5

How Ecosystems

Work

SOLVING LAKE WASHINGTON’S

ECOLOGICAL IMBALANCE

L

ake Washington is a large, deep freshwater lake on

the eastern boundary of the city of Seattle. During

the early part of the 20th century, Lake Washington

came under increasingly intense environmental

pressures as the Seattle metropolitan area expanded

and suburban sewage treatment plants began releasing

treated sewage (effluent) into the lake. In 1955 scientists

and a pollution control commission determined that

the effluent was raising the levels of dissolved nutrients,

particularly phosphorus, to the point of serious

pollution. These nutrients were supporting vast mats

of cyanobacteria (photosynthetic bacteria) that formed

a smelly green scum over the surface of the water. The

bacteria that decompose cyanobacteria multiplied

explosively, consuming vast quantities of oxygen,

until the lake’s deeper waters could no longer support

organisms such as fishes and small invertebrates.

Scientists predicted that the lake’s decline could be

reversed by stopping the pollution. They outlined steps

necessary to save the lake, including comprehensive

planning by the region’s suburbs and complete

elimination of sewage discharge into the lake. After

initial defeats, the state legislature passed a bill aimed

at controlling the lake’s pollution; at the time the bill

provided for the most ambitious and most expensive

pollution-control project in the United States.

Treated sewage was first diverted away from the lake

in 1963, and the last sewage was diverted in 1968. The

lake’s condition began to improve (see graphs). Water

transparency returned to normal within a few years, and

cyanobacteria eventually disappeared. By 1975 the lake

was back to normal, and today the lake remains clear.

graphingactivity