Epilithic diatoms distribution in four tributaries of the Gorganrud River (NE Iran)

Gorganrud is one of the most important river basins of northeast Iran in Golestan Province with various tributaries receiving different discharges. Due to the importance of diatoms in water quality assessments, this study was carried out to investigate general floristic survey and spatial distribution of diatoms in relation to water parameters in four tributaries of the Gorganrud River. Diatom samples were collected between 2018 and 2019 from stones by brushing with toothbrush. Diatoms and water sampling were done in monthly intervals. In total 73 taxa belonging to 30 genera were found. The most species rich genera were Gomphonema (9 spp.), Nitzschia (8 spp.) and Navicula and Surirella (each with 6 spp.). The most frequent and abundant taxa were Achnanthidium minutissimum, A. gracillimum, Cymbella excisa, Diatoma moniliformis, Gomphonema olivaceum, Nitzschia dissipata and Rhoicosphenia abbreviata. Results of the PERMANOVA test based on relative abundance of taxa only showed the significant differences between diatom composition of the Oghan with Zarringol and Chehelchay streams. Cluster analysis showed that the samples from the four streams were classified in the same groups. CCA revealed that TDS and pH are the main factors affected the diatom distribution in this study. In general, the studied streams were dominated by cosmopolitan taxa and the differences are related to the quantity of common taxa and the presence or absence of rare taxa.