VERTICAL DISTRIBUTION AND TRANSPORT OF TEXTILE MICROFIBERS (MFS) IN THE MEDITERRANEAN WATER COLUMN

Microfibers (MFs) are ubiquitous in natural environments and are considered as an anthropogenic litter that has become a global concern. Approximately 2 million tons of MFs are released into the ocean every year and once in the marine environment, it can be hypothesized that their accumulation and transport are largely controlled by oceanographic processes. We collected water column samples for MFs separation and polymeric characterization at 38 stations located in the Central-Western Mediterranean Sea and Tyrrhenian Sea from the surface to the seafloor (down to 3465 m). The aim of our study was to investigate MFs distribution in different water masses and the transport through different Mediterranean basins. MFs were detected in all samples collected with a median concentration of 3.8 MFs/L (IQR: 3.1 MFs/L) and a median fiber length and diameter of 1023 µm (IQR: 1007 µm) and 15 µm (IQR: 3 µm) respectively. Polymeric analysis highlighted cellulosic-based fibers as the dominant polymer (85%) in the whole water column over synthetic fibers (5%) and processed cellulose (8%), while fibers of animal origin were detected occasionally. MF concentration was locally characterized by vertical variations according to water mass characteristics, and by changes in size, with surface and deep fibers being generally longer and larger than mid-water fibers. Mediterranean surface waters showed homogeneous MF content (range 3.0-4.9 MFs/L), while intermediate and deep waters were characterized by two opposite trends. Intermediate waters showed decreasing MFs concentration moving from the Eastern to the Western Mediterranean basin (9.6-1.6 MFs/L) while MF content increased in deep waters at the same stations (3.7-9.1 MFs/L). Along the Sardinia Channel, we estimated a negative MF flux of 6 X 10 13 MF/day entering the Tyrrhenian Sea at intermediate depths, and a positive flux of 12 X 10 13 MF/day in the deep-water mass. We also estimate a 10-fold flux of synthetic fibers leaving the Tyrrhenian Sea, compared with the daily inflow. Our findings suggest that: i) there is a predominant MFs input on surface waters by atmospheric deposition; ii) MFs vertical distribution is size-dependent; iii) MFs transport between Mediterranean basins is mediated by Levantine waters; iv) a potential export of MFs to deep waters occurs in the Tyrrhenian Sea.