| Popis: |
Summary Small lakes and ponds are valuable for several reasons, such as biodiversity conservation and recreational purposes (fishing, boating etc.). During the past decades, many shallow lakes and ponds have been subject to a considerable degree of eutrophication as a result of human activities, which can cause a transition to a turbid, phytoplankton-dominated water state, often resulting in severe blooms of potentially toxic cyanobacteria. The aim of this study was to investigate the ecological state of eight Brussels ponds two years after biomanipulation (i.e. pond drawdown and fish removal), as well as the effects of pike addition, by means of assessing changes in phytoplankton, zooplankton, macrophytic vegetation communities and main nutrient concentrations. In order to investigate the biomanipulated ponds within a broader context, 23 additional ponds were studied during the same period, including two ponds that were biomanipulated during previous years. 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) to hypereutrophic (turbid) 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 first year after fish removal, biomanipulation was very successful in all but one pond. The removal of plankti-benthivorous fish disposed the large zooplankton from predation and allowed them to control phytoplankton biomass through intense grazing. The ecological quality improved markedly in most of the ponds. During the period after biomanipulation however, juvenile planktivorous fish returned in some ponds, feeding on large zooplankters. In vegetated ponds, the effect of planktivorous fish could be buffered by the presence of submerged macrophytes, through competition with phytoplankton for nutrients and providing shelter for zooplankton that grazes on phytoplankters. In non vegetated ponds, the reintroduction of planktivorous fish immediately resulted in the disappearance of large Cladocera and as such shifted the ecosystem back to the turbid state. It seems that vegetation can indeed buffer the effect of fish reappearance, although this is probably not the case when nutrient loading is too high. In such case, nutrient reduction is the only way to improve water quality and establish a clear-water state. The addition of pike in all biomanipulated ponds was not sufficient to control juvenile planktivorous fish, as suggested by the zooplankton and phytoplankton data in some ponds that, despite pike addition, returned to the turbid state in 2008. A simulation of a worst-case scenario performed with a simplified growth model, has shown that indeed pike addition was insufficient. Based on these results, we advise to add pike at reproductive age in sufficient numbers, that will produce the required amount of young pike in spring to control juvenile planktivorous fish. Taking into account the importance of submerged macrophytes for pike reproduction and for stabilization of the system after biomanipulation, measures should be taken to restore submerged vegetation in those ponds where macrophytes did not recover. Such measures can be the reintroduction of propagules (i.e. seeds, oospores,rhizome fragments) from sediment of ponds with a dense vegetation and by lowering grazing pressure on young shoots by providing protection or lowering the number of birds. |
