• Category
  • Length Range
    80-93 µm
  • Width Range
    14-17 µm
  • Striae in 10 µm
  • Synonyms
    Navicula legumen var. decrescens Grunow in Van Heurck 1880
  • Reported As
    Pinnularia legumen (Krammer and Lange Bertalot 1986, pg. 413, Pl. 184, Fig 2)



Valves are linear-lanceolate, with a slightly triundulate valve margin. The central valve expansion is slightly wider than the distal expansion. The apices are rostrate. The axial area is linear-lanceolate and 1/4-1/3 of the valve width. The central area is large and elliptic to rhombic. The raphe is lateral. Proximal sections of the raphe are slightly expanded and inclined toward the primary valve side. Terminal raphe fissures are shaped like question marks. Striae are radiate in the central valve, becoming strongly convergent toward the apices.


This taxon occurs in the benthos of lakes of northeastern US.

Original Description

  • Basionym
    Navicula legumen var. decrescens
  • Author
    Grunow in Van Heurck 1880

Original Images

Pinn Decrescens Orig Ill
Pinn Descrescens Orig Descr

Cite This Page

Friedman, N., Potapova, M. (2021). Pinnularia decrescens. In Diatoms of North America. Retrieved May 20, 2024, from https://diatoms.org/species/pinnularia-decrescens


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