This observation led Welch (1981) to con-
amount of dilution water declined.
clude that continual low-rate inputs of dilution water over the entire summer
were preferable to very high but irregular rates which are above the amount
that can produce a decline in nutrients or a washout of cells.
When input of
dilution water stopped (August
undiluted high-nutrient water rapidly
replaced the diluted lake water.
Another effect of dilution in Moses Lake, in addition to creating nutri-
ent limitation, was the effect of cell washout (Cooke et al. 1986).
When
water was pumped from Parker Horn to Pelican Horn, a sharp decrease in algal
biomass occurred, particularly when the water exchange rates reached
-1
0.09 day . Similarly for Parker Horn; cell washout became a significant
-1
factor when the mean flushing rate was 10 percent day
In the remainder of
-1
the lake, where flushing averaged 1.4 percent day , cell washout was probably
not a significant factor because cell growth rates, at maximum, can exceed
50 percent day
The cost of water for dilution at Moses Lake was zero since water
already designated for downstream irrigation was simply routed through the
lake.
The pump for Pelican Horn cost 4,000 (1983 price), plus overhead for
If the water had had a cost similar to that of a typical Washing-
ton domestic supply, the 2-month cost of dilution water would have been about
million.
Green Lake, Washington
Dilution of Green Lake, in metropolitan Seattle, WA, began in 1962.
Domestic water was added at a
sufficient to increase the water exchange
-1
rate from an estimated 0.8 to 2.3 year
Over the 1965-68 period, the flush-
ing rate, based on dilution water only, ranged from 0.88 to 2.4 year
'(Welch
1981, Cooke et al. 1986).
Chlorophyll
phosphorus concentration, and water
transparency improved dramatically, and the fraction of algal biomass composed
of blue-greens declined substantially.
Water quality declined in the 1970s
when dilution was reduced, and blooms of algae returned in 1982 when no
dilution water was added.
High costs were incurred because domestic water is
expensive.
It was calculated, using the mass balance models described
63
-1
10 m year
that 7.6
of water would be needed
(269
to reduce the mean concentration of phosphorus in this medium-sized lake
(area
ha, mean depth, 3.8 m) to 20
P
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