## 1 Introduction  
Transitional waters are extremely complex ecosystems [1; 2; 3]. The Water Framework Directive of the European Communities (European Communities, 2000. Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000) defines them as \"superficial bodies of water near the mouths of rivers which have a partially saline character due to their proximity to coastal waters, but which are substantially influenced by freshwater flows\". Their high productivity provides habitats, refuge areas and food sources for a wide range of aquatic animals from resident brackish to freshwater and marine migratory species [4]. Transitional waters support important ecosystem services, including good water quality, fisheries, aquaculture and tourism, as well as agricultural activities in their watersheds [5]. Anthropic activities expose these ecosystems to continuous internal and external disturbance [6; 7; 8; 2], including nitrogen (N) pollution arising from agricultural and urban activities, which poses potential threats to biodiversity and ecosystem functioning [3; 9; 10]. In addition, an increase in N-loads can significantly compromise water quality, promoting the development of micro and macroalgal blooms [11; 12]. This, in turn, could alter the species composition and feeding behaviour of the aquatic animal community, from primary consumers to top predators. Changes in the availability and quality of basal food sources can affect the distribution of organisms and the feeding links between trophic levels, with effects on the stability and structure of the entire food chain [2; 12; 13]. Increased N-loads could thus also compromise, either directly or indirectly, the persistence of ecologically and economically important fish species [14]. In the Mediterranean area, the European eel, _Anguilla anguilla_ ([PERSON], 1758), and the annular seabream, _Diplodus annularis_ ([PERSON], 1758), are widespread and among the most important fishery resources [15; 16]. However, in the last two decades, European eel populations have collapsed due to low recruitment and habitat alteration, and the species has been classified as 'critically endangered' since 2014, according to the International Union for Conservation of Nature [17]. It is known that both fish species are generally characterised by a high degree of omnivory and trophic plasticity depending on the composition and abundance of the available prey [18; 19]. Specifically, the annular seabream, _Diplodus annularis_, is a demersal omnivorous species, feeding opportunistically on a wide variety of prey including zoobenths, algae and plants. The European _Anguilla anguilla_ is a generalist predator feeding mainly on invertebrates and fish but it also exhibits scavenger behaviour, feeding on dead animals including fish. These trophic traits can be expressed differently by individuals within the population [18; 20; 21]. Due to their omnivory, the trophic strategies of these species can directly reflect variations in the inputs determining the trophic status of the waters and thus the quality and availability of potential prey. Thus, understanding the patterns underlying the trophic choices of these fish species and their associated food webs is crucial for ecosystem management and the conservation of their habitats. Several studies have been carried out on the diet of eels and seabream, often based on gut content analysis [19; 22; 23; 24; 25]. However, gut content analysis provides only a snapshot of a consumer diet, which is assumed to vary over time [7; 26; 27]. Furthermore, individuals often have no recognisable prey in their stomach, and description of the trophic links between species thus requires large samples [28]. Carbon (\(\delta^{13}\)C) and nitrogen (\(\delta^{15}\)N) stable isotope analysis is increasingly becoming useful tool for detecting organic and inorganic matter sources and understanding species' foraging behaviour and the relationships between organisms. It is thus useful for reconstructing food webs in aquatic ecosystems [7; 29; 30; 31].