Trophic State and Eutrophication - Freshwater Ecology: Concepts and Environmental Applications - page 350

of ecosystem valuation, economic analysis of the costs and benefits of nu-
trient control can be difficult (Carpenter et al., 1998).
Control of non-point sources can have benefits beyond the local effects
of lowering lake, stream, and groundwater eutrophication. Recently, a
large anoxic zone has appeared in the Gulf of Mexico, and the reduced O 2
has damaged fisheries in the region. This anoxic zone likely is caused by
river-borne nutrients (Turner and Rabalais, 1994). Furthermore, there is
more N and P in rivers worldwide but less increase in silicon. This shift in
stoichiometry and nutrient amount has led to increases in algal productiv-
ity and shifts in composition of near-shore marine plankton communities
(Justic et al., 1995a). Thus, increased non-point source nutrient pollution
has increased eutrophication in marine coastal regions throughout the
world (Justic et al., 1995b).
One of the first steps toward lowering phosphorus input into water-
sheds from point sources is generally a ban on phosphate-containing deter-
gents. This restriction can cut in half the
phosphate entering sewage works. In these
situations, detergents for automatic dish-
washers and automatic car washes are gen-
erally exempt because of the reduced efficacy
of low-phosphate alternatives.
Control of point sources generally puts
the majority of the financial burden on fewer
institutions (e.g., a municipal sewage treat-
ment plant or a specific factory) than does
the control of non-point sources. Removing
phosphorus from waste streams can be
costly. One method of removal involves
chemical treatment with alum or Fe 3 to
precipitate the phosphate. The precipitate is
then allowed to settle and the low P water is
released. This method generally can bring
effluent concentrations down to 0.2-1 mg P
liter 1 (Clasen et al., 1989).
Nitrogen can be removed by converting
ammonium to ammonia gas by raising the
pH. The solution is stripped of ammonia by
bubbling gas through it, then the water is
neutralized and released. Alternatively, waste
can go through an aerobic treatment to con-
vert the nitrogen to nitrate, followed by an
anoxic phase in which nitrate is used in den-
itrification. The resulting N 2 gas enters the
atmosphere. Finally, wetlands can be used to
remove nitrogen, as discussed later.
Removing N and leaving P in a system
may not solve eutrophication problems be-
cause many species of cyanobacteria that form
undesirable blooms can utilize N 2 gas via fix-
ation and do not need nitrate or ammonium

benefits in increased returns of adult sockeye
were about $12 million per year (Stockner and
MacIsaac, 1996). These estimates suggest that
fertilizing nutrient-poor coastal lakes to levels
similar to those thought to occur historically,
or to those in pristine ecosystems, is econom-
ically feasible.
A fertilization project in the southwest United
States was less successful. Lake Mead is a
large reservoir in Nevada and Arizona that sup-
ports a sports fishery valued at approximately
$7 million per year. Fish production in Lake
Mead has decreased; largemouth bass (Mi-
cropterus salmoides) harvest has declined
more than 90% since the 1960s. It was hypoth-
esized that the closure of Glen Canyon Dam in
1963 lowered nutrient input into Lake Mead
and led to declines in fish production. A large-
scale fertilization experiment was initiated over
a 4-year period to assess the effect of in-
creased nutrient input on fish production and
water quality. This experiment resulted in a
moderate decrease in water quality (increased
taste and odor problems and chlorophyll a).
However, increases in zooplankton and forage
fish were not significant (Vaux et al., 1995). Ap-
parently in this case more fertilization would
be necessary to stimulate fish production but
could cause degradation of the quality of the
drinking water from Lake Mead. Furthermore,
there is no guarantee that more fertilization
would lead to increased fish production.