Fuzzy Logic Approach for Limnological Assessment Using Biological Condition Gradients in the Aras River Basin (Türkiye)

ABSTRACT Developing a robust framework for assessing stream ecological status (ES) and establishing defensible nutrient criteria remains a key challenge in aquatic ecosystem management. We applied a fuzzy logic‐based approach to evaluate the biological condition gradient (BCG) of 17 streams in the Aras River Basin using biological (Trophic Index of Turkey [TIT], Trophic Index [TI]) and chemical (total phosphorus [TP], electrical conductivity [EC]) metrics during summer, fall and spring seasons. Diatom community and integrated metrics were used to assess ES of aquatic ecosystems. Biological condition gradient scores ranged from 0.23 (Sarısu Stream, fall) to 0.81 (Akçakale Stream, spring), reflecting conditions from degradation to high integrity. Strong correlations were found between BCG and TIT ( r = −0.93, p < 0.01), TI ( r = −0.90, p < 0.01), TP ( r = −0.75, p < 0.01) and EC ( r = −0.84, p < 0.01), highlighting BCG's sensitivity. The results indicate diverse ecological statuses across spatiotemporal scales, emphasizing the need for integrated biological and chemical assessments. This approach enhances ecological evaluation precision and offers a scalable methodology for watershed management. Given increasing anthropogenic and climatic pressures, further refinement of BCG could provide critical insights for adaptive management and policy in Türkiye and beyond. Aligning ecological assessments with predictive modelling underscores BCG's potential as a key tool for monitoring, conserving and restoring freshwater ecosystems. We recommend using BCG due to its superior accuracy and flexibility.