Screening for microplastic particles in plankton samples: How to integrate marine litter assessment into existing monitoring programs?

• Monitoring samples collected for zooplankton analysis can be used for MP counts.
• Depth-resolved samples reveal vertical heterogeneity in MP distribution.
• High copepod stocks (and high grazing) coincide with increased MP at depth.
• The vertical distribution of MPs varies seasonally.
• No clear differences between coastal and open sea areas in the Baltic Sea.

Microplastics (MPs) are a newly recognized type of environmental pollution in aquatic systems; however no monitoring of these contaminants is conducted, mostly due to the lack of routine quantification. In the net samples collected with a 90-μm WP2 net, pelagic MP abundance was quantified by light microscopy and evaluated as a function of inshore–offshore gradient, depth, and season; the same samples were used for zooplankton analysis. The MP abundance was ∼102–104 particles m−3, with no significant inshore–offshore gradient during summer but increasing offshore in winter. MP abundance in deeper layers was positively affected by zooplankton abundance in the upper layers and significantly lower during winter compared to summer. These findings indicate heterogeneity of MP distribution due to biotic and abiotic factors and suggest that samples collected for other purposes can be used for quantification of MPs in the Baltic Sea, thus facilitating integration of MP assessment into existing monitoring schemes.

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