• Category
  • Length Range
    12-55 µm
  • Width Range
    5-8 µm
  • Striae in 10 µm



Frustules are linear-clavate. In valve view, the headpole is rostrate. A narrow sternum extends the length of the apical axis of the valve. Striae are grouped together, 1-6 between costae. Numerous costae are present, positioned along the transverse axis. In some specimens, costae are not visible near the footpole. A rimoportula is present near the headpole, and usually visible under the light microscope. Cells often grow in fan-shaped colonies. Internal valves are common in this taxon.


Meridion circulare var. constrictum was found in a prairie stream north of Iowa Lakeside Lab, Dickinson County, Iowa. Specimens from the Iowa Lakeside Lab Herbarium confirmed this taxon in soils, duffs from prairie swales, moss, a plankton tow from “No Name” creek in Dickinson County, the surface of a small puddle and in sand from the Little Sioux River.

This species has a fairly cosmopolitan distribution, but is most commonly found in streams and creeks. Williams (1985) described the taxon as being sited in Great Britain, France, Hungary, USA, New Zealand, France, and Lapland. As the cells divide, they form fan-like colonies growing on plants and rocks (Ralfs, 1843). Meridion circulare var. constrictum is less common than the nominate variety and considered to survive outside of permanent water bodies in locations such as puddles (Stancheva, 2006).

Credit: Natalie Hoidal
Live colony from Northern prairie stream at Iowa Lakeside Lab, Dickinson County, Iowa.
Credit: Natalie Hoidal
Live colony from Northern prairie stream at Iowa Lakeside Lab, Dickinson County, Iowa

Original Description

" M. constrictum. Lateral surfaces constricted below the apex, transversely striated : the ends of the striae forming puncta along the margins of the front view.

  • Basionym
    Meridion constrictum
  • Author
    Ralfs 1843

Original Images

Meridion Circulare Constrictum2
Meridion Circulare Constrictum

Cite This Page

Hoidal, N. (2013). Meridion circulare var. constrictum. In Diatoms of North America. Retrieved July 15, 2024, from https://diatoms.org/species/meridion_circulare_var._constrictum


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Meridion circulare var. constrictum 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.