Hatem Zaghden
email: hatem.zaghden@gmail.com

Badreddine Barhoumi
email: barhoumibadredine@yahoo.fr

Lobna Jlaiel

Catherine Guigue

Lassaad Chouba

Soufiane Touil

Sami Sayadi
email: sami.sayadi@gmail.com

Marc Tedetti
email: marc.tedetti@mio.osupytheas.fr

Catherine Guigue D , Lassaad

Occurrence, origin and potential ecological risk of dissolved polycyclic aromatic hydrocarbons and organochlorines in surface waters of the Gulf of Gabès (Tunisia, Southern Mediterranean Sea)

Keywords: PAHs, PCBs, OCPs, ecological risk, Gulf of Gabès, Mediterranean Sea 44 45 46 47 48 49 50

come   L'archive ouverte pluridisciplinaire

ecological and biogeochemical importance within the Mediterranean Sea, and at the same 102 time, its high level of anthropogenic pressure, the Gulf of Gabès remains very poorly 103 documented in terms of concentrations of organic pollutants in the dissolved phase of 104 seawater, in particular organochlorine (OCs) compounds such as PCBs and OCPs. In this 105 context, the objectives of this work were i) to describe the spatial distribution of dissolved 106 PAHs, PCBs and OCPs (concentrations, molecular patterns) in surface coastal waters of the 107 Ghannouch-Gabès area (Gulf of Gabès, Tunisia) and compare this distribution with that of 108 core physico-chemical and biogeochemical parameters, ii) to assess the main origins/sources 109 of these organic contaminants, and iii) to determine their potential ecological risk. To our 110 knowledge, this work is the first to provide together levels of dissolved PAHs, PCBs and 111 OCPs in coastal marine waters of Tunisia. The Gulf of Gabès is located in the Southeastern part of Tunisia (Southern Mediterranean 116 Sea), and spreads over 400 km of coastline (more than 50% of the Tunisian coastline) 117 between the city of Chebba in the North and the Tunisian-Libyan border in the South (Fig. 1).

118

The Gulf of Gabès, which accounts for ~ 30% of the Tunisian coastal waters, is characterized 119 by a wide continental shelf, shallow waters (isobaths 50-m deep are reached 110-km 120 offshore), high amplitude (up to 2-m) tides, and hydrodynamics driven by tides and 121 anticyclonic winds [START_REF] Béjaoui | Synthesis review of the Gulf of Gabes (eastern Mediterranean Sea, Tunisia): morphological, climatic, physical oceanographic, biogeochemical and fisheries features[END_REF][START_REF] Zayen | Microplastics in surface waters of the Gulf of Gabes, southern Mediterranean Sea: Distribution, composition and influence of hydrodynamics[END_REF]. The Gulf of Gabès hosts coastal 122 cities, such as Sfax and Gabès, with concentrated industrial, harbour and agriculture activities.

123

The Ghannouch-Gabès area in the central part of the Gulf, holds one of the biggest industrial 124 zones in Tunisia. This industrial complex includes the Ghannouch commercial harbour, the 125 phosphoric acid industry of the Tunisian Chemical Group, the Gabès Fluorine Chemical 126 Industry, a thermal power plant, petrochemical infrastructures, as well as industries related to agribusiness, building materials, energy production and textiles [START_REF] El Zrelli | Heavy metal contamination and ecological risk assessment in the surface sediments of the coastal area surrounding the industrial complex of Gabes city, Gulf of Gabes, SE Tunisia[END_REF], 2018;[START_REF] Fourati | Sources and spatial distribution of dissolved aliphatic and polycyclic aromatic hydrocarbons in surface coastal waters from the Gulf of Gabès (Tunisia, Southern Mediterranean Sea)[END_REF]. The phosphoric acid industry of the Ghannouch industrial complex has been discharging the phosphogypsum, an acidic by-product of phomanufacturing, into the Gabès coastal waters over the past several decades. The solid phosphogypsum issued from the production of phosphoric acid, diluted with seawater to form gypsum water, is conveyed by several pipes to the South of the Ghannouch commercial harbour where the gypsum water mixes with the cooling water discharged by all the chemical industry units, and finally ends up at sea through a unique canal [START_REF] Darmoul | Effets des rejets industriels de la région de Gabès (Tunisie) sur le milieu marin récepteur[END_REF]. Between 1000 and 13,000 t of phosphogypsum is released daily this way into the sea [START_REF] Béjaoui | Modélisation de la dispersion du phosphogypse dans le golfe de gabès[END_REF][START_REF] El Zrelli | Heavy metal contamination and ecological risk assessment in the surface sediments of the coastal area surrounding the industrial complex of Gabes city, Gulf of Gabes, SE Tunisia[END_REF]. In addition to this phosphogypsum discharges, several wadis carry a variety of urban,

Sampling and in situ measurements

Samples were collected at 27 stations in the Ghannouch-Gabès coastal area during the MERITE cruise from 30 October to 3 November 2017. Stations were distributed along coastal-offshore transects (also corresponding to North-South transects), i.e., S1-S3, S20 in front of El Melah wadi (Northern Ghannouch), S4-S6, S21 in front of El Demna wadi (Northern Ghannouch), S7-S12, S23 in front of Ghannouch industrial complex/commercial harbour and phosphogypsum effluent outlet, S13-S15 in front of fishing harbour and Gabès wadi, and S16-S18, S27 in front of Mersit wadi (Southern Gabès). Also, S19 was positioned as the north most station, S22 close to the Ghannouch industrial complex, in front of Ettine wadi/fluorine discharges, S24 and S25 between the Ghannouch industrial complex and the fishing harbour, and S26 in front of Griaa wadi (Fig. 1, Table S1). Stations S1-S18 (> 5-m depth) were sampled on board the vessel "Rahma" on 30 October (S1-S6), 1 November (S7-S12) and 3 November (S13-S18). The most coastal stations S19-S27 (< 5-m depth) were were then purified using silica-alumina columns, internal standards were added before the 764 analysis which was subsequently performed using gas chromatography coupled with a mass 765 spectrometer (GC-MS) and an electron capture detector (GC-ECD) (Text S1, Table S2). PCBs and 30 ng for OCPs). They ranged from 63 to 114% (mean of 87%), 7 to 117% (mean 777 of 85%) and 20 to 115% (mean of 79%) for PAHs, PCBs and OCPs, respectively (Table S2).

778

The surrogate standard (added to perform the quantification) recoveries ranged from 69 to 779 92% and from 66 to 84% for PAHs and OCs, respectively (Table S3). Method detection limits 780 (MDLs) defined as mean blank value + 3 × standard deviation (SD) ranged from 0.04 to 2.66 781 ng L -1 , 0.03 to 0.42 ng L -1 and 0.10 to 0.40 ng L -1 for PAHs, PCBs and OCPs, respectively 782 (Table S2). For compounds that were not detected in blanks, instrument detection limits

783

(IDLs), determined as a signal-to-noise ratio (S/N) of 3, were substituted. highest values (> 260 mV) were found in the Southern part (S13-S18) (Fig. 2f).

869

The coastal stations S23-S25 were also characterized by the highest POC and PON 870 concentrations (73.2-89.6 µM and 8.9-10.9 µM, respectively), the highest SPM values (15.0-871 16.8 mg L -1 ), and the lowest δ 15 N values (-1.2-1.9‰) (Table S5). On the other side, the 872 lowest concentrations of POC, PON and SPM were recorded in the Northern and offshore 873 sites (S3, S4, S8, S11) (Table S5). Also, salinity, FDOM, O2 concentration, POC, PON and 874 SPM concentrations were positively (linearly) correlated to each other (r = 0.65-0.97, n = 15, 875 p < 0.0001-0.009), while these latter were all negatively correlated to pH (r = -0.70--0.93, n = 876 15, p < 0.0001-0.004). Turbidity and TChla concentration, albeit not measured in stations 877 S19-S27, showed higher values in the Southern stations (S13-S18) than in the Northern ones 878 (Table S5). Overall, these results highlight coast-offshore and South-North gradients of the 879 different physico-chemical and biogeochemical parameters, with a substantial influence of the 880 phosphogypsum effluent outlet on the local biogeochemistry.

Spatial distribution of total dissolved PAHs, PCBs and OCPs

882

The spatial distribution of concentrations of Σ16 PAHs (sum of 16 compounds), Σ20 PCBs

883

(sum of 20 congeners) and Σ10 OCPs (sum of 10 compounds) in surface waters (at ~ 0.5-m 884 depth) is provided Fig. 3. Detailed information about individual compounds is given in Table 885 S6. Concentration of Ʃ16 PAHs was the highest in the vicinity of Ghannouch industrial 886 complex: at stations S8 and S9 (62.7-71.2 ng L -1 ), in front of Ghannouch 887 harbour/phosphogypsum discharges, at station S22 (64.9 ng L -1 ), in front of Ettine 888 wadi/fluorine discharges, at stations S4, S19, S20 (40.5-59.8 ng L -1 ), Northern Ghannouch,

889

and at station S24 (46.1 ng L -1 ), between Ghannouch and Gabès harbours (Fig. 3a, Table 1).

890

Concentration of Σ20 PCBs showed a quite different distribution with the highest values 891 found off Ghannouch-Gabès area (at S9: 33.7 ng L -1 and S11: 31.4 ng L -1 ), Northern 892 Ghannouch (at S1: 22.1 ng L -1 ), and in front of fishing harbour and Gabès wadi (at S13: 28.3 893 ng L -1 ) (Fig. 3b, Table 1).

894

For OCPs, concentration of Σ4 HCHs displayed a distribution pattern relatively similar to 895 that of PAHs, with the highest values observed around the Ghannouch industrial complex and 896

Northern Ghannouch: at stations S8 (14.8 ng L -1 ), S22 (9.4 ng L -1 ), S19 and S20 (6.4-7.0 ng 897 L -1 ) (Fig. 3c, Table 1). Interestingly, Σ16 PAH and Σ4 HCH concentrations were positively 898 correlated (r = 0.72, n = 27, p < 0.001). The highest concentration of Σ6 DDTs observed was 899 in Northern Ghannouch (at S4-S6 and S19: 11.3-13.2 ng L -1 ), and in front of phosphogypsum 900 discharges (at S8: 12.0 ng L -1 ), as well as Southern Gabès, in front of Griaa wadi (at S26: 10.9 901 ng L -1 ) and in front of Mersit wadi (at S16: 11.7 ng L -1 ) (Fig. 3d, Table 1). It appeared that the

Molecular distribution of dissolved PAHs, PCBs and OCPs 910

Among the 16 PAHs targeted in this study, four were not detected in the samples: BaP,

911

DahA, IcdP, and BghiP (Fig. 4a). The most abundant PAHs were Nap (on average 63% of 912 total PAHs, ranging from 27% at S7 to 82% at S19) followed by Phe (on average 12%,

913

ranging from 7% at S8, S19, S22 to 25% at S4), and Flt (on average 7%, ranging from 1% at 914 S8, S22 to 36% at S7), while other compounds only accounted for 0-5% on average of total 915 PAHs (Fig. 4a). With respect to size, the concentrations of LMW-PAHs in surface waters of S7). This difference may be due to the fact that this latter S7).

981

The concentrations of total dissolved (Ʃ20) PCBs (2.9-33.6 ng L -1 ) we determined here in 982 the Gulf of Gabès (Fig. 3b, Table 1) were in most cases higher than those recorded in other 983 environments, such as the Open Baltic Sea (0.02 ± 0.01 ng L -1 for Ʃ7 PCBs) (Cornelissen et 984 al., 2008) and the Tokyo Bay, Japan (0.04-0.6 ng L -1 for Ʃ209 PCBs) [START_REF] Kobayashi | Spatial distribution and partitioning of polychlorinated biphenyls in Tokyo Bay, Japan[END_REF].

985 However, they were of the same order of magnitude than those recorded in the Singapore's S7). Urbanization, human activities and 990 particularly, intense industrialization in this area may explain the higher PCB concentrations 991 compared to the other environments.

992

The concentrations of total dissolved (Ʃ10) OCPs (5.3-26.7 ng L -1 ) found here (Fig. 3c and 993 d, Table 1) were in the same range than those measured in Hangzhou Bay, China (1.3-26 ng 994 L -1 for Ʃ10 OCPs) [START_REF] Li | Residues and distribution of organochlorine pesticides in water and suspended particulate matter from Hangzhou Bay, East China Sea[END_REF]) and Singapore's northeastern and southwestern coastlines 995 (4.9-22 ng L -1 for Ʃ12 OCPs) [START_REF] Basheer | Persistent Organic Pollutants in Singapore's Coastal Marine Environment: Part I, Seawater[END_REF], but lower than that recorded in Jinzhou 996 Bay, China (4.2-136.8 ng L -1 for Ʃ19 OCPs) [START_REF] Yao | Distribution and sources of polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) in surface waters of Jinzhou Bay in China[END_REF] (Table S7). Large differences 997 in pesticide concentrations in global surface waters may be explained mainly by differences in 998 agricultural activities, given the strong connection between OCPs and agriculture (Montory et 999 al., 2017). Although all these comparisons should be taken with caution due to the different number of compounds analyzed, the tendency that emerges is that the concentrations we measured in this work in the Ghannouch-Gabès area (Southern Gulf of Gabès) were in the mid-range of those reported in other coastal environments (Table S7).

Sources/origins

Compositional profiles for PAHs in water samples are illustrated in Fig. 4a. LMW-PAHs were abundant at all sampling sites, representing on average > 80% of Ʃ16 PAHs; HMW-PAHs, on the other hand, were present in low concentrations and contributions to Ʃ16 PAHs. This is typical for surface waters because of the relatively high solubility of LMW-PAHs (log Kow is 3-5 for LMW and 5-7 for HMW) [START_REF] Moeckel | Relationship between the concentrations of dissolved organic matter and polycyclic aromatic hydrocarbons in a typical UK upland stream[END_REF][START_REF] Shen | Residues of organochlorine pesticides (OCPs) and polycyclic aromatic hydrocarbons (PAHs) in waters of the Ili-Balkhash Basin, arid Central Asia: Concentrations and risk assessment[END_REF].

The evaluation of PAH sources by sampling site was achieved by considering published indicators (Table 1) and the analysis of bivariate cross plots, which reflect characteristic PAHs ratios (Fig. S1). Because PAHs naturally partitioned between dissolved and particulate water phases in line with their molecular weight/hydrophobicity, the dominance of the LMW compounds in the present samples (especially Nap and Phe), which are much more soluble than the HMW ones, cannot be used as an origin/source criterion. For that, isomeric compound ratios, Ant/(Ant+Phe) and Flt/(Flt+Pyr) are more reliable, although they might be affected by weathering processes between the emission points and the sampling area [START_REF] Katsoyiannis | Model-based evaluation of the use of polycyclic aromatic hydrocarbons molecular diagnostic ratios as a source identification tool[END_REF]. According to Table 1 and Table S4, PAHs were found to be both pyrolytic and petrogenic origin, with a predominance of petrogenic sources (63% of the stations had a ratio of Flt/(Flt+Pyr) between 0.4 and 0.5, and 89% of the stations had a ratio of Ant/(Ant+Phe) < 0.1) [START_REF] Ravindra | Seasonal and site specific variation in vapour and aerosol phase PAHs over Flanders (Belgium) and their relation with anthropogenic activities[END_REF][START_REF] Soclo | Origin of polycyclic aromatic hydrocarbons (PAHs) in coastal marine sediments: case studies in Cotonou (Benin) and Aquitaine (France) areas[END_REF][START_REF] Yunker | PAHs in the Fraser River basin: a critical appraisal of PAH ratios as indicators of PAH source and composition[END_REF]. The plot of isomeric ratios, Ant/(Ant+Phe) versus Flt/(Flt+Pyr) (Fig. S1), confirm the presence of the last referred sources in surface waters. This reflects the multitude of sources of PAHs in this anthropized coastal area of the Gulf of Gabès through the utilization of petroleum products and incomplete combustion of fossil fuels and biomass [START_REF] Stogiannidis | Source Characterization of Polycyclic Aromatic Hydrocarbons by Using Their Molecular Indices: An Overview of Possibilities[END_REF].

Petrogenic PAHs could be introduced via accidental oil spills, discharges from routine ship and harbour activities/operations, and urban runoff, while pyrogenic PAHs may enter via exhausts and residues issued from the combustion related to the various urban, industrial and agriculture activities occurring in this region. Moreover, PAHs, particularly LMW-PAHs prevailing in this study, can reach surface waters of the Gulf of Gabès via atmospheric transport over long distances from the Gabès city and other areas. Interestingly, the stations located either in front of (S7-S9, S23) or close to (S22, S24) Ghannouch industrial complex/commercial harbour and phosphogypsum effluent outlet (Fig. 1, Table S1) presented dissolved PAHs mainly of pyrogenic (combustion) origin, as seen from their Ant/(Ant+Phe) (0.12-0.21) and Flu/(Flu+Pyr) ratios (0.43 and 0.90) (Table 1, Table S4). Moreover, these stations (S7-S9, S22-S24) were marked by higher concentrations of total dissolved PAHs (19.5-71.2 ng L -1 ; mean and median: 49 and 56 ng L -1 ) compared to the other stations (17.6-59.8 ng L -1 ; mean and median: 31 and 28 ng L -1 ). This could be the sign of a specific PAH fingerprint due to the whole Ghannouch industrial complex activity. 1, Fig. 4b), which is in agreement with previous observations and may be explained by their higher solubility in water compared to PCBs of higher degree of chlorination [START_REF] Li | Residues and distribution of organochlorine pesticides in water and suspended particulate matter from Hangzhou Bay, East China Sea[END_REF][START_REF] Montuori | Polychlorinated biphenyls and organochlorine pesticides in water and sediment from Volturno River, Southern Italy: occurrence, distribution and risk assessment[END_REF][START_REF] Moret | Determination of polychlorobiphenyl congeners (PCBs) in the surface water of the Venice lagoon[END_REF]. Less chlorinated PCBs are more amenable to long range atmospheric transport than heavier ones [START_REF] Wania | A global distribution model for persistent organic chemicals[END_REF]. This suggests that atmospheric transport could be the most important PCB source in the studied region. Furthermore, Tri-CBs, which are mainly used as an insulating liquid for transformer oils and power capacitors [START_REF] Tang | Distribution, sources and ecological risks of organochlorine compounds (DDTs, HCHs and PCBs) in surface sediments from the Pearl River Estuary, China[END_REF], can leaked into the soil and end up in surface waters via surface runoff and land use [START_REF] Duan | Distributions and sources of polychlorinated biphenyls in the coastal East China Sea sediments[END_REF]. Surprisingly, CB-180 which is a heptachloro-congener, characterized by a higher number of chlorine atoms (and thus a lower solubility in water), was one of the most abundant PCBs in our samples (27% of total PCBs on average). Generally, CB-180 is not detected or detected at very low levels in the dissolved phase of natural waters [START_REF] Montuori | Polychlorinated biphenyls and organochlorine pesticides in water and sediment from Volturno River, Southern Italy: occurrence, distribution and risk assessment[END_REF][START_REF] Moret | Determination of polychlorobiphenyl congeners (PCBs) in the surface water of the Venice lagoon[END_REF]Rabodonirina et al., 2015). Indicator PCBs were detected in the surface waters of the Gulf of Gabès with detection frequency varied from 19 to 100%, and concentrations ranged from 1.8 to 28.4 ng L -1 . The levels of Ʃ7 IN-PCBs were strongly correlated with total PCBs (Ʃ20 PCBs) (Pearson r = 0.97, p < 0.0001) and a conversion factor of 4 can be used to estimate total PCB concentrations (Σ209 PCBs) from 7 IN-PCBs (i.e., Σ209 PCBs = 4 × 7 IN-PCBs).

Similar conversion factor was applied by previous studies for sediment and road dust [START_REF] Anh | Characterization of 209 polychlorinated biphenyls in street dust from northern Vietnam: contamination status, potential sources, and risk assessment[END_REF][START_REF] Hoai | Recent levels of organochlorine pesticides and polychlorinated biphenyls in sediments of the sewer system in Hanoi, Vietnam[END_REF]. PCBs are used in industry as a heat exchanger liquid in transformers and capacitors, and as additives in paints, carbonless copy papers and plastics.

As in numerous other countries, equipment containing PCBs have been widely used in

Tunisia from the 1970s to late 1980s (an import ban of those equipment being occurred in 1986). Generally, sources of PCBs in marine environment are wastes of electrical transformers, oil spillage, and any historical use of PCB [START_REF] Mitra | Characterization, source identification and risk associated with polyaromatic and chlorinated organic contaminants (PAHs, PCBs, PCBzs and OCPs) in the surface sediments of Hooghly estuary, India[END_REF]. Recently, PCBs have been recorded in Tunisian (Bizerte) coastal areas: in surface sediments, mussels, fishes and atmospheric particles (Barhoumi et al., 2014a, b, c;[START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF][START_REF] Ben Ameur | Concentration of polychlorinated biphenyls and organochlorine pesticides in mullet (Mugil cephalus) and sea bass (Dicentrarchus labrax) from Bizerte Lagoon (Northern Tunisia)[END_REF]. In these compartments (sediments, fishes, atmosphere), the presence of tetrachloro-, pentachloro-, hexachloro-and even heptachloro-congeners was consistent with a contribution of commercial (Aroclor) PCB mixtures [START_REF] Takasuga | Isotope dilution analysis of polychlorinated biphenyls (PCBs) in transformer oil and global commercial PCB formulations by high resolution gas chromatography-high resolution mass spectrometry[END_REF]. In the same way, our PCB molecular distribution may reflect the influence of commercial PCB mixtures.

Various approaches based on the ratio of parent DDTs and their metabolites have been used to assess whether the DDT contamination in a given area can be considered recent, and whether the degradation of DDTs occurred rather under aerobic or anaerobic condition [START_REF] Hitch | Unusual persistence of DDT in some Western USA soils[END_REF][START_REF] Hong | Horizontal and vertical distribution of PCBs and chlorinated pesticides in sediments from Masan Bay, Korea[END_REF][START_REF] Lee | Distribution of organochlorine pesticides in sediments from Kyeonggi Bay and nearby areas, Korea[END_REF][START_REF] Liu | Organochlorine pesticides in surface sediments and suspended particulate matters from the Yangtze estuary, China[END_REF] (Table S4). Here, since parent DDTs were not detected at all in our samples, the ratios (DDE+DDD)/ƩDDTs were always equal to 1. This point outs the historical/ancient character of the DDT contamination in the Gulf of Gabès. Moreover, more than half of stations displayed DDD/DDE ratio < 1 (Table 1), which suggests that DDT was rather biotransformated into DDE under aerobic conditions [START_REF] Galanopoulou | DDTs and other chlorinated pesticides and polychlorinated biphenyls pollution in the surface sediments of Kerstsini Harbor, Saronikos gulf, Greece[END_REF][START_REF] Mostafa | Assessment of persistent organochlorine residues in sediments of Hadramout coastal area, Gulf of Aden, Yemen[END_REF]. This is in accordance with the oxidizing conditions prevailing in surface waters of the Gulf of Gabès, as indicated by O2

concentrations and redox values (Table S5). Therefore, our results indicate that DTTs origin from historical load since only their metabolites have been detected. Prior to their ban in Tunisia early 1984s [START_REF] Apek | Association for environmental protection in Kerkennah[END_REF], DDTs were largely used as pesticides in agriculture in the environment of the Gulf of Gabès, thus the current flux may come from places where they have remained via runoff and wadis or the sediment resuspension. The highest DDT concentrations measured at stations S4 (12.1 ng L -1 ), S5 (11.35 ng L -1 ), S6 (13.23 ng L -1 ) and S19 (11.5 ng L -1 ) located in front of El Demna and El Melah wadis (Northern Ghannouch), and station S16 (11.66 ng L -1 ) located in front of Mersit wadi (Southern Gabès), may confirms the last statement. In addition, antifouling paints used in boat maintenance could be another source of DDTs in surface water of the Gulf of Gabès, evidenced by the high DDT concentration measured at station S8 (11.97 ng L -1 ) located in front of Ghannouch harbour/phosphogypsum discharges.

HCHs usually have two formulations: technical-grade (predominantly constituted by α-HCH) and lindane (> 99% γ-HCH) [START_REF] Iwata | A new view on the divergence of HCH isomer compositions in oceanic air[END_REF]. The isomer ratio α-/γ-HCH can be used as an indicator of the HCH sources, i.e., technical HCH has α-HCH/γ-HCH ratios ranging between 3 and 7, whereas those of lindane are < 1.0 or close to 1.0 [START_REF] Willett | Differential toxicity and environmental fates of hexachlorocyclohexane isomers[END_REF]. α-HCH was found in only three sites, in the Gulf of Gabès, whereas γ-HCH was detected at all sites (Table S6), thus yielding ratios lower than technical HCHs. This reflects the fact that HCHs were predominantly of lindane origin. Lindane was usually used in agriculture for seed treatment for beans, barley and wheat, as well as a lesser degree as a pharmaceutical to treat lice and scabies [START_REF] Barhoumi | Organochlorines in the surface sediments of the Bahiret el Bibane lagoon (Tunisia, Southwestern Mediterranean Sea)[END_REF]. Given their high vapour pressure [START_REF] Shunthirasingham | Spatial and temporal pattern of pesticides in the global atmosphere[END_REF], γ-HCH was probably entered the Gulf via atmospheric transport. Additionally, high detection frequency (85%) of β-HCH was observed, suggested that HCHs were applied in these areas in the past and had degraded, because β-HCH is more stable than other HCH isomers and accumulates in the environment over time [START_REF] Malik | Levels and distribution of persistent organochlorine pesticide residues in water and sediments of Gomti River (India)-a tributary of the Ganges River[END_REF]. Direct discharges, surface runoff from surrounding agricultural soils, along with long-range transport may account for the HCHs pollution in this area.

As shown in Fig. 4c, DDTs and HCHs displayed different patterns. This could indicate different types of human activities (agriculture) prevailed in the area, as well as different input sources, e.g., wet and dry deposition sources, surface runoff, and atmospheric transport. DDT and HCH patterns could be also influenced by the physico-chemical properties of isomer compounds along with environmental conditions. In addition, redox potential, topography, and hydrodynamic condition together with other factors play important roles in controlling the distribution and fate of DDTs and HCHs [START_REF] Su | Organochlorine pesticides in marine environment of quanzhou bay, southeast China[END_REF].

Potential influence of the industrial effluent and ecological risk assessment

Our results show a significant influence of the phosphogypsum effluent on the local physico-chemistry and biogeochemistry, with the highest values of salinity, FDOM, O2, SPM, POC, PON, and the lowest values of pH observed in the most coastal stations located between the Ghannouch and Gabès harbours (S23-S25; Fig. 1), and the opposite pattern (i.e., the lowest values of salinity, FDOM, O2, SPM, POC, PON, and the highest values of pH) found in the Northern and offshore sites, leading to marked coast-offshore and South-North gradients (Fig. 2, Table S5). Hence, the influence of the phosphogypsum effluent mostly appeared South of the Ghannouch industrial complex, in the inter-harbour (Ghannouch-Gabès) zone. These observations are in accordance with recent works, which pointed out a substantial effect of the Ghannouch phosphogypsum effluent on seawater or surface sediment chemistry, with lower pH values and higher concentrations in various major, trace and metal elements in stations situated close to/South of the discharge outlet, and in the inter-harbour area [START_REF] El Kateb | Impact of industrial phosphate waste discharge on the marine environment in the Gulf of Gabes (Tunisia)[END_REF][START_REF] El Zrelli | Heavy metal contamination and ecological risk assessment in the surface sediments of the coastal area surrounding the industrial complex of Gabes city, Gulf of Gabes, SE Tunisia[END_REF], 2018). The fact that the phosphogypsum followed a North to South direction, and accumulate in the area between Ghannouch and Gabès harbours is consistent with results from [START_REF] Zayen | Microplastics in surface waters of the Gulf of Gabes, southern Mediterranean Sea: Distribution, composition and influence of hydrodynamics[END_REF], who reported, during our sampling period and in our sampling area, the occurrence of a tidally-averaged coastal current flowing from the North to the South, very likely in relation with wind and the regional current. Dissolved organic contaminants (PAHs, PCBs and OCPs) did not closely follow the spatial distribution of salinity, FDOM, O2, SPM, POC and PON (Fig. 3), neither were correlated to these parameters as seen from the PCA (Fig. 5). They displayed less marked coast-offshore and North-South gradients (particularly for PCBs and DDTs), although PAHs and HCHs presented higher concentrations in the Ghannouch/effluent zone (Fig. 3). This was due to the fact that dissolved PAHs, PCBs and OCPs in coastal surface waters of the Gulf of Gabès very likely originated from various sources including probably the phosphogypsum effluent but also many others as industrial effluents, wadis, runoffs, sediment resuspension, and the atmosphere with aerosol deposition and gas exchanges [START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF][START_REF] Fourati | Sources and spatial distribution of dissolved aliphatic and polycyclic aromatic hydrocarbons in surface coastal waters from the Gulf of Gabès (Tunisia, Southern Mediterranean Sea)[END_REF][START_REF] Zouch | Effect of acidic industrial effluent release on microbial diversity and trace metal dynamics during resuspension of coastal sediment[END_REF].

Concentrations of the targeted PAH, PCB and OCP molecules in surface waters of the Gulf of Gabès were generally low. As a consequence of their environmental persistence, bioaccumulation and toxicity, the continued exposure to these POPs can pose adverse effects to animals and human health, even at very low levels [START_REF] Kim | Evolutionarily adapted hormesisinducing stressors can be a practical solution to mitigate harmful effects of chronic exposure to low dose chemical mixtures[END_REF]. Therefore, the potential risk of those chemicals on aquatic organisms was evaluated. TEFs were applied here to quantify the current mixture of PAHs carcinogenicity in seawater samples (Table 2). The results show that in terms of total TEQs∑PAHs, the measured values were found in the range between 0.04 and 0.51 ng TEQ L -1 (mean of 0.14 ng TEQ L -1 ) which were lower than the threshold values established for BaP in water, suggesting that at the ongoing concentrations the evaluated mixture does not seem able to start carcinogenic responses in aquatic organisms [START_REF] Abdel-Shafy | A review on polycyclic aromatic hydrocarbons: source, environmental impact, effect on human health and remediation[END_REF]. TEQs∑PAHs observed in surface waters of the Gulf of Gabès is comparable to that found in the Atlantic Iberian northwest coastline (Rocha and Rocha, 2021).

It is also important to evaluate if the measured concentrations of PAHs pose ecological risks. As shown in Table 2, RQ(MPCs) of individual PAHs in waters from the study area were all < 1, indicating relatively low ecosystem risk [START_REF] Cao | Distribution and ecosystem risk assessment of polycyclic aromatic hydrocarbons in the Luan River, China[END_REF]. RQ(NCs) of Chr was <1

indicating almost no risk to the ecosystem, whereas RQ(NCs) of the other PAHs was > 1 in at least 7% of samples indicating moderate risks. Concerning the mix of the twelve PAHs (RQΣPAHs), as the value of RQƩPAHs(NCs) are ≥ 1 or < 800 and those of RQƩPAHs(MPCs) are ≈0, it is concluded that, overall, the analyzed PAHs have a low risk to aquatic organisms.

Regarding organochlorines, it can be seen that all RQCMCs values for individual OCPs were < 1 and RQCCCs > 1 suggesting that risk from measured OCPs is moderate to severe, however, RQCCCs for PCBs are < 1 indicating a low risk (Table 3).

Conclusion

The results obtained in this study provided the first detailed analysis of dissolved PAH and OCs (including PCBs and OCPs) distributions in surface water collected from 27 stations in the Gulf of Gabès (Tunisia). PAH levels were lower than those found in other coastal waters around the world, while OC levels were moderated. The tendency that emerges is that the concentrations we measured in this work in the Ghannouch-Gabès area (Southern Gulf of Gabès) were in the mid-range of those reported in other coastal environments. They displayed less marked coast-offshore and North-South gradients (particularly for PCBs and DDTs), although PAHs and HCHs presented higher concentrations in the Ghannouch/effluent zone.

Risk assessment based on toxic equivalents (TEQs) indicated that DDTs could induce the highest risks for benthic organisms, whereas ecological risks from PAHs and PCBs were potentially lower. This work provides baseline information for further studies about this Gulf.

Since PAHs and OCs levels were only studied in autumn, more studies throughout the whole year with increasing samples sizes are needed to fully understand the levels of PAHs and OCs and the ecological risk assessment of this area.

Figure captions

Figure 1. Location of the study stations in the Ghannouch-Gabès coastal area in the Gulf of Gabès (Tunisia, Southern Mediterranean Sea). The detailed characteristics of the stations are given in Table 1.

  guaranteed through the implementation of strict QA/QC procedures. Before 768 use, all glassware was intensively cleaned and baked at 450 °C for 6 h. All the material that 769 could not be baked was washed with 1 M HCl, ultrapure water, n-hexane:acetone mixture 1:1 770 (v/v) and then dried at room temperature. Procedural blanks (to determine the contamination 771 introduced during extraction and cleanup) were processed in the same way as the real samples 772 and were included with each batch of 5 samples. Most of the analytes were not detected in 773 blanks, except Nap, Acy, Ace, Flu, Phe, Ant, Flt, Pyr, BaA and Chr. Their amounts were not 774 significant relative to that of the water samples (< 7%). The recoveries of target compounds 775 were checked using standard mixtures spiked in ultrapure water (50 ng for PAHs, 20 ng for 776

  881

902

  spatial distributions of PAHs, PCBs, HCHs and DDTs were relatively different from those of 903 the physical and physico-chemical parameters described above (i.e., salinity, pH, FDOM, O2 904 concentration and redox potential) because the contaminant concentrations exhibited less 905 pronounced coast-offshore gradients, and were less driven by the phosphogypsum effluent 906 outlet. PAHs and OCPs (particularly HCHs) nonetheless seemed to show more similarities 907 with physico-chemical parameters than PCBs, especially regarding their higher concentrations 908 in the Ghannouch/effluent zone.

  909

  967study targeted more compounds (Ʃ29 PAHs) including alkylated compounds, which are 968 recognized to account for a large part of the pool of dissolved PAHs in natural waters(Guigue 969 et al., 2014;[START_REF] Wang | Oil spill identification[END_REF]. Also, dissolved PAH concentrations were in the lower range 970 of those reported in coastal Bohai Bay, China (48-607 ng L -1 for Ʃ16 PAHs)(Tong et al., 971 2019), various coastal areas of the Southern, Northern and Northwestern Mediterranean Sea, 972 such as the Gulf of Tunis, Tunisia (139-1008 ng L -1 for Ʃ24 PAHs)(Mzoughi and Chouba, 973 2011), the Marseilles coastal area, France (1.9-151 ng L -1 for Ʃ17 PAHs)(Guigue et al., 2011, 974 2014) and the Alexandria coast, Egypt (13-120 ng L -1 for Ʃ7 PAHs) (El Nemr and Abd-Allah, 975 2003), but were higher than those observed in the Open Western and Eastern Mediterranean 976 Sea (0.16-9 ng L -1 for Ʃ19 PAHs)[START_REF] Berrojalbiz | Biogeochemical and physical controls on concentrations of polycyclic aromatic hydrocarbons in water and plankton of the Mediterranean and Black Seas[END_REF], the Northwest Atlantic coast of 977 the Iberian Peninsula (9.3 ± 0.6 ng L -1 for Ʃ16 PAHs)(Rocha and Rocha, 2021), the European 978 Atlantic coastline, Portugal (8.3-9.5 ng L -1 for Ʃ16 PAHs)(Rocha et al., 2021) and Barcelona, 979 Spain and Banyuls-sur-Mer, France (0.4-8.6 ng L -1 for Ʃ15 PAHs)[START_REF] Garcia-Flor | Enrichment of organochlorine contaminants in the sea surface microlayer: An organic carbon-driven process[END_REF] 980 (Table

986

  northeastern and southwestern coastlines (0.22-20.41 ng L -1 for Ʃ8 PCBs)(Basheer et al., 987 2003), and they were lower than those found in the Daya Bay, China (91.1-1355.3 ng L -1 for 988 Ʃ12 PCBs)(Zhou et al., 2001), and the Panama Canal and California Coast, USA (bdl-18,450 989 ng L -1 for Ʃ19 PCBs)[START_REF] Menzies | Baseline occurrence of organochlorine pesticides and other xenobiotics in the marine environment[END_REF] (Table

Figure 2 .

 2 Figure 2. Spatial distribution of a) temperature, b) salinity, c) pH, d) fluorescent dissolved

Figure 3 .

 3 Figure 3. Spatial distribution of concentrations of total dissolved a) PAHs (sum of 16

Figure 4 .

 4 Figure 4. Relative abundance (in %) of a) 12 PAHs, b) 9 PCBs congeners, and c) 7 OCPs in

Figure 5 .

 5 Figure 5. Principal component analysis (PCA), based on the Spearman 's correlation matrix,

  Figure 2

  O2 concentration and FDOM, and low concentrations of total dissolved PAHs, PCBs Gabès area (Fig.3a, Table1) were of the same order of 965 magnitude but were lower than those recently measured in the neighboring area byFourati et

	935 959	respectively, and maximal contributions of 64% (at S7) and 51% (at S11), respectively (Fig.
	936 960	4c). They were followed by γ-HCH, which represented on average 19% with minimal and and OCPs (Fig. 5).
	937 961	maximal values of 4% (at S11, S14) and 51% (at S12), respectively. The sum of the
	938 962	remaining compounds (α-HCH, β-HCH, δ-HCH, 4,4′-DDE) accounted on average for 22% of 4. Discussion
	939 963	total OCPs (Fig. 4c). The DDD/DDE ratio was quite variable, ranging from 0.21 (S11) to 3.74 4.1. Concentration levels
	940 964	(S7), with mean and median values of 1.30 and 0.80, respectively (Table 1). Since 2,4′-DDT The concentrations of total dissolved (Ʃ16) PAHs (17.6-71.2 ng L -1 ) recorded here in the
	941 916	and 4,4′-DDT were not detected, the (DDE+DDD)/ƩDDTs ratios were all equal to 1 (data not surface waters of the Ghannouch-
		the Gulf of Gabès ranged from 15.8 to 62.4 ng L -1 and for HMW-PAHs, the range was 1.4 to reported in Table 1).
	950
	926 951	different groups of stations. Group n°1 (S2, S3, S5, S7, S10, S12, S14, S17, S18, S21 and
	927 952	values of 0% (at S3 and S26 for PCB-18, and at S12, S14 and S15 for PCB-52) and maximal S27) is rather characterized by high values of pH, low values of salinity, O2 concentration and
	928 953	values of 41% (at S20 for PCB-52) or 59% (at S14 for PCB-18). PCB-101, -28 and -153 FDOM, and low concentrations of total dissolved PAHs, PCBs and OCPs. Group n°2 (S1, S6
	929 954	displayed lower mean relative abundances representing 14, 10 and 6% (respectively) while and S11) is depicted by quite high values of pH, low values of salinity, O2 concentration and
	930 955	the sum of the remaining congeners (PCB-118, -128, -138) only depicted 6% of total PCBs FDOM, and high concentrations of total dissolved PAHs, PCBs and OCPs. Group n°3 (S4,
	931 956	(Fig. 4b). S8, S9, S13, S16, S19, S20, S22) is defined by lower values of pH, higher values of salinity,
		Among OCPs of interest, 2,4′-DDT, 4,4′-DDT and 4,4′-DDD were not at all detected. 2,4′-O2 concentration and FDOM, and high concentrations of total dissolved PAHs, PCBs and
	934

917

23.3 ng L -1 . The Flt/(Flt+Pyr) ratio was 0.56 on average and ranging in values from 0.40 918 (S10) to 0.90 (S7), while the Ant/(Ant+Phe) ratio was 0.07 on average with values ranging 919 from 0.01 (S1) to 0.21 (S8) (Table

1

).

920

Of the 20 targeted PCB congeners, 11 were not detected in the samples. These were PCB-

921 8,

). The relative abundance of 922 the 9 congeners detected was highly variable from station to station. PCB-180 alone 923 accounted for 27% of the PCBs on average, with minimal contribution of 1% (at S10) and 924 maximal contribution of 71% (at S26). PCB-18 and -52 were also detected in large amounts.

925

The two congeners represented 18 and 19% of PCBs on average, respectively, with minimal 932 DDD and 2,4′-DDE compounds were major species, with average contributions to total OCPs 933 of 31 and 28%, respectively, minimal contributions of 0% (at S12, S15) and 4% (at S7, S18), 942

3.4. PCA classification of stations 943

Spearman PCA was applied on various parameters including salinity, pH, O2 944 concentration, FDOM, as well as concentrations of Σ16 PAHs, Σ20 PCBs and Σ10 OCPs (Fig. 945

5). The first principal component (PC1), which explained 39% of total variability within 946 samples, mainly received the contribution of variables salinity, pH, FDOM and in a lesser 947 extent O2 concentration and Σ10 OCPs. The second principal component (PC2) accounted for 948 21% of total variability within samples and was mostly related to variables Σ16 PAHs, Σ20 949 PCBs, followed by O2 concentration and Σ10 OCPs. PCA allowed the discrimination of 957 OCPs. Finally, group n°4 is characterized by roughly low values of pH, high values of salinity, 966 al. (2018) (i.e., 9-198 ng L -1 ; Table

  The dissolved phase was mostly enriched by PCBs displaying a low degree of chlorination,

i.e., tri-(CB-18, CB-28; 28% of total PCBs on average), tetra-(CB-52; 19% of total PCBs on average), penta-(CB-101; 19% of total PCBs on average), and hexa-CBs (CB-153; 19% of total PCBs on average) (Table

Table captions Table 1 .

 captions1 Total concentrations of dissolved PAHs, PCBs, OCPs, HCHs, DDTs and associated ratios from seawater samples collected at ~ 0.5-m depth in coastal area of the Gulf of Gabès (Tunisia, Southern Mediterranean Sea).

Table 2 .

 2 Toxic equivalence quantities (TEQs) and mean values of RQ(NCs) and RQ(MPCs) of PAHs from seawater samples collected at ~ 0.5-m depth in coastal area of the Gulf of Gabès (Tunisia, Southern Mediterranean Sea).

Table 3 .

 3 Mean values of RQ (CCCs) and RQ (CMCs) for OCPs and PCBs from seawater samples collected at ~ 0.5-m depth in coastal area of the Gulf of Gabès (Tunisia, Southern Mediterranean Sea).

Table 3 .

 3 Mean values of RQ (CCCs) and RQ (CMCs) for OCPs and PCBs from seawater samples collected at ~ 0.5-m depth in coastal area of the Gulf of Gabès (Tunisia, Southern Mediterranean Sea). Average Measured Concentration for OCPs and PCBs in this study (ng L -1 ).

		γ-HCH	DDEs	DDDs	PCBs
	CMCs (ng L -1 )	950	1100	1100	-
	CCCs (ng L -1 )	-	1	1	4
	MEC*	2.286	4.295	3.568	10.771
	RQCMCs	0.002	0.004	0.003	-
	RQCCCs	-	4.295	3.568	0.769

*

Acknowledgements

We are grateful to the National Institute of Marine Science and Technology (INSTM) for their logistic support, as well as the captain and the crew of the vessel "Rhama" for their technical help during the cruise. We acknowledge P. Raimbault, N. Garcia, and the Plateforme Analytique de Chimie des Environnements Marins (PACEM platform) of the Mediterranean Institute of Oceanography (MIO) for POC, PON and isotopic analyses, as well as C. Sammari (INSTM) for the cruise organization and management. We warmly thank M.M. Serbaji from the National Engineering School of Sfax (ENIS) for the production of the map (Figure 1).

Funding

This work was financially supported by the CNRS-INSU MISTRALS MERMEX-MERITE project and the IRD French-Tunisian International Joint Laboratory (LMI) "COSYS-Med". CRediT authorship contribution statement Hatem Zaghden, Badreddine Barhoumi and Marc Tedetti: Conceptualization, Investigation, Methodology, Data curation, Writing -original draft, and Writing -review & editing. Lobna Jlaiel: Methodology and Data curation. Catherine Guigue: Writingreview & editing. Lassaad Chouba: Conceptualization and Data curation. Soufiane Touil and Sami Sayadi: Supervision

Declaration of competing interest

Publishing of this paper will cause no conflict of interest with any people, institutions or legislation.

ratios from seawater samples collected at ~ 0.5-m depth in coastal area of the Gulf of Gabès 1745 (Tunisia, Southern Mediterranean Sea).