This dataset presents all the fossil diatom species identified in a 109 cm-long lake sediment core from a thermokarst lake in Bylot Island, Nunavut, Canada. The core was collected in June 2015 at « Gull Lake » (also named BYL-66 in other publications) from the still-present ice cover with a 7 cm-diameter percussion corer (Aquatic Research Instruments). Slides were prepared at the Aquatic Paleoecology Laboratory of CEN for 60 samples; each cm for the upper 12 cm of the core, and then each 2 cm towards core bottom. For each sample, about 400 diatom valves were counted along transects using a light microscope at a 1000x magnification.
