Sea surface microplastics in Slovenian part of the Northern Adriatic

MPB-08106; No of Pages 8 Marine Pollution Bulletin xxx (2016) xxx–xxx

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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul

Sea surface microplastics in Slovenian part of the Northern Adriatic Tamara Gajšt a, Tine Bizjak a, Andreja Palatinus b, Svitlana Liubartseva c, Andrej Kržan a,d,⁎ a

University of Nova Gorica, School of Environmental Sciences, Vipavska 13, 5000 Nova Gorica, Slovenia Institute for Water of the Republic of Slovenia, Dunajska c. 156, 1000 Ljubljana, Slovenia c Euro-Mediterranean Center on Climate Change, via M. Franceschini 31, 40128 Bologna, Italy d National Institute of Chemistry, Department for Polymer Chemistry and Technology, Hajdrihova 19, 1000 Ljubljana, Slovenia b

a r t i c l e

i n f o

Article history: Received 28 July 2016 Received in revised form 11 October 2016 Accepted 13 October 2016 Available online xxxx Keywords: Slovenian sea Adriatic Sea Marine litter Sea surface Microplastics Markov chain model

a b s t r a c t Plastics are the most common material of marine litter and have become a global pollution concern. They are persistent in the environment where they gradually degrade into increasingly smaller particles–microplastics (MP). Our study presents results of sea-surface monitoring for MP in the Slovenian part of the Trieste Bay in the Northern Adriatic Sea. In 17 trawls conducted over a 20-month period we found a high average concentration of 406 × 103 MP particles/km2. Over 80% of the particles were identified as polyethylene. The significant variability of MP concentrations obtained on different sampling dates is explained by use of surface current maps and a recently developed Markov chain marine litter distribution model for the Adriatic Sea. © 2016 Published by Elsevier Ltd.

1. Introduction Millions of tons of plastics have been produced since the 1940s and 1950s, when mass production of plastics started in earnest (Barnes et al., 2009; Thompson et al., 2009; Andrady, 2011; Ivar do Sul and Costa, 2014). In 2014 the annual global production reached a record 311 million tons (Plastics Europe, 2015). Although plastics represent only approximately 10% of waste generated, the proportion of plastic debris accumulating in the environment is much greater (Barnes et al., 2009). Recent estimations of mismanaged plastic waste entering the ocean range within 4,8–12,7 million tons per year (Jambeck et al., 2015). According to the global assessment by Eriksen et al. (2014), 250 thousand tons of plastics are floating at sea. The global standing stock of small floating plastic debris (b 200 mm) has been estimated in the range of 93–236 thousand tons for 2014 (van Sebille et al. 2015). To obtain the latter values not only have the authors used a rigorous statistical framework to standardize a global plastic dataset measured by surface-trawling plankton nets, but also coupled the ocean circulation models for spatial interpolation of the observations. Because of their properties plastics remain in the environment for long periods, leading to the possibility that all plastics ever introduced into the environment still remain there (Barnes et al., 2009; Thompson et al., 2005).

⁎ Corresponding author at: University of Nova Gorica, School of Environmental Sciences, Vipavska 13, 5000 Nova Gorica, Slovenia. E-mail address: [email protected] (A. Kržan).

In the environment, larger plastic items gradually fragment into smaller pieces (Andrady, 2011). Plastic particles smaller than 5 mm in size are commonly described as microplastics (MP) (Arthur et al., 2009; Lusher et al., 2013). Two main sources of MP in the oceans are a) particles produced, manufactured and marketed in the size range of microplastics such as production pellets (primary MP) and b) fragmentation of larger debris due to weathering (secondary MP) (Andrady, 2011). The demand for plastic is predicted to continue (World Economic Forum, Ellen MacArthur Foundation and McKinsey & Company, 2016) and this will likely result in a further increase of MP pollution in the environment (Barnes et al., 2009; Thompson et al., 2005). The highest concentrations of plastic waste are predicted to occur in highly populated, shallow and enclosed waters such as the Mediterranean (van Sebille et al., 2015; Barnes et al., 2009; Cózar et al., 2015) and it is suggested that floating MP can be transported from sea surface to sea sediments (Alomar et al., 2016). High concentrations of MP pollution were observed even in areas far-away from densely populated areas such as Cabrera Island - a natural reserve (Alomar et al., 2016) and Lake Hovsgol in Mongolia (Free et al., 2014). Moreover, MP were also found to accumulate in Arctic Sea ice (Obbard et al., 2014). In the past decade, the number of published studies on MP in the marine environment has increased significantly (Barboza and Gimenez, 2015) with results indicating that MP are omnipresent in marine environments. Due to their large surface-to-volume ratio and their chemical composition MP adsorb and concentrate chemical pollutants from surrounding waters. Consequently, MP act as a reservoir for toxic chemicals

http://dx.doi.org/10.1016/j.marpolbul.2016.10.031 0025-326X/© 2016 Published by Elsevier Ltd.

Please cite this article as: Gajšt, T., et al., Sea surface microplastics in Slovenian part of the Northern Adriatic, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.031

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in the environment (Ivar do Sul and Costa, 2014; Zarfl and Matthies, 2010). It was suggested that plastics may even increase the environmental persistence of adsorbed pollutants (Teuten et al., 2009). A growing number of reports show that MP are ingested by a range of marine species (e.g., Lazar and Gracan, 2011; Ivar do Sul and Costa, 2014). It is speculated that ingested MP act as vectors in the transfer of organic pollutants to living organisms and the food web (Teuten et al., 2009). The Adriatic Sea is a land-locked sea that collects a third of the fresh water flowing into the Mediterranean, mainly due to the river Po, thus acting as a dilution basin. The Slovenian part of the basin is the eastern part of the Gulf of Trieste at the northernmost end of the Adriatic. The area is a shallow (approx. depth of 20 m) semi-enclosed basin with a “flat” bathymetry in the southern part (Malačič et al., 2012). Surprisingly, little information is available on the occurrence of marine litter and MP in the Adriatic Sea and, in particular, in its northern part. Vianello et al. (2013) analyzed sediments in the Lagoon of Venice. Their results showed the highest concentrations of MP in areas influenced by freshwater inputs. The only published study on the Slovenian coast is an analysis of MP in beach sediments (supra- and infralittoral) by Laglbauer et al. (2014). The authors found a large majority of MP to be fibers followed by fragments and film particles, whereas no granules were found. Based on their median concentrations: 170 and 178 particles/kg of dry sediment, the investigated beaches were assessed as moderate to very dirty according to the Clean Coast Index (CCI) (Alkalay et al., 2007). A recently published model for the transport of floating plastic debris in the Adriatic (Liubartseva et al., 2016) predicted the northern part of the basin and the Northern Adriatic coast of Italy as the most polluted parts of the sea. In addition, a strong seasonal fluctuation of particles concentrations was indicated. The purpose of our study was to analyze the abundance and occurrence of sea surface MP in the Slovenian part of the Gulf of Trieste in order to obtain the first assessment of this type of pollution for the area. 2. Materials and methods 2.1. Experimental Sea-surface sampling was conducted using an epineuston net with a mouth opening of 0,6 m × 0,15 m, 1,5 m in length, and a mesh size of 300 μm. The net was towed alongside the boat, beyond the wake of the vessel, with an average speed of approximately 3 knots for about 20 min. The mouth opening of the net was kept half-submerged. Samples were collected in a removable cod-end by means of rinsing from the outside of the net. The content of the cod end was transferred onto a metal sieve (300 μm mesh size) to reduce the volume and carefully transferred (by spray bottle) to glass containers, where it was fixed with ethanol to prevent organic matter from decomposing, and stored. The locations of sampling transects along which we collected 17 samples during the period December 2012–August 2014 are shown in Fig. 1. Exact sampling information including the dates and distances for each trawl are given in Table 1. Taking into consideration that the samples were collected by trawling the epineuston net in the straightest line possible, the sampling distances were calculated with the haversine formula (Sinnott, 1984) based on the latitude and longitude values of the recorded start and end point for each sample. The concentration of MP in the sea is affected by a number of environmental factors. Collignon et al. (2012) showed that the presence of strong winds prior to sampling has an effect on the concentrations of plastic particles found. Kukulka et al. (2012) showed the effect of wind-mixing on plastic concentration. The sampling routes were selected taking into account the sea surface current prediction by the NAPOM model (North Adriatic Princeton Ocean Model; Ličer et al., 2012) prepared by the Slovenian Environment Agency. The routes were selected in various directions relative to the currents (same direction, opposite to, perpendicular).

To prevent contamination of the samples before and during the analysis, several measures were used (Hidalgo-Ruz et al., 2012). Cotton clothes and laboratory coats were worn to minimize contamination by synthetic fibers. The laboratory was aired before the analysis and then closed to minimize airflow that could increase airborne contamination. All the equipment and the working counters were cleaned prior to analysis and particles were stored in clean covered (during work) or sealed (for storage) petri dishes. The separation of MP was conducted manually by visual examination of the samples using a stereomicroscope (20 × and 40 × magnification). Each sample was checked by two persons working in sequence in order to ensure consistent and complete collection of MP. Particles were then photographed on a Leica DMS1000 digital microscope with a coded zoom and a motorized scanning stage. By analyzing the images of the particles with Leica Application Suite (LAS), we were able to determine the 2D projected area of the particles and their size, which was determined based on the longest diagonal. The microscope digital camera was also used to collect images from a larger predefined area (e.g. petri dish), that were composed to form a single image (Fig. 5) suitable for image analysis. A representative set of 850 particles (14%) was analyzed for material type using a nearinfrared spectrometer (SIRoGran, IoSys, Ratingen, Germany). Statistical analysis was conducted with Microsoft Excel 2016 and SPSS version 23 (IBM Corp., 2015). 2.2. Model To better understand our results we (post-festum) obtained maps with statistically calculated marine debris concentrations using a recently developed model for the Adriatic Sea by Liubartseva et al. (2016). Two-dimensional Markov chain model is based on a combination of Lagrangian code MEDSLIK-II (De Dominicis et al., 2013) with sea surface currents from the Adriatic Forecasting System (Guarnieri et al., 2010) and wind from the European Centre for Medium-Range Weather Forecasts. Modeling of marine debris distributions takes into consideration the main terrestrial and maritime litter inputs in the Adriatic (Liubartseva et al. 2016). The Markov chain model provides the horizontal resolution of 2,2 km, and the temporal resolution of one day. Sea surface currents, wind, turbulent diffusion and the Stokes drift are considered as the main transport mechanisms. Additionally, beaching of plastics and a probability of washing back into the water are taken into account. 3. Results and discussion MP up to 5 mm in size were found in all collected sea surface samples and particles larger than 5 mm were found in all but four. Combined, all 17 samples contained a total of 6086 particles smaller than 5 mm and 391 particles larger than 5 mm. The total dry weight of the particles was 4,7 g of which particles smaller than 5 mm in size weighed 0,4 g. Particle numbers, concentrations and weights for all samples are shown in Table 1. The statistical analysis of our results is affected by the small number of samples. Normal Q-Q plot (Fig. S1) shows that obtained concentrations of MP are not normally distributed but skewed. Non-parametric Kruskal-Wallis H test (Table 2) showed a statistically significant difference between the concentration of MP found on different sampling days (χ2(3) = 9692, p = 0,021 and η2 = 0,60,575). The η2 furthermore shows that over 60% of differences can be attributed to different sampling dates. The mean ranks for each sampling dates were 9,33 on 14.12.2012, 14,4 on 8.5.2013, 7 on 14.3.2014 and 4,5 on 6.8.2014. A follow-up Kruskal-Wallis H test (Table 2) conducted between pairs of sampling dates showed statistically significant differences between concentrations of MP in samples collected on 8.5.2013 and 14.3.2014 and 8.5.2013 and 6.8.2014 (p b 0,05; η2 N 60%). The difference in MP concentration between samples collected on 14.12.2012 and 8.5.2013 was also substantial, but outside the commonly used 0,05

Please cite this article as: Gajšt, T., et al., Sea surface microplastics in Slovenian part of the Northern Adriatic, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.031

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Fig. 1. Zooming the area of MP monitoring: the Mediterranean and Adriatic Seas (a); enlarged sub-domain of the northern Adriatic Sea (b), where the area of monitoring is contoured in red; and locations of 17 sea surface samplings (c). The color of transects corresponds to different sampling dates: yellow - 14.12.2012; red - 08.05.2012; green - 14.03.2014; and blue 06.08.2014. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

significance value (χ2(1) = 3756, p = 0,053 and η2 = 0,5366). Differences in mean rank values between other pairs of sampling dates were statistically less significant with a few times higher p values. From the statistical analysis we can conclude that the MP concentration in samples collected on 8.5.2013 significantly differs from MP concentration in samples from other three sampling dates. The variation of MP concentration was much lower for samples collected on the same sampling date (Table 3). Standard deviation (s) of MP concentration for all sampling days is roughly 10-times higher than MP concentration observed on 14.12.2012, 14.3.2014 and

6.8.2014. The samples collected in May 2013 and particularly samples T15 and T17 stand out with high concentration of MP and consequently have higher s values. Samples T15, T17 and T13 also contained the highest number of particles larger than 5 mm, indicating generally higher debris loads. On the other hand, the lowest concentrations were found in three samples (T23, T24, T25) collected in August 2014, when also very low numbers of larger particles were collected. The relatively good agreement of concentrations between samples collected on the same date leads us to conclude that the results accurately reflect the concentration of MP on those dates. Based on this

Table 1 Sampling information and particle concentrations for all samples collected and analyzed in this study. Sample Date T10 T11 T12 T13 T14 T15 T16 T17 T18 T19 T20 T21 T22 T23 T24 T25 T26

Distance (km) Nr. of micro particles found Nr. of particles N5 mm Particles/km2 Particles/m3 Weight of micro particles [g] Weight (N5 mm) [g]

14.12.2012 2,07 2,32 1,67 8.5.2013 1,37 1,99 1,62 1,93 0,54 14.3.2014 3,17 1,94 2,00 1,70 1,80 6.8.2014 2,49 2,62 2,36 0,49

389 196 151 505 594 1.993 197 1.004 275 259 171 81 109 21 42 29 70

0 20 5 58 29 178 12 67 3 9 2 0 2 2 4 0 0

313.193 140.688 150.265 612.193 497.104 2.050.876 169.969 3.097.566 144.575 223.027 142.553 79.347 100.664 14.084 26.677 20.506 236.752

4,18 1,88 2,01 8,19 6,63 27,34 2,23 41,32 1,93 2,97 1,90 1,06 1,35 0,19 0,36 0,27 3,17

0,004 0,015 0,008 0,02 0,008 0,187 0,01 0,131 0,001 0,011 0,004 0,001 0,006 0,004 0,004 0,002 0,0002

0 0,1 0,005 2,005 0,185 1,237 0,005 0,756 0,003 0,022 0,001 0 0,003 0,028 0,005 0 0

Please cite this article as: Gajšt, T., et al., Sea surface microplastics in Slovenian part of the Northern Adriatic, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.031

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Table 2 Results from Kruskal-Wallis H test for all the sampling dates and separately for each pair of sampling dates. Kruskal-Wallis H test Sampling dates

χ2

p

η2

Mean ranks

14.12.2012/8.5.2013/14.3.2014/6.8.2014 14.12.2012/8.5.2013 14.12.2012/14.3.2014 14.12.2012/6.8.2014 8.5.2013/14.3.2014 8.5.2013/6.8.2014 14.3.2014/6.8.2014

9,692 3,756 1,089 2 5,771 4,86 1,5

0,021 0,053 0,297 0,157 0,016 0,027 0,221

0,60,575 0,5366 0,1556 0,3333 0,6412 0,6075 0,1875

9,33/14,4/7/4,5 2,33/5,8 5,67/3,8 5,33/3 7,8/3,2 6,8/2,75 3/3,75

assumption we conclude that in the sampled area, the spatial and temporal variability of concentrations of MP is very high. Since there was no obvious explanation for the wide range of results we examined various possible factors. First we considered winds and currents on sampling dates. We assumed that currents should be more important since MP particles are submerged and directly driven by currents in the upper mixed layer, although waves caused by wind can also affect their movement. Daily wind maps for all sampling dates are shown in Fig. S2 (supplementary information). The conditions on the sampling dates differed significantly, with medium to very light winds in various directions, however we were not able to discern any clear correlation between wind situations and observed MP concentrations. We also obtained hourly NAPOM-based current maps shown for all transects in Fig. S3 (supplementary information). One can conclude that on December 14, 2012, (relatively low MP concentrations) the southwestern currents from Trieste and the Soča (Isonzo) river outflow were strongly intensified along the Italian coast, transporting the plastic particles from those sources away from the sampling area. In contrast, on May 8, 2013, (very high MP concentrations) the coastal jet tended to bifurcate directing the transport of particles from Trieste and Soča toward the sampling area. On March 14, 2013 (low MP concentrations) again the currents from Trieste and Soča were shifted to the Italian coast, and they did not recirculate to the north. For T21 with the lowest MP concentration of the sampling session the currents were directed from the coast in front of a nature reserve (Sečovlje salt fields), which is probably the least polluted coastal area in Slovenia. On August 6, 2014, (lowest MP concentration) the sampling areas were largely unaffected by currents from the Trieste/Soča area except for the most outlying T26, which demonstrated the highest MP abundance of the samplings. Based on this analysis cases we can discern a correlation between high MP concentrations and southwestern coastal currents from the Trieste–Soča river area. It is also significant that, on our sampling dates, the model did not indicate currents that could bring debris from the Po river or the eastern Adriatic coast. Significant spatial variations have been reported in other studies as well. For example, high concentrations of MP with clear spatial variations were found in the sediments of the Belgian coastal zone (Claessens et al., 2011). To better understand the situation we proceeded to obtain maps with statistically calculated marine debris concentrations using a recently developed model for the Adriatic Sea by Liubartseva et al. (2016). Calculated (daily resolution) marine debris concentration maps with marked sampling locations are shown in Fig. 2. The maps

show predicted high concentrations of plastic debris close to the Slovenian coast on May 8, 2013, the day on which we obtained the highest quantity of MP. For the other three sampling dates, when the amount of MP found was several times lower, areas of high modeled plastic debris concentrations were either further north or west of the sampling locations. On Aug. 8, 2014, when we recorded the lowest concentrations of MP, the sampling locations were in a particularly litter-free area, except for the last transect of the day (T26), which fell in a higher concentration area and where we observed the highest MP concentration of the day. The comparison of our results with the modeled distribution of debris concentration shows a good qualitative agreement with the measured MP concentrations. This may be somewhat surprising considering the generalizations necessary for the modeling, the inherent locational uncertainty (the model distributions have a horizontal resolution of approximately 2.2 km) and the relatively low (daily) temporal resolution of the maps. We believe that the use of the sea-current situation and modeled debris concentrations offers a good explanation of the very diverse results we obtained on real samples, showing the value and usefulness of advanced modeling tools. The northern tip of the Adriatic Sea however is a particular case: it is highly enclosed and shallow, with a well-pronounced mesoscale circulation, perhaps making it more suitable for such analysis. The average abundance of MP from all samples was 472 × 103 ± 201 × 103 particles/km2 (6,29 ± 2,68 particles/m3) where the highest concentration observed was 3,1 × 106 particles/km2 (41,3 particles/ m3) and the lowest 14,1 × 103 particles/km2 (0,19 particle/m3). The overall average comes with a very high standard deviation, which is a consequence of the extremely broad range of results discussed above. Due to the approximately one-half submerged sampling net opening the values in particles/m3 units come with a higher uncertainty than results expressed as surface concentrations (particles/km2). The submersion of the mouth opening is affected by various factors such as the movement of the waves, and it is consequently difficult to estimate the volume of the seawater sampled through the net. A better estimation could probably be achieved with the use of a flow meter, as in a study by Lusher et al. (2015). At least 10% of collected MP particles from each sample (850 in total) were analyzed for their chemical composition using near infra-red spectroscopy. The results of the analysis are shown in Fig. 3. N80% of the particles were identified as polyethylene (PE), 14% of the particles were not identified and the rest were either polypropylene (PP), polyolefin (PO), polystyrene (PS), polyvinyl chloride (PVC) or acrylonitrile butadiene

Table 3 Mean (x̅), standard deviation (S), standard error of the mean (SE), median (x͂), minimum (min) and maximum (max) values for all the sampling dates together and separately.

x̅ S SE x͂ Min Max

All sampling dates

14.12.2012

Particles/km2

Particles/m3

Particles/km2

Particles/m3

Particles/km2

8.5.2013 Particles/m3

Particles/km2

14.3.2014 Particles/m3

Particles/km2

6.8.2014 Particles/m3

471.767 828.154 200.857 150.265 14.084 3.097.566

6,29 11,05 2,68 2,01 0,19 41,32

201.382 96.950 55.974 150.265 140.688 313.193

2,69 1,29 0,75 2,01 1,88 4,18

1.285.542 1.243.855 556.269 612.193 169.969 3.097.566

17,14 16,59 7,42 8,19 2,23 41,32

138.033 55.063 24.625 142.553 79.347 223.027

1,84 0,73 0,33 1,90 1,06 2,97

74.505 108.287 54.143 23.592 14.084 236.752

1,00 1,45 0,73 0,31 0,19 3,17

Please cite this article as: Gajšt, T., et al., Sea surface microplastics in Slovenian part of the Northern Adriatic, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.031

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Fig. 2. Daily maps of plastic debris concentrations calculated by the model (Liubartseva et al., 2016) with marked sampling transects.

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Please cite this article as: Gajšt, T., et al., Sea surface microplastics in Slovenian part of the Northern Adriatic, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.031

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T. Gajšt et al. / Marine Pollution Bulletin xxx (2016) xxx–xxx Table 4 Results of image analysis showing the average area and size of the particles for each sample (ordered by descending average size) with corresponding standard deviations (S) and standard errors of the mean (SE).

Fig. 3. Graphical representation of the results obtained using NIR spectroscopy with particle number and the corresponding percentage (note that the percentage is rounded to 0% if there were b3 particles analyzed).

styrene (ABS). The particles, which were classified as unidentified were smaller in size (below 0,3 mm) or colored black (on which NIR cannot be used) and would need to be analyzed by alternative methods used for the identification of polymers such as Fourier transform infrared micro-spectroscopy (μ-FTIR) or Raman spectroscopy (Hidalgo-Ruz et al., 2012). In most published studies the main types of sea surface MP reported were mostly PE, PP and PS (Phuong et al., 2016). PE and PP are predominant due to their buoyancy due to densities lower than seawater. Considering the results of the chemical analysis, which positively identified 86% of the analyzed particles as plastic, we conclude that the corrected average abundance of MP in all samples was 406 × 103 MP particles/km2 (5,41 particles/m3), with the highest concentration 2,66 × 106 MP particles/km2 (35,5 MP particles/m3) and the lowest concentration of 12,1 × 103 MP particles/km2 (0,16 particles/m3). We have generated a KML file to visualize trawling paths in Google Maps. Each trawling path includes additional information: sample ID, date of sampling, trawling distance and calculated MP concentration where we have taken into account the percentage (86%) of positive plastic particle identification (MP concentration = [particles/km2 from Table 1] ∗ 0.86). All analyzed particles were photographed using a digital microscope and image analysis was performed. Particle sizes and the projected 2D area obtained are shown in Table 4. The average size of the MP particles from all samples was 2,69 ± 0,04 mm and the average area was 3,09 ± 0,08 mm2. The total 2D projected area of the particles was 25,2 cm2. Fig. 4 shows the size distribution of the MP that were analyzed by NIR spectroscopy. The distribution shows a relatively well-centered Gaussian form with the highest number of MP in the size range between 2,2 and 2,71 mm. Image analysis was also conducted on the images of the particles that were not analyzed with NIR, however, the results are not as accurate due to the fact that the particles were not photographed individually (unlike those analyzed by NIR) and due to their arrangement on the image: overlapping or particles in physical contact could be sometimes be considered as one individual particle, which probably lead to an overestimation of their size/area (Fig. 5). The results of this study suggest that the amount of MP in the Slovenian section of the Northern Adriatic Sea is higher (but in the same order of magnitude), than in most similar sea surface MP studies conducted in the world's oceans, other parts of the Mediterranean Sea and several freshwater ecosystems. In a study conducted in the northwestern Mediterranean (Collignon et al., 2012), the mean concentration of MP found was 116 × 103 MP particles/km2, approximately 3,5-times lower than in our study. In another study conducted in the

Sample ID

Number of particles analyzed

Area (mm2)

S

SE

Length (mm)

S

SE

T13 T17 T15 T19 T11 T12 T18 T20 T25 T16 T10 T21 T22 T23 T24 T14 T26

38 147 409 24 37 18 4 13 4 43 24 5 26 11 22 34 13

4,36 5,29 3,69 3,63 3,15 2,89 2,39 1,95 2,86 1,87 2,19 1,68 1,65 1,52 1,76 1,04 0,67

2,14 3,60 2,68 2,04 2,93 1,86 1,63 1,27 3,71 1,34 2,07 0,59 2,09 1,87 1,92 0,59 0,32

0,35 0,30 0,13 0,42 0,48 0,44 0,81 0,35 1,86 0,20 0,42 0,26 0,41 0,56 0,41 0,10 0,09

3,63 3,48 3,03 2,91 2,82 2,73 2,58 2,56 2,36 2,27 2,11 1,99 1,97 1,87 1,84 1,66 1,42

1,08 1,39 1,25 0,78 1,42 0,83 0,75 0,70 1,44 0,90 0,96 0,42 1,13 1,18 1,03 0,57 0,52

0,18 0,11 0,06 0,16 0,23 0,20 0,37 0,19 0,72 0,14 0,19 0,19 0,22 0,36 0,22 0,10 0,14

Mediterranean in the Gulf of Oristano (Sardinia, Italy, de Lucia et al., 2014) the average concentration of particles found was 0,15 MP particles/m3 or 36-times lower than in our study, and on average 130 × 103 (MP and meso plastics (5–50 mm)) particles/km2 were found in a study conducted in the western Mediterranean Sea (Faure et al., 2015). In the North Pacific Central Gyre (310 × 103 plastic particles smaller than 4,76 mm/km2 (Moore et al., 2001)) the concentration of particles found was lower but similar to our results. The average abundance of particles found in the South Pacific Subtropical Gyre (25 × 103 MP particles/km2 (Eriksen et al., 2013)) was 16-times lower. Moore et al. (2002) observed slightly higher average concentration of MP (8 particles/m3) in the coastal waters of California, while two studies conducted along the southeastern and southern coast of Korea reported higher concentrations of MP (Kang et al., 2015; Song et al., 2014). In studies that used different sampling methodologies, as they sampled sub-surface seawater, the reported concentration of particles was higher than in our research. Concentrations of MP in the Atlantic ranged from 13 to 501 MP particles/m3, with almost half of them identified as PE or PP (Enders et al., 2015). In the northeastern Pacific Ocean and coastal British Columbia, the MP concentration ranged from 8 to 9200 particles/m3 (Desforges et al., 2014). According to Laglbauer et al. (2014) the majority of litter found on Slovenian beaches is made of plastic, which is in agreement with the high concentrations of MP particles we found. However, we believe additional studies would need to be conducted to determine the correlation between the MP pollution of the two connected ecosystems. The majority of particles in our study was identified as PE. As the density of PE (LDPE and HDPE) is lower than that of sea water and PE is the most widely used type of plastic (Andrady and Neal, 2009) this result is not unexpected. For example, in 2012 and 2013 the largest demand for plastics in Europe was for PE (46%), followed by PP (30%), PS (11%) and polyurethane (11%) (Plastics Europe, 2014). 4. Conclusion The presented study shows that the average concentration of MP particles (406 × 103 MP particles/km2 or 5,41 MP particles/m3) measured on the surface of the Slovenian section of the Adriatic Sea is among the highest concentrations recorded in the Mediterranean Sea. The area experiences significant locational and temporal variation in MP pollution, which we were able to link to surface currents in the area. Measured MP concentrations corresponded very well with the numerical model (Liubartseva et al., 2016) predicting marine debris concentrations in the Adriatic Sea indicating the validity of the model and

Please cite this article as: Gajšt, T., et al., Sea surface microplastics in Slovenian part of the Northern Adriatic, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.031

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Fig. 4. The size distribution of MP particles analyzed by NIR spectroscopy. Each histogram bin range equals to 0,51 mm.

demonstrating its utility. The results are also in agreement with studies identifying the Mediterranean Sea as one of the world's hotspots for marine litter (Lebreton et al., 2012; Suaria and Aliani, 2014; van Sebille et al., 2015). Chemical analysis identified N80% of analyzed particles as PE. The concentration of MP particles found in each of the samples varied significantly.

Since the samples were collected at different time intervals, in different seasons, and under different weather conditions before (and during) the samplings, which can all affect the distribution of MP in the environment, we believe that more sampling should be carried out on a regular basis to better assess the MP pollution.

Fig. 5. Image analysis of the particles that were also analyzed with NIR (diameter of the opening 6 mm) (a) and the image analysis made on the particles photographed together in a glass Petri dish (diameter 80 mm) (b) where the red and yellow square mark examples of particle overlap that may be cause for possible errors. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Gajšt, T., et al., Sea surface microplastics in Slovenian part of the Northern Adriatic, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.031

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Please cite this article as: Gajšt, T., et al., Sea surface microplastics in Slovenian part of the Northern Adriatic, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.031