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Do lagoon area sediments act as traps for polycyclic aromatic hydrocarbons? 1
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Author names and affiliations 3
Dr Mauro Marini (corresponding author) 4 National Research Council (CNR) 5 Institute of Marine Science (ISMAR) 6 Largo Fiera della Pesca, 2 7 60125 Ancona 8
ITALY 9 [email protected] 10 tel +39 71 2078840 11
fax +39 71 55313 12
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Dr Emanuela Frapiccini 14 National Research Council (CNR) 15
Institute of Marine Science (ISMAR) 16 Largo Fiera della Pesca, 2 17
60125 Ancona 18 ITALY 19 [email protected] 20
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1. Introduction 22
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Several natural and anthropogenic processes can lead to the formation of polycyclic 24
aromatic hydrocarbon compounds (Wakeham et al., 1980), whose main inputs are 25
pyrolytic and petrogenic (Means et al., 1980; Lipiatou and Saliot, 1991). Each source 26
generates a characteristic PAH distribution pattern due to the different chemical-27
physical behaviour of these compounds (Mitra et al., 1999). PAH behaviour in a marine 28
system is the result of different factors, such as PAH sources and physicochemical 29
properties, water and sediment movement, size fraction and environmental conditions 30
(Baumard et al., 1999; Wang et al., 2001; King et al., 2004). Through the study of the 31
probable source of these compounds, it is possible to identify PAH distribution in a 32
certain area (Baumard et al., 1998; Mitra et al., 1999; Franco et al.,2006). Once PAHs 33
appear in the marine environment, they are present in the water column then, due to 34
their high hydrophobicity and molecular mass (Mackay, 1991), they tend to accumulate 35
in sediment and biota. In the marine environment they can be studied mainly in three 36
matrices: water column, marine organisms and sediments. Sedimentary hydrocarbons 37
have received special attention because these compounds are readily sorbed onto 38
particulate matter, in fact bottom sediments are considered as a reservoir of hydrophobic 39
contaminants (Medeiros et al., 2005). The level of PAH in sediments varies, depending 40
on the proximity of the sites to areas of human activity and on the PAH biodegradation 41
(Bihari et al., 2007). The study of these compounds is needed because they have shown 42
differences in their stability, transport mechanisms and fate, because of their physical-43
chemical properties, distribution constants, half-life times and origin (Bouloubassi and 44
Saliot, 1993). Various studies have been carried out on PAHs in Mediterranean and 45
3
Adriatic marine sediments (Baumard, et al., 1998; Alebic-Juretic, 2011; Bouloubassi, et 46
al., 2012), in particular, this work is focused on the Italian Adriatic coast, since it is 47
characterised by the presence of several rivers that discharge organic compounds (Tesi 48
et al., 2007) in the sea and by transitional areas such as lagoons. Coastal lagoons are 49
vulnerable systems, located between the land and the sea, enriched by both marine and 50
continental inputs and are among the most productive aquatic ecosystems (Nixon, 51
1998). The coastal lagoon that has been examined in this study is the Lesina lagoon 52
(Fig.1). This area has been frequently investigated in the last few years (Roselli et al., 53
2009; Specchiulli et al., 2009; Specchiulli et al., 2010; Lugoli et al., 2012; D’errico et 54
al., 2013). However, the effects of the coastal lagoon characteristics on PAH sorption in 55
sediment haven’t been studied yet. Up to now, several studies on the different sorption 56
properties of the sediments, the sorption kinetics and the various influencing factors 57
have been performed (Karickhoff et al., 1979; Barret et al., 2010; Yang and Zheng, 58
2010). It has been demonstrated that the changes in salinity are significant for increase 59
in equilibrium sorption constants (Means, 1995, Tremblay et al., 2005). Xia et al. 60
(2006) have been focused on the PAH sorbed which increases with the sediment 61
content. The purpose of this work is to understand the PAH behaviour in the lagoon 62
areas through the determination of PAH distribution and PAH sorption. Specifically, 63
how certain characteristics of the lagoon sediments such as particle-size, organic matter, 64
salinity and vegetative sediments, may affect the PAH behaviour in the transitional 65
areas compared to their behaviour in the open sea. For this reason two different areas 66
have been compared: a closed transitional environment (Lesina lagoon) and a coastal 67
marine environment (offshore Ravenna harbour) in order to see how PAHs behave 68
before they reach the sea in crossing a transitional lagoon area. 69
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2. Material and methods 71
72
2.1. Study areas 73
74
The lagoon of Lesina (Fig. 1), situated on the Southern Adriatic coast of Italy (41.88 °N 75
and 15.45 °E), is characterised by shallow water (0.7 – 1.5 m) and limited exchanges 76
with the sea. Due to its shallow depth, the Lesina lagoon is strongly influenced by 77
meteorological and climatic conditions, continental inputs and low tidal exchange. The 78
lagoon is connected to the sea by two tidal channels: one to the west (about 2 Km long) 79
and the other to the east (about 1 Km long) (Roselli et al., 2009). It receives freshwater 80
inputs from urban wastewaters, intensive aquaculture and agricultural activities, 81
determining a very important input of organic and inorganic contaminants, which cause 82
eutrophication events, characteristic in the coastal lagoon (Specchiulli et al., 2009). To 83
find out which characteristics of the lagoon affect the PAH accumulation in the 84
sediment, the Lesina lagoon has been divided into two basins: a western and an eastern 85
one, showing well known different hydrological and physical-chemical characteristics. 86
Indeed, about 80% of the annual freshwater budget is discharged into the eastern part of 87
the lagoon, consequently, a trophic and salinity gradient from the western to the eastern 88
part of the basin was established (Roselli et al., 2009). For a better understanding of 89
PAH behaviour in transitional environment such as the Lesina lagoon, also the 90
accumulation, distribution of the PAHs in a coastal sea area sediment, offshore Ravenna 91
harbour in the Northern Adriatic Sea, (Fig. 1) have been evaluated. This area has been 92
chosen since it is strongly influenced by the contribution of fresh water that flows along 93
5
the western Adriatic coast and by river water from the Po Valley (Marini et al., 2002; 94
Campanelli et al., 2011). Some characteristics of the two study areas have been 95
described in Table 1. 96
97
2.2. Sample collection and preparation 98
99
The Lesina lagoon marine sediments were collected in autumn 2010. Because of the 100
Lesina lagoon shallowness and the high heterogeneity of the area, thirteen sediments 101
were sampled: five in the western basin and eight in eastern one (Fig. 1). For a 102
comparison purpose we also studied sediment samples taken in the western Adriatic 103
Sea. Precisely, offshore Ravenna harbour (5 Km from Ravenna) six marine sediment 104
samples were collected, in autumn 2010, by a box corer at a depth inferior to 20 cm. 105
The sediment samples collected in both study areas were homogenized and were stored 106
at –18 °C prior to process in the laboratory. Fig. 2 shows the sediment sample 107
classification according to Shepard (1954). The sediment samples were air-dried and 108
then 10 g of dry sediment was weighed with an analytical balance. For each sediment 109
sample (Lesina Lagoon and offshore Ravenna harbour) the water content and the PAH 110
concentration were defined. The sorption experiments were carried out in one Ravenna 111
sediment (R1) and in two Lesina lagoon sediments (Les2 and Les9 respectively for the 112
western and eastern basin). These sediment samples were chosen because they were the 113
less contaminated ones (Fig. 3). 114
115
116
2.3. PAH extraction and chemical analysis 117
6
118
The PAH extraction from sediment samples was carried out with methylene chloride 119
solvent (20 mL) by three cycles of 15 min each of ultrasonic baths. The PAH enriched 120
solvent was centrifuged (1500 rpm for 15 min) and the suspended part was then 121
removed by rotary evaporation (35 °C). The dry residue was recovered with acetonitrile 122
(0.5 mL). This process was followed by the chromatographic analysis. The PAHs were 123
analysed with a high performance liquid chromatography (HPLC Ultimate3000, 124
Dionex). A mixture of PAHs was separated on a 4.6 x 150 mm analytical reverse phase 125
column C16 3µm 120 Å. Eluting PAHs were detected with a fluorescence detector 126
(RF2000, Dionex) for the quantitative analysis and together with (in line) a PDA-100 127
Photodiode Array Detector for the qualitative analysis. Acenaphthylene cannot be 128
analysed with fluorescence detection, so it is analysed with a PDA-100 Photodiode 129
Array Detector. A mixture of acetonitrile and water (from 40:60 to 90:10), distilled and 130
further purified by a Mill-Q system (Millipore, Billerica, MA, USA), was used as the 131
mobile phase, delivered with a gradient program at 1.5 mL min-1
(IOC-Unesco, 1982). 132
The detection limit (estimated as two time background noise) of the method was 0.04 – 133
0.4 ng g-1
for PAH. 134
135
2.4. Sorption to sediment 136
137
The sediment samples of both study areas were employed for sorption experiments to 138
evaluate the water-sediment distribution coefficient (Kd). For each sediment sample 139
(R1, Les2 and Les9) a batch test was prepared using five different initial solute 140
concentrations (559 mg L-1
, 224 mg L-1
, 112 mg L-1
, 56 mg L-1
and 28 mg L-1
). They 141
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were performed in the following ratio 1:10, 1:25, 1:50, 1:100 and 1:200 from a standard 142
PAH solution (EPA 610 PAH Mix), using methyl chloride as solvent. The mass of dry 143
sediment which was used in each batch test was of 1.0 ± 0.1 g with a final solution 144
volume of 10 mL (Means, 1995), comprising 0.4% formalin to inhibit microbial 145
activity. The samples, inserted into glass tubes without caps and sealed with parafilm, 146
were equilibrated in a shaking table in the dark and at a temperature of 25.0 ± 0.5 °C. 147
The equilibrium time wasn’t calculated by sequential sampling but according to the 148
equilibrium achievement of the low-water-solubility compounds, generally achieved 149
within 24 h (Karickhoff et al., 1979; Barret et al., 2011). After reaching the equilibrium, 150
the glass tubes were centrifuged for 30 min at 3000 rpm. The PAH extraction from the 151
aqueous phase was performed by methylene chloride, then a liquid-liquid separation 152
was made. The solution was concentrated on a rotary evaporator and the dry residue 153
was recovered with acetonitrile. Analysis of extracts were performed using HPLC as 154
explained above (2.3). Blank samples containing sorbate solution but no sediment were 155
also prepared in triplicate for each concentration. The quantity of PAH sorbed to the 156
sediment phase in the samples was calculated by the difference between the PAH 157
concentrations in the water phase in the blank samples and those from the sorption 158
samples containing sediment (Kohl and Rice, 1999). Sorption isotherms were 159
established for all the 16 PAHs. The curve was fitted by the three isotherm equations: 160
the linear model, Freundlich model and Langmuir model (Trevisan et al., 1995; 161
Businelli et al., 2000; Yang and Zheng, 2010). Kd represents the sorption capacity of the 162
whole sediment. Instead, if only one characteristic of the sediment is considered, for 163
example the organic carbon, Kd is substituted by Koc. Koc is the partition coefficient 164
corrected for organic carbon content of the sediment (Means, 1995). 165
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166
2.5. PAH quantitation 167
168
PAH quantitation was performed using the external standard calibration procedure. 169
Calibration curves were established using a serial dilution (1:50, 1:100 and 1:200 with 170
methylene chloride) from a standard PAH solution (EPA 610 PAH Mix), purchased 171
from Supelco, Bellefonte, PA, USA. Standard PAH solution (1:1) contains a mixture of 172
sixteen priority pollutant PAHs, with a known concentration. Methods employed were 173
validated by intercalibration. Recovery rates were obtained for each individual PAH on 174
two sediment samples certified for PAH: IAEA code 383 (IAEA/MEL/65, 1998) and 175
IAEA code 408 (IAEA/MEL/67, 1999). These certified samples were extracted and 176
analysed following the same procedure as for the sediment samples. PAH recoveries on 177
sediment samples certified IAEA code 383 varied between 42% (for acenaphtene) and 178
93% (for indeno[1,2. PAH,3-c, d]pyrene)recoveries on sediment samples certified 179
IAEA code 408 varied between 51% (for benz[a]anthracene) and 88% (for anthracene),. 180
The percentage standard deviations varied between 2% (for benzo[b]fluoranthene) and 181
24% (for anthracene). All concentrations were expressed on a dry weight basis and no 182
corrected with the recovery data. 183
184
3. Results and discussion 185
186
3.1. PAH molecular distribution 187
188
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An average PAH molecular distribution in the two study areas has been carried out to 189
explain the contribution of each compound on total PAH load (Fig. 4a). The standard 190
deviation of the relative percentage values has been calculated for the thirteen sediment 191
samples of the Lesina lagoon and for six sediment samples of 5 Km offshore Ravenna 192
harbour. Sediment samples of the Lesina lagoon have recorded that the PAH load is 193
dominated by 4 ring compounds (Fig. 4b): fluoranthene and pyrene, which together 194
have reached about 44% of the total. Three sites (Les4, Les7 and Les8) have revealed a 195
PAH molecular distribution dominated by 5/6 ring PAHs, although lower total PAH 196
concentration has appeared (≤ 100 ng g-1
d. w., Fig. 3). While, naphthalene, 197
acenaphthylene, acenaphthene and fluorene have been recorded below detection limits. 198
PAH molecular distribution obtained in this work may be to compared to other 199
Mediterranean coastal lagoon, where the PAH group profile substantiates a 200
predominance of high molecular weight over low molecular weight PAHs (Frignani et 201
al., 2003; Culotta et al., 2006; Perra et al., 2009). A different PAH molecular 202
distribution has been shown in offshore Ravenna harbour marine sediments, in fact, 2/3 203
ring PAHs have been found in these sites, in particular, the phenanthrene compound has 204
contributed with 21% to total PAH load; fluoranthene and pyrene compounds follow. 205
206
3.2. Sediment PAH concentration 207
208
The total PAH concentration was determined by the sum of each organic compound 209
concentration (∑PAH) in the surface sediment layers of the Lesina lagoon and of 5 Km 210
offshore Ravenna harbour. ∑PAH in Lesina lagoon sediments was found to vary 211
between 4 – 4486 ng g-1
dry weight (mean 866 ± 1236 ng g-1
d. w.), besides, it varies 212
10
quite widely between stations (Fig. 3). In the western basin of the Lesina lagoon the 213
PAH concentration has appeared higher along the southern shore (Les1 e Les3). . These 214
discharge areas have represented the dominant vector of PAH inputs in the lagoon. The 215
site Les1 near livestock farms and fish farms follows with 1081 ng g-1
. On the contrary, 216
in the eastern basin of the Lesina lagoon the southern shore sediments have shown a 217
lower PAH concentration (≤ 100 ng g-1
d. w.), except for Les5 (929 ng g-1 d. w.) and 218
Les10 (1390 ng g-1
d. w.) that are located near drainage pumping stations, which may 219
have increased the PAH level. Therefore, eastern basin central area stations have 220
resulted as the ones with the highest PAH concentration, showing a PAH level of 1559 221
and 1189 ng g-1
dry weight, respectively for Les11 and Les12. The total PAH 222
concentrations obtained in this work are medium low compared with the other 223
Mediterranean coastal lagoons (Specchiulli et al., 2009) 224
∑PAH in offshore Ravenna harbour sediments has appeared lower than the lagoon one, 225
showing a less variable range: 130 – 550 ng g-1
dry weight (mean 321 ± 187 ng g-1
d. 226
w., Fig. 3). The main difference between the two study areas has been the 2/3 ring PAH 227
concentration, dominant in all Ravenna stations suggesting probable oil inputs, but 228
absent in the Lesina sediments (Fig. 4b). This may be explained by the nearness of the 229
Ravenna harbour and by oil spill from boats, which could be responsible for the release 230
of petroleum in the surrounding environment (De Luca et al., 2004; King et al., 2004). 231
The Les 2, Les 9 and R1sediments were chosen because they were the less 232
contaminated ones (Les 2 and Les 9 < 100 ng g-1 d.w. while R1 was 130 ng g-1 d.w., 233
Fig.3). 234
235
236
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3.3. Sorption studies 237
238
From Table 2 it can be seen that the sorption isotherms of 16 PAHs are all well fitted 239
with the three equation models. Since the fitting results of the linear isotherm model are 240
the best, only the Kd values calculated with the linear model have been considered. The 241
carried out sorption tests suggested that the Kd values changed depending on the 242
different PAH compounds. For this reason, a distinction between lower molecular 243
weight PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene and 244
anthracene) and higher molecular weight PAHs (fluoranthene, pyrene, crysene, 245
benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benz[a]pyrene, 246
dibenz[a,h]anthracene, indeno[1,2,3-cd]pyrene and benzo[ghi]perylene) has been made. 247
The higher molecular weight PAHs were sorbed more in sediments in comparison with 248
the lighter ones (Witt, 1995). The sorption capacity of the lower molecular weight 249
PAHs in various sediments follows this sequence: Kd eastern > Kd western > Kd 250
offshore Ravenna harbour. While, the sorption capacity of the higher molecular weight 251
PAHs follows this other sequence: Kd western > Kd eastern > Kd offshore Ravenna 252
harbour (Table 3). This last sequence can be compared with TOC sequence: TOC 253
western > TOC eastern > TOC offshore Ravenna harbour (Table 1). Indeed, only the 254
higher molecular weight PAHs have shown a significant correlation between Kd and 255
TOC (r = 0.998; n= 3; p < 0.05). Since the sorption capacity was correlated with the 256
TOC, the Kd values of the higher molecular weight PAHs could be normalized to TOC 257
(Means, 1995). By the Koc calculation it has been observed that the sorption capacity 258
was affected by the TOC of the sedimentary matrix. Therefore, the TOC was the most 259
significant factor which controlled the PAH sorption in sediment. The sediment Koc 260
12
values of the eastern basin resulted higher compared with the western basin ones, for all 261
the analysed PAHs (Fig. 5). While, the offshore Ravenna harbour sediments showed 262
higher Koc values in lower molecular weight PAHs in comparison with the western 263
basin ones, suggesting a greater molecular persistence in the coastal sediments. 264
265
3.4. Sediment particle size and organic carbon content 266
267
The partitioning of PAH in sediments is linked to several more or less strong 268
correlations with different sediment textural features. It has been demonstrated that the 269
concentration of PAHs in sediments was affected by the chemical composition of the 270
samples such as the organic matter and water content (Kim et al., 1999). A more muddy 271
sediment is characterized by high values of PAHs (Belahcen et al., 1997). Other studies 272
underlined the role of grain-size fractions (Readman et al., 1982). The sorption results 273
have generally accepted that organic carbon content (TOC) is important to control the 274
accumulation of organic pollutants in sediments. Moreover, a positive correlation (r = 275
0.839, n = 6, p < 0.05) between the concentration of total PAHs and TOC has been 276
observed in the Lesina lagoon sediments (Les 3; Les 5; Les 7; Les 11; Les 12; Les 13). 277
While, with regard to the correlations between the concentration of PAHs and the 278
particle size (sand, silt or clay), the obtained values showed a correlation no statistically 279
significant (r = 0.282, n = 13). This may be explained by a lack of correlation between 280
pelite and organic matter and by the TOC distribution and by the presence of the 281
different PAH sources in the Lesina lagoon. 282
In the eastern basin central areas and in the nearest urban center site (Les3) the TOC 283
increased. In fact, in the Les3 site high TOC equivalent to 5.28% was recorded, while, 284
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in the eastern basin central area a range between 3.80% – 4.67% TOC was observed 285
(Specchiulli et al., 2010). Furthermore, in the highest TOC areas, a major PAH 286
concentration was present (Fig. 4a). A different situation can be noticed in the sampled 287
coastal marine environment, where offshore Ravenna harbour sediments have revealed 288
a strong correlation between PAH and fine grain-size (r = 0.972, n = 6, p < 0.01). The 289
TOC in offshore Ravenna harbour sediments ranges 0.83% ± 0.15 and increases 290
offshore (Tesi et al., 2007). The highest PAH concentration was found in the R2 site 291
and in the more offshore sediments (R4 and R5), where the result of the clay and silt 292
percentage was above 90% (Fig. 4b). A positive correlation (r = 0.967, n = 6, p < 0.01) 293
of the organic carbon with the finest sediment fraction was confirmed in this area. Thus, 294
PAH accumulation has proved to be strongly associated with the finest sediment 295
fraction. This strong correlation showed that clay or clay and silt have had a great 296
influence on PAH distribution, confirming their preferential sorption to organic material 297
and sediments with high clay percentage, as demonstrated by Zang et al. (2004). 298
299
3.5. Salinity 300
301
Salinity is one of the most fluctuating environmental factors that might affect PAH 302
degradation and PAH accumulation in sediments (Tam et al., 2002). It is acknowledged 303
that when the sea water salt concentration increases the PAH solubility decreases; 304
causing a PAH transfer from the aqueous phase to the solid one (Xia and Wang, 2008), 305
consequently the microorganism degrading skill decreases (Nedwell, 1999). The salinity 306
variability is significant in a shallow lagoon environment. The Lesina lagoon salinity 307
has been determined by freshwater input, precipitation, evaporation, morphology 308
14
(Marolla et al., 1995) and the exchange efficiency of the tidal channels (Fabbrocini et 309
al., 2005). In this work, according to Specchiulli et al. (2010) and Roselli et al. (2009), 310
a lower salinity has been observed in the eastern basin of the lagoon (7 – 16, in winter) 311
in comparison with the western basin (> 19, in winter). This probably happens because 312
the eastern area of the lagoon receives the freshwater inputs, mainly along the southern 313
shore, collecting agricultural drainage water from a pumping station located south of 314
Lesina (Specchiulli et al., 2010). In both Lesina lagoon basins a lack of correlation 315
between PAH concentration and salinity values has been recorded. Several physical 316
conditions, such as salinity, may have caused the different microorganism adaptation 317
patterns (Spain et al., 1980; Xia and Wang, 2008). Tam et al. (2002) have shown that 318
the percentage of degraded phenanthrene has varied in relation to the different values of 319
salinity, therefore, in the presence of high or low salinity degradation bacteria have been 320
inhibited so, it can be presumed that this could happen also in the Lesina lagoon. In 321
offshore Ravenna harbour sediments a strong relation between PAH concentration and 322
salinity has been recorded (r = 0.816, n = 6, p < 0.05). So, it can be observed that, in 323
this area, the salt gradient increases gradually from the coast to the open sea affecting 324
the PAH concentration in offshore marine sediments (Marini and Frapiccini, 2013). 325
326
3.6. Vegetative sediments 327
328
The Lesina lagoon is characterized by a community of macrophytobenthos (Ruppia 329
cirrhosa and Nanozostera noltii), mainly distributed in the eastern and central parts of 330
the basin (Roselli et al., 2009). Here, the two sampled sites (Les11 and Les12) have 331
shown a high total PAH concentration: 1559 and 1189 ng g-1
, respectively. As 332
15
demonstrated by Zhang et al. (2004) the highest PAH concentration has been recorded 333
in vegetative sediment samples. PAH sorption in these central areas may be due to a 334
higher presence of TOC values and clay contents in sediments with mangrove 335
vegetation than in those without. However, the sorption results have shown how the 336
organic carbon contained in the sediment was the most significant factor that controls 337
the sorption, to the disadvantage of other less significant factors (particle-size) (Yang 338
and Zheng, 2010; Hassett et al., 1980). Therefore, the PAHs discharged in the Lesina 339
lagoon are probably sorbed more in vegetative sediments than in the ones without 340
vegetation. A minor PAH accumulation has been observed in the Ravenna area since no 341
eelgrass prairies are present there (Barletta et al., 2003). 342
343
4. Conclusion 344
345
The present study has compared two separate areas: a coastal lagoon (Lesina lagoon) 346
and a coastal marine area (offshore Ravenna harbour) in order to evaluate the PAH 347
behaviour in the marine sediments of both areas. It has been demonstrated that in a 348
transitional environment such as the Lesina lagoon, where several factors depending on 349
area heterogeneity, come into relation, the PAH distribution and sorption have been 350
mainly affected by TOC in comparison with the particle size of the sediment. Through 351
the Koc calculation, it can be observed that the eastern basin sediment has had a greater 352
sorption ability than the western one. This is due to a higher TOC in the eastern basin of 353
the Lesina lagoon, which is also increased by the presence of vegetative sediments. 354
These have enabled the PAH sorption in the eastern sediments. Salinity may be 355
considered a factor which affects the PAH behaviour also in the lagoon environment. 356
16
However, comparing both study areas, the salinity gradient effect on PAH accumulation 357
has appeared weaker in the lagoon sediments than in the coastal area ones. 358
The relations observed between PAH distribution and sorption and the examined 359
parameters (grain size, TOC, salinity and vegetative sediments) have resulted stronger 360
in the coastal Ravenna marine area compared with the Lesina lagoon one. This is so 361
because the transitional environments, as the Lesina lagoon, are greatly heterogeneous 362
areas with a high variability of the abiotic factors. The above-mentioned heterogeneity 363
may become a confusing factor and contribute to the influencing of PAH behaviour. In 364
fact, in these areas such behaviour is different compared with the well-known marine 365
area PAH distribution patterns, confirmed in the offshore Ravenna harbour sediments. 366
Therefore, the results have shown that transitional areas contribute to the increasing of 367
the PAH accumulation in the sediment turning it into a trap for organic contaminants 368
such as PAHs. 369
370
Acknowledgments 371
372
We would like to thank Raffaele D’Adamo for the sediment sampling in Lesina Lagoon 373
and Antonietta Specchiulli to compare the total organic carbon values in the Lesina 374
Lagoon. This research is supported by the Bandiera RITMARE Project - La Ricerca 375
Italiana per il Mare – coordinated by National Research Council and financed by Italian 376
University and Research Ministry, National Research Program: 2011-2013. 377
378
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