The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

Didymosphenia geminata
Didymosphenia geminata

Copyright Info
Didymosphenia geminata (Lyngb.) M. Schmidt

Common name: didymo

Synonyms and Other Names: Echinella geminata, Gomphonema geminatum, Didymosphenia geminatum [synonyms for the Great Lakes native strain]; Styllaria geminata, Dendrella geminata, Lyngbyea pulvinata var. geminata; Didymo

Taxonomy: available through www.itis.govITIS logo

Identification: Cells are distinguished by their large silica cell wall (frustule) shaped like a curved bottle (MISIN 2017). Cells feature a raphe, which is a structure used for movement on surfaces. Extracellular stalks that form nuisance blooms are secreted from the raphe (Spaulding and Elwell 2007). While microscopic, Didymosphenia geminata forms dense mats of extracellular stalks that are pale yellowish brown to white in color. Nuisance mats resemble fiberglass and have the texture of wet wool, but do not feel slimy to the touch ( Pennsylvania Sea Grant 2013).

Size: cells to 140 micrometers

Native Range: Northern Europe and Asia.  Possibly northern North America.  

Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

Nonindigenous Occurrences:

Table 1. States with nonindigenous occurrences, the earliest and latest observations in each state, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Didymosphenia geminata are found here.

StateFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
AK199820015Anchorage; Chulitna River; Lower Kenai Peninsula; Lower Susitna River; Upper Kenai Peninsula
CA200520233Lower American; Mono Lake; North Fork Feather
CO199420034Blue; Cache La Poudre; Eagle; St. Vrain
ID199920195Lower Boise; Lower Clark Fork; St. Joe; Upper Coeur d'Alene; Upper Spokane
MD201620161North Branch Potomac
MA201320142Housatonic; Westfield River
MI199420223Lake Superior; Manistee; St. Marys
MT199920113Flint-Rock; Middle Clark Fork; St. Marys
NH200720071Black River-Connecticut River
NY200720163East Branch Delaware; Hudson-Hoosic; Upper Delaware
PA200720244Lackawaxen; Pine; Upper Delaware; Youghiogheny
UT200020001Upper Weber
VT201420142Passumpsic River; Winooski River
VA201420142Upper Dan; Upper James
WA199620113Duwamish; Nooksack; Skokomish
WY201120111Upper North Platte

Table last updated 7/13/2024

† Populations may not be currently present.

Ecology: Didymosphenia geminata can be found in freshwater rivers, streams and lakes. This taxon prefers cool water of low conductivity (Patrick and Reimer 1975) and may be useful as indicators of increases in salt concentrations in the Great Lakes (E. F. Stoermer, personal communication).  The species is predominantly benthic but has been observed in planktonic samples (Blanco and Ector 2009). It prefers clear streams and rivers with shallow, moderately-flowing water with a pH of 7 or above and rocky substrate (MISIN 2017; Sea Grant Pennsylvania 2013; Spaulding and Elwell 2007). Historically the species was restricted to low-nutrient waters but has recently seen large range expansions reportedly occurring in eutrophic rivers, showing much greater tolerance for nutrient and flow conditions than previously expected. This may be attributed to a genetic variant with broader tolerances than the original species.  Nuisance benthic growths are only known to occur in flowing water.  While this species seems to be confined to cold areas, it reaches its highest biomass at higher water temperatures of ~20°C (Blanco and Ector 2009). It has also been observed at temperatures of 27°C (Spaulding and Elwell 2007).  

Didymosphenia geminata is capable of sexual reproduction, but recent nuisance blooms occurring globally have been a result of asexual vegetative cell division (Bothwell and Spaulding 2008). When the cell divides, the stalk divides as well (Spaulding and Elwell 2007).  Didymosphenia geminata typically blooms in streams with low to undetectable levels of phosphorous.  Short-term nitrogen and phosphorus additions followed by nutrient limitation have been reported to increase cell densities and division rates, possibly accelerating mat formation. The addition of nitrogen and/or phosphorus can stimulate cell division and increases the frequency of dividing Didymosphenia geminata cells (Bray et al. 2017; Jackson et al. 2016). As nutrient  concentrations (particularly dissolved phosphorus) decrease, stalk production tends to increase  (Jackson et al. 2016).  Current research suggests that stalk production allows for better access to light in addition to providing habitat for nutrient cycling and phosphorus acquisition via phosphatase production (Bray et al. 2017).

Didymosphenia geminata is a food source for macroinvertebrates (Spaulding and Elwell 2007). Mats formed in blooms create microenvironments that can be colonized by diatoms and other organisms (Blanco and Ector 2009). Recent studies on blooms have revealed supersaturated concentrations of dissolved oxygen in D. geminata mats, suggesting that other photosynthetic organisms exist within these mats, allowing Didymosphenia geminata to compete with other algae for both nutrients and light (Blanco and Ector 2009; Spaulding and Elwell 2007).

Means of Introduction: Didymosphenia geminata has been shown to survive outside of the stream environment. Cells are able to survive and remain viable for 40 days in cool, dark, damp conditions. Angling equipment, boot tops, neoprene waders, and felt-soles provide a particularly suitable environment for cells to remain viable. Cells can hitchhike on this equipment and other recreational equipment into new bodies of water (Spaulding and Elwell 2007). Freshwater diatoms are dispersed through the flow of water and transport by other organisms, primarily waterfowl (Kristiansen 1996).

Status: Expanding range in the Great Lakes.  Didymosphenia geminata is considered native to the Great Lakes (Stoermer 1965 collection).  Lack of evidence for the native status has led to the idea that recent blooms of D. geminata are due to introductions or range expansions, or possibly a separate strain of the species that is not native (Taylor and Bothwell 2009). In recent history, Didymosphenia geminata has seen dramatic range expansions globally, including in North America. Taylor and Bothwell (2009) argue that observational and experimental evidence shows that changing environmental conditions, specifically low phosphorus concentrations, are causing nuisance blooms which are responsible for drawing attention to the species.

Impact of Introduction:
Summary of species impacts derived from literature review. Click on an icon to find out more...


Didymosphenia geminata grows in thick mats that cover all surfaces and substrates (Bickel and Closs 2008; Bothwell and Spaulding 2008; Ejaz et al. 2021), reducing the growth of other algae (Ejaz et al. 2021). When the mats break loose and float, they can clog fishing nets (Bothwell and Spaulding 2008) and water filters and hydro plants (Ejaz et al. 2021). The presence of didymo is associated in changes in macroinvertebrate assemblages and densities (Clancy et al. 2020; Gillis and Chalifour 2010). Its presence also alters water movement such that it disrupts the egg beds (redds) and impacts adult survivorship of important fishery fish such as Salmo trutta (brown trout) and Oncorhynchus mykiss (rainbow trout) (Bickel and Closs 2008; Bothwell and Spaulding. 2008). Didymo presence is also associated with a decrease in angler waterbody use (of infested waters) and angler fishing rates (Root and O’Reilly 2012; Beville et al 2012) and has impacted local economies that rely on fishing tourism when excursions are canceled (Bothwell and Spaulding 2008). Didymosphenia geminata has proven effective at removing heavy metals from sewage and industrial waste (Wysokowski et al. 2017).

Remarks: Records from the Great Lakes region prior to 1990 or that are unicells with short stalks rather than mat-forming colonies are not included in the mapping data as they are presumed to be native.  

References: (click for full references)

Bailey, J.W. (1842): A sketch of the infusoria, of the family Bacillaria, with some account of the most interesting species which have been found in a recent or fossil state in the United States. Part II. Naviculacea. - Amer. J. Sci. Arts 42: 88–105.

Bellavance, M. (2006): Didymo, une nouvelle menace…? Ou pourquoi nos habitudes seront changées dès l’an prochain! - Bull. Liais. Assoc. Pêch. Sport. Saumon Riv. Rimouski 4: 3–5.

Beville, S.T., G.N. Kerr, and K.F.D. Hughey. 2012. Valuing impacts of the invasive alga Didymosphenia geminata on recreational angling. Ecological Economics 82:1-10. https://doi.org/10.1016/j.ecolecon.2012.08.004.

Bhatt, J.P., A. Bhaskar, and M.K. Pandit. 2008. Biology, distribution and ecology of Didymosphenia geminata (Lyngbye) Schmidt an abundant diatom from the Indian Himalayan rivers. Aquatic Ecology 42:347-353.

Bickel, T.O., and G.P Closs. 2008. Impact of Didymosphenia geminata on hyporheic conditions in trout redds: reason for concern? Marine and Freshwater Research 59(11):1028-1033.

Blais, S. (2008): Guide d’identification des fleurs d’eau de cyanobactéries. Comment les distinguer des végétaux observés dans nos lacs et nos rivières. - Direction du suivi de l’état de l’environnement, ministère du Développement durable, de l’Environnement et des Parcs, Montreal.

Blanco, S. and L. Ector. 2009. Distribution, ecology and nuisance effects of the freshwater invasive diatom Didymosphenia geminata (Lyngbye)M. Schmidt: a literature review. Nova Hedwigia 88(3-4):347-422.

Bothwell, M.L., and S.A. Spaulding. 2008. Proceedings of the 2007 International Workshop on Didymosphenia geminata. Canadian Technical Report of Fisheries and Aquatic Sciences, Nanaimo, BC.

Boyer, C.S. (1927): Synopsis of North American Diatomaceae. Part II. Naviculatae, Surirellatae. - Proc. Acad. Nat. Sci. Philadelphia Suppl. 79: 229–583.

Bray, J., C. Kilroy, P. Gerbeaux, and J.S. Harding. 2017. Ecological eustress? Nutrient supply, bloom stimulation and competition determine dominance of the diatom Didymosphenia geminata. Freshwater Biology 62:1433-1442.

Briggs, S.A. (1872): The diatomaceae of Lake Michigan. - The Lens 1: 41–44.

Britton, M.E. (1944): A catalog of Illinois algae. - Northwestern Univ. Stud. Biol. Sci. Med. 2: 1–177.

Burkholder, P.R. (1931): Studies in the phytoplankton of the Cayuga Lake Basin. - Bull. Buffalo Soc. Nat. Sci. 15: 21–182.

Byle, J. 2014. Ecological Drivers and Species Interactions of Whirling Disease. Unpublished M.S. thesis. University of Colorado, Boulder, CO.

Chase, H.H. (1902): Flora of Michigan diatomaceae. - In: POLLOCK, J.B. (ed.): Fifth annual report of the Michigan Academy of Science: 166–183. Michigan Academy of Science, Alma.

Clancy, N. G., J. Brahney, J. Curtis, and P. Budy. 2020. Consequences of Didymo blooms in the transnational Kootenay River basin. Department of Watershed Sciences at Utah State University, Logan, UT.

Clearwater SJ; Jellyman PG; Biggs BJF; Hickey CW; Blair N; Clayton JS, 2007. Didymosphenia geminata experimental control trials: Stage Three Phase One (assessment of the effectiveness of GemexTM, a chelated copper formulation, in a small river). NIWA Report: HAM2007-060. NIWA Report: HAM2007-060.

CÔTÉ, C. (2007): La prolifération de l’algue Didymo au Québec, qu’en est-il ? - Bull. Féd. Québec. Gest. Zecs 9: 14–15.

Cullis JDS; McKnight DM; Spaulding SA, 2015. Hydrodynamic control of benthic mats of Didymosphenia geminata at the reach scale. Canadian Journal of Fisheries and Aquatic Sciences, 72(6):902-914. http://www.nrcresearchpress.com/doi/full/10.1139/cjfas-2014-0314

Day, D. (1882): Plants of Buffalo and vicinity. - Bull. Buffalo Soc. Nat. Sci. 4: 65–279.

De Toni, G.B. (1891): Sylloge algarum omnium hucusque cognitarum. Vol. II. Bacillarieae; sectio I. Raphideae. - Typis Seminarrii, Patavii.

Duthie, H.C. (1973): A checklist of the freshwater algae of Ontario, exclusive of the Great Lakes. - Naturaliste Canad. (Quebec) 103: 83–109.

Ejaz, H., E. Somanader, U. Dave, H. Ehrlich, and M.A. Rahman. 2021. Didymo and Its Polysaccharide Stalks: Beneficial to the Environment or Not?. 2(1), pp.69-79. Polysaccharides 2(1):69-79. https://doi.org/10.3390/polysaccharides2010005.

FOX, J.L., T.O. ODLAUG & T.A. OLSON (1969): The ecology of periphyton in western Lake Superior. Part 1. Taxonomy and distribution. - Water Resources Res. Center Bull. 14: 1–127.

FOX, J.L., T.A. OLSON & T.O. ODLAUG (1967): The collection, identification, and quantitation of epilithic periphyton in Lake Superior. - Proc. 10th Conf. Great Lakes Res. 1: 12–19.

FOX, J.L., T.A. OLSON & T.O. ODLAUG (1973): The collection, identification, and quantitation of epilithic periphyton in Lake Superior. - Proc. 10th Conf. Great Lakes Res. 10: 12.

Gillis, C.A., and M. Chalifour. 2010. Changes in the macrobenthic community structure following the introduction of the invasive algae Didymosphenia geminata in the Matapedia River (Québec, Canada). Hydrobiologia 647(1):63-70. https://doi.org/10.1007/s10750-009-9832-7.

Gillis, C. and I. Lavoie. 2014. A preliminary assessment of the effects of Didymosphenia geminata nuisance growths on the structure and diversity of diatom assemblages of the Restigouche River basin, Quebec, Canada. Diatom Research 29(3):281-292.

Hoddle, M. 2017. Didymo (or Rock Snot) Didymosphenia geminata (Bacilariophyceae:Cymbellales). http://cisr.ucr.edu/didymo_rock_snot.html. Accessed on 08/16/2017.

HOHN, M.H. (1951): A study of the distribution of diatoms in western New York State. - Mem. Cornell Agric. Exploit. Sta. 308: 1–37.

Jackson, L.J., L. Corbett, and G. Scrimgeour. 2016. Environmental constraints on Didymosphenia geminata occurrence and bloom formation in Canadian Rocky Mountain lotic systems. Canadian Journal of Fisheries and Aquatic Sciences 73:964-972.

James DA; Bothwell ML; Chipps SR; Carreiro J, 2015. Use of phosphorus to reduce blooms of the benthic diatom Didymosphenia geminata in an oligotrophic stream. Freshwater Science, 34(4):1272-1281. http://www.jstor.org/stable/10.1086/683038?seq=1#page_scan_tab_contents

Jellyman PG; Clearwater SJ; Biggs BJF; Blair N; Bremner DC; Clayton JS; Davey A; Gretz MR; Hickey C; ; Kilroy C, 2006. Didymosphenia geminata experimental control trials (product screening and testing) and stalk degradation studies. NIWA Project MAF06504, Client Report CHC2006-128. New Zealand: National Institute of Water And Atmospheric Research.

Kristiansen, J. 1996. Dispersal of freshwater algae - a review. Hydrobiologia 336:151-157.

KÜTZING, F.T. (1849): Species Algarum. - Brockhaus, Leipzig.

LINDSEY, A.A., D.V. SCHMELZ & S.A. NICHOLS (1969): Natural areas in Indiana and their preservation. - The Indiana Academy of Science, Indianapolis.
MARSH, C.D. (1895): On the Cyclopidae and Calanidae of Lake St. Clair, Lake Michigan, and certain of the inland lakes of Michigan. - Bull. Michigan Fish Comm. 5: 1–24.

MDDEP-MRNF (2007): Scientific Advisory Committee on Didymosphenia geminata, 2007. What is Didymo and how can we prevent it from spreading in our rivers? - Ministère du Développement durable, de l’Environnement et des Parcs et Ministère des Ressources naturelles et de la Faune, Québec.

Midwest Invasive Species Information Network (MISIN). 2017. Didymo Didymosphenia geminata . http://www.misin.msu.edu/facts/detail.php?id=229. Accessed on 08/10/2017.

MILLER, C.R. (1915): Fresh-water algae occurring in the vicinity of the island of Montreal. - Canad. Rec. Sci. 9: 391–425.

MOFFAT, M.C. (1994): An ultrastructural study of Didymosphenia geminata (Bacillariophyceae). - Trans. Amer. Microscop. Soc. 113: 59–71.

NELSON, R.R., T.O. ODLAUG, B.O. KROGSTAD, O.R. RUSCHMEYER & T.A. OLSON (1973): The effects of enrichment on Lake Superior periphyton. - Water Resources Research Centre Bull. 59: 1–182.

PALMER, C.M. (1930): Algae of Indiana - additions to the 1875–1928 check list. - Proc. Indiana Acad. Sci. 40: 107–109.Patrick, R. and C. W. Reimer 1975. The diatoms of the United States, exclusive of Alaska and Hawaii, Volume 2, Part 1-Entomoneidaceae, Cymbellaceae, Gomphonemaceae, Epithemaceae. Academy of Natural Sciences of Philadelphia Monograph No. 13, 213 pp.

Pennsylvania Sea Grant. 2013. Didymo Didymosphenia geminata. https://seagrant.psu.edu/sites/default/files/Didymo2013_reduced_0.pdf. Accessed on 08/15/2017.

River Alliance of Wisconsin. 2009. Didymo / Rock Snot (Didymosphenia geminata) A Threat to Wisconsin's Waters. https://www.wisconsinrivers.org/images/Documents/AIS/Didymo_WT910_Final_4.pdf. Accessed on 08/18/2017.

Root, S., and C.M. O'Reilly. 2012. Didymo control: increasing the effectiveness of decontamination strategies and reducing spread. Fisheries 37(10):440-448.

Spaulding, S.A. and L. Elwell. 2007. Increase in Nuisance Blooms and Geographic Expansion of the Freshwater Diatom Didymosphenia geminata. USGS In Cooperation with the Federation of Fly Fishers, Reston, VA. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev3_015176.pdf.

Stoermer, E. 1965.  Unpublished collection, [cited on Great Lakes Diatom Home Page].

STOERMER, E.F. (1975): Comparison of benthic diatom communities in Lake Michigan and Lake Superior. - Verh. Int. Vereinigung Limnol. 19: 932–938.

STOERMER, E.F. (1980): Characteristics of benthic algal communities in the upper Great Lakes. U.S. Environmental Protection Agency, Ecological Research Series EPA 600/3-80-073. - Environmental Research Laboratory, Duluth, Minnesota.

STOERMER, E.F. (1993): Evaluating diatom succession: some pecularities of the Great Lakes case. - J. Paleolimnol. 8: 71–83.

Stoermer, E. 1995.  Great Lakes Diatom Home Page.  http://www.umich.edu/~phytolab/GreatLakesDiatomHomePage/top.html

STOERMER, E.F. & R.G. KREIS (1978): Preliminary checklist of diatoms (Bacillariophyta) from the Laurentian Great Lakes. - J. Great Lakes Res. 4: 149–169.

STOERMER, E.F., Y.Z. QI & T.B. LADEWSKI (1986): A quantitative investigation of shape variation in Didymosphenia geminata (Lyngbye) M. Schmidt (Bacillariophyta). - Phycologia 25: 494–502.

STOERMER, E.F., R.G. KREIS & N.A. ANDRESEN (1999): Checklist of diatoms from the Laurentian Great Lakes II. - J. Great Lakes Res. 25: 515–566.

Taylor, B.W. and M.L. Bothwell. 2009. The Origin of Invasive Microorganisms Matters for Science, Policy, and Management: The Case of Didymosphenia geminata. Bioscience 64(6):531-538.

THOMAS, B.W. & H.H. CHASE (1887): Diatomaceae of Lake Michigan as collected during the last 16 years from the water supply of the city of Chicago. - Notarisia 2: 328–330.
WARD, H.B. (1896): A biological examination of Lake Michigan in the Traverse Bay Region. - Bull. Michigan Fish. Commiss. 6: 1–100.

Wysokowski, M., P. Bartczak, S. Zóltowska-Aksamitowska, A. Chudzinska, A. Piasecki, E. Langer, V.V. Bazhenov, I. Petrenko, T. Noga, A.L. Stelling, and H. Ehrlich. 2017. Adhesive stalks of diatom Didymosphenia geminata as a novel biological adsorbent for hazardous metals removal. CLEAN–Soil, Air, Water 45(11):1600678. https://doi.org/10.1002/clen.201600678.

Other Resources:
Great Lakes Waterlife

Author: Sturtevant, R., K. Hopper, and C.R. Morningstar

Revision Date: 1/15/2024

Citation Information:
Sturtevant, R., K. Hopper, and C.R. Morningstar, 2024, Didymosphenia geminata (Lyngb.) M. Schmidt: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?SpeciesID=2856, Revision Date: 1/15/2024, Access Date: 7/13/2024

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.


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Citation information: U.S. Geological Survey. [2024]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [7/13/2024].

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