Water physical and chemical properties override urbanization in shaping diatom taxonomic and functional diversity in urban ponds

Abstract Urban ponds are increasingly exposed to both local environmental changes and regional urbanization pressures, which threaten aquatic biodiversity, making it urgent to understand drivers of freshwater biodiversity for effective conservation and biomonitoring. In this study, we examined the taxonomic and functional diversity of benthic diatoms, bioindicators of water quality, in 68 urban ponds in three Finnish cities, the Helsinki capital region, Tampere, and Turku. We found that (1) diatom species and functional compositions significantly differed among the regions, but urbanization did not affect either diatom taxonomic or functional diversity. (2) Taxonomic diversity metrics were unresponsive to local environmental factors, whereas functional diversity metrics responded to water pH, temperature, and nutrient levels. (3) Both diatom species and functional composition were affected by water pH, water color, and water temperature, with nonlinear responses in traits. Functional, rather than taxonomic, diversity metrics are more sensitive to water physical and chemical properties, providing complementary insights gained from trait‐based approaches into community structure and resilience. Our findings suggest that urbanization per se does not necessarily reduce diatom diversity; instead, water quality within ponds plays a critical role in structuring diatom communities. Maintaining or improving in‐pond conditions can sustain diverse and functionally rich diatom communities, reinforcing their value for biomonitoring in urban landscapes.