Chemical and biological quality of the Belgian Semois basin

The fresh water of the Belgian Semois catchment area was studied at 25 stations from the point of view of several complementary aspects: chemical composition, diatoms and benthic invertebrates. The methods used were the organic pollution index (IPO, LECLERCQ and VANDEVENNE, 1987), a diatom index (ID, LECLERCQ, unpublished) and the global normalized biological index (IBGN, AFNOR, 1992). The chemical and diatom analyses enabled us to describe several kinds of natural water subject to increasing mineralization and presenting various levels of pollution and eutrophication. We present a detailed quality map for the basin managers. Diatoms seem to be the most reliable bioindicators for these parameters. Invertebrates are less interesting, as they are practically insensitive to water mineralization (at least, at the family level which is the only possible identification level in routine monitoring); moreover, invertebrates may be more sensitive to the substrate than to organic pollution, which can lead to erroneous quality evaluations.