Microplastic content variation in water column: The observations employing a novel sampling tool in stratified Baltic Sea

A new tool was developed for microplastics (MPs) investigation in the water column. It can collect several cubic meters of water from predefined water layers down to 100 m. The tool was tested in the Baltic Sea during the period of spring thermocline formation. Strong MPs stratification was observed at all of the sampled stations. On coastal stations (~30 m deep), stratification with high fibers content was associated with the proximity of terrestrial sources and estuarine discharges, while on off-shore stations the variability of MPs was related to vertical thermohaline structure. Mean MPs content was the 32.2 (SD 50.4) pcs/m3. Elevated MPs concentrations were observed in subsurface, near-bottom and thermohaline layers compared with intermediate layers. The heterogeneity of MPs distribution suggests that MPs particles can be retained above the density-gradient layers in coastal seas and the World Ocean. •Vertical distribution of microplastics in the water column of the Baltic Sea is investigated.•A novel combined CTD + pumping system is employed.•Density gradients at thermo- and haloclines retain microplastics.•Elevated fibers concentrations were related to freshwater discharges and beaches.•Geotextiles from coastal structures are the sources of fibers in marine environment.