Abstract
In order to analyze the distribution and evolution of the aquatic vegetation and ecological status of the Semois-Chiers basin (Semois sub-basin and Chiers sub-basin), macrophyte surveys were conducted at 48 sites in 2007 and 2013. Environmental parameters were also measured in order to characterize the waterbodies in terms of physico-chemical properties and anthropogenic pressure. The two-way clustering and indicator value (INDVAL) methods were used to assess groups of sites according to their macrophytic composition and species communities. The results showed a clear difference between streams in the Lorraine area (calcareous watercourses) and in the Ardennes (siliceous). Within each natural region, those with natural vegetation of high ecological status were separated from those dominated by resistant species. The Macrophytical Biological Index for Rivers (IBMR) was calculated for the sites visited in 2010 and 2013 and the results show a trend towards an increase of IBMR values of polluted sites. For the latter, the Wilcoxon test was performed to assess the significance of the difference in quality between 2010 and 2013. This showed a statistically significant difference (p-value = 0.035). Our results showed similarities with previous data, as well as some differences. The differences observed might indicate a gradual change in the composition of the vegetation in the study area, which was caused by changes in environmental conditions. They could also reflect a lack of information about the ecology of certain groups of plants, mainly bryophytes and macroalgae that were not considered in previous studies. Despite the measures implemented under the EU’s Water Framework Directive (WFD), the current vegetation of the Semois river differs little from that observed in 1996. The headwaters of a Semois river, described in previous studies as polytrophic and devoid of vegetation, show a slight improvement, with the appearance of macrophytic species. In some parts of the Chiers sub-basin, however, resistant species observed in 1999 persist.
Highlights
Under the European Union’s Water Framework Directive (WFD), the ecological status of a water body is evaluated on the basis of three main factors: biology, physical chemistry and hydromorphology
The specific objectives of the study were to: i) analyze the aquatic vegetation at 48 sites on various waterbodies in the Semois-Chiers basin; ii) define the characteristics of the macrophytic communities at these sites and identify their eco-chemical indicators; and iii) analyze trends in the IBMR values and macrophytic community in order to identify any changes in the status of these sites resulting from measures implemented under the WFD
Within the five groups defined, the results showed a clear difference between the Lorraine groups (2 and 3) and the Ardennes ones (4 and 5). Those with natural vegetation of high ecological quality were separated from those dominated by resistant species
Summary
Under the European Union’s Water Framework Directive (WFD), the ecological status of a water body is evaluated on the basis of three main factors: biology, physical chemistry and hydromorphology. In the Semois-Chiers basin, information on aquatic macrophytes is limited, apart from that in the work reported by [1]-[22]. Thoen et al [23] showed that there was a longitudinal succession of species along the Semois river and that this succession was determined by eco-chemical factors They described macrophytic groups over a distance of 200 km based on an analysis of 218 botanical records. Studies were conducted on vascular plants and some information was provided on bryophytes and macroalgae In these studies, the reference conditions were not specified and the communities studied included both sensitive and resistant species. The reference conditions were not specified and the communities studied included both sensitive and resistant species Despite these studies, the Semois-Chiers area remains under-surveyed. The reference macrophytic communities were waterbodies classified by the Walloon Government as being areas showing little or no human impact [28]
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