La végétation aquatique et du bord de l'eau dans le bassin versant du Bocq (Condroz, Belgique) The aquatic and bank vegetation in the Bocq catchment (Condroz, Belgium)

Résumé

La végétation aquatique et du bord de l'eau du Bocq (Condroz, Belgique) et de ses trois principaux affluents a été décrite à partir de plusieurs centaines de relevés, à la fois de végétation et de milieu (environ 60 variables). En plus, des analyses détaillées d'eau ont été faites dans 21 prélèvements. Les tableaux de données ont été étudiés de manière séquentielle : - l'analyse des totaux marginaux, - l'analyse des motifs des lignes, - la simplification des tableaux, - et les analyses multivariables comme l'analyse en composantes principales, l'analyse des correspondances, la gradation non métrique multidimensionnelle et diverses techniques de classification automatique.

Différents groupements aquatiques ou des berges sont alors décrits, cartographiés et mis en relation avec différents facteurs de l'environnement. La nette influence de la pente, du degré de minéralisation, de la teneur en dérivés d'azote et de phosphate et de l'action humaine, sur le répartition des macrophytes sont facilement révélées. Les résultats sont comparés avec ceux obtenus par le système de Braun-Blanquet ou d'autres approches.

Mots-clés:

• Macrophytes,
• relevés de végétation,
• hélophytes,
• analyses chimiques,
• analyses multivariables,
• groupements végétaux,
• pollution

Abstract

The Bocq river (45 km long) and its main tributaries, the Crupet river (11 km long), the Petit Bocq (10 km long) and the Leignon river (11 km long) flow in the Condroz, to the south of the Sambre and Meuse line. The streams generally run across pastures, woods or villages : cultures are not important. The concentration of hydrogen carbonate in water is generally high.

River vegetation was observed in a set of 362 relevés, having a lengh between 10 and 100 m. The streams were divided into 1 km sections and in each section, three relevés were chosen, in three different environmental conditions except in the Crupet river and in every top section of the three other streams in which all the relevés were contiguous. For each relevé, the aquatic and amphibious macrophytes were listed and the abundance of some species was estimated with a 1-5 index. Several environmental features were noted such as width and depth of the channel, the main types of substrate, the general slope, the characteristics of the bank, the type of edge vegetation, the type of landscape, the rock type, the sources of damage and chemical parameters. Some of these variables were divided in 0-1 modes, so only qualitative variables were used. Detailed physico-chemical and chemical analyses were carried out from 21 samples, in 1987 and 1988. Our aim was to give a simple and complete description of vegetation and environment, so that it would be possible to observe the evolution (natural or not) of the stream.

For each stream, species x relevés and environmental features x relevés tables were established. The data processing was sequential, in four steps : - the examination of the marginal distributions, - the pattern analysis of the lines, - the simplification of the tables by suppressing the rare and unstructured species or variables, - the replacement of the columns by a smaller number of blocks (one or two block sites were used and for each site, only the structured species or variables were kept), - the multivariate analyses of the simplified tables with principal component analysis, correspondence analysis, non-metric multidimensional scaling and cluster analysis (only with transformed data from the multivariate analyses). Vegetation or vegetation and environment tables were used.

The principal component analysis of the chemical parameters revealed the importance of the calcium hydrogen carbonate complex (first axis), of the nutrients like N and P (second axis) and of Ion Na (third axis).

The following groupings were then defined and mapped : grouping with Caltha palustris and Lychnis flos-cuculi, with Juncus effusus, with Myosotis scorpioides and Scraphularia auriculata, with Cirsium oleraceum and Petasites hybridus, with Berula erecta and Callitriche platycarpa, with Elodea canadensis, with Callitriche obtusangula, with Sparganium emersum, with Fontinalis antipyretica and Ranunculus fluitans for the Bocq; with Ranunculus fluitans, with Fontinalis antipyretica, with Alopecurus geniculatus and Apium nodiflorum, with Nasturtium officinale and Rumex conglomeratus, with Festuca gigantea and Fraxinus excelsior, with Polygonum bistorta, with Polygonum amphibium, with Typha latifolia, with Cardamine amara and Chrysosplenium opppusitifolium for the Crupet river; with Alnus incana, with Polygonum hydropiper, with Scrophularia aurilata for the Petit Bocq; with Epilobium parviflorum and Lotus uliginosus, with Callitriche obtusangula, with Fontinalis antipyretica for the Leignon river.

The main Matures which influence vegetation are : the general slope (or current velocity), the calcium hydrogen carbonate complex, nutrients like N and P and anthropic influence. The effluents of Ciney are responsible of a severe organic pollution in the bottom part of the Leignon river and in several km of the Bocq. Our results are compared with those obtained with the Braun-Blanquet system or other approaches.

Keywords:

• Macrophyts,
• vegetation relevés,
• helophyts,
• chemical analyses,
• multivariate analyses,
• vegetation groupings,
• pollution