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No. 320: Microplastic-like particles in seabed sediments from inner Danish waters 2015

Strand, J., Lundsteen, S. & Murphy, F. 2019. Microplastic-like particles in seabed sediments from inner Danish waters 2015. Aarhus University, DCE – Danish Centre for Environment and Energy, 22 pp. Scientific Report No. 320. http://dce2.au.dk/pub/SR320.pdf

Summary

The presence of microplastic-like particles in the size range of 20-5000 µm have been studied in sediment collected at twelve stations in the period October - December 2015 in the inner Danish waters covering areas in the Kattegat, Belt Sea, Sound and western Baltic Sea. The content and composition of microplastic-like particles in all samples with volume of 50 ml were determined only by visual identification using microscopy after the samples had been chemically digested to remove natural organic matter, density fractionated with saturated saline solution and size fractionated in six size classes using metal test sieves. Visually identified particles were stored for later chemical analysis by µFT-IR spectroscopy.

The study showed that particles identified as microplastic-like occurred in all twelve sediment samples. Out of all the identified microplastic-like particles 71–100 % were fibres, i.e. elongated filaments. Fibres were the dominant type of microplastic-like particles followed by films/fragments which contributed up to 29 % of the microplastic–like particles found. The contents of microplastic-like particles were in the range 202-3511 particles per kg dry weight and 300-1340 particles per litre wet sediment. In addition, another group of the particles was characterised as of “uncertain” origin, because they most likely consisted of remains from natural organic matter, but it could not be fully excluded that they consisted of synthetic polymers. The levels of microplastic-like particles in this study were within the range of microplastics in sediment found in two previous danish studies.

Microplastic-like particles in the size range of 20-300 µm dominated contributing 62-100 % of all plastic-like particles in the sediment samples. The smallest particles in the size fraction 20-38 µm contributed with 8-42 % of the particles identified as microplastic-like in the different samples. No particles with sizes > 1000 µm were found in the samples.

Regarding colour composition, blue microplastic-like particles were with 53 % on average the most frequent group, followed by transparent and black particles with 22 % and 18 % on average. Some particles with other colours e.g. white, red, grey, brown, green and violet were also identified as microplastic-like.

A relationship was found between microplastic-like particles and sediment contents of total organic carbon (TOC), supporting the findings of Strand et al. (2013), that microplastic-like particles in the inner Danish waters mainly are deposited in sedimentary areas together with natural organic matter and not so much in erosion zones, where sediments mainly contain more dense sand and silt particles.