• Category
  • Length Range
    12-50 µm
  • Width Range
    2.8-5.0 µm
  • Striae in 10 µm
    16-21 in the center valve



The ventral margin is straight in smaller specimens and weakly concave in larger specimens. The dorsal margin is convex. The apices are acutely rounded, appearing 'nose-like' due to the inset position of the helictoglossae. The helictoglossae are set in equidistant from the apices (symmetric to the transapical axis), giving the appearance of an indentation in the LM. One apical rimoportula is present, just dorsal to the center of the apex, and is expressed externally as a round pore. The distal ends of the raphe lie on the valve mantle and the terminal raphe nodules are well set in from the apices. In girdle view the frustules are rectangular to quadratic and the inset helictoglossae are observable.

Several forms and variations in length (L), width (W) and stria density are currently included in the broad concept of E. incisa as presented by Patrick and Reimer (1966; L 15-50 µm, W 4-7 µm, 13-17 stria/10 µm), Krammer and Lange-Bertalot (1991, 13-50 µm, W 4-6 µm, 12-17 stria/10 µm), Siver et al. (2005, L 12-28 µm, W 3-4 µm, 16-17 stria/10 µm; also narrow specimens W 2-2.5 µm), and Furey et al. (2011, 13-50 µm, W 2.8-5.0 µm, 12-17 stria/10 µm). Some of the narrow specimens have been described as new species, such as E. canicula. Specimens that have broader, more rounded apices, such as valves similar in form to Taf. 161, Figs 13-15 in Krammer and Lange-Bertalot (1991) may later be described as new species, but for now should be tracked as a variation in valve form. Lange-Bertalot et al. (2011) have further described new species from Europe, such as E. incisadistans, that share similarities in form (including the inset helictoglossae) that should be compared with the North American flora as taxonomic work on this species continues. Eunotia incisadistans has broader valves with lower stria density.


Eunotia incisa is common in acidic, low nutrient, freshwater environments ranging from bryophyte epiphytes to ponds and small lakes, and can occasionally occur in high abundances (Patrick and Reimer 1966, Camburn and Charles 2000, Siver et al. 2011, Krammer and Lange-Bertalot 1991). Populations are often noted to contain several forms suggesting additional studies are warranted in this group (Krammer and Lange-Bertalot 1991, Siver et al. 2005, Peeters and Ector 2018).

North American populations of Eunotia incisa have been illustrated and reported from several microhabitat types in the Carolina Bays (Gaiser and Johansen 2000) and common as a Fontinalis epiphyte in Long Branch Creek (Camburn et al. 1978), South Carolina. Furey et al. (2011) found it in moderate to high abundance as epiphytic on bryophytes in 1st-5th order streams below 1605 masl in the Great Smoky Mountains National Park. Camburn and Charles (2000) report it from sediment cores taken in New York and Adirondack lakes. Bahls (2011) illustrated populations of Eunotia incisa from small ponds and lakes in Montana and Washington. Reavie and Kireta (2015) report it in protected wetlands in the Laurentian Great Lakes especially around Lakes Superior, Michigan, and Ontario. In the Great Lakes Network National Parks, it is found as rare in sediment samples form many GLKN lakes, but notably common in Outer Lagoon (Apostle Isands National Lakeshore) and Shoepack Lake (Voyageurs National Park). In the eastern US, E. incisa is reported as two forms from acidic oligotrophic ponds on Cape Cod, Massachusetts Siver et al. (2005), and from a few acidic and oligotrophic waterbodies on the Atlantic Coastal Plain (Siver and Hamilton 2011).

In Canada, Eunotia incisa is widely reported from stained softwater lakes in Quebec and Labrador (Fallu et al. 2000) and rivers in Quebec (Lavoie et al. 2008).

Many groups have used Eunotia incisa as an indicator organism and published regional environmental optima, including:

Gaiser and Johansen (2000): pH 4.6, water depth 17 cm

Reavie and Kireta (2015): TP 25 µg/l, Cl- 10 µg/l

Camburn and Charles (2000): pH 5.21, ANC 12 µeq/l, TP 6.28 µg/l

Furey et al. (2011): Found between pH 3.9-5.6.

Siver et al. (2005): the common form optima of pH 5.2, TP 11.2; the narrow form pH 5.2, TP 10.8

Siver and Hamilton (2011): pH 5.3, TP 12.9

Fallu et al. (2000): DOC opt 10.5 mg/l, alk 22 µeq/l

Original Description

  • Author
    W.Sm. ex W.Greg. 1854

Original Images

Eincisa  Orig Ill
Eincisa  Origdes

Citations & Links



  • Index Nominum Algarum
  • North American Diatom Ecological Database
    NADED ID: 33026


Dec 14, 2023 - Addition of Autecology

From 2011 until 2023 the autecological information on this taxon page was limited. As of this update, information on ecology, biogeography, and environmental optima have been added. - Mark Edlund

Cite This Page

Furey, P., Edlund, M., Bishop, I., Allen, L., Spaulding, S. (2011). Eunotia incisa. In Diatoms of North America. Retrieved May 18, 2024, from https://diatoms.org/species/eunotia_incisa


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