Evaluation of non-indigenous biological assessment tools using benthic macroinvertebrates in a regulated river in the semi-arid region of Iran

Benthic macroinvertebrates are widely used as key indicators in the biomonitoring of aquatic ecosystems, particularly in fragile ecosystems downstream of dams. Nevertheless, the use of non-indigenous biological tools requires regional evaluation, especially for developing countries and rivers with regulated flow in semi-arid regions. The Zayandehrud River is a perennial river in central Iran. Since 1953, it has been subject to inter-basin water transfer projects, and its flow is regulated by reservoirs and diversion dams. This river has recently become increasingly intermittent and fragile due to anthropogenic activities, highlighting the need for tailored assessment methods. This makes it a critical case study for evaluating benthic macroinvertebrate biological tools (BMBTs). Different BMBTs, including community composition, biotic indices (BMWP, ASPT, and LIFE), and the FFGs approach, were evaluated for the Zayandehrud River. According to the results, the macroinvertebrate communities downstream of the Zayandehrud Reservoir Dam (ZRD) showed homogenization of beta diversity due to river regulation, with significant spatial variation in Shannon diversity and community composition. Specifically, BMWP and ASPT indices effectively demonstrated the impact of flow interruptions and regulation. In contrast, the LIFE index, Shannon diversity index and FFGs approach do not accurately represent the environmental conditions, especially drying up of the Zayandehrud River downstream of the Cham-Aseman Diversion Dam (CDD). These methods likely failed because their assumptions (continuous flow and specific sensitivity traits) are inconsistent with the intermittent nature of semi-arid rivers and the desiccation tolerance of their taxa. Consequently, BMBTs must be applied cautiously to avoid misclassifications and misunderstandings in assessing regulated rivers. This underscores the urgent need for regionally adapted BMBTs to inform effective water management and policy decisions.