Environmental Engineering Reference
In-Depth Information
5. Project relationship between in-lake total phosphorus
and algal biomass
1. Identification of problem lake
10
2
2. Characterize system, describe lake morphology, measure
nutrient inputs, describe watershed
10
1
10
0
3. Identify nutrient control strategies that are feasible
Riparian protection
10
0
10
1
10
2
10
3
Total P ( g L
-1
)
Improved sewage
treatment, P bans
Lower agricultural
fertilizer additions
6. Project effect on lake clarity with chl-Secchi depth
relationships
8
7
6
5
4
3
4. Project relationship between in-lake
nutrient concentrations and management effects on nutrient loading
2
1
0
10
0
10
1
10
2
10
2
Chl
a
( g L-1)
10
1
7. Assess if costs of control strategies can be justified on
basis of in-lake improvements
10
0
10
0
10
1
10
2
L
8. Post control monitoring to asses if lake has improved
according to expectations.
z
(
−
)
FIGURE 17.9
A simplified diagram of the steps that can be used to modify eutrophication
in a lake.
Oude, 1991), and bioassays (see Chapter 16) indicate that colimitation by
N and P is likely in many lakes.
Lowering
external loading
(supply of nutrients from outside the sys-
tem) is generally necessary to control eutrophication. The lowering of ex-
ternal loading usually incurs some cost; thus, lake managers may need to
estimate the amount of improvement in water quality that will result from
a set amount of nutrient control. This estimation involves calculation of
in-lake P concentrations and subsequent algal biomass. The following is a
general, steady-state equation to calculate in-lake total P concentration:
L
z
TP
(
)
where TP is the total phosphorus in mg m
3
g liter
1
),
L
is the P load-
(
ing in mg m
2
year
1
,
z
is the mean depth in meters,
is the flushing rate
in year
1
, and
is the sedimentation rate in year
1
, approximately equal
to 10/
z
.
This equation represents one of the simplest cases. It accounts for
sources and losses of P in the lake. The source is loading from rivers,
groundwater, and atmosphere. Losses are from washout (flushing and
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