SOME FACTORS AFFECTING DISTRIBUTION OF DIATOM ASSEMBLAGES IN PYRENEAN SPRINGS

1. The composition and abundance of diatom assemblages were studied together with physiographical and physicochemical variables in twenty-eight springs in the Central Pyrenees during the period 1987-88. 2. Characteristic groupings of diatom taxa are revealed by Principal Components Analysis. One group, made up of assemblages of the taxa Navicula cincta, Caloneis spp. and Nitzschia elliptica, is characteristic of springs with waters of high ionic strength. Certain diatom taxa are characteristic of hard water springs, either in quiet waters (Denticula tenuis, Achnanthes minutissima, Eunotia spp.) or fast flowing ones (Fragilaria spp., Diatoma hiemale). Other taxa are representative of relatively quiet, softwater springs: Anomoeoneis brachysira, Tabellaria flocculosa, Aulacoseira distans and Fragilaria vaucheriae. 3. Five different spring types were identified, with respect to diatom assemblages, using discriminant analysis. The first consisted of high mountain springs with cold, fast waters and low ionic strength. The second group, distributed throughout the Pyrenees, had similar environmental characteristics to the first group but with intermediate water velocity and slightly higher mineral content. The third contained springs in calcareous regions of the Pyrenees, mainly slow-flowing and with hard waters. The fourth was composed of softwater springs, whilst the fifth consisted of two springs which were subject to stressful conditions (limitation of light, abundance of nitrates or high salinity). 4. Ionic strength and current velocity appeared to be the dominant environmental factors affecting diatom distribution in Pyrenean springs. These factors also seem to determine diatom communities in some other geographical areas. In some springs, however, peculiar environmental factors (dim light, salinity, and high temperature) affect the structure of diatom assemblages.