In the laboratory of phytobenthos, benthic diatom assemblages are analyzed in order to assess the quality of running waters as required by the Water Framework Directive.

Benthic diatoms (Phytobenthos) are an important biological quality element for assessing the ecological status of rivers. They grow attached to various substrates and are strongly affected by physicochemical and biological disturbances that occur in rivers.

Diatoms are microscopic unicellular algae that occur mostly as single cells, although some join to form colonies. They are ubiquitous organisms present in a wide range of marine and fresh water habitats. Diatoms are important components of planktonic and benthic communities and among the most important primary producers as they frequently constitute the base of aquatic food chains.

Diatoms belong to the algae class Bacillariophyceae (phylum of Bacillariophyta) and probably are the most numerous among eukaryotic aquatic organisms, accounting over 450 genera of living diatoms with over 100 000 species. They have certain features which make them unique amongst the algae. One of the particular features is the siliceous cell wall (frustule) which is made of two halves (valves). The shape, morphology and ornamentation of the valves are very important for exact diatom identification and taxonomy. Diatom communities by virtue of their short life cycle respond rapidly and predictably to environmental change, providing highly informative assessments of water quality in streams and rivers.

According to the Water Framework Directive 2000/60/EC (WFD), the rivers ecological status has to be classified into five ecological quality classes

and the classification schemes for Phytobenthos (type-specific) communities should be based mainly on species composition and their abundance. These parameters can be quantified and expressed as a numerical value using diatom indices.

Several indices have been developed, most of which are calculated with a weighted average equation using relative abundances of all taxa in a sample from a site, species pollution sensitivity and their indicator value.

The IPS (Specific Pollution Sensitivity Index) index has been applied successfully in most of the European countries and it has proved to be the most accurate of the diatom water quality indices in Mediterranean rivers.

IPS scores range from 1 to 20, being 1 the worst quality and 20 the best. Values are ranged into 5 quality categories in correspondence with the classes which are provided by the WFD.