Synthesis What Happens after a Biomanipulation

As stated in the introduction, the main theory behind biomanipulation is still focused on pelagic processes. However, both littoral and benthic factors, such as submerged macrophytes and benthic feeding fish, strongly affect the success of a biomanipulation, suggesting that these processes should receive equal consideration. Instead of being the only mechanism involved, alterations in the food chain may be viewed as triggers that initiate other processes.

A biomanipulation has several 'primary effects' such as an increase in zooplankton abundance within 1-2 years (Figure 6). This results in a reduced algal biomass, which in turn leads to improved light conditions and improved possibilities for submerged macrophytes to establish. The macrophytes absorb nutrients, leading to a further reduction in algal biomass and a 'positive spiral' is created (Figure 6). Furthermore, the reduction in benthic feeding fish results in reduced resuspension of sediment particles and thereby to a reduced turbidity, which improves light penetration, and also reduces the damage to macrophytes. Less feeding by fish at the sediment surface also reduces the nutrient transport from sediment to water, thereby reducing the algal growth potential. Moreover, once the macrophytes have established they also stabilize the sediment surface,

What happens?

Turbidity (-) P from sediment Zooplankton (+) Fish competition

What happens?

Turbidity (-) P from sediment Zooplankton (+) Fish competition

Submersed macrophytes (+)

"Positive spiral" Algae (-)

"Positive spiral" Algae (-)

Juvenile fish (+)

Figure 6 An overview of important processes triggered by a reduction in cyprinid fish abundance (biomanipulation). Primary positive effects are increased zooplankton abundances and reduced turbidity and internal nutrient (P) loading from the sediment. These in turn have effects on other variables in the system by creating a 'positive spiral' of reduced algal growth, improved light penetration and growth of submerged macrophtes. The removal of fish also causes reduced competition among planktivorous fish which leads to a higher survival of juvenile fish; a process that may counteract the positive effects.

Juvenile fish (+)

Figure 6 An overview of important processes triggered by a reduction in cyprinid fish abundance (biomanipulation). Primary positive effects are increased zooplankton abundances and reduced turbidity and internal nutrient (P) loading from the sediment. These in turn have effects on other variables in the system by creating a 'positive spiral' of reduced algal growth, improved light penetration and growth of submerged macrophtes. The removal of fish also causes reduced competition among planktivorous fish which leads to a higher survival of juvenile fish; a process that may counteract the positive effects.

thereby further reducing the resuspension. These processes strengthen the 'positive spiral.' However, another 'primary effect' of a fish reduction is reduced food competition among fish (Figure 6). If the biomanipulation is intense enough and the density of piscivorous fish is high enough, the recruitment of young fish may not become a problem. However, if an insufficient amount of fish is removed, the young fish, recruited years following the biomanipulation, will have access to an almost unlimited food resource, since competition with larger fish is negligible. However, this 'negative spiral' may, at least partly, be avoided by addition of piscivorous fish.

Some lakes present better conditions than others for using biomanipulation as a therapeutic tool. Low external and internal phosphorus loading and large areas suitable for colonization of submerged macro-phytes increase the probability of a successful biomanipulation. Another important conclusion is that the former completely dominant view that the only mechanism involved in a biomanipulation is the pelagic food chain (fish-zooplankton-algae) should be less pronounced. Instead this process may be viewed as the trigger for secondary, mainly benthic and littoral, processes, such as establishment of submerged macrophytes and reduction in benthic feeding by fish. A final conclusion may be that a well-planned biomanipulation in combination with a reduction in the external nutrient input is an attractive lake rehabilitation method likely to be successful in improving the water quality in most types of eutrophic lakes.

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