U bent hier

Biological quality of the main waterways and ponds

The improving trend observed between 2004 and 2010 for the biological quality of the waterways and the Canal continued especially for the Senne in 2013, even though its overall quality is still far from the objective of "good ecological potential". For the other waterways, the quality appears to have stabilised. With regards to the three ponds studied since 2004, all of which are located in the Woluwe valley, their ecological quality has not evolved greatly in recent years. Good ecological potential is achieved for certain elements of biological quality. The poor scores for the fish index can be applied right across the territory.

A monitoring network based on 5 groups of biological quality elements

Evaluations of the biological quality were carried out in 2004, 2007, 2009, 2010 and 2013, except for fish in 2009 and 2010. Future evaluations will be carried out every 3 years, in compliance with the Water Framework Directive (WFD) and the recommendations of experts. The various sampling points are located on the Senne, the Canal, the Woluwe (waterways and ponds), and one of its tributaries, the Roodkloosterbeek. Given their small scale, the WFD does not impose an evaluation for Brussels ponds, but the directive is nonetheless carried out for three of them using management grants. In 2007, two new monitoring sites on the Senne, after the South plant and before the North plant, were added in order to monitor the impact of discharge from the water treatment plants. In 2009, the Neerpedebeek, Molenbeek, Vogelzangbeek and Linkebeek were also evaluated (and whose results were published in the previous edition of the State of the Environment).

Five biological groups were taken into account:

  • phytoplankton (generally microscopic algae suspended in water),
  • macrophytes (plants such as reeds),
  • phytobenthos (living micro and macro algae which are attached to or near to the riverbed),
  • macro-invertebrates (insects and larva, worms, crustaceans, etc.)
  • and fish.

The evaluation of the biological quality of each index is based in particular on a comparison of the observed situation with reference conditions. These equate to the optimal situation ("maximum ecological potential"), taking into account deterioration of the natural physical conditions caused by human activity (see methodological sheet).

Evaluation of the biological quality of surface water in the Brussels Region

The maps below illustrate the evaluations obtained for the 11 measuring points subjected to monitoring since 2004 or 2007. The evaluation summarised below derives from the results of the study carried out during the 2013 measuring campaign (Van Onsem et al., 2014).

Evolution of the biological quality of the main Brussels waterways and Woluwe ponds
Source: Brussels Environment, dpt. Reporting and environmental impacts, 2015


With regards to the Senne at the exit to the Region, the improvement in biological quality observed between 2004 and 2007 only continued for macrophytes. The biological quality of the three other biological groups did not show any pronounced trends between 2007 and 2013, whereas the physico-chemical quality evolved positively over the same period (see "General physico-chemical quality of surface water"). It can be observed nonetheless that the 2013 results are encouraging when compared to those of 2010, both in terms of the quality of water and habitat (progression to a higher category from "poor" to "moderate" or from "bad" to "poor" for the macrophytes and phytobenthos indices (except one of the two sites) and macro-invertebrates). However, only the results from future measuring campaigns will make it possible to determine whether there has been a genuine evolution or simple fluctuations in biological quality. At the entry to the Region, the situation remains stable and has even changed for the better for macro-invertebrates. These few positive trends at the entry and exit to the Region do not mean that we can claim to have achieved good potential however. Besides, although fish are observed both upstream and downstream of the Flemish Region, their presence is never recorded in the Brussels Region. There are many obstacles to overcome to reverse this situation: barriers to fish migration, profound hydromorphological upheaval (concreted riverbanks, coverings), but also numerous point source or diffuse source discharges which the Senne receives both during dry periods (processed discharge from water treatment plants) and rainy periods (outlet for numerous stormwater overflows).
The Canal shows average biological quality in general, for whichever index is considered, and has consistent quality between the entry and exit to the Region. The values obtained for the macro-invertebrates and fish indices show an increasing trend: good potential is even respected for macro-invertebrates at the exit to the territory. However, fish populations are severely affected by the domination of invasive species, which is often the case in waterways. Moreover, the values obtained for the phytoplankton and phytobenthos indices have shown a decreasing trend since the last campaign, although this has not been reflected in a change of quality grade.

The situation is mixed for the Woluwe. Biological quality achieves good potential for macrophytes and phytobenthos, and the trend for these two indices is positive. In contrast, the quality relating to macro-invertebrates and fish has evolved unfavourably ("average" and "poor" respectively in 2013): the habitat has regressed for these two indices. For macro-invertebrates, this deterioration has been accompanied by a loss of quality grade. However, for the first time in 2013, American crayfish (an invasive species) have been observed: they could pose a threat for macrophytes and macro-invertebrates.

The biological quality of the Roodkloosterbeek deteriorated in 2013 compared to 2009 and 2010. The causes of this deterioration do not appear to be the water quality, judging by the good score achieved for phytobenthos (the only index to achieve good potential and experience a positive trend). All the other indices have fallen by one quality grade. Although the macro-invertebrate index has in any case achieved average quality, the macrophytes and fish indices are graded as bad quality. The characteristics of the habitat (state of the banks and riverbed, significant shadow) would prevent the settling of macrophytes. The presence of the American crayfish probably also plays a role.

The pond in the Sources Park (ETA 3 on the map) shows poor quality for macro-invertebrates (which was still average in 2010), but average for the other indices. The values show a stable evolution for phytoplankton and positive evolution for macrophytes and fish. The good score for the fish index should be qualified, given that this pond is used as a fishing pond.

The long pond in the Woluwe Park (ETA 2) and the Boitsfort pond (ETA 1) achieve good potential for macrophytes and macro-invertebrates, and average quality for phytoplankton. This positive assessment can be correlated to the bio-manipulation carried out in 2007 and 2005 respectively, whose effects were still felt in 2013. Although, unlike the long pond, these beneficial effects appear to be slowing down for the Boitsfort pond, which has experienced a deterioration in quality and turbid water since 2009-2010. As for the score for fish, it remains bad or poor.

Bio-manipulation: an operation which is beneficial for the quality of ponds if it is carried out and monitored effectively

As indicated above, one of the clear improvements observed in two of the three ponds is attributed to the management measure taken in the context of the Blue Network programme: bio-manipulation. An apt moment to look back on the purpose and impacts of this measure, which has been implemented and studied at other Brussels ponds.

Bio-manipulation has the following objectives: achieve clear water by taking action on the food chains within the aquatic ecosystem (turbidity is linked to phytoplankton), increase the ecological quality of ponds and ensure the stability of the aquatic ecosystem if possible. It consists of a winter drying-out (enabling the oxygenation and mineralisation of sludge) and a partial or total elimination of fish (see the documented sheet). Normally, this then results in a positive effect in terms of water transparency. Since light penetrates more deeply, the submerged vegetation can be restored. If the submerged vegetation coverage is sufficient, piscivorous fish are reintroduced in order to stabilise the state of the aquatic ecosystem.

The bio-manipulative initiatives conducted at the 13 Brussels ponds between 2005 and 2009 were characterised by a significant improvement in their ecological quality in the short term, with the exception of one pond (VUB & APNA, 2010). However, success over the medium term is far from guaranteed: 6 ponds have actually reverted to having high concentrations of phytoplankton. Various causes of failure have been identified (the amount of fish removed was insufficient, the fish were reintroduced too soon, the nutrients content - total phosphorus in particular - was too high).

The knowledge obtained with regards to bio-manipulation (VUB & APNA, 2010) has confirmed the benefits of this method (in comparison with 17 ponds which were not bio-manipulated). An intervention strategy has also been implemented to identify the appropriate restorative initiative for the specific context at each Brussels pond. A regular follow-up of the ponds for which bio-manipulation had positive effects appears to be crucial, given the rapid evolution dynamic observed.

Positive impacts on biodiversity from the improvement of the biological quality of water

The improvement in the indices of biological quality in the Woluwe valley are the reason behind the remarkable evolution of populations of odonates (dragonflies and damselflies) between 2006 and 2013, both in terms of the number of species and their conservation status, according to researchers (see Green spaces and biodiversity "Monitoring of species").

What is the outlook for waterways?

For waterways, initiatives concerning hydromorphology (in particular the free circulation of fish) or the limiting of emissions of pollutants (dredging of sediment, reducing the operation of stormwater overflows, eliminating illegal discharges) would appear to be the first steps before any ecological restoration, and essential for making any significant progress in their biological quality.

Although the severe hydromorphological pressures on the Brussels waterways are inherent to the urbanised character of the Region (cfr. the list of deteriorations in chapter 2 of the second water management plan, 2015), it is certainly possible to reduce these pressures in some areas so as to recreate habitats which are favourable to living aquatic communities, and which serve as a basis for evaluating biological quality. To ensure that these initiatives are planned, the second water management plan (currently being adopted) anticipates the drafting of an inventory of the hydromorphological status of the Senne and the Woluwe.

With regards to limiting discharges of pollutants, the regional policy of collecting and processing waste water discharges will continue. At the same time, the quality management policy for run-off rain water as well as the policy of recovering clear water in the hydrographic network are becoming more and more significant (cfr. the second water management plan). Another measure for limiting discharges of pollutants is the removal of sediment from the Senne: dredging operations were carried out in the upstream section in summer 2013, and planned to be carried out in the downstream section in 2016. It will be interesting to observe whether this initiative will be accompanied by positive impacts on biological quality during future measuring campaigns.

A threat: the spread of invasive species

The presence of invasive exotic species could be problematic for the management and restoration of aquatic ecosystems. American crayfish were observed in the Roodkloosterbeek, and for the first time, during the monitoring campaign in 2013, in the Woluwe. Moreover, numerous living aquatic communities in the Canal are dominated by invasive species (see Green spaces and biodiversity "Invasive exotic species").

Datum van de update: 30/05/2020


Study(ies) and report(s)

VUB & INBO - VAN ONSEM S., BREINE J. & TRIEST L., March 2014. « De ecologische kwaliteit van waterlopen, kanaal en vijvers in het Brussels Hoofdstedelijk Gewest in 2013: fytoplankton, fytobenthos, macrofyten, macro-invertebraten & vissen ». Study performed on behalf of Brussels Environment. 117 pp. (without the annexes) (.pdf, in Dutch only)

VUB & ULB - TRIEST L., VAN ONSEM S., JOSENS G. & CROHAIN N., March 2012. « Beoordeling van de ecologische kwaliteit van waterlichamen in het Brussels Hoofdstedelijk Gewest in uitvoering van de Europese Kaderrichtlijn Water ». Study performed on behalf of Brussels Environment. 207 pp. (.pdf, in Dutch only)

VUB & APNA - DE BACKER S., PERETYATKO A., TEISSIER S., TRIEST L., March 2010. « Ecologische beoordeling van het effect van biomanipulatie op langere termijn in enkele vijvers in het Brussels Hoofdstedelijk Gewest ». Study performed on behalf of Brussels Environment. 52 pp. (.pdf, in Dutch only)

VUB, INBO & ULB - TRIEST L., BREINE J., CROHAIN N. & JOSENS, G., January 2008. « Evaluatie van de ecologische staat van sterk veranderde en artificiële waterlichamen in het Brussels Hoofdstedelijk Gewest zoals bepaald in de Kaderrichtlijn Water 2000/60/EG ». Study performed on behalf of Brussels Environment. 228 pp. (.pdf, in Dutch only)

VUB, IBW & ULB - VAN TENDERLOO A., TRIEST L., BREINE J., BELPAIRE C., JOSENS G. & GOSSET, G., December 2004. « Uitwerking van een ecologische-analysemethodologie voor sterk veranderde en kunstmatige waterlichamen in het Brussels Hoofdstedelijk Gewest zoals in toepassing van de Kaderrichtlijn Water 2000/60/EG ». Study performed on behalf of Brussels Environment. 192 pp. (.pdf, in Dutch only)