Effect of seasonal variability on the development and application of a novel Multimetric Index based on benthic macroinvertebrate communities – A case study from streams in the Karun river basin (Iran)

Using multimetric indices (MMIs) for monitoring the ecosystem health of rivers is common practice. Increasing the applicability of MMIs needs understanding of the impacts of natural spatial and temporal variability on community structure in relation to effects of anthropogenic pressure gradients. In this study, we tested the impact of seasonal variability in environmental conditions and macroinvertebrate community structure on the development and application of MMIs using streams in the Karun river basin as a case study. We sampled and measured physical and chemical variables, physical habitat characteristics, and macroinvertebrate communities from 53 stream sites during four seasons. We defined pressure gradients for each season and also by combining all seasons’ data. Then, we assessed the applicability of three strategies recommended for tackling the effect of seasonal variability on MMIs development and application: (i) by developing seasonal MMIs (MMIspr,11 MMIspr: Multimetric index for spring; MMIsum,22 MMIsum: Multimetric index for summer; MMIaut,33 MMIaut: Multimetric index for autumn; MMIwin44 MMIwin: Multimetric index for Winter;), (ii) by aggregating data across seasons (MMIcomb55 MMIcomb: Multimetric index for whole year;), and (iii) by using the signal-to-noise ratio (MMIS/N66 MMIS/N: Multimetric index based on S/N ratio.). Our results showed that high seasonal variability in the streams of the Karun river could affect the MMIs’ core metrics composition and performance. Developing seasonal MMIs was the best strategy to tackle the seasonal variability as indicated by highest precision of MMIs, responsiveness to anthropogenic pressure gradient and sensitivity to impairment. Based on the results of this study, seasons with base-flow (summer and autumn) were most suited for the development of MMIs. The main reason might be the stability in natural drivers (e.g., stable river flows) shaping macroinvertebrate community structure during these seasons. In general, our results showed the MMIs developed based on macroinvertebrate communities could efficiently discriminate reference sites from the impacted sites and, therefore, can be used as a valuable tool for bioassessment in the streams of the Karun river basin and similar systems