Urban sentinels: quantifying marginal effects of impervious cover on Brachionus–Keratella dominance

Urbanization, often quantified as impervious cover, alters catchment hydrology and limnological conditions, yet sensitive bioindicators and quantitative metrics for small urban freshwater ecosystems remain limited. We evaluated whether rotifers with widespread distribution ( Brachionus and Keratella ) can serve as indicators of urbanization across 18 ponds in Berlin along an impervious-cover gradient. Rotifers comprised 11 genera and were dominated by Brachionus and Keratella . Total rotifer abundance was higher in ponds with high (183.9 ± 235.2 ind l −1 ) vs lower (107.8 ± 89.8 ind l −1 ) urbanization level, with Brachionus reaching more than three times the mean abundance in highly urban sites. A beta regression model explained 56% of the variation in a Brachionus/Keratella index proposed here, showing positive effects of impervious cover and dissolved oxygen, and a negative effect of elevation. A five-percentage-point increase in impervious cover resulted in a 15.6% increase in the index at low baseline cover (5%), but only 6.5% increase at higher baseline cover (60%), indicating a nonlinear response. Future studies should incorporate interannual variability and nutrient dynamics to further refine this rotifer-based metric, enhancing its ecological applicability and predictive capacity as a sensitive indicator of urbanization in small ponds.