Environmental Engineering Reference
In-Depth Information
Measures to reduce these fi sh stocks need to be repeated
to prevent re-establishment of their populations. An
improvement in the underwater light climate (i.e. more
visibility) indicates that the reduction of planktivore
and benthivore fi sh is succesful and that the top-down
cascading effects are re-established (Hosper 1997;
Meijer 2000): phytoplankton biomass is controlled by
zooplankton which in turn are not heavily predated
by planktivores because piscivores control them.
cacy of restoration measures are also likely to vary
with lake depth (see Moss 1998). In addition, because
shallow lakes are more likely to be colonized by macro-
phytes, a shift to a clear-water state in these lakes will
occur earlier than in the deeper ones.
3. Lessons l earned
Gulati
et al
. (2008) attribute the failure of lake bioma-
nipulation measures to one or more bottlenecks,
including (1) ineffective biomass reduction of plank-
tivorous and benthivorous fi sh and the inability to
sustain fi sh at low levels, (2) inadequate reduction of
external P and a concomitant increase of P release
from sediments, (3) poor edibility of cyanobacteria to
zooplankton, (4) reduction of macrophytes by fi sh and
waterfowl and (5) recurrent failure of introduced
northern pike (piscivorous fi sh). Among these, bottle-
necks (1), (2) and (4) are crucial to overcome for lake
biomanipulation to be successful (see also Gliwicz
2005). Below, we highlight the importance of selected
biotic factors for applying biomanipulation as a lake
restoration measure.
2.
Historical o verview
Lake biomanipulation as a restoration measure started
in the early 1970s. State-of-the-art reviews are given
by Gulati and van Donk (2002) and Jeppesen
et al
.
(2007a). From these studies, it becomes clear that in
almost all biomanipulated lakes in Europe, the stand-
ing stock of planktivorous fi sh was effectively reduced
but
Daphnia
populations did not increase in the years
thereafter. Both poor food quality (high cyanobacterial
densities) and/or predation by remaining planktivores
probably contributed to the failure of the daphnids to
develop high population densities. Northern pike (
Esox
lucius
), which was introduced to control the planktivo-
rous fi sh, mainly bream, was rarely a success. These
studies showed that the size-selective predation on
larger zooplankton by the planktivorous fi sh led to a
decrease of daphnids and to their grazing on phyto-
plankton, so that water quality did not improve (Gulati
& van Donk 2002). In other words, the
trophic cascade
between phytoplankton and zooplankton was largely
decoupled (Kasprzak
et al
. 2007). In both Denmark and
the Netherlands, the effect of fi sh removal was less
obvious because lakes tended to return to the turbid
state unless fi sh removal was repeated annually. Other
factors that caused biomanipulation to fail were (1)
insuffi cient reduction in external P loading (e.g. Benn-
dorf
et al
. 2002), (2) continuation of internal P loading
and (3) the failure of macrophytes to appear.
There are more cases of long-term failure than
success for lake biomanipulation, mainly because the
nutrient input effects on the pelagic food web tend to
persist even after the top-down manipulation measures
(McQueen
et al
. 1986). Thus, reduction of nutrients
from the lake's catchment, rather than in-lake only, is
an important prerequisite for success of biomanipula-
tion. In deeper lakes the failure of biomanipulation
measures is also caused by high P loading (Benndorf
et al
. 2002 ). Because sediment - water interactions
differ with depth, the nutrient dynamics and the effi -
Importance of fi sh
In shallow lakes, it is relatively easy to manipulate fi sh
(Lammens 1999) and produce rapid effects through
fi shing on the planktivores and benthivores. After
fi shing, the standing stock usually ranges from 150 kg
FW ha
− 1
to 20 kg FW ha
− 1
(Lammens
et al
. 2002 ).
Maintenance of a fi sh stock at low densities is desirable
but generally hard to achieve. This is because there is
less competition between fi sh individuals for food
leading to higher growth. Furthermore, because the
amount of P per unit body weight in fi sh is quite high,
P recycling by fi sh seems to retard lake restoration
(Sereda
et al
. 2008). Studies on Loosdrecht lakes
showed that the P regenerated from fi sh was about
140% of the external P loading (van Liere & Janse
1992). Thus, nutrients and algal concentrations are
expected to decline upon removal of planktivores. For
fi sh removal to be successful, a substantial reduction of
the fi sh stock must take place.
Role of m acrophytes
Macrophytes stabilize lakes after biomanipulation
(Coops & Hosper 2002 ). They infl uence various proc-
esses in shallow lakes (Figure 18.7). Firstly, macro-
phytes can accumulate relatively large amounts of N
