What is a Successful Biomanipulation

Although the decision whether a rehabilitation effort is successful or not is subjective, some more or less general criteria may be put forward. One criterion for a successful biomanipulation is that the algal turbidity decreases and the water becomes clearer. Another criterion is that the amount of cyanobacteria decreases. The problems with cyanobacteria are that they tend to form nuisance 'blooms' that may be toxic. A third criterion lies in the stability of the improvements, that is, if the effects are only temporary or if they last for a longer period. On the basis of these criteria and of diagnostic variables such as (a) increased Secchi depth (water clarity), (b) reduced algal chlorophyll, (c) increased biomass of zooplankton, and (d) increased coverage of submerged vegetation, the success of specific biomanipulation measures undertaken may be roughly quantified. Biomanipulations may be ranked from those where all (100%) of the chosen diagnostic variables (a-d) were improved, to those where only a few, or even none, of the variables improved. If biomanipulation through fish reduction really has a positive effect on lake status, the portion of the diagnostic variables that are still improved after, for example 5 years, might be expected to be positively related to the intensity of the measure; that is, to the portion of fish removed from the lake. This would be a critical test of whether the fish removal is the probable cause of the observed

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Figure 5 The portion of planktivorous fish removed in 14 biomanipulation cases plotted against the portion (%) of the diagnostic variables (water clarity, chlorophyll, submersed macrophytes, zooplankton (Daphnia), total phosphorus, and cyanobacteria) that were improved. The figure shows that there is a positive relation between the effort (i.e., how much fish that is removed) and the gain; here expressed as the portion of 'diagnostic variables' that were still improved after 5years.

10 20 30 40 50 60 70 80 90 100 Fish reduction

Figure 5 The portion of planktivorous fish removed in 14 biomanipulation cases plotted against the portion (%) of the diagnostic variables (water clarity, chlorophyll, submersed macrophytes, zooplankton (Daphnia), total phosphorus, and cyanobacteria) that were improved. The figure shows that there is a positive relation between the effort (i.e., how much fish that is removed) and the gain; here expressed as the portion of 'diagnostic variables' that were still improved after 5years.

improvements. As seen in Figure 5, which is based on 14 biomanipulation studies, there is indeed a relationship between the proportion of improved diagnostic variables and the proportion (%) of the planktivorous fish community that was removed from the lake. In other words, the intensity of the measure is proportional to how 'successful' the biomanipulation is likely to become (Figure 5). A major conclusion is that if the planktivorous fish proportion is reduced by less than 50%, for example, because of a restricted budget, biomanipulation as a rehabilitation method should not be recommended since only a limited number of the diagnostic variables are expected to be found to have improved 5 years after the biomanipulation (Figure 5). However, if the budget allows a reduction of 75% of the cyprinid fish assemblage, the probability is high that the biomanipulation will be viewed as a success even after 5 years (Figure 5). Moreover, also the intensity of the fish reduction is important; that is, a fish reduction with 10 tons during one year is not the same as 1 ton each year during 10 years, since the fish community is able to compensate mortality with reproduction if the intensity is too low.

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