## 4 Discussion
### Harbor-Pollution Monitoring and Model Simulations  
[PERSON] et al. [30] and [PERSON] et al. [5] provide a comprehensive overview of sea-bottom-monitoring methods, and [PERSON] et al. [31] define detailed requirements for monitoring that addresses trends in the amount of litter deposited on the seafloor as part of the European Marine Strategy Framework Directive. Presently, the official monitoring focusses on shallow coastal waters, marginal seas and the deep-sea floor. Our approach, focusing on sea floors in urban areas, addresses complementary areas and pollution hotspots.  
For shallow waters, underwater visual surveys by scuba divers are recommended and detailed protocols exist [31]. Presently, the scuba-diver method delivers the most reliable data for harbors and marinas, as well. Recently, it has been applied to detect lost fishing gear on harbor sediments in the Baltic Sea. This method is associated with high efforts and costs, which do not allow its integration into regular monitoring programs.  
Usually, monitoring is intended to provide an insight into the pollution process, meaning how much litter is deposited per area and time-unit. A combination of scuba divers and our method could meet this demand. Scuba divers could be used to clean harbor floor areas from litter. Afterwards, the ongoing pollution process on these areas couldbe observed with our method, and a combination with dredging methods could also be beneficial. However, our approach is certainly suitable as a screening method to get a first insight into pollution levels and major items. The results allow the definition of pollution reduction measures and can serve for assessing the effectiveness of these measures.  
For the model simulations, we used an advanced spatially high-resolved 3D flow model that is very well adapted to the Warnow Estuary [35]. In contrast, the assumption that a litter resuspension and bottom transportation takes place at a bottom shear stress of 8.25 \(\times\) 10\({}^{-7}\) Pa is a strong simplification. This does not take into account important factors, such as the different shape, size or density of litter particles. Therefore, the model simulations are only a first step towards a better understanding of bottom transport and have to be treated with caution. If we assume that what the model suggests is close to reality, we see that under common wind conditions, the resuspension and transport of litter above the sediment does not play an important role. During storms, a resuspension and reallocation of litter can take place even in sheltered harbor areas (e.g., Alter Stadthafen). Therefore, the observed litter concentrations above the sediment are likely to be affected by storms and represent an uncertain pollution time period. The low observed bottom-litter concentrations in city harbors suggest, that events, such as the Hanse Sail or the Kiel Week, are not important pollution events. This can be misleading because during storms litter reallocation and sediment cleaning may have taken place. All harbor-monitoring methods suffer from this uncertainty. The lack of comparable data and literature do not allow us to critically reflect on our results in more detail.