148
Nutrients, Algae and Herbivores - the Paradox of Enrichment Revisited
with strict grazer control of algal biomass, unless the dilution rate is above the
persistence boundary (D > DJ; in that case, grazer extinction is most likely to
result (except for a small set of initial conditions located within the focal
attraction basin).
In order to make the loading diagram compatible with measured loading
figures, we will assume that a fraction of the P load is lost before entering the
pelagic zone of the lake, as suggested by Fig. 2.2. Using the average load decay
of 22%, estimated from the data set used by Prairie (1988), the resulting
loading diagram (Fig. 5.16) bears a clear resemblance to the loading limits
constructed by Vollenweider (1976). Although the upper limit of the shaded
area in Fig. 5.16 intersects the "excessive" loading limit of Vollenweider
(1976), this does not necessarily mean that biomanipulation cannot work in
eutrophic lakes. In fact, even the most heavily eutrophicated lakes considered
by Vollenweider (1976) are within the loading limits for stable grazer control
on algal biomass in Fig. 5.16.
Case Studies. As pointed out by Benndorf (1987), the omission of basic limnological information on water renewal and nutrient loading is a major
weakness shared by many biomanipulation studies. The set of available data
for testing the loading criteria proposed here is therefore very limited. Still,
the few documented cases of food web manipulations where reliable loading
figures also exist (Fig. 5.17) seem to conform reasonably well with the pattern
predicted by the present model in six out of seven cases. However, since the
publication of Sanni and WrervAgen (1990), it has been reported that the
shallow lake Mosvatn, where the model predicted that biomanipulation
should fail, has been invaded by large stands of macrophytes. As such, the
biomanipulation of Mosvatn should perhaps be reclassified as unsuccessful,
albeit not due to the kind of mechanisms that are considered in the present
model.
Benndorf (1987) proposed a biomanifulation-efficiency threshold at an
areal phosphorus loading of 0.5-1 (g P) m- year-· in lakes with mean depths of
5-10 m. This threshold level corresponds quite nicely with the volumetric
loading limit around 0.1 (g P) m- 3 year-· predicted by the present model for
lakes with water residence times > 1 year. The two lakes considered by
Benndorf (1987), Bautzen reservoir and Grafenhain, are both classified in
conformance with the conclusions of Benndorf (1987).
The failure of biomanipulation to improve water quality in the two
hypereutrophic Danish lakes, S0bygud S0 and Vreng S0 (Jeppesen et al.
1990), is also predicted by the model. The model predicts that the flushing
rates in these two lakes are so high that it is unlikely for a persistent grazer
population controlling algal biomass to become established, even with major
reductions in the external P loading. Of the three lakes in this material that
have undergone major reductions in the external P loading (Bautzen
Reservoir, Vreng S0, and Gjersj0en), it is only in Gjersj0en that load reduction
appears to have improved the odds for successful biomanipulation.
Nutrients, Algae and Herbivores - the Paradox of Enrichment Revisited
with strict grazer control of algal biomass, unless the dilution rate is above the
persistence boundary (D > DJ; in that case, grazer extinction is most likely to
result (except for a small set of initial conditions located within the focal
attraction basin).
In order to make the loading diagram compatible with measured loading
figures, we will assume that a fraction of the P load is lost before entering the
pelagic zone of the lake, as suggested by Fig. 2.2. Using the average load decay
of 22%, estimated from the data set used by Prairie (1988), the resulting
loading diagram (Fig. 5.16) bears a clear resemblance to the loading limits
constructed by Vollenweider (1976). Although the upper limit of the shaded
area in Fig. 5.16 intersects the "excessive" loading limit of Vollenweider
(1976), this does not necessarily mean that biomanipulation cannot work in
eutrophic lakes. In fact, even the most heavily eutrophicated lakes considered
by Vollenweider (1976) are within the loading limits for stable grazer control
on algal biomass in Fig. 5.16.
Case Studies. As pointed out by Benndorf (1987), the omission of basic limnological information on water renewal and nutrient loading is a major
weakness shared by many biomanipulation studies. The set of available data
for testing the loading criteria proposed here is therefore very limited. Still,
the few documented cases of food web manipulations where reliable loading
figures also exist (Fig. 5.17) seem to conform reasonably well with the pattern
predicted by the present model in six out of seven cases. However, since the
publication of Sanni and WrervAgen (1990), it has been reported that the
shallow lake Mosvatn, where the model predicted that biomanipulation
should fail, has been invaded by large stands of macrophytes. As such, the
biomanipulation of Mosvatn should perhaps be reclassified as unsuccessful,
albeit not due to the kind of mechanisms that are considered in the present
model.
Benndorf (1987) proposed a biomanifulation-efficiency threshold at an
areal phosphorus loading of 0.5-1 (g P) m- year-· in lakes with mean depths of
5-10 m. This threshold level corresponds quite nicely with the volumetric
loading limit around 0.1 (g P) m- 3 year-· predicted by the present model for
lakes with water residence times > 1 year. The two lakes considered by
Benndorf (1987), Bautzen reservoir and Grafenhain, are both classified in
conformance with the conclusions of Benndorf (1987).
The failure of biomanipulation to improve water quality in the two
hypereutrophic Danish lakes, S0bygud S0 and Vreng S0 (Jeppesen et al.
1990), is also predicted by the model. The model predicts that the flushing
rates in these two lakes are so high that it is unlikely for a persistent grazer
population controlling algal biomass to become established, even with major
reductions in the external P loading. Of the three lakes in this material that
have undergone major reductions in the external P loading (Bautzen
Reservoir, Vreng S0, and Gjersj0en), it is only in Gjersj0en that load reduction
appears to have improved the odds for successful biomanipulation.