Ecological evaluation of pike addition in the biomanipulated ponds of the Brussels Capital Region

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.