Biofilm growth on plastics in rivers and in the ocean plays a key role for the understanding of the location of plastic particles in the water column. Most studies focus on microbiology and do not address parameters such as biofilm thickness as a function of time as well as biofilm density, which are relevant input parameters to model the transport of plastic particles. Therefore, we have conducted own experiments on biofilm samples taken from plastics in the river Warnow during different seasons of the year to characterize the biofilm growth and the related weathering of the polymer samples. Here, we will present results of these experiments according to the temporal evolution of the biofilm properties under real conditions using physical measurements like the wettability and spectroscopic properties. We will also demonstrate the impact of environmental conditions like solar radiation and temperature on the biofouling growth and what implications from this for the density of the plastic particles exist.