Ecological follow-up of the active biological management of some ponds administered by the B.I.M.

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.