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United Nations Environment Programme
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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).

  1. Creation of sedimentation zones using macrophytes in backwater areas of the reservoir in order to reduce nutrients loads transported from the catchment during floods.
  2. 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.
  3. Dredging sediments from areas of significant accumulation in order to reduce internal loading of nutrients.
  4. 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.
  5. Introduction of zebra mussels, which filter seston and consolidate loose bottom substrates, protecting the sediment from resuspension.
  6. Introduction of pikeperch to counteract an observed population decline, at a rate of 500-1,000 fingerlings per ha.
  7. Introduction of 500-1,000 pike fingerlings per ha, in areas well separated from the stocking areas of pikeperch to avoid predation.
  8. Selective removal of cyprinids to maintain their biomass below 50 kg/ha.
  9. 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.
  10. 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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