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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 -


6. RESTORATION OF STREAMS FOR WATER QUALITY IMPROVEMENT AND FISHERY ENHANCEMENT

According to European Union (EU) Water Framework Directive (WFD), stream restoration for water quality improvement should consider the restoration of the ecological integrity of the whole stream ecosystem. Restoring the ecological integrity of the stream system means enhancing the biological diversity and natural instream processes. The new dimension of this philosophy has been provided by the concept of Ecohydrology (Zalewski 2000) which suggests the use of ecosystem properties as a management tool toward enhancement of the resilience and resistance of stream ecosystem to stress. Thus, ecohydrology may be proposed as a new tool for implementation for the Integrated Water Resources Management (IWRM) and for WFD. With the restoration of key stream processes such as flow patterns, patterns of sediment erosion and deposition, nutrient cycling, and species succession water quality can be enhanced. The first step in stream restoration, then, is to restore the natural stream morphology as a basic element of water quality and fishery improvement.

 

A. Optimisation of the physical stream structure

In order to improve the quality of stream water and enhance its native biota, it is necessary to restore the natural complexity of the stream channel structure. Rehabilitation of the natural stream channel structure will restore the proper functioning of stream ecosystem.

Stream biota depend upon the structural complexity of the stream habitat. Thus, biota may be an essential indicator of the status of the stream ecosystem. The higher trophic level organisms, such as fish, may provide an integrated assessment of watershed conditions. Further, this means that fish communities can serve as a sensitive indicator of the relative health of its aquatic ecosystem and its surrounding watershed (Karr et al. 1986). The link between fishery and environmental status forms the basis for utilising fish communities for directly monitoring water resource quality.

Therefore, the main aim of restoring stream channel structural complexity is to reintroduce the diversity of main channel features, such as depth, flow, substrate, and cover - both instream and riparian - that provide fish habitat and that comprise the physical attributes of the riverine ecosystem.

B. Restoration of a pool-riffle-run sequence

Several hierarchical frameworks for stream habitat classification within a watershed context have been developed. One such example is shown in Figure 6.1. Fish mainly respond to changes in the mesoscale hydraulic habitat, comprised of pools, runs, and riffles, and to their primary characteristics - depth, flow, and substrate composition - as well as to their secondary characteristics - woody debris and vegetation. Fish do not respond to these variables independently, but rather in combination and in some hierarchical manner (Rabeni and Jacobson 1993). However, from a practical standpoint, it is often easier to determine the responses based upon pool vs. riffle preferences than to correlate fish distribution or abundance with a number of related, underlying factors.

The influence of habitat type on the fish community parameters - estimated as biomass and diversity - in small upland and lowland rivers showed that pools and riffles on both stream types maintained higher levels of fish biomass and fish diversity then the transition zones, or runs, situated between the pools and riffles (Łapińska 1996, Zalewski et al. 1998) (Figure 6.2). This phenomenon is related to the greater habitat complexity associated with the pools and riffles. The pools provide a diversified depth gradient, while riffles offer diversified substrate sizes (Schlosser 1987).

C. Optimisation of instream cover by reintroduction of large woody debris (LWD)

Large woody debris (LWD), through its impact on physical processes within the stream ecosystem (e.g., hydrological, hydraulic, sedimentological, and morphological processes), plays a critical role in enhancing and maintaining habitat for biota.

Fig. 6.1. Hierarchical organisation of stream systems and their habitat subsystems, as a framework  for analysing processes at various scales (adapted from Frissell et al. 1986) (lager image)

Fig. 6.2. The influence of habitat type on fish biomass and species diversity  (after Łapińska 1996, changed) (lager image)

From the point of view of the fish community, complex woody debris structures provide a variety of microhabitat types, including refugia, that can support a wide range of organisms at different stages in their life cycles (Angermeier and Karr 1984). For fishes, the main ecological function provided by woody debris as an instream structure was in the provision of microhabitat. These functions are summarised in Table 6.1.

Table 6.1. Functions of woody debris as instream microhabitat (definitions adapted from Fausch 1993)
Overhead cover Structure that decreases predation risk by directly obstructing the visual predation on fish by aerial predators, and that provides shade, reducing the visibility of fish to horizontally positioned predators
Visual isolation Structure that directly obstructs horizontal visual contact between fishes, thereby reducing predator-prey interaction and antagonistic inter- and intra-specific behaviours
Velocity refuge Structure that provides areas of low velocity amongst swifter currents, thereby minimising the energy costs to fishes of maintaining favourable stream positions

Large woody debris as structure

Fish communities respond quickly to changes in the amount of large woody debris (LWD) in a river. Experimental results show significant (about 50%) reductions in fish biomass and diversity after removing LWD from a uniform stream channel (Figure 6.3a). The results also showed significant changes in fish community structure. Restoration of the LWD in the river channel, however, resulted in a rapid recovery of the community to pre-disturbance conditions (Figure 6.3b).

Fig. 6.3a. The effect of large woody debris (LWD) removal on fish community biomass and species diversity  (Shannon Index H'  not transformed) (after Łapińska 1996, changed) (lager image)

Fig. 6.3b. The effect of large woody debris (LWD) removal and restoration  on fish community structure (after Łapińska 1996, changed) (lager image)

Large woody debris as a factor in fish interactions

Habitat complexity, associated with woody debris, appears to be critical in interspecific relationships within the fish community. Negative relationships between habitat complexity and the foraging success by predatory fishes have been already demonstrated (Everett and Rhuiz 1993, Łapińska et al. 2001). In enclosure experiments that tested the influence of littoral zone type, including zones with large woody debris, on the growth and behaviour of pikeperch (Stizostedion lucioperca L.) and their prey (roach, Rutilus rutils (L.)), Łapińska et al. (2001) showed that the growth rate of pikeperch was negatively affected by the presence of woody debris in littoral zone due to decreased predation efficiency (Figure 6.4). These results suggest that large woody debris can stabilise predator-prey relationships in freshwater ecosystems.

Fig. 6.4. The effect of large woody debris (LWD) as a stream bank structure on predator growth rates (after Łapińska et al. 2001, changed) (lager image)

 

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