Biomanipulation of eutrophic ponds: the importance of submerged vegetation recovery

Thirteen phytoplankton-dominated eutrophic ponds in Brussels (Belgium) were biomanipulated (complete emptying and fish removal) in order to restore the ecological quality and reduce the risk of cyanobacterial blooms. Data on nutrients, phytoplankton, macrophytes and zooplankton were obtained before and after biomanipulation. Fish removal initially resulted in a drastic reduction of phytoplankton biomass and a shift to the clear-water state in twelve ponds. Lower phytoplankton biomass was associated with a marked increase in density and size of large cladocerans and/or a recovery of submerged vegetation.
During the period after biomanipulation, the ponds have shown very different dynamics, mainly defined by planktivorous fish recolonization and presence of submerged vegetation. In ponds where fish remained absent, large cladocerans were perfectly capable of controlling phytoplankton biomass, regardless of the presence of vegetation. All ponds that were not
recolonized by fish as such remained in the clear-water phase.
As a result of fish recolonization in some of the ponds, a strong decrease of density and size of large cladocerans was observed, even in vegetated ponds, suggesting that submerged macrophytes did not provide any shelter for large zooplankton against fish predation. Despite a strong impact of fish predation on large cladocerans, only non-vegetated ponds returned to a turbid, phytoplankton-dominated state. In ponds with a submerged vegetation cover of > 30 %, submerged macrophytes inhibited an increase of phytoplankton growth, most likely through nutrient limitation, as suggested by the lowered nutrient concentrations in vegetated ponds.