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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.

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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.gov

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.  

Alaska	Hawaii	Puerto Rico & Virgin Islands	Guam Saipan

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

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...

Ecological	Economic	Other

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)

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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.

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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.

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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.

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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.

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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.

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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

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Other Resources:
Great Lakes Waterlife

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

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.