DIATOM ASSEMBLAGES IN SOME MEDITERRANEAN TEMPORARY STREAMS IN SOUTHEASTERN SPAIN

Attached algal assemblages were studied for one year in four streams o f the Alicante province (Southeastern Spain). All the sampling points were located between 250 and 500 meters above sea level and were subje ct to disturbance effects of reservoir discharges and/or seasonal spat es. Water varied from oligo- to mesotrophic. Algal biomasses, expresse d as chlorophyll-a content, dry weight (DW) and ash-free dry weight (A FDW) were high. No differences in algal biomass were observed between shaded and unshaded sites. Chlorophyll-c is the dominant form of chlor ophyll in winter and spring, and chlorophyll-b in summer, indicating a n 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 per iodicity of these species. When discharge was high raphid diatoms were dominant, especially Achnanthes minutissima, but this species was som etimes replaced by Amphora pediculus in streams with a more constant f low. 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 ye ar. Centric diatoms (Pleurosira laevis and Cyclotella me neghiniana) m ay form blooms in ponds in summer. The life form of diatoms and the di versity of their assemblages is influenced by discharge.