This study aimed to investigate macroinvertebrates biodiversity indices in Chalus River in Mazandaran province and the impact of spring floods on these indices during spring and summer in 2015. Environmental parameters including daily discharge data during 2014-2015, temperature, dissolved oxygen, nitrate, phosphate, pH, TDS and EC were measured in river branches and main channel of the river at nine stations. Benthic macroinvertebrates were collected by surber sampler at each station, then identified and counted. Results showed significant differences in biological indices of EPT, EPT/Chironomidae, ASPT, HFBI, Shannon-Wiener and Simpson diversity between two seasons (p<0.05). Among environmental parameters, the discharge level showed the most negative correlation with biological indices (p<0.05). Furthermore, the amount of nitrate and phosphate indicated the most positive and significant correlation with indices. Based on RDA results, parameters including temperature, oxygen, nitrate and pH in spring, and discharge level in summer were the most important affecting factors. Results of the present research showed that the spring floods affect directly or indirectly on environmental parameters such as DO and pH that can consequently  have influence on biological indices of benthic macroinvertebrate in Chalus River. Generally, steady changes in temperature, precipitation, and stream hydrology affect stream biota, but such effects may be less important compared to the impact of flood events.

Keywords: Macroinvertebrates, Chalus River, Spring floods

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