Environmental Engineering Reference
In-Depth Information
SEDIMENT
RESUSPENSION
PHYTOPLANKTON
Turbidity
Herbivory
(fish, birds)
Nutrients
SUBMERGED
PLANTS
Waves (wind)
Allelop. subs.
Feeding
Refuge
PLANKTIVORE and
BENTHIVORE FISH
ZOOPLANKTON
Figure 18.7
A diagrammatic representation of the mechanisms and factors causing sediment resuspension and turbidity in
shallow lakes in relation to macrophytes (submerged plants). Different feedback mechanisms and their relative intensities are
indicated with arrows (Allelop. subs. = allelopathic substances). (From Gulati and van Donk 2002.)
and P from both water and sediments (Barko & James
1998), reducing bioavailability of these elements for
algae (van Donk & van de Bund 2002). Moreover, the
algae (periphyton) adhering to macrophytes act as
major nutrient sinks. Secondly, the macrophytes
provide refuge for larger bodied zooplankton and
young fi sh against fi sh predation (Moss 1998), and
thus promote zooplankton grazing. Thirdly, macro-
phytes consolidate bottom sediments and reduce their
resuspension by fi sh and wind. Finally, both denitrifi ca-
tion in the macrophyte beds and the release of allelo-
pathic substances by macrophytes are reported to
adversely affect phytoplankton (Figure 18.7), although
the mechanisms for such negative feedbacks are as yet
unclear (van Donk & van de Bund 2002). Thus, all
these characteristics related to macrophyte develop-
ment and growth lead to improved underwater light
climate in lakes. For macrophytes to be able to stabilize
lakes after biomanipulation, it is important that they
can develop into a stable population. This depends on
the amount of light reaching the bottom, the presence
of propagules (plant material) in the bottom, predation
by waterfowl and the softness of the sediment (Scheffer
1998 ).
large - bodied zooplankton decreases. The decreased pre-
dation effects cascade in the food web and lead to a
marked increase in larger-bodied zooplankton, espe-
cially daphnids, as well as their grazing pressure on
algae (Potthof
et al
. 2008) leading to a clear-water
phase. Clear-water conditions have been reported for
several lakes in western Europe following reductions of
planktivorous fi sh (e.g. Gulati 1990; Jeppesen
et al
.
1999) and in the United States (Schrage & Downing
2004). It can thus be concluded that reduction of the
planktivorous fi sh stock helps to restore the large-bodied
zooplankters, particularly those comprising
Daphnia
species. Such a change will lead to a reduction in algae
biomass and improvement of water transparency.
Role of z ebra m ussels
The zebra mussel,
Dreissena polymorpha
, a bio - invader
in many lakes in temperate regions, is a potential tool
for lake management (Dionisio Pires
et al
. 2010 ).
Because zebra mussels are not found everywhere in the
world, the use of these bivalves is less general than
other techniques, like external P reduction, which
apply universally. Use of zebra mussel in lake mana-
gement therefore only applies to temperate regions.
Dionisio Pires
et al
. (2004) demonstrated that adult
mussels can act as biofi lters: the mussels clear the lake
from cyanobacteria at much higher rates than other
particles in the water (Figure 18.8). In some shallow
Role of z ooplankton
In lake restoration studies, the standing stock of plank-
tivorous fi sh is always reduced so that predation on
