Pinnularia quadratarea var. constricta f. interrupta

Pinnularia quadratarea var. constricta f. interrupta H.Heiden 1905 was quite difficult to identify and the process involved generous help of more experienced diatomists. P. quadratarea has tons of varieties and forms, and most of them are not well documented. This form was described in Atlas der Diatomaceenkunde with a single drawing from waters near Greenland. It was seen again in the Hudson Bay and the Laptev Sea in 1980s and assumed to be a sea ice species. I found it in plankton in the Bering Sea, which agrees with its sea ice association.
Most useful paper: Poulin, M. and A. Cardinal (1982) Sea ice diatoms from Manitounuk Sound, southeastern Hudson Bay ( Quebec, Canada). I. Family Naviculaceae. Canadian Journal of Botany 60: 1263-1278.