• Category
  • Length Range
    7.5-22 µm
  • Width Range
    1.8-3.2 µm
  • Striae in 10 µm
    16-24 in the center, 20-24 at the ends



Frustules are narrow rectangles in girdle view. Ventral margin is concave. In smaller specimens the ventral margin is slightly concave or straight. Dorsal margin is convex. Apices are protracted, and frequently strongly dorsally reflexed. Apices of shorter specimens are more obtusely rounded. Helictoglossae are positioned close to the apices. Distal raphe end curves a short distance on to the valve face.

There is some variability in valve morphology of E. exigua between populations. For example, some populations are less silicified than others or the ventral margin is less convex. (compare first two specimens with last four specimens). Lange-Bertalot et al (2011) report broader specimens from Europe (3 to 4 µm) though earlier Krammer and Lange-Bertalot (1991) include narrower specimens (2 ?) 2.5 to 4 (5) µm. Patrick and Reimer (1966) report a range in valve width of 2 to 4 µm.

See further discussion around E. exigua taxonomy in Lange-Bertalot et al. (2011). Eunotia exigua sensu lato, a complex of closely related taxa, has recently had several taxa removed, such as E. cataractarum, E. orthohedra, E. exigua var. tridentula, E. exigua var. undulata, E. kruegeri, and others. Both habitat and a careful examination of valve outline and morphological characteristics will help distinguish taxa. More taxonomic work is needed.


Eunotia exigua is an indicator of acidic conditions in the Great Smoky Mountains where it is common in streams, waterfalls and wetwalls.

In Europe, taxa included within the E. exigua species complex are considered to occur in sites with anthropogenic acidification (Alles et al. 1991) and found in waters with increased aluminum and sulfate concentrations.

Original Description

H. exiguum. Brébisson in litt. — H. minutum, laevissimum, fascias brevissimas formans, articulis a latere secundario arcuatis, apicibus productis, obtusissimis. — Long. 1/250—1/150´´´. — In Gallia. Specimina comm. amic. De Brébisson (v. s.)

  • Basionym
    Himantidium exiguum
  • Author
    Bréb. in Kütz. 1864

Original Images

Eunotia exigua orig illus
Eunotia Exigua  Grunow1862  Descr

EMAP Assessment

EMAP Distribution

Eunotia Exigua 40
Map 40

Response Plots

Eunotia Exigua

Citations & Links



  • Index Nominum Algarum
  • CAS
  • North American Diatom Ecological Database
    NADED ID: 33015

Cite This Page

Furey, P. (2012). Eunotia exigua. In Diatoms of North America. Retrieved July 21, 2018, from https://diatoms.org/species/eunotia_exigua


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Eunotia exigua from all the stream reaches where it was present. Note that the relative abundance scale is the same on each plot. Explanation of each environmental variable and units are as follows:

ELEVATION = stream reach elevation (meters)
STRAHLER = distribution plot of the Strahler Stream Order
SLOPE = stream reach gradient (degrees)
W1_HALL = an index that is a measure of streamside (riparian) human activity that ranges from 0 - 10, with a value of 0 indicating of minimal disturbance to a value of 10 indicating severe disturbance.
PHSTVL = pH measured in a sealed syringe sample (pH units)
log_COND = log concentration of specific conductivity (µS/cm)
log_PTL = log concentration of total phosphorus (µg/L)
log_NO3 = log concentration of nitrate (µeq/L)
log_DOC = log concentration of dissolved organic carbon (mg/L)
log_SIO2 = log concentration of silicon (mg/L)
log_NA = log concentration of sodium (µeq/L)
log_HCO3 = log concentration of the bicarbonate ion (µeq/L)
EMBED = percent of the stream substrate that is embedded by sand and fine sediment
log_TURBIDITY = log of turbidity, a measure of cloudiness of water, in nephelometric turbidity units (NTU).
DISTOT = an index of total human disturbance in the watershed that ranges from 1 - 100, with a value of 0 indicating of minimal disturbance to a value of 100 indicating severe disturbance.