The colonization of the invasive diatom Didymosphenia geminata in Icelandic rivers

The diatom species Didymosphenia geminata (Lyngb.) M. Smith has been forming nuisance growths in some rivers in Iceland since the early 1990s (JONSSON et al. 2000). The algae covers stony river bed as a grayish, woolen cover and can eliminate macrophytes and moss. When the species was first found in Iceland, information from the literature of similar mass development in other countries was sparse, but since then time the nuisance appearance of D. geminata has been expanding on the global scale and the invasive behaviour has been recognized (SPAULDING & ELWELL 2007). lceland is a volcanic island predominantly formed of basaltic rock of Quaternary and Tertiary age. Using the type ofthe bedrock as a criteria for land zone classification (GuoJONSSON 1990), conductivity is generally highest in rivers within the neovolcanic zone and in long direct run-off rivers originating in well-vegetated highland catchments, whereas conductivity is lower in short run-off rivers of the Tertiary basalt formation (ÜISLASON et al. 1998, JONSSON et al. 2000). Rivers running from well-vegetated moors and rivers