Samples of surficial sediments (upper 1-2 cm) were collected from 37 shallow lakes of Wapusk National Park using a coring tube (38-mm internal diameter). Lakes were located in three different ecozones spanning a latitudinal and vegetational gradient, from NE to SW: the Coastal Fen (n=21), the Interior Peat Plateau (n=10), and the Boreal Spruce Forest (n=6). Hydrochloric acid (HCl) and a mixture of sulphuric (H2SO4) and nitric (HNO3) acids were successively used to respectively remove carbonates and digest organic matter from the samples. Samples were rinsed several times with distilled water after each step. About 1 ml of the resulting diatom solutions were dried onto glass coverslips, which were then mounted onto microscopic slides. About 500 valves per sample were identified and enumerated along random transects using an optical microscope. Identification was carried out to the finest taxonomic level possible at a 1000x magnification. Results are expressed as the relative abundance (%) of each taxon within one lake.