Occurrence of Organic Colloids in the Stratified Estuary of the Krka River (Croatia)

Keywords: colloidal organic carbon, dissolved organic carbon, halocline, Krka Estuary

Along a vertical profile of a stratified estuary, we determined organic carbon in the following fractions: low molecular weight (LMW-OC<0-02 gin), colloidal (0-02 gm<COC<0-7 gm), dissolved (DOC<0-7 gm) and particulate (POC>0.7 gm). The results showed abundant concentrations of organic colloids accounting for: 40% of total organic carbon (TOC) in the overlying brackish water (salinity 2), 22% in the underlying marine water (salinity 38) and 19% ( + 5 % ) for intermediate salinity samples (salinity 3-31). Upon contact with seawater (in the salinity range 2-5) both DOC and COC concentrations decreased drastically (44 and 47 ]aM respectively), whereas POC slightly increased (I 1 gM), being consistent with previous laboratory and field studies, and indicating that at low salinities, the colloidal fraction is actively involved in the DOC physicochemical aggregation. In the halocline, we observed that organic colloids and relatively degraded particles accumulated together in the lowest part of the interface (salinity 31). Because previous studies in the same area have shown accumulation of degraded cells and fragments in the bottom of the halocline, we suggest that large colloids (COC in this study) at the interface probably originated from fragmentation of non-living organisms or aggregates. Furthermore, it is likely that these processes which are enhanced in a highly stratified estuary, are partly responsible for the DOC non-conservative dilution observed along this profile.

Introduction

In aquatic environments colloids are defined as material in the range 0.001-1 g m (Void & Vold, 1983;[START_REF] Goldberg | Modern chemistry and chemical technology applied to the ocean and its resources[END_REF]. T h e r e b y , they comprise a large spectrum of organic and inorganic species including clays, macromolecules, aggregates as well as viruses and some bacteria and flagellates [START_REF] Sharp | Size classes of organic carbon in seawater[END_REF][START_REF] Cauwet | Organic chemistry of sea water particulates. Concepts and developments[END_REF]; S t u m m & M o r g a n , 1981; [START_REF] Rassoulzadegan | Predator-prey interactions of nanozooplankton and bacteria in an oligotrophic marine environment[END_REF]Bergh et al., 1989). Although several studies have reported a significant concentration of colloids particularly in particle-rich m e d i a such as the oceanic euphoric layer, the coastal or estuarine waters [START_REF] Sigleo | Composition of estuarine colloidal material: organic components[END_REF] oPresent address: Laboratoire de Microbiologie Marine, CNRS UPR 223, Campus de Luminy Case 907, 13 288 Marseille Cedex 9, France. 0272-7714/95/010105+ 10 $08.00/0 ¢~ 1995 Academic Press Limited [START_REF] Koike | Role of sub-micrometre particles in the ocean[END_REF][START_REF] Wells | Occurrence of small colloids in sea water[END_REF][START_REF] Benner | Bulk chemical characteristics of dissolved organic matter in the ocean[END_REF][START_REF] Ogawa | Comparison of two methods for measuring dissolved organic carbon in sea water[END_REF]Semprr6 et al., 1994), very little is known about their distribution and how they might affect biogeochemical processes in natural waters. However, in the estuary numerous studies reported physicochemical aggregation phenomena of high molecular weight terrigenous-derived dissolved organic matter (DOM) [START_REF] Sholkovitz | Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater[END_REF][START_REF] Mayer | Geochemistry of humic substances in estuarine environments[END_REF][START_REF] Whitehouse | Organic carbon and colloids in the Mackenzie River and Beaufort Sea[END_REF]Semprr6 & Cauwet, 1993) highlighting a colloid-*particle transfer which may precede sedimentation [START_REF] Pauc | Flocculation et potentiel de surface des mat~riaux en suspension et environnement d'embouchure[END_REF][START_REF] Mccave | Size spectra and aggregation of suspended particles in the deep ocean[END_REF]. On the other hand, it has been proposed that some organic colloids may have originated from fragmentation of particles [START_REF] Lampert | Release of dissolved organic carbon by grazing zooplankton[END_REF][START_REF] Cho | Major role of bacteria in biogeochemical fluxes in the ocean's interior[END_REF][START_REF] Karl | Downward flux of particulate organic matter: a particle decomposition paradox[END_REF][START_REF] Koike | Role of sub-micrometre particles in the ocean[END_REF][START_REF] Smith | Intense hydrolytic enzyme activity on marine aggregates and implications for rapid particle dissolution[END_REF]Semprr6 et al., 1994) indicating in this case a particle~colloid pathway. Moreover, recent discoveries indicated possible assimilation of colloidal D O M by choanoflagellates as well as tunicates [START_REF] Marchant | Uptake of submicrometre particles and dissolved organic material by Antarctic choanoflagellates[END_REF][START_REF] Tranvik | Uptake and utilization of' colloidal D O M ' by heterotrophic flagellates in seawater[END_REF]. Therefore, qualifying and quantifying processes involving the colloidal fraction, may help to a better understanding of the microbial food-web as well as the organic matter transfer in estuary.

The highly stratified estuary of the Krka River (Croatia), is well suited for investigations dealing with the transformations of organic matter because different processes such as primary production, degradation of particles by bacteria as well as physicochemical aggregation/disaggregation have been observed along a vertical salinity gradient established in less than 1 m [START_REF] Zutic | A film of organic matter at the fresh water/sea-water interface of an estuary[END_REF][START_REF] Vilicic | Vertical distribution of phytoplankton in a stratified estuary[END_REF][START_REF] Biscan | Surface properties of suspended solids in stratified estuaries (Krka River estuary and Rh6ne River delta)[END_REF][START_REF] Cauwet | Carbon inputs and biogeochemical processes at the halocline in a stratified estuary: Krka River, Yugoslavia[END_REF][START_REF] Fuks | Bacterial abundance and activity in the highly stratified estuary of the Krka River[END_REF]Legovic et al., 1991a;Svetlicic et al., 1991). In spring 1990, we simultaneously collected water samples along a vertical profile from the Krka Estuary and analysed them for organic carbon content in the so-called ' low molecular weight ', ' colloidal ', ' dissolved ' and ' particulate ' fractions. By comparison between these parameters and previous studies, we discuss the physicochemical and biological processes which may involve large organic colloids in the size range 0"02-0"7/am.

Materials and m e t h o d s

In May 1990, seven water samples were collected from the Krka Estuary (Figure 1; station E) above, below and at 4-10-cm intervals vertically in the halocline. For the sampling a scuba diver used a multicharmel peristaltic pump which avoids disturbance by air bubbles [START_REF] Kniewald | Marine sampling by scuba diving. 3. Sampling procedures for measurement of mercury concentrations in estuarine waters and sea water[END_REF][START_REF] Cauwet | Carbon inputs and biogeochemical processes at the halocline in a stratified estuary: Krka River, Yugoslavia[END_REF]. In this study, the halocline which was defined as the interval in which ~S / ~Z > 5 m ~ (Vukojevic, 1983, cited in Legovic et al., 1991b), separated the upper brackish water layer from the lower marine layer at c. 2 m below the surface. After collection, the water was filtered under reduced vacuum with an all-glass filter holder through a 47-ram-diameter pre-heated (45 °C) glass fibre filter (GF/F). The filtration was stopped after I 1 to avoid colloid adsorption on the filter. The rinsed filters were dried at 40 °C for 24 h and were used later for total suspended matter (TSM) and POC determinations, whereas the filtrates were collected in pre-heated (450 °C) graduated glass bottles (1 1) tightly closed with a Teflon-lined cap.

The subsamples were subsequently ultrafiltered under a laminar flow air bench through a 0-02 lam laboratory synthesized tubular membrane of zirconium/aluminium oxide [START_REF] Larbot | New inorganic ultrafiltration membranes: titania and zirconia membranes[END_REF] by the method described previously (Semprrr, 1991;Semprr6 et al., 1994). The laboratory experiments (with Dextran solutions) indicated that the membrane cutoff was around 500 000 dalton. Briefly, 1 1 of each pre-filtered sample was 1.7 times concentrated by cross-flow ultrafiltration on the membrane 15o51'36"E 43o47'30"N Figure 1. Location of the Krka Estuary (Croatia) and sampling station (E). inserted in a Teflon/Kel F unit. These conditions facilitated a good conditioning of the membrane and avoided membrane clogging. Ultrafiltrate and concentrate were collected in pre-heated (450 °C) graduated glass bottles (1 l) and closed with a Teflon-lined cap. Total procedure time for each sample was c. 5 h. Aliquots (8 ml) of filtered and ultrafiltered samples (ultrafiltrate+concentrate) were taken in pre-heated (450 *C) 10-ml glass vials poisoned with HgC12 ( 1 0 m g 1 -~), covered with a Teflon-lined cap, and stored at 4 *C in the dark. Therefore, by the separative methods described above, organic carbon was operationally fractionated in the following fractions: L M W -O C < 0 -0 2 ~trn; 0 . 0 2 < C O C < 0 . 7 lam; D O C < 0 . 7 ~tm; POC>0.7 lain.

For D O C (ultrafiltrate, concentrate and initial GF/F filtrate) measurements, after acidification (pH 2) with a 2 M HC1 solution, the samples were bubbled for 5 min, with CO2-free pure air to purge inorganic carbon. Dissolved organic carbon was measured by means of the uv-persulphate oxidation method [START_REF] Collins | An automated photochemical method for the determination of dissolved organic carbon in sea and esruarine waters[END_REF] modified by Cauwet (1984). The standard deviation was c. 3/aM for <83/aM and less than 2% above. The carbon blank of the water used for the standard solutions (83,167,250,333 IxM) Cauwet et al., 1990a;Sempdr6 & Cauwet, 1993;[START_REF] Cauwet | HTCO method for DOC analysis in seawater: influence of catalyst on blank estimation[END_REF]. Dissolved inorganic carbon (DIC) was measured by a uv-persulphate analyser with a standard deviation of 8 ~tM. After removal of carbonates, P O C was determined by G F / F filter combustion ( L E C O CS 125 analyser) with a standard deviation of 2%.

Results and d i s c u s s i o n

Description of the halocline, particle characteristics Table 1 gives the physical parameters (depth collection, salinity) as well as D I C and P O C concentrations and P O C % [(POC x 100)FFSM] of the water samples along the profile. T h e D I C distribution showed a normal [START_REF] Meybeck | River transport of organic carbon to the ocean[END_REF][START_REF] Cauwet | Carbon inputs and biogeochemical processes at the halocline in a stratified estuary: Krka River, Yugoslavia[END_REF] conservative behaviour [Figure 2(a); r2=0.96, n=7] establishing confidence in our sampling precision. T h e P O C concentration range was 3 2 -1 0 8 ~tM (average 67 laM) comprising 15-26% (average 19%) of T S M and 21--44% (average 32%) of total organic carbon ( T O C ) . These high P O C % for corresponding T S M data reflect a situation in which aquatic production d o m i n a t e d the charge of the river [START_REF] Meybeck | River transport of organic carbon to the ocean[END_REF][START_REF] Ittekot | Global trends in the nature of organic matter in river suspensions[END_REF]Cauwet et al., 1990b).

Particulate organic carbon concentrations increased with the salinity from the brackish water to the b o t t o m of the halocline, thus, the strong salinity gradient (from 3 to 31 in 22 cm) and particulate content (POC and P O C % ) of the profile [Figure 2(b,c)] are in good agreement with those described previously [START_REF] Zutic | A film of organic matter at the fresh water/sea-water interface of an estuary[END_REF]Cauwet, 1991, and references therein;Legovic et al., 1991a, b). (1) A well-defined halocline (at c. 2 m below the surface) separating the overlaying brackish water (first sample salinity 2) and the underlying marine water (salinity 38). (2) A particle accumulation in the b o t t o m of the halocline (depth= 2.08-2.12 m; salinity 24-31) as a consequence of the density and shear stress barrier [START_REF] Cauwet | Carbon inputs and biogeochemical processes at the halocline in a stratified estuary: Krka River, Yugoslavia[END_REF]. However, it is important to notice that P O C % m a x i m u m ( d e p t h = 2 . 0 6 m ; salinity 19) was situated above the P O C m a x i m u m ( d e p t h = 2 . 0 8 m; salinity 24) indicating that the particles are preferentially degraded in the lowest part of the interface. This later observation is consistent with the results of [START_REF] Zutic | A film of organic matter at the fresh water/sea-water interface of an estuary[END_REF] and [START_REF] Vilicic | Vertical distribution of phytoplankton in a stratified estuary[END_REF] ""1'"'1' '"1'"'1' '"1 '"' I" "1'"' I 1 and2) and if not shown are smaller than the symbol size. salinity (2); C O C comprised 2 2 -5 4 % (average 32%) of D O C (Table 2) and 14-40% (average 22%) of T O C (Table 3), indicating that organic colloids represented a significant fraction of organic carbon during the spring season in the Krka Estuary. Interestingly, the results show a clear non-conservative behaviour of D O C (Figures 3 and4) as well as C O C and L M W -O C (Figure 3), a point which will be discussed later.

h a l o c l i n e , a c c u m u l a t i o n o f c h l o r o p h y l l d e g r a d a t i o n p r o d u c t ( p h a e o p h y t i n ) as well as h e t e r o t r o p h i c activity, t h e c h l o r o p h y l l b e i n g p r o d u c e d in t h e o v e r l y i n g w a t e r .

Dissolved and colloidal organic carbon distribution

In the overlying brackish water the colloidal fraction was relatively the most a b u n d a n t ( C O C / D O C = 5 4 % ; C O C / T O C = 4 0 % ) . We found that u p o n contact with seawater (salinity 2-5) the brackish-water D O C decreased sharply by 44 laM, moreover, Tables 2, 3 and Figure 3 indicate that this can principally be attributed to its colloidal fraction. This fact has been observed in other estuaries by different authors [START_REF] Sholkovitz | Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater[END_REF]; 2). [START_REF] Whitehouse | Organic carbon and colloids in the Mackenzie River and Beaufort Sea[END_REF]Semprr6 & Cauwet, 1993), and certainly reflects the physicochemical flocculation of some terrigenous-derived dissolved organic mafter (mainly humic acids) through colloidal state into particles due to ionic strength changes [START_REF] Sholkovitz | Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater[END_REF][START_REF] Mayer | Geochemistry of humic substances in estuarine environments[END_REF]. This phenomenon might explain why we observed [Figure 2(b)] in the salinity 5 sample, a small increase of POC concentration (11 laM), the rest of the newly generated particles having probably sunk towards the sediment through the interface. This assumption seems consistent with the previous work of [START_REF] Hadzija | The carbohydrates in relation to mineralogic and granulometric composition of surface sediments in the Karst Estuary (River Krka Estuary, Yugoslavia)[END_REF] indicating a preferentially terrigenous source of sedimented organic material (rich in carbohydrate-humic substances complexes) in the estuarine sediments. Slightly above the halocline (depth=2.06 m; salinity 19) we observed a D O C submaximum due to the L M W -O C . It is important to notice that at the same level, we observed the highest organic content of particles [see P O C % , Figure 2(c)]. Although in this study, we do not have simultaneous biological data available, several authors in previous work located the chlorophyll maximum above the halocline as well [START_REF] Zutic | A film of organic matter at the fresh water/sea-water interface of an estuary[END_REF][START_REF] Vilicic | Vertical distribution of phytoplankton in a stratified estuary[END_REF]. If these informations were confirmed together, they may suggest a production of L M W -O C by healthy phytoplankton above the interface.

From the higher part of the halocline (depth=2-06 m; salinity 19) to its lowest part (depth = 2" 12 m; salinity 31), the LM'Og-OC concentrations decreased slightly from 121 to 98 jaM, whereas C O C increased slightly from 32 to 41 laM. These results showed that at the interface level (depth=2.12 m; salinity 31), organic carbon is in colloidal and particulate states for, respectively, 17 and 44% (Table 3). Actually, the residence time of particles as well as the number of heterotrophic bacteria in an interface is larger than anywhere else in the water column [START_REF] Mayer | Aggregation of colloidal iron during estuarine mixing: kinetics, mechanisms, and seasonality[END_REF][START_REF] Zutic | A film of organic matter at the fresh water/sea-water interface of an estuary[END_REF]Legovic et al., 1991a), then particulate organic matter including dead organisms and also various aggregates, represents a potential source for transformations such as flocculation, disaggregation or D O M releases by organisms. However, it has been reported that in seawater, autolysis of dead ceils releases mainly low molecular weight compounds [START_REF] Carlucci | Diel production and microheterotrophic utilization of dissolved free amino acids in waters off Southern California[END_REF] whereas the ultrafiltration membrane we used in this study, retains mostly large colloids with a molecular weight higher than 500 000 dalton (Semprr6 et al., 1994).

Although in estuaries, physicochemical aggregation of dissolved organic matter into colloids induced by seawater usually appears mainly at low salinifies (salinity less than 10; [START_REF] Sholkovitz | Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater[END_REF][START_REF] Mayer | Geochemistry of humic substances in estuarine environments[END_REF], we can not totally disregard the flocculation process in saline waters [START_REF] Baskaran | Scavenging of thorium isotopes by colloids in seawater of the Gulf of Mexico[END_REF][START_REF] Moran | Short residence time of colloids in the upper ocean estimated from 238U-234Th disequilibria[END_REF]. However, we feel that the apparent increase of organic colloids near the interface may be due to the fragmentation of larger particles by physical disaggregation [START_REF] Karl | Downward flux of particulate organic matter: a particle decomposition paradox[END_REF] or biological processes (such as micro-organism degradation as well as interspecies grazing; [START_REF] Lampert | Release of dissolved organic carbon by grazing zooplankton[END_REF][START_REF] Koike | Role of sub-micrometre particles in the ocean[END_REF][START_REF] Smith | Intense hydrolytic enzyme activity on marine aggregates and implications for rapid particle dissolution[END_REF]. Furthermore, Koike et al. (1990) reported that the grazing of bacteria by fagellates in the open ocean is responsible for some submicrometre particle (essentially organic) production. Interestingly, Sigleo et al. (1982) gave a similar composition (30% carbohydrates, 10% lipids and 10% nucleotides) for colloidal material and phytoplankton in the Patuxent River. It is likely that these processes occur together at the interface in proportions difficult to determine.

Non-conservative distribution of D O C

These phenomena may certainly explain partly the clear non-conservative D O C behaviour (Figure 4; r 2 < 0" 1, n = 7) along the vertical profile at this season. Several authors have already reported no clear relationship between salinity and D O C [START_REF] Cauwet | Carbon inputs and biogeochemical processes at the halocline in a stratified estuary: Krka River, Yugoslavia[END_REF][START_REF] Suzuki | Dissolved organic carbon enigma: implications for ocean margins[END_REF][START_REF] Miller | Preliminary study of D O C in the Tamar Estuary, UK, using OV-persulphate and H T C O techniques[END_REF]Semprr6 & Cauwet, 1993). On the other hand, although the flocculation of the colloidal fraction of D O C was generally admitted, D O C conservative dilution has been observed many times [START_REF] Moore | The behaviour of dissolved organic material, iron and manganese in estuarine mixing[END_REF][START_REF] Mantoura | Conservative behaviour of riverine dissolved organic carbon in the Severn Estuary: chemical and geochemical implications[END_REF][START_REF] Whitehouse | Organic carbon and colloids in the Mackenzie River and Beaufort Sea[END_REF]. Dissolved organic carbon conservative dilution concept along an estuary is usually related to internal physicochemical and biological processes [START_REF] Mantoura | Organic film at the halocline[END_REF]. Flowing from a karstic environment, the Krka River generally has clear water and a very low D O C concentration constant throughout the year, therefore, the biological production in the estuary will play an important role in the carbon budget [START_REF] Cauwet | Carbon inputs and biogeochemical processes at the halocline in a stratified estuary: Krka River, Yugoslavia[END_REF]. However, upon the first contact with seawater, and probably because of the sensitive sampling technique we observed the fate of D O C by physicochemical processes (29% as regard to the theoretical dilution by seawater salinity 5). Moreover, the special character of the interface (which acts as a kind of density barrier) probably contributes to biologically regulate the concentration of colloidal and low molecular weight fractions of D O C slightly above and in the halocline. All these organic matter transformations which in other kinds of estuaries may occur near the sediment or in the bottom nepheloid layer (Semp~r~ et al., 1994) might firstly not be detected by surface water sampling, and secondly affect the D O C mass balance in the estuary.

Figure 2 .

 2 Figure 2. Vertical profiles for (a) salinity, Co) particulate organic carbon (POC>0.7 pm) and (c) organic content of total suspe, ndcd mattcr [POC%=(100 x POC)FFSM]. POC standard deviation is within 2%, horizontal error bars are smaller than the symbol size.

T

  a b l e 2 gives D O C a n d C O C d a t a for t h e s a m p l e s . M a s s b a l a n c e s m e a s u r e d o n t h e s a m p l e s i n d i c a t e d t h a t a n a v e r a g e o f 105 :t: 2 % w a s f o u n d in t h e c o n c e n t r a t e a n d u l t r a f d t r a t e f r a c t i o n s i n d i c a t i n g t h a t c o n t a m i n a t i o n a n d loss w e r e m i n i m i z e d d u r i n g t h e p r o c e d u r e . D i s s o l v e d o r g a n i c c a r b o n c o n c e n t r a t i o n s r a n g e d f r o m 103 to 147 laM ( a v e r a g e 133 IxM) a n d are s i m i l a r w i t h t h o s e a l r e a d y p u b l i s h e d ( C a u w e t , 1 9 9 1 ) . T h e C O C c o n c e n t r a t i o n r a n g e w a s 3 2 -7 9 ~tM ( a v e r a g e 4 3 ~tM) b e i n g m a x i m u m at l o w

  Vertical profiles for (a) salinity, (b) dissolved organic carbon (DOC<0"7 lam), (c) colloidal organic carbon (0-02<COC<0.7/am) and (d) low molecular weight organic carbon (LMW-OC<0.02 lam). Horizontal error bars are instrument standard deviations (indicated in Tables

  Dissolved organic carbon (DOC) concentration versus salinity for the water samples collected from the Krka Estuary, May 1990. Horizontal error bars are instrument standard deviations (indicated in Table

TABLE 1 .

 1 Depth collection, salinity, particulate organic carbon (POC) and dissolved inorganic carbon (DIC) in water samples collected from the Krka Estuary, May 1990

	Depth (m)	Salinity	POC (laM)	POC%	DIC(laM)
	1.50	2	48 + 1	18	3815 + 8
	1.90	3	36 + 1	16	3931 4-8
	1.96	5	59 4-1	17	3591 4-8
	2.06	19	82 4-2	26	3239 4-8
	2-08	24	108 4-2	22	2951 4-8
	2.12	31	106 4-2	20	2934 4-8
	5.00	38	32 4-1	15	2488 4-8
	POC% = (POC x 100)/TSM.

was estimated to be 15 JaM by pure air circulation inside the system and subtracted from the standards. In this study, C O C represented (after volume corrections) the difference between DOCconcCnt~at e and D O C ultrafiltrate and thus its uncertainty was estimated to c. 4 laM from the square root of the sum of the standard deviation calculation. O u r uv-persulphate unit might have underestimated D O C values by a factor of 1.0-1.2 (

TABLE 2 .

 2 Depth collection, salinity, dissolved organic carbon (DOC), colloidal organic carbon (COC) and low molecular weight organic carbon (LMW-OC) in the water samples collected from the Krka Estuary, May 1990

	Depth (m) Salinity DOC (~tM) COC (~tM) LMW-OC (~tM) COC/DOC (%) % InitialDOC
	1"50	2	147 ± 3	79 ± 3	75 + 2	54	108
	1-90	3	134 :l: 3	46 + 4	99 4-2	34	108
	1-96	5	103 4-2	32 4-3	80 4-2	31	108
	2-06	19	147 4-3	32 + 4	121 4-2	22	104
	2-08	24	144 4-3	40 4-4	109 4-2	28	103
	2-12	31	133 4-3	41 4-4	98 + 2	31	104
	5.00	38	121 4-2	33 4-4	92 + 2	27	103
			DOC<0.7 jam; 0-02<COC<0-7 ~tm; LMW-OC<0-02 ~m.
			% Initial DOC=(DOCult~m~,c+COC) x 100/DOCi~au.j ..... .

TABLE 3 .

 3 Depth collection, salinity, and relative abundance of particulate organic carbon (POC), colloidal organic (COC) and low molecular weight organic carbon (LMW-OC) in the water samples collected from the Krka Estuary, May 1990

	Depth (m)	Salinity POC/TOC (%) COCfTOC (%) LMW-OC/TOC (%)
	1.50	2	25	40	38
	1.90	3	21	27	58
	1"96	5	36	20	49
	2.06	19	36	14	53
	2.08	24	43	16	43
	2.12	31	44	17	41
	5.00	38	21	22	60
	POC>0-7 p.m; DOC<0"7 lam; 0-02 ~tm<COC<0.7 lam; LMW-OC<0"02 ~tm;
	POC+DOC=TOC.
	Salinity
	5 10152025303540
	0.20
	1.40
	1-60
	~. 1-80
	2.0(
	2.2(
	5.01
	5.20

Acknowledgements

This study is part of a French-Yugoslav research programme on stratified estuaries, supported by EEC contract No. CI1-0544-F. We thank D. Petricoli for sample collection, Professors I. Koike, H. Ogawa and N. Ohkuchi from the O. R. I. Tokyo Japan for useful comments on the manuscript, Drs B. Cosovic, N. Vdovic, V. Vojvodic and V. Zutic from the Centre For Marine Research of the Rudjer Boskovic Institute, Zagreb Croatia for helpful discussions. The authors are also very grateful to Manami Motoya and Dr J. Sanderson for the English correction of the manuscript and to the anonymous referees for improving the quality of the manuscript.


Michel Gourgand
email: gourgand@isima.fr

Nathalie Grangeon
email: grangeon@isima.fr

Nathalie Klement
email: klement@isima.fr

An analogy between bin packing problem and permutation problem: a new encoding scheme

The bin packing problem aims to pack a set of items in a minimum number of bins, with respect to the size of the items and capacity of the bins. This is an NP-hard problem. Several approach methods have been developed to solve this problem. In this paper, we propose a new encoding scheme which is used in a hybrid resolution: a metaheuristic is matched with a list algorithm (Next Fit, First Fit, Best Fit) to solve the bin packing problem. Any metaheuristic can be used but in this paper, our proposition is implemented on a single solution based metaheuristic (stochastic descent, simulated annealing, kangaroo algorithm). This hybrid method is tested on literature instances to ensure its good results.

Introduction

The bin packing problem has been introduced by [START_REF] Johnson | Near-optimal bin packing algorithms[END_REF]. It considers a set of N items, each item with a given size w i , and several bins with a same capacity C. The aim of this problem is to pack all of the items in a minimum number of bins. The sum of the size of the items packed in a bin has to be smaller than the capacity of the bin. Each item has to be packed in one bin.

This problem can be met in industrial application or computer network design and memory allocation [START_REF] Chandra | Bin packing with geometric constraints in computer network design[END_REF]. [START_REF] Bortfeldt | Container loading problems: A state-of-the-art review[END_REF] makes a state-of-the-art review about the container loading problem: the bin packing problem can be used to pack a set of cargo into a minimum number of containers. It can also be used to solve assembly line balancing problems [START_REF] Wee | Assembly line balancing as generalized bin packing[END_REF] or multiprocessor scheduling problems [START_REF] Coffman | An application of bin-packing to multiprocessor scheduling[END_REF].

In [START_REF] Gourgand | Activities planning and resource assignment on multi-place hospital system: Exact and approach methods adapted from the bin packing problem[END_REF] we use the bin packing problem to model the problem of activities planning and resources assignment, called the HCT problem. The problem considers a system composed of resources and of a set of activities to assign. Each activity needs a resource to be treated and a time slot, a period when it will be done. Each activity has a known process time. Each activity starts in one period and finishes in the same period. Resources have a planning defining their available time: the resource opentime. Each resource cannot treat all the activities, there is a list of incompatibilities between activities and resources.

The aim is to assign each activity to one resource and one period. Activities have to be done as soon as possible, so the assigned periods have to be the smallest possible. The aim of the HCT problem is not to make a precise schedule of the activities but to assign a period to each activity.

The HCT problem has to respect some constraints: resources have to be able to process their assigned activities according to the incompatibilities ; activities have to be assigned to a resource during the opentime of this resource ; each activity has to be assigned to one resource and one period. Figure 1 gives an example of an assignment in such an application. Activities are assigned to resources during periods. A couple (resource, period) can be seen as a bin.

Table 1 summarizes the analogies between the bin packing problem and this application. A new constraint in our application is the incompatibility constraint between resources and activities. The HCT problem can be seen as an extension of the bin packing problem with incompatibility constraints. In Section 2, a brief state of the art about approximate algorithms used to solve the bin packing problem is made. In this paper, we propose an encoding scheme, which is used in the hybridization between a metaheuristic and a list algorithm. The used method is described in Section 3, as its theoretical justification. The experimental results on literature instances are presented in Section 4. This paper is ended by a conclusion and some further works.

State of the art

Given the size of the real problems that can be solved thanks to the bin packing problem, exact methods quickly reach their limit about computational time. Moreover, [START_REF] Garey | Computers and Intractability: A Guide to the Theory of NP-completeness[END_REF] shows that it is an NP-Complete decision problem. Approximate algorithms seem to be a good way to solve this problem.

The most popular list algorithms have been developed by [START_REF] Johnson | Near-optimal bin packing algorithms[END_REF]. Items are considered one by one according to a list. Next Fit (N F ) is the most intuitive method for the bin packing problem. The maximum number of items is packed into the current bin. If the available space in the current bin is smaller than the size of the considered item, this bin is closed and a new bin is opened and becomes the new current bin. First Fit (F F ) is different from N F in that none of the bins is closed. Each item is packed into the first bin which can contain it. Once an item cannot fit in any bin, a new bin is opened. Best Fit (BF ) consists in packing each item into the best bin which is the one with the smallest available space after packing the considered item into it.

The use of metaheuristics to solve the bin packing problem starts in the 1990s. In most published works, a classical encoding scheme S is used where S(i) is the index of the bin where item i is placed. [START_REF] Alvim | A hybrid improvement heuristic for the one-dimensional bin packing problem[END_REF] proposes a hybrid metaheuristic based on tabu search. [START_REF] Falkenauer | A hybrid grouping genetic algorithm for bin packing[END_REF] proposes a hybrid grouping genetic algorithm which uses the heuristic F F D [START_REF] Johnson | Near-optimal bin packing algorithms[END_REF] and the dominance criterion from [START_REF] Martello | Lower bounds and reduction procedures for the bin packing problem[END_REF]. To a better use of grouping genetic algorithm, it uses the classical encoding, augmented with a group part which defines the used bins. [START_REF] Loh | Solving the one-dimensional bin packing problem with a weight annealing heuristic[END_REF] applies a new approach method: the weight annealing. It uses the classical encoding scheme. [START_REF] Levine | Ant colony optimization and local search for bin packing and cutting stock problems[END_REF] uses a hybrid ant colony optimization, inspired by Falkenauer's works. It does not individualize the items, they are designated by their size and not their index: S = (w i ) i∈N with w i the size of item i. In these papers, the hybrid part consists in matching a known metaheuristic (like genetic algorithm, tabu search method, ant colony optimization) to an additional improvement method such a local search.

Another encoding scheme has been exploited. [START_REF] Reeves | Hybrid genetic algorithms for bin-packing and related problems[END_REF] uses a permutation coding to hybridize the genetic algorithm. [START_REF] Stawowy | Evolutionary based heuristic for bin packing problem[END_REF] uses a permutation with separators representation: it is a list of n items and l separators of bins, integers from range {1, ..., n} represent the item, integers {n + 1, ..., n + l} represent the separators. It developed a simplified version of the genetic algorithm: a simple evolutionary based heuristic.

Most of the works on the bin packing problem uses population based metaheuristics.

Proposed hybrid method

The method proposed in this paper uses a hybridization between a metaheuristic and a list algorithm (Next Fit, First Fit, Best Fit). This method is performing in two search spaces: Ω where a solution represents a list of items and S where a solution represents the assignment of each item to a bin. The aim of this purpose is to reduce the size of the set of solutions where the metaheuristic is performing.

Encoding of the set of solutions

The used encoding is inspired by the one used in the permutation problems. A solution (X i ) i∈N ∈ Ω is a list of items as in a permutation problem with X i an item index. A list algorithm L, such as N F , F F or BF , is applied to the list of items X to determine the assignment of the items to the bins: (Y i ) i∈N ∈ S as the classical scheme, with Y i the bin index assigned to item i. Then a cost function H is applied to the assignment Y . The general scheme of the encoding is the following:

X ∈ Ω -→ Heuristic L L(X) = Y ∈ S -→ Criteria H H(Y )
Figure 2 summarizes the considered sets of solutions.

-Ω is the set of all the lists of items. card(Ω)= N ! -Ω ′ is the set of all the possible assignment of items to bins, without checking the capacity constraint. card(Ω ′ )= N N -C is the set of the admissible solutions: a solution is admissible if the capacity constraint is respected. C ⊆ Ω ′ . -S is the set of all the solutions built by the application of a list algorithm on a list of items. S ⊆ Ω ′ and S ⊆ C.

Proof that S contains the set of optimal solutions: Let be Y * any optimal solution. Y * = (Y * 1 , ..., Y * N ) with Y * i the bin index assigned to item i. A list of items assigned in each bin is deduced. By concatenation of these lists, an ordered list of all the items is determined: X * ∈ Ω. By applying the heuristic F F or N F to X * , Y * is found, so Y * ∈ S. As a consequence the set of the optimal solutions written C * is included in S. Let be S B the set of solutions ∈ Ω ′ built by the heuristic BF , S F the set built by F F and S N the set built by N F . We have the following assumption:

C * ⊆ S B ⊆ S F ⊆ S N ⊆ C ⊆ Ω ′
. By exploiting this encoding scheme, the search space is reduced and it is assured that the optimal solution can be reached. This proposition presents some advantages:

-Admissibility of the solutions is ensured by the list algorithms.

-Many metaheuristics already exist to solve the permutation problems, with their neighborhood systems for single solution based metaheuristics and their crossover operator and/or mutation for population based metaheuristics. -The metaheuristic is performed on a smaller set S than the original one Ω ′ .

-The set S all the optimal solutions. -S conforms with the property of accessibility.

-This method is a combination of simple existing methods easy to implement.

General framework

The general framework can be used with any metaheuristic. Algorithm 1 shows the principle of hybridization between a local search and a list algorithm where V : Ω -→ Ω denotes the neighborhood system and H : S -→ N the cost function.

Algorithm 1: Principle algorithm of the hybridization 1 Let be X ∈ Ω the initial solution 2 Y = L(X): a list algorithm is applied to the list X 3 while necessary do 4 Choose uniformly and randomly X ′ ∈ V (X)

5 Y ′ = L(X ′ ) 6 if H(Y ′ ) ≤ H(Y ) then 7 X = X ′ 8 Y = Y ′
Neighborhood system V (X): Several classical neighborhood systems for permutation problems can be used; P i,j : the item at position i permutes with the one which is at position j. |P i,j | = N.(N -1)

2

; I i,j : the item at position i is inserted at position j. |I i,j | = N.(N -1). In both cases, V satisfies the accessibility and reversibility properties.

Cost function H(Y ): The most intuitive cost function is the number of used bins. But many solutions have the same value of cost function. To avoid it, [START_REF] Falkenauer | A genetic algorithm for bin packing and line balancing[END_REF] proposes a new cost function H which characterizes the average bin utilization, defined by Equation [START_REF] Johnson | Near-optimal bin packing algorithms[END_REF].

H(Y ) = - j∈N (F j (Y )/C) k M (Y ) (1)
Where F j (Y ) = N i=1 w i .δ j,Yi , ∀j ∈ {1, N } is the sum of the sizes of the items packed in bin j (we use the Dirac function δ, δ a,x = 1 if a = x, 0 otherwise), M (Y ) is the number of bins used by the solution Y , k > 1 is a constant which defines how much a solution with equally filled bins is preferred over one in which some bins are rather full and other rather empty. A good value is k = 2 [START_REF] Falkenauer | A hybrid grouping genetic algorithm for bin packing[END_REF].

Experimentation

Hybridization has been tested with both metaheuristics: simulated annealing or kangaroo algorithm (iterated local search).

Originally, the inhomogeneous simulated annealing was used by [START_REF] Metropolis | Equation of state calculations by fast computing machines[END_REF] to simulate the physical annealing in metallurgy. Unfavorable transitions are accepted with a probability e -H(Y ′ )-H(Y )

T

. Simulated annealing converges in probability to the set of optimal solutions if neighborhood system V satisfies the accessibility and reversibility properties [START_REF] Aarts | Simulated Annealing: Theory and Applications[END_REF]. The initial temperature T 0 is chosen such as all the transitions are authorized at the beginning, i.e. e -H(Y ′ )-H(Y )

T 0 ≃ 1, ∀(Y, Y ′ )
according to the algorithm proposed by [START_REF] Aarts | Simulated Annealing: Theory and Applications[END_REF]. The decreasing factor α is computed by α = IterM ax ( Ta T0 ). T a is the latest temperature close to 0. Kangaroo algorithm is an iterated local search. This algorithm consists in a stochastic descent, but if there is no improvement of the current solution after a number A ≥ |V |.ln(2) [START_REF] Fleury | Méthodes stochastiques et déterministes pour les problèmes NPdifficiles[END_REF] of iterations, a jump is made. To make this jump, a solution is chosen in a neighborhood system W different from V . Kangaroo algorithm converges in probability to the set of optimal solutions if neighborhood system W satisfies the accessibility property. W consists in applying a given number of times neighborhood system V , it satisfies the accessibility property.

We compared all the possible combinations: choice of the metaheuristic (kangaroo algorithm or simulated annealing), choice of the list algorithm (N F , F F or BF ) and choice of the neighborhood system (P i,j or I i,j ).

All of the referenced papers in the state of the art used the same instances, except [START_REF] Reeves | Hybrid genetic algorithms for bin-packing and related problems[END_REF]. In the instances, the bin capacity is equal to 150 and sizes of items between 20 and 100. Four sizes of problems are used: 120 items, 250, 500 and 1000. For each size, twenty different instances have been created. "u120" to "u1000" define the size of the instance. The results are about the sum of the differences between the optimal solution and the solution found by the considered method for all the instances of the same size. For instance, for the twenty instances of size "u120", [START_REF] Falkenauer | A hybrid grouping genetic algorithm for bin packing[END_REF] finds the optimal solutions + 2 bins in total.

Table 2 compares our two best methods SABFP (Simulated Annealing + Best Fit + P i,j ) and KABFP (Kangaroo Algorithm + Best Fit + P i,j ) to the bibliographic ones. SABFP and KABFP use the Best Fit heuristic and the neighborhood system P i,j . At the opposite, our hybridization does not work very well with the list algorithm Next Fit or with the neighborhood system I i,j (which disturbs too much the current solution).

This method gives good results. Our strength is the easy implementation of our proposition. It can be easily used on a lot of applications (resource constrained scheduling, assembly line balancing, multiprocessor scheduling).

u120 +2 0 0 0 0 0 u250 +3 +2 0 0 +1 +2 u500 0 0 0 0 +2 +2 u1000 0 0 0 0 +4 +4

Conclusion

In this paper, we propose a hybridization between a metaheuristic and a list algorithm to solve the bin packing problem. Some methods already exist to solve it and perform very well. The main advantage of our proposition is its intelligibility because it combines two simple well-known methods. This method allows us to work on two different search spaces. This eases the used method. Several reasons justify this encoding scheme: it is not useful to check the admissibility of the solutions, many methods already exist to solve permutation problems, the set used for the metaheuristic is reduced and contains all the optimal solutions. This resolution method used on the bin packing problem is currently applied on real applications such a problem of activities planning and resources assignment. We can also solve more sophisticated models thanks to this encoding scheme. Indeed, more constraints can be added to the list algorithms: incompatibilities between items, between items and bins, precedence.

Hybridization between a metaheuristic and a list algorithm can be used with any metaheuristic. In further work, hybridization will be used with population based metaheuristic. The use of particle swarm optimization is the next step of our approach.

Fig. 1 .

 1 Fig. 1. Example of our application

Fig. 2 .

 2 Fig. 2. Sets of solutions

Table

.

  Analogies between the bin packing problem and the HCT problem.

		Bin packing problem	Problem of activities planning and resources assignment
		Item	Activity
	Data	Bin	Couple (resource, period)
		Size of an item	Process time of an activity
	Objective To assign items in bins	To assign activities to periods and resources
	Constraints	Capacity of the bins ∅	Resources opentime Incompatibility constraint
	Criterion	To minimize the number of used bins	To minimize the number of used couples (resource, period)

Table 2 .

 2 Results of the different methods to solve the bin packing problem

	Prob	[9]	[12]	[8]	[11] SABFP KABFP


Yong Li

Li Yong

A risk perspective on Chinese student migration in France 1

Keywords: student mobility, study abroad, labour market experiences, migration decisions, risk, uncertainty, individualisation, China, France

Over the past two decades, the global flow of Chinese student migration has changed significantly. Overseas Chinese students are torn between the desire to return to China and a fear of returning. Based on a study of employability among Chinese students who arrived in France in the 2000s, the study adopts a sociological perspective of risk in analysing student mobility. The findings show that risk and uncertainty shape the migration process for Chinese students. Overseas Chinese students are engaged in an individualisation process in which they are vulnerable to social change, and they must address risk in different areas of life upon graduation. Thus, many students find achieving a traditional ideal of success difficult, and they must postpone their return to China.

INTRODUCTION

Since the late 1990s, French campuses have observed an influx of Chinese students. From 1998 to 2005, the number of Chinese students in French public universities increased twelvefold, ten times more than the increase in the number of other foreign students [START_REF] Fabre | Les étudiants étrangers dans l'enseignement supérieur français: Une croissance soutenue par les Asiatiques [Foreign students in French higher education : A growth sustained by the Asians[END_REF].

Today, France is the seventh most popular country as a destination for Chinese students.

Traditionally, the migration of Chinese students has been studied in terms of students' return (or not) to their home country after the complete their studies. In France, recent studies show that Chinese students possess a strong desire to return to their country of origin-a stronger desire than other foreign students-yet have a fear of returning (Ennafaa & Paivandi, 2008a[START_REF] Ennafaa | Les étudiants étrangers en France: Enquête sur les projets, les parcours et les conditions de vie [Foreign students in France : Survey on projects, life-courses and living conditions[END_REF]. It may appear puzzling that many Chinese students want to remain abroad after completing their education despite an unfavourable economic climate that does not favour their integration into the labour market.

This study of employability among Chinese students who arrived in France in the 2000s reveals that the overseas experience of Chinese students may be analysed from a sociological perspective of risk. The central hypothesis is threefold: 1) risk and uncertainty are structural elements of the immigration process; 2) the risk society and individualisation thesis of Ulrich Beck [START_REF] Beck | Risk society: Towards a new modernity[END_REF][START_REF] Beck | Individualization: Institutionalized individualism and its social and political consequences[END_REF]) offers an appropriate framework with which to analyse recent developments in Chinese students' migration to Western countries; and 3) the post-graduation experiences of Chinese students in France correspond to an individualisation of risk management. These students find achieving a traditional ideal of success increasingly difficult. This chapter is largely conceptual but uses data collected between 2007 and 2012 from biographical interviews with Chinese graduate students who remained in France after their college studies. Seventeen Chinese employees (9 women and 8 men) working in the Upper-Normandy region (comprising the northern départements of Eure and Seine-Maritime and encompassing the northeastern portion of Normandy) or in Paris were interviewed with the "snowball" method. The respondents were between 25 and 33 years of age, arrived in France after 2000, and worked there for less than five years.

RETURN AND NON-RETURN OF STUDENTS: CURRENT TRENDS AND ISSUES

Return to the home country is a common theme in modern migrant narratives; however, the meaning of such a return varies across generations, and the possibility of return strongly depends on the legal status and qualifications of migrants as well as the socioeconomic and political environments of migrants' home countries [START_REF] Al-Rasheed | The myth of return: Iraqi Arab and Assyrian refugees in London[END_REF][START_REF] Bolognani | The myth of return: Dismissal, survival or revival? A Bradford example of transnationalism as a political instrument[END_REF][START_REF] Sinatti | Mobile transmigrants' or 'unsettled returnees'? Myth of return and permanent resettlement among Senegalese migrants[END_REF][START_REF] Zetter | Reconceptualizing the myth of return: Continuity and transition amongst the Greek-Cypriot refugees of 1974[END_REF]. For some migrants, their return to their home country is indefinitely postponed, potentially developing into a myth [START_REF] Sayad | L'immigration ou les paradoxes de l'altérité : L'illusion du provisoire[END_REF].

For the Chinese students interviewed for this study who arrived in France in the 2000s, a successful return to China is often the goal of their venture because it justifies socioeconomically motivated migration. However, some graduates find realising their dream difficult because a migration journey is characterised by risk and uncertainty.

For over half a century, the return of foreign students to their countries of origin has been a concern for both researchers and policymakers. The question now assumes a special meaning in France, where the government is attempting to integrate foreign students into its general immigration policy [START_REF] Deneuve | Migrations internationales. Symposium conducated at the meeting of the Commissariat Général du Plan[END_REF][START_REF] Héran | Le temps des immigrés: Essai sur le destin de la population française[END_REF][START_REF] Héran | Immigration, marché du travail, intégration: Rapport du séminaire présidé par François Héran[END_REF].

Discussions on this subject are informed by ethical considerations [START_REF] Dumitru | L'éthique du débat sur la fuite des cerveaux [Ethics of the brain drain debate[END_REF], changes in theoretical perspectives, and political orientations. The study of Chinese student mobility must therefore encompass several recent developments.

First, the global flow of Chinese student migration has changed significantly over the last two decades. Since China's opening up in 1978, most Chinese students who have gone overseas to study have decided to stay remain the developed world. However, since 2000, the number of students returning to China has been increasing significantly. In the United States, this movement is illustrated by "a sustained drop in the number of doctoral students from China … who planned to remain in the United States beginning in the late 1990s" (Saravia & Miranda, 2004, p. 608). In Europe, the number of Chinese graduates returning to their home country has also increased in recent years. This increase in returning Chinese students is partly due to the sharp decrease in job opportunities owing the economic slowdown in the West. In addition, China's surging economy and unprecedented investment in its innovative system have created favourable conditions at home [START_REF] Hao | A tale of sea turtles: Job-seeking experiences of Hai Gui (highskilled returnees) in China[END_REF][START_REF] Welch | Returnees and diaspora as source of innovation in Chinese higher education[END_REF]. Further, beyond automatic market regulation, the Chinese state has played an important role in this movement by mobilising discursive, political, and financial resources to motivate highly skilled scientists and professionals to return to China [START_REF] Jonkers | Mobility, migration and the Chinese scientific research system[END_REF][START_REF] Nyíri | Expatriating is patriotic? The discourse on 'new migrants' in the People's Republic of China and the identity construction among recent migrants from the PRC[END_REF][START_REF] Nyíri | From class enemies to patriots: Overseas Chinese and emigration policy and discourse in the People's Republic of China[END_REF][START_REF] Welch | Enter the Dragon: The internationalization of China's higher education system[END_REF][START_REF] Welch | Returnees and diaspora as source of innovation in Chinese higher education[END_REF][START_REF] Zweig | Competing for talent: China's strategies to reverse the brain drain[END_REF]. If the feeling of duty to serve the nation appears prevalent among the Chinese intellectual diaspora [START_REF] Liu | New migrants and the revival of overseas Chinese nationalism[END_REF][START_REF] Nyíri | Mobility and cultural authority in contemporary China[END_REF]Nyíri & Breidenbach, 2005), the meaning of returning home has radically changed over the last few decades. For the "sea turtles" of the reform era [START_REF] Wang | 海 归 时 代 [Returning times[END_REF][START_REF] Wang | 当代中国海归 [Contemporary Chinese Returnees[END_REF][START_REF] Wang | 中国留学人才发展报告[The report on the development of Chinese overseas educated talents[END_REF], unlike twenty years ago, returning to China no longer means sacrificing one's professional future for the nation and subscribing to a particular ideological position. By joining a prosperous economy at home, returnees may find that their self-interest (whether material or symbolic) and the needs of the nation coincide [START_REF] Cai | Deploying the Chinese knowledge diaspora: A case study of Peking University[END_REF].

Second, this change in the flow of migration raises questions regarding the country of origin and the migrants themselves, particularly regarding capitalising on foreign experiences in the Chinese labour market. In the earlier era of China's opening, simply returning with a foreign degree was sufficient to ensure a good job. Now, however, with the increasing number of returnees and an increase in graduate unemployment [START_REF] Lian | 蚁族:大学毕业生聚居村实录 [Ant tribes: A record of inhabited village of university graduates[END_REF][START_REF] Rocca | Quand la Chine redécouvre la question sociale [When China rediscovers the social question[END_REF], merely having a foreign degree is no longer sufficient. Today's labour market in China is adopting an increasingly mature, discriminating attitude towards recruiting international graduates (Hao & Welch, 2012, p. 243). Overseas-educated graduates have certain advantages with regard to employment compared with their local counterparts, including much better access to a local Hukou 2 in a major Chinese city (e.g., Beijing or Shanghai), which accords returnees more freedom than local graduates in their job-seeking efforts. However, returnees are confronted with problems that their local counterparts do not face, such as striking a balance between the knowledge and attributes that they obtained overseas and their home culture and filling the gap between their wage expectations and the expectations of their employers [START_REF] Ascencio | L'insertion professionnelle des Chinois francophones dans les entreprises françaises en Chine [The employment of French-speaking Chinese in French companies in China[END_REF][START_REF] Hao | A tale of sea turtles: Job-seeking experiences of Hai Gui (highskilled returnees) in China[END_REF][START_REF] Welch | Returnees and diaspora as source of innovation in Chinese higher education[END_REF]. If graduates from top-ranking universities with work experience can cater to employers' needs and can succeed in securing higher positions in the labour force, graduates from less recognised higher education institutions, without professional experience, may have difficulty finding a job [START_REF] Hao | A tale of sea turtles: Job-seeking experiences of Hai Gui (highskilled returnees) in China[END_REF][START_REF] Welch | Returnees and diaspora as source of innovation in Chinese higher education[END_REF]. Every day, dramatic stories of young unemployed returnees generate stronger collective disappointment regarding overseas studies, as illustrated by the popular saying, "The Sea Turtle has become seaweed" (haigui bian haidai). However, recent studies on employability among returnees suggest that a quality international education nevertheless positively influences an individual's career and life in China and that the so-called "devaluation of the sea turtle" has been greatly exaggerated by the media [START_REF] Hao | A tale of sea turtles: Job-seeking experiences of Hai Gui (highskilled returnees) in China[END_REF][START_REF] Wang | 中国留学发展报告 [Annual report on the development of study abroad in China[END_REF]. However, media hype has had a real effect on the decisions of Chinese graduates, as students who follow with the news may be fearful of returning to China.

Third, over the last two decades, a "paradigm shift" has changed the manner in which international migration studies and student migration are approached in the social sciences.

Because of the contribution of transnationalism, scholars now emphasise movement and mobility as well as the connectivity between the country of origin and the host country as developed by migrants [START_REF] Schiller | Towards a definition of transnationalism. Introductory remarks and research questions[END_REF]. In a similar vein, transmigrants maintain significant ties across national borders and live their lives simultaneously in multiple locations [START_REF] Portes | Introduction: The debates and significance of immigrant transnationalism[END_REF][START_REF] Sinatti | Mobile transmigrants' or 'unsettled returnees'? Myth of return and permanent resettlement among Senegalese migrants[END_REF]. The immigration process is increasingly perceived as circular and reversible rather than as linear and unidirectional.

Moreover, the transnational or cosmopolitan character of returning home is emphasised: returning to one's country of origin does not presage the end of migration. A return can be the starting point of new mobility [START_REF] Baas | In-betweeness: The (dis)connection between here and there. The case of Indian student-migrant in Australia[END_REF][START_REF] Maillard | Circulation des cerveaux entre la Chine et la Californie: Étudiants, ingénieurs, entrepreneurs et capital-risqueurs chinois dans la Silicon Valley [Brain circulation between China and California : Chinese students, engineers, entrepreneurs and venture capitalists[END_REF][START_REF] Varrel | Back to Bangalore[END_REF]. Regarding student migration, the "brain drain" paradigm that was developed in the 1960s has gradually yielded to new paradigms that are characterised by a "connexionist" approach. Since the late 1980s, researchers have attempted to explain that brain drain is not always a definite loss for the country of origin by conceptualising new approaches, such as "brain gain", "transit brain drain", "delayed return", and even "brain mobility". After all, the shifts in academic approaches to migration are rooted in deeper ideological shifts. The nexus of "return migration and development" has always been the basis of reasoning regarding academic approaches to returning student migration [START_REF] De Haas | Migration and development: A theoretical perspective[END_REF]. Recent views celebrating migrants as grass-roots actors of development are partially driven by neoliberal ideologies that emphasise individuals' agency rather than the states' responsibility to solve structural problems of development [START_REF] De Haas | The migration and development pendulum: A critical view on research and policy[END_REF].

It is interesting to study the ways in which Chinese graduates themselves consider the issues regarding whether to return to China. Although international mobility has become a mundane experience for a growing number of individuals, when they are unsure about the outcomes of mobility in terms of social well-being, they are gripped by disillusion or vague fear. Students thus try to find a safe anchorage (e.g., a geographical, social identity) in our era of "liquid" modernity [START_REF] Bauman | Liquid moderniy[END_REF]. A tension exists between the desire for mobility (be it international or transnational) and the desire for security. Since the 1990s, many studies have observed a "wait and see" strategy among overseas Chinese students. Instead of a definite return to the country of origin, students are seeking secure status in the host country (e.g., a "residence card") while exploring opportunities to return to China [START_REF] Cheng | 代 中国留学生 研究 [The study of contemporary overseas Chinese students[END_REF][START_REF] Bail | La nouvelle immigration chinoise au Japon: Circulation des élites transnationales et reconfiguration des citoyennetés nationales et locales [The new Chinese immigration in Japan : Movements of transnational elites and reconfiguration of national and local citizenships[END_REF], 2009;[START_REF] Liu-Farrer | Making careers in the occupational niche: Chinese students in corporate Japan's transnational business[END_REF][START_REF] Zweig | China's brain drain to the United States: Views of overseas Chinese students and scholars in the 1990s[END_REF][START_REF] Zweig | Redefining the brain drain : China's 'diaspora option[END_REF]. In France, naturalised students are actively involved in activities related to China [START_REF] Cheng | Non-remaining and non-returning: The Mainland Chinese students in Japan and Europe since the 1970s[END_REF]. For such students, the increase in transnational activities can be perceived to result from a collective strategy of risk management in the sense that Chinese students, even if they wish to return to China, prefer to "play it safe". By constructing a social space outside China, and by building overseas networks, Chinese students prepare an exit for themselves if their plan to return fails.

Chinese students' decisions regarding the immigration process thus appear to be affected not only by risk and uncertainty but also material interests, cultural norms, collective representations, and moral obligations. Thus, the concepts of risk and uncertainty can be appropriately included in discussions on the return and non-return of Chinese students in France.

RISK, UNCERTAINTY, AND IMMIGRATION

Risk shapes and is shaped by migration [START_REF] Williams | Migration, risk, and uncertainty: Theoretical perspectives[END_REF]. The concept of risk differs from the concept of uncertainty because the former refers to known uncertainty [START_REF] Knight | Risk, uncertainty and profit[END_REF][START_REF] Zinn | Literature review: Sociology and risk. SCARR working paper 2004/1[END_REF]. Risk can be understood in terms of the probability of known results. According to [START_REF] Williams | Migration, risk, and uncertainty: Theoretical perspectives[END_REF], a degree of uncertainty always exists in immigration, which comes from two sources: first, migrants' imperfect knowledge about the actual conditions in both the place of departure and possible destinations and, second, the unpredictability of the future. All future changes involve some uncertainty because the likelihood of particular outcomes cannot be known with certainty. Because migration decisions are generally based on at least some knowledge of the destination, decisions are rarely, if ever, made in full ignorance; thus, migration should be understood as a process associated with risk assessment in conditions of partial knowledge (Williams & Baláž, 2012, pp.168-169).

Risk and uncertainty have been widely discussed in the social sciences [START_REF] Williams | Migration, risk, and uncertainty: Theoretical perspectives[END_REF][START_REF] Zinn | Risk as an interdisciplinary research area[END_REF]. Sociological theories emphasise the social construction of risk, in which "the individual's perception and response to risk can only be understood against the background of their embeddedness in a sociocultural background and identity as a member of a social group, rather than through individual cognition" (Taylor-Gooby & Zinn, 2006, p. 37). Ulrich Beck's theory of the "risk society" [START_REF] Beck | Risk society: Towards a new modernity[END_REF][START_REF] Beck | World risk society[END_REF] attempts to interpret the growing risk awareness and the societal and individual responses to risk in a general framework of social change within modernity. In the following section, the scope of Beck's argument is outlined, and the relevance of Beck's argument to Chinese students in France is then discussed. As early as the mid-1980s, Beck proclaimed the advent of a second modernity, which was characterised by an increase in modernisation risks and the individualisation of society in the developed West [START_REF] Beck | Risikogesellschaft: Auf dem Weg in eine andere Moderne[END_REF][START_REF] Beck | Risk society: Towards a new modernity[END_REF][START_REF] Beck | Politics of risk society[END_REF]Yan, 2010, p. 489). However, this second modern transformation also involves other parts of the world [START_REF] Beck | World risk society[END_REF][START_REF] Beck | Beyond class and nation: Reframing social inequalities in a globalizing world[END_REF][START_REF] Beck | World at Risk[END_REF][START_REF] Beck | Remapping social inequalities in an age of climate change: For a cosmopolitan renewal of sociology[END_REF][START_REF] Beck | Varieties of second modernity: The cosmopolitan turn in social and political theory and research[END_REF].

First, qualitatively different risks are emerging as the unintended side effects of industrialised modernity [START_REF] Zinn | Risk as an interdisciplinary research area[END_REF]. These side effects are exemplified by the exponential growth of new technology, which poses risks that are not fully understood and that cannot be controlled (e.g., smog, radiation, climate change, and genetically modified food). Moreover, such risks have become less temporally and spatially bound, and they display "a tendency to globalisation" (Beck, 1992, p. 13;1999, 2009). These changes can be understood in terms of a shift from risk to greater uncertainty. Therefore, the traditional methods of risk management such as reliance on family support or insurance do not apply [START_REF] Williams | Migration, risk, and uncertainty: Theoretical perspectives[END_REF].

Second, the emergence of a risk society is connected to the complementary processes of individualisation in the sphere of work, family life, and self-identity. According to Beck, Individualization is understood as a historical process that increasingly questions and tends to break up people's traditional rhythm of life -what sociologists call a normal biography. As a result, more people than ever before are being forced to piece together their own biographies and fit in the components they need as best they can. They find themselves bereft of unquestionable assumptions, beliefs or values and are nevertheless faced with the tangle of institutional controls and constraints which make up the fiber of modern life (welfare state, labor market, education system, etc.). To put it bluntly, the normal life-history is giving way to the do-it-yourself life-history. (Beck & Beck-Gernsheim, 2002, pp. 56-57) Although some sociologists are sceptical about the value of the risk society theory in migration studies, risk theory may provide useful insight into how societies address the risks associated with migration, as long as specific contexts are taken into account.

From this perspective, the recent waves of emigration of upper-middle class Chinese to Western countries may be perceived as a component of reflexive modernisation because these individuals are fleeing not traditional misery but the side effects of compulsive modernisation in China. The typical risks of so-called advanced countries ("new civilisation risks", e.g., climate change, ecological destruction, economic inequality, unemployment, global pandemics, an ageing society) coexist with the risks of industrial and traditional societies [START_REF] Han | Redefining second modernity for East Asia: A critical assessment[END_REF]. According to recent surveys, Chinese citizens are primarily concerned with domestic and social risks, and they tend to consider the risks of everyday life (e.g., poverty, traffic accidents, problems with obtaining care for illnesses, difficulties in accessing housing) to be the most serious risks (N. Liu, 2009;[START_REF] Liu | Composite risks and composite management in transnational society[END_REF]. Thus, among the "push" factors of this emigration of professional and entrepreneurial Chinese to the "safe haven" of the West, the risks generated by the "deficiencies of modernity" are the most significant (e.g., food poisoning, health system crises, soaring real estate prices).

The next section of the paper focuses on Beck's individualisation theory. This framework moves the discussion from the "cataclysmic democracy" of catastrophic environmental risks to the complexity of risk in everyday life (Williams & Baláž, 2012, p.173) and the ways in which individuals address these risks.

CHINESE STUDENT MOBILITY AS AN INDIVIDUALISATION PROCESS

Discussing the individualisation process on the institutional level, Beck describes this concept in three dimensions: Disembedding, removal from historically prescribed social forms and commitments in the sense of traditional contexts of dominance and support (the 'liberating dimension'); the loss of traditional security with respect to practical knowledge, faith and guiding norms (the 'disenchantment dimension'); and -here the meaning of the word is virtually turned into its opposite -re-embedding, a new type of social commitment (the 'control' or 'reintegration dimension') (Beck, 1992, p. 128) (italics in original).

In the text that follows, this model is used to interpret the recent trends in Chinese student migration as an individualisation process.

Disembedding, from state assignment to market competition

The democratisation of overseas studies, promoting self-financing education since the end of the 1990s, implies a transfer of responsibility for education investment from the state to young people and their families [START_REF] Pan | Education abroad, human capital development, and national competitiveness: China's brain gain strategies[END_REF], which should be perceived in light of a redefinition of state-society-individual relationships in late socialist China [START_REF] Hoffman | Autonomous choices and patriotic professionalism: On governmentality in late-socialist China[END_REF][START_REF] Hoffman | Patriotic professionalism in urban China: Fostering talent[END_REF][START_REF] Yan | The Chinese path to individualization[END_REF].

Before the 1980s, in the era of the planned economy, Chinese university graduates were assigned jobs upon graduation by the government in a top-down process, which guaranteed them a job but determined where they should live and what they should do [START_REF] Hao | A tale of sea turtles: Job-seeking experiences of Hai Gui (highskilled returnees) in China[END_REF][START_REF] Hoffman | Patriotic professionalism in urban China: Fostering talent[END_REF]. After the reform and opening of China in the 1970s, in the first period of student migration, government-sponsored overseas studies prevailed, in which the goals, type of training, duration of overseas stay, and future jobs of students were arranged by the state.

The period since the early 1990s has witnessed all forms of privatisation of property, labour, and the self (Zhang & ONG, 2008). The state has been moving towards more and less "distanced" modes of governing [START_REF] Rose | Governing "advanced" liberal democracies[END_REF] such as marketing the labour distribution, fostering a self-enterprising ethic, and emphasising individual choice and autonomy rather than state distribution, as illustrated most explicitly by patriotic professionalism [START_REF] Hoffman | Autonomous choices and patriotic professionalism: On governmentality in late-socialist China[END_REF][START_REF] Hoffman | Patriotic professionalism in urban China: Fostering talent[END_REF]. The government gradually loosened its control over graduates, which induced students to respond more readily to the needs of a rapidly changing labour market by finding jobs on their own. Although finding a job upon graduation was previously difficult for some students, the rapid expansion of the higher education system in 1998 has rendered finding a job extremely difficult for many of today's graduates (Hao & Welch, 2012, p. 247). In this context, students and their families increasingly regard international education is by as a rational investment in human capital to meet the demands of a labour market that is becoming increasingly competitive.

Accordingly, China has experienced a Chuguo Re (going-out fever) since the government loosened the restrictions on self-financed overseas studies in the late 1990s [START_REF] Xiang | Emigration from China: A sending country perspective[END_REF]. The privatisation of studying abroad has also privatised the risks that are involved in international educational mobility for students and their families. Individuals must make choices and decisions regarding diverse countries of destination, universities, degree programmes, and so forth, based on their partial knowledge of both the host country and the home country. They must take the entire responsibility for failures, delays, and incidents that affect them during their journey.

However, the Chinese students who were interviewed for this study tended to have positive perceptions of the risks posed by migration during their overseas study project. They appear to be what [START_REF] Lyng | Edgework, risk and uncertainty[END_REF] called edgeworkers. They believe that overseas studies not only increase their self-esteem through their ability to manage these risks successfully but also contribute to peer esteem. In addition, acquiring another language and experiencing international exposure improve the value of their curriculum vitae on the labour market. Overseas study experiences may raise a "normal" biography via geographical and social mobility into an elective or "do-ityourself" biography (King & Ruiz-Gelices, 2003, p. 232). Moreover, our respondents perceive the risks in migration as a source of excitement that invigorates an otherwise well-adjusted and predictable life. Experiencing another lifestyle and confronting risks on one's own in a foreign country are two common themes of the "romanticism" of studying abroad, as indicated in the narratives of students and in the popular literature on this subject in China (Hu, 2004, pp. 152-156).

Disenchantment, the loss of traditional certainty and security

As students venture into a globalised life-world, the traditional security with respect to practical knowledge, faith, and guiding norms loses a large portion of its relevance. During their stay abroad, the students must set goals for themselves, build their own life courses, and give meaning to their experiences, sometimes under pressure and with conflicting constraints.

However, risk and uncertainty do not come only from the "difficulties of integration" into the host society as foreigners. Rather, risk and uncertainty primarily derive from the centrifugal forces of the individualisation process in the host society itself [START_REF] Castel | La montée des incertitudes : Travail, protections, statut de l'individu [The rise of uncertainty : Work, protection, status of the individual[END_REF], as well as the crisis of employment and higher education. In European countries, the labour markets are becoming more flexible and precarious, creating forms of underemployment and economic niches that are saturated with risks. The traditional work-oriented training systems are struggling to survive. Some educational degrees are "deadlocks" because they lead nowhere in the labour market. When Chinese graduates try to enter the French labour market, they go through a "twilight zone" of underemployment subject to risks, including temporary unemployment, few jobs on the labour market, additional training, part-time jobs, and unpaid internships.

Over the last ten years, official discourse in China suggests that Chinese students should first accumulate adequate overseas work experience before returning to China, although in the context of the economic recession in Europe, a great number of students have no chance of finding a job related to their initial training. Thus, with the overall decrease in the volume of work on the French labour market, a large number of Chinese graduates are unlikely to find work after their training in the host country despite all their good intentions. Some graduates cannot even find an internship, and they are hence forced to seek opportunities in China. This study shows that many Chinese graduates find themselves trapped in a "double-bind" between the need to succeed and the inability to succeed in the prescribed forms [START_REF] Roulleau-Berger | Introduction : Nouvelles migrations chinoises et marchés du travail européens [Introduction : New Chinese migrations and European labour markets[END_REF].

With the increase in foreign degrees in China, the benefit of overseas studies should not be taken for granted because the extent of the benefit largely depends on an individual's personal trajectory. The Chinese graduates who were interviewed for this study were increasingly feeling the need to combine their foreign degrees with other elements (e.g., overseas work experience, financial capital, social relations) to find a good job when they return to China. For instance, Liu, a 35-year-old tourist professional with a PhD in biology in Paris, discussed the drastic changes in China's labour market that forced him to abandon his initial return project:

Those who can find a job in China are not satisfied, and they are not sure about their future. They decide to go abroad and use foreign training as a means or a shortcut to success. They can study for two or three years abroad just to get covered with gold, to become a true piece of gold on their return to China. Many people have gone abroad with this idea. But the reality has changed. Today, when someone returns to China, the situation she faces is not comparable to that of 10 years ago. The labour market has changed dramatically! Companies put a big question mark on the value of diplomas obtained abroad.

Re-embedding, in search of new forms of social integration

The re-embedding dimension refers to the interactions of migrants with new forms of social control or social integration programmes established in both the country of origin and the host country. In France, the government has sought to implement a selective immigration policy in recent years to identify "good immigrants" who can contribute to the collective good. However, by asking immigrants to demonstrate their willingness to integrate and their adaptability to a greater extent, this policy can burden people who are already weakened by various difficulties (e.g., linguistic, economic, professional) [START_REF] Soysal | Citizenship, immigration, and the European social project: Rights and obligations of individuality[END_REF]. Conversely, China has increasingly focused on a "diaspora option" to allow greater freedom of movement internationally and to deploy Chinese nationalism in an effort to entice highly skilled overseas Chinese to bring their skills back home, even if not permanently [START_REF] Meyer | Network approach versus brain drain: Lessons from the diaspora[END_REF][START_REF] Welch | Higher education and global talent flows: Brain drain, overseas Chinese intellectuals, and diasporic knowledge networks[END_REF][START_REF] Zweig | Competing for talent: China's strategies to reverse the brain drain[END_REF][START_REF] Zweig | Redefining the brain drain : China's 'diaspora option[END_REF]. At the beginning of the new millennium, in response to the increase in the Chinese knowledge diaspora, China's policy shifted direction from huiguo fuwu (return and serve the homeland) to the more flexible weiguo fuwu (serve the homeland) by implementing an array of Key Overseas Talent Recruitment schemes targeting the large pool of highly talented overseas Chinese. To take advantage of this new climate, top universities have introduced nonresident fellowship schemes, sometimes termed yaling moshi (dumbbell model), which encourage joint appointments in China and abroad [START_REF] Cai | Deploying the Chinese knowledge diaspora: A case study of Peking University[END_REF][START_REF] Welch | Enter the Dragon: The internationalization of China's higher education system[END_REF][START_REF] Welch | Returnees and diaspora as source of innovation in Chinese higher education[END_REF]. Although it grants various privileges to excellent overseas talent, the state stipulates that ordinary Chinese students provide for their own futures by participating in market competition, which is already quite fierce in China [START_REF] Pan | Education abroad, human capital development, and national competitiveness: China's brain gain strategies[END_REF][START_REF] Xiang | A ritual economy of 'talent': China and overseas Chinese professionals[END_REF].

In this new global context, the question of who is and who is not eligible for membership and rights in national communities is increasingly perceived in terms of personal skills, experience, disposition, and chance. From the perspective of a life course, some transitions in the migration journey (e.g., from school to work) are highly significant in initiating advantageous or disadvantageous life course developments. Some transitions lead to cumulative processes in the subsequent life course and, eventually, path dependence (Wingens, de Valk, Windzio, & Aybek., 2011, p. 13). Student migration thus becomes a continuous process of differentiation and individualisation.

In this context, young returnees who spend a number of years abroad developing some mastery of Western knowledge and culture often face issues reintegrating into their home society. Some returnees struggle with effectively combining the notion of "living for others" with the notion of "living for oneself". Although they are increasingly dependent on the employment market, they nevertheless face traditional familial pressure and duties, friendship reciprocity, and social norms. Other returnees struggle with the choice of whether to work in state, private, or foreign enterprises or whether to start their own businesses. Returnees also vacillate between wanting to fulfil their individual aspirations and wanting to meet their sense of social responsibility (i.e., to support the nation). [START_REF] Hoffman | Autonomous choices and patriotic professionalism: On governmentality in late-socialist China[END_REF] argues that young Chinese professionals harbour neoliberal ideas of self-development as well as late-socialist patriotism. One can sip a Starbucks latte and work for a foreign company while also caring for the nation and exalting nationalism [START_REF] Welch | Returnees and diaspora as source of innovation in Chinese higher education[END_REF].

Studying in France and afterwards? Problematic transitions

This section examines the dilemma that many Chinese students face regarding the dissonance between a traditional ideal of success and the students' new conditions in a global risk society that hamper the realisation of this ideal. The ideal of success is a concept that has emerged from the analysis of the students' interview responses. It is a polysemic notion. First, the respondents view overseas studies as a self-development and self-transformation enterprise [START_REF] Hoffman | Autonomous choices and patriotic professionalism: On governmentality in late-socialist China[END_REF][START_REF] Hoffman | Patriotic professionalism in urban China: Fostering talent[END_REF]) that both improves one's human capital (i.e., being more competitive, performing better) and cultivates oneself (i.e., being more cultured, open-minded, cosmopolitan). Second, the interviewees view their overseas experiences to foster their social advancement. These two aspects are closely linked: self-transformation explains one's career advancement; however, self-fulfilment becomes meaningful only when it permits upward social mobility.

For young Chinese students who are engaged such a career advancement project, the success of this project is measured by a double comparison: first, a comparison with oneself, as individuals claim to have obtained a more favourable position through overseas studies than they would have occupied had they remained in China, and second, a comparison with others, as returnees are expected to surpass non-migrant members of their original social milieu (e.g., university friends or former business colleagues). This ideal of success influenced the Chinese graduates' decisions throughout the immigration process. For the interviewees in this study, returning to China could be synonymous with a triumphant homecoming. Graduates justify returning to China by their career ambitions ("I can reach executive positions only in China"), cultural convenience ("I am Chinese; I feel more comfortable living in China"), and family attachment ("There is a Chinese saying: 'When parents are alive, do not travel to distant lands'"). According to some respondents, had they wanted to settle permanently in a foreign country, they would not have come to France, where a residence permit is less accessible than in countries with "immigration traditions" (e.g., Canada, Australia). Conversely, a return to China is possible only when the graduates have already "proven themselves" abroad and when they can expect a prosperous career in China.

The Chinese graduates believe that they have a moral obligation to succeed: going abroad nearly always requires important financial support from their parents, and this material and emotional investment must be returned; in short, they must meet the strong expectations of a successful return from all sides [START_REF] Hu | Le métier d'étudiant étranger : Le cas des étudiants chinois non spécialistes de français en France [The life of foreign student : The case of Chinese students nonspecialised in French studies in France[END_REF].

When returning to China becomes both desirable and demanding, the returnee should be well prepared. Thus, most Chinese students in this study consider working for a few years (2 to 5 years) before returning to China. This deferred return project reveals Chinese graduates' awareness of the risks in the context of the country of origin. Graduates must carefully weigh the advantages and disadvantages. Because professional experiences in Western companies are considered rewarding, Chinese students believe that the years of work in France may strengthen their risk resilience in their future mobility.

Although this "study-short term overseas work experience-return" scheme is a widely held belief among Chinese students in France, the actual development of the life stories of Chinese graduates may greatly differ from this ideal. In a context characterised by risk and uncertainty, any minor incident in the migration process can derail an individual from his originally designed pathway to success.

For Chinese students in France, the postgraduate period involves numerous transitions in different areas of life: a transition into the labour market, a transition into adult life (for those who are single), the choice of a work place, and access to membership in the country of residence. The students would obviously face these transitions in China; however, abroad, the major challenge is to simultaneously manage these dynamics that imply different constraints and expectations.

The notion of timing is important, and it rests on the societal validity of age norms. On-time transitions are generally advantageous, or they at least do not involve negative consequences.

Off-time transitions constitute asynchronicities within and between trajectories and produce potentially disadvantageous consequences [START_REF] Wingens | The sociological life course approach and research on migration and integration[END_REF]. Student migrants must not only integrate within the labour market but also succeed in a series of transitions in their life course.

Nothing better illustrates the difficulty of this task than the professional integration of Chinese graduates in France. To obtain a job in France, Chinese graduates must overcome all forms of institutional exclusion, ethnic discrimination, and occupational segmentation [START_REF] Hargreaves | Multi-ethnic France: Immigration, politics, culture and society[END_REF]. Many young Chinese engineers find jobs in IT services (computer engineering service companies) that are characterised by high unemployment rates and precarious careers. Indeed, the newcomers to the IT sectors were among the first to lose their jobs in the wake of the financial crisis in France in September 2008.

The risks in the French labour market generate another risk for Chinese graduates: insecurity of residence in the host country. The recent shift in French immigration policy is marked by increasing restrictions on the residence rights of foreigners [START_REF] Huddleston | Migrant integration policy index III France[END_REF]. In the case of labour migration, the right of residence for migrants is conditional on their work situation. In the first years after leaving a university, the greatest concern of Chinese graduates is securing their stay in France.

Risks and vulnerabilities are also introduced in the private sphere, especially in intimate relationships: how does one build a lasting relationship in the context of uncertainty and mobility? Indeed, the spectre of delaying marriage haunts many Chinese girls who believe that they are already late in starting a family, according to traditional age norms [START_REF] Li | La construction sexuée du parcours d'insertion des diplômés chinois en France [The gendered construction of the integration path of Chinese graduates[END_REF].

All of these risks are intertwined. In Paris, when a Chinese employee loses her job, she is afraid of being unable to renew her residence permit. She must rethink her entire life project, including plans for marriage, parenthood, a family reunion with her parents, returning to China, and so forth. In contemporary China, public discourse tends to glorify Chinese students abroad as vanguards of modernity and tends to romanticise their migration. The general belief is also that they can succeed in any difficult circumstances [START_REF] Nyíri | Mobility and cultural authority in contemporary China[END_REF][START_REF] Xiang | A ritual economy of 'talent': China and overseas Chinese professionals[END_REF].

Follow-up interviews revealed that many of the interviewees who claimed that they would return to China "in 2 or 3 years" did not actually do so. In a changing global socio-economic landscape, individuals' career plans quickly become obsolete, as exemplified by the vicissitudes of Chinese "expatriates" (waipai). Traditionally, Chinese workers in France seek to acquire the status of returnee (after at least three years of work in France). This status allows them to expect a salary that is two or three times higher than that in China. Individuals who are naturalised in France can return to China with a French expatriation contract (Ascencio, 2006, p. 392). In recent years, deterred by the cost of expatriation, French companies in China are increasingly turning to local candidates. Further, a new strategy involves selecting elite students from China, financing their education through a training agreement, and then sending them back to China, of course, with a salary that is standard in China. This "localisation of human resources" tends to flatten the wage gap between the "expatriates" and "locals" and ultimately suppresses most "expatriate" jobs. Chinese engineers and professionals in France seeking an "expatriate" contract find themselves competing with their compatriots in China, who are also brilliant but are less expensive [START_REF] Li | Transformation des marchés du travail et dynamiques de l'immigration étudiante chinoise en France [Transformation of labour markets and dynamics of Chinese student immigration in France[END_REF].

By returning to China, many Chinese graduates wish to take advantage of the qualification differentials between advanced countries and China either to obtain access to higher professional positions (if not a higher salary) than what they may expect in their host countries or to obtain promotions more quickly than their local counterparts. Currently, although important skills differences remain between China and France, the respondents in this study acknowledge that as ordinary employees in France, they have little chance of entering management positions upon their return to China. Indeed, the graduates' physical absence from the Chinese homeland for some years may strain or sever social relationships, leading to a loss of guanxi. The loss of such relationships can be particularly influential in the Chinese context in which changes occur at rapid speed and in which personal relationships play an instrumental role in accessing key positions [START_REF] Hao | A tale of sea turtles: Job-seeking experiences of Hai Gui (highskilled returnees) in China[END_REF][START_REF] Leung | Read ten thousand books, walk ten thousand miles': Geographical mobility and capital accumulation among Chinese scholars[END_REF]. Therefore, many respondents fear that after returning to China, they must first prove themselves in subordinate jobs in Chinese companies, just as their local competitors have done. For those who already had professional experience in China and who studied abroad to advance faster than their Chinese counterparts, the idea of returning and competing with local graduates indicates the failure of their initial project.

CONCLUSION

This chapter demonstrates that risks and uncertainties are structural elements of the migration phenomenon. The paradigm of risk helps account for new trends in Chinese student immigration in France-and elsewhere. Based on the concept of individualisation, this study theorises about and examines the dynamic interplay between macro-level institutional changes and changes in individual life courses. Thus, with the transformations of the global labour market and immigration system, the migration experiences of Chinese students may be increasingly perceived as a process of risk management in everyday life.

Nevertheless, individualisation does not mean that individuals are freed from societal constraints. Instead, people are more dependent than ever on institutions and are more vulnerable to social change. Institutional crises are increasingly experienced as individual crises, and they must be treated as such. Part of the distress experienced by the Chinese graduates in this study can be explained by the gap between the ideal of success through mobility and the difficulties of the students in achieving their goals in an era of a global risk society. This article is a first attempt to apply the theory of the risk society to research on student mobility and migration. The study was conducted among only students who remained in France after completing their education in the host country. The interviewees' choices regarding returning to China or remaining in France should be considered "expressed preferences", not a prediction of their eventually actions [START_REF] Xiao | Return to China or Remain in the U.S.?[END_REF]. Future studies on this subject should focus on risk and uncertainty in immigrants' decision making. Today, the ability to travel across national borders has become critical for individuals, in terms of not only obtaining access to "goods" but also obtaining a more favourable position in the global redistribution of "bads" and risks [START_REF] Beck | Why 'class' is too soft a category to capture the explosiveness of social inequality at the beginning of the twenty-first century[END_REF]Curran, 2013aCurran, , 2013b). If we recognise that human beings are "reflexive" actors, then people can modify their actions to avoid perceived hazards, although their perception of risks is not automatic but socially conditioned (Curran, 2013a). This view questions the effectiveness of Chinese graduates' risk-taking and risk-avoidance strategies. Can these strategies really improve their life chances? Do these strategies enhance their risk position in an unequal risk society?

The Chinese household registration that determines eligibility for social benefits, including schooling for one's child.

He was previously a Teaching and Research Temporary Attaché (Attaché temporaire d'enseignement et de