Floating marine macro litter: Density reference values and monitoring protocol settings from coast to offshore. Results from the MEDSEALITTER project

Monitoring Floating Marine Macro Litter (FMML) is a global priority, stressed within international programs, and regulated for the European Seas by the Marine Strategy Framework Directive. Although some well-defined common protocols exist for the assessment of beach litter and ingested litter, methodologies for FMML monitoring still vary, leading to some inconsistent results and hampering the global assessment of this threat. Within the MEDSEALITTER project (2016–2019), field experiments were implemented to define optimal monitoring parameters for FMML visual monitoring at different spatial scales, by assessing the influence of platform speed, strip width, observers experience, weather conditions, and litter size on its detectability. Along with the results of these experiments, we present the FMML density ranges detected across the over 20,000 km surveyed, highlighting a decreasing gradient from river mouths to coastal areas and the open sea, and providing a valuable contribution to the assessment of FMML in the Mediterranean Sea. •Floating Marine Macro Litter is a main anthropogenic pressure on marine ecosystems.•Regulations plastic items are implemented worldwide and need standardized assessment.•Field experiments allowed to define optimal parameters for FMML visual monitoring.•Precise protocol settings and recommendations for small/large platforms are described.•FMML density ranges highlighted a decreasing gradient from river mouths to open sea.