## 3 Results  
### Community Composition and Isotopic Signatures  
A total of 8752 samples comprising basal resources, invertebrates and fish were collected from the three lakes, 8645 (148 taxa) of which were invertebrates and fish (Table 1, Table S1). Malacostraca (Amphipoda, Decapoda and Isopoda), Gastropoda, Anthozoa, Bivalvia, Polychaeta and Ophiuroidea together made up 93.09 \(\pm\) 3.86 % of invertebrates. Invertebrate abundance was lower in IP than the other two lakes (Table 1, paired-\(\chi^{2}\) test, \(\chi^{2}\) at least 20.46, \(p\)-value always \(<\)0.0001, Table S2). The composition of both the invertebrate and fish community also varied (contingency table, \(\chi^{2}\) at least 170.2, \(p\) always \(<\)0.001, Cramer's V at least 0.46, Tables S2 and S3). The abundance of some taxa, such as Decapoda and Anthozoa, decreased, while that of others (such as Amphipoda) increased with the pollution level of the lake (paired-\(\chi^{2}\) test, \(\chi^{2}\) at least 48.36, \(p\)-value always \(<\)0.0001). The number of fish taxa varied, i.e., 23 in LP, 17 in IP and 7 in HP. The relative abundance of fish differed between lakes (paired-\(\chi^{2}\) test, \(\chi^{2}\) at least 34.21 \(p\)-value always \(<\)0.0001) and was lowest in HP. The standard length of _Anguilla anguilla_ was lower in HP (\(35.10\pm 3.63\) cm) than the other two lakes (\(48.81\pm 6.07\) cm in LP, \(47.66\pm 2.12\) cm in IP) (Mann-Whitney test with Bonferroni correction in cases of multiple comparisons, U = 13.0, \(p\)-value always \(<\)0.05). Similarly, _Diplodus annularis_ had an average standard length of \(6.25\pm 0.14\) cm in HP, which was lower than LP(8.76 \(\pm\) 0.31 cm) and IP (9.43 \(\pm\) 0.41 cm) (Mann-Whitney test with Bonferroni correction in cases of multiple comparisons, U = 2.0, \(p\)-value \(<\)0.05). Among the basal resources, detritus showed depleted \(\delta^{13}\)C values, while primary producers were \(\delta^{13}\)C-enriched (Figure 2). \(\delta^{13}\)C-enrichment was also observed in pelagic fish with specialist diets such as _Atherina boyeri_ (\(\delta^{13}\)C = \(-\)15.34 \(\pm\) 0.08%o in LP, \(-\)17.45 \(\pm\) 1.35%o in IP and \(-\)16.14 \(\pm\) 0.90%o in HP). Since neither the mean nor the variance (\(\sigma^{2}\)) of \(\delta^{13}\)C in the primary producers differed significantly either within each lake or between lakes (one-way ANOVA and associated [PERSON]'s test for homogeneity of variances, F at least 0.1412, \(p\)-value always \(>\)0.05), we concluded that the presence of a salinity gradient within a lake could not have an effect on the isotopic variability of the baseline. \(\delta^{15}\)N values of primary producers increased with eutrophication (one-way ANOVA, F: 5.80, \(p<\)0.01). The isotopic differences observed in basal resources reflected those observed in the whole community (Table 1, Figure 3; Kruskal-Wallis, Hc at least 127.1, \(p\)-value \(<\) 0.001; for \(\delta^{13}\)C Mann-Whitney with Bonferroni correction for multiple comparisons, U at least 45,590, \(p\)-value always \(<\)0.001 and for \(\delta^{15}\)N Mann-Whitney, U at least 421.57 with Bonferroni correction for multiple comparisons, \(p\)-value always \(<\)0.001, Figure 3) and in _A. anguilla_ and _D. annularis_. The \(\delta^{13}\)C and \(\delta^{15}\)N isotopic signatures of _Anguilla anguilla_ and _Diplodus annularis_ differed between lakes (Kruskal-Wallis, Hc at least 35.5, \(p\)-value \(<\)0.001). \(\delta^{13}\)C values were higher in the least polluted lake (Table 2; Mann-Whitney with Bonferroni correction for multiple comparisons, U at least 3392.5, \(p\)-value always \(<\)0.001), while \(\delta^{15}\)N values increased with eutrophication (Table 2; Mann-Whitney with Bonferroni correction for multiple comparisons, U at least 555, \(p\)-value always \(<\)0.001). Specifically, in _A. anguilla_, the more generalist of the two species, \(\delta^{13}\)C values reflected the shift of inputs from marine to terrestrial origin passing from the least to the most eutrophic lake (Figure 2 and Tables 1 and 2).