Environmental Engineering Reference
In-Depth Information
tion. First we shall assess the major types of distur-
bances of lakes, identifying the problems and main
symptoms, and then discuss lake restoration strategies
and techniques in use. Finally we draw some conclu-
sions and discuss research and development prospects
for the future.
diffuse agricultural runoff from catchment areas into
lakes. Accordingly, the rates of algal production and
nutrient accumulation in lake sediments rise and the
effects of eutrophication will persist and accumulate
over time (Rast & Thornton 1996).
In temperate regions, increases in light and water
temperature in eutrophic lakes, especially in the
months of spring, generally result in increased algal
growth rates and production, dominated by cyano-
bacteria. Consequently, the underwater light condi-
tions deteriorate. In eutrophic lakes, many genera of
cyanobacteria (e.g. fi lamentous genera: Planktothrix
( Oscillatoria) , Anabaena , and Aphanizomenon ; and the
cosmopolitan, colony - forming species Microcystis aeru-
ginosa ) gradually come to dominate because they form
blooms and scums. The ability of cyanobacteria to
dominate phytoplankton over a wide range of nutrient
levels, as well as to fi x atmospheric nitrogen, together
with their relatively low palatability to zooplankton,
allows them to attain high densities and biomass.
Thus, they can persist and dominate the phytoplank-
ton of shallow, eutrophic lakes throughout the main
growth period (spring-autumn), and even overwinter.
Lastly, several species of cyanobacteria can produce
potent toxins, which constitute a menace to both
human and animal health (Codd 2000).
Rast and Holland (1988) provide a decision-making
protocol for developing and implementing eutrophica-
tion control measures in lakes and reservoirs (Figure
18.1). This scheme addresses the various concerns and
cost-effectiveness to be considered when selecting fea-
sible, remedial measures. A practical framework for the
management and control measures of lakes and reser-
voirs is to (1) assess the available information, (2) iden-
tify eutrophication problems, (3) analyse costs and
expected benefi ts of the alternative management strat-
egies and adequacy of institutional and regulatory
frameworks for implementation of a given strategy and
then (4) select a control strategy and publicity meas-
ures and evolve mechanisms to minimize the recur-
rence of eutrophication problems.
18.2 DISTURBANCE FACTORS
AFFECTING LAKES
Continuously rising inputs of nutrients, organic
matter, silt and still more toxic contaminants, have led
to sharp deterioration of water quality in many lakes
and reservoirs over recent decades. This entails
increased growth of algae and aquatic plants, and
accumulation of organic matter, including detritus,
leading to reduced water transparency. Water trans-
parency is considered an indicator of lake and reservoir
health. The most persistent and widespread water
quality problem related to human use of lakes and res-
ervoirs is eutrophication , but acidifi cation remains
a problem in some parts of the world. Here we discuss
both problems, presenting their main causes and
symptoms. This will set the stage for discussing restora-
tion strategies in the following section.
18.2.1
Eutrophication
Eutrophication is a natural ageing process of lakes that
causes a steady increase in production of biomass due
to a gradual accumulation of nutrients and a slow
decrease in lake depth. In classical terms, eutrophica-
tion is the enrichment of water by inorganic plant
nutrients, especially N and P. Increasing eutrophica-
tion in the early 1970s of the Great Lakes and many
other lakes and reservoirs in the United States and
Canada, along with many lakes in western Europe,
raised widespread public concern. In view of
P-enrichment in lakes and related problems of eutroph-
ication, many state agencies funded large-scale
research projects, such as the Experimental Lake Area
in Winnipeg, Canada (e.g. Blanchfi eld et al . 2009 ). The
eutrophication models thus developed provided insight
into the quantitative relationships between nutrient
loading rates and algal biomass and production (Vol-
lenweider 1987). What emerges clearly is that the
main causal factors for eutrophication are direct, point
discharges of human and animal wastes, as well as
18.2.2
Acidifi cation
Fortunately, acidifi cation problems in lakes and reser-
voirs in Scandinavian countries and the United
Kingdom are less grave today than they were in the late
twentieth century. In the United States, however, acidi-
fi cation is still an important environmental hazard in
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