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The effect of biomanipulation (i.e. pond drawdown and fish removal) on the ecological status of eight Brussels ponds biomanipulated in 2007 was studied by means of assessing changes in phytoplankton, zooplankton and macrophytic vegetation as well as nutrient concentrations. To broaden the context, 14 additional ponds were studied during the same period, including two ponds that were biomanipulated during previous years to assess the effects of biomanipulation on a longer term. All the ponds studied are eutrophic to hypereutrophic when considering total phosphorus and therefore have a high potential for phytoplankton biomass development, including cyanobacterial blooms. However, phytoplankton biomass covered a range from oligotrophic (clear water) to hypereutrophic (turbid water) conditions indicating that other environmental factors play a more important role in phytoplankton control than nutrients. This implies that the ponds of the Brussels region have a considerable potential for restoration by means of biomanipulation. The biomanipulation results confirmed the importance of fish in determining ecological quality of ponds and indicated that when pond ecosystems are impaired by eutrophication, considerable degree of their ecological quality can be restored through fish community manipulation. The fish removal resulted in a drastic decrease in phytoplankton biomass in all but one biomanipulated ponds. The decrease in phytoplankton biomass was generally associated with a substantial increase in large Cladocera densities and size and, in some cases, restoration of submerged vegetation. Large Cladocera and submerged vegetation appeared to be the main factors controlling phytoplankton growth. In the absence of fish, large cladocerans could control phytoplankton even when submerged vegetation was not restored. Such ponds, however, are more vulnerable to recolonisation by fish than those where submerged vegetation was restored. They have shown marked fluctuations in phytoplankton biomass associated with fish reintroduction. Therefore, restoration of submerged vegetation is essential for assuring the longer term success of biomanipulation. It is also important for ecological quality restoration because the vegetated ponds showed greater improvements in the ecological quality than the non-vegetated ones. Addition of piscivorous fish can make the biomanipulated ponds more resistant to the backward shifts to the turbid state and subsequent deterioration of ecological quality. The effect of piscivore addition might depend on whether submerged vegetation was restored or not as a result of biomanipulation. To make future management interventions more focused and cost-effective, further research on the response of the pond ecosystems to different biomanipulation techniques is needed. Although a useful management tool, biomanipulation on itself can not reverse the process of eutrophication, it can only mitigate some of its consequences. Additional measures aiming at reducing nutrient loading can facilitate the biomanipulation efforts. For successful biomanipulations the ponds should remain within the bistable nutrient range which still permits the shift to the clear water state. Beyond this range, manipulation of fish community will have little, if any, effect on ecological quality of ponds. |
