Environmental Engineering Reference
In-Depth Information
suitable, natural substratum for the larvae of the
zebra mussels (Reeders & bij de Vaate 1990) augurs
well for the return of these large grazers after lake
biomanipulation.
Future lake-restoration plans, for example in the
Netherlands, typically envisage near-nature develop-
ment, emphasizing that a lake is an integral part of
a landscape comprising other aquatic, semi-aquatic
and terrestrial ecosystems (see papers in Nienhuis &
Gulati 2002). Such a measure includes reinforcing the
shoreline vegetation of lakes to prevent wind- and
wave-induced erosion and improving the propensity
of the land-water transition to develop a natural
biodiversity. The water authorities have also started
to excavate several 20 - 40 m deep pits within the
shallower lake parts to allow wind-induced shifting
and burial of the loose, nutrient-rich lake sediments
into these pits to retard in-lake nutrient release rates
from wind-induced resuspension of the sediments.
Moreover, creation of artificial islands to reduce the
wind fetch factor and erosion is planned in some
lakes. Feasibility plans are also under way to deploy
water-level management, encourage the shoreline
macro-vegetation and to develop greater natural
evolution of the aquatic and semi-aquatic ecosystems.
The plans envisage extending the upper and lower
limits of the permissible annual water-level fluctu-
ations and exploring the effects, especially of transient
water-level draw downs (Coops & Hosper 2002).
Near-natural water levels that allow wider fluctuations
than the current 'fixed' levels are considered the best
option. However, in the light of long-term climate
change and its consequences for hydrology and
water-management practices, the impact of flooding
and recession on the ecosystems as well as water use
by humans need to be investigated thoroughly.
Sustainability of the positive effects on water qual-
ity is central to remedial measures. The experience
gained from the failures and occasional successes
of the last two decades should make it possible to
develop more-enduring strategies for greater sustain-
able restoration of our lake ecosystems. Lastly, the long-
term aims of water management should be to create
and maintain sustainable and healthy aquatic envir-
onments that possess optimal properties for their
assigned functions.
12.4 Conclusions and perspectives
Although in many European countries freshwater
lakes and reservoirs constitute a small fraction of the
land area, their importance for human health, re-
creation and national economies is indisputable. There
is obviously an urgent need to further curtail, divert
and treat the unwarranted inputs into the lakes and
reservoirs of nutrients, organics, silt and contaminants
from the watershed. Despite more than two decades
of nutrient-reduction measures in runoff waters,
many lakes still exhibit an insufficient improvement
in water quality. We now understand well that
sustainability of the positive effects on water quality
is central to the remedial measures. An important
principal cause for this hold-up of responses to
corrective measures is the large stockpiles in lake
sediment of P, which due to its slow release allows
the eutrophication symptoms to persist. Secondly,
inadequate nutrient reductions in the runoff waters
and the virtually unabated and diffuse inputs from
the agricultural grounds in the catchment act as a
major bottleneck to lake restoration. It is therefore
difficult to predict the response of an ecosystem
under restoration. Thus, further manipulations of
both chemical and biological processes are needed to
sustain the positive effects of the corrective measures.
However, monitoring of water quality and restoration
of in-lake processes alone will be futile exercises if
not carried out in a watershed context. This latter
involves documenting the entire landscape setting,
including habitat type, hydrological regime, soil
properties, topography and invasive species, all of which
can hamper restoration measures. Moreover, chances
of improving a lake are often better if the chosen
measures complement each other.
