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Newsletter and Technical Publications
Freshwater Management Series No. 5
Guidelines for the Integrated Management of
the Watershed
- Phytotechnology and Ecohydrology -
A complex approach to reservoir restoration
The restoration project for the lowland Sulejow
Reservoir in Central Poland serves as an example of how the restoration of a
given water body would be adjusted to take into account the prevailing biotic
and abiotic conditions influencing a specific reservoir ecosystem. The
restoration project consists of a series of activities, both planned and
already implemented, that utilise the biological properties of the lake
ecosystem in such a way as to improve water quality in the reservoir. This
approach could be similar to those used in similar ecosystems, and is presented
diagrammatically in Figure 8.9.
- Reduction of the
external and internal nutrient loads
The starting point of the restoration project was
activities connected with the reduction of nutrient loads from agriculture
areas situated in the upstream river valley and direct catchment of the
reservoir. These activities included the creation of sedimentation zones within
backwater areas of the riverine floodplain (Figure 8.11: point 1) and of
riparian buffer zones along reservoir shoreline (Figure 8.9: point 2).
- Prevention of
sediment resuspension
To reduce the rate of sediment resuspension in shallow
water areas by wave action, mobile sediments were removed (Figure 8.9: point 3)
or consolidated using rooted macrophytes and/or colonial mussels (e.g., zebra
mussel, Dreissena polymorpha) (Figure
8.9: points 4 and 5). It is worth noting that, in the case of reservoirs, the
main factor responsible for the lack of macrophytes in the littoral zone is
water level fluctuations. Thus, in order to counteract the effects of waves and
increase the environmental heterogeneity essential for maintaining diverse and
abundant fish and zooplankton communities, it was necessary to artificially
create and maintain shoreland wetlands and macrophyte buffers, as one of the
biomanipulation actions.
- Alteration of
fish community structure
To achieve an optimal
fishery - in terms of yield and community structure - from the point of view of
water quality, both direct actions such as stocking (Figure 8.9: points 6 and
7) and selective removal of specific fishes by increased fishing pressure
(Figure 8.9: points 8 through 10), as well as indirect actions, were required.
Indirect actions included alteration of fish spawning success and recruitment
(Figure 8.10).
- Creation of sedimentation zones using macrophytes in
backwater areas of the reservoir in order to reduce nutrients loads transported
from the catchment during floods.
- Creation of riparian buffers along the reservoir
shoreline and appropriate siting of tourist infrastructure in order to reduce
nutrients load from the direct reservoir catchment.
- Dredging sediments from areas of significant
accumulation in order to reduce internal loading of nutrients.
- Introduction of macrophytes in constructed, anchored
floating islands to provide refuges for fish and zooplankton and enhance
predator-prey interactions. These macrophyte "pods" also compete with
phytoplankton for nutrients and decrease light, thereby decreasing the
likelihood of intense algal blooms.
- Introduction of zebra mussels, which filter seston and
consolidate loose bottom substrates, protecting the sediment from resuspension.
- Introduction of pikeperch to counteract an observed
population decline, at a rate of 500-1,000 fingerlings per ha.
- Introduction of 500-1,000 pike fingerlings per ha, in
areas well separated from the stocking areas of pikeperch to avoid predation.
- Selective removal of cyprinids to maintain their
biomass below 50 kg/ha.
- Control of fish spawning success by use of
species-specific spawning substrates, and by modifying water level changes
within the reservoir during the spawning period.
- Removal of excessive numbers of fry from the
littoral zone of the reservoir during June and July; these fry can be relocated
to other ponds and lakes requiring stocking.
| A) |
Water level
manipulation: Water level manipulation during the spawning period is a
productive way of using the changes in reservoir water level for the management
of fish communities (Zalewski et al.
1990a, 1990b) (Figure 8.9: point 9; Figure 8.11). However, in many reservoirs,
changes in water level occur around the time of spawning, isolating fish from
the flooded vegetation in the littoral zone that acts as spawning substrate
(Ploskey 1985, Zalewski et al. 1990a,
1990b). |
| B) |
Provision of
preferred spawning substrates: An alternative for managing fish spawning
success is to provide the target fish species with its preferred spawning
substrate (Figure 8.9: point 10; Figure 8.12). |
| C) |
Modification of
food stocks: In the temperate zone, most juvenile fishes intensively forage
upon zooplankton at some point in their life histories. Thus, the structure and
abundance of fry communities may, via their pressure on zooplankton, strongly
influence the intensity of algal blooms. Manipulation of water levels and/or
spawning substrates not only alters the future composition of adult fish
communities but also results in immediate changes in the food web and dynamics
of the trophic cascade (Frankiewicz et al. 1996). |
| D) |
Reduction in
numbers of excessive fry: Juvenile
fish can negatively affect water quality by increasing nutrient resuspension
while feeding on benthic prey when planktonic prey organisms are not available.
Thus, there may be a need to control juvenile fish numbers by altering
reproductive success or removal of excessive fry by transferring them to other
lakes (Figure 8.9: point 10). |
| E) |
Stocking of predators: Predatory fishes play special role in
the biomanipulation of lowland European reservoirs. The key predators in the
pelagic and littoral zones are pikeperch (Stizostedion
lucioperca) and pike (Esox lucius),
respectively. However, one of the major factors limiting their recruitment is
cannibalistic pressure on juvenile specimens by older fish (Frankiewicz
et al. 1999) (Figure 8.13). To avoid
this predation, appropriately sized juveniles should be stocked, and stocking
should be done at sites remote from known habitat areas of larger individuals
and in areas of numerous prey organisms (Figure 8.9: points 6 and 7). This may
be done by choosing stocking areas near the spawning grounds of cyprinids, or
by placing eggs within artificial spawning substrates previously used in the
control of planktivorous fishes. Notwithstanding, some cannibalistic predation
pressure is unavoidable. |
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