Algae form an important component of aquatic ecosystems and diatoms are probably the most widespread and abundant of all groups of benthic algae. Diatoms (Bacillariophyta) are unicellular and unique amongst photosynthetic organisms in having a rigid siliceous wall (frustule) around the cytoplasm. They are classified to species level primarily on the basis of symmetry and ornamentation of the frustule. In many European countries and North America, diatoms have been used successfully as indicators of eutrophication in rivers. Few studies have investigated the composition of epilithic diatom assemblages in Irish rivers (Foged 1977, Heuff and Horkan 1984, and Ní Chatháin 2002). This project aimed to address this deficiency by investigating the composition and distribution of epilithic diatom assemblages in lowland base-rich rivers in Ireland. Twenty-five lowland rivers were sampled in August and September 2002.
To fulfil the obligations of the Water Framework Directive, a river typology within Ecoregion 17 had to be produced. The objective of this study was to survey 50 sites within the Republic of Ireland that had been previously classified as high quality by the Irish EPA, to determine whether they were of high ecological status (and thus could be used as reference conditions) and to use these spatial reference sites to develop the river typology.
Benthic diatom samples were collected from 45 potential reference river sites in Ireland.
Diatom samples were also collected from 24 lowland eutrophic sites. The main objective for
sampling the potential reference sites was to determine the type-specific biological typologies represented by these sites. The lowland sites were sampled as part of another project and are used here for comparative purposes. A larger number of taxa was identified from the reference sites (175 species) in comparison with the lowland sites. (71 species). Distinct differences in diatom distribution and diversity between the reference sites and the lowland sites were revealed by DCA analysis. DCA analysis also showed that the original typological classification of the reference sites was not reflected to any extent in the distribution of benthic diatoms between these sites. However, there was some differentiation between the reference river sites with respect to diatom distribution along axis 1 of the DCA ordination and this was strongly correlated with water conductivity.