Microplastic (MP) in the environment is a topic of great global concern. As plastic production continues to increase, more and more plastic is being released into the environment. Plastic often ends up in the sea and can pose threats for various animal species, for instance through entanglement or ingestion of MPs. So far, MP research has focused on the marine environment. Little is known about rivers, although they are regarded as the transport media of MP and therefore represent an essential pathway connecting the mainly land-based sources of MP pollution and the sea as a sink. This thesis investigated the Lower Weser with regard to MP concentration, distribution of different polymer types as well as their size and shape. For this purpose, surface water samples were taken at twelve stations. Two samples of different size fractions (10 to 500 μm and 0.5 to 5 mm) were taken at each station aiming to achieve accurate results by applying size-adapted state-of-the-art methods such as Fourier transform infrared spectroscopy (FTIR). In preparation for the measurement of size fraction <500 μm, an enzymatic treatment was applied, which aims at the removal of organic matter, but is non-destructive to MPs. In size fraction >500 μm, low concentrations from 0.09 to 1 m-3 were found, whereas they were significantly higher with 145 to 2756 m-3 for size fraction <500 μm. The highest MP concentration was detected in Bremerhaven. A total of 24 different polymer types were identified, with polyethylene (PE) and polypropylene (PP) being predominant in size fraction >500 μm and varnish being predominantly present in size fraction <500 μm. In both size fractions, the trend can be observed that MPs mainly occur as very small-sized particles. Comparisons with other studies turned out to be difficult as there are no standardized and automatized methods concerning sampling, processing and identification. Standardized methods are highly recommended for future research enabling the comparability as well as objectivity of studies.