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Ecological quality of the main waterways and ponds

Between 2004 and 2010, the overall ecological quality of the Zenne, the Kanaal and the Woluwe remained relatively stable (ranging from bad to good according to the waterway in question). Nevertheless, monitoring indicates that progress has been achieved in the Woluwe and Zenne for a number of specific bio-indicators. As for the 3 ponds that have been studied since 2004 (all 3 forming part of the Woluwe valley), 1 has reached "good ecological potential" since 2007 while the other 2 are of moderate ecological quality.


Pursuant to the European Water framework directive (WFD), every Member State must implement networks for monitoring water quality and taking the necessary measures to ensure that their bodies of surface water reach a "good ecological and chemical condition" by 2015. The evaluation of the ecological quality of the waterways depends, specifically, on analysing the composition and presence of various groups of biological indicators in relation to certain reference circumstances. The latter correspond with the natural condition; for significantly modified waterways (Zenne and Woluwe) or man-made ones (Kanaal), the reference is the optimum situation ("maximum ecological potential") which takes into account the changes brought about by human activities in terms of natural physical conditions. For this last category of waterways, the target of "good ecological conditions" is replaced by the "good ecological potential" target.
Four large groups of biological indicators must be considered: there is the water flora that contains the macrophytes (higher plants such as reeds) and the phytobenthos (plant-based plankton that lives on the water bed, such as diatoms), secondly the phytoplankton (generally microscopic water plants in suspension in the water), and then the macro-invertebrates (insects and larvae, worms, molluscs,...) and finally the fish.

Evaluation of the ecological quality of the surface water in the Brussels Capital Region (BCR)

The assessment of the ecological quality concerns the years 2004 and 2007 and (aside from fish) also the years 2009 and 2010. In accordance with the guidelines and recommendations of experts, the next evaluation will be carried out in 2013 and then once every three years. The various sampling points are located along the Zenne, the Kanaal and the Woluwe (waterway and ponds) and 1 of its tributaries. The WFD does not impose any evaluation for the ponds of BCR given their limited size. However BCR also assesses the quality of the ponds for management purposes. In 2009, the Neerpedebeek, the Molenbeek, the Vogelzangbeek and the Linkebeek were also evaluated.
The maps below illustrate the evaluations obtained for the 9 measurement points that have been monitored since 2004. The number of sites where the general ecological quality of the water was found to be moderate to good, rose from 3 to 4 between 2004 and 2010. The evaluation principle that applies is however extremely stringent because it is based on the group of biological indicators that obtained the lowest score. Furthermore, we had to limit the scope of the overall evaluation to the biological indicators for which data were available. From the review of the change per group of bio-indicators, it appears that in most measurement points the ecological quality remained identical or progressed. In some cases - large pond at Bosvoorde, long pond at Woluwepark - progress was significant. A slight improvement also occurred for the Zenne: the appearance of macrophytes from 2009 on, both at entry to and egress from Brussels, as well as insects (chironomidae) and molluscs, in 2007 and 2010 respectively at egress from the Region.

Ecological quality of the most important Brussels waterways and ponds of the Woluwe (2004-2007-2009-2010): overall evaluation and evaluation per group of bio-indicators
Source: Bruxelles Environnement-Leefmilieu Brussel, Sub-department Water, 2011
Ecological quality of the most important Brussels waterways and ponds of the Woluwe (2004-2007-2009-2010): overall evaluation and evaluation per group of bio-indicators

With regard to the sites that were only sampled during the last analysis campaign:

  • The overall ecological quality of the pond of de Verdronken Kinderen was found to be moderate and that of the pond Ten Reuken was good (2009) (both ponds are located in the Woluwe valley);
  • The overall ecological quality of the Neerpedebeek and the Vogelzangbeek was found to be bad; that of the Linkebeek was inadequate and that of the Molenbeek (Laarbeekbos) was moderate (2009). A possible explanation of these results is the fact that these waterways still receive discharges that come from households, agriculture or the roads network.

Various measures within the context of the Blue Network programme contributed to the clear improvements that have been observed in various ponds: notably the winter drying-out which oxygenates and mineralises the silt (large pond at Bosvoorde in 2004 and 2006, long pond in the Woluwe park in 2007, pond Ten Reuken in 2002, pond of de Verdronken Kinderen in 2001-2002), the surveillance of the fish populations, the ecological management of riverbanks and pruning,...


Date de mise à jour: 24/06/2020