Indicator value of lotic water mites (Acari: Hydrachnidia) and their use in macroinvertebrate-based indices for water quality assessment purposes

Abstract

The Water Framework Directive (WFD) of the European Union includes benthic macroinvertebrates for the ecological assessment of running waters. The invertebrate-based Star-ICMi index, adopted in 2010, does not include Hydrachnidia (water mites) in its complex formulation. However, Hydrachnidia are associated with many environmental variables and may be useful for stream ecological characterization. We have therefore assessed the bioindicator value of Hydrachnidia in streams of central Italy, and the ability of two mite-containing (the PTH and the newly formulated PTHfam indices) and four mite-excluding biotic indices (Star-ICMi, BMWP, ASPT, and IBE) to characterize 216 sites ranging from semipristine to impacted. Mite bioindicator value was high at family level. Index reliability was consistently low for the PTH and ASPT indices. Distribution of mite families across quality classes (QCs) was fully separated only for the PTHfam index. QC assignment remained reliable at high index values (i.e., at high ecological status) for the PTHfam and BMWP indices, while logarithmic correlations between the PTHfam and the other indices suggest that the latter may misrepresent sites in high, good, and moderate ecological status sensu WFD. Further studies on the PTHfam index are warranted in light of its simplicity, high reliability, and low sampling and taxonomic identification effort.