Modelling Seasonal Dynamics and Bioindicator Responses in a Tropical Monsoon River: Insights from the Chaliyar River, Western Ghats, India

Freshwater ecosystems sustain rich biodiversity and provide vital services, yet they are increasingly strained by human activities and shifting climate patterns. This study focuses on the Chaliyar River in Kerala’s Western Ghats biodiversity hotspot to identify seasonally responsive indicators of ecosystem health by integrating physicochemical, microbial, and biological observations.

Field sampling was conducted across three hydrological seasons of 2024, Pre-monsoon, Monsoon, and Post monsoon, was carried out at seven sites spanning forested headwaters to urban downstream zones. 23 water quality parameters and benthic macroinvertebrate assemblages were analysed to assess temporal and spatial variability. Significant differences (p < 0.05) in alkalinity, hardness, nitrate, COD, BOD, and microbial load highlighted monsoon driven dilution and nutrient enrichment. Benthic macroinvertebrate analysis (26 families, 8 orders; n = 1,267) showed notable variation in %EPT (H = 12.80, p = 0.0016), FBI (H = 8.83, p = 0.012), and functional groups such as scrapers (H = 8.38) and shredders (H = 9.95), confirming their sensitivity to monsoonal disturbance and post monsoon recovery.

In Multivariate analysis, significant seasonal variation (Kruskal–Wallis, p < 0.05) was observed for temperature (H = 9.88), alkalinity (H = 10.21), magnesium hardness (H = 13.44), nitrate (H = 12.88), COD (H = 10.15), and coliforms (H ≈ 11.24). Spearman correlation identified three gradients: ionic–alkalinity (EC–TDS ρ = 0.83), nutrient–organic enrichment (nitrate–phosphate ρ = 0.73), and redox–oxygen (Fe–DO ρ = –0.68). PCA explained 69.3% variance, separating premonsoon (ionic enrichment) and monsoon (organic loading). CCA showed BOD, nitrate, and coliforms controlling tolerant taxa, while DO favoured EPT assemblages (71.3% total inertia). NMDS (stress = 0.12, PERMANOVA p = 0.002) confirmed distinct seasonal clustering, establishing hydrological regulation as the key driver of biotic and abiotic patterns.

By combining in situ biological monitoring with detailed physicochemical assessment, this study establishes a good framework for evaluating biodiversity in tropical rivers. The findings aid in developing standardized, scalable metrics for freshwater biodiversity that align with global biodiversity observation systems. They also point to the potential for linking field-based indicators with new eDNA and remote-sensing methods for thorough ecological monitoring.