Floods and Footprints: Diatom Communities Track Urban Pollution Beneath Monsoon‐Driven Variability in the Tropical River System

ABSTRACT This study was conducted in the Mula‐Mutha River Basin (MMRB), located in the northern Western Ghats of Maharashtra, India, to assess whether monsoon‐driven floods (seasonal hydrological variability) and anthropogenic footprints (spatial environmental gradients) exert stronger control on diatom assemblages. To achieve this, diatom and water samples were collected across three hydrological seasons (pre‐monsoon, monsoon and post‐monsoon) over 2 years (2022 and 2023). Results showed that diatom richness and diversity remained relatively stable across seasons and river stretches, whereas community composition varied significantly among river stretches but showed weak seasonal differentiation. β‐diversity analysis revealed that species turnover was the dominant component, indicating strong spatial heterogeneity in diatom assemblages along the river continuum. Null model analyses indicated that stochastic processes were more prominent during seasonal high‐flow conditions, while deterministic processes associated with environmental filtering were more evident in impacted midstream regions. Indicator taxa analysis further revealed stretch‐specific assemblages, with pollution‐tolerant taxa dominating impacted midstream regions and more sensitive taxa associated with upstream sites. Diatom‐environment relationships identified dissolved oxygen and electrical conductivity as the primary physicochemical drivers influencing diatom community structure. Variation partitioning showed that spatially structured environmental variables explained a modest proportion of community variation, while a large unexplained fraction suggested additional influence of stochastic processes and unmeasured factors. Overall, the results indicate that diatom community dynamics in the Mula‐Mutha River Basin are predominantly governed by stochastic processes under seasonal flooding, with localised deterministic influences linked to anthropogenic footprints in impacted midstream regions. Importantly, strong seasonal hydrochemical variability did not translate into strong seasonal separation of diatom assemblages in multivariate community composition, highlighting that diatom assemblages provide a spatially integrated signal of environmental and anthropogenic gradients in monsoon‐driven tropical rivers.