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In order to determine the ecological status of surface waters of different types in a particular ecoregion, the reference conditions for each type should be identified to compare with observed conditions, as suggested by the Water Framework Directive (WFD). In this study, 36 candidate reference river sites of the Sakarya Basin (Türkiye) were sampled three times in 2018, and the biological assessment of these sites was evaluated using 18 different diatom indices. In addition, nine different environmental variables were also sampled combined with epilithic diatoms. Based on the canonical correspondence analysis (CCA), alkalinity and pH were determined as the most significant environmental variables for diatom assemblages. Among the diatom indices, Biological Diatom Index (IBD), Artois‐Picardie Diatom Index (IDAP), Descy's Index (DES) and Sládeček's Index (SLA) showed significant correlations with alkalinity and pH according to multiple linear regression (MLR) results. IBD index showed the highest response to the pressures and indicated high or good ecological status of the sampling sites. Moreover, rivers with high water quality status have a high correlation with alkalinity, pH and pollution‐sensitive species. On the other hand, good water quality rivers were highly correlated with total phosphorus and intermediate pollution‐sensitive or pollution‐tolerant species. The 13 reference sites established in this study can be used to assess the ecological status by comparison with the same types of river bodies for further studies in the basins of Türkiye.

In this study, a water-soluble metal-free phthalocyanine (SPC) containing sodium 2-mercaptoethanesulfonate substituents at the peripheral positions was used to investigate the algaecidal properties and oxidative effects on the growth of two microalgal species, Arthrospira platensis and Chlorella vulgaris . Although OD at 560 nm and chlorophyll- a content were decreased in Arthrospira platensis during 7 days depending on dose and time, increases in both OD at 750 and chlorophyll- a content at 8 ppb (parts per billion) concentration on the 7th day were observed in Chlorella vulgaris . However, total SOD (superoxide dismutase) and GR (glutathione reductase) enzyme activity of A. platensis cultures did not display any alteration in all concentrations, SOD activity displayed an increase significantly at 2 ppb concentration, and GR activity showed increases at 1, 2, and 4 ppb concentrations in C. vulgaris application. In A. platensis application, APX (ascorbate peroxidase) activity decreased at 0.50 ppb, 1 ppb, and 1.5 ppb concentrations. In addition, C. vulgaris application showed decreases at all concentrations. When MDA content increased at all concentrations, the H 2 O 2 content increased only at significatly 0.125 ppb concentration in A. platensis cultures. Both MDA (malondialdehyde) and H 2 O 2 (hydrogen peroxide) content of C. vulgaris cultures showed a statistically significant decrease at all concentrations compared to control. Free proline decreased at 0.25 ppb, 0.50 ppb, 1 ppb, and 1.5 ppb concentrations in A. platensis application, and it decreased at all the concentrations of C. vulgaris application. It concluded that this compound has inhibition effects on A. platensis , but it supports growth in C. vulgaris . Therefore, this synthesized phthalocyanine compound (SPC) should be consumed carefully, and the contamination to aquatic ecosystems should be prevented.