Trophic State and Eutrophication - Freshwater Ecology: Concepts and Environmental Applications - page 348

EXAMPLE 17.1.
Using Loading Equations to Predict Response to Nutrient Control

If a lake manager is able to lower mean total summer phosphorus inputs to
a lake by 50% from an initial loading value of 1 g m 2 year 1 total P, what
will be the expected decrease in chlorophyll given a mean depth of 10 m, a
flushing rate ( ) of 2 years 1 , and a sedimentation ( ) rate of 1 year 1 ? How

does this translate into increased Secchi depth?
First, we need to solve for initial chlorophyll (in practice, this will prob-
ably be a measured value). To do so, we solve for TP concentration first, then
use the equation relating TP to chlorophyll. To calculate TP, do not forget to
convert P loading (L) into mg m 2 year 1 and that mg m 3 g liter 1 :

1
(

0
2

0 0

1)

g liter 1 TP 10 33.3 P

If we rearrange the equation relating chlorophyll to TP, we get

10 1.46 log 10 TP

1.09

g chl liter 1 chl 13.6

g liter 1 . Us- A 50% decrease in loading will reduce the TP by half to 16.7

g liter 1 chl. In-
spection of Fig. 17.8, which was constructed using slightly different equa-
tions, allows the calculated relationships to be checked. If we use the
nomogram in Fig. 17.8, we can also see that the Secchi depth is expected to
increase from approximately 1.5 to 2.7 m. An additional issue is variance; it
is beyond the scope of this discussion, but a significant amount of variance
occurs in the empirical relationships and this source of uncertainty must be
considered in an actual management situation.

ing the second equation, this concentration will yield 4.9

tems, and that nutrient control methods may decrease the probability of a
noxious bloom but not preclude the possibility.
Not all the results of eutrophication are bad. Fish biomass may be
greatest in eutrophic lakes (Fig. 17.6). Some lakes are fertilized artificially
to increase fish production (Sidebar 17.2). Eutrophic lakes are also more
resistant to the effects of acid precipitation because they are buffered by
metabolic activities (Davison et al., 1995) and photosynthesis tends to in-
crease the pH.

MITIGATING LAKE EUTROPHICATION

Eutrophication management can begin with control at the nutrient
source (treating the cause) or with in-lake treatment (treating the symptom;
Table 17.2). Treating the cause of eutrophication by controlling nutrient
sources is generally most cost-effective over the long term. Nutrients can
come from point sources, such as sewage outfalls, factory effluents, septic
tanks, and waste flowing from the surface of intensive livestock operations.