Increase in nuisance blooms and geographic expansion of the freshwater diatom Didymosphenia geminata

The diatom Didymosphenia geminata (Lyngbye) Schmidt is emerging as an organism with an extraordinary capacity to impact stream ecosystems on a global scale. In recent years, streams in New Zealand, North America, Europe, and Asia have been colonized by unprecedented masses of “didymo” and its extracellular stalks (fig. 1). This diatom is able to dominate stream surfaces by covering up to 100 percent of substrate with thicknesses of greater than 20 cm, greatly altering physical and biological conditions within streams. This species is expanding its geographic range in North America and the rate that nuisance blooms are reported by the public and local media are increasing, yet little scientific investigation of the phenomenon in North America has been initiated.