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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 -


C. Biomanipulation for water quality improvement in the lowland reservoirs

Shallow, lowland reservoirs, which accumulate nutrients transported by their inflowing rivers, are especially susceptible to intensive algal blooms, including toxic blue-green algal blooms (see Zalewski 1999). The most effective way to control this symptom of eutrophication is to eliminate its cause(s). Generally, this requires the dual approach of reducing the incoming nutrient loads (mainly of phosphorus and nitrogen) and implementing biological controls on algal growth through manipulating the biota within the reservoir (biomanipulation). To maximise the effectiveness of these latter actions in any given reservoir, the hierarchy of factors determining its actual trophic state should be analysed. Among these factors, both hydrological and hydrochemical conditions, as well as the biotic characteristics of the ecosystem, should be identified.

Modification of most abiotic factors that influence water quality is practically difficult, while others fall within the competence of engineers. Conversely, modification of many biotic factors affecting the food web is possible at little cost, and can contribute to profitable spinoffs in reservoir systems.

Increased abundances of predatory fishes, through their pressure on zooplanktivorous fish, can result in the maintenance of large cladocerans within the water column, thereby controlling algal abundance and contributing to water quality improvements. Such activities are termed "biomanipulation" (Shapiro et al. 1975). Biomanipulation utilises the trophic cascade phenomenon, which transmits changes within a given trophic level to lower trophic levels (Carpenter et al. 1985). Taking into account the direction of the observed changes within the biological communities, McQueen et al. (1986) termed such transfers "top-down" cascades. This effect was demonstrated in the extreme by the complete removal of fish from a water body, resulting in the dominance of large form zooplankton, whose effective filtering resulted in increased water transparency. More frequently, however, similar effects are achieved by the introduction of new predatory fishes, the reinforcement of existing predatory fish populations, the selective removal of zooplanktivorous fishes, or the introduction of phytoplanktivorous fishes.

Biomanipulation - Changing the biological structure of an ecosystem in order to improve water quality (Shapiro et al. 1975).
Cascade effect - Transmitting within a given trophic level to lower trophic levels (Carpenter et al. 1985).

Notwithstanding, a number of limitations to biomanipulation has been identified. For instance, in cases where the planktivorous fish population was dominated by deep-bodied species, even numerous predators were not able to reduce substantially their numbers due to difficulties in efficiently predating upon a prey with such an anti-predatory shape. In such cases, the effects of biomanipulation at the top of the trophic pyramid were not transmitted to the lower levels. Similarly, herbivorous fishes have frequently stimulated algal development, instead of controlling it by excreting nutrients in readily available form. The classic methods of biomanipulation do not always reduce the occurrence of algal blooms or improve water quality, especially under conditions of high phosphorus concentrations (e.g., DeMelo et al. 1992, Drenner and Hambright 1999). In such cases, predatory fish species were rapidly replaced by omnivorous species as the level of eutrophication increased (e.g., Persson et al. 1991). As a result of the failure of some biomanipulations during the 1980s and 1990s, greater attention has been placed on analysing and integrating top-down effects with bottom-up effects. These latter reflect the nutrient dynamics within reservoirs (McQueen et al. 1989).

The utility of biomanipulation as a tool for the restoration and maintenance of high quality water resources has been discussed at length is recent years (e.g., DeMelo et al. 1992, Sarnelle 1996). Drenner and Hambright (1999), in reviewing 41 biomanipulation experiments, found that 61% resulted in a permanent improvement in water quality. Of the balance, only about 15% did not show any positive effects within a one-year period. This suggests that, in the majority of lakes and reservoirs, there are real possibilities for effective biomanipulation. However, to take advantage of these possibilities, it is necessary to develop a broad knowledge of a given ecosystem, which, combined with regular monitoring, will allow the application of several biomanipulation techniques. The most difficult systems to control in this manner are large lakes, but, even in such ecosystems, biomanipulation can yield positive results. A good example of this is the restoration of Lake Erie (Makarewicz and Bertram 1991), where a combination of bottom-up (reduced nutrient loads) and top-down (introduced salmonids and re-established walleyed pike populations) effects, supported by a zebra mussel invasion, resulted in evident improvements in water quality. Similar successes were achieved in other water bodies, especially in cases where it was possible to utilise the stabilising effects of macrophytes (e.g., Blindow et al. 1993).

The stabilising role of macrophytes in reservoir ecosystems

The characteristics of littoral habitats, especially in shallow lowland reservoirs, are of great importance in determining the dynamics of biological processes throughout the entire ecosystem (see Ploskey 1985). There is a wealth of literature showing the effects of littoral macrophytes and inundated terrestrial vegetation on both bottom-up and top-down processes in lakes and reservoirs (e.g., Werner and Hall 1988, Savino and Stein 1989, Chick and McIvor 1994, Zalewski et al. 1995). These effects are usually realised via the direct and indirect effects of littoral macrophytes on fish population dynamics.

The indirect effects include the affect that aquatic macrophytes can have on the food chain by negatively influencing planktonic algae. Phytoplankton populations suffer from competition for nutrients, shading, and allelopathy. Macrophytes can also decrease the amounts of nutrients available to phytoplankton, reduce water movement, and limit resuspension of sediment. Moreover, some plants can oxidise the sediment surface, limiting phosphorus release into the water column. Beds of macrophytes positively affect invertebrates, providing shelter and food. The invertebrates associated with macrophytes usually form denser and more diversified communities than those in areas lacking macrophytes, since these invertebrates can utilise not only phytoplankton but also periphyton and decomposing plant material as food sources. The varying species of invertebrates utilising these food resources may act to stabilise the long-term predator-prey interactions that affect individual invertebrate-food organism relationships.

The direct effects of aquatic macrophytes in the littoral zone relate to their utility as refuges and/or foraging grounds, spawning substrates, and nursery areas for most fish species. The types and densities of vegetation within the littoral zone and temporarily-flooded terrestrial areas influence, to a great extent, both fish species composition and abundance in the reservoir. This is because macrophyte beds usually differ in structural complexity, water chemistry, and composition of invertebrate communities. The structural complexity created by the presence of macrophytes may influence the outcome of competitive interactions between fish species, as in the case of the perch (Perca fluviatilis) and the roach (Rutilus rutilus) (e.g., Winfield 1986). Compared with the roach, perch forage more effectively between macrophytes, achieving higher growth rates and abundances in these areas. In structurally simple habitats, the roach is the more efficient forager (e.g., Persson and Greenberg 1990). The occurrence of littoral macrophytes determines the density of typical ambush predators, such as the pike (Esox lucius), and greatly affects their foraging success. In turn, the presence of piscivorous fishes may strongly influence the distribution of juvenile and small forage fishes among the macrophyte patches, and between vegetated and nonvegetated habitats (e.g., Werner and Hall 1988). Differences in predatory pressure and food distribution among macrophyte patches can encourage fish assemblages to become more diverse within the littoral zone of reservoirs than in unvegetated or open water areas. To fulfil the conflicting demands of different fish species for feeding and predation avoidance, a littoral zone with an intermediate degree of structural complexity would seems to be the optimal situation (Savino and Stein 1989).

Other aspects of the role of shoreline habitat in water quality management of lakes are connected with fish spawning strategies. In the temperate zone, many reservoir fishes use littoral macrophytes and flooded terrestrial vegetation as spawning substrates (Fernando and Holcik 1991). For some of them, like the pike, the absence of such spawning grounds may be a major limiting factor. Also, the abundance of perch has been found to be correlated with the availability of submerged shoreland vegetation during spring and early summer (Zalewski et al. 1990a, 1990b, 1995). Macrophytes not only serve as the substrate for egg attachment, but their presence also diminishes egg mortality, protecting them against wind and wave action, siltation, and incidental predation.

 

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