Dataset of Water physiochemistry overrides urbanisation in shaping diatom taxonomic and functional diversity in urban ponds

We collected diatoms from 68 ponds across three Finnish urban regions in 15 – 28 May 2024: 38 ponds in the Helsinki capital region, 15 ponds in Tampere, and 15 ponds in Turku. To sample diatoms, we randomly selected 10 stones in each study pond and carefully removed the biofilm from a 25 cm² area of a stone with a toothbrush and a template. The 10 subsamples were combined into a composite sample per site. In the laboratory, samples were purified using 30% hydrogen peroxide, centrifuged at 1,250 rpm with distilled water for 20 minutes three times, and mounted on Naphrax slides.

To collect diatom species trait data, we extracted trait data from literature, with genus-level traits used when species-specific data were unavailable. We created a species-trait matrix with biovolume as continuous positive values and other traits coded as presence (1) or absence (0).