Is diatom richness responding to catchment glaciation? A case study from Canadian headwater streams

Due to global change affecting glaciers worldwide, glacial streams are seen as threatened environments deserving specific scientific interest. Glacial streams from the Coast Range and Rocky Mountains in British Columbia and at the border to Alberta were investigated. In particular glacial streams and downstream sites in the Joffré Lakes Provincial Park, a near by mountain river and two large glacial streams in the Rocky Mountains (Kootenay Range, Jasper National Park) were studied. Regardless of a high variability of catchment glaciation (1 to 99%) thin organic biofilms with firmly attached diatom frustules of the genera Achnanthidium, Psammothidium, Encyonema, Gomphonema and fragilaroid taxa were found in all cases. In spite of fundamentally different geological conditions between the Coast Range sites and the Rocky Mountain sites, the pioneer taxon Achnanthidium minutissimum (with a slimy long ecomorph) was dominating quantitatively in most of the glacier stream samples together with the rheobiontic Hannaea arcus. Individual glacier stream samples were characterized by the dominance of Achnanthidium petersenii and Gomphonema calcifugum/Encyonema latens. The diatom community analysis (cluster analysis) revealed the expected separation of glacier stream sites and sites of the lower segments of the river continuum (e.g., dominance of Diatoma ehrenbergii in the mountain river). In the Joffré area, the total species richness of turbid glacial streams close to the glacier mouth was significantly lower than in the more distant sites. The two largest glacial streams in the Rocky Mountains showed divergent results with a remarkable high species richness (43 taxa) at the Athabasca River origin (Columbia Icefield) and low diversity in Illecillewaet river (9 km downstream the glacier mouth). From the biogeographical point of view the dominant taxa comprised mainly widespread pioneer species coping best with the unstable conditions, while the subdominant taxa comprised taxa specific for pristine arctic-alpine or high altitude habitats (e.g., Psammothidium grischunum). Almost 50% of the taxa were classified as oligo- to oligo-mesotraphentic and approximately 20% as endangered or extremely rare.