Operational DGT threshold values for metals in seawater from protected coastal areas in Sardinia (Western Mediterranean)

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Operational DGT threshold values for metals in seawater from protected coastal areas in Sardinia (Western Mediterranean) Barbara Marras∗, Natalia Montero, Alessandro Marrucci, Alexandre Bettoschi, Marco Atzori, Marco Schintu Dipartimento di Scienze Mediche e Sanità Pubblica, Università Degli Studi di Cagliari, Cagliari, Italy

ARTICLE INFO

ABSTRACT

Keywords: Passive sampling DGT Trace metals Western Mediterranean DGT threshold

Diffusive gradients in thin films (DGTs) were used for monitoring metal (Cd, Cu, Ni, and Pb) concentrations in protected and non-protected coastal areas in Sardinia (Western Mediterranean). The deployment of DGTs in relatively undisturbed areas enabled calculation of operational DGT threshold values, which can be used for assessments of the environmental quality of coastal areas. The DGT thresholds were defined as the median metal concentrations that were found in protected areas, which ensured consideration of the natural variability of the different study sites. The calculated DGT thresholds were 11.6 ng L−1 for Pb, 5.1 ng L−1 for Cd, 63 ng L−1 for Cu and 152 ng L−1 for Ni. A comparison of the calculated DGT thresholds with previous DGT studies in the area demonstrated their suitability for identifying sites of environmental concern in the Western Mediterranean.

1. Introduction Coastal areas are very productive ecosystems that have a variety of habitats and rich biodiversity, thus presenting important social and economic values (UNEP, 2006; Martínez et al., 2007; Russi et al., 2016; FAO, 2018). Regarding contamination of the marine environment, the European Directives (the Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD)) state that contaminant levels should not give rise to pollution effects (European Commission, 2000, 2008). However, traditional monitoring, consisting of collections of spot water samples and followed by analysis of contaminants in the laboratory, might not be the best approach for coastal waters (Mills et al., 2011). The main limitations are the unrepresentativeness of the spot water samples and the uncertainties associated with the concentrations of contaminants that are below the detection limits of analytical techniques (Madrid and Zayas, 2007; Belzunce-Segarra et al., 2012). Thus, to obtain reliable information for chemical concentrations in the marine environment, high-intensity water sampling should be carried out, which is very time consuming and costly (Vrana et al., 2005). Passive samplers have been proposed as an alternative to spot sampling for obtaining high-quality data of contaminant levels in dynamic systems (Allan et al., 2006; Zabiegala et al., 2010; Lohmann et al., 2012). Passive sampling is based on the in situ deployment of devices that are capable of accumulating, via diffusion into a receiving

∗

phase, contaminants dissolved in water (See Greenwood et al., 2007; Kot-Wasik et al., 2007). The contaminants are subsequently extracted in the laboratory and their concentrations are measured, allowing calculations of time-weighted average (TWA) concentrations in water or equilibrium pore-water concentrations in sediment (e.g., Vrana et al., 2005, 2010). Specifically, diffusive gradients in thin films (DGT) (Davison and Zhang, 1994; Zhang and Davison, 2015) are the most widely used passive samplers for metals (Menegário et al., 2017). Metals have become a global problem due to their toxicity, wide sources, non-biodegradable properties and accumulative behaviour (e.g., Naser, 2013; Khan et al., 2018). Accordingly, Cd, Pb, Ni and Hg are considered as priority pollutants within the WFD (European Commission, 2013). The distribution of metals between different compartments of the environment and their impacts on the biota are determined mainly by their chemical forms (Vignati et al., 2009). Considering this, several studies have demonstrated that the uptake of metals by organisms is better explained by the free-ion activities of metals or by the labile metal forms, rather than by the total concentrations (Campbell, 1995; Zhao et al., 2016). Thus, it has been suggested that DGT-labile metal concentrations might represent the fraction that is potentially bioavailable to the biota (e.g., Schintu et al., 2008, 2010; Estrada et al., 2017; Vannuci-Silva et al., 2017). DGTs have been widely used for the measurement of trace metals in freshwater (e.g., Odzak et al., 2002; Gimpel et al., 2003; Tusseau-Vuillemin et al., 2007; Uher et al., 2017). In the marine environment, DGTs have been used for the measurement

Corresponding author. E-mail address: [email protected] (B. Marras).

https://doi.org/10.1016/j.marpolbul.2019.110692 Received 6 May 2019; Received in revised form 23 October 2019; Accepted 24 October 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Barbara Marras, et al., Marine Pollution Bulletin, https://doi.org/10.1016/j.marpolbul.2019.110692

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of metals in estuaries (e.g., Dunn et al., 2007; Montero et al., 2012) and coastal waters (e.g., Munksgaard and Parry, 2003; de Souza et al., 2014). Passive samplers have been suggested as the first step of a tieredapproach to identify areas of environmental concern (see Miège et al., 2015). This approach is based on the calculation of threshold values, and measurements that exceed these values could be used as early warning signals of potential contamination (Reagan et al., 2018). However, the lack of large datasets for passive samplers has hindered the determination of environmental quality standards that enable risk assessments for coastal waters (Rodríguez et al., 2018). Another approach for assessing the water quality of the marine environment could be the calculation of operational DGT threshold values. These values could be used for determining the degree of contamination of areas affected by anthropogenic activities (e.g., Montero et al., 2012). Previous studies with DGTs in the Mediterranean Sea (i.e., Scoullos et al., 2006; Lafabrie et al., 2007; Schintu et al., 2008, 2010, 2018; Sakellari et al., 2013; Caro et al., 2015; Cindrić et al., 2017) have mainly been conducted in areas impacted by different contaminant sources (e.g., industries, harbours, mining, and urban runoff). However, it is important to measure DGT-labile metal concentrations in pristine or relatively undisturbed areas, which can be used as reference sites for establishing DGT threshold values (Pedreira et al., 2017). In the present study, DGT-labile metal (Cd, Cu, Ni, and Pb) concentrations were measured in four protected coastal areas (i.e., marine protected areas (MPA) and national parks (NP)) and in four non-protected coastal areas (i.e., sites exposed to different sources and degrees of pollution) in Sardinia (Western Mediterranean), aiming (i) to measure DGT-labile metal concentrations for a wide variety of coastal sites and (ii) to establish operational DGT threshold values for trace metals in the Western Mediterranean. To the best of our knowledge, this is the first time that DGT-labile metal concentrations measured in different MPAs in the Western Mediterranean have been reported. The calculated DGT threshold values can be used to establish the contamination levels of coastal areas affected by anthropogenic activities.

navigation are allowed); partial (free navigation, traditional fishing and sport fishing with some restrictions are allowed) (Villa et al., 2002; RAS, 2014a) (Fig. 1). The MPA of Asinara (96.7 km2) is within the Porto Torres territory and includes the island of Asinara (52 km2), which is also a national park and the surrounding sea, and extends for varying lengths of 300–1200 m. La Maddalena Archipelago National Park falls entirely within the territory of La Maddalena. The park covers more than 210 km2, one quarter of which is on land and the rest is at sea. The archipelago includes 7 major islands and several islets and rocks. Almost the entire park is a Site of Community Importance (SCI) (European Commission, 1992) (e.g., Spargi), with the exception of the urban nuclei of several islands, such as La Maddalena and Santo Stefano, where stations LM2 and LM3, respectively, are located. The MPA ‘Tavolara - Punta Coda Cavallo’ covers an area of 160 km2 (94% is marine surface). This SCI consists of the islands of Tavolara (6 km2), Molara and Molarotto. The Gulf of Olbia holds one of the largest ferry ports in the Mediterranean (2 million passengers per year), an important trading harbour (4 million tons of traffic volume every year) and mussel aquaculture sites (Marchini et al., 2016). To determine the effects of these activities, two stations (OL2 and OL3) were placed within the Gulf of Olbia. The MPA ‘Peninsula of Sinis - Island of Mal di Ventre’ covers an area of 270 km2 (OR1). Additionally, two stations, OR2 and OR3, were placed at the mouth of the Scolmatore Channel (e.g., an artificial channel connecting Cabras lagoon with the Gulf of Oristano) and the Tirso river (152 km), respectively. The sampling stations (CA1 and CA2) in the Gulf of Cagliari were intended to monitor anthropogenic inputs from Cagliari and its hinterland (approximately 500,000 inhabitants) and potential contamination arising from the oil refinery and harbour. 2.2. DGT deployment, processing and analysis Different sampling campaigns were carried out in the period 2010–2015 in five areas located along the Sardinian coastline. Detailed explanations of the sampling campaigns are shown in Table 1. For the case of Asinara (AS1, AS2, AS3) and La Maddalena (LM1, LM2, LM3), several sampling campaigns were performed, while only one sampling campaign was conducted for the remaining stations. All glassware and plexiglass holders, used in the laboratory and for DGT deployments, respectively, were soaked in a 10% HNO3 acid bath overnight and were then rinsed with Milli-Q water before use. Deionized water, double distilled in the laboratory and purified with a Zeener Power I Water Purification System (Human Corporation, South Korea), was used during all field and laboratory procedures. Open pore DGTs (0.8 mm; polyacrylamide + Chelex-100), were supplied by DGT Research Ltd. (Lancaster, UK). The plexiglass holders, each containing three DGT devices, were immersed at the field locations in the water column at a depth of approximately 3 m, for periods ranging from 3 to 22 days (n = 110 DGTs). The temperatures were measured at deployment and retrieval time, using a YSI multiprobe (Geoves, mod. MICROHYD1, B&C electronics) and the immersion times were recorded to the nearest minute. The samplers deployed at LM3 in March 2011, at CA3-CA5 in October 2011, at AS2 in July and September 2015 and at AS1 in September 2015 were lost due to vandalism or strong hydrodynamic conditions. When retrieved, the DGT exposure windows were cleaned with deionized water and each device was stored in an individual clean plastic bag. The DGT probes were transported to the laboratory and stored at 4 °C until analysis. No visible biofouling was observed on the surfaces of the DGT devices, even for the longest deployment period (22 days). Thus, effects from biofouling on the accumulated metals were not expected, which is in accordance with the observations of other long-term deployment studies (14–30 days) carried out in the Mediterranean Sea (Schintu et al., 2008, 2018; Baeyens et al., 2018). In the laboratory, the DGTs were disassembled and the binding

2. Material and methods 2.1. Study area and sampling sites DGT-labile metal (Cd, Cu, Ni, and Pb) concentrations were measured during several sampling campaigns that were performed in the period 2010–2015, in the framework of the INTERREG Project MOMAR (Marine Monitoring). Five sampling areas were selected along the Sardinian coastline: Asinara Island, the La Maddalena Archipelago and the Gulfs of Olbia, Cagliari and Oristano (Fig. 1, Table 1). The AS1-AS5 (Asinara), OL1 (Tavolara in Olbia) and OR1 (Sinis-Seu in Oristano) stations are located in marine protected areas (MPAs) and LM1 (Spargi in La Maddalena) is in a national park. Additionally, several stations were selected in coastal areas that showed different levels of exposure to anthropogenic impacts (e.g., tourism, urban nucleus, ship traffic, and agriculture): LM2 and LM3 in the La Maddalena Archipelago, OL2 and OL3 in the Gulf of Olbia, CA1 and CA2 in the Gulf of Cagliari and OR2 and OR3 in the Gulf of Oristano. Sardinia's MPAs include a total area of nearly 630 km2 of sea in some of the most pristine coastal stretches in the Mediterranean (MATTM, 2010). MPAs have been designated to preserve their morphological, oceanographic and biological characteristics and to mitigate the impacts of anthropogenic activities (Guidetti et al., 2014). In Sardinia, MPAs may have three levels of protection to ensure that the most sensitive marine habitats remain undisturbed: integral (i.e., the environment is entirely protected and only activities related to the conservation of the natural resources or for scientific purposes are accepted); general (i.e., the construction of new buildings is forbidden, but traditional production methods such as agriculture and fishing, and the construction of strictly necessary infrastructures and low-speed 2

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Fig. 1. Location of the DGT deployment stations at the five sampling areas selected along the Sardinian coastline: A) Asinara Island, B) La Maddalena Archipelago, C) Gulf of Olbia, D) Gulf of Cagliari and E) Gulf of Oristano.

resins were eluted with 1 M HNO3 acid solution (> 24 h; Ultrapurgrade nitric acid 60%, Merck Millipore, Germany). The resulting acid extracts were analysed, following EPA (1994), by graphite furnace atomic absorption spectroscopy (GFAAS Varian GTA120-AA240Z) using the Zeeman background correction, for cadmium (Cd), lead (Pb), copper (Cu) and nickel (Ni). The calibrations were performed by means of mono-element solutions (1000 ppm) that were diluted to the concentrations of the samples. Quality control (QC) was assured by means of multi-element solutions that were prepared analogously to the calibration samples. The instrumental detection limits (IDL) were as follows: 0.05 μg L−1 for Cd, 0.2 μg L−1 for Cu, 1 μg L−1 for Ni and 0.5 μg L−1 for Pb. The average DGT-labile metal concentrations in water (Cw) during the deployment time were calculated following the procedure of Zhang and Davison (1995). The diffusion coefficients provided by DGT Research (2003) were temperature-corrected. The detection limits of the DGT technique (DGT-DL) were calculated as the mean ± 3 × SD of the mass measured in the DGT blanks (n = 15): 0.42 ng for Pb, 4.2 ng for Cu and 2.5 ng for Ni (Dabrin et al., 2016). Cd concentrations in blank eluates were below the instrumental detection limit, so, IDL/2 (0.025 μg L−1) was used for the calculation of the DGT-DL (0.04 ng).

2.3. Statistical analysis Graphical presentations of the DGT results, reported as the means ± standard error of the mean (SEM), were created with Microcal Origin software. The DGT blank values (ranging from below the instrumental detection limit, and up to 10% of the accumulated mass in the deployed DGTs) were not subtracted from the calculation of labile metal concentrations. Statistical analyses were carried out by the open source Jamovi software (www.jamovi.org). Shapiro-Wilk and Levene's tests were applied for the assessment of normality and homogeneity of variance, respectively. When necessary, DGT concentrations were log-transformed and analysis of variance (ANOVA) or Student's t-test were used for establishing significant differences among stations. The significance level was set at α = 0.05, corresponding to a confidence level of 95%. 3. Results and discussion 3.1. DGT-labile metal concentrations in protected and non-protected coastal areas The metal concentrations measured at protected and non-protected coastal areas are shown in Fig. 2. The ranges of metal concentrations measured for the protected coastal areas (Pb: 4.5–23 ng L−1, Cd: 3

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Table 1 Location of DGT sampling stations, deployment dates and days of exposure. Average temperatures during the deployment time are quoted. Sampling areas

Stations

Tag

Lat. (N)

Long. (E)

Month sampled

Deployment time (days)

Average T (°C)

Asinara Island

Ponte Bianco

AS1

41.073950

8.339467

Cala Reale

AS2

41.061211

8.291325

Punta Marcuzza

AS3

41.045483

8.265667

Spalmatore West Coast

AS4 AS5

40.984954 40.992300

8.232500 8.215417

May 2015 July 2015 March 2011 May 2015 May 2015 July 2015 September 2015 March 2011 September 2015

14 17 22 14 14 17 12 22 12

21.2 24.5 15.0 21.2 21.2 24.5 21.4 15.0 21.4

Spargi

LM1

41.233851

9.355595

Arsenal

LM2

41.211394

9.428949

S.Stefano

LM3

41.202351

9.420144

September 2010 March 2011 September 2010 March 2011 September 2010

3 15 3 15 3

22.3 14.0 22.3 14.0 22.3

Gulf of Olbia

Tavolara Gulf of Olbia Commercial port

OL1 OL2 OL3

40.895070 40.920838 40.920250

9.674426 9.538424 9.518760

June 2011 June 2011 June 2011

4 4 4

22.1 22.1 22.1

Gulf of Cagliari

Capo S.Elia Sarroch

CA1 CA2

39.178583 39.079800

9.158150 9.060050

October 2011 October 2011

21 17

20.3 20.1

Gulf of Oristano

Sinis - Seu Scolmatore Tirso river

OR1 OR2 OR3

39.898782 39.899612 39.885363

8.407023 8.483637 8.536664

June 2011 June 2011 June 2011

4 4 4

21.0 21.0 21.0

La Maddalena Archipelago

Fig. 2. DGT metal concentrations (Pb, Cd, Cu, and Ni) measured during different sampling campaigns (2010–2015) at several stations located in protected and nonprotected coastal areas. Data are expressed as the mean ± SEM. 4

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0.4–9.3 ng L−1, Cu: 32–288 ng L−1, and Ni: 41–397 ng L−1) were lower than in non-protected coastal areas (Pb: 14.5–81 ng L−1, Cd: 0.5–24 ng L−1, Cu: 40–876 ng L−1, and Ni: 91–525 ng L−1). However, the relatively high concentrations found at some stations that were located within protected areas required a deeper understanding of the local processes taking place at each site.

the vicinity of this station (Schintu et al., 2015). Unfortunately, the DGTs deployed in March at LM3 were lost, so it is difficult to know if this was a local process, affecting only LM2, or if it would have influenced the entire bay. 3.1.3. Gulf of Olbia For Olbia, significantly lower concentrations were measured at OL1 (Pb 13.2 ng L−1, Cd 7.4 ng L−1, Cu 176 ng L−1, and Ni 153 ng L−1) compared to OL2 (Pb 71 ng L−1, p = 0.005; Cd 9.9 ng L−1; Cu 579 ng L−1, p = 0.006; and Ni 321 ng L−1, p < 0.001) and OL3 (Pb 29 ng L−1; Cd 24 ng L−1, p = 0.0008; Cu 876 ng L−1, p = 0.002; and Ni 208 ng L−1, p = 0.010). OL1 showed the lowest metal concentrations, explained by the presence of this station at Tavolara Island, which is in the MPA (Fig. 1C). However, the metal concentrations at OL1 were relatively high when compared to other environmentally protected areas (e.g., AS4, AS5, and LM1) and are probably explained by inputs from the adjacent Olbia Harbour (e.g., Galgani et al., 2011). The other two stations, OL2 and OL3, were located within the Gulf of Olbia, which is affected by the presence of different contaminant sources (e.g. harbour, industrial and aquaculture activities). In principle, an outward decrease in metal concentrations should be expected, as OL2 is located closer to the mouth, and thus is influenced by mixing with relatively cleaner ocean water (e.g., Schintu et al., 1991). This was the case for Cd and Cu, that presented higher concentrations at OL3 (Cd 24 ng L−1, p = 0.028; and Cu 876 ng L−1) than at OL2. It should be pointed out that the relatively high DGT-Cu concentrations measured at OL3 and OL2 were the highest values for all sampling areas. Accordingly, Richir et al. (2015) measured several metals in samples of the seagrass Posidonia oceanica that were collected in 13 Mediterranean countries (110 stations) and observed that the highest Cu concentrations were present in Olbia. High DGT-Cu concentrations associated with harbours have been previously observed (Montero et al., 2012; Costa and WallnerKersanach, 2013; Caro et al., 2015) and these values were explained by the leaching of Cu from the antifouling paints used on boat hulls (See Daehne et al., 2017). In contrast to those observed for Cd and Cu, OL2 showed the highest Pb (71 ng L−1, p = 0.014) and Ni (321 ng L−1, p = 0.004) concentrations. The OL2 station is located at the mouth of the Padrongianus River, and the importance of the river as a source of these metals to the environment cannot be ignored (e.g., Schintu et al., 1991).

3.1.1. Asinara Island All stations sampled at Asinara Island were located within the MPA. AS1, AS2 and AS3 were located inside the Gulf of Asinara, while AS4 and AS5 were placed in the channel connecting the Gulf of Asinara with the offshore waters, and thus are influenced by water exchange with the open ocean (Fig. 1A). Consequently, the lowest concentrations of Pb (6.0 ng L−1) and Cu (32 ng L−1) were found at AS4 and the lowest concentrations of Cd (3.4 ng L−1) and Ni (41 ng L−1) were found at AS5. For Asinara Island, during normal environmental conditions, the predominant wind-driven anticlockwise flow brings offshore water into the gulf. However, under reverse climatic conditions, mainly in summer, a reverse flow occurs and some inputs from the southern part of the Gulf of Asinara (e.g., the Porto Torres industrial area and harbours) could be expected (Virdis et al., 2012; RAS, 2014b), which will affect the stations located within the gulf and those present in the channel differently (Fig. 1A). Accordingly, in March, there were no significant differences between AS2 and AS4, while in September, AS3 (Cd 8.1 ng L−1, Cu 229 ng L−1, and Ni 397 ng L−1) presented significantly higher Cd, Cu and Ni concentrations than AS5 (Cd 3.4 ng L−1, p = 0.002; Cu 58 ng L−1, p = 0.004; and Ni 41 ng L−1, p = 0.002). Additionally, significantly higher metal concentrations were measured at AS2 in May than in March: Pb (March 9.7 ng L−1, and May 22 ng L−1; p = 0.017), Cu (March 41 ng L−1, and May 64 ng L−1; p = 0.049) and Ni (March 129 ng L−1; and May 148 ng L−1; p = 0.026). Overall, the highest concentrations in the study area were measured at AS1, AS2 and AS3 in the summer months: Pb (May: AS1 20 ng L−1, AS2 22 ng L−1, AS3 21 ng L−1), Cd (May: AS1 8.6 ng L−1, AS3 9.3 ng L−1), Cu (Sept: AS3 229 ng L−1) and Ni (July: AS1 396 ng L−1, AS3 367 ng L−1; Sept: AS3 397 ng L−1). Previous studies in the area of Porto Torres have measured high metal concentrations in the sediments (Schintu et al., 2015) and in mussels (Benedicto et al., 2011) and DGT metal concentrations (Pb 75 ng L−1, Cd 9.0 ng L−1, and Ni 378 ng L−1) similar to those found in the present study (Lafabrie et al., 2007). This reinforces the idea that the higher metal concentrations measured at AS1-AS3, in summer, might have an origin in the Porto Torres industrial area.

3.1.4. Gulf of Cagliari For Cagliari, the CA1 and CA2 stations were located at opposite sides of the gulf (Fig. 1D). However, similar concentrations were measured at CA1 (Pb 23 ng L−1, Cd 5.6 ng L−1, Cu 40 ng L−1, and Ni 246 ng L−1) and CA2 (Pb 20 ng L−1, Cd 5.3 ng L−1, Cu 43 ng L−1, and Ni 165 ng L−1). Schintu et al. (2008) deployed DGTs in three consecutive years (2004–2006) in the Gulf of Cagliari and measured Pb (4–45 ng L−1), Cd (1–5 ng L−1) and Cu (5–52 ng L−1) concentrations that were in the range of the measurements reported in the present study. However, lower Ni (33–80 ng L−1) concentrations were measured in that study. Schintu et al. (2008) deployed DGTs at sites located far from the coastline (500–3000 m), and the higher Ni concentrations measured in this study are very likely explained by activities carried out in the vicinity of the sampling stations. The CA1 station is along the route of cruises departing from Cagliari Harbour and CA2 is close to an oil refinery, which have been previously identified as nickel emission sources into the environment (Becagli et al., 2011; EEA, 2013; Harari et al., 2016; Tornero and Hanke, 2016). The potential role of these pressures on the measured nickel concentrations cannot be ignored, but further studies are necessary to confirm these results.

3.1.2. La Maddalena Archipelago Two sampling campaigns were carried out in the La Maddalena Archipelago, in September 2010 and March 2011. In September, there were no significant differences between the metal concentrations measured at LM1 (Pb 23 ng L−1, Cd 1.7 ng L−1, Cu 50 ng L−1, and Ni 111 ng L−1) and those found at LM2 (Pb 34 ng L−1, Cd 0.6 ng L−1, Cu 52 ng L−1, and Ni 108 ng L−1) and LM3 (Pb 42 ng L−1, Cd 0.5 ng L−1, Cu 68 ng L−1, and Ni 91 ng L−1), regardless of the different levels of protection of the studied islands: Spargi (LM1; under environmental protection), La Maddalena (LM2; urban nucleus, harbour, and military area) and Santo Stefano (LM3; military area) (Fig. 1B). In contrast, in March, significantly higher concentrations were measured at LM2 for Pb (81 ng L−1; p < 0.001), Cd (6.7 ng L−1; p < 0.001), Cu (242 ng L−1; p = 0.017) and Ni (181 ng L−1; p = 0.005) than at LM1 (Pb 4.5 ng L−1, Cd 0.4 ng L−1, Cu 46 ng L−1, and Ni 148 ng L−1). Additionally, higher metal concentrations were found at LM2 in March than in September (Pb p = 0.039, Cd p = 0.009, Cu p < 0.001, and Ni p = 0.013). Winter is more prone to strong rainfalls and rough seawater than summer, so the increase in metal concentrations at LM2 might be explained by the local inputs from stormwater runoff (Dunn et al., 2007) and/or remobilization of the contaminated sediments present in

3.1.5. Gulf of Oristano For Oristano, the OR1 (Pb 11.6 ng L−1, Cd 3.0 ng L−1, Cu 288 ng L−1, and Ni 383 ng L−1) station, located in the MPA, exhibited metal concentrations very similar to or greater than OR2 (Pb 5

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14.5 ng L−1, Cd 3.4 ng L−1, Cu 250 ng L−1, and Ni 268 ng L−1), which is beyond the limits of the MPA (Fig. 1E). On the other hand, the Pb, Cu and Ni concentrations were two-fold higher at OR3 (Pb 21 ng L−1, Cd 3.6 ng L−1, Cu 515 ng L−1, and Ni 525 ng L−1) than at the other stations (Cu: ORI1 vs ORI3 p = 0.014; OR2 vs OR3 p = 0.009; Ni: OR2 vs OR3 p = 0.026). The high metal concentrations found at OR3 are explained by the location of this station. On one hand, it receives inputs from the Tirso River, which drains an area characterized by agricultural and industrial activities. On the other hand, it is affected by the inputs from the nearby tourist and industrial harbours present in the area (RAS, 2006). In this regard, Schintu et al. (2009) measured high metal concentrations in sediments collected close to OR3 and Magni et al. (2006) observed genotoxic effects, associated with metals, in the native mussels of this area of the gulf. OR1 is located in a general protection area, with limited anthropogenic activities. Thus, the relatively high Cu and Ni concentrations measured at this station are probably the result of metals arriving from the gulf. Cucco et al. (2006) concluded that the Sirocco winds force the water to circulate from the basin through the northern part of the gulf by following the coastline, and thus, reaching OR1. This explains the similar concentrations found at ORI1 and ORI2, with no significant differences for any of the metals under study.

Mediterranean. Only three studies refer to DGT deployments in areas that might be considered as reference sites, in terms of their trace metal levels, in the Western Mediterranean. Schintu et al. (2008) measured DGT metal concentrations in the ranges of 10–15 ng L−1 for Pb, 2–7 ng L−1 for Cd, 7–80 ng L−1 for Cu and 45–120 ng L−1 for Ni in Capo Carbonara MPA (eastern Sardinia). The operational DGT thresholds calculated in the present study are within those ranges, except for Ni. The lower Ni concentrations found by Schintu et al. (2008) in the Capo Carbonara MPA may be explained by the combination of the position of the sampling station (e.g., 500–3000 m from the coastline) along with the reduced contamination sources on that part of the island. However, it should be mentioned that Schintu et al. (2008) also deployed DGTs at three sites in Sardinia that were affected by different contamination sources and the highest Ni concentrations were measured at the station located in the Capo Carbonara MPA. Thus, considering that Sardinia is a very famous tourist attraction in the Mediterranean, the relatively high Ni concentrations found in some protected coastal areas might be explained by nickel inputs (e.g. combustion or leaking fuel oil) from recreational vessels (EEA, 2013). The DGT thresholds were also comparable to the DGT metal concentrations measured by Schintu et al. (2018) at a reference site (Pb 1.0–7.0 ng L−1, Cd 2.8–10.0 ng L−1, Cu 33–213 ng L−1, and Ni 48–263 ng L−1) in Giglio Island (northern Tyrrhenian Sea) and by Baeyens et al. (2018) in a three-week cruise that was conducted in the open sea between Île du Levant and Corsica (Cd 7.0 ng L−1, Cu 60 ng L−1, and Ni 188 ng L−1). Thus, the calculated DGT thresholds seem to adequately represent the relatively undisturbed areas of the Western Mediterranean. The next step is to check if the calculated DGT thresholds can be used to differentiate between coastal areas presenting different types and degrees of contamination. The comparison of the DGT thresholds with the metal concentrations measured in the present study enabled identification of areas within protected coastal areas that may need further attention: AS1 (Cu and Ni) and AS3 (Cu and Ni) in Asinara Island, OL1 (Cu) in the Gulf of Olbia and OR1 (Cu and Ni) in the Gulf of Oristano. This information can be used to find the contamination sources and to establish site-specific management programmes for each protected coastal site. Additionally, the application of DGT thresholds identified the most problematic metals for the non-protected coastal areas: Pb in the La Maddalena Archipelago, Pb and Cu in the Gulf of Olbia and Cu and Ni in the Gulf of Oristano. As one of the aims of this study was to extend the use of calculated operational DGT thresholds (Pb 11.6 ng L−1, Cd 5.1 ng L−1, Cu 63 ng L−1, and Ni 152 ng L−1) for water quality assessments in the Western Mediterranean, these values were compared with previous DGT studies in the area. Higher DGT metal concentrations were measured by Schintu et al. (2010) at five sites located in southwest Sardinia (Pb 820–3130 ng L−1, Cd 90–600 ng L−1, and Cu 1450–2230 ng L−1); by Lafabrie et al. (2007) (Pb 38–75 ng L−1, Cd 6–16 ng L−1, and Ni 197–1380 ng L−1) in the north-western Mediterranean (Porto Torres, Canari, Livorno) and by Caro et al. (2015) in southern France (PortCamargue marina; Cu 1060–12,550 ng L−1). All of these sites were affected by diverse anthropogenic activities (e.g., harbours, industrial activities, and mining). Based on these results, it seems that calculated operational DGT thresholds, even if they are based on a limited number of samples, could be used to identify sites of environmental concern in the Western Mediterranean. The DGT thresholds were also lower than the DGT metal concentrations measured in areas affected by different pollution sources in the eastern Mediterranean: Scoullos et al. (2006) (Cd 5.4–9.2 ng L−1, Cu 51–280 ng L−1) and Sakellari et al. (2013) (Cd 6.4–9.8 ng L−1, Cu 64–311 ng L−1) in the Aegean Sea and Cindrić et al. (2017) (Pb 27–47 ng L−1, Cd 7.4–9.4 ng L−1, Cu 122–278 ng L−1, and Ni 262–293 ng L−1) in the Adriatic Sea. Although it was out of the scope of this study, it seems that the use of calculated DGT thresholds could potentially be extended to other areas of the Mediterranean. However, considering that background metal levels are site-specific (Tueros et al.,

3.2. DGT threshold levels in the Western Mediterranean In the present study, DGTs were deployed in environmentally protected areas, namely, MPAs and national parks. As previously discussed, Sardinian MPAs have different levels of protection (integral, general and partial), which together with hydrodynamic factors, have influenced the relatively high metal concentrations measured at some stations located within the protected areas (See section 3.1.). This emphasizes the complexity of finding relatively undisturbed areas in the Western Mediterranean that enable the calculation of DGT threshold values (Pedreira et al., 2017; Abessa et al., 2018). Thus, with the aim of providing numerical thresholds that can be used for environmental assessments of coastal areas in the Western Mediterranean, operational DGT thresholds were calculated as the median concentrations for all stations located within protected areas. The use of the median, instead of the mean, reduces the effects of extreme values, while including the natural variability of the different study sites (Burke, 2001). The calculated DGT thresholds for the Western Mediterranean are 11.6 ng L−1 for Pb, 5.1 ng L−1 for Cd, 63 ng L−1 for Cu and 152 ng L−1 for Ni (Fig. 3). To test the suitability of the calculated DGT threshold values, the estimated thresholds were first compared with previous DGT studies performed in relatively undisturbed areas in the Western

Fig. 3. Boxplots showing the range of metal concentrations measured by DGTs in protected coastal areas. Within each boxplot, solid lines, calculated as the median, represent the operational DGT thresholds for the Western Mediterranean. 6

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2008), these DGT thresholds should be used cautiously for water quality assessments of areas beyond the Western Mediterranean.

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4. Conclusions This work demonstrates the usefulness of diffusive gradients in thin films (DGTs) for measuring metal concentrations at a wide variety of coastal sites in Sardinia and for discriminating between the sites presenting different levels of contamination. The DGT metal concentrations measured in relatively undisturbed areas were used for the calculation of operational DGT threshold values in the Western Mediterranean. The DGT thresholds were calculated as the median metal concentrations found in environmentally protected areas, which ensured the consideration of the natural variability of the different study sites. A comparison of the calculated DGT thresholds with previous DGT studies in the area demonstrated their suitability for identifying sites of environmental concern in the Western Mediterranean. The use of these thresholds could potentially be extended to other coastal areas in the Mediterranean. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been conducted within the framework of the EU funded INTERREG Project MOMAR (Integrated system for monitoring and control of the marine environment). It was also supported by a Project on the management of Marine Protected Areas (2011–2015) funded by Regione Autonoma della Sardegna (L.R. August 7, 2007, N.7) (Scientific coordinator: Prof. Marco Schintu). We would like to thank the personnel from the MPAs of Asinara, Tavolara, the Peninsula of Sinis and La Maddalena Archipelago National Park for their technical support during these projects. References Abessa, D.M.S., Albuquerque, H.C., Morais, L.G., Araújo, G.S., Fonseca, T.G., Cruz, A.C.F., Campos, B.G., Camargo, J.B.D.A., Gusso-Choueri, P.K., Perina, F.C., Choueri, R.B., Buruaem, L.M., 2018. Pollution status of marine protected areas worldwide and the consequent toxic effects are unknown. Environ. Pollut. 243, 1450–1459. Allan, I.J., Vrana, B., Greenwood, R., Mills, G.A., Roig, B., Gonzalez, C., 2006. A “toolbox” for biological and chemical monitoring requirements for the European Union's Water Framework Directive. Talanta 69, 302–322. Baeyens, W., Gao, Y., Davison, W., Galceran, J., Leermarkers, M., Puy, J., Superville, P.-J., Begery, L., 2018. In situ measurements of micronutrient dynamics in open seawater show that complex dissociation rates may limit diatom growth. Sci. Rep. 8 Article 16125. Becagli, S., Sferlazzo, D.M., Pace, G., di Sarra, A., Bommarito, C., Calzolai, G., Ghedini, C., Lucarelli, F., Meloni, D., Monteleone, F., Severi, M., Traversi, R., Udisti, R., 2011. Evidence for ships emissions in the Central Mediterranean Sea from aerosol chemical analyses at the island of Lampedusa. Atmos. Chem. Phys. Discuss. 11, 29915–29947. Belzunce-Segarra, M.J., Montero, N., Gonzalez, J.-L., Larreta, J., Franco, J., Borja, A., 2012. Advantages of using passive samplers in comparison with spot sampling for metal evaluation in estuarine waters: an example from the Bay of Biscay (Northeastern Spain). In: Nriagu, J., Pacyna, J., Szefer, P., Markert, B., Wuenschmann, S., Namiesnik, J. (Eds.), Heavy Metals in the Environment. Maralte B.V., Voorschoten, pp. 77–90. Benedicto, J., Andral, B., Martínez-Gómez, C., Guitart, C., Deudero, S., Cento, A., Scarpato, A., Caixach, J., Benbrahim, S., Chouba, L., Boulahdidi, M., Galgani, F., 2011. A large scale survey of trace metal levels in coastal waters of the Western Mediterranean basin using caged mussels (Mytilus galloprovincialis). J. Environ. Monit. 13, 1495–1505. Burke, S., 2001. Missing values, outliers, robust statistics and non-parametric methods. LCGC Eur. Online Suppl., Stat. Data Anal. 19–24. Campbell, P.G.C., 1995. Interactions between trace metals and organisms: critique of the free-ion activity model. In: Tessier, A., Turner, D. (Eds.), Metal Speciation and Bioavailability in Aquatic Systems. J. Wiley & Sons, New York, NY, pp. 45–102. Caro, A., Chereau, G., Briant, N., Roques, C., Freydier, R., Delpoux, S., Escalas, A., ElbazPoulichet, F., 2015. Contrasted responses of Ruditapes decussatus (filter and deposit

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