Microplastics accumulate to thin layers in the stratified Baltic Sea

Area of Science:

• Environmental Science
• Marine Biology
• Oceanography

Background:

• Water stratification in the Baltic Sea, caused by density and salinity differences, impedes vertical mixing.
• This stratification can influence the sinking rates and distribution of microplastics within the water column.

Purpose of the Study:

• To investigate the accumulation of microplastics at the halocline and thermocline layers in the Baltic Sea.
• To quantify microplastic concentrations in these specific stratified water layers.

Main Methods:

• Water samples were collected using a 100 μm plankton net for vertical transects and a 30 L water sampler at specific depths.
• Microplastics were analyzed using imaging Fourier transform infrared spectroscopy (FTIR).

Main Results:

• Microplastics were found to accumulate in thin layers within the halocline and thermocline.
• Average concentrations were 0.92 ± 0.61 MP m⁻³ (237 ± 277 ng/m⁻³) in the water column between layers and 0.44 ± 0.52 MP L⁻¹ (106 ± 209 ng/L⁻¹) at the boundaries.
• Polyethylene, polypropylene, and polyethylene terephthalate were the predominant microplastic types.

Conclusions:

• Microplastic accumulation occurs in stratified layers, specifically the halocline and thermocline.
• Biofilm formation may affect microplastic sinking rates, obscuring trends with particle density.
• Comprehensive microplastic assessment requires sampling deeper water layers, not just surface waters, in stratified aquatic environments.