Performance of different biotic indices and sampling techniques for the assessement of water quality in the lowland stretch of the river Tiber.

The lowland stretches of rivers are usually the ones where a macroinvertebrate species collection is more difficult to obtain because of the depth and turbidity of the water. Since the choice of the index and of the specific sampling technique is critical for the final result, there is a strong interest into setting up reliable methods for assessing the biological water quality. The aim of this paper is to elucidate the performances of four biotic indices and the influence of four sampling methods on the water quality assessment of a deep and turbid stretch of a river. We collected macroinvertebrates seasonally at 10 sampling sites along the final stretch of the Tiber River using four sampling methods (including hand net, wood and plastic artificial substrates). At the same time, chemical, physical, and bacteriological variables were recorded monthly along the same river stretch. Considering those variables, sampling sites were grouped into three clusters with different water quality, using the chronological clustering method. The same result was obtained from macroinvertebrate data using the same method. We then examined the ability of four biotic indices widely used in Italy to discriminate between the three clusters. The indices are: BMWP' (British Monitoring Working Party, modified for Spain), ASPT' (Average Score Per Taxon, modified for Spain), EBI (Extended Biotic Index) and IBE (Indice Biotico Esteso). BMWP' and ASPT' showed the best results. BMWP' and ASPT' scores – but not those of EBI and IBE – were sensitive to the different sampling methods utilised. We concluded that in this type of river stretch, BMWP' and ASPT' could be more useful than EBI and IBE although the relative efficiency of the chosen sampling method should be taken into account.