Variability and course of occurrence of ice cover on selected lakes of the Gnieźnieńskie Lakeland (Central Poland) in the period 1976–2015Bogumił Nowak, Dominik Nowak and Mariusz PtakE3S Web Conf., 44 (2018) 00126DOI: https://doi.org/10.1051/e3sconf/20184400126