## 4 Discussion
### Harbor Pollution--State, Causes and Management  
All harbors in and around Kiel and Rostock have sea-floor-litter concentrations below 0.2 particles/m\({}^{2}\). Marinas in semi-urban surroundings of the cities show concentrations below 0.1 particles/m\({}^{2}\). The only exception is the relatively small Ludewigsbecken in Rostock, which shows higher pollution (1.75 particles/m\({}^{2}\)). It is likely that this higher concentration is still the result of recent reconstruction work. Centuries of human activities in and around the city harbors of Rostock and Kiel, such as frequent social activities as well as large-scale harbor events such as the Hanse Sail and the Kiel Week, with millions of visitors annually, do not seem to significantly increase the observed harbor-floor litter pollution. In all these harbors, bottom-cleaning activities are unknown and certainly do not take place regularly. Our monitoring method certainly underestimates the present pollution, as only recently deposited items have been identified. In 2019, Tunisia counted 9.4 million tourist arrivals [45]. Tourism focusses on several coastal resorts and the season is between June and September. The three Tunisian harbors of Yasmine Hammamet, Monastir and Bizerte, are tourism hotspots throughout the summer season. These intensive human activities are reflected in the observed sea-floor pollution which is about seven times higher than German harbors. In city harbors, such as Bizerte and Montazah, the concentration exceeds 3 particles/m\({}^{2}\) sea floor. In Tunisian touristic harbors, measures to reduce emissions are implemented, but especially in cities, deficits in waste management and an insufficient public awareness of litter problems still seems to exist. Our concentrations observed in marinas are in the range of 100 to far above 1000 times higher than those of macro-litter studies in Baltic and Mediterranean coastal sea-floors [10; 46; 47]. An overview by [PERSON] et al. [48] shows areas, such as the Gulf of Aqaba in the Red Sea, with bottom-litter concentration up to 2.8 pieces/m\({}^{2}\). However, our results indicate that harbor-bottom pollution is a problem especially in the African city harbors. In the cities of Kiel and Rostock in Germany an improved waste-management system has been implemented in recent years. This especially affected the harbor events (Hanse Sail, Kiel Week). It includes a deposit system for cups, the mandatory use of degradable tableware, free waste deposit containers for all ships, reusable fence fasteners, low-emission fireworks, nightly ground cleaning and an optimized waste-bin distribution and emptying system [49]. These activities are largely a result of the European Union's ban on single-use plastic items such as plastic plates, cutlery, straws, balloon sticks and cotton budswhich came into force in 2021 [50]. Further, the Kiel Week received the Platinum Level Certification of the Clean Regatta Program. The 20 criteria include the elimination of single-use items (e.g., elimination of single-use water bottles, plastic straws, bags, dinnerware, water-refill stations) and a responsible waste management (e.g., green team, proper waste-bin placement and paperless signage for event management). The Kiel Week aims at a sustainable events certification according to ISO 20121 [51] and Rostock is following a similar pathway. In Germany, already since 2003, a deposit for plastic bottles exists, and it was expanded in 2021. Before, only 1 L bottles were charged with a 0.25 EUR deposit, but today, a deposit of at least 0.15 EUR exists for the vast majority of all plastic bottles. From 2022, certain types of plastic bags became prohibited and customers have to pay for most thicker plastic bags. These laws may explain the lack of plastic bottles and bags on sea floors in Germany. Our results suggest that the existing waste management systems and the legal frameworks are suitable to reduce emissions and reduce harbor-bottom litter pollution.