146
D. Absalon et al.
According to the Water Framework Directive (WFD, Directive 2000/60/EC),
dam reservoirs are treated as heavily modified water bodies, so until 2021, they
are required to reach at least good ecological status. As reservoirs are considered to
be heavily modified sections of rivers, they are mostly not very similar to rivers. Each
river, as regards ecology, is an open ecosystem, which is in a state of dynamic equilibrium [5, 6]. According to the river continuum concept (RCC) by [7] breaking its continuity undoubtedly influences changing hydrological conditions, which can significantly shape physical, chemical and biological processes determining the quality of
water in the reservoir and thus in the river downstream of the dam [8–11]. In general,
particularly in reservoirs with long retention time, the flow velocity is minimum,
what makes them more similar to lakes. Like lakes, dam reservoirs should be characterised with primary production reflecting their processing capacity, which means that
biomass production in a reservoir or lake should not exceed its destruction capacity
in the course of natural processes. Imbalance of this equilibrium by increasing the
content of compounds of allochtonic or autochthonic origin in water as a consequence
leads to the changes triggering the deterioration of the chemical status and ecological
status of the reservoir [12]. Dam reservoirs, despite being man-made structures, have
developed rich ecosystems with a high diversity of living organisms [13–16]. The
functioning of reservoirs significantly modifies balance and flow of energy [17] and
matter in the World Ocean. The evidence for that is many times smaller transport of
all kinds of load in the river downstream of the dam [18]. This is directly connected
with reservoir accumulation [4, 19–23]. Most of the bed load remains in the reservoir. Suspended load and wash load is retained only in part. Effectiveness of dam
reservoirs in trapping sediments is in some cases significant and amounts to 70–90%
of the reservoir’s storage capacity [24]. It is assumed that about 30–40% of matter
transported in the form of suspension by rivers in the world does not reach seas,
oceans, and some big lakes and is trapped in man-made dam reservoirs at least for
the period of existence of this infrastructure [25].
For the purposes of implementing the Water Framework Directive, a distinction
is made between heavily modified and artificial surface water bodies, in accordance
with the applied parameters, into such category of surface water bodies, which is the
closest to the given artificial or heavily modified water body. Not all dam reservoirs
may be treated as lakes. Elements differentiating dam reservoirs from lakes depend on
parameters characteristic of reservoirs, the most important of which is regarded as the
rate of water exchange. On this basis, dam reservoirs may be divided into reolimnic,
transitory, and limnic [12]. Reolimnic reservoirs—with the water retention period
shorter than 20 days—are reservoirs, which in extreme cases are not much different
than rivers with a low flow velocity. Transitory reservoirs—with the retention period
of 20 do 40 days—are reservoirs which in the river part are more similar to rivers
and in the lake part (close to the dam) to lakes. Limnic reservoirs—with the retention
period of over 40 days—are more similar to lakes.
D. Absalon et al.
According to the Water Framework Directive (WFD, Directive 2000/60/EC),
dam reservoirs are treated as heavily modified water bodies, so until 2021, they
are required to reach at least good ecological status. As reservoirs are considered to
be heavily modified sections of rivers, they are mostly not very similar to rivers. Each
river, as regards ecology, is an open ecosystem, which is in a state of dynamic equilibrium [5, 6]. According to the river continuum concept (RCC) by [7] breaking its continuity undoubtedly influences changing hydrological conditions, which can significantly shape physical, chemical and biological processes determining the quality of
water in the reservoir and thus in the river downstream of the dam [8–11]. In general,
particularly in reservoirs with long retention time, the flow velocity is minimum,
what makes them more similar to lakes. Like lakes, dam reservoirs should be characterised with primary production reflecting their processing capacity, which means that
biomass production in a reservoir or lake should not exceed its destruction capacity
in the course of natural processes. Imbalance of this equilibrium by increasing the
content of compounds of allochtonic or autochthonic origin in water as a consequence
leads to the changes triggering the deterioration of the chemical status and ecological
status of the reservoir [12]. Dam reservoirs, despite being man-made structures, have
developed rich ecosystems with a high diversity of living organisms [13–16]. The
functioning of reservoirs significantly modifies balance and flow of energy [17] and
matter in the World Ocean. The evidence for that is many times smaller transport of
all kinds of load in the river downstream of the dam [18]. This is directly connected
with reservoir accumulation [4, 19–23]. Most of the bed load remains in the reservoir. Suspended load and wash load is retained only in part. Effectiveness of dam
reservoirs in trapping sediments is in some cases significant and amounts to 70–90%
of the reservoir’s storage capacity [24]. It is assumed that about 30–40% of matter
transported in the form of suspension by rivers in the world does not reach seas,
oceans, and some big lakes and is trapped in man-made dam reservoirs at least for
the period of existence of this infrastructure [25].
For the purposes of implementing the Water Framework Directive, a distinction
is made between heavily modified and artificial surface water bodies, in accordance
with the applied parameters, into such category of surface water bodies, which is the
closest to the given artificial or heavily modified water body. Not all dam reservoirs
may be treated as lakes. Elements differentiating dam reservoirs from lakes depend on
parameters characteristic of reservoirs, the most important of which is regarded as the
rate of water exchange. On this basis, dam reservoirs may be divided into reolimnic,
transitory, and limnic [12]. Reolimnic reservoirs—with the water retention period
shorter than 20 days—are reservoirs, which in extreme cases are not much different
than rivers with a low flow velocity. Transitory reservoirs—with the retention period
of 20 do 40 days—are reservoirs which in the river part are more similar to rivers
and in the lake part (close to the dam) to lakes. Limnic reservoirs—with the retention
period of over 40 days—are more similar to lakes.