**1. Introduction**

Transitional waters are extremely complex ecosystems [1–3]. 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 [2,6–8], 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 a ffect the distribution of organisms and the feeding links between trophic levels, with e ffects 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* (Linnaeus, 1758), and the annular seabream, *Diplodus annularis* (Linnaeus, 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 zoobenthos, 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 di fferently 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 managemen<sup>t</sup> and the conservation of their habitats.

Several studies have been carried out on the diet of eels and seabream, often based on gu<sup>t</sup> content analysis [19,22–25]. However, gu<sup>t</sup> 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 (δ13C) and nitrogen (δ15N) 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 [7,29–31]. The isotopic ratio of these elements in consumer tissues reflects that of the assimilated food sources in a predictable way [7,32]. δ13C signatures vary considerably among primary producers, generally with lower values in marine than terrestrial aquatic vegetation. This makes it possible to disentangle the contribution of various basal sources to food networks [7,31,33–36]. The δ15N values gradually increase with each trophic level, thus providing information on the position of organisms in the food web [31,37,38]. In parallel, the δ15N values of primary producers reflect the nature (organic or inorganic) and the source of nitrogen inputs (natural or anthropogenic) in a predictable way. δ15N is thus also useful for tracking anthropogenic N pollution in water bodies and across trophic levels in food webs [11,39–41].

The main purpose of this study was to describe and analyse the diets and food webs of the eel *Anguilla anguilla* and the annular seabream *Diplodus annularis* in three neighbouring Mediterranean coastal lakes characterised by di fferent eutrophication levels. It is known that energy flows and the transfer of nutrients depend primarily on the foraging choices of each organism within the community [31]. Similarly, the high trophic generalism and omnivory generally observed in *A. anguilla* and *D. annularis* [19,24,25,42,43] can be the result of di fferent foraging strategies adopted by each individual within their respective populations.

In order to obtain highly detailed information and to consider variability in the use of resources by *A. anguilla* and *D. annularis*, the diet of the two species was obtained from trophic links of each individual within a population as determined by means of the Isotopic Trophic Unit (ITU) approach [31]. Isotopic Trophic Units are defined as groups of individuals with similar isotopic signatures occupying the same position in the δ13C-δ15N niche space [31].

We studied the diet of each population in detail without excluding a priori any food source in the area. We hypothesised that di fferences in invertebrate and fish assemblages across lakes with di ffering trophic status could determine patterns of variation in the trophic niche width of the two fish species depending on their specific feeding habits. Specifically, we sought to verify whether a lower abundance and diversity of species at higher trophic levels caused *A. anguilla* to become more generalist and *D. annularis* to become more specialized.
