**Changing Isotopic Food Webs of Two Economically Important Fish in Mediterranean Coastal Lakes with Di** ff**erent Trophic Status**

**Simona Sporta Caputi 1, Giulio Careddu 1, Edoardo Calizza 1,2,\*, Federico Fiorentino 1, Deborah Maccapan 1, Loreto Rossi 1,2 and Maria Letizia Costantini 1,2**


Received: 22 February 2020; Accepted: 13 April 2020; Published: 16 April 2020

**Abstract:** Transitional waters are highly productive ecosystems, providing essential goods and services to the biosphere and human population. Human influence in coastal areas exposes these ecosystems to continuous internal and external disturbance. Nitrogen-loads can a ffect the composition of the resident community and the trophic relationships between and within species, including fish. Based on carbon (δ13C) and nitrogen (δ15N) stable isotope analyses of individuals, we explored the feeding behaviour of two ecologically and economically important omnivorous fish, the eel *Anguilla anguilla* and the seabream *Diplodus annularis*, in three neighbouring lakes characterised by di fferent trophic conditions. We found that *A. anguilla* showed greater generalism in the eutrophic lake due to the increased contribution of basal resources and invertebrates to its diet. By contrast, the diet of *D. annularis,* which was mainly based on invertebrate species, became more specialised, focusing especially on polychaetes. Our results sugges<sup>t</sup> that changes in macroinvertebrate and fish community composition, coupled with anthropogenic pressure, a ffect the trophic strategies of high trophic level consumers such as *A. anguilla* and *D. annularis*. Detailed food web descriptions based on the feeding choices of isotopic trophospecies (here Isotopic Trophic Units, ITUs) enable identification of the prey taxa crucial for the persistence of omnivorous fish stocks, thus providing useful information for their managemen<sup>t</sup> and habitat conservation.

**Keywords:** food webs; Mediterranean coastal lakes; nitrogen pollution; stable isotopes; trophic relationships; *Anguilla anguilla*; *Diplodus annularis*
