Cymbellonitzschia diluviana Hustedt (Bacillariophyceae): Habitat and auxosporulation

The diatom Cymbellonitzschia diluviana Hustedt is an important indicator species in interglacial deposits but is rarely reported from contemporary sites. The presence of large populations in Lough Neagh, N. Ireland, made possible a study of its ecology and auxosporulation. The apical length of cells in the lake was usually between 7 and 25 µm, but in culture there was a broader range between 5 and 42 µm. Amongst smaller cells (< 11 µm), some (3 %) were found with raphes on the dorsal (i.e. convex side) rather than usual ventral side. At cell division, both daughter cells had hantzschioid symmetry. C. diluviana is found in alkaline waters (pH 7.6–9.0) but not usually where there is significant calcite deposition. Its present rarity is probably because its preferred habitat is now less common. In L. Neagh, C. diluviana was found on exposed rocky and sandy shores, but was particularly abundant in shifting sand bars. This highly abrasive and erosive environment requires cells to have good adhesion and the ability to withstand extended periods of burial. C. diluviana is essentially a ruderal species, adapted to habitats with high disturbance.