Fragilaria intermedia Grun. 1881

Length: 45

Width: 3.5

Striae:11

Collection 804

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Grun. in Van Heurck, Syn. Diat. Belgique, pl. 45, figs. 9-1, 1881 (Patrick and Reimer 1966).

The frustules form long, ribbon-like colonies and are attached at swollen central portions of the valve faces (Stoermer and Yang 1969). The valves are linear to lanceolate in shape with rostrate to subcapitate ends. The axial area is narrow and linear with a distinct border. The central area is a unilateral fascia and not visibly thickened. Often, indistinct or rudimentary striae can be resolved.

Synonyms (Patrick and Reimer 1966)

Synedra vaucheriae Kutz.

Fragilaria vaucheriae (Kutz.) Peters.
Early work discusses this taxon's similarity to Synedra vaucheriae Kutz. [Fragilaria vaucheriae (Kutz.) Peters.], based on isotype observations and proposed synonomy (Petersen 1938). In the Lake Michigan populations, the two taxa are easily distinguished using several morphological characteristics (thicker striae, more strongly silicified valve structure, and the absence of a distinctly thickened border around the central area)(Stoermer and Yang 1969). This question warrants further investigation.

Ecology (Stoermer and Yang 1969)

This taxa has variable populations in Lake Michigan. Historical populations have been recorded primarily from littoral areas; however, Stoermer and Yang (1969) report large populations in offshore plankton. These offshore populations are more elongate and lanceolate rather than linear and may represent a different form or variety.

Size ranges and morphology

In samples from Lake Michigan, the length of this species ranges from 25-65 micrometers. The breadth at the widest area ranges from 3-5 micrometers. Striae are coarse and cross lineate with 8-13 striae in 10 micrometers (Stoermer and Yang 1969).