Distribution of Microplastics in Marine Species in the Schleswig-Holstein Wadden Sea
Within the project, a total number of 12 species were studied at four sites along the Wadden Sea coast of Schleswig-Holstein. The species included demersal and pelagic fish species (Atlantic herring - Clupea harengus, viviparous eelpout - Zoarces viviparus, European plaice - Pleuronectes platessa and common sole - Solea solea) as well as benthic species represented by molluscs (soft-shell clam (sand gaper) - Mya arenaria), common cockle - Cerastoderma edule, blue mussel - Mytilus edulis, Pacific oyster - Magallana gigas, common periwinkle - Littorina littorea), polychaetes (lugworm - Arenicola marina) and arthropods (green shore crab - Carcinus maenas and brown shrimp - Crangon crangon). These were sampled twice, in October 2017 and May/June 2018, with 16 to 66 individuals and examined for microplastics.
After sampling, the benthic individuals were incubated for 24 h in filtered seawater to determine the amount of microplastic particles from faecal residues. After dissection of the tissue, the samples were prepared by destroying the biological organic material with a digestion solution of potassium hydroxide and sodium hypochlorite followed by filtration. Microplastic particles were identified by staining with Nile red and subsequent examination under a fluorescence microscope. A subset of the identified particles were also examined for their polymer type using μRaman spectroscopy.
The results show that all investigated species except common sole are affected by microplastic contamination. The proportion of affected individuals was 38.9% (demersal chordates), 42.3% (pelagic chordates) and 88.2% of all invertebrates. In invertebrates, microplastic particles were most frequently found in individuals of molluscs (96.6% of all individuals), followed by polychaetes (92.7%) and arthropods (64.8%).
With regard to the concentrations of microplastics (in particles per g net weight), the following median values were determined in decreasing order: Common periwinkle (5.50), lugworm (2.27), viviparous eelpout (1.72), common cockle (1.69), blue mussel (1.38), followed by Atlantic herring (0.95), Pacific oyster (0.86), green shore crab (0.45), soft-shell clam (0.40), brown shrimp (0.12) and European plaice (0.01).
The dominant type of microplastics was broken fragments of larger items, which also show a clearly different pattern than fibres in the frequency distribution of the particle sizes. The number of particles recorded increases steadily with decreasing particle size across all particle types. Considering this distribution in relation to fragments separated by species, it becomes evident that Atlantic herring in particular has a different distribution pattern. The median particle size for herring is 287 μm. In comparison to all other species, whose median values lie between 107–168 μm, a significant difference can be observed, which indicates an active uptake of particles of a certain size.
With regard to seasonal differences, the results are difficult to assess, especially due to varying weights of individual animals. Also with regard to the spatial distribution of the four locations List, Pellworm, Büsum and Friedrichskoog only weak tendencies of increasing concentrations with increasing geographical latitude can be determined. Significantly higher values are observed for the species soft-shell clam, green shore crab and blue mussel at the southernmost site of Friedrichskoog. The polymer types analyzed by μRaman spectroscopy on 113 particles are dominated by polyethylene, ethylene-vinyl acetate copolymer, polyethylene terephthalate and polypropylene, which together comprise over 80% of the polymer spectrum.
Based on the results, several species can be identified as suitable biological indicators for future monitoring programs. From the point of view of statistical representativeness, species with high individual weights such as soft-shell clam and lugworm should be considered in particular. However, other species could also be integrated, provided that a sufficient number of individuals are sampled and analyzed as composite samples and the background signals from environmental factors, in particular the mudflat sediment, are recorded.
