Urban impact in the history of water quality in the Stockholm archipelago.

From the historical perspective, the increasing number of inhabitants in the city of Stockholm has had a negative effect on water quality in the surrounding lakes and coastal region. Government control and measures taken to improve water quality have been in progress since the mid-19th century. Water conditions in the 19th century, compared to later years is difficult to assess from the first chemical and biological investigations, due to infrequent sampling and the different parameters and methods used. However, a retrospective evaluation of water quality can be made on the basis of results from plankton investigations, which began in the early 20th century. The occurrence of the cyanobacteria Planktothrix agardhii, which indicates nutrient-rich conditions, was surveyed during summers with similar temperature conditions throughout the 20th century. The results show that eutrophied conditions probably prevailed in the Stockholm archipelago from the beginning of 20th century until the early 1990s. In the last decade of the 20th century, water quality appeared to be better than 100 years earlier. Today's better water conditions are most probably an effect of proper measures taken in wastewater treatment.