The fate of marine microplastics in the swash zone.

This study provides the first directly comparable estimates of seawater contamination with microplastics (MPs, 0.3-5 mm) and mesoplastics (MePs, 5-25 mm) during the five sampling expeditions in November and December 2024 in the south-eastern part of the Baltic Sea. Bulk seawater sampling was carried out in two nearby locations simultaneously during different stages of storm activity on sandy and pebbly sea shores. The polymeric composition of the microparticles was identified using μ-Raman and FTIR spectroscopy. The maximum abundance of MPs in seawater near pebbly and sandy shores was 1360 and 1300 items/m3, respectively. The maximum content of MPs was 12,160 items/m3 in water samples with sand suspension near pebbly shores. The average abundance of MePs was 35.6 times higher during the storm compared to the post-storm period at both stations combined. Two key trends were observed during the post-storm period: (i) a decrease in average fiber length, and (ii) an increase in the ratio of MPs to MePs. This confirms that storms in swash zone lead to the fragmentation of large plastic particles into microplastics.