## 4 Discussion  
Our results indicate that anthropogenic inputs affected the composition and abundance of the lake animal community. Specifically, the diversity of fish decreased, and trophic choices of the eel _Anguilla anguilla_ and the seabream _Diplodus annularis_ changed, with increasing eutrophication. _A. anguilla_, which was more piscivorous than the seabream at low and intermediate eutrophication, increased its consumption of invertebrates and basal resources. On the other hand, the seabream, which fed more on invertebrates, increased its preference for polychaetes. The high selectivity for polychaetes in the highly eutrophicated lake could be due to the facilitated capture of these preys and their good contribution to the energy supply of their predators compared to other aquatic invertebrates [66; 67; 68]. Changes in the feeding choices of the two fish species resulted in increased interspecific niche overlap, suggesting that eutrophication may have strong bottom-up effects on interspecific interactions [69; 70]. Previous research demonstrates that the \(\delta^{15}\)N values of the aquatic biota reflect anthropogenic nitrogen inputs from the surrounding terrestrial areas [71; 72; 10; 71]. In our study, increased eutrophication was associated with higher \(\delta^{15}\)N in the fish community, as previously observed by [PERSON] et al. [2] and [PERSON] et al. [3] for invertebrates and primary producers respectively. Our results indicate that in transitional waters, individual fish \(\delta^{15}\)N could be a useful indicator of anthropogenic N transfer along food webs [10; 73], while the range of \(\delta^{13}\)C in the fish population could reflect the diversity of C inputs, emphasising the need for ecological monitoring in these productive ecosystems. The distinct carbon isotopic signatures (\(\delta^{13}\)C) of primary producers (e.g., terrestrial vs. aquatic vegetation) allowed us to discern the origin of the organic matter contributing to the nutrient pool of water bodies [7; 33; 34; 74]. Specifically, depleted \(\delta^{13}\)C organic matter values indicated the contribution to the organic matter pool of allochthonous (terrestrial) carbon, while enriched \(\delta^{13}\)C indicated autochthonous primary production, as also observed in other aquatic ecosystems [7; 34; 35; 74]. Although it was not in the remit of this study to investigate the cause, carbon enrichment was also visible in pelagic and strictly specialist species such as the sand smelt, _A. boyeri_. In addition, neither the mean nor the variance of \(\delta^{13}\)C of primary producers differed significantly between sites within the same lake or between different lakes. This allowed us to exclude possible interferences in the isotopic baseline arising from environmental parameters such as the salinity and oxygenation levels of the respective area. The larger contribution of terrestrial organic matter in HP may be due to the large input of fresh water that this lake receives from the hinterland, as indicated by the low salinity generally observed in this lake compared to the other two. These results are consistent with what has been observed in similar environments [7; 74; 75]. Large Carbon Ranges suggest multiple carbon sources at the base of the food web [7; 35; 62; 63; 64]. The supply of organic matter from multiple sources in the three lakes was evident in the \(\delta^{13}\)C values of _A. anguilla_, which shifted from marine to terrestrial input with the increasing pollution. Its larger Carbon Range in the eutrophic lake indicates that this species integrated both autochthonous and allochthonous carbon pathways, while in the unpolluted lake it relied mainly on the autochthonous one. It is acknowledged that increased N-loads promote significant changes in aquatic productivity [76] that could potentially affect the composition of the prey community [77; 78]. This in turn might be reflected in the feeding behaviour of consumers at all trophic levels [2; 7; 79].