Fragilaria pinnata is widely distributed in freshwater systems (Patrick and Reimer 1966). In the Great Lakes, specimens of this species were scattered in collections from nearshore areas from Lake Michigan. Occasionally this species was found in low numbers in offshore collections, especially during the fall months (Stoermer and Yang 1969).
In Lake Michigan, the length of Fragilaria pinnata ranges from 5-30 micrometers. The breadth ranges from 2-6 micrometers and the striae numbers range from 8-11 striae in 10 micrometers (Stoermer and Yang 1969).