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.