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Attached algal assemblages were studied for one year in four streams of the Alicante province (Southeastern Spain). All the sampling points were located between 250 and 500 meters above sea level and were subject to disturbance effects of reservoir discharges and/or seasonal spates. Water varied from oligo- to mesotrophic. Algal biomasses, expressed as chlorophyll-a content, dry weight (DW) and ash-free dry weight (AFDW) were high. No differences in algal biomass were observed between shaded and unshaded sites. Chlorophyll-c is the dominant form of chlorophyll in winter and spring, and chlorophyll-b in summer, indicating an alternation between chlorophytes (summer) and diatoms (winter-spring). Even though the diatom flora was basically the same in all streams, there were several differences in the dominant species and in the periodicity of these species. When discharge was high raphid diatoms were dominant, especially Achnanthes minutissima, but this species was sometimes replaced by Amphora pediculus in streams with a more constant flow. The dynamics of diatom assemblages are related to a lessening of the flow and an increase in the ionic content of water. The diversity of the samples forms a bimodal curve with peaks in autumn and spring. Only a few species were present in high cell numbers throughout the year. Centric diatoms (Pleurosira laevis and Cyclotella me neghiniana) may form blooms in ponds in summer. The life form of diatoms and the diversity of their assemblages is influenced by discharge.