Boxplots summarizing the 4,4'-DDD:4,4'-DDE ratios of all studied lakes in the 5-year periods are depicted in Fig. . Ratios are mostly above one until ca. 1985. The median of ratios peaks in ca. 1955 after which it declines until ca. 1970. Then it slowly increases, reaching a secondary peak in ca. 1980, and drops beneath one in ca. 1990. In ca. 1995, it shortly peaks slightly above one, and returns to almost the same value in ca. 2000. 4,4'-DDD:2,4'-DDD ratios stay mostly between values of two and three over the length of the observed period from ca. 1950 to ca. 1995 (Fig. ). Light gray, hatched boxplots show values that have to be considered with caution, either because only a limited number of lakes is represented, or the occurrence of values at that time is not considered historically sound (see section 2.12).