Longitudinal and seasonal differentiation of epilithic diatom communities in the uppermost sections of two mountain spring-fed streams

The present study was part of a project on the regional variability of several components of the biota in 30 mountain springs in the Adamello-Brenta Regional Park (Southern Alps, ltaly). The longitudinal distribution and seasonal changes in community structure of algae other than diatoms have already been published (CANTONATI 1999). Here we present the parallel study on epilithic diatoms. Springhead diatom communities have been shown to differ considerably in both species composition and higher diversity from the assemblages in the downstream spring streams. The species characterizing the springheads have been frequently found in springs and comparable habitats, but they are neither crenobiontic nor restricted to pristine running waters. Achnanthidium minutissimum, A. pyrenaicum and Hannaea arcus, which increased in abundance downstream, are well-known rheobionts. For Odontidium mesodon, which dominated only in the least shaded site, Cox (1990) also reported high light requirements. With respect to seasonality, most diatoms, which attach to the substratum and are less disturbed by its instability, have to be considered as constants. On the contrary, macroalgae are mostly variants with marked seasonal peaks at times of highest discharge (nutrient peaks) or highest substratum stability. In this study seasonal patterns of diatom species may also be less pronounced since absolute quantities were not determined (only relative abundances were considered).