Original paper

Assessment of rivers using diatoms: effect of substrate and evaluation method

Mendes, Tânia; Almeida, Salomé F.P.; Feio, Maria J.


The aim of this work is to clarify the effect of using diatom assemblages from different substrates, and applying different methods, to assess water quality. This issue is of major importance for environmental monitoring especially where the proposed substrate (usually stones) is absent and when diverse methods are used. For this, different substrates (stones, macrophytes and sediment) and evaluation methods (two predictive models - MoDi and DIATMOD and a diatom index - IPS - “Indice de Polluossensibilité Specifique“) were tested on the assessment of 52 stream sites, based on diatom assemblages. The predictive models are based on epilithic diatoms but while MoDi uses the relative abundance of diatom species (frequency) and was developed for central Portugal, the DIATMOD is based on the presence/absence of species and was implemented for the entire country. The three methods were applied to 29 samples of diatom communities from three different substrates (epilithon, epiphyton and epipsammon/epipelon) collected in 12 potentially disturbed sites located in central Portugal. Additionally, the epilithic diatoms of 40 reference sites were used for comparison. We found that the epilithic diatom assemblages differed from the remaining two in terms of multivariate patterns (MDS, PERMANOVA) but not in terms of absolute taxa richness or diversity (Shannon-Weaver Index; PERMANOVA). Despite these differences, water quality classifications among the three substrates had good agreement (kappa classes: moderate-very good; average % of total agreement per substrate: 48.2 - 89.6 %) within the same method. However, they varied between methods for the same substrate (kappa classes: null-low; average % of total agreement per substrate: 19 - 25 %). The assessment method that best translated a sites anthropogenic degradation was the MoDi model, which is responding to nutrients and organic contamination (e.g. total P, CQO, and CBO5), land use (e.g. urban areas and intensive agriculture) and also habitat modifications (HQA and morphological conditions). The use of relative abundance, as in the MoDi, and the fact that the model was developed specifically for the study area probably contributed to the higher accuracy of this method. Although widely used the IPS did not perform as well as the MoDi probably because the index, was originally designed for a different European region (France). These results showed that in the absence of a certain substrate, alternative substrates may be used for water quality assessment with diatoms without a significant interference in the results. The use of assessment methods (indices or models) that were not designed or adapted to the study area may result in lower sensitivity. The use of relative abundance data instead of presence-absence data seems to increase the methods sensitivity to disturbance and is therefore recommended.


diatomssubstratespredictive modelsdiatom index ipstype of data