Are quality biological indices universally useful?: Limits of benthic based indices in a brackish and arid sub-basin from Argentina

Abstract

The water quality of rivers belonging to an arid sub-basin in northern Argentina was evaluated in a longitudinal gradient, during the period of maximum dryness in 2017. The composition and structure of the phytobenthos were studied using biotic indices. A total of 126 spp. were determined, with a predominance of diatoms and Cyanobacteria. Most of the diatom species found were meso to polyhalobus and tolerant to organic contamination. Despite the rivers of the basin cross areas with very different land uses, no differences were recorded in phytobenthos abundances between sites. The Pantle & Buck Saprobity index classified the sub-basin in the α-mesosaprobability range, and the diatomaceous eutrophication/pollution index assigns poor quality to most of the sites. Salinity should induce phycoflora homogenization that limits the sensibility of biotic indices to anthropogenic impacts.