Lake Washington lies parallel to Puget Sound and is separated from it by a line of hills. In 1916, the level of Lake Washington was permanently lowered, with the construction of a locked ship canal connecting the lake with Puget Sound. It is a large, deep lake (18km long and 65 metres deep).
1. Clear water, benthic fish species and macrophytic vegetation
2. Eutrophic, blue-green algae, turbid, loss of benthic fish, transmission of typhoid to humans
High phosphorus and nitrogen input from sewage increased turbidity and density of phytoplankton and zooplankton. The level of dissolved oxygen decreased with an associated loss of benthic fish species and macroinvertebrate prey species.
State 1: From the 1890's, untreated sewage was piped directly into Lake Washington, and by 1922, there were 30 outfalls, servicing 50,000 people. At the same time, it was also the source of drinking water for many people. In 1907, a serious outbreak of typhoid occurred.
State 2: Following a public vote, in an attempt to improve the water quality of the lake, the sewage was diverted from the lake into Puget Sound from 1958. Secondary treatment plants were established, which decomposed the organic materials, but still released inorganic nutrients. As the sewage was removed from the lake, Daphnia, which feeds on the algae, increased in number helping to remove the algal blooms as well as providing a food supply for fish. The quality of this deep, cold lake improved quickly after being flushed with low-phosphate water.
Contact
Jacqui Meyers
Email
jacqui.meyers@csiro.au
CSIRO Sustainable Ecosystems,
PO Box 284,
Canberra ACT 2601
Keywords
Pollution, biomanipulation, emprical data, model
