• Category
  • Length Range
    9.6-15.6 µm
  • Width Range
    5.5-7.3 µm
  • Striae in 10 µm
  • Reported As
    Achnanthes lanceolata var. omissa (Patrick and Reimer 1966)
  • Genus Considered



Valves are elliptic to elliptic-lanceolate, with broadly rounded ends. The raphe valve has a narrow axial area. The raphe is straight, with expanded proximal ends. Striae are radiate and curved, increasingly so toward the apices. The central area is bounded by 1-2 short striae on either side, which are widely spaced. The rapheless valve has a rhombic axial area that is wider than on the raphid valve. Striae on this valve also radiate throughout the valve, but are more dense at mid-valve than they are on the raphe valve. A cavum is present, which extends to the apical axis.

We consider this taxon to be conspecific with Achnanthes lanceolata var. omissa (Patrick and Reimer 1966). A recent SEM study (Chudaev et al. 2015) showed that this species has no terminal raphe fissures and therefore it was transferred to Platessa. The presence of the cavum and predominantly unseriate striae, are however not characteristic of Platessa. Since this species has an unusual combination of characters, its generic position is uncertain.


Planothidium joursacense has been observed in samples across a broad holoarctic range, from the Laurentian Great Lakes , northern European lakes, to Bering Island in the north Pacific. This taxon is understood to be indicative of higher quality lake and stream systems. In the Laurentian Great Lakes, this taxon has an estimated phosphorus optimum of 47.5 ug/L total phosphorus (Reavie et al. 2004). A large population of P. joursacense was recently documented in samples collected from the Popple River in northeastern Wisconsin at the USGS Status and Trends site. In a recent European intercalibration exercise species was assigned the quality status “good” (Kelly et al. 2014) for high alkalinity lakes.

Popple River Near Fence
Credit: Photo credit: USGS National Water Information System (NWIS), retrieved from https://waterdata.usgs.gov/usa/nwis/uv?04063700
USGS gage on the Popple River, near Fence, WI

Original Description

Achnanthes joursacense nov. sp. (Pl. XI, fig. 26 et 27). — De forme largement elliptique ; longueur 15 à 20μ, largeur 8 à 10μ; valves à structure semblable et à cloison en fer à cheval; valve supérieure à pseudo-raphé étroitement lancéolé, unilatéralement stauronéiforme, les stries mariquant sur l'espace limité par la cloison; stries rayonnantes, courbes, au nombre, de 9 en 10μ au milieu, un peu plus serrées aux extrémités; valve inférieure à aires axiale et centrale nulles; stries courbes, rayonnantes, au nombre de 9 en 10μ au milieu et un peu plus serrées aux extrémités, non raccourcies ni absentes au milieu de la valve. Se distingue de l'Achnanthes lanceolata var. elliptica Cl,, par sa forme plus large, par sa valve inférieure non stauronéiforme, et par le plus grand écartement des stries sur les deux valves.

  • Basionym
    Achnanthes joursacense
  • Author
    Hérib. 1903
  • Length Range
    15-20 µm
  • Width
    8-10 µm

Original Images

Pljoursacense  Orig Ill
Pljoursacense  Orig Descr

Citations & Links



  • Index Nominum Algarum
  • North American Diatom Ecological Database
    NADED ID: 155016

Cite This Page

Bishop, I., Potapova, M. (2017). Planothidium joursacense. In Diatoms of North America. Retrieved June 13, 2024, from https://diatoms.org/species/planothidium_joursacense


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Planothidium joursacense 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.