Abundance and vertical distribution of the phytobenthic community within a pool and riffle sequence of an alpine gravel stream

In the 2(nd) order mountain brook Oberer Seebach (Austria), diatoms and cyanoprokaryotes dominate the microphytobenthos, with the diatoms forming the richest species group. Comparative investigations of different habitats clearly show seasonal variations in algal species composition and biomass throughout the year. Pool habitats hold a higher number of taxa than riffle sites. In addition, phytobiomass is two- to nearly fourfold higher in pools. Based on investigations of the chlorophyll-a vertical profiles within the coarse gravel sediments, the light discontinuity layer was determined to be situated at 7-10 cm sediment depth. Vertical distribution showed a maximum of taxa a few centimeters below the sediment surface. The same diatom taxa were found throughout all sampling dates, sediment depths and sampling sites. The results of exposed perforated metal tubes clearly indicate that the occurrence of phytobenthos in aphotic zones is primarily controlled by saltatory movements of the upper sediment layers driven by discharge.