| Popis: |
During the past decades, many shallow lakes and ponds have been subject to a considerable degree of eutrophication as a result of human activities. Increased nutrient loading can cause a transition from a clear to a turbid, phytoplankton-dominated state, often resulting in severe blooms of potentially toxic cyanobacteria. Brussels ponds in general show a considerable variation in phytoplankton biomass, despite the overall high nutrient concentrations (eutrophic to hypereutrophic when considering total phosphorus). As factors other than nutrients seem to determine phytoplankton biomass in these ponds, they have a good potential for restoration by means of biomanipulation. The aim of this study was to follow-up the effect of biomanipulation (partial or complete water drawdown, fish removal and/or pike addition) performed on 18 Brussels during 2005 - 2010 by means of assessing changes in phytoplankton, zooplankton, macrophytic vegetation communities and main nutrient concentrations. Additionally, two turbid, cyanobacteria-dominated ponds in which sediment was removed (no water drawdown) in spring 2010 were also studied. In 14 out of 15 ponds in which a complete fish removal and water draw down was applied, a decrease of phytoplankton biomass was observed to a certain extent during the 1st year after biomanipulation. Coinciding with the increase in transparency, large cladocerans generally increased in size and density. Submerged vegetation was restored in several ponds and often consisted of several macrophyte species. On the longer term, the situation after biomanipulation varied a lot from pond to pond and depended on several factors. An important factor for stabilization of the obtained clear-water state on the longer term is the recovery of submerged vegetation. Submerged vegetation is known to have the potential to restrain phytoplankton biomass through several mechanisms. Especially when fish recolonized the ponds and reduce zooplankton grazing through predation, submerged macrophytes can serve as a bufferagainst phytoplankton biomass increase and prevent a shift back to the turbid state. Submerged macrophytes can potentially prevent a phytoplankton biomass increase after fish recolonization, but only if nutrients are sufficiently low. Above a certain threshold of nutrient concentration in the water column (a rough average TP concentration of 350 µg P L-1 was obtained from data on Brussels ponds), macrophytes are less able to efficiently control phytoplankton biomass during the whole summer, often resulting in a collapse of macrophytes, coinciding with a shift back to the turbid state. Pike additions should be carried out aiming at establishing a stable, reproducing pike population, contributing to the stabilization of a clear-water state on the longer term rather than to obtain an immediate reduction of recolonizing planktivorous fish. Care should be taken that pike are added in clear-water vegetated ponds, as pike hunt by sight and need vegetation for their reproduction, mainly for attachment of the eggs and to provide shelter for the young pike to protect them from older pike. The biomanipulation results confirm the importance of fish in determining the ecological quality of eutrophic ponds and indicate that when pond ecosystems are impaired by eutrophication, a considerable degree of their ecological quality can be restored through manipulation of the fish community. Taking into account the importance of submerged vegetation recovery, efforts should be made to enhance their restoration after biomanipulation. Also, reduction of nutrients to a certain extent remains important when a longer term effect of biomanipulation is desired. |
