## 1 Introduction  
It is estimated that 250,000 metric tons of plastic float in the seas and that macro- and meso-plastic (>5 mm) has a weight share of 86% in it [1]. Marine macro-plastics (>25 mm) can be distinguished into floating and sinking material, depending on their density. Bio-fouling with marine biota increases the density of plastics and within weeks turns floating into sinking plastic [2]. Floating material can be transported by wind and currents over long distances [3], but a large share is accumulated at beaches in the surroundings of emission pathways from surface-wave activity [4]. Sinking plastics can be accumulated on the sea bottom, especially in sheltered pays or in the deep sea. [PERSON] et al. [5] provide a comprehensive overview of the present global state. However, large shares of plastics that accumulate on sandy coastal sea bottoms are subject to wave-induced resuspension and in the end largely end up at the coastline, as well [6]. The accumulation of amber at beaches visualizes this process [7]. Altogether, near the coast, beaches and sheltered coastal areas, such as bays and harbors/ports, are likely hotspots for plastic accumulation [8; 9]. The emitted quantities of macro-plastics are correlated to human activities, population densities and modified by human behavior and the effectiveness of waste-management systems. Large rivers, coastal urban areas as well as untreated sewage and stormwater are considered to be major pathways for macro-plastic to the marine environment [10]. Additionally, intensively used touristic beaches can also serve as a source for macro-plastic in the marine environment and not only as a sink [10]. Human activities, especially in and around urban coastal areas, have to be regarded as of highest importance for marine plastic emissions [11]. In most parts of the Baltic Sea, coastal city centers do not host industrial ports any more, but the city seasides have been transformed into leisure and recreation areas, hosting leisure-boat marinas and offering water access for locals and tourists. Further, during the last decades, the Baltic Sea became an inner European Union sea. This development fueled water-related tourism, and the number of sport-boat ports increased. Additionally, many city harbors developed additional infrastructures to host the increasing number of cruise ships and ferry lines as well as the associated tourism. In 2017, over 5 million passengers visited the Baltic ports onboard cruise ships. The number of calls was about 2500 cruise ships and is still increasing [12]. Additionally, in 2023, 86 regular ferry routes existed in the Baltic Sea [13], and in 2020 about 67 million passengers embarked and disembarked from the over 200 larger commercial Baltic Sea ports [14]. Around the Baltic Sea about 95 coastal cities are located as well as many smaller seaside resorts, and the vast majority have a city port and host marinas. This number underlines the increasing potential importance of city- and sport-boat ports as plastic-emission and pollution hotspots in the Baltic Sea region, which is similar to other regions [15]. In the recent assessment of the state of the Baltic Sea, marinas and leisure harbors are considered as a pressure [16], and marine litter is taken into account as an indicator. However, reliable data on litter (plastic) sea-floor pollution in harbors are scarce [17]. From a fishing port in Turkey, over 300 kg of seabed litter was extracted in 2017 [18]. In 2020, in Newport harbor (USA) scuba divers collected over 2000 kg of marine litter [19]. In 2022, in Mulhein harbor (Germany) three divers collected 200 kg of marine litter within 2 h [20]. From the city port of Rostock (Germany), 550 kg of marine litter was removed during one day in 2022 [21]. These data indicate that a largely unknown pollution problem in harbors seems to exist. In Alexandria, Egypt, [PERSON] et al. [22] describe high microplastic concentrations at beaches and floating near the water surface in harbors.