Is the IBMR macrophytic index a reliable tool for water quality monitoring?

Macrophytes contribute to the functioning of aquatic ecosystems, and their sensitivity to pollution allowed their use as biological indicators. It is in fact generally true that some of these species are very sensitive to biocidal products, organic compounds and excess of nutrients. Along riparian and lacustrine ecosystems, several types of plant communities dominated by aquatic macrophytes may be found with different structure and floristic composition, due to the interactions of biotic, abiotic and anthropogenic components. Most of them can be referred to the phytosociologic classes Charetea Fukarek ex Krausch 1964, Potametea Klika in Klika & Novák 1941 and Lemnetea O. Bolos & Masclans 1955, and to the Annex I Habitats 3140, 3150, 3260 of 92/43/EEC Directive. Following the enactment of the European Water Framework Directive (WFD, 2000/60/EC), studies on use of biotic indices have been widely developed and new methodologies have emerged in order to determine the ecological quality of water bodies, by calculating the Ecological Quality Ratio (EQR) or the ratio between the values of the biological parameters observed for a given water body and those detected at the reference conditions applicable to that. The indices based on the use of aquatic macrophytes give indications on the overall water quality and the level of alteration of water bodies, in particular due to organic pollution; they are now widely used in several European countries, especially Great Britain, France and Austria. Among the various methods, one of the most widely accepted is the Index Macrophytique Biologique en Rivière or IBMR, used to determine the trophic status of the continental part of natural and artificial rivers, essentially related to the content of ammonium, orthophosphate and events of organic pollution. The actual effectiveness of this index, however, has not been properly verified and some studies have reported the absence of a significant correlation between IBMR and nutrients (especially ammonium and phosphates!), highlighting various problems that emerged in using this index in the assessment of watercourses quality. In this scenario, and especially considering that the index IBMR has been identified by the Italian authorities as the official national methodology to be used to control water environments in accordance with the WFD, a study was conducted to investigate its effectiveness in general, and in particular in the Italian territory. The methodology has been applied to streams and rivers of Central Italy with different conservation status and rather diversified hydrological, physical and geolitological conditions, in order to represent the variability of a large territory. In the period 2011/13, 36 sampling sites along 15 water bodies belonging to 6 sub-basins of the River Tiber, covering a total area of 11,707 km2, were investigated. The values obtained were compared with the chemical and physical data collected in the same period at the same sampled sites and subjected to correlation analysis. Only a handful of considered chemical and physical parameters were significantly related to changes in the value of IBMR. In particular, it is remarkable the absence of significant correlations of IBMR with some parameters related to trophic status, such as P, nitric N, nitrous N, Nitrites, Ammonia. Moreover, it was shown that the correlation is particularly weak when some stations have chemical or physical parameter values markedly differing from the others. This could mean that the IBMR has been calibrated over a range of physical and chemical parameters with less variable values than those found in the rivers and streams of Central Italy. On the basis of the obtained results, it seems that IBMR is a rather weak tool to rely on, for water quality monitoring in the considered territory.