Flood effects on macrobenthic communities in the lowest zone of Gharesou River – Golestan Province

This study examines the impact of a flood event on macrobenthic biodiversity indices in the lowest zone of the Gharesou River, which flows into the Gorgan Gulf, in the southeastern Caspian Sea. Samples were collected after short summer drought in six surveys, from October 2018 to December 2019, with three surveys before and three surveys after the flood event of March 2019. Regarding the spatial distribution, the samples were collected at four sites (stations), with two replicates at each site. Eight types of macrobenthic taxa at the levels phylum (Nematoda), class (Ostracoda), family (Discorbidae, Chironomidae, Lumbriculidae, Naididae, and Tubificidea) and genus (Streblospio sp.) and two types of zooplankton, i.e. Cyclops sp. and Daphnia sp., were identified. The abundance of macrobenthic taxa increased gradually in the post-draught period, while it reduced and reached the lowest point in the summer after the flood event (August 2019). It was also observed that the flood significantly increased the organic matter in the riverbed (p < 0.05). The study of Gharesou River over post-drought and post-flood situations indicated that flood reduced the abundance of macrobenthic taxa, while macrobenthic taxa over post-drought had a higher recovery rate than that over post-flood periods. A potential explanation for this observation is that benthic taxa were more tolerant to short summer drought than extreme floods. In general, a steady and continuous flow could prevent undesired fluctuations in diversity indices.