Hypoxia has spread in Tiefer See (NE Germany) due to increased human activity. The onset of hypoxia indicated by varve preservation is dated to ~1920 at the lake’s depocenter, which responds faster and more severely to the reduction in oxygen level. The spread of hypoxic conditions is a gradual process that has lasted nearly 100 years, and the chemistry of the sediments shows that the depletion of oxygen in the lake started several decades before the onset of varve preservation.