This study, the first such attempt in Turkey, aimed to investigate the distribution of diatom species related to the variation of total phosphorus (TP) in eight basins to determine trophic weight and indicator values of diatom species and to develop the Trophic Index-Turkey (TIT) for assessing the ecological status of freshwaters. The distribution of the diatom species in 225 running water bodies from 8 basins of Turkey was determined in 2014 (summer and fall) and 2015 (spring and summer) in association with TP variation. Trophic values for 219 species were determined during this study and many of these are presented for the first time. Optima and tolerance TP values were calculated for each species in the different aquatic ecosystems using weighted average regression. Diatom species such as Clevamphora ovalis, Caloneis amphisbaena, Gyrosigma acuminatum, Navicula tripunctata, and Nitzschia umbonata showed high trophic weight values, while Achnanthidium minutissimum, Cymbella excisa, Didymosphenia geminata, Hannaea arcus, and Meridion circulare had low trophic weight values. These findings provide supplementary information about the biotic integrity of the water bodies and could be used to develop the TIT for the bioassessment of freshwater ecosystems, especially running water bodies.

Diatoms are an important part of aquatic biodiversity and the main component of phytobenthos. They play a key role in aquatic ecosystems and indicate water quality. The European Union Water Framework Directive requires that phytobenthos be used for the ecological quality assessment of water. This study evaluated the ecological status of various watercourses in the western Mediterranean basin of Turkey using a multivariate approach and presents a new trophic index Turkey (TIT) based on diatom assemblages. Twenty-five running water bodies were seasonally monitored for biological and physicochemical analyses from summer 2014 to summer 2015. A total of 102 species belonging to 22 genera were recorded. Cymbella excisa Ku¨tzing, Gomphonella parvula (Ku¨tzing) Rabenhorst, Ulnaria ulna (Nitzsch) Compe`re, and Cocconeis communis f. placentula (Ehrenberg) Chmielevski were the most commonly found species. A canonical correspondence analysis was used to examine the relationship between species and environmental factors. The most effective explanatory factors, including nitrate nitrogen, electrical conductivity, altitude, total nitrogen, orthophosphate, and calcium carbonate significantly influenced the ecological preferences of diatom species in the ecosystems. TIT values ranged between 1.53 in Kaya creek and 2.73 in Dalaman stream (A8). Ecological status of water bodies was assessed using an ecological quality ratio based on trophic index Turkey (EQR-TIT) for each of the stations. EQR-TIT ranged from 0.44 in Dalaman stream (A8) to 0.99 in Kocabu¨k creek during the study period. TIT had a high correlation coefficient (R 2 = 0.77) with log total phosphorus and may be an appropriate diatom metric to assess the ecological status of water bodies.

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