According to the European Union Water Framework Directive requirements, diatom metrics were used to assess the ecological status of surface waters in the Gaziantep central catchment (Turkey). A total of 42 diatom taxa were identified. A few environmental factors (especially lead, copper, orthophosphate, and chromium) played significant roles on the distribution of diatom assemblages among the sampling stations. The first two axes of the canonical correspondence analysis elucidated 91.6 % of the species–environment correlations with 13.9 % of the cumulative variance of species. The applied diatom indices (TIT – Trophic Index Turkey, TI – Trophic Index, and EPI-D – Eutrophication and/or Pollution Index-Diatom) showed different results in the evaluation of the ecological status of sampling stations. The results of TIT and TI indicate that Kıratlı creek has a poor ecological condition, while a good ecological condition was reported by the EPI-D index. This creek is associated with relatively high nutrient values (e.g., 91.1 µg L–1 P-PO4, 8.6 mg L–1 N-NO3, 0.59 mg L–1 N-NO2, and 97.5 mg L–1 SO4) and characterized by pollution-tolerant taxa (e.g., Amphora ovalis Cocconeis placentula Cyclotella meneghiniana Gomphonema parvulum Fragilaria biceps, and Navicula trivialis). The results of the Spearman correlation analysis reveal that TIT has a significant positive correlation with P-PO4 (p < 0.01, r = 0.780), conductivity (p < 0.01, r = 0.769), N-NO3 (p < 0.01, r = 0.714), and N-NO2 (p < 0.01, r = 0.778). Our results show that TIT is a suitable diatom metric to assess the ecological status of surface waters in the Gaziantep central catchment and for the Mediterranean region in general.
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