Microplastics in the Hamburg port area—an analysis of sediment depth profiles along the upper Elbe river, Germany

Harbours pose a unique environment of increased anthropogenic pressure and artificial river morphology that are of specific interest concerning microplastic release and accumulation. To address the specific situation in the Hamburg port area, a study in cooperation with the Hamburg Port Authority (HPA) was conducted. Sediment samples at different depth levels were taken at seven sites with similar flow velocity and underlying morphology. Two sites are located upstream and downstream of the port of Hamburg, while four sites are located in the centre area of the port. One additional site takes into account an estuary of a receiving stream flowing into the upper river Elbe. For the analysis of microplastic concentrations in the samples, the biogenic organic matter was removed by oxidative digestion followed by wet sieving and density separation. For identification, the Nile red staining method in combination with fluorescence microscopy was applied. A subset of identified synthetic polymers was investigated for polymer composition via µRaman spectroscopy. In addition, sediment parameters such as grain size distribution, organic matter and water content were analysed. In total, a number of 31 sediment samples divided into different depths below riverbed level were examined. In brief, 11,280 microplastic particles could be identified. Both the highest and lowest number of particles were detected at centre port sites ranging from 60 to 21,799 microplastics per kilogramme dry weight. Fragments are the dominating particle morphology throughout all locations, except for one centre harbour site where microbeads are most common. Frequently appearing synthetic polymers were detected to be polyvinyl chloride (34%) and polyethylene terephthalate (28%). Within this study, a significant correlation between microbead concentrations and the percentage of sand fractions (coarse, middle and fine sand) was detected.