ANALYSIS OF A MANNED MARS MISSION WITH NUCLEAR ELECTRIC PROPULSION (NEP) SYSTEM

The goal of this paper is to analyze the performance of a ion NEP system for a manned mission to Mars. The objectives are to evaluate whether NEP could reduce journey's time and mass of propellant compared to both chemical and Nuclear Thermal Propulsion (NTP). To these authors' knowledge this is the first time that a manned Mars mission using only NEP has been analyzed in detail.

The trajectory assumed consists in an acceleration spiral to leave LEO, followed by acceleration with constant thrust after reaching escape speed. Thereafter the spacecraft decelerates until reaching the ∆V needed to capture a Low Martian Orbit (LMO) through a spiral. This trajectory was calculated for thruster power varying from 100 to 300 MW. Mission times are found to depend strongly on power. The gain in shortening times more than compensate for the increase in the mass budget.

Reactor sizing has been done using Los Alamos National Laboratory realistic data for Particle Bed Reactors. The shield uses Be, LiH and W and its architecture has been designed based on a simplified model obtained with the neutronic MCNP code. After optimization, reactor mass, including shielding, is estimated between 4600 kg (333 MWth), to 5370 kg (1000 MWth).

The NEP system consists of a cluster of 20 to 60 ion thruster modules, depending on power, each with thrust 15.55 N, beam diameter 0.4 m and power 5 MWe. Total thruster power assumed after thermal to electric conversion is 100 MWe (150 MWe, 200 MWe and 300 MWe). Specific impulse with Xe propellant is 32,780 s, much higher than that of a NTP system using hydrogen.

The spacecraft mass budget found at convergence includes structure, propulsion system, propellant, environmental control and life support system, water, avionics, crew, crew accommodation (including food), and modules for Mars landing and travel back to Earth. The conclusions indicate that NEP is likely a serious contender for more economical manned Mars missions.

INTRODUCTION

Although the recently released Augustine Commission report takes a rather negative view of the economic feasibility of a US-driven Manned Mars Mission (M3) in the next decades with current NASA budgets, the interest in Mars exploration remains high.

M3 have been considered for a long time since the Von Braun-inspired "Collier's" article of the '50s. Recent ones are the NASA Mars reference mission [START_REF] Drake | Reference Mission Version 3.0 Addendum to the Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team[END_REF] and the very detailed Russian study [START_REF] Istc | [END_REF] commissioned by ESA, and including nuclear electric propulsion (NEP) options. More recently, the ESA Future EU Space Architecture presented in 2009 investigated chemical as well NP alternatives, but only as a system study, not in any propulsion detail. Among its findings is that NP may become a key requirement for human missions to Mars.

It is now apparent that galactic and cosmic radiation (GCR) is probably the bottleneck of human interplanetary travel. The energy spectrum of GCR is such as to pose excessive risks for extended space travel [3;4]. Countermeasures based on shielding or MHD are commonly thought not to be effective in view of the energy composing the GCR spectrum [START_REF] Czysz | Future Spacecraft Propulsion Systems[END_REF] and reaching > 10 15 eV.

Basic physics tells that the radiation dose is (flux) x (time). Not being able to reduce the flux below a safe threshold (of order 1-5 mSv/yr, vs. 200 to 650 mSv/yr for GCR, depending on solar cycle) implies the only alternative is to reduce travel time.

Conventional chemical propulsion (CP) has vacuum specific impulse Isp ≈ 450-460 s. A reasonable mass ratio (MR) forces therefore Hohmann trajectories, with round trip times lasting 3-4 years or more, depending on Mars stay time and ephemerides (position relative to Earth). Hence the interest for NP, [START_REF] Ferraro | 60 th International Astronautical Congress[END_REF] Manuscript presented at the 60 th International Astronautical Congress, Daejeon, Republic of Korea, [START_REF] Ferraro | 60 th International Astronautical Congress[END_REF] Copyright by IAF 2 since in principle the Isp (equal to exhaust velocity for an ideal expansion) is potentially much higher than that of CP. In fact, to 0-th order, the potential energy E and the exhaust velocity Ve (=Isp for ideal expansion) of a rocket are linked by E = ½ m (Ve) 2 Thus Isp scales as (E/m) 1/2 . Chemical E/m is limited to about 10 7 J/kg, while nuclear is about a factor 10 6 to 10 7 larger. Thus the Isp in NP could be of order 10 3 larger, with a corresponding increase in MR. In addition, if thermodynamic acceleration (e.g., as in thermal NP) is replaced by Coulomb or Lorentz acceleration (as in gridded ion engines and MPD thrusters) Ve (and Isp) limitations due to thermodynamics and materials may be removed [5;6]. Of course, because the product thrust times Isp is proportional to the exhaust (jet) power, at fixed reactor power raising Isp means lowering thrust. Thus the trade-off is between high thrust (controlling mainly travel time) and low Isp, or (vice-versa) high Isp (controlling MR) and low thrust. The particular mission will determine the choice. For a M3 where trip time must be as short as possible cost will be the main driver, as spacecraft and all auxiliaries (including any cargo) must be lifted to LEO, a very costly proposition.

The goal of this paper is, in fact, to investigate whether a M3 is feasible with NP, based on the mass and power budgets resulting from using this technology, in particular NEP. Compared to the ISTS Russian study mentioned (assumed power ~ 10 MW), here power is in the O(100) MW. This analysis was inspired by a 2008 study led by A. Lorenzoni (at the Italian Space Agency ASI), where two student teams at the School of Aerospace Engineering of Rome compared M3 using CP and thermal NP (paper IAC -09.C4.7. -C3.5.2 reports their findings). Here the same M3 is analyzed using NEP, to compare the mass budget and performance; to these authors' knowledge, this is the first time that a M3 is analyzed using NEP as the only propulsion means.

Because of time and space limitations, only the main details of the study are reported here, among them electric thruster sizing in Section 1, nuclear reactor sizing and power in Section 2, the powered trajectory (that is, non-Hohmann) in Section 3, and the mass budget in Section 4. Final considerations attempting a comparison with CP are reported at the end of this paper.

NEP: POWERED PROPULSION SYSTEM

Advantages of Electric Propulsion

Electric propulsion (EP) thrusters, and ion thrusters in particular, are a propulsion technology already used for sending spacecraft to the Moon (ESA's Smart-1 mission) and to the asteroids (JAXA's Hayabusa mission). The methods for ionizing the propellant and accelerating ions vary, but all take advantage of the Coulomb force. Even relatively small potential differences can create high velocity exhaust gases, resulting in large Isp. Accordingly, mass or propellant requirements drop according to the Tsiolkovski's equation (rocket equation)

( ) log / log o eff e o f eff e o M V v M M v M M -  = =  (1.1)
where veff is the effective exhaust velocity, or Isp, Mo and Mf are the masses of the spacecraft at the beginning and end of the maneuver, and ΔM = Mo -Mf is the mass of propellant consumed. The rocket equation illustrates the importance of achieving high veff, providing adequate power is available. For a given mission ΔV, low exhaust velocity must be compensated by large│ΔM│, implying a large increase of vehicle size and cost. It is the dependence of ΔV on the logarithmic term in the above equation (for constant veff) that has driven EP development. In terms of Isp, comparing EP with the best cryogenic chemical propulsion (CP) shows very clearly the superiority of the former, e.g., see [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF]. At the same time, raising Isp increases the amount of power of the exhaust ion jet (scaling with the cube of Isp) that must be supplied by an external power source. This escalates rapidly with Isp and with the thrust required by the mission.

Gridded Ion Engines (GIE)

In the vast area of EP technologies, that based on Coulomb acceleration of a ionized propellant is arguably the more mature. Ion engines consist of a discharge chamber closed at one end, and with a set of perforated and accurately aligned grids at the other, hence their Gridded Ion Engine (GIE) name. In their discharge chamber gaseous propellant is injected and ionized, forming a plasma. The positive ions in the plasma diffuse towards the grids and are extracted and accelerated by the potentials relative to the grid(s). The space charge of the emerging ion beam is neutralized by electrons provided by an external cathode (the socalled neutralizer). Various ionization strategies characterize different GIEs (see [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF], for a thorough review, used here to report key aspects of GIEs).

Figure1. Schematic diagram of a GIE [6]

A triple-grid system is typical of current GIEs, see Figure 1. It operates in an accelerate/decelerate mode to enhance ions throughput at the voltages employed. In Figure 1 the thruster body is at a potential VB appropriate to the ion beam velocity (Isp) desired. This is thus the potential of the inner, or screen, grid. The next grid, the acceleration, is at a negative potential to focus each single ion 'beamlet' and to enable its extraction. Note that each beamlet is in fact an electric ionic current. Ions are slightly decelerated to the space potential, via a low negative voltage applied to the outer deceleration grid that couples the ion beam to space plasma [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF].

The external neutralization of the ion beam is done by a weak external plasma created by means of a DC discharge between the neutralizer cathode and an adjacent keeper electrode closely coupled to the space plasma. Electrons are extracted from this plasma by the ion beam as necessary to neutralize its space charge. This is a natural process involving no active control [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF]. An important function of the outer grid is to minimize the erosion of the accel grid due to the bombardment by low energy charge-exchange ions. These ions are generated by interactions between beam ions and the neutral gas atoms, both escaping from the discharge chamber and also created by the neutralizer [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF].

A major advantage of GIE is that plasma production, ion extraction and acceleration, and ion beam neutralization are separate processes, simplifying GIE design. As shown later, further separation of functions is possible in a 4-grid configuration for ultra-high Isp operation, where ion extraction and acceleration can be examined independently [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF].

Engine Parameters

GIE do have limitations dictated by physics and technology. Some [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF] Perveance is the ability of the grids to handle (pass through) current:

Perveance = 2 2 / 1 2 / 3 2 9 4 d AT m e V I g i o T B       =  (1.3)
where ( )

ac B T V V V + =
, o is the dielectric constant of free space, d is the ion acceleration distance, A is the nominal grid area, and Tg is its effective 'transparency' [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF].

Beam diameter

The beam diameter of the engine may go from a few cm to a maximum (tested) 40 cm, see Table 1. Up to 50 cm is feasible, but the development of grid systems of this magnitude is difficult, due to problems associated with maintaining grid integrity during launch, when the launcher is subject to vibrations, and during thruster operation, typically at high and variable temperature. Assuming a specific impulse of 32,780th s (the highest tested with Xenon propellant) and assuming high perveance, GIE thrust varies with diameter as in Figure 5.

Figure 5 Thrust as a function of thruster diameter [6]

The M3 considered here and the effect of GCR dictate minimum trans-Mars time for astronauts; thus thrust must be maximized, with the [main] constraint that propellant mass to LEO orbit (ultimately, cost) must be capped. Preliminary estimates indicate that thrust should be O(10 2 ) N, to be subdivided in a number of GIE modules, each with a [conservative] diameter 40 cm. Note that actual thruster cross section shape may change to compact the entire GIE cluster. Thrust power (scaling with the product specific impulse times thrust) varies also with the thruster diameter, see Figure 6. A range 100-300 MWe was therefore assumed and its effect on the present M3 investigated. 300 MW may seem today inordinately large, but note that in the late '60s the Phoebus 2A reactor built by LASL was tested at 4.2 GW for 12 minutes.

Figure 6 Power consumption as a function of thruster diameter [6]

Specific Impulse

The specific impulse, or ve, of a GIE depends on the beam acceleration potential and the atomic mass mi of the propellant according to between acceleration grids increases. To constrain mass and cost, the engine must have Isp as high as feasible, so the ion beam acceleration potential assumed here is 70 kV.

Propellants

Isp depends on the atomic mass of the propellant. Any inert material easily vaporized and ionized can be used, but currently Xe is the preferred propellant. As a general rule, high Isp requires low atomic mass. For VB = 70 KV Isp varies with propellant as in Figure 8.

Figure 8 Specific impulse as a function of atomic mass of the propellant [6]

However, thrust increases with increasing atomic mass (decreasing Isp), see Figure 9: This trend can be canceled by increasing IB, that is, by increasing perveance. At constant VB , this can be achieved only by picking a 'more negative' grid acceleration, that is increasing Vac (at the expense of grid life), or by eliminating the inter-grid gap.

In fact:

( ) ac B T V V V + = (1.5) and Perveance = 2 / 3 T B V I (1.6)
Because thrust increases also with VB, the best performance is presumably achieved by combining high values of VB and mi with high perveance, and using xenon. This choice is a reasonable compromise between thrust, Isp, power and thus mission time.

Effects Due To Perveance

Perveance depends on Isp: increasing Isp means also to increase perveance, since both are controlled mainly by VB. This said, perveance depends also on the accel grid potential, Vac, therefore it can also be raised by more negative Vac, e.g., Vac = -5 kV. Of course, more negative Vac will worsen the erosion caused by chargeexchange ions. Allegedly, carbon grids are capable to reduce or solve this problem [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF].

The perveance of a GIE grid system is a very important parameter influencing maximum performance for a given ion beam diameter and specific impulse, see Figures 10 and11

Power Consumption

With Isp and thrust T appropriate to a M3 the resulting power P (scaling as the product of Isp times T) may be expected to be very large. Electric power scales of course as total voltage times total beam current IB. With the numbers estimated or chosen here for the M3, electric power may be in the 100 to 300 MW range. Roughly speaking, the nuclear reactor thermal power may be a factor 3 higher. For what said about grid size, handling such power in a single engine is unfeasible. Individual GIE modules will have to be assembled in a cluster, a single engine having, say, 5 MW power., for a total of 20 to 60 thrusters. Clustering reduces risks posed by engine malfunctions and enables thrust steering without gimbaling devices.

Final Thruster Sizing

Based on all the considerations made so far, the following set of sizing parameters is derived or assumed

Dimensioning the core

In this M3 we have chosen a Particle Bed Reactor (PBR) because it is the type of solid-core reactor that shows the highest performance; in fact it is lighter and more compact than a traditional reactor using rods or pins. PBR represent the current best choice fulfilling requirements [START_REF] Lenard | A review of rector configurations for space nuclear electric propulsion and surface power considerations[END_REF].

The core of the reactor consists of spherical particles of fuel, 235 UC2 or 235 UO2, with a diameter of about 500μm. These are contained in a matrix and supported by hexagonal moderator blocks, assembled in a cylindrical configuration. The particles structure consists of a fuel nucleus covered by graphite and by an external inert layer of zirconium hydride. The particles are packed in two coaxial porous cylinders where coolant flows. The inner cylinder is made of a carbon compound to resist high temperature, while the external is of aluminium alloy. The cylindrical moderator bulk, containing the fuel elements, is made of beryllium or lithium hydride. This fuel may reach up to 3500 K at 60 atm. The advantages of PBR are high fuel density and therefore high power density (up to 40 MW/dm 3 ). These make PBR suitable for interplanetary missions.

In the preliminary design of the nuclear reactor many requirements and constraints must be considered [START_REF] Lenard | A review of rector configurations for space nuclear electric propulsion and surface power considerations[END_REF]:

• the reactor must remain critical with a reactivity coefficient keff = 1.000 ( +0.005, -0.000). Dimensioning the PBR core used results of from the Los Alamos reactor code (1986) [START_REF] Humble | Space Propulsion Analysis and Design, Ch.5 and Ch.8[END_REF]. This code evaluates criticality and required power using the Monte Carlo method. To analyze a large range of power the analysis was made with 7, 19, and 37 fuel elements. Knowing power requirements the dimensions are obtained directly. In the particular case in which two solutions are found, the configuration with the lighter mass is chosen. The expressions to calculate core radius are : The following charts represents volume and mass reactor as function of thermal power. The charts above show that for thermal power < 250 MWth the lightest configuration is with 7 fuel elements, for power between 250 e 750 MWth is 19 fuel elements, while for thermal power above 750 MWth the best configuration is with 37 fuel elements. Here the final choice must consider also shield mass and other subcomponents, as illustrated below.

Dimensioning the radiation shield

To dimension the shield different materials and configurations were analyzed using the Monte Carlo Neutronics Program [START_REF] Lenard | A review of rector configurations for space nuclear electric propulsion and surface power considerations[END_REF]. Beyond the shield, neutrons fluence falls with the inverse of square distance from reactor, while the gamma dose with the inverse of distance to the 2.3 power. Moreover, it was found that at the edge of the shadow projected by the shield the dose and the fluence change strongly with the radial position: the rate of change is approximately exponential with radial distance. From this analysis a simplified model was developed to estimate gamma dose and neutrons fluence beyond the shield. The model uses the best configuration using classic materials, LiH, W and Be: Be is a material that reflect neutrons; W is used for its high attenuation of gamma rays and its high cross section in absorbing neutrons. Lithium hydride completes the attenuations of the neutron flux: in particular, hydrogen slows down them, while lithium has a large cross section. In Table 2 Setting the values of beryllium length, fluence and dose, the above expressions can be inverted to obtain tungsten and lithium hydride length: The maximum values of fluence and dose were assumed, respectively, 4.455 x 10 9 n/cm 2 and 15 rad. This choice is based on the threshold values that can be absorbed by humans [START_REF] Del Rossi | Radiation, Doses and Risks in Nuclear Propulsion[END_REF]. Biological effects due to absorption of radiation do not depend only from the dose, but also from the type of radiation and from its density. In fact gamma rays, X rays and beta particle are less dangerous than the heavier alpha particles and fission fragments. Radiations interact with human cells, causing death in acute cases, and DNA damage for long exposure.

ln ln n nW W n LiH nLiH LiH nW LiH W nLiH D F CC L            -+ = -                (2.9) ln ln n nLiH LiH n W nW W nW LiH W nLiH D F CC L            - = -                (2.

Dimensioning of pressure vessel and electric power generation system

The reactor is inside a pressure vessel, and its mass was also calculated based on standard empirical engineering formulae [START_REF] Humble | Space Propulsion Analysis and Design, Ch.5 and Ch.8[END_REF]: The internal pressure is assumed 6 MPa, and peak pressure twice as much. The mass factor depends on material: for completely metal vessel is 2500 m. The vessel is assumed cylindrical with two hemispherical end caps.

0 i tot pv v pV m g  = (2.
Heat from core is absorbed by a coolant gas, here a mixture of 10% helium and 90% xenon, with a total mass flow rate = 8.9 kg/s. This hot flow produces mechanical work via turbomachinery driving an alternator to produce electric power. The power conversion system is a simple Brayton cycle with 33% efficiency. The mass of the turbomachinery is also calculated with empirical formulae [START_REF] Lenard | A review of rector configurations for space nuclear electric propulsion and surface power considerations[END_REF]: At this stage the objective is to choose the reactor configuration minimizing mass. A core with 7 fuel elements is chosen for the 333MWth reactor, and with 19 fuel elements for the 500MWth, 667 MWth and 1000 MWth reactors.

(

ORBIT TRANSFER

Key requirements

The main problem for this M3 is the interplanetary flight time, influenced by the GCR (health risks) and driving fuel mass and cost. The low thrust of NEP is another problem, because present-day all orbital changes are based on impulsive manoeuvres, standard in CP, but unfeasible with the low thrust of EP. In fact, low and constant thrust causes different types of losses: 1) Gravitational losses, due to the angle between the local horizon of the spacecraft and the tangent to the trajectory (called "flight angle") that cannot be kept zero during the entire manoeuvre 2) Steering losses, due to the attitude control system 3) Misalignment losses, due to misalignment between velocity and thrust vectors A MATLAB™ program to calculate and chart the Earth-Mars legs and back, was used with the following input:

-spacecraft mass, specific impulse and thrust -altitude and parameters of starting and final orbits -day of departure from LEO.

-days of acceleration -days of breaking Power = 300 MWe => thrust = 933 N All the configurations assume same Isp (32,780 s) and we assumed the starting mass of 150 ton, but after many iteration we had the following results: The mass consumption includes all consumables for a Mars-direct trajectory, a crew of four and includes also 10% safety margins.

Power
The total mass consumption does not include the mass consumed during the crew stay on Mars, assumed sent in advance via a cargo mission.

The total fuel consumption includes a 10% safety margin.

MASS BUDGET

The mass budget consists in the total mass of the spacecraft with all its systems, subsystems and modules of the mission. This calculation is fundamental, because the cost of a mission depends strongly on the total mass to be launched to orbit and then to be accelerated to Mars.

Currently, he whole spacecraft cannot be launched in a single launch. (Presumably, that could be enabled by a future version of ARES 5.) In any case, the spacecraft would probably have to be assembled in LEO. Note also that with CP propulsion two cargo missions were estimated to be necessary in advance of the human expedition in the NASA's reference mission (V.2 and V.3) of the '90s. Here the goal was to design the human part of the mission. Accordingly, its mass is divided into two parts: a fixed mass that doesn't change significantly during the trip, and the propellant mass.

In the following sections these masses are reported after having been calculated in detail; for brevity these details are not shown, but will be published later on in a dedicated paper.

Fixed mass

This section lists the components constituting the fixed mass, including all electrical and structural systems and subsystems, the propulsion system and the Martian modules. These mass estimates are based on assuming 100 MW, 150 MW, 200 MW and 300 MW of electric power available, influencing reactor and propulsion system mass.

Structure, modules and avionics

Structure

The spacecraft structure supports all other subsystems and ensures the necessary strength during its life. It provides protection to the astronauts from GRC and from meteorites.

Modules [START_REF]Architettura di una missione abitata su Marte[END_REF] • MAS: it is the module where the crew will live during the journey to Mars and the stay on the planet. After capturing LMO (Low Mars Orbit), will reach Mars' surface with a parachute and the propulsion system. The mass budget includes therefore also the thermal shield needed during the entry in the Martian atmosphere. The following table shows all the elements composing the module. Avionics Includes all the electrical systems and subsystems necessary to pilot the spacecraft; it has a mass of 600 kg.

ECLSS, EVA subsystem, Crew

ECLSS [START_REF]Architettura di una missione abitata su Marte[END_REF] The Environmental Control and Life Support System is the system that reproduces Earth-like atmosphere in the spacecraft. It includes water, nitrogen, oxygen and all the subsystems necessary to create a livable spacecraft environment. In particular it generates the internal pressure and controls the local temperature and the air cycle.

Water and food

Water is utilized in many systems and subsystems and to wash and drink. The calculations also include food for the astronauts, see Table 4.2 [START_REF]The Mars Direct Plan[END_REF]. The crew consumable mass per day is 32.5 kg.

EVA subsystem [START_REF]Architettura di una missione abitata su Marte[END_REF] The Extravehicular Activity (EVA) subsystem spacesuit operates when the astronauts leave the spacecraft. The spacesuit keeps pressure constant; controls temperature and rejects heat; provides the aircycle; allows drinking and urinating; monitors vital parameters; and grants telecommunication(s) with the modules.

Crew

The mission foresees a four-person crew.

Propulsion system

The propulsion system includes the nuclear reactor and its shielding, the turbo machinery and the generator to power the electrical thrusters. These electrical are assumed 30 m away from the reactor and its shield.

Total mass

Fixed mass of the spacecraft is composed from MAS, CAP, structures, avionics, ECLSS (only the system without oxygen and water), crew and crew elements. This masses are the same for all power:

MAS (ton)

24.35

CAP (ton) 7

Structure (ton) 14. The mass of the propulsion system, radiator and propellant changes with power (100 MW, 150 MW, 200 MW, 300 MW). In the first case there are twenty 5-MW ion thrusters, thirty in the second, forty in the third and sixty in the last case. Each engine weighs ~50 kg.

CONCLUSIONS

The combination of Isp and power of the GIE NEP system for the M3 considered here results in predictions for both mass and mission times that are significantly better than with other CP and NTR propulsion systems. Especially with respect to CP, the mass saving is noteworthy, as well as the fact that using NEP increases power, but does not increase mass proportionately, while mission time is substantially reduced. These results were obtained with detailed calculations for the mass budget, after reactor and thruster sizing based on current technology and knowhow. The Isp assumed for the GIE system is, in fact, rather conservative, as there is no conceptual or engineering obstacle to raise it by a factor 2 or more in the near term [START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF]. This said, there are many engineering issues connected with NEP for a M3 that were not discussed here because of time and space limitations; among them reactor integrity (the effect of radiation on materials), thruster life, how to refuel a compact reactor while it is operating or 'hot', and many others. They will be examined in the future. Based on the results presented, a NEP-powered M3 appears not only feasible, but also more convenient than CP-and likely also NTR-powered missions in terms of cost, besides being the only way to drastically reduce travel time and thus GCR dose for the crew. This last is gradually being recognized the bottleneck of interplanetary human missions. To enable a future NEP M3, investing in this propulsion technology is necessary. That is an unlikely prospective in the current financial climate, but would spare much time and effort to our future generations.

4 )

 4 Here the range of VB is of order 1-70 kV. Propellants tested, see below, have atomic weight between 1 AMU of hydrogen to 200 AMU of mercury. Most GIE use 131 AMU xenon; and its Isp is in Figure7.

Figure 7

 7 Figure 7 Specific impulse of Xenon as a function of net ion accelerating potential [6]At fixed atomic mass, the Isp increases with increasing applied potential, since the potential VB

Figure 9

 9 Figure 9 Thrust as a function of atomic mass of the propellant [6]

  :

Figure 10 6 Figure 11

 10611 Figure 10Thrust as a function of Vac for a 30 cm beam diameter thruster[START_REF] Fearn | The Application of Ion Thrusters to High Thrust, High Specific Impulse Nuclear Electric Missions[END_REF]

1 )

 1 Ion beam accelerating voltage: VB = 70 KV 2) Accel grid voltage: Vac = -5 KV 3) Mass of xenon propellant: mi = 131 AMU 4) Individual beam diameter: d = 40 cm 5) overall efficiency: ηmt = 0.9 6) Individual module power consumption: P = 5 MW Performance calculated for each GIE thruster are: 7) Specfic impulse: Si = 321 560 m/s = 32780 s 8) Thrust: T = 15.55 N 9) Rate of flow of propellant: m  = 4.8 × 10 -5 Kg/s 10) Ion beam current: IB = 35.7 A 11) Absolute voltage: VT = 75 KV 12) Perveance: 1.74 × 10-6 A/V 3/2 13) Perveance parameter : Pg = 4.5 × 10 -14 14) Effective transparency of the screen grid: Tg = 500 mA/Kg

Figure 2 . 1 Volume of reactor as function of thermal power for 7

 217 Figure 2.1 Volume of reactor as function of thermal power for 7, 19 and 37 fuel elements [8]

  13) with pi= vessel peak pressure (Pa) Vtot=vessel total volume (m 3 ) Φv= vessel mass factor(m)

  Starting from this input the program reads a subroutine with Earth, Moon and Mars positions at the starting location and time, and then integrates the three velocity components of all bodies under the effect of Newton's Law and Gravitation among all bodies (a n-body problem). Earth and Mars orbits are assumed unperturbed and elliptic, inclined with respect to the Sun equator. The program includes interaction among all bodies. For simplicity, thrust and velocity vectors are assumed always aligned The spacecraft starts from a 400 km LEO and arrives in a low Mars orbit (200 km LMO). The program calculates total ΔV and fuel mass consumed by dividing the trans-Mars leg into two segments: in the first the spacecraft accelerates to the maximum speed possible, in the second the spacecraft slows down to capture the LMO. Thrust for the four thruster electric powers assumed is given below

Figure 3 . 1 : 12 Figure 3 . 2 :

 311232 Figure 3.1: Flight time as function of power

Figure 3 . 3 :

 33 Figure 3.3: Thrust-Mass ratio as function of power

Lower limit Upper Limit Comments

  are reported in Table1.1:

	Parameter
	Beam diameter
	(cm)
	Beam diameter	~ 3	~ 40	Upper limit
	(cm)			due to grid
				distortion
	Beam	10	70,000	Values > 5
	accelerating			kV need 4
	potential (V)			grids
	Beam current	-	150	Determined
	density			by
	(mA/cm 2 )			perveance
	Grid perveance	-	4500	Limit
	factor, Pg ( *)			determined
				by
				Qinetiq's
				T5 thruster
	Thrust density	-	30	Determined
	(mN/cm 2 )			by
				perveance
	Power density	-	4500
	(W/cm 2 )
	Specific	2000	150,000	Determined
	impulse (s)			by max
				voltage,
				assumed to
				be 70 kV,
				and ion
				mass
	Mean ion mass	~ 4.5	~ 200	Minimum
	(AMU)			value is
				hydrogen
				compound,
				maximum
				is mercury
	Thruster	-	~ 800	Determined
	temperature for			by solenoid
	Kaufman-type,			and RF
	RIT-type and			coils, also
	ECR thrusters			wiring
	(deg C)
	Thruster		~ 300	Determined
	temperature for			by
	MESC			permanent
	thrusters (deg			magnets
	C)

Table 1 .1 Some current parameter limits. Adapted from [6]

 1

	(*) The perveance parameter, Pg, perveance per unit
	area, is defined as	g P	=	2 g AT / 1 i d m B I	2	=	9 o  4	2 e	3 T V	/	2
	(1.2)

Table 1 .2 Performance of the overall propulsion system 2. NUCLEAR REACTOR

 1

	1/2
	15) Area of thruster: A = 1172 cm 2
	16) Current density: JB = 30.46 mA/cm 2
	17) Power density: Pd = 4266 W/cm 2
	18) Thrust density: Td = 13.26 mN/cm 2
	The performance of the entire propulsion system is
	therefore in Table 2
	Engine number	20	30	40	60
	Power
	consumption	100	150	200	300
	(MW)
	Specific impulse (s)	32780	32780	32780 32780
	Thrust (N)	311	467	622	933
	Rate of flow of	1.07 ×	1.6 ×	2. ×	3.2 ×
	propellant (Kg/s)	10 -3	10 -3	10 -3	10 -3
	Ion beam current (A)	713.4	1070.5 1427	2140.2 A
	Perveance (A/V 3/2 )	3.5 × 10 -5	5.2 × 10 -5	7 × 10 -5	1 × 10 -4
	Area (cm 2 )	23442	34828	46884 70326

  • the reactor must remain subcritical in the worst accidental conditions, also when immersed in water or wet sand, and in dry sand. • nuclear fuel U 235 is enriched at 93.15% to reduce fuel mass.

	200
	1 50 100 150 • considering the results obtained from reactors (MW) 1 2 3 4 already built, the minimum fuel burn up must be al least 4%. The assumptions considered in dimensioning the reactor are: Consumption ➢ Core: cylindrical; ➢ Radius of shield equal to radius of core; ➢ Thermal efficiency of turbomachinery: 33%; Power ➢ Reactor average density: 1600 kg/m 3 ; it includes 5 coolant pipes, moderators, reflectors and all of Negative Accel Grid Potential (kV) the components except the pressure vessel [8].
	➢ Coolant volume: about 16% of total core
	volume.
	➢ Pressure drop of the propellant gas in duct: 3.5%
	of total pressure.
	➢ Payload and housing module: at 30m from
	reactor. This value is a good trade-off between
	distance to reduce radiation from reactor and
	mass of boom.
	➢ Shell deformation: < 2% at operating pressure
	and temperature.

Ferraro et al. 2009 Manuscript presented at the 60 th International Astronautical Congress, Daejeon, Republic of Korea, 12-16 October 2009. Copyright by IAF 7

Table 2 .1 Core radius and core height for different fuel configurations

 2

		10 --4 6 1.3261 (10) 0.1735 47.625 3 core P P -core core 42 9.0958(10) 7.1665 (10) core R = core PP + -+ -	+	(2.1)
	for 7 fuel elements;
				core R	2.655 (10) = -	12 --5 9 8.946 (10) core P +	4 core P
				5 --3 3 7.427(10) core PP core 2 -+ 1.1703 (10)	(2.2)
				2.2955 -+ core P	313.34
	for 19 fuel elements;
				core R	=	4.905 (10)	11 -	4 core P	-	2.881 (10)	7 -	3 core P	+	(2.3)
				6.2522(10) +	42 core PP 0.5992 core --	+	252.28
	for 37 fuel elements.
	Similarly, core height H is from :
			H	core	=	2.83 (10)	42 core P -	+	0.5203	core P	+	26.06	(2.4)
												for 7 elements;
	H	core	4.027 (10) = -	52 core P -	+	0.1427	core P	+	17.9883	(2.5)
												for 19 elements;
	H	core	6.502 (10) = -	62 core P -	+	0.05009	core P	+	18.335	(2.6)

Table 2 .2 Core volume and core mass for different fuel configurations

 2

Table 2 .3 Attenuation lengths for neutron fluence and gamma dose [7]

 2

		.3 below the attenuation coefficients of
	materials for gamma rays and neutrons are reported:
	Material	Neutrons attenuation	Gamma ray attenuation	Density
	Be	5.8cm	35cm	1850 kg/m 3
	W	5.2cm	2.0cm	19300 kg/m 3
	LiH	5.6cm	33cm	757 kg/m 3
	The simplified expressions to calculate neutrons fluence
	and gamma dose are [7]:

Table 2 .4 Shield geometry and mass for 333 MWth and 500 MWth reactors

 2 The results of shield dimensioning for a 333 MWth reactor and for a 500 MWth reactor, obtained after analyzing different PBR configurations are shown in table 2.4:

		333 MWth reactor 500MWth
		7	19	7	19
		elements	elements	elements	elements
	Rshield(m)	0.315	0.395	0.311	0.356
	Shield Area (m 2 )	0.312	0.489	0.304	0.399
	LBe (cm)	18	18	18	18
	LLiH (cm)	58.71	58.71	59.33	59.33
	LW (cm)	29.53	29.53	29.83	29.83
	mshield (kg) 2023	3169	1990	2607

th International Astronautical Congress, Daejeon, Republic of Korea, 12-16 October 2009. Copyright by IAF 9

Table 2 .5 Shield geometry and mass for 667 MWth and 1000 MWth reactors

 2

Table 2 .6 Radiators Mass for Different Reactor Pth Finally

 2 we have assumed that the support structure is 10% of total inert mass (core shield and pressure vessel).

	tm m	=	200 * (	power level in kWe	) / 100)	(2.14)
	At these power levels the alternator is slightly heavier
	than turbomachinery: its mass is conservatively estimated

Table 2 .7 Reactor and power generation masses for 333 MWth and 500 MWth reactors

 2 Ferraro et al. 2009 Manuscript presented at the 60 th International Astronautical Congress, Daejeon, Republic of Korea, 12-16 October 2009. Copyright by IAF

		667 MWth reactor	1000 MWth reactor
		19	37	19	37
		elements	elements	elements	elements
	core +
	shield	2924	7031	2945	3750
	mass (kg)
	pressure
	vessel	246.7	568.4	272.2	270.6
	mass (kg)
	Turbo
	machinery	632.5		774.6
	mass (kg)
	Alternator mass (kg)	758.9		929.5

Table 2 .8 Reactor and power generation masses for 667 and 1000 MWth reactors

 2

Table 3 . 1 Leg duration, mass and fuel consumption

 3

	100	MWe

Table 3 . 2 Leg duration, mass and fuel consumption

 3

	Power	200	MWe
	Outward journey:
	Initial mass	140	ton
	Days	125
	Fuel consumption	20.8	ton
	Mass consumption	2.3	ton
	ΔV	51.8	km/s
	Arrival mass	116.8 ton
	Stay on Mars:
	Days	285
	Mass consumption	5.3	ton
	Return journey:
	Initial mass	116.8 ton
	Days	110
	Fuel consumption	18.4	ton
	Mass consumption	2.0	ton
	ΔV	54.9	km/s
	Arrival mass	96.4	ton
	Total days	520
	Total fuel consumption	43.1	ton
	Total mass consumption	4.4	ton
	Structures and propulsive system mass 89.8	ton
	Exceeded mass	2.7	ton

Table 3 . 3 Leg duration, mass and fuel consumption

 3

	Power	300	Mwe
	Outward journey:
	Initial mass	160	Ton
	Days	100
	Fuel consumption	25	Ton
	Mass consumption	1.9	Ton
	ΔV	54.7	km/s
	Arrival mass	133.01 Ton
	Stay on Mars:
	Days	205
	Mass consumption	3.8	Ton
	Return journey:
	Initial mass	133.01 Ton
	Days	80
	Fuel consumption	20.0	Ton
	Mass consumption	1.5	Ton
	ΔV	60.1	km/s

Table 3 . 4 Leg duration, mass and fuel consumption

 3

Table 4 . 1 •

 41 CAP: it is the capsule for the re-entry on Earth.• MTT: it's the module for the Mars-Earth transfer. It is formed from CAP and MAS.

	Component	Mass (ton)
	Structure of MAS	8.5
	Mechanisms	4.05
	Landing propellant 9
	Propeller and Tank 2.2
	Module Parachute 0.6
	TOTAL	24.35

Ferraro et al. 2009 Manuscript presented at the 60 th International Astronautical Congress, Daejeon, Republic of Korea, 12-16 October 2009. Copyright by IAF 13

Table 4 .2 (* Mass includes 10% safety margins)

 4 Each has been assumed to weigh 110 kg. Training equipment is included in the mass budget. This mass is reported in table 4.3.

		Daily need	Percent	100MWe	150MWe	200MWe	300MWe
		per person (kg)	Recycled %	reactor*	reactor*	reactor*	reactor*
	Journey time (days)			335	282	235	180
	Oxygen (kg)	1	80	294.8	248.2	206.8	158.4
	Dry food (kg)	0.5	0	737.0	620.4	517.0	396
	Whole food (kg)	1	0	1474.0	1240.8	1034.0	792
	Drinking water (kg) 4	80	179.2	992.6	827.2	633.6
	Washing water (kg) 26	90	3832.4	3226.1	2688.4	2059.2
	Total (kg)	32,5	87	6227.6	5242.4	4368.6	3346.2

2

  Ferraro et al. 2009 Manuscript presented at the 60 th International Astronautical Congress, Daejeon, Republic of Korea, 12-16 October 2009. Copyright by IAF

	Avionics (ton)	0.6
	Crew and Crew support (ton)	9
	ECLSS (fixed) (ton)	4.8
	Total (ton)	59.95
		14

Table 4 . 3

 43

Table 4 . 4

 44 The weights of reactor, radiators and propellant are shown in the following table with the other results. The following table lists all the main masses.

	Power (MWe)	100	150	200	300
	Fixed mass (ton)	59.95	59.95	59.95	59.95
	ECLSS total (ton)	6.23	5.24	4.37	3.35
	Reactor (ton)	4.56	4.99	4.98	5.37
	Electric Thrusters (ton)	0.73	1.09	1.45	2.18
	Radiators (ton)	11.70	17.50	23.40	35.10
	Propellant (ton)	30.66	38.77	43.13	49.55
	Total (ton)	113.82 127.54 137.28 155.49
	Initial mass (ton)	120.5	130.0	140.0	160.0
	Excess mass (ton)	6.68	2.46	2.71	4.51


Audrey M Darnaude
email: a.darnaude@cefas.co.uk

Fish ecology and terrestrial carbon use in coastal areas: implications for marine fish production

Keywords: benthic food webs, flatfish, population dynamics, river run-off, stable isotopes

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

Introduction

Successful fisheries management depends on our ability to identify and explain causes of population fluctuations and changes. Relationships between climatic change and modification of fisheries have been explained for some pelagic species [START_REF] Cushing | Climate and Fisheries[END_REF][START_REF] Cushing | The long-term relationship between zooplankton and fish[END_REF][START_REF] Yañez | Pelagic fish stocks and environmental changes in the south-east Pacific. Global Versus Local Changes in Upwelling Systems[END_REF]. However, the effects of large and irregular year-to-year variations in environmental parameters induced by weather conditions on demersal fish resources are less well understood (Salen-Picard et al. 2002). Temperature and fluctuation in planktonic primary productivity have been the main environmental factors studied [START_REF] Nielsen | Can changes in the fisheries yield in the Kattegat (1950-92) be linked to changes in primary production?[END_REF], but less attention has been paid to the effects of fluctuations in macrobenthic communities as fish food resources.

Land-based run-off has for long been thought to enhance demersal fishery production in various coastal areas where fish abundance follows river discharge variations [START_REF] Kerr | Marine Coastal Eutrophication: the Response of Marine Transitional Systems to Human Impact: Problems and Perspectives for Restoration[END_REF][START_REF] Cushing | The long-term relationship between zooplankton and fish[END_REF][START_REF] Lloret | Fluctuations of landings and environmental conditions in the north-western Mediterranean Sea[END_REF]. This link has been attributed mainly to enhanced planktonic production by river nutrient loads [START_REF] Cushing | Climate and Fisheries[END_REF][START_REF] Nielsen | Can changes in the fisheries yield in the Kattegat (1950-92) be linked to changes in primary production?[END_REF][START_REF] Maslowski | Effects of trophic conditions on benthic macrofauna in the vicinity of the River Swina mouth (Pomeranian Bay; southern Baltic Sea)[END_REF]. However, efficient exploitation of river-borne particulate organic matter (POM) by some marine benthic organisms has been shown recently [START_REF] Bouillon | Sources of suspended organic matter and selective feeding by zooplankton in an estuarine mangrove ecosystem as traced by stable isotopes[END_REF]Darnaude et al. 2004b), with active uptake by coastal benthic food webs sustaining demersal fish (Darnaude et al. 2004a). As terrestrial POM is brought to the sea mainly during river floods, this explains the link between river discharge and local production of certain demersal fish (Salen-Picard & Arlhac 2002;Salen-Picard et al. 2002).

However, the potential of river floods to enhance multispecies demersal fisheries, which will vary according to fish ecology during benthic life (Salen-Picard et al. 2002), still needs to be assessed. Our previous work has shown that dietary differences explain most of the interspecific differences in terrestrial POM uptake by fish (Darnaude et al. 2004a). Because terrestrial POM use by marine macrobenthos varies according to availability in the environment, terrestrial POM transfer to demersal fish is also likely to depend on their distribution, due to spatial differences in terrestrial POM deposition in the coastal zone (Darnaude et al. 2004b).

In the present study, carbon and nitrogen stable isotopes were used to assess intra-( juveniles vs. adults) and interspecific variations in terrestrial POM uptake in the five main flatfish (Arnoglossus laterna, Buglossidium luteum, Citharus linguatula, Solea lascaris and Solea solea) present off the Rhone River delta (NW Mediterranean). To assess the conditions for fish exploitation of terrestrial POM during juvenile and adult stages, these data were compared with diet and depth distribution data obtained from the same fish. Stable isotopes were then used to describe coastal benthic food webs within fish depth ranges of life. Provided that distinct isotopic signatures can be identified for the primary producers, stable isotopes constitute a powerful tool for the assessment of the main organic matter source(s) that sustain animal feeding [START_REF] Vander Zanden | Variation in δ 15 N and δ 13 C trophic fractionation: implications for aquatic food web studies[END_REF]. When combined, carbon and nitrogen stable isotopes allow the description of food web structure and organic matter transfer within animal communities [START_REF] Kaehler | Trophic structure of the marine food web at the Prince Edward Islands (Southern Ocean) determined by δ 13 C and δ 15 N analysis[END_REF][START_REF] Polunin | Feeding relationships in Mediterranean bathyal assemblages elucidated by stable nitrogen and carbon isotope data[END_REF][START_REF] Takai | Carbon sources for demersal fish in the western Seto Inland Sea, Japan, examined by δ 13 C and δ 15 N analyses[END_REF]. The carbon isotope signature (δ 13 C), which increases from prey to predator by only 1-2‰, provides clues about the origin of the consumers' food [START_REF] De Niro | Influence of diet on the distribution of carbon isotopes in animals[END_REF][START_REF] Wada | The use of stable isotopes for food web analysis[END_REF], whereas the nitrogen signature (δ 15 N), which increases by 2•5-4•5‰ from food to consumer, helps define the organisms' trophic levels [START_REF] Minagawa | Stepwise enrichment of 15 N along food chains: further evidence and the relation between δ 15 N and animal age[END_REF][START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF]. The assessment of the role of fish ecology in determining the consequences of terrestrial POM inputs on fish growth and reproduction is necessary in order to predict benthic fisheries responses to changes in river discharge.

    

This study was performed on the coastal soft bottoms located seawards of the Rhone River delta (43°14′-43°22′ N, 4°54′-5°03′ E). The Rhone River is the most important run-off to the Mediterranean. Its inputs in nutrients play a key role in enhancing NW Mediterranean's productivity. However, they are relatively constant from year to year [START_REF] Lochet | Transfers between a eutrophic ecosystem, the river Rhône, and an oligotrophic ecosystem, the north-western Mediterranean Sea[END_REF], whereas terrestrial POM inputs by the river follow its discharge [START_REF] Pont | Les débits solides du Rhône à proximité de son embouchure: données récentes (1994-95)[END_REF][START_REF] Sempéré | Carbon inputs of the Rhône River to the Mediterranean Sea: Biogeochemical implications[END_REF]. Terrestrial POM forms the major portion of terrestrial matter brought to the sea during river floods. On average, 6•2 × 10 6 t year -1 of terrestrial particulate material is discharged in the river prodelta [START_REF] Zuo | Accumulation rates and sediment deposition in the northwestern Mediterranean[END_REF]. In this area, macro-and microphytobenthic productions are low [START_REF] Bodoy | Evaluation simultanée des biomasses et productions primaires phytoplanctonique et microphytobenthique en milieu côtier[END_REF]. Marine phytoplankton and Rhone River POM are the main sources of organic matter for benthic ecosystems [START_REF] Cauwet | Contribution of the Rhône River to organic carbon inputs to the northwestern Mediterranean Sea[END_REF]Darnaude et al. 2004a). More than 50% of sediment organic matter is of terrestrial origin irrespective of depth, but terrestrial POM sedimentation and availability are maximal at 30 -50 m [START_REF] Bouloubassi | Carbon sources and cycle in the western Mediterraneanthe use of molecular markers to determine the origin of organic matter[END_REF]Darnaude et al. 2004b). Sediment type and benthic community composition differ mainly between 0 and 20 m (sandy bottoms with a dominance of molluscs and polychaetes) and 30-100 m (muddy bottoms with polychaete-dominated communities) (Darnaude et al. 2004b). Flatfish are abundant throughout, and consist mainly of Soleidae (S. solea, S. lascaris and B. luteum), Bothidae (mainly A. laterna) and Citharidae (C. linguatula).



Samples were collected during 2000-01, at the end of a 4-year period of moderate discharge for the Rhone River [START_REF] Darnaude | Apports fluviaux en zone côtière et réseaux trophiques marins benthiques: transfert de matière organique particulaire terrigène jusqu'aux poissons plats au large du Rhône[END_REF]. Juvenile and adult flatfish and their main prey were collected seasonally (winter, spring, summer) within three depth ranges (0-20 m, 30-50 m and 70-100 m) that reflect the main types of benthic environments occupied by the fish. To limit sampling-linked biases in fish distribution, diet and size, fish were collected over 24-h cycles of trawling (hauls of 30 min duration at 2 knots speed at 2-h intervals). Both a small commercial otter trawl targeting adult fish (swept area = 4 × 3 m; cod end mesh size = 20 mm) and a small experimental beam-trawl targeting juvenile fish (swept area = 1•5 × 0•5 m; cod end mesh size = 8 mm) were employed. Fish were counted and their total lengths recorded to assess juvenile and adult bathymetric distribution for each species. Fish guts were removed and preserved in 10% neutralized formaldehyde solution for stomach content analysis, and dorsal white muscle samples were taken for isotope analysis. Macrobenthic preys were collected using a pneumatic suction-sampler at 0-5 m depth by SCUBA diving and using a Smith-McIntyre grab at greater depths (10 -20, 30 -50 and 70 -100 m). Macrobenthos samples were kept alive in filtered water from the sampling site for 24 h to allow gut evacuation. They were then sorted by taxon (mainly to the class or order level) under a binocular microscope and stored separately, after dissection of large individuals to remove hard parts and digestive tracts. At this stage, polychaetes were determined to species level to allow sorting them by trophic group [START_REF] Fauchald | The diet of worms: a study of polychaete feeding guilds[END_REF]. Samples for isotope analysis were kept frozen before being processed. Rhone River POM and offshore seawater POM (marine phytoplankton) were chosen as proxies of the terrestrial and marine POM supplies to the benthic ecosystems studied. They were sampled in winter, spring and summer, i.e. during the main periods of high and low flow of the river [START_REF] Sempéré | Carbon inputs of the Rhône River to the Mediterranean Sea: Biogeochemical implications[END_REF], to ensure obtaining mean values that reflected seasonal variability in their isotopic signatures. For each season, Rhone River water samples were collected 6 km upstream of the river mouth. Seawater was sampled in the Gulf of Marseilles, outside the Rhone River influence (Frioul island: 43°15′ N, 5°18′ E; salinity > 37•8). Water samples were prefiltered on 250 µm-mesh sieves to remove zooplankton and detritus. POM samples (three per site per season) were obtained by filtering the corresponding water samples (3 × 20 L per site per season) on previously combusted and weighted Whatman GF/F glass fibre filters. Samples were kept frozen prior to stable isotope analysis.

 

Global (annual) diets of juvenile and adult A. laterna, B. luteum, C. linguatula, S. lascaris and S. solea were determined from the stomach contents of at least 30 individuals (≥ 10 per season) per species, life stage and depth. Prey items were identified and counted under a binocular microscope, following the procedure described in [START_REF] Darnaude | Food partitioning among flatfish (Pisces: Pleuronectiforms) juveniles in a mediterranean coastal shallow sandy area[END_REF]. They were sorted down to the class or order level, with the exception of polychaetes, which were identified to species and pooled by trophic group following [START_REF] Fauchald | The diet of worms: a study of polychaete feeding guilds[END_REF]. Prey category dry weights were recorded after 24 h drying at 45 °C. They were used to calculate prey weight percentages in flatfish diet (W%) at each depth.

  

All samples were freeze-dried. Water POM samples (on GF/F filters) were cut into small pieces. Prey and fish samples were ground into a fine powder (< 60 µm) using a mortar and pestle. For small prey, several individuals were pooled and homogenized together to obtain enough material for analysis. Samples from large invertebrates (polychaete and mollusc soft tissues) and fish (white muscle) were analysed without any prior treatment. Samples of water POM and small invertebrates (bivalve juveniles, crustaceans, ophiuroids, etc.) were divided into two subsamples. The subsample for carbon isotope analysis was acidified with 1% HCl solution to remove carbonates (they present higher δ 13 C values than organic carbon; De Niro & Epstein 1978), rinsed with distilled water and oven-dried at 40 °C for 24 h. The nitrogen isotope analysis subsample was not acidified, as acidification modifies δ 15 N [START_REF] Pinnegar | Differential fractionation of δ 13 C and δ 15 N among fish tissues: implications for the study of trophic interactions[END_REF]. Three prey category samples and 10 fish group samples were analysed per depth. 13 C/ 12 C and 15 N/ 14 N ratios in the samples were determined using continuous-flow isotope mass spectroscopy (CF-IRMS) [START_REF] Preston | Interfacing an automatic elemental analyser with an isotope ratio mass spectrometer: the potential for fully automated total nitrogen and nitrogen-15 analysis[END_REF]. Weighed samples of material (1 mg) were oxidized and the N 2 and CO 2 passed to a single inlet dual collector mass spectrometer (PDZ Europa ANCA SL 20-20 system). Isotope ratios were expressed as parts per thousand (‰) differences from a standard reference material:

δX = [(R sample /R standard ) -1] × 10 3
where δ = the measure of heavy to light isotope in the sample, X = 13 C or 15 N and R = the corresponding ratio ( 13 C/ 12 C or 15 N/ 14 N).

International Standard references are Vienna Pee Dee Belemnite (vPDB) for carbon, and atmospheric N 2 for nitrogen. However, to calibrate the system and compensate measurement drift with time, cornflour and cod (Gadus morhua L.) white muscle, with known signatures, were used as internal reference materials when analysing water samples and animal material, respectively. Experimental precision, based on the standard deviations of the internal reference replicates, was 0•12‰ for δ 13 C and 0•13‰ for δ 15 N.

 

To assess main modifications in POM sources' isotopic signatures during the year, average δ 13 C and δ 15 N values of Rhone River POM and marine phytoplankton (offshore seawater POM) were determined for both periods of high (winter, spring) and low (summer) river discharge. Student's t-tests were performed independently on the calculated δ 13 C and δ 15 N to test within-year signature changes by source, and among-sources signature differences for each period.

In aquatic systems, δ 13 C and δ 15 N generally increase by 1-2‰ and 2•5 -4•5‰ from food to consumer, respectively [START_REF] De Niro | Influence of diet on the distribution of carbon isotopes in animals[END_REF][START_REF] Minagawa | Stepwise enrichment of 15 N along food chains: further evidence and the relation between δ 15 N and animal age[END_REF][START_REF] Wada | The use of stable isotopes for food web analysis[END_REF][START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF]. These values were used to assess food web structure off the Rhone River. The comparison of flatfish and benthic prey isotopic signatures with the δ 13 C and δ 15 N values obtained for Rhone River POM and marine phytoplankton allowed the description of benthic food webs at each depth (δ 13 C vs. δ 15 N graphs) and estimation of the average (mean ± SD) trophic increases in isotopic signatures for the whole study area (for detail, see [START_REF] Darnaude | Apports fluviaux en zone côtière et réseaux trophiques marins benthiques: transfert de matière organique particulaire terrigène jusqu'aux poissons plats au large du Rhône[END_REF]. On average, δ 13 C increased by 1•80 ± 0•21‰ from prey to predator off the Rhone River, while the mean trophic increases in δ 15 N were 2•60 ± 0•05‰ and 3•20 ± 0•10‰ for herbivores and carnivores, respectively.

Intra-and interspecific differences in fish isotopic signature were tested by two-way (species × life stage) s performed separately on δ 13 C and δ 15 N values. One-way (depth range) s were also performed to test depth-related δ 13 C variations in the juveniles and adults of C. linguatula and in the adults of A. laterna, B. luteum and S. solea. In each case, data were log-transformed to stabilize variance, and post-hoc comparisons of means were performed using Student-Newman-Keuls tests. Only differences in isotopic signatures associated with P < 0•05 were considered to be statistically significant. Because significant differences in carbon signatures in fish can arise not only from differences in food sources (terrestrial vs. marine) but also from differences in trophic levels (δ 15 N), differences in δ 13 C do not allow the assessment of the extent of terrestrial-POM uptake by the fish. To solve this problem, fish trophic levels (TL) in the case of an exclusively marine POM-based diet were calculated using the equation:

TL = [(δ 15 N fish -(δ 15 N phytoplankton + 2•6))/3•2] + 2
where 2•6 and 3•2 = average trophic increases in δ 15 N (‰) found in the study area for herbivores and carnivores, respectively.

They were then used to obtain corresponding theoretical marine carbon signatures (δ 13 C th ) for all fish groups by using the equation:

δ 13 C th = δ 13 C phytoplankton + [(TL -1) × 1•8]
where 1•8 = average trophic increase in δ 13 C (‰) observed in the benthic communities of the study area.

The comparison of fish theoretical and observed δ 13 C values allowed estimating the extent of their terrestrial POM uptake off the Rhone River.

Results

 -

Different depth distributions were observed between the five species, reflecting differences in habitat-use. Depth distribution changed with age in A. laterna, B. luteum and S. solea. In these three species, most juveniles (> 98%) occurred at 0-20 m depth. Adults occurred at all depth ranges, but most of them (55% in A. laterna, 62% in B. luteum and 47% in S. solea) were found at 30 -50 m. In S. lascaris and C. linguatula, juveniles and adults had similar distributions. However, S. lascaris was taken only on nearshore sandy bottoms (0-20 m depth), whereas C. linguatula occurred only offshore, on the muddy bottoms of 30-100 m in depth.

The five species showed important differences in food preferences and diet adaptability (Table 1, Fig. 1) despite occupying the same habitats. Buglossidium luteum had the last specialized diet, with similar proportions of polychaetes, molluscs (mainly bivalves) and crustaceans. Solea solea, S. lascaris and A. laterna also preyed on a wide variety of benthic organisms but showed clear preferences for polychaetes, bivalves and crustaceans, respectively. In C. linguatula the diet was highly specialized, consisting almost exclusively of fish and crustaceans (mysids and shrimps).

In all cases, ontogenetic changes in diet were observed; however, the extent of differences between juvenile and adult fish varied according to species (Table 1). Ontogenic changes in diet were clearest in B. luteum and S. lascaris. In B. luteum, juvenile diet was dominated by crustaceans (cumaceans, amphipods and copepods) and adult diet by polychaetes (deposit feeders and carnivores). In S. lascaris, juveniles ate mainly polychaetes (suspension feeders), whereas adults fed essentially on bivalves (siphons). In the three other species, the main prey categories remained the same for both life stages, but with some modifications in the size and nature of the prey ingested. In A. laterna, both life stages consumed mainly crustaceans (W% > 68%). However, juvenile A. laterna fed maily on cumaceans and amphipods, while the adults consumed mainly shrimps, brachyurans and mysids. In S. solea, polychaetes dominated the diet of both juveniles and adults (W% > 34%) but, as the fish matured, subsurface deposit-feeding and carnivorous polychaetes progressively replaced surface deposit-feeding polychaetes in the diet. In C. linguatula, fish were the main prey item irrespective of the age (W% > 64%), but prey size increased with growth, and shrimps were progressively replaced by mysids. Diet varied with depth, especially in adult A. laterna, B. luteum and S. solea (Fig. 1). In these three fish groups, mollusc and crustacean consumptions were maximal at 0 -20 m and at 70 -100 m, respectively, and a common peak in polychaete ingestion was observed at 30 -50 m for B. luteum and S. solea. In C. linguatula, main preys were always fish, shrimps and mysids, but their respective contributions to the diet varied with depth and the adults ate polychaetes only at 30-50 m depth.

  

Nitrogen and carbon isotopic signatures differed significantly between species (P < 0•001) and life stage (P < 0•01), which suggested intra-and interspecific differences in trophic levels and food origin (Table 2). In all species, δ 15 N increased significantly (P < 0•01) between juveniles and adults, suggesting increased trophic level with growth.

However, the level of increase varied among species, with a minimum (< 1•0‰) in the two sole species and a maximum (> 2•6‰) in C. linguatula. Consequently, interspecific differences in nitrogen signature varied by life stage. Among juveniles, δ 15 N was significantly lower (< 9‰) in A. laterna, B. luteum and C. linguatula than in S. solea and S. lascaris (> 10‰). Among adults, A. laterna (δ 15 N = 10•1‰) and C. linguatula (δ 15 N = 11•6‰) had significantly lower and significantly higher nitrogen signatures, respectively, than the other species. Carbon isotopic signature was similar for juveniles and adults in A. laterna, C. linguatula and S. solea, but differed significantly (P < 0•05) between juveniles and adults in B. luteum and S. lascaris (Table 3). δ 13 C was significantly lower in juvenile S. solea (-20•0‰), and higher in juvenile C. linguatula and B. luteum (>-17•6‰) than in A. laterna and S. lascaris. For adults, carbon signature differed significantly between all species. Adult S. solea had the

4•5 0•2 16•4 1•2 - - - 0•9 - - Leptostraceans 3•4 - - - - - - - - - Mysids < 5 mm - - - 4•2 29•8 2•2 6•5 3•0 5•5 - 5 -10 mm - 10•4 - - - 7•0 - 5•3 - - Cumaceans 47•5 0•6 17•4 1•3 - - 13•7 2•2 - - Amphipods 15•9 5•7 17•4 8•3 - - 6•4 4•2 17•7 - Shrimps Alpheidae - - - - - 0•2 - - - 1•1 Crangonidae < 20 mm - 33•0 - 9•8 5•4 - - - - 0•1 20 -40 mm - 6•1 - - - 15•9 - - 11•0 2•2 Pagurids - 0•9 - - - - - - - 0•6 Brachyurans - 11•4 - 2•4 - - - - - 1•3 Molluscs 7•2 1•7 26•8 15•6 - + 7•8 79•7 26•6 7•8 Bivalves Juveniles < 10 mm 7•2 - 10•4 - - + 1•0 - 6•0 + Siphons - 1•7 - 14•8 - - 6•8 78•4 20•6 5•4 Gastropods - + 16•4 0•8 - - - 1•3 - 2•4 Echinoderms - + + + - - - 1•8 1•5 8•7 Ophiurids - + + + - - - 1•8 1•5 3•0 Holothurians - - - - - - - - - 5•7 Phoronidians - 3•8 - + - - - - 3•2 - Fish (Gobiids) - 0•2 + + 64•8 73•5 - - - 0•1 < 30 mm - + + + 64•8 1•1 - - - - 30 -50 mm - 0•2 - - - 72•4 - + - 0•

  

Carbon and nitrogen signatures varied significantly during the year for both POM sources (Table 3). However, carbon signature was always significantly lower (> 3•5‰) for Rhone River POM (annual mean δ 13 C = -26•11 ± 0•23‰) than for marine phytoplankton (annual mean δ 13 C = -22•36 ± 0•23‰). This allowed the transfers of these two POM sources to be traced within food webs and the extent of terrestrial POM uptake by the fish to be determined.

Isotopic signatures indicated the incorporation of material of terrestrial origin in the flesh of all fish species, at least during one benthic life stage (Fig. 2). δ 13 C was only slightly lower than would be expected in an exclusively marine diet for adult C. linguatula and for juvenile and adult A. laterna, but was particularly low in juvenile S. lascaris, adult B. luteum, and juvenile and adult S. solea. Bathymetric differences in diet and terrestrial POM use by the benthos explained most of the differences in isotopic signature observed among fish groups. At 0-20 m, terrestrial POM exploitation by benthic prey was reduced, and only deposit-feeding and carnivorous polychaetes exploited mainly terrestrial POM for their growth (Fig. 3). The juveniles and adults of A. laterna, the juveniles of B. luteum and the adults of S. lascaris (mainly bivalve and crustacean feeders, Table 2, Fig. 1) were located at the top of the marine phytoplanktonbased food web. Solea solea juveniles, which ingested reliance on marine primary production, but with a partial exploitation of terrestrial POM. At 30 -50 m depth, terrestrial POM was exploited by bivalve juveniles, brachyurans and Crangonidae shrimps, and all polychaetes exploited mainly river-borne material for growth (Fig. 4). The low contribution of polychaetes , PC primary consumers, POM particulate organic matter, pol. polychaetes (C carnivorous, DS surface deposit feeding, DsS subsurface deposit feeding, S suspension feeding), SC secondary consumers. Detailed isotopic signatures (means ± SD) of benthic prey can be found in Darnaude et al. (2004b).

to the diet of C. linguatula (W% < 2%) (Fig. 1) was responsible for its position at the top of the marine POM-based food web. However, the consumption of Crangonidae and the occasional ingestion of polychaetes by adult C. linguatula at this depth resulted in a slightly lower δ 13 C than would be expected in an exclusively marine diet (Fig. 2). Prey isotopic signatures fully explained the differences in carbon signature observed for the adults of S. solea, B. luteum and A. laterna. Increased consumption of polychaetes (W%) resulted in decreased δ 13 C (Fig. 1). Adult S. solea (polychaetes W% > 88%) were at the top of the terrestrial POM-based food web, while adult A. laterna (polychaetes W% < 25%) occurred near the top of the marine phytoplankton-based web. Adult B. luteum ( polychaetes W% = 70%) were intermediate in both webs.

At 70-100 m depth, benthic organisms showed little exploitation of river POM, and only subsurface deposit feeding polychaetes depended on terrestrial material for growth (Fig. 5). Their absence in the diet of C. linguatula juveniles and adults and their very low contribution to the diet of adult A. laterna and B. luteum (W% < 1%) (Fig. 1) explained why these fish were positioned at the top of the marine POM-based food web. Conversely, adult S. solea, that ingested substantial amounts of subsurface deposit feeding polychaetes (W% > 29%), were positioned between the marine and the terrestrial POM-based food webs. Thus, terrestrial POM uptake was least in C. linguatula, whose diet always included very few prey able to exploit terrestrial material for growth. It was greatest in S. solea, where deposit feeding polychaete ingestion by juveniles and adults was high and where adults were found mainly at 30 -50 m depth. In B. luteum, A. laterna and S. lascaris, main terrestrial POM uptake was intermediate. It occurred in the life stage showing the highest polychaete ingestion and/or present at 30-50 m depth.

Discussion

Reliable isotopic studies of terrestrial organic matter transfer within coastal food webs, i.e. based on the simultaneous analysis of at least two elements, are rare [START_REF] Camusso | Spatial variations in trace metal and stable isotope content of autochtonous organisms and sediments in the river Po[END_REF][START_REF] Melville | Spatial analysis of stable isotope data to determine primary sources of nutrition for fish[END_REF]. Few include benthic top predators such as flatfish [START_REF] Deegan | Evidence for spatial variability in estuarine food webs[END_REF][START_REF] Kwak | Food web analysis of southern California coastal wetlands using multiple stable isotopes[END_REF][START_REF] Paterson | A stable carbon isotope study of the food-web in a freshwater-deprived south African estuary, with particular emphasis on the ichthyofauna[END_REF], and fewer still suggest a significant influence of river-borne material on fish populations [START_REF] Lee | Carbon dynamics of Deep Bay, eastern Pearl River estuary, China. II: Trophic relationship based on carbon and nitrogen stable isotopes[END_REF][START_REF] Sheaves | Short-circuit in the mangrove food chain[END_REF]. The present study demonstrated the importance of terrestrial carbon use by juvenile and adult bottom-dwelling fish, even in periods of moderate river discharge. This was detected only as a result of the comprehensive reconstruction of fish benthic food webs at different depths, which was facilitated by the presence of only two main POM supplies (river POM and marine phytoplankton) in Fig. 4. Plot of δ 13 C vs. δ 15 N (mean‰ ± SE) obtained for the benthic food web components of the juveniles ( juv.) and adults (ad.) of Citharus linguatula ( CL) and the adults of Arnoglossus laterna ( AL), Buglossidium luteum ( BL) and Solea solea ( SS ) at 30 -50 m depth. PC primary consumers, POM particulate organic matter, pol. polychaetes (C carnivorous, DS surface deposit feeding, DsS subsurface deposit feeding, S suspension feeding), SC secondary consumers. Detailed isotopic signatures (means ± SD) of benthic prey can be found in Darnaude et al. (2004b).

the study area (Darnaude et al. 2004a). The number of potential POM sources for the benthos is usually higher in estuarine ecosystems (Riera 1998;[START_REF] Lee | Carbon dynamics of Deep Bay, eastern Pearl River estuary, China. II: Trophic relationship based on carbon and nitrogen stable isotopes[END_REF]. Therefore, it is possible that a non-negligible uptake of terrestrial POM by marine benthic fish also occurs in other studied areas (e.g. Deegan & Garrit 1997;Kwack & Zedler 1997) where system complexity prevents its detection by stable isotope methods.

      

Because of the interspecific and depth differences in terrestrial POM use by the benthos observed off the Rhone River (Darnaude et al. 2004b), flatfish depth distribution and diet can play a key role in determining their sensitivity to Rhone River floods. Flooding events have been shown to result not only in an increase in benthic prey densities, but also in modifications of benthic community compositions, mainly in favour of deposit feeding and carnivorous polychaetes [START_REF] Salen-Picard | Changements à long terme dans une communauté benthique d'un golfe méditerranéen (golfe de Fos)[END_REF][START_REF] Massé | Long-term changes in sand-bottom macrofauna along the coast of Provence (northwest Mediterranean Sea)[END_REF]. These effects should be the greatest at 30-50 m depth, where most of the terrestrial POM is deposited [START_REF] Bouloubassi | Carbon sources and cycle in the western Mediterraneanthe use of molecular markers to determine the origin of organic matter[END_REF]Darnaude et al. 2004b).

Juvenile flatfish habitat usually corresponds to the coastal margins of the adult habitat, which reduces intraspecific competition and favours juvenile growth [START_REF] Gibson | The impact of habitat quality and quantity on recruitment of juvenile flatfishes[END_REF]). However, not all flatfish follow this general pattern. Some species display similar depth distributions throughout their benthic lives [START_REF] Marinaro | Ecologie alimentaire de deux soles d'Algérie, Pegusa nasuta (Pallas) et P. lascaris (Risso) (Téléostéens, soléidés)[END_REF][START_REF] Garcia-Rodriguez | Contribution to the knowledge of Citharus linguatula (Linnaeus, 1758) (Osteicthyes: Heterosomata) in the Iberian Mediterranean[END_REF][START_REF] Bolle | Recruitment variability in dab (Limanda limanda) in the southeastern North Sea[END_REF]). Among the five species studied, both strategies were observed. In S. lascaris and C. linguatula, juveniles and adults were found within the same depth ranges. By contrast, S. solea, B. luteum and A. laterna juveniles occurred at 0-20 m whereas adults were found from 10 to 100 m depth. As terrestrial POM uptake by the benthos peaks at 30-50 m, the shallow distribution of S. solea, B. luteum and A. laterna juveniles and of all life stages of S. lascaris prevent their benefiting from the greatest river input-linked increases in prey density, unlike adult S. solea, B. luteum and A. laterna and all the life stages of C. linguatula.

Fish diet composition depends mainly on prey detection mode and gut morphology [START_REF] De Groot | On the interrelationship between morphology of the alimentary tract, food and feeding behaviour in flatfishes (Pisces: Pleuronectiformes)[END_REF]. Soleidae are morphologically adapted to the ingestion of vulnerable prey with low mobility, such as polychaetes or bivalves. Conversely, Bothidae and Citharidae are visual hunters capable of capturing and ingesting larger and more mobile prey, such as crustaceans and fish. This explains the dominance of polychaetes and bivalves in the diets of S. solea, B. luteum and S. lascaris and the predominance of crustaceans and fish in the diets of A. laterna and C. linguatula. Modification of diet with growth is mainly due to differences in metabolic needs Fig. 5. Plot of δ 13 C vs. δ 15 N (mean‰ ± SE) obtained for the benthic food web components of the juveniles ( juv.) and adults (ad.) of Citharus linguatula (CL) and the adults of Arnoglossus laterna (AL), Buglossidium luteum (BL) and Solea solea (SS ) at 70 -100 m depth. PC primary consumers, POM particulate organic matter, pol. polychaetes (C carnivorous, DS surface deposit feeding, DsS subsurface deposit feeding, S suspension feeding), SC secondary consumers. Detailed isotopic signatures (means ± SD) of benthic prey can be found in Darnaude et al. (2004b).

between juveniles (small size, energy allocated to growth) and adults (larger size, energy allocated to growth and reproduction) [START_REF] Nikolskii | Fish Population Dynamics[END_REF][START_REF] Gerking | Feeding Ecology of Fish[END_REF]. However, in flatfish it also reflects the progressive adaptation to benthic life [START_REF] Sorbe | Rôle du benthos dans le régime alimentaire des poissons démersaux du secteur Sud Gascogne[END_REF]. In general, the ingestion of small crustaceans (copepods, cumaceans) decreases with growth, and food progressively diversifies until large benthic invertebrates are taken by the adults [START_REF] Rogers | Prey partitioning among juvenile flatfish with particular reference to the Soleidae[END_REF]. Our results confirm this pattern, but ontogenetic changes in diet varied between species. In C. linguatula, prey size increased without a pronouced modification in diet composition. In the four other species, however, ontogenetic changes in diet involved not only a shift in prey size, but also a modification of the taxa ingested. As deposit feeding and carnivorous polychaetes are the only benthic invertebrates to principally exploit terrestrial POM, consumers of these species, such as adult S. solea and B. luteum, would benefit most from increased river POM inputs. All life stages of S. lascaris and adult A. laterna may also benefit from river floods, as an increase in terrestrial POM availability may result in a significant increase in the density of suspension feeding polychaetes, bivalves and Crangonidae shrimps. Conversely, Rhone River floods would have little impact on juvenile and adult C. linguatula, whose prey do not substantially exploit terrestrial POM.

Regarding feeding adaptability, the five species showed very different strategies. Citharus linguatula had a very specialized diet and a low trophic adaptability, as confirmed by the constancy of its diet irrespective of depth-range. By contrast, S. solea, B. luteum, A. laterna and S. lascaris consumed a wide range of prey items, and seemed able to adapt their diet according to prey availability in their immediate environment. They ingested highest amounts of bivalves at 0 -20 m, polychaetes at 30 -50 m and crustaceans at 70 -100 m, the depths at which these prey items were the most abundant (Darnaude et al. 2004b). Such 'opportunistic' feeding behaviour should allow all life stages of S. solea, S. lascaris, B. luteum and A. laterna to exploit the selective increases in benthic prey abundances linked to terrestrial POM inputs, notably during periods of high river discharge.

As predator size determines the size-range of prey in the diet [START_REF] Cohen | Body size of animal predators and animal prey in food webs[END_REF]), fish trophic level usually increases with size, a pattern observed in all five species studied. Trophic levels varied from 3•1 to 3•7 in juveniles and 3•5 -4•0 in adults. The increase in trophic level with growth was low in the two sole species, due to an adult diet composed mainly of primary consumers (deposit feeding polychaetes or bivalves). Therefore, despite their different sizes, the adults of B. luteum, S. solea and S. lascaris showed similar trophic levels (around 3•8). This has important implications with regard to the intensity of terrestrial POM transfer to fish. Due to energy loss within trophic pyramids, biomass decreases inversely proportional to animal position in the food web [START_REF] Pimm | Food Webs[END_REF]. Therefore, the consumption of primary consumers that directly exploit terrestrial POM for growth (deposit-feeding polychaetes, bivalves) would be more advantageous for fish than the ingestion of secondary consumers that rely on it (carnivorous polychaetes, brachyurans, shrimps). According to the trophic levels observed in the five species, adult S. Solea, B. luteum and S. lascaris should be favoured more by increased terrestrial POM use by their prey, than adult A. laterna or C. linguatula.

   

Terrestrial POM constitutes only a fraction of Rhone River inputs to the sea. The river's high dissolved organic matter and nutrient inputs influence primary production intensity over the entire NW Mediterranean basin (Tusseau-Vuillemain 1998), thereby determining food quantity and quality for larval fish. However, unlike particulate inputs, dissolved inputs are relatively constant from year to year [START_REF] Cauwet | Apports du Rhône en carbone et azote dissous et particulaire de juin 1994 à juin[END_REF] and local annual primary production is largely independent of river discharge [START_REF] Lefèvre | Review of gross community production, primary production, net community production and dark community respiration in the Gulf of Lions[END_REF]. River floods should therefore have little impact on fish larval life.

The potential impacts of river floods on fish population dynamics and their consequences for local fisheries would therefore depend on the benthic life stage(s) affected by terrestrial POM uptake. In fish, year-class strength is thought to be determined mainly in the late larval and early juvenile stages, where most mortality occurs [START_REF] Cushing | Climate and Fisheries[END_REF]. As changes in natural mortality could lead to large differences in cohort size [START_REF] Leggett | Recruitment in marine fishes: is it regulated by starvation and predation in the eggs and larval stages?[END_REF][START_REF] Bailey | Contrasting years of prey levels, feeding conditions and mortality of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska[END_REF], factors that favour survival at the juvenile stages can strongly influence population dynamics. In flatfish, food quality and quantity have stronger impacts on juvenile growth and survival than density-dependent regulation, temperature and sediment type [START_REF] Fonds | Metabolism, food consumption and growth of plaice (Pleuronectes platessa) and flounder (Platichthys flesus) in relation to fish size and temperature[END_REF][START_REF] Rogers | Environmental factors affecting the distribution of sole Solea solea (L.) within a nursery area[END_REF][START_REF] Van Der Veer | The 'maximum growth/optimal food condition' hypothesis: a test for 0group plaice Pleuronectes platessa in the Dutch Wadden Sea[END_REF][START_REF] Rogers | Population density and growth rate of juvenile sole Solea solea (L.)[END_REF]. Increased juvenile prey abundance should therefore highly favour growth and survival, increasing the number of fish recruiting to the adult stock. In this case, increased fish catches would be expected in the years directly following a flooding event, with a species-dependent response time lag reflecting age at recruitment to the fishery. This has already been shown for S. solea (Salen-Picard et al. 2002;Darnaude et al. 2004a). Observations of juvenile diet from the present study suggest a similar response in S. lascaris, A. laterna and B. luteum.

Interannual variation in reproductive success can also have an important influence on fish population dynamics, as it controls benthic recruitment through the amount of eggs released in the environment [START_REF] Miller | An overview of the second flatfish symposium: recruitment in flatfish[END_REF]. Feeding influences both size at first maturity [START_REF] Morgan | The relationship between fish condition and the probability of being mature in American plaice (Hippoglossoides platessoides)[END_REF]) and adult growth and condition [START_REF] Bromley | The influence of feeding regime on sexual maturation, fecundity and atresia in first-time spawning turbot[END_REF]. Fecundity increases with size in fish [START_REF] Horwood | The Bristol Channel sole Solea solea (L.): a fisheries case study[END_REF] and female condition influences egg quantity [START_REF] Horwood | The effect of feeding levels on the fecundity of plaice (Pleuronectes platessa)[END_REF] and quality [START_REF] Rijnsdorp | Reproductive variability in North Sea Plaice, sole and cod[END_REF]. As older and larger females show higher fecundity and produce more recruits with high survival rates [START_REF] Cardinale | The relationship between stock and recruitment: are the assumptions valid?[END_REF], the condition and growth of spawning fish, which is linked closely to their food supply, can have an impact on future yearclass strength in fish. Therefore, terrestrial POM uptake by adult fish after a flood would further optimize species reproductive success for several years, resulting in a long-term increase in fish catches, with a time lag depending on spawning frequency and age at recruitment to the fishery. This phenomenon, already demonstrated for the common sole S. solea (Salen-Picard et al. 2002;Darnaude et al. 2004a), should also occur in B. luteum and, to a lesser extent, A. laterna and S. lascaris.

Thus, dependent on fish depth distribution, food preference, dietary adaptability, trophic level and terrestrial POM uptake in moderate river discharge conditions, Rhone river floods can be predicted to have little impact on C. linguatula populations but would increase the four other species' local stocks for several years, with a maximum impact on S. solea.

Conclusion

While the present study suggests that river floods will enhance fishery production of most of the species studied for several years, further studies linking time variation in river discharge, prey abundance, fish fecundity and fish recruitment are still required before the exact nature of the flatfish response can be predicted accurately. However, the use of carbon and nitrogen stable isotopes allowed the detection and description of terrestrial carbon incorporation into five marine flatfish off the Rhone River delta in conditions of moderate river discharge. Interspecific differences in fish diet and habitat-use fully explained the intensity of terrestrial POM uptake during benthic life. As fish ecology will determine a species ability to exploit or not the increases in benthic prey production linked to terrestrial POM inputs, fish ecology controls species' sensitivity to modifications in river discharge and must be assessed carefully before estimating the impacts of river floods for coastal fisheries production. Such impacts should be maximal in large polychaete feeders with low trophic level, high trophic adaptability and wide bathymetric distribution, such as Solea solea.

Fig. 1 .

 1 Fig. 1. Depth variation in diet (expressed as dry weight percentage of prey in stomach contents) in the juveniles and adults of Citharus linguatula and in the adults of Arnoglossus laterna, Buglossidium luteum and Solea solea.

Fig. 2 .

 2 Fig. 2. Terrestrial vs. marine POM use during juvenile ( juv.) and adult (ad.) growth in Arnoglossus laterna (AL), Buglossidium luteum (BL), Citharus linguatula (CL), Solea lascaris (SL) and Solea solea (SS ). Comparison of obtained (mean δ 13 C and δ 15 N ± SE in ‰) and theoretical (dark line in bold) isotopic signature allowed estimation of the degree of terrestrial material uptake by the fish (for details see Materials and methods).

Fig. 3 .

 3 Fig.3. Plot of δ 13 C vs. δ 15 N (mean‰ ± SE) obtained for the benthic food web components of the juveniles ( juv.) and adults (ad.) of Arnoglossus laterna (AL), Buglossidium luteum (BL), Solea lascaris (SL) and Solea solea (SS ) at 0-20 m depth. Cr, Crangonidae (20 -40 mm), PC primary consumers, POM particulate organic matter, pol. polychaetes (C carnivorous, DS surface deposit feeding, DsS subsurface deposit feeding, S suspension feeding), SC secondary consumers. Detailed isotopic signatures (means ± SD) of benthic prey can be found inDarnaude et al. (2004b).

Table 1 .

 1 Global diet (expressed as dry weight percentage of prey in stomach contents) obtained for the juveniles (Juv.) and adults (Ad.) of Arnoglossus laterna, Buglossidium luteum, Citharus linguatula, Solea lascaris and S. solea off the Rhone River. N = sample size; +, values less than 0•01%; -, prey category not found

		A. laterna	B. luteum	C. linguatula	S. lascaris	S. solea
		Juv.	Ad.	Juv.	Ad.	Juv.	Ad.	Juv.	Ad.	Juv.	Ad.
	N	58	596	32	92	31	51	47	32	33	97
	Size range (total length, cm)	20 -49	81-132	32-48	83 -126	63 -91	161-237	49 -96	160 -212	32 -92	264-374
	Depth range of life (in m)	0-20	0 -100	0 -20	0 -100	30 -100	30 -100	0 -20	0 -20	0-20	0-100
	Polychaetes	21•5	26•0	21•9	56•8	-	1•1	65•6	2•9	34•5	75•9
	Surface deposit feeders	2•1	4•7	-	+	-	-	6•6	0•4	24•4	3•0
	Subsurface deposit feeders +	0•3	-	17•9	-	0•6	-	-	10	54•1
	Suspension feeders	11•5	13•5	-	0•9	-	-	59•0	1•7	-	5•5
	Carnivores	7•9	7•5	21•9	38•0	-	0•5	-	0•8	+	13•3
	Sipunculids	-	-	-	-	-	-	-	-	-	2•2
	Crustaceans	71•3	68•3	51•2	27•4	35•2	25•3	26•6	15•6	34•2	5•3
	Copepods

Table 2 .

 2 Carbon and nitrogen isotopic signatures (mean δ 13 C and δ 15 N ± SE, in‰) in the five flatfish studied off the Rhone River. -, fish group absent within

	depth range
		0 -20 m depth	30 -50 m depth	70 -100 m depth	global: 0-100 m
		δ 13 C	δ 15 N	δ 13 C	δ 15 N	δ 13 C	δ 15 N	δ 13 C	δ 15 N
	A. laterna
	Juveniles	-18•93 ± 0•46	8•52 ± 0•16	-18•93 ± 0•46	8•52 ± 0•16	-	-	-18•93 ± 0•46	8•52 ± 0•16
	Adults	-16•75 ± 0•25	10•49 ± 0•47	-18•23 ± 0•26	10•10 ± 0•14	-18•25 ± 0•25	9•69 ± 0•62	-18•23 ± 0•26	10•10 ± 0•14
	B. luteum
	Juveniles	-17•52 ± 0•15	8•77 ± 0.18	-17•52 ± 0•15	8•77 ± 0.18	-	-	-17•52 ± 0•15	8•77 ± 0.18
	Adults	-18•47 ± 0•31	11•18 ± 0•21	-19•04 ± 0•22	11•07 ± 0•09	-17•99 ± 0•52	10•97 ± 0•31	-19•04 ± 0•22	11•07 ± 0•09
	C. linguatula
	Juveniles	-	-	-17•60 ± 0•08	8•98 ± 0•08	-17•58 ± 0•09	8•85 ± 0•08	-17•60 ± 0•08	8•98 ± 0•08
	Adults	-	-	-17•64 ± 0•08	11•63 ± 0•11	-17•67 ± 0•10	11•67 ± 0•10	-17•64 ± 0•08	11•63 ± 0•11
	S. lascaris
	Juveniles	-18•71 ± 0•15	10•69 ± 0•12	-18•71 ± 0•15	10•69 ± 0•12	-	-	18•71 ± 0•15	-10•69 ± 0•12
	Adults	-16•96 ± 0•22	11•10 ± 0.17	-16•96 ± 0•22	11•10 ± 0.17	-	-	-16•96 ± 0•22	11•10 ± 0.17
	S. solea
	Juveniles	-20•03 ± 0•33	10•07 ± 0•14	20•03 ± 0•33	10•07 ± 0•14	-	-	-20•03 ± 0•33	10•07 ± 0•14
	Adults	-19•07 ± 0•23	11•00 ± 0•13	-20•14 ± 0•38	11•06 ± 0•15	-20•06 ± 0•38	11•05 ± 0•23	-20•14 ± 0•38	11•06 ± 0•15

Table 3 .

 3 Temporal variation in isotopic signature observed for the Rhone River particulate organic matter (POM) and marine phytoplankton (offshore seawater POM); N = sample size; t-test = results of Student's tests realized on isotopic signatures in each case. Probabilities (P) inferior to 0•05 indicate significant within-year signature changes by source or among-sources signature differences for each period

	Period of high	Period of low
	river discharge	river discharge
	(winter-spring)	(summer)
	N = 6	N = 3	t-test

significant amounts (W% > 34%) of deposit feeding polychaetes, had a very different position on the graph, denoting a substantial dependency on terrestrial POM source for their growth. Adult S. solea and B. luteum and juvenile S. lascaris (deposit feeding or carnivorouspolychaete feeders) had intermediate positions, indicating

Acknowledgements

Thanks are expressed to the crews of the OV Antedon and Pr Georges Petit for their help in sampling, to Dr Chantal Salen-Picard for polychaete determination and counting, to Mrs Gillian Taylor for her assistance with stable isotope analysis and to Dr Mireille Harmelin-Vivien, Chantal Salen-Picard and Ewan Hunter for their useful comments on the manuscript.