An evaluation of surface micro and meso plastic pollution in pelagic ecosystems of western Mediterranean Sea

In the 90’s, the discovery of high concentrations of plastic debris in the oceans generated interest and concern of general public as well as scientific community. Since, the accumulation of plastic debris in the North Pacific and North Atlantic gyres has been widely reported. Surprisingly, only few quantitative studies exist concerning the Mediterranean Sea. The purpose of this study was to examine the distribution, abundance and characteristics of surface micro (size 0.33 – 5.0 mm) and meso (size 5.0 – 50 mm) plastic debris, as compared to the abundance of organic material, in the western Mediterranean Sea. 41 samples have been collected during 2 cruises in September 2011 and August 2012 in regions distributed between positions 38-44°N and 001-010°E (Gulf of Lion, Eastern Spanish coast, Balearic Islands, Sardinia, Corsica). Sample collection was performed using a surface net tow of 330 μm mesh size. Samples were sieved to separate particles smaller than 5 mm from larger ones; plastic pieces were extracted from the plankton and organic matter, and sorted in 6 categories: fragments, pellets, thin films, foams, lines and others (tar, etc.). Each were counted and weighted, as was the organic matter once dried. The concentrations of micro-plastic debris were measured in term of particles number and particles mass per surface unit. The average values of 130’000 parts/km2 and 58’000 mg/km2, respectively were measured, with highest concentrations reaching values superior to 420’000 parts/km2 and 140’000 mg/km2. A qualitative analysis mainly revealed the presence of fragments (77% in mass), but also thin films (13%), foams (7%), pellets (2%) and lines (2%). The average ratio of micro-plastic mass to dry organic material mass has been measured at 1.04. A fraction of the samples was collected in low wind conditions (u10 < 4 m/s), while the other part was collected under medium wind conditions (4 ≤ u10 ≤ 11 m/s). As the vertical distribution of micro-plastic is known to be highly sensitive to wind stress, the values have been corrected and re-interpreted using the methodology proposed by Kukulka & al. 2012. The resulting corrected values suggest that the employed sampling process underestimated the surface micro-plastic concentration by an average factor of 1.55. The average concentrations of meso-plastic debris reached the number of 5’500 parts/km2 and the mass of 120’000 mg/km2, mostly fragments (57%) and thin films (34%). The results revealed that (1) plastic marine debris are widely distributed in western Mediterranean sea, (2) plastic debris concentrations in the Mediterranean Sea are of the same order of magnitude than those measured in the North Atlantic or North Pacific subtropical gyres and (3) plastic debris concentrations are about the same order of magnitude than dry organic material, arguing in favor of the hypothesis of potential misinterpretation and ingestion of these particles by plankton feeders in the sea. Kukulka T, Proskurowski G, Moret-Ferguson S, Meyer D, Lavender Law K, 2012. Geophysical Research Letters, vol 39