## 1 Introduction  
Anthropogenic inputs of nitrogen (N) and phosphorus (P) to coastal ecosystems became the primary cause of eutrophication and subsequent coastal ecosystem degradation worldwide, reducing biodiversity and the potential of marine ecosystems to provide valuable ecosystem goods and services [1, 2, 3]. However, while marine restoration actions are common, their success rates are highly variable, especially with continuous eutrophication [4]. Various policy frameworks like the European Water Framework directive (WFD) (WFD, 2000/60/EC) and Marine Strategy Framework Directive (MSFD) (MSFD, 2008/56/EC) aim to hinder its progress and to return to a good ecological status. The WFD marks a decisive turn in European water policy, management practices, and restoration trends. After 20 years of implementation in two management cycles, the WFD had been successful in setting up a governance framework for integrated water management for more than 110,000 water bodies in the EU, including transitional and coastal waters. However, as of today, WFD implementation has shown only limited success, especially in coastal waters of the Baltic Sea in regard to counteracting eutrophication and returning to a good ecological status [5]. This can be attributed to several reasons. First, land-basedmeasures are difficult to implement, since the major nutrient input emerges from diffuse sources like agriculture or from the atmosphere [6]. In addition, water quality improvements are delayed by decades due to leaching from \"legacy sources\" in soils, sediments, and groundwater [7]. Furthermore, despite management plans, eutrophication in most European coastal waters is expected to increase in the upcoming decades due to human population growth and intensified agriculture practice [8; 9]. Within coastal waters, internal mitigation measures, also known as \"water-based\", \"sea-based\", or \"end-of-pipe\" solutions, have come into focus to actively support restoration efforts [8; 10; 11]. As a consequence, the eight European Baltic Sea littoral states signed a minister declaration to intensify measures toward the good ecological status and good environmental status of the Baltic Sea, which includes the promotion of sustainable sea-based measures [12]. Recommendations for sea-based measures to combat eutrophication already exist, and measures can be differentiated into hydrological engineering (mechanical measures) (HE), geoengineering (chemical measures) (GG), and ecological engineering (EE) ([PERSON] and [PERSON], 2018) [9]. In particular, ecological engineering has gained a lot of attention during recent years. The cultivation of filter feeders like the blue mussel (_Mytilus_ spp.) as a mitigation measure has been highly discussed and is partly practiced already [13; 14; 15; 16; 17]. Longline mussel cultivation can be a sustainable method for producing highly valuable proteins for human consumption or animal stock and simultaneously function as a nutrient retention measure to relieve coastal waters of surplus nutrients [14; 18; 19]. However, blue mussel cultivation also has its limitations based on its physiological needs (e.g., salinity, temperature, and Chl. a concentration) and can also be associated with negative environmental impacts (e.g., increased bio deposition) [20; 21]. Another example for an ecological engineering approach is floating wetlands. Floating wetlands are artificial islands planted with emergent macrophytes to remove excess nutrients from the water via the root system through incorporation into plant biomass and bacterial denitrification [22; 23]. Nevertheless, these internal measures are hardly considered within the scope of the implementation of the WFD and are therefore not listed in the recent \"Program of Measures\" (POM) for the German coastal waters. The introduction and implementation of internal measures is often hampered, because of several concerns and a lack of information.