Environmental Engineering Reference
In-Depth Information
Fig. 12.10
A diagrammatic
representation of the mechanisms
and factors causing sediment
resuspension and turbidity in
shallow lakes in relation to
macrophytes (submerged plants).
After lake restoration the increase
in macrophytes plays an important
role in reducing sediment
resuspension and turbidity and
improving underwater light climate.
Different feedback mechanisms
and their strength are indicated
with arrows. (Allelop. subs.
SEDIMENT
RESUSPENSION
PHYTOPLANKTON
Turbidity
Herbivory
(fish, birds)
Nutrients
SUBMERGED
PLANTS
Waves (wind)
Allelop. subs.
Feeding
Refuge
PLANKTIVORE and
BENTHIVORE FISH
=
allelopathic substances.) From
Gulati and van Donk (2002).
ZOOPLANKTON
(Horppila
et al.
1998). The decrease in P input due to
a fish-stock reduction of 150 kg of fresh weight ha
−1
in Lake Wolderwijd, the Netherlands, equalled
c
.60%
of the external loading (Meijer
et al.
1994a, 1994b).
Secondly, such a reduction strongly relieves predation
on large-bodied zooplankton so that their grazing pres-
sure on phytoplankton and detritus increased, and the
light climate improved. By their active foraging for
benthic prey in the lake sediments, the planktivores
also resuspend sediments, causing an increase in
turbidity and a deterioration of the light climate. This
sediment resuspension also stimulates aerobic min-
eralization of P, which may be refixed in Fe complexes
if redox potential is high. Model studies on shallow
Dutch lakes reveal that fish-induced resuspension
causes more than 50% of the turbidity in shallow lakes
(Meijer
et al.
1990).
The reductions in the standing stocks of fish, both
planktivores and benthivores, often stimulate recruit-
ment of the YOY (young of the year) fish, thereby con-
siderably cancelling out the positive effects. Both this
and the inadequate fish-biomass reductions might
explain the failure of biomanipulation measures in
many Dutch lakes and elsewhere. That a reduction of
planktivore biomass and its maintenance at < 50 kg
of fresh weight ha
−1
will indeed enhance the chances
of success needs reaffirming in future studies. How-
ever, most failed studies reveal that it is extremely
difficult to maintain over long periods a consistently
low standing stock of piscivores, such as northern pike,
in biomanipulated lakes. The pike, for reasons not
yet fully understood, fail to develop even a moderate
population size on introduction into these lakes.
Role of macrophytes
The stabilizing role of macrophytes on lakes after
biomanipulation is now well established (van Donk
et al.
1993, Jeppesen 1998; see papers in Kufel
et al.
1997; see reviews by Coops 2002 and van Nes 2002).
Macrophytes influence various processes in shallow
lakes (Fig. 12.10). Because of their huge biomass rel-
ative to phytoplankton, macrophytes can accumulate
large amounts of N and P from both water and
sediments (Barko & James 1998) and thereby reduce
the bioavailability of these nutrients for algae (Gulati
& van Donk 2002, van Donk & van de Bund 2002).
Moreover, macrophytes and the colonizing periphyton
act as major nutrient sinks throughout most of the
vegetative period. Secondly, they provide refuge for
larger-bodied zooplankton and young fish against
fish predation (Moss 1990, 1998), and thus promote
zooplankton grazing (Timms & Moss 1984). Thirdly,
they considerably reduce fish-induced bioturbation, as
well as restrict the wind-induced resuspension of the
bottom sediments; that is, they increase sedimentation
of phytoplankton and detritus. All these features lead
to improved underwater light climate in lakes (Barko
& James 1998). Consequently, one of the first posit-
ive effects in the biomanipulated lakes is the seasonally
