1 2 3 4 5 6 7 8 Impact of wind-driven circulation on contaminant 9 dispersion in a semi-enclosed bay 10 11 12 Camille Mazoyera,b,∗, Heleen Vannestea, Christiane Dufresnec, Yann 13 Ourmieresa, Marcello G. Magaldid,e, Anne Molcarda 14 a 15 Univ Toulon, Aix Marseille Univ., CNRS/INSU, IRD, MIO UM 110, Mediterranean 16 Institute of Oceanography, La Garde, France bUniversit´ede Toulon, IMATH, EA 2134, Avenue de l’Universit´e,83957 La Garde, 17 France 18 cInstitut des sciences de la mer, Universit´edu Qu´ebec `aRimouski, 310 all´eedes 19 Ursulines, C.P. 3300, Rimouski, Qu´ebec, G5L 3A1, Canada 20 dIstituto di Scienze Marine, S.S. di La Spezia, Consiglio Nazionale delle Ricerche, Forte 21 Santa Teresa, Pozzuolo di Lerici (SP), I-19032, Italy 22 e Department of Earth and Planetary Sciences, The Johns Hopkins University, Olin 23 Hall, 34th and North Charles Streets, Baltimore, MD 21218, USA 24 25 26 27 Abstract 28 29 30 Mediterranean semi-enclosed bays are often exposed to high levels of con- 31 taminants originating from anthropogenic activities in the bay. To assess 32 33 their fate and impact on the environment, it is essential to investigate coastal 34 35 circulation regimes which may play an important role in the dispersion of 36 contaminants across the bay and beyond. In this study, a high resolution 37 38 coupled hydrodynamic - passive tracer model was combined with ADCP ob- 39 40 servations, to identify major circulation patterns and associated dissolved 41 contaminant dispersion pathways in the contaminated semi-enclosed bay of 42 43 Toulon (South of France, NW Mediterranean Sea). Two dominant circulation 44 45 patterns and two derived ones could be identified, driven by winds (Mistral 46 47 ∗Corresponding author 48 Email address: [email protected] (Camille Mazoyer ) 49 50 51 Preprint submitted to Estuarine Coastal and Shelf November 25, 2019 52 53 54 55 56 57 58 59 60 61 62 63 64 and easterly winds) and offshore waters intrusions. Medium to strong Mistral 65 events (superior to 6 m s−1) with a WNW direction cause a bi-layer pattern 66 67 with surface waters flowing out of the bay and marine waters entering at 68 69 depth. Less frequently, Western Mistral winds of medium to strong strength 70 (superior to 6 m s−1) may generate an anticyclonic circulation. During east- 71 72 erly wind conditions (> 6 m s−1), an inward flow can be observed which 73 74 is sometimes reinforced by offshore waters intrusions, probably from the lo- 75 cal boundary current, the Northern Current (NC). Furthermore, dissolved 76 77 contaminant dispersion pathways were simulated under typical wind forcing 78 79 conditions with three source points of copper (Cu) that were identified based 80 on surface Cu observations. While most of the WNW Mistral wind events 81 82 transport dissolved copper plumes across and out of the bay, contaminant 83 84 dispersion can remain confined to the bay under certain west mistral con- 85 ditions. Conversely, during easterly wind events, contaminants are exiting 86 87 the bay as a narrow vein along the Saint-Mandrier peninsula, before prob- 88 89 ably converging on the NC offshore. Accordingly, this study demonstrates 90 the important impact of hydrodynamic-driven processes on the dispersion of 91 92 contaminants within a semi-enclosed bay. 93 94 Keywords: Semi-enclosed bay, wind-driven circulation, contaminant 95 dispersion, MITgcm, Toulon Bay 96 97 98 1. Introduction 99 100 101 Today, marine coastal areas suffer from an extremely high anthropogenic 102 pressure, which is the result of a continuous development of human activi- 103 104 ties in these areas for centuries. These activities have lead to pollution hot 105 106 107 2 108 109 110 111 112 113 114 115 116 117 118 119 120 spots, endangering the water quality of ports, beaches, aquaculture zones 121 and marine protected areas, challenging local governments to find a balance 122 123 between economical and environmental interests. To this end, Toulon Bay 124 125 (South of France, NW Mediterranean Sea) is an area of interest as it is a 126 semi-enclosed bay harbouring the largest French naval base while located at 127 128 less than 20 km from the Marine National Park of Port-Cros (Fig.1). The 129 130 core of this marine protected area includes two islands within the Hyeres Bay, 131 namely Porquerolles and Port-Cros Islands. The surrounding waters, host- 132 133 ing a unique marine ecosystem, are highly monitored to maintain its quality, 134 135 protect its biodiversity and promote a sustainable development of tourism 136 and local communities. Its proximity to the highly industrialised Toulon city 137 138 requires a better understanding of contaminant distribution within the bay 139 140 and its surroundings, to assess its potential impact on the water quality of the 141 national park. The long history of naval, industrial and commercial activities 142 143 in the Toulon Bay has important environmental consequences for the bay. 144 145 High concentrations of trace metals (e.g. lead, copper, zinc, ...) have been 146 measured in sediments (Tessier et al., 2011; Pougnet et al., 2014) and surface 147 148 waters (Dang et al., 2015; Jean et al., 2012; Coclet et al., 2018). For both 149 150 matrices, the contamination is the strongest in the enclosed north-western 151 part of the bay (i.e. the small Bay (SB)) where surface sediments contain up 152 153 to ca. 1000 times more mercury compared to background values, while up to 154 155 80, 40 and 31 times more copper, zinc and lead, respectively (Tessier et al., 156 2011). Surface waters on the other hand, are ca. 40 and 70-fold enriched 157 158 in respectively dissolved lead and copper (Coclet et al., 2018), compared to 159 160 trace metal concentrations observed in the open Mediterranean Sea (Tovar- 161 162 163 3 164 165 166 167 168 169 170 171 172 173 174 175 176 Sanchez et al., 2014). While Toulon Bay might be considered as one of the 177 most polluted marine areas of the northern Mediterranean sea (Tessier et al., 178 179 2011), contamination might be exported to offshore waters through hydro- 180 181 dynamic processes and reach sensitive areas as the Marine National Park of 182 Port-Cros. 183 184 Very few studies have focused on the hydrodynamic processes within the 185 186 Toulon Bay. Millot et al.(1981) and Tine(1981) were the first to use current 187 profiler data to quantify water exchanges through the channel between the 188 189 small (SB) and the large bay (LB), while Duffa et al.(2011), Dufresne et al. 190 191 (2014) and Dufresne et al.(2018) recently constructed a hydro-sedimentary 192 model of the Toulon Bay, to create a post-accidental management tool in the 193 194 event of radionuclide releases to the environment. These studies showed that 195 196 the circulation is strongly dependent on wind conditions, causing for instance 197 reversals in the water column in the channel connecting the SB with the LB 198 199 (Dufresne et al., 2014). While these studies focused on the quantification of 200 201 the exchanges between the SB and the LB, here we investigate the circulation 202 in the LB under various wind conditions and the resulting exchanges with 203 204 offshore waters. 205 206 The aim of this study is two-fold. The first objective is to identify dom- 207 inant circulation patterns inside the bay, using an ocean circulation model 208 209 (MITgcm) and ADCP observations in the Toulon Bay. The second objective 210 211 is to investigate contaminant transport, by defining dispersion patterns and 212 assess the potential impact on key areas within the bay and beyond. 213 214 After a description of the studied site, the model configuration and in 215 216 situ observations, we describe the wind-driven circulation within the bay. 217 218 219 4 220 221 222 223 224 225 226 227 228 229 230 231 232 An analysis of the contaminant distribution as observed in March 2017 is 233 discussed and the model is used as a tool to simulate the dispersion of dis- 234 235 solved contaminants based on wind conditions. 236 237 238 2. Material and methods 239 240 2.1. Study area 241 242 The water circulation of the southeast French Mediterranean coast, is 243 244 characterized by a strong geostrophic boundary current flowing along the 245 246 North Western Mediterranean Sea coast counter-clockwise, the Northern 247 Current (NC). This current has been largely studied especially focusing on its 248 249 behaviour upstream in the Ligurian basin (Taupier-Letage and Millot, 1986; 250 251 Alberola et al., 1995a; Sammari et al., 1995; Bethoux et al., 1982), down- 252 stream at the eastern side of the Gulf of Lion (Alberola and Millot, 2003; 253 254 Conan and Millot, 1995; Flexas et al., 2002), and along its shelf (Auclair 255 256 et al., 2001; Petrenko, 2003; Rubio et al., 2009; Lapouyade and Durrieu de 257 madron, 2001). However few studies on the NC circulation were conducted in 258 259 the Var region, which can be seen as an area of transition (Ourmi`ereset al., 260 261 2011; Guihou et al., 2013; Declerck et al., 2016). These studies showed that 262 in this area the NC has a meso and sub-mesoscale activity which can create 263 264 exchanges between the shelves and offshore waters. For instance Declerck 265 266 et al.(2016) modelled intrusions into the Hyeres Bay, nearby the Toulon 267 Bay. They showed that the NC, through meanders and vortices dynamics, 268 269 can either act as a barrier or favour in- and off-shore water mixing into semi- 270 271 enclosed bays as Hyeres Bay. Here, we will focus on the Toulon bay, analyze 272 its inner wind-driven circulation and the possible offshore waters intrusions 273 274 275 5 276 277 278 279 280 281 282 283 284 285 286 287 288 that may be driven by the Northern Current that flows off the bay. 289 Toulon Bay is a shallow semi-closed area (about 10 km x 15 km), divided 290 291 into two bays, separated by a seawall of 1.4 km long modeled impervious on 292 2 293 the whole water column: a small bay (SB; ≈ 10 km ) which hosts mostly all 294 the anthropogenic activities and a large bay (LB; ≈ 42 km2) connected to 295 296 the offshore sea (Fig.1). The LB is delimited by Cape Carqueiranne to the 297 298 east, and by the Saint-Mandrier peninsula to the west. At the south of the 299 LB, the bathymetry is very steep with a canyon of ca. 600 m deep. 300 301 Since there is no significant tide motion (tidal amplitude of approximately 302 303 20 cm) (Millot et al., 1981; Alberola et al., 1995b) in the Toulon bay, its 304 circulation is supposed to be mostly wind-driven with possible boundary 305 306 current intrusions. The wind forcing has an important spatial and temporal 307 308 variability with strong events (Millot et al., 1981; Dufresne et al., 2014). 309 Three wind situations can be distinguished: Mistral, easterly wind and calm 310 311 weather. Toulon is surrounded by mountains that deflect these winds: the 312 313 Mistral blows from the west/north-west, especially in winter and spring. It 314 is a cold, dry and strong wind unlike the easterly wind (from north-east to 315 316 south-east) that usually brings clouds, rain and waves. 317 318 319 2.2. Model description 320 The M.I.T General Circulation Model (MITgcm) is a primitive equation 321 322 ocean circulation model (Marshall et al., 1997). It is a z-coordinate finite- 323 324 volume model that solves the incompressible Navier-Stokes equations under 325 Boussinesq approximation, on an Arakawa-C grid. In this model, the free 326 327 surface equation is based on a pressure correction method. For this study, 328 329 we use the hydrostatic approximation. The time scheme is a quasi second 330 331 6 332 333 334 335 336 337 338 339 340 341 342 343 344 order Adams Bashford and the horizontal advection scheme is the third-order 345 DST (direct spacetime) flux limiter. The horizontal subgrid-scale mixing is 346 347 parametrised with the biharmonic Leith viscosity (see Leith, 1968, 1996). 348 −5 2 −1 349 Vertical background viscosity is set with Az = 10 m s , a standard value. 350 We use the turbulent closure scheme K-profile parameterisation (KPP) from 351 352 Large et al.(1994), to model vertical sub-grid scale mixing. For atmospheric 353 354 forcing, we use the bulk formulation from Large and Yeager(2004), and open 355 boundary conditions are prescribed and relaxed by a sponge layer. Free-slip 356 357 boundary conditions are applied at the bottom and on the lateral boundaries. 358 359 360 2.3. MITgcm high-resolution configuration TBAY100 361 The Toulon Bay domain spans a longitudinal range from 5.88 ◦ to 6.13 ◦E 362 363 and a latitudinal range from 43.04 ◦ to 43.13 ◦N (Fig.1). The horizontal 364 365 mesh has 200x100 grid points with 100 m resolution. There are 75 depth 366 levels with a 50 cm minimum layer thickness at the surface, increasing to 367 368 about 30 meters near the seabed. Minimum depth is 3 meters and maximum 369 370 depth is 602 meters in the canyon at the south boundary (Fig.1). The 371 timestep is 5 seconds. Outputs are saved every hour to catch most of the 372 373 dynamics variability. The TBAY100 configuration has two open boundaries 374 375 (south and west) and is forced with a NEMO-based configuration (Declerck 376 et al., 2016) which is a 1/192 degrees configuration of the Var coast (here- 377 378 after NIDOR192), including the Hyeres Bay and the “ˆIles d’Or” islands (i.e. 379 380 Porquerolles, Port-Cros and Levant; Fig.1). Indeed, TBAY100 is the third 381 level of nested coarser models (GLAZUR64, NIDOR192). GLAZUR64 and 382 383 NIDOR192 have been validated during previous studies (Ourmi`ereset al., 384 385 2011; Guihou et al., 2013). The rigorous downscaling set-up guarantees good 386 387 7 388 389 390 391 392 393 394 395 396 397 398 399 400 confidence in the last and highest resolution configuration TBAY100. The 401 horizontal velocities and mass fields open boundary conditions are uploaded 402 403 daily. The operational global model ARPEGE from Meteo-France is used as 404 405 atmospheric forcing: it has a 1/10 degrees spatial resolution (about 9.5 km 406 at our latitude) and a 3 hours temporal resolution. 407 408 The starting date is the 1st of February 2016, after a 1 month simula- 409 410 tion (spin-up evaluated to 15 days). Initial conditions are computed from 411 NIDOR192 output. 412 413 414 2.4. MITgcm ptracers package 415 416 We used in this study the ptracers package from the MITgcm model. 417 This is a passive tracer package, in which the dissolved tracers do not affect 418 419 actively the physics of the ocean as they do not affect the water density. 420 421 Tracers are initialized with precise quantities (for details see Section4), and 422 are released every time step. 423 424 425 2.5. ADCP 426 427 Horizontal velocity data from Acoustic Doppler Current Profilers (ADCP, 428 429 600 kHz, RDInstruments) were also used. Sampling frequency was 10 min. 430 Two ADCPs were moored in the south of the LB between January 2012 431 432 and April 2012 (see Fig.1B ). Their location were chosen to measure the 433 434 exchanges between the LB and offshore. ADCP 1 was located near Cape 435 Cepet and ADCP 2 was located near Cape Carqueiranne. The ADCPs were 436 437 bottom-moored at 32 m, and 29 m respectively. The water column was 438 439 divided into 35 1m-thick bins with the first bin at 2 m above the bottom. 440 441 442 443 8 444 445 446 447 448 449 450 451 452 453 454 455 456 2.6. Diagnostics - composites pictures 457 458 Previous studies on the area (Millot et al., 1981; Dufresne et al., 2014) con- 459 cluded that atmospheric forcings strongly influence hydrodynamics into the 460 461 Toulon Bay. Strong weather conditions cause highly variable currents (due 462 463 to various processes such as offshore swell, wind gusts,... ) combined with 464 offshore waters intrusions and temporal reversals. The wind rose depicted in 465 466 (Fig.2) is computed from the Meteo-France ARPEGE model dataset used 467 468 as external atmospheric forcings for the oceanic model (TBAY100). Two 469 major wind regimes can be defined accordingly: 470 471 472 1. A medium to strong Mistral (258.75 degrees to 326.25 degrees): a west 473 to north-west wind with an intensity between 6 and 24 m s−1 (24.9 % 474 475 of the time); 476 477 2. A medium to strong easterly wind (56.25 degrees to 101.25 degrees): a 478 south-east to a north-east wind with an intensity between 6 and 24 m 479 480 s−1 (9 % of the time); 481

482 −1 483 When the wind intensity is lower than 6 m s , we consider to be under 484 calm conditions (i.e. no wind). The circulation within the bay is therefore 485 486 analyzed for each wind condition. We averaged the horizontal velocity fields 487 488 associated with the specific wind regime, for the surface layer (0-5m) and 489 across the capes section (red line on Fig.1) defining the LB limit. These 490 491 composites pictures are a powerful tool to highlight an average circulation 492 493 and its variability under specific conditions. 494 495 496 497 498 499 9 500 501 502 503 504 505 506 507 508 509 510 511 512 2.7. Datasets 513 514 Table1 gives an overview of all data presented, and their date ranges. As 515 one of our main goal is to study contaminant dispersion, we have decided to 516 517 simulate the year 2016-2017 to coincide with dissolved copper observations 518 519 available for March 2017. Simulations are validated by a comparison with 520 ADCP observations obtained in 2012 (see section 2.5). In order to focus 521 522 on the wind-driven circulation, the velocities simulated by the model and 523 524 measured by the ADCP have been filtered. Data have been processed with 525 the PL66 filter (Limeburner et al., 1985), in order to filter all motions under 526 527 25h, including inertial oscillations, diurnal and semidiurnal tidal constituents 528 529 and eigen motions. 530 In the next sections, we will first identify the main circulation patterns 531 532 induced by regional wind characteristics and provide a quantitative valida- 533 534 tion using ADCP observations. Subsequently, results of 2 days simulations 535 with idealized wind conditions will be presented showing the principal con- 536 537 taminant dispersion pathways in the Toulon Bay. These simulations benefit 538 539 from the 1-month spin-up of the longer simulation TBAY100-PHYS. Even- 540 tually, the TBAY100-TRAC 14-month simulation will allow the assessment 541 542 of long-term temporal variability in contaminant dispersion, by running a 543 544 continuous contaminant release from 3 sources representing the main civil 545 ports in the SB (black dots on Fig.1B). 546 547 548 549 550 551 552 553 554 555 10 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 Table 1: The data ranges of the presented data. 583 584 Data (observations or simulations) Dates 585 586 ADCP Cepet January-April 2012 587 ADCP Carqueiranne January-April 2012 588 589 Seawater samples 20 March 2017 590 591 Simulation TBAY100-PHYS (physics) February 2016 - April 2017 592 Simulation TBAY100-TRAC (tracers) February 2016 - March 2017 593 594 Idealized simulations / 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 11 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 Figure 1: A) Map of Toulon Bay and its surrounding area (SB: small bay, LB: large 652 653 bay, S.-M. P.: Saint-Mandrier peninsula, G. P.: Giens peninsula, Porq.:Porquerolles, P.- 654 Cros: Port-Cros). The black box indicates the geographical extent of the model domain 655 656 of TBAY100. B) Zoom of Toulon Bay with bathymetry (m). The white line between 657 the SB and the LB indicates the impervious seawall on the whole water column. Orange 658 dots indicate ADCP locations: Cape Cepet and Carqueiranne. The red line indicates the 659 660 position of the vertical cross-section presented in Figs.6 and8. Black dots indicate the 661 positions of the three contaminant sources: Toulon civil port (source 1), la Seyne-sur-Mer 662 civil port (source 2), and Saint-Mandrier civil port (source 3). Red boxes: locations for 663 664 which temporal contaminant dispersion is discussed in section 4.0.2. 665 666 667 12 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 Figure 2: Windrose for winds used as external forcings from Meteo-France ARPEGE 707 model, from February 2016 to May 2017. Directions are divided into 16 slices of 22.5 708 709 degrees each. Mistral wind is composed of west-northwest winds with three main direc- 710 tions: d12, d13, d14 from 258.75 degrees to 326.25 degrees. Easterly wind is composed 711 of east-northeast winds with three main directions: d3, d4 and d5 from 56.25 degrees to 712 713 123.75 degrees. Note the windrose shows the direction from where the wind blows. 714 715 716 717 718 719 720 721 722 723 13 724 725 726 727 728 729 730 731 732 733 734 735 736 3. Circulation regimes 737 738 3.1. Surface composite pictures 739 740 Composite pictures of surface currents (0-5m) for Mistral and easterly 741 742 wind, obtained with model results, show opposite current directions induced 743 by the two wind regimes (Fig.3a,c). Overall, Mistral drags the surface 744 745 water southeastwards (Fig.3a) enhancing the surface current intensity near 746 747 Cape Cepet and Cape Carqueiranne, whereas easterly wind pushes it towards 748 the north-west (Fig.3c). While Mistral events drive the surface circulation 749 750 across the entire bay, the influence of easterly wind in the LB is limited 751 752 (indicated by the small arrows on Fig.3c). However in the southern part of 753 the model domain, current intensities are higher and have a stronger gradient 754 755 compared to Mistral conditions. This may be due to intrusions of offshore 756 757 waters that amplify the local wind-driven current. To evaluate the relative 758 importance of these intrusions, we computed a transport index across two 759 760 transects (Fig.3c and d), inspired from the method used by Barrier et al. 761 762 (2016) for the quantification of intrusions in the Gulf of Lion. While T1 (Fig. 763 4) allows us to trace the occurrence of surface offshore waters intrusions near 764 765 the southern boundary of the model domain, T2 identifies water intrusions 766 767 in the LB. The northward meridional current velocity was used to calculate 768 the positive transport T+ across each transect over the surface (2 to 10 m 769 770 depth to focus on the core of the water intrusions). The transects locations 771 772 were chosen where the current intensities are large combined with a standard 773 deviation greater than 0.1 m s−1 for T1 and 0.05 m s−1 for T2 (Fig.3c,d). 774 775 The T1 transect ends westward at the canyon location which is a natural 776 777 bathymetric guidance of offshore water intrusions. Transport is normalised 778 779 14 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 Figure 3: Left panels (a,c) are composite pictures of surface currents (m s−1). Right 819 panels (b,d) are standard deviations of surface currents for the composite pictures (m 820 821 s−1). Top panels (a,b) are for medium to strong Mistral (6 - 24 m s−1), bottom panels 822 (c,d) are for medium to strong easterly wind (6 - 24 m s−1). The black lines T1,T2 are 823 824 the cross-sections for the meridional transport estimations. 825 826 827 828 829 830 831 832 833 834 835 15 836 837 838 839 840 841 842 843 844 845 846 847 + 848 by its temporal mean < T >: 849 T(t)+ 850 I(t) = + (1) 851 < T > 852 We consider intrusions to occur when index I(t) ≥ 1, i.e. when the 853 854 cross-section positive transport exceeds its mean. Fig.4 represents the time 855 evolution between February 2016 and April 2017 of the wind (a), T1 (b) 856 857 and T2(c). As expected, most (94% ) of the easterly wind episodes (grey 858 859 boxes) are associated with T1 intrusions. More than 13 periods reveal strong 860 intrusions with a high T1 index (up to 6-7). 861 862 Hence, during medium to strong easterly wind periods, offshore surface 863 864 waters are pushed toward the LB, bending in a cyclonic circulation at the bay 865 entrance. Whether or not the offshore waters enter the bay can be evaluated 866 867 by the T2 index. During three episodes (episodes 1, 2 and 6 in Fig.4), a T2 868 869 index higher than 3 highlights the presence of an intrusion within the LB. 870 At other times (episodes 3,4 and 5), the T2 index remains below the index 871 872 value 3, indicating that the current mainly bypasses the LB. T2 intrusions 873 874 have been estimated to occur in the LB 78% of the time during easterly wind 875 episodes. 876 877 To evaluate the influence of the wind on the intrusion into the LB, a 878 879 scatter plot in Fig.5 was used to represent the T1 and T2 indexes according 880 to the wind intensity and direction. 881 882 Very high values of T1 occur 16% of the time for E-NE strong winds (all 883 884 points circled by the black line in Fig.5) corresponding to a non intrusion 885 circulation (T2 small), while high values of T2 (and T1 larger than 1) occur 886 887 with medium (around 8 m s−1)) north-easterly wind (90-120 degrees) (all 888 889 points circled by the red line in Fig.5b). The wind direction combined with 890 891 16 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 Figure 4: Model results during February 2016 - April 2017 period including (a) wind in- 930 tensity (m s−1) and direction (from which the wind blew) from Meteo-France ARPEGE 931 932 model; (b) index intrusion for transect T1; (c) index intrusion for transect T2. Locations 933 of T1 and T2 transects are black lines in Fig.3. Grey boxes are medium to strong east- 934 erly wind periods. The horizontal dotted line corresponds to the detection threshold of 935 936 intrusions. 937 938 939 940 941 942 943 944 945 946 947 17 948 949 950 951 952 953 954 955 956 957 958 959 960 its intensity seem to be the main forcings triggering intrusions of offshore wa- 961 ter in the LB. The remaining 6% correspond to situations without intrusion 962 963 near the southern boundary nor into the LB. 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 Figure 5: Scatter plot of wind speed (m s−1) versus wind direction (deg). The colorbar 982 983 indicates the value of T1 index (a) or T2 index (b). Red circle corresponds to winds 984 with an entrance in the domain and in the LB. Black circle corresponds to winds with an 985 986 entrance in the domain but no entrance in the LB. Red arrow corresponds to a wind of 8 987 m s−1 and black arrow corresponds to a wind of 16 m s−1. 988 989 990 991 3.2. Cross-section composite pictures 992 After having investigated surface circulation, this section aims at studying 993 994 the circulation through depth. To study the impact of wind conditions on 995 996 the entire water column, circulation has been studied along a vertical cross- 997 section between Cape Cepet and Cape Carqueiranne (red line in Fig.1B). 998 999 For a medium to strong Mistral wind (6-24 m s−1), results show a bi-layer 1000 1001 circulation into the bay (Fig.6a), as it was shown for certain time periods by 1002 1003 18 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 Duffa et al.(2011). We clearly see here that the surface layer flows southward 1017 (blue) towards the open sea with a maximum mean value of 0.18 m s−1 near 1018 1019 Cape Carqueiranne, whereas an incoming flow enters (red) the LB at the 1020 1021 bottom of the water column (10-40 m). The average inversion depth is at 1022 ca. 10 m depth at the center of the channel, and it can deepens for strong 1023 1024 winds. 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 Figure 6: Left panels (a,c) are composite pictures of meridional velocity on a vertical 1044 1045 cross-section between Cape Cepet and Cape Carqueiranne (red line in Fig.1B). Right 1046 panels (b,d) are standard deviations of the meridional velocities for the composite pictures. 1047 −1 1048 Top panels (a,b) are for medium to strong intensity Mistral (6 - 24 m s ), bottom panels 1049 (c, d) are for medium to strong easterly wind (6 - 24 m s−1). 1050 1051 1052 In order to evaluate the occurrence of the bi-layer circulation, the merid- 1053 ional velocities were integrated at the surface (0-10 m) (blue box in Fig.8a) 1054 1055 and below the inversion depth (20-40 m) (red box in Fig.8a) in order to 1056 1057 avoid its variability. Results show that the bi-layer pattern, defined by a 1058 1059 19 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 inward surface flux, and outward deep flux, and according to our criteria, oc- 1073 curs for 81% of the Mistral events. The standard deviation on the outcoming 1074 1075 current along Cape Carqueiranne (Fig.6b) suggests that velocity variations 1076 1077 are more important in this outflow compared to cape Cepet. A bathymetric 1078 effect is a probable explanation for this current acceleration near this cape 1079 1080 induced by stronger wind. Overall the standard deviation of the meridional 1081 1082 current across the vertical section has the same order of magnitude of the 1083 current itself. This can suggest that the bi-layer system may not be the only 1084 1085 possible scheme. 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 Figure 7: Left panel (a) is a depth integration of meridional velocity on a cross-section 1099 between Cape Cepet and Cape Carqueiranne (red line in Fig. 1B) through number of 1100 Mistral events (6 - 24 m s−1). Right panel (b) is the standard deviation of the meridional 1101 1102 velocity throught time. Red and blue boxes are locations where STD ≥ 1.5. 1103 1104 The transport across this section (Fig.7b) for all the events correspond- 1105 1106 ing to the Mistral regime is globally in the direction of the wind near the 1107 1108 capes and upwind in the middle of the section. The same observation was 1109 first shown for an idealized long narrow lake by Csanady(1973), where the 1110 1111 transport is downwind in shallow parts of the lake and upwind in deeper 1112 1113 parts. This observation has also been made in the Gulf of Trieste a Mediter- 1114 1115 20 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 Figure 8: Composite pictures of meridional velocity on a vertical cross-section between 1158 Cape Cepet and Cape Carqueiranne (red line in Fig.1B). Top panel (a) is for a bi-layer 1159 −1 1160 circulation (Mistral 6 - 24 m s )), bottom (b) is for an anticyclonic circulation (Mistral 1161 6 - 24 m s−1). Red and blue boxes are used for transport analysis (red for positive and 1162 blue for negative). 1163 1164 1165 1166 1167 1168 1169 1170 1171 21 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 ranean semi-enclosed basin (Malaˇciˇcet al., 2012), in coastal lagoons (Fian- 1185 drino et al., 2017) and in a shallow estuarine lake (Schoen et al., 2014) for 1186 1187 wind-driven circulation. 1188 1189 The Toulon Bay bi-layer pattern typical of the classic “wind driven chan- 1190 nel circulation” is the dominant pattern during Mistral wind conditions, oc- 1191 1192 curring 81% of the Mistral-driven circulations. The composite picture result- 1193 1194 ing in this BLP is depicted in Fig.8a. The events are selected according to 1195 the transport value in the upper/lower layers imposed negative at the surface 1196 1197 and positive at depth (blue/red boxes in the figure). 1198 1199 Looking closer around the capes (boxes in Fig.7b), the standard devia- 1200 tions of the integrated meridional velocity have very high values, exceeding 1201 1202 1.5 m2 s−1, revealing inversions of the currents along the capes. In partic- 1203 1204 ular, another possible circulation pattern can result from the Mistral wind 1205 regime, when the inflow (upwind) at the western Cape Cepet emerges and 1206 1207 fills the whole water column. By extracting the remaining events, having an 1208 1209 inflow at the western Cape, and an outflow at the eastern Cape (Fig.8b, 1210 red boxes), an anticyclonic circulation within the LB is revealed during 13% 1211 1212 of the Mistral events. 1213 −1 1214 Medium to strong easterly winds (6-24 m s ) appear to generate an 1215 inward flow into the LB on the east side (Fig.3c), with a core positioned at 1216 1217 the surface near Cape Carqueiranne (Fig.6c). The outflowing current follows 1218 1219 the bathymetry at the westside of the cross section, near Cape Cepet, with 1220 its core at ca. 50 meters depth. A small confined inward current appears 1221 1222 to occur at the edge of the cross-section (Cape Cepet), although standard 1223 1224 deviations (Fig.6d) show that its behaviour is very variable. Indeed, this 1225 1226 1227 22 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 current can also disappear for instance when the surface current along the 1241 entire section enters the LB (not shown). Note that a significant fraction of 1242 1243 the surface current bypasses the LB. 1244 1245 The analysis of the composite pictures described above, not only corrobo- 1246 rates the previously identified highly variable nature of the hydrodynamics in 1247 1248 the Toulon Bay (Duffa et al., 2011; Dufresne et al., 2014), but it also clearly 1249 1250 allows to associate circulation patterns with typical wind characteristics. 1251 1252 3.3. Main circulation patterns 1253 1254 A qualitative and quantitative analysis of the model results, using the 1255 1256 composite pictures described in section 3.1 and 3.2, allows the identification 1257 of four distinct circulation patterns, depending on the wind regimes and the 1258 1259 offshore waters intrusions: 1260 1261 1262 1. The Bi-layer pattern (BLP; Fig.9a): the predominant circulation pat- 1263 tern during medium to strong Mistral events (81%). It occurs mainly 1264 1265 with a west-northwest wind, with a median intensity of 8.88 m s−1. It 1266 1267 is characterised by a unidirectional surface outflow to the southeast to- 1268 wards the open sea inducing a bottom current entering the Toulon Bay 1269 1270 (Fig.8a). The maximum velocities observed near Cape Carqueiranne 1271 1272 are due to a cape effect; 1273 2. The Anticyclonic pattern (ACP, a derived circulation from the BLP 1274 1275 pattern; Fig.9b): a barotropic inward flow near Saint-Mandrier penin- 1276 1277 sula that follows the bathymetry in an anticyclonic way and leaves 1278 the LB near Cape Carqueiranne. This circulation pattern occurs 13% 1279 1280 of medium to strong Mistral wind events. It occurs with a west to 1281 1282 1283 23 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 Figure 9: Composites pictures of surface velocities (m s−1) showing the 4 circulation 1335 patterns: a) BLP pattern, b) ACP pattern, c) IP pattern, d) NIP pattern. A wind rose is 1336 1337 presented for each pattern. 1338 1339 24 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 −1 1352 west-northwest wind, with a median intensity of 8.22 m s and mostly 1353 during winter periods meaning the absence of stratification can play a 1354 1355 role for its occurrence. In most cases, a small confined current flows 1356 1357 southeastwards along the Saint-Mandrier peninsula (Fig.8b); 1358 3. The Intrusion pattern (IP; Fig.9c) : an inward flow near Cape Car- 1359 1360 queiranne, caused by an offshore waters intrusion into the LB during 1361 −1 1362 easterly wind events (with median intensities of 9.15 m s ). This pat- 1363 tern has been highlighted by a transport analysis to detect offshore 1364 1365 waters intrusions into the LB. It has been identified to occur for 78% 1366 −1 1367 of the time when easterly winds exceed 6 m s and for wind from 80 1368 to 120 degrees most of the time. South of the open boundary line, this 1369 1370 circulation pattern has a cyclonic character, probably mainly driven by 1371 1372 the along-coast pathway of the NC. These offshore waters intrusions 1373 depend on the upstream circulation features, and more specifically on 1374 1375 whether an NC intrusion into the Hyeres Bay has occurred (Declerck 1376 1377 et al., 2016). 1378 1379 4. The Non Intrusion pattern (NIP; Fig.9d) : this derivated regime from 1380 the IP pattern has also an anti-clockwise circulation south of the open 1381 1382 boundary line, induced by medium to strong easterly wind events with 1383 1384 a predominant northeast direction (60 to 90 degrees) to with a median 1385 intensity of 15.18 m s−1, which is greater than for the IP pattern. It 1386 1387 occurs 16% of the time when easterly winds exceed 6 m s−1. This 1388 1389 pattern is characterized by strong current intensities located mainly 1390 south of the LB and by no waters intrusions in the LB. 1391 1392 1393 It is worth mentioning that the criteria used to identify the four distinct 1394 1395 25 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 circulation patterns can be differently chosen, and may give slightly different 1409 percentage of occurrence. However, the four patterns are clearly present in 1410 1411 the simulations, and this will be confirmed with the distribution of contam- 1412 1413 inant (section4). 1414 1415 3.4. Validation with in-situ observations 1416 1417 The Toulon Bay model TBAY100 is the third level of nested coarser 1418 1419 models (GLAZUR, NIDOR) (see section 2.3), with accurate forcings at the 1420 southern and western boundaries. Nevertheless, validations with in-situ ob- 1421 1422 servations is necessary and will emphasis our confidence in the previous re- 1423 1424 sults. In-situ current observations in such coastal areas are scarse. ADCP 1425 data were only available from January to April 2012. The model results are 1426 1427 compared to these ADCP observations by sorting and averaging the veloc- 1428 1429 ities obtained during the two typical wind conditions of the area. Surface 1430 (top 2.5 m) and bottom currents (28 m) resulting composites, computed by 1431 1432 the model and recorded by both ADCPs at Cape Carqueiranne and Cepet, 1433 1434 are presented in the current rose diagrams in Fig. 10 as average percentages 1435 through the ADCP deployment period and the total period of the model sim- 1436 1437 ulation. The figure is split in 4 boxes: left ones correspond to Cape Cepet 1438 1439 ADCP analysis, right ones correspond to Cape Carqueiranne; upper blue 1440 boxes correspond to Mistral wind regime, and lower orange ones to easterly 1441 1442 wind regime. In each box 4 current roses are designed for the observation, 1443 1444 the model, the surface and the bottom. 1445 At Cape Cepet, during medium and strong Mistral winds, (upper left 1446 1447 box) surface modelled and observed currents flow towards the same direction, 1448 1449 i.e. south-east (Fig. 10a and b), although modelled intensities are stronger 1450 1451 26 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 Figure 10: Current roses (m s−1) for ADCP observations and model output at location 1497 1498 Cepet (left-hand side), and Carqueiranne (right-hand-side). Blue boxes (top) are for 1499 medium to strong Mistral conditions, orange boxes (bottom) for medium to strong easterly 1500 1501 wind condition. For each box, observations (left) and model output (right) are represented 1502 for surface (top) and bottom (bottom) currents. Note current roses show the direction 1503 towards which the current flows. 1504 1505 1506 1507 27 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 −1 −1 1520 (0.35 m s versus 0.20 m s ). At the bottom, current intensities and 1521 directions are overall the same, though observations show more variability 1522 1523 in direction (Fig. 10c and d). For medium and strong easterly wind events 1524 1525 (lower left box), differences appear between modelled and observed surface 1526 currents (Fig. 10i and j). While the observations show a north-westwards 1527 1528 and south-southeastward flow with a maximum intensity of ca. 0.20 m s−1, 1529 1530 modelled currents flow towards the west to south-west at a significant higher 1531 velocity. At the bottom, currents have similar directions, flowing towards the 1532 1533 south-east, with slightly more intense modelled currents (Fig. 10k and l). A 1534 1535 possible explanation for the mismatches between the ADCP and model data 1536 (mostly situations in Fig. 10i and j), is the bathymetry approximation in 1537 1538 the TBAY100 model, in particular near the capes, where the bathymetry is 1539 1540 shallow and steep. We observe in the model outputs that current variability 1541 is very important near the Cape Cepet location and that this location is near 1542 1543 the area where the current bypasses the LB in the model. 1544 1545 At Cap Carqueiranne, during medium and strong Mistral winds (upper 1546 right box), surface currents are stronger than at Cape Cepet and can reach 1547 1548 up to 0.25 m s−1 during this period (Fig. 10e and f). Surface modelled and 1549 1550 observed currents have similar directions and are flowing to the south-east 1551 out of the large bay. At the bottom, modelled and observed currents are 1552 1553 weaker than at the surface (a maximum of ca. 0.15 m s−1) but they have 1554 1555 almost the same directions, i.e. south-southeast for both model and obser- 1556 vations (Fig. 10g and h). Accordingly, model and observations are very 1557 1558 coherent at Cape Carqueiranne for Mistral conditions both in intensity and 1559 1560 direction. During medium and strong easterly wind (lower right box), sur- 1561 1562 1563 28 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 −1 1576 face currents flow north-westwards with a maximum intensity of 0.3 m s 1577 for both model outputs and observations (Fig. 10m and n), which may cor- 1578 1579 respond to an intrusion of offshore waters into the LB. Overall, the observed 1580 1581 surface currents at the Cape Carqueiranne location seems to be more re- 1582 sponsive to wind, as they appear to be stronger than at Cape Cepet, and are 1583 1584 approximately downwind for both wind conditions. Finally, bottom currents 1585 1586 during medium and strong easterly wind are weaker compared to the surface 1587 currents and have the same maximum intensity of 0.15 m s−1 for modelled 1588 1589 data and observations. While the observed currents do not have a dominant 1590 1591 direction, the modelled currents flow north-westwards and south-eastwards 1592 (Fig. 10o and p). 1593 1594 To summarize, the model results are coherent with most of the observed 1595 1596 velocities direction at the ADCP locations. In terms of intensity, the model 1597 overestimates most of the time the occurrence and the intensity. The most 1598 1599 important discrepancies appear at the western side of the LB boundary, 1600 1601 at Cape Cepet, during easterly wind regimes, in particular at the surface. 1602 However the current rose of the ADCP (Fig. 10i) and of the model, do 1603 1604 not show clear dominant currents, indicating that the circulation is very 1605 1606 variable in this particular location. This variability is confirmed by the model 1607 results suggesting (Fig3a & 8b) that for the anticyclonic circulation, offshore 1608 1609 waters may enter or not into the LB, leading to a different response at the 1610 1611 ADCP location. Moreover we should recall that the ADCP experiment lasted 1612 4 months in 2012 and the model simulation from February 2016 to April 1613 1614 2017. Interannual and seasonal variability could also explain part of the 1615 1616 discrepancies. 1617 1618 1619 29 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 If we compare the schematic circulations suggested by the model outputs 1633 and presented in the previous sections, during mistral regime the results are 1634 1635 very coherent with both the BLP and the ACP. During easterly wind regime, 1636 1637 the NIP appear at the bottom but not clearly at the surface. More ADCP 1638 moorings would be necessary to validate this circulation suggested by the 1639 1640 simulations. 1641 1642 In conclusion, several factors influence the hydrodynamics of the Toulon 1643 Bay: the preceding conditions, offshore waters intrusions, wind intensity and 1644 1645 direction. The important impact of wind on hydrodynamics within small 1646 1647 semi-enclosed bays, was also observed in the Gulf of Fos, where wind events 1648 have a stronger influence on hydrodynamics than the shelf offshore current 1649 1650 (Ulses et al., 2005). Even in small basins under strong tidal influence, wind 1651 1652 events may interfere on local circulation, such as in Southern Australia (Black 1653 et al., 1993). The circulation patterns identified in this section highlight 1654 1655 the possible export paths for contaminants which are discussed in the next 1656 1657 section. 1658 1659 1660 4. Contaminant distribution 1661 1662 Now that the circulation has been validated, the second objective is to 1663 1664 better understand the distribution of contaminant in the area. 1665 Chemical analysis of surface seawater samples from the Toulon Bay, ob- 1666 1667 tained within the projects METFLUX, PREVENT and the MERMEX pro- 1668 1669 gram by N. Layglon (MIO, University of Toulon) following an established 1670 method (Jean et al., 2012), have revealed the presence of elevated copper 1671 1672 concentrations in March 2017, varying from 3 nM in the LB to 113 nM in 1673 1674 1675 30 1676 1677 1678 1679 1680 1681 1682 1683 1684 1685 1686 1687 1688 the SB (Fig. 11). While the distribution of dissolved copper is relatively uni- 1689 form in LB (and close to background concentrations for the Mediterranean 1690 1691 Sea, 1.6 nM; (Tovar-Sanchez et al., 2014)), steep concentration gradients 1692 1693 were observed in the SB with maximum values measured in the docks of the 1694 civil (113 nM) and navy (67 nM) ports of Toulon and in the civil ports of 1695 1696 La Seyne-sur-Mer (40 nM) and Saint-Mandrier (82 nM). This is in agree- 1697 1698 ment with previous observations (e.g. Coclet et al., 2018), hence these ports 1699 can be considered as point sources of dissolved copper to the bay. Common 1700 1701 sources of copper in harbours are antifouling paints which are applied to the 1702 1703 hulls of boats to prevent growth of fouling organisms (e.g. Schiff et al., 2007). 1704 Since the ban of triorganotin formulations, most modern marine antifouling 1705 1706 paints contain a Cu-based biocidal pigment (Turner, 2010) which has led to 1707 1708 increased levels of dissolved copper in coastal waters, especially in and near 1709 ports (e.g. Biggs and D’Anna, 2012; Warnken et al., 2004). Accordingly, an- 1710 1711 tifouling paints are hypothesised to be a major source of dissolved copper to 1712 1713 the Toulon Bay. 1714 To understand and evaluate the impact of the elevated levels of dissolved 1715 1716 copper on the water quality of the Toulon Bay, distribution patterns were 1717 1718 modelled under the influence of different wind conditions and seasonal vari- 1719 ability was assessed via a 14-month simulation. Copper has been shown to 1720 1721 have a high affinity for organic matter in the marine environment, forming 1722 1723 dissolved organic copper species (e.g. Buck et al., 2007). Its distribution can 1724 thus be modelled as a passive tracer. Based on the surface distribution map 1725 1726 of dissolved copper (Fig.11), copper was released as a passive tracer at three 1727 1728 sources in the TBAY100 model: the civil ports of (1) Toulon, (2) la Seyne- 1729 1730 1731 31 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 (nM) 1745 1746 1747 Source 1 1748

1749 Source 2 1750 1751

1752 Source 3 1753 1754 1755 1756 1757 1758 Figure 11: Sea surface distribution of copper (nM) measured on 20 March 2017 (av. wind 1759 speed and direction: 2.4 m −1 SE). Positions of the three contaminant sources in the 1760 1761 TBAY100 model: Toulon civil port (source 1), la Seyne-sur-Mer civil port (source 2), and 1762 Saint-Mandrier civil port (source 3). 1763 1764 1765 sur-Mer and (3) Saint-Mandrier (see Fig.1B for the precise location in the 1766 TBAY100 model). As the majority of the berths in the ports are occupied 1767 1768 year-round, copper leaching from anti-fouling paints is considered to be a 1769 1770 constant process and was therefore modelled as a continuous source. The 1771 quantity of copper released per time step at each source (Cu ; nmol 1772 source 1773 dt−1) was calculated based on the capacity of each port (i.e. number of 1774 1775 berths) using the following equation: 1776 1777 1778 Cusource = Nberths × Smean × AR × dt (2) 1779 1780 where N is the number of berths per port, S is the standard esti- 1781 berths 1782 mate for wetted hull surface area (i.e. 41 m2) of a typical recreational boat 1783 1784 (12 m long with a beam of 4 m) used by paint manufacturers (i.e. the length 1785 1786 1787 32 1788 1789 1790 1791 1792 1793 1794 1795 1796 1797 1798 1799 1800 by the beam width by 0.85, a standard conversion factor; Earley et al.(2014) 1801 ) and AR is an average release rate of antifouling paint (6.9 nmol m−2 s−1). 1802 1803 The latter is based on estimates for cumulative passive copper leaching (i.e. 1804 1805 without considering cleaning activities and boat usage) during a typical paint 1806 life cycle of three years for 2 types of paint (4164 ug cm−2 or 1388 ug cm−2 1807 1808 yr−1 which corresponds to 6.9 nmol m−2 s−1), which was derived from in 1809 1810 situ measurements in San Diego Bay (Earley et al., 2014). Accordingly, AR 1811 provides a minimum baseline for leaching rates of Cu from antifouling paints 1812 1813 into the marine environment. 1814 1815 Table 2: Estimates of copper release rates at the three identified point sources in Toulon 1816 Bay. N is the number of berths per port, and Cu is the quantity of copper released 1817 berths 1818 per time step (nmol/5s) 1819 1820 Nberths Cu 1821 (nmol/5s) 1822 1823 Source 1 (Toulon civil port) 982 1.5x106 1824 1825 Source 2 (la Seyne-sur-Mer) 300 4.5x105 1826 6 1827 Source 3 (Saint-Mandrier) 800 1x10 1828 1829 1830 1831 4.0.1. Wind dependent contaminant distribution patterns 1832 As shown in section 3.3 several circulation patterns could be correlated 1833 1834 with two main wind directions: the Mistral and the easterly wind. The 1835 1836 circulation patterns were depending on the wind intensity and direction for 1837 the former and the existence of offshore waters intrusions for the latter. The 1838 1839 influence of these four circulation patterns on the dispersion of dissolved con- 1840 1841 taminants in the Toulon Bay was examined by running idealized simulations, 1842 1843 33 1844 1845 1846 1847 1848 1849 1850 1851 1852 1853 1854 1855 1856 imposing a constant wind during 2 days instead of using the ARPEGE atmo- 1857 spheric model as external wind forcings. Care was taken to select two days 1858 1859 with wind conditions similar to the imposed one, to avoid too important nu- 1860 1861 merical instabilities, and a possible model blow-up. The same patterns were 1862 obtained for multiple dates for each of the four wind case scenarii and asso- 1863 1864 ciated circulation patterns: 1) Mistral 8 m s−1 (ACP), 2) Mistral 16 m s−1 1865 1866 (BLP), 3) easterly wind with offshore waters intrusion (IP) and 4) easterly 1867 wind without offshore waters intrusion (NIP), underlining the representative- 1868 1869 ness of these contaminant distribution patterns for each scenario. 1870 −1 1871 At medium wind intensity (8 m s ), the plume of dissolved contaminants 1872 released in the SB spreads into almost the entire LB (Fig. 12-A1). Overall 1873 1874 concentration maxima occur at the surface in the entire bay with exception of 1875 1876 a small eddy, which is formed at the exit of the SB. This contaminant plume 1877 is originated from source 2 and 3 that are evacuated from the SB during the 1878 1879 time span of 2 days. The waters exiting the SB encounter an incoming vein 1880 1881 flowing westward into the LB and mix because of this front preventing them 1882 from spreading in the whole LB. The SB waters bypass this front by following 1883 1884 an anticyclonic motion into the LB, spreading the contaminant at the surface 1885 1886 of the LB (Fig. 12-A2). The dispersion of the contaminants released at the 1887 Toulon civil port (source 1) remains confined to the northeast sector of the 1888 1889 SB. Mistral winds of high intensity (16 m s−1) export dissolved contaminants 1890 1891 to the LB and beyond, until the Giens peninsula, in 2 days time (Fig. 12- 1892 B1) demonstrating the wind-driven circulation in the Toulon Bay area. The 1893 1894 majority of the contaminants remain at the surface (Fig. 12-B2). 1895 1896 In the case of easterly winds, simulations have demonstrated that the 1897 1898 1899 34 1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 nmol/m² m 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 Figure 12: On the left (A1, B1, C1, D1), test cases of contaminant release for the 3 1944 sources source 1, source 2, source 3. Integration of tracers on the whole column water, 1945 1946 logarithmic scale (nmol m−2). On the right (A2, B2, C2, D2), depth where the tracer 1947 concentration is the maximum (m). Different circulations with constant wind conditions 1948 are simulated: ACP with medium mistral (A), BLP with strong mistral (B), IP (offshore 1949 1950 waters intrusion into the LB) with medium easterly wind (C) and NIP (without intrusion) 1951 with strong easterly wind (D). 1952 1953 1954 1955 35 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 presence or absence of offshore waters intrusions is the determining factor 1969 defining the contaminant distribution patterns. It is correlated with wind 1970 1971 characteristics (intensity and direction), so that a medium SE wind induces 1972 1973 an entrance of offshore waters into the LB whereas a strong NE wind tends to 1974 prevent an intrusion of waters into the LB. During easterly wind conditions 1975 1976 with an intrusion into the LB (corresponding to the intrusion pattern and a 1977 1978 wind of medium intensity), the contaminant plume is exiting the Toulon Bay 1979 at depth along a narrow route around the Saint-Mandrier peninsula (Fig. 1980 1981 12-C1), following the bathymetry. Passed this cape, pollutants resurface on 1982 1983 the south coast of the peninsula while remaining at mid-water depths further 1984 offshore. Once at this point, pollutants are likely taken up in the NC and 1985 1986 quickly exported westwards along the coast. Without offshore waters intru- 1987 1988 sions in the LB, with a strong easterly wind, contaminants are distributed 1989 over the entire LB and are evacuated following the passageway along the 1990 1991 Saint-Mandrier peninsula (Fig. 12-D1). While the dissolved contaminants 1992 1993 remain on the surface, in the SB they sink to the bottom once passed the 1994 seawall. Passing the peninsula, the contaminant plume mainly follows the 1995 1996 bathymetry. 1997 1998 In conclusion, the model shows that, outside of the Toulon Bay, espe- 1999 cially the south coast of the Saint-Mandrier peninsula is directly subjected 2000 2001 to frequent plumes of dissolved contaminants (for 2 out of 4 scenarii, which 2002 2003 corresponds to medium and strong easterly wind periods or 9% of the total 2004 time). As the contaminants show a tendency of staying in surface layers, 2005 2006 extra vigilance may be required for the quality of surface waters in this 2007 2008 area. Furthermore, dissolved contaminants, originating from Toulon Bay, 2009 2010 2011 36 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 may reach the Marine National Park of Port-Cros, under mainly strong Mis- 2025 tral winds driving a bi-layer circulation pattern which is estimated to occur 2026 2027 ca. 20% of the time, given that medium to strong Mistral correspond to 2028 2029 24.9% of the total time. Accordingly, the impact of Toulon Bay’s contami- 2030 nated waters on the National Park appears possible, which warrants further 2031 2032 investigation using a model with a domain that extends to the National Park. 2033 2034 2035 4.0.2. Seasonal variability of contaminant dispersion 2036 A 14-month simulation was conducted to assess the seasonal variability 2037 2038 of dissolved contaminant distributions in the Toulon Bay. Therefore the 2039 2040 Cu distribution map in Fig. 11 (based on 42 observations) was used as 2041 initial condition and copper was released at a continuous rate (Table2) from 2042 2043 the three point sources. The main interest of this simulation is to evaluate 2044 2045 the potential impact of contaminant release on surface waters (top 5 m of 2046 the water column) in recreational zones such as near beaches and in areas 2047 2048 exploited for aquaculture, where water quality may impact public health. 2049 2050 Accordingly, Fig. 13 shows the temporal variation in dissolved copper at 2051 the highly frequented beaches of Mourillon (Large Bay), the aquaculture site 2052 2053 Lazaret (Small Bay) and, for comparison, the northern SB close to sources 2054 2055 1 and 2 (Fig.1B). The model results show that at the three locations, the 2056 surface waters are more concentrated in dissolved copper (up to 5 times the 2057 2058 background signal) during spring and early summer. This may be due to the 2059 2060 fact that this period is generally characterized by winds with a lower intensity 2061 (Fig. 13), which implies weaker surface currents and longer residence time 2062 2063 (Dufresne et al., 2014). As dissolved contaminant dispersion is wind-driven in 2064 2065 the SB, the dispersion of dissolved copper is reduced and an accumulation can 2066 2067 37 2068 2069 2070 2071 2072 2073 2074 2075 2076 2077 2078 2079 2080 be observed in the entire SB. On the other hand, medium Mistral wind (6-10 2081 m s−1) with an anticyclonic pattern (Fig.9b), is probably responsible for the 2082 2083 observed elevated copper concentrations in the waters of Mourillon (Fig. 12- 2084 2085 A2). Accordingly, the observed pattern of a degraded water quality in spring- 2086 summer in the Toulon Bay, merits to be tested in the future with observations 2087 2088 to validate the coupled hydrodynamic and passive tracer model as a useful 2089 2090 tool for pollution management. The low dissolved copper concentrations in 2091 winter, are mostly likely due to the stronger wind intensities (> 15 m s−1) 2092 2093 and the frequent alternation between Mistral and easterly winds promoting 2094 2095 water exchanges between SB and LB as demonstrated by Dufresne et al. 2096 (2014). 2097 2098 The 1-year simulation is not sufficient for the contaminant continuously 2099 2100 released at the source points to impact the far east side of the bay. Therefore, 2101 the temporal variability of contaminant dispersion will not be discussed for 2102 2103 the beaches located at the far east side of the TBAY model domain. 2104 2105 Nonetheless, copper concentrations in these areas remain close to back- 2106 ground values for the Mediterranean Sea (i.e. 1.6 nM; Tovar-Sanchez et al. 2107 2108 (2014)) after one year. Although seasonal variability was the focus of the 2109 2110 14-month simulation, a validation of the tracer model was performed, com- 2111 paring the Cu concentration map with the model output for the same day, 20 2112 2113 March 2017. This comparison shows that the surface Cu concentrations are 2114 2115 underestimated compared to the measured values (Fig. S1 and S2 in Supple- 2116 mentary Material), while the concentration gradient across the Toulon Bay 2117 2118 is reproduced. The estimates are especially low in the small bay which is 2119 2120 most likely due to the minimum Cu leaching rates used in the model (section 2121 2122 2123 38 2124 2125 2126 2127 2128 2129 2130 2131 2132 2133 2134 2135 2136 2137 2138 2139 2140 2141 2142 2143 2144 Small Bay Medium and strong Mistral Lazaret Medium and strong easterly wind 2145 Mourillon 2146 25 ) 2147 -1 20 2148 2149 15

2150 10 )

2151 -1 5 2152 25 2153 0 s (m intensity Wind 2154 20 2155 15 2156 2157 10 2158 2159 5 2160 0 2161 l (nmol Surface concentration

2162 01/03/16 01/05/16 01/07/16 01/09/16 01/11/16 01/01/17 01/03/17 2163 2164 Figure 13: Surface distribution of Copper (nmol l−1) during a 14 months simulation with 2165 the model TBAY100, in three studied sites, corresponding to the boxes Fig.1. Wind 2166 2167 intensity (m s−1) for medium and strong wind events: black dots for Mistral events, grey 2168 dots for easterly wind events. 2169 2170 2171 2172 2173 2174 2175 2176 2177 2178 2179 39 2180 2181 2182 2183 2184 2185 2186 2187 2188 2189 2190 2191 2192 4). The latter do not take into account boat cleaning activities and boat 2193 traffic, two activities that are particularly concentrated in the small bay (but 2194 2195 difficult to quantify) and are known to have an impact on Cu leaching from 2196 2197 antifouling paints. Accordingly more efforts are needed in quantifying Cu 2198 leaching rates to improve copper distribution modelling studies in coastal 2199 2200 areas, where antifouling paints are likely a major source of Cu. 2201 2202 2203 5. Conclusions with discussion 2204 2205 The aim of this study was to investigate circulation patterns and scenarii 2206 2207 of contaminant dispersion depending on weather conditions, thanks to the 2208 2209 very high resolution configuration TBAY100 (configuration set up with the 2210 MITgcm ocean model). As previous studies focused on the exchange between 2211 2212 SB and LB, this study focused on the circulation occurring in the LB and 2213 2214 the exchanges with offshore waters, to assess the potential impact of the 2215 heavily contaminated waters of Toulon Bay on the surrounding environment, 2216 2217 including the Marine Port-Cros National Park. 2218 2219 Model simulations have highlighted that the circulation in Toulon Bay is 2220 mainly wind-driven. However, offshore waters intrusions do occur and can 2221 2222 induce strong currents inside the bay. These intrusions may be NC intrusions 2223 2224 as in the Hyeres Bay upstream and an assessment of this hypothesis would be 2225 part of a future work. The tides contribute only for 3-4 cm on average of the 2226 2227 free surface elevation, with a maximum of 20 cm in our zone, as confirmed by 2228 2229 local observations (Rey et al., 2019). Therefore, we can say that the tide has 2230 a minor impact compared to other forcings. Overall, four circulation patterns 2231 2232 could be identified. Medium to strong Mistral events do most frequently (81 2233 2234 2235 40 2236 2237 2238 2239 2240 2241 2242 2243 2244 2245 2246 2247 2248 %) generate a Bi-layer pattern and less often (13 %) an Anticyclonic pattern. 2249 In both patterns, the plume of dissolved contaminants spreads at the sur- 2250 2251 face over the entire LB, but in summer period, the contaminant plume flows 2252 2253 towards the Giens peninsula. The anticyclonic circulation appears mostly 2254 during winter periods suggesting that the absence of stratification can play 2255 2256 a role for its occurrence. The impact of stratification on this schematic cir- 2257 2258 culation will be addressed in a future work. When the wind episode is long 2259 enough and considering the frequent occurrence of the Bi-layer pattern, there 2260 2261 could be exchanges of contaminants with the Hyeres Bay and the Port-Cros 2262 2263 National Park. Medium to strong easterly winds induce offshore waters in- 2264 trusions into the LB (78 %) associated with a cyclonic circulation (Intrusion 2265 2266 Pattern). Under these conditions, contaminants are spreading into a nar- 2267 2268 row vein above the bottom, flowing around Saint-Mandrier peninsula before 2269 probably meeting the NC offshore. At the same time, it can also spread 2270 2271 inside the LB before being carried offshore Saint-Mandrier peninsula if there 2272 2273 is no offshore waters intrusion into the LB (Non Intrusion Pattern). 2274 In addition, medium to strong Mistral events typically create upwelling 2275 2276 events in the north-western Mediterranean. These events also occur down- 2277 2278 stream in the Gulf of Lion (Millot, 1979; Barrier et al., 2016) and the Bay of 2279 Marseille (Pairaud et al., 2011). They are known to strongly impact the bio- 2280 2281 geochemistry of surrounding waters (Ross et al., 2016). To our knowledge, 2282 2283 there is no evidence in the scientific literature for these upwellings in the 2284 area near Toulon, but recent observational data is confirming such phenom- 2285 2286 ena (Rey et al., 2019). Some upwelling episodes could be identified in our 2287 2288 simulations, but a specific study on these phenomena will have to be carried 2289 2290 2291 41 2292 2293 2294 2295 2296 2297 2298 2299 2300 2301 2302 2303 2304 out, focusing on vertical movements, at very short time and space scale. 2305 As to optimise predictions of contaminant distribution in this exception- 2306 2307 ally polluted area, an increment and adjustment of locations of moored 2308 2309 ADCP locations is envisaged in the near future, as well as an assessment 2310 of non hydrostatic effects on the circulation in Toulon Bay. Moreover, other 2311 2312 suggestions for future work are: evaluating the dispersion of contaminants 2313 2314 toward the Marine National Park Port-Cros using a model domain incor- 2315 porating the park and implementing a sedimentary package into the model 2316 2317 as to model also dispersion patterns for contaminants with a high particle 2318 2319 affinity such as lead. 2320 2321 2322 6. Acknowledgements 2323 2324 This work was granted access to the HPC resources of Aix-Marseille Uni- 2325 2326 versit´efinanced by the project Equip@Meso (ANR-10-EQPX-29-01) of the 2327 program “Investissements d’Avenir” supervised by the Agence Nationale de 2328 2329 la Recherche. The TBAY100 simulations were also performed using GENCI- 2330 2331 IDRIS (Grant A0030101707). Mazoyer C. PhD grants is supported by the 2332 French Ministry of Research. This work was supported by the EU-funded 2333 2334 Project IMPACT (PC IFM 2014-2020, Prot. ISMAR n. 0002269). The au- 2335 2336 thors would like to thank C´elineDuffa at IRSN (Institut de Radioprotection 2337 et de Sˆuret´eNucl´eaire)for kindly providing ADCP data. We would like to 2338 2339 thank Nicolas Layglon for providing the copper concentration results which 2340 2341 were obtained during his Master 2 training at MIO (University of Toulon) 2342 as part of the METFLUX and PREVENT projects and the MERMEX pro- 2343 2344 gram. We also thank the SHOM for providing the bathymetry of the Toulon 2345 2346 2347 42 2348 2349 2350 2351 2352 2353 2354 2355 2356 2357 2358 2359 2360 Bay. We dedicated this study to our colleague and friend Dr. C´edricGarnier. 2361 2362 2363 References 2364 2365 Alberola, C., Millot, C., 2003. Circulation in the French mediterranean coastal zone 2366 2367 near Marseilles: the influence of wind and the Northern Current. Continental 2368 Shelf Research 23, 587–610, lU. 2369 2370 Alberola, C., Millot, C., Font, J., Jan. 1995a. 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Validation of tracer simulation Observations

Source 1

Model output

Source 1

Figure S1: Validation of tracer simulation. A) Surface distribution of dissolved Cu concentrations measured on March 20th 2017 at 1 m depth, normalized to the maximum concentration value measured i.e. at source 1, Toulon Port (log10 scale). B) Modelled surface distribution of dissolved Cu at 1 m depth on March 20th 2017 averaged over the duration of sample collection (12h, from 6:00 till 18:00), normalized to the maximum concentration value simulated i.e. at source 1, Toulon Port (log10 scale). Colour filled dots in both figures are the Cu concentrations measured at observation points (at 0.5 m depth) normalized to the maximum concentration measured i.e. at source 1, Toulon Port. Figure S2: Correlation between the measured and estimated Cu surface concentrations at the observation sites shown in figure 1. The outlier was excluded from the regression analysis.