Trophic State and Eutrophication - Freshwater Ecology: Concepts and Environmental Applications - page 357

input much but does lower loads of biochemical oxygen demand and path-
ogenic bacteria. Older sewage treatment plants released significant amounts
of ammonium, which is toxic to aquatic life. More advanced sewage plants
oxygenate treated water, and most ammonium is converted to nitrate by
nitrification, leading to different chemical and biological patterns down-
stream from the sewage outfall (Fig. 17.13B). The most advanced sewage
plants remove nitrogen by stripping ammonium or denitrification.
Little attention has been paid to eutrophication in wetlands. Eutroph-
ication in the Everglades is a serious concern. Eutrophication control strate-
gies in a European wetland were discussed in Sidebar 13.3.
CASE STUDIES OF EUTROPHICATION
Examples of pollution control can aid understanding of general eco-
logical concepts and their application to solving eutrophication problems.
This section outlines some successes and failures of lake managers that il-
lustrate the complexity of the issues involved and the variety of problems
that have arisen.
Lake Washington
The case of Lake Washington is one of the great triumphs for limnol-
ogists, lake managers, and environmentalists. This lake has experienced a
strong recovery based on scientific understanding of limnological proper-
ties and particularly the efforts of the late limnologist Prof. W. T. Ed-
mondson (Biography 17.2). A fascinating account has been written of the
political and scientific aspects of cleaning up this lake (Edmondson, 1991).
Lake Washington is a large (28-km long and 65-m deep) lake that
forms the eastern border of the city of Seattle, Washington, and its sub-
urbs. Lake Washington is a monomictic lake (summer stratification) that
was historically oligotrophic. As human population in the lake's watershed
grew, pollution of the lake increased (Table 17.4). In 1936, the city of Seat-
tle diverted its sewage from the lake, and by 1963 all major sewage inputs
to the lake were halted.
The phosphorus input from sewage dumped into the lake caused a de-
crease in lake clarity. Following the halt of sewage input into the lake,
phosphorus levels decreased significantly (Edmondson and Lehman, 1981),
populations of the eutrophic cyanobacterium Oscillatoria rubescens de-
creased (Fig. 17.15), and a species of Daphnia typical of oligotrophic wa-
ters became abundant again. Thus, removal of the nutrient input from
sewage allowed the lake to return to an oligotrophic state.
In this case, O 2 never completely disappeared from the hypolimnion of
the lake, and nutrient control brought about rapid reversal of eutrophica-
tion without release of excessive P from the sedimentary FePO 4 . The ex-
cess phosphorus added to the lake in the past remains buried in the sedi-
ments and some has been washed out of the lake. The lake now receives
heavy recreational use and maintains a reasonable clarity and absence of
algal blooms.