Montana Field Guide

Morphological Category – Symmetric biraphid

Frustules flat, almost always coming to rest in valve view due to shallow mantles (1.5-2.0 µm deep). Valves broadly lanceolate with acutely rounded ends in all but the smallest specimens. Axial area narrow; central area broad, rectangular, reaches nearly to the valve margins. A single, shortened stria present on each side of the central area. External raphe fissures straight and threadlike. Raphe. Proximal raphe ends slightly inflated and relatively distant from one another. Terminal raphe fissures bent diagonally towards the valve margin, characteristically in close contact with a single areola at each apex, making the raphe fissures appear as hook-like terminations. Striae broad, lineate, radiate and slightly curved about the central area; parallel or slightly convergent towards the apices. Striae extend onto the valve mantle.

Size Range
Length 8-14 µm. Width 3-4 µm. Striae in 10 µm 14-18.

Type Locality: Davis Creek near Busby, Big Horn County, Montana, USA

Useful Link:
Diatom Glossary [Diatoms of North America website]

Species of Hippodonta have more strongly silicified frustules, broader profiles in girdle view, and simpler external raphe systems that lack the hooked or strongly deflected terminal fissures characteristic of Navicula species. Navicula goersii is distinguished from N. perminuta by its subrostrate rather than obtusely rounded ends. It differs from N. exilis by its smaller size, coarser striation, and rectangular shape of the central area. It differs from N. veneta by its smaller size, subrostrate ends, and distinctly transverse central area.

Found in prairie streams and rivers.

Warm, alkaline waters with moderate to high nutrients and conductivity, which are typical of the sagebrush and grassland prairies of the western United States. Common diatom associates of N. goersii are Diploneis puella, Navicula caterva, Biremis circumtexta, Navicula canalis , Fallacia tenera, Nitzschia reversa, and Pleurosigma delicatulum.

WATER CHEMISTRY
Abundance-weighted mean values for selected variables: C = 2039 µS/cm; pH = 8.4; T = 18.4 °C; total nitrogen = 1.810 mg/L; total phosphorus = 0.137 mg/L

Diatoms typically reproduce by cell division (mitosis) and occasionally by meiosis—sexual reproduction in which female and male gametes combine to form a specialized zygote called an auxospore. Repeated divisions result in cells of a population becoming progressively smaller and smaller. When cells reach a critically small size, sexual reproduction is initiated, resulting in an auxospore and initial cells that are the largest attainable for the species, after which cell division and size reduction resume (Amato 2010).

Apparently secure in the Northwestern Great Plains.