Marine Pollution Bulletin 86 (2014) 494–504
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Floating debris in the Mediterranean Sea Giuseppe Suaria, Stefano Aliani ⇑ CNR-ISMAR (Institute of Marine Sciences – National Research Council), Forte S. Teresa – 19032, Pozzuolo di Lerici, SP, Italy
a r t i c l e
i n f o
Article history: Available online 10 August 2014 Keywords: Marine litter Floating debris Mediterranean Plastic pollution
a b s t r a c t Results from the first large-scale survey of floating natural (NMD) and anthropogenic (AMD) debris (>2 cm) in the central and western part of the Mediterranean Sea are reported. Floating debris was found throughout the entire study area with densities ranging from 0 to 194.6 items/km2 and mean abundances of 24.9 AMD items/km2 and 6.9 NMD items/km2 across all surveyed locations. On the whole, 78% of all sighted objects were of anthropogenic origin, 95.6% of which were petrochemical derivatives (i.e. plastic and styrofoam). Maximum AMD densities (>52 items/km2) were found in the Adriatic Sea and in the Algerian basin, while the lowest densities (10 cm) debris on the surface of the central and western Mediterranean Sea, with the main goal of providing new information on the abundance, distribution and composition of marine litter in the Mediterranean region. Debris abundance and distribution are also discussed in relation to natural and anthropogenic processes such as river discharge, coastal population density, oceanographic features, fishing, shipping and touristic activities. The abundance and distribution of floating debris in the world’s ocean has been widely estimated using surface net trawls (e.g. Gregory et al., 1984; Ryan, 1988; Day et al., 1990; Shaw and Day, 1994; Moore et al., 2001; Yamashita and Tanimura, 2007; Zhou et al., 2011; Law et al., 2010; Collignon et al., 2012; Van Cauwenberghe et al., 2013). However, such surveys typically focuses only on small litter items (2 cm) sighted off the starboard side of the track-line. Sightings were all performed during daylight hours, by naked eye and only under good weather conditions (Beaufort sea state 21 kts and wave height >2 m), when the detection probability within the transect width was severely compromised, were removed from the dataset and were not considered in further analysis. Meteo-marine data such as wind speed and direction, sea surface temperature and salinity, were automatically recorded by the ship’s data logger. The survey effort was split into 30-min transects (mean length: 9.21 ± 1.05 km) in order to standardize fatigue for the observer, enhance the number of replicates and better account for the patchy distribution of debris at sea. The exact distance covered during each transect was calculated from GPS start and stop positions using the haversine formula (Sinnott, 1984). After being recorded, every item was allocated to one out of four size classes (100 cm) and to one of two major type categories: Anthropogenic Marine Debris (AMD) and Natural Marine Debris (NMD). AMD was further subdivided into styrofoam (expanded polystyrene), plastic (mainly fragments, plastic bags, bottles and containers) and others (e.g. manufactured wood, aluminum cans, rubber strips, glass bottles, paper and cardboard), while NMD was classified as wood (mainly logs, trunks, branches and canes), algae (mainly branches of Cystoseira spp. and Sargassum spp.) or others (e.g. egagropili of Posidonia oceanica, cuttlebones, bird feathers, sponges or pumice). The survey area was subdivided into 14 sectors (Fig. 1) following the METEOMEDTM subdivisions of the Mediterranean Sea for marine weather forecasts: Central Adriatic (A), Southwestern Adriatic (B), Southeastern Adriatic (C), Strait of Otranto (D), Northwestern Ionian Sea (E), Sicilian Sea (F), Strait of Sicily (G), South Tyrrhenian Sea (H), Central Tyrrhenian Sea (I), Corsica Channel (M), Sea of Sardinia (K), Balearic Sea (L), Algerian Basin (M) and Sardinia Channel (N). The number of surveyed transects per sector varied from 3 to 38, mainly due to different weather conditions, navigational schedules and operational needs during the cruises. Location of all transects and sectors is reported in Fig. 1. 2.2. Abundance estimates A simplified distance sampling technique was used to estimate AMD and NMD densities taking into account the decrease in litter detectability with increasing distance from the observer. In order to do so, during the last cruise (Ichnussa13) the perpendicular distance of all sighted objects was estimated by knowing that the distance from an item when abeam, equals the distance traveled
496
G. Suaria, S. Aliani / Marine Pollution Bulletin 86 (2014) 494–504
Fig. 1. Map of the central-western Mediterranean Sea showing the study area, the location of all transects and sectors and the distribution of AMD (black bars) and NMD (white bars) densities (expressed as number of items/km2) in all surveyed transects.
across the length of each transect. Densities of AMD and FMD were then calculated by simply dividing the total number of sighted objects by the effective area surveyed during observations, using the following equation:
D¼
n ESW L
ð1Þ
with L being the length of the transect and n the number of sighted objects (Buckland et al., 2005). Mean densities of anthropogenic and natural debris (expressed as numbers of items per km2) were computed for each sector and plotted in Fig. 3 as mean values ± s.e. 2.3. Statistical analysis
Fig. 2. Proportion of debris sightings based on the perpendicular distance from the observer. Detections probability is assumed to be 1 on the track-line (0–10 m). Dashed line show the computed effective strip width (ESW).
by the ship between two consecutive bearings of the same object taken at 45 and 90 to the bow. Every item was then assigned to one of 8 distance categories (0–10 m, 10–20 m, 20–30 m, 30–40 m, 40–50 m, 50–60 m, 60–100 m, >100 m) and the relative proportion of sighted objects for each category was plotted in Fig. 2. Using the software DISTANCE 6 (Thomas et al., 2010), distance measurements were used to calculate an overall effective strip width (ESW) according to standardized line transect method (Buckland et al., 2005). By definition, the number of items detected beyond the ESW, equals the number of items missed within it (Thomas et al., 2006). Therefore, the effective area surveyed was assumed to correspond to a strip of width ESW (31.05 ± 2.45 m)
Non-normal distribution of data was tested by Shapiro–Wilk test on the residuals from one-way ANOVA. Non-parametric Kruskal– Wallis test was used to determine whether the abundance of AMD and NMD differed significantly within the study area. Mann–Whitney U test for multiple pairwise comparisons was run to identify specific differences in the abundance of floating debris between all sectors. Spearman’s non-parametric correlation coefficient was used to identify significant correlation between the abundance of natural and anthropogenic debris in the whole basin as well as within different sectors. All the analyses were carried out using the free software PAST v 3.01 (Hammer et al., 2001). 3. Results 3.1. Abundance and distribution 167 transect counts were performed for a total of ca. 83 h of observation, covering an overall survey length of 1538.2 km.
G. Suaria, S. Aliani / Marine Pollution Bulletin 86 (2014) 494–504
1402 floating objects were sighted throughout the entire study area and no debris items were detected in 10% of the transects. Ship’s log revealed that 12 transects were made during adverse weather conditions (wind speed >21 kts), so they were removed from dataset and were not considered in density calculations. AMD was present in 87% of all transects, with densities ranging from 0 to 162.1 items/km2 and a mean total density of 24.9 ± 2.4 items/km2 across all sampled locations. Maximum densities of AMD were observed in the Adriatic Sea (sectors A and B) and in the Algerian basin (sector M), which had mean values of 54.6 ± 11.1, 52.1 ± 11.4 and 52.9 ± 11.4 items/km2 respectively. The lowest densities of AMD were found in the Central Tyrrhenian Sea (sector I) and in the Sicilian Sea (sector F), where mean values of 4.9 ± 2.8 items/km2 and 6.3 ± 2.6 items/km2 were observed. In all the other sectors mean AMD densities ranged from 10.9 to 30.7 items/km2. Assuming comparable densities of anthropogenic litter in the eastern part of the basin, we estimated that during our survey approximately 62 106 macro-litter items were afloat on the surface of the entire Mediterranean Sea. This represents a 17-fold increase since the last estimate of 3.6 106 debris items made over 25 years ago by McCoy (1988). NMD was found in 42% of all transects, with densities ranging from 0 to 117.5 items/km2 and a mean abundance of 6.9 ± 1.2 items/km2 across the whole study area. The highest concentrations of NMD (32.2 ± 12.9 items/km2) were observed in the Corsica Channel (Sector J), between Corsica and Elba island, where a large number of tree trunks, canes and pieces of wood of different
497
sizes were sighted, suggesting a strong terrigenous input of marine debris. This was also the only sector in which NMD was more abundant than AMD. Intermediate NMD densities (ranging from 6.6 ± 2.2 to 15.6 ± 4.4 items/km2) were found in the Adriatic Sea (Sectors A, B and C) and in sectors K, L and M. Conversely, NMD was present in very low abundances (
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Floating debris in the Mediterranean Sea Giuseppe Suaria, Stefano Aliani ⇑ CNR-ISMAR (Institute of Marine Sciences – National Research Council), Forte S. Teresa – 19032, Pozzuolo di Lerici, SP, Italy
a r t i c l e
i n f o
Article history: Available online 10 August 2014 Keywords: Marine litter Floating debris Mediterranean Plastic pollution
a b s t r a c t Results from the first large-scale survey of floating natural (NMD) and anthropogenic (AMD) debris (>2 cm) in the central and western part of the Mediterranean Sea are reported. Floating debris was found throughout the entire study area with densities ranging from 0 to 194.6 items/km2 and mean abundances of 24.9 AMD items/km2 and 6.9 NMD items/km2 across all surveyed locations. On the whole, 78% of all sighted objects were of anthropogenic origin, 95.6% of which were petrochemical derivatives (i.e. plastic and styrofoam). Maximum AMD densities (>52 items/km2) were found in the Adriatic Sea and in the Algerian basin, while the lowest densities (10 cm) debris on the surface of the central and western Mediterranean Sea, with the main goal of providing new information on the abundance, distribution and composition of marine litter in the Mediterranean region. Debris abundance and distribution are also discussed in relation to natural and anthropogenic processes such as river discharge, coastal population density, oceanographic features, fishing, shipping and touristic activities. The abundance and distribution of floating debris in the world’s ocean has been widely estimated using surface net trawls (e.g. Gregory et al., 1984; Ryan, 1988; Day et al., 1990; Shaw and Day, 1994; Moore et al., 2001; Yamashita and Tanimura, 2007; Zhou et al., 2011; Law et al., 2010; Collignon et al., 2012; Van Cauwenberghe et al., 2013). However, such surveys typically focuses only on small litter items (2 cm) sighted off the starboard side of the track-line. Sightings were all performed during daylight hours, by naked eye and only under good weather conditions (Beaufort sea state 21 kts and wave height >2 m), when the detection probability within the transect width was severely compromised, were removed from the dataset and were not considered in further analysis. Meteo-marine data such as wind speed and direction, sea surface temperature and salinity, were automatically recorded by the ship’s data logger. The survey effort was split into 30-min transects (mean length: 9.21 ± 1.05 km) in order to standardize fatigue for the observer, enhance the number of replicates and better account for the patchy distribution of debris at sea. The exact distance covered during each transect was calculated from GPS start and stop positions using the haversine formula (Sinnott, 1984). After being recorded, every item was allocated to one out of four size classes (100 cm) and to one of two major type categories: Anthropogenic Marine Debris (AMD) and Natural Marine Debris (NMD). AMD was further subdivided into styrofoam (expanded polystyrene), plastic (mainly fragments, plastic bags, bottles and containers) and others (e.g. manufactured wood, aluminum cans, rubber strips, glass bottles, paper and cardboard), while NMD was classified as wood (mainly logs, trunks, branches and canes), algae (mainly branches of Cystoseira spp. and Sargassum spp.) or others (e.g. egagropili of Posidonia oceanica, cuttlebones, bird feathers, sponges or pumice). The survey area was subdivided into 14 sectors (Fig. 1) following the METEOMEDTM subdivisions of the Mediterranean Sea for marine weather forecasts: Central Adriatic (A), Southwestern Adriatic (B), Southeastern Adriatic (C), Strait of Otranto (D), Northwestern Ionian Sea (E), Sicilian Sea (F), Strait of Sicily (G), South Tyrrhenian Sea (H), Central Tyrrhenian Sea (I), Corsica Channel (M), Sea of Sardinia (K), Balearic Sea (L), Algerian Basin (M) and Sardinia Channel (N). The number of surveyed transects per sector varied from 3 to 38, mainly due to different weather conditions, navigational schedules and operational needs during the cruises. Location of all transects and sectors is reported in Fig. 1. 2.2. Abundance estimates A simplified distance sampling technique was used to estimate AMD and NMD densities taking into account the decrease in litter detectability with increasing distance from the observer. In order to do so, during the last cruise (Ichnussa13) the perpendicular distance of all sighted objects was estimated by knowing that the distance from an item when abeam, equals the distance traveled
496
G. Suaria, S. Aliani / Marine Pollution Bulletin 86 (2014) 494–504
Fig. 1. Map of the central-western Mediterranean Sea showing the study area, the location of all transects and sectors and the distribution of AMD (black bars) and NMD (white bars) densities (expressed as number of items/km2) in all surveyed transects.
across the length of each transect. Densities of AMD and FMD were then calculated by simply dividing the total number of sighted objects by the effective area surveyed during observations, using the following equation:
D¼
n ESW L
ð1Þ
with L being the length of the transect and n the number of sighted objects (Buckland et al., 2005). Mean densities of anthropogenic and natural debris (expressed as numbers of items per km2) were computed for each sector and plotted in Fig. 3 as mean values ± s.e. 2.3. Statistical analysis
Fig. 2. Proportion of debris sightings based on the perpendicular distance from the observer. Detections probability is assumed to be 1 on the track-line (0–10 m). Dashed line show the computed effective strip width (ESW).
by the ship between two consecutive bearings of the same object taken at 45 and 90 to the bow. Every item was then assigned to one of 8 distance categories (0–10 m, 10–20 m, 20–30 m, 30–40 m, 40–50 m, 50–60 m, 60–100 m, >100 m) and the relative proportion of sighted objects for each category was plotted in Fig. 2. Using the software DISTANCE 6 (Thomas et al., 2010), distance measurements were used to calculate an overall effective strip width (ESW) according to standardized line transect method (Buckland et al., 2005). By definition, the number of items detected beyond the ESW, equals the number of items missed within it (Thomas et al., 2006). Therefore, the effective area surveyed was assumed to correspond to a strip of width ESW (31.05 ± 2.45 m)
Non-normal distribution of data was tested by Shapiro–Wilk test on the residuals from one-way ANOVA. Non-parametric Kruskal– Wallis test was used to determine whether the abundance of AMD and NMD differed significantly within the study area. Mann–Whitney U test for multiple pairwise comparisons was run to identify specific differences in the abundance of floating debris between all sectors. Spearman’s non-parametric correlation coefficient was used to identify significant correlation between the abundance of natural and anthropogenic debris in the whole basin as well as within different sectors. All the analyses were carried out using the free software PAST v 3.01 (Hammer et al., 2001). 3. Results 3.1. Abundance and distribution 167 transect counts were performed for a total of ca. 83 h of observation, covering an overall survey length of 1538.2 km.
G. Suaria, S. Aliani / Marine Pollution Bulletin 86 (2014) 494–504
1402 floating objects were sighted throughout the entire study area and no debris items were detected in 10% of the transects. Ship’s log revealed that 12 transects were made during adverse weather conditions (wind speed >21 kts), so they were removed from dataset and were not considered in density calculations. AMD was present in 87% of all transects, with densities ranging from 0 to 162.1 items/km2 and a mean total density of 24.9 ± 2.4 items/km2 across all sampled locations. Maximum densities of AMD were observed in the Adriatic Sea (sectors A and B) and in the Algerian basin (sector M), which had mean values of 54.6 ± 11.1, 52.1 ± 11.4 and 52.9 ± 11.4 items/km2 respectively. The lowest densities of AMD were found in the Central Tyrrhenian Sea (sector I) and in the Sicilian Sea (sector F), where mean values of 4.9 ± 2.8 items/km2 and 6.3 ± 2.6 items/km2 were observed. In all the other sectors mean AMD densities ranged from 10.9 to 30.7 items/km2. Assuming comparable densities of anthropogenic litter in the eastern part of the basin, we estimated that during our survey approximately 62 106 macro-litter items were afloat on the surface of the entire Mediterranean Sea. This represents a 17-fold increase since the last estimate of 3.6 106 debris items made over 25 years ago by McCoy (1988). NMD was found in 42% of all transects, with densities ranging from 0 to 117.5 items/km2 and a mean abundance of 6.9 ± 1.2 items/km2 across the whole study area. The highest concentrations of NMD (32.2 ± 12.9 items/km2) were observed in the Corsica Channel (Sector J), between Corsica and Elba island, where a large number of tree trunks, canes and pieces of wood of different
497
sizes were sighted, suggesting a strong terrigenous input of marine debris. This was also the only sector in which NMD was more abundant than AMD. Intermediate NMD densities (ranging from 6.6 ± 2.2 to 15.6 ± 4.4 items/km2) were found in the Adriatic Sea (Sectors A, B and C) and in sectors K, L and M. Conversely, NMD was present in very low abundances (
