Effects of floods on macroinvertebrate communities in the Zarin Gol River of northern Iran: implications for water quality monitoring and biological assessment

Background The timing, magnitude, and duration of extreme hydrological disasters have the potential to threaten the species diversity and river habitats. On August 8, 2014, and August 19, 2017, disastrous floods struck mountainous regions of Iran's Zarin Gol River basin. Macroinvertebrate communities were studied at seven upstreams prior to the floods in June 2014 and 2017 and after the floods in September 2014 and 2017 and 9 months after the second flood in June 2018. The effects of floods on macroinvertebrate communities, recovery rate of macroinvertebrate community resilience and influencing factors were investigated. Results Despite the fact that extreme floods were the only reason of the disruption, the effects of biological water quality assessment metrics after the disaster were comparable to those of heavily polluted waters. Biological indicators revealed that the communities were unaffected prior to the floods, and the water quality remained within acceptable limits. Following the disasters, the density of macroinvertebrates declined, and biological indicators demonstrated the severe depletion of water quality. Community indicators (species richness, percent model affinity (PMA), and Ephemeroptera, Plecoptera, and Trichoptera (EPT) richness) rebounded after 9 months (June 2018), demonstrating that Zarin Gol River's macroinvertebrate communities could recover even after the severe impact of the floods. As a result, macroinvertebrate samples taken from flood-affected areas revealed that extreme flooding, rather than a decrease in water quality, causes a loss of diversity and abundance. Conclusion Because of shifts in hydrological regimes in streams around the world, understanding the short-term impacts of strong flooding and the comparatively quick recovery of macroinvertebrate ecosystems has important consequences for bio-assessment programs after severe floods.