Context-Dependent Effects of Monsoon Seasonality on Riverine Fish Assemblages: Case Study From Bharathapuzha River, India

In tropical monsoon rivers, understanding how powerful seasonal hydrology interacts with localized anthropogenic degradation to shape fish assemblages is a central ecological and management challenge. This study investigated fish community dynamics in the Bharathapuzha River, India, using a comprehensive spatio-temporal dataset from 108 sampling events across nine sites and three seasons. Environmental heterogeneity was characterized through hierarchical cluster analysis and principal component analysis, revealing distinct spatial groupings and dominant axes of variation, including flow dynamics (explaining 23.9% of environmental variation) and habitat integrity (explaining 22.8% of environmental variation). Fish community composition showed significant differences based on study site (explaining 33.7% of community variance) and season (explaining 16.4% of community variance), with a crucial and significant interaction between study site and season (explaining 17.6% of community variance). This interaction demonstrated that the monsoon's influence on fish assemblages in the Bharathapuzha River is profoundly context-dependent, varying significantly among sites. Redundancy analysis, explaining 11.6% of adjusted community variance, further identified physical habitat structure as a greater unique driver (5.7%) compared to water quality (3.5%). These findings establish that fish assemblages in the Bharathapuzha River are primarily structured by the intricate interplay between its natural flow regime and site-specific habitat conditions, providing a nuanced understanding essential for localized conservation.