Ulva (Enteromorpha) intestinalis Linnaeus, 1753

Common Name: Green alga, grass kelp

Synonyms and Other Names:

gut weed, Conferva intestinalis, Enteromorpha intestinalis, Enteronia simplex, Fistularia intestinalis, Ilea intestinalis, Hydrosolen intestinalis, Scytosiphon intestinalis, Scytosiphon intestinalis var. nematodes, Solenia intestinalis, Tetraspora intestinalis, Ulva bublosa var. intestinalis, Ulva compressa var. intestinalis, Ulva enteromorpha var. intestinalis.

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Identification: Thalli of this species of green alga are yellow green to vibrant or dark green and tubular, hollow, wrinkled, convolute, intestine-like, and crumpled looking. Individual cells are often relatively round or ovoid but sometimes may be rectangular or polygon-shaped. They are generally arranged randomly but in some cases can form disorganized rosettes. Plants may be branched or unbranched. Samples from the Portage River, Ohio are branched. Branching may be inversely related to salinity. Near the Detroit River in the Great Lakes drainage, there have been two forms recorded, namely U. intestinalis f. maxima and U. intestinalis f. cylindracea (Blomster et al. 1998, Catling and McKay 1980, Hadi et al. 1989, Kapraun 1970, Taft 1964). In Ohio, U. intestinalis colonies grow up to 20 cm long. Cells are around 10–25 by 16–18 microns (Taft 1964).

Size: colonies to 20 cm long

Native Range: Ulva intestinalis is a relatively cosmopolitan species known to form blooms in a diverse range of habitats around the world (Cummins et al. 2004). With respect to its introduction to the Great Lakes drainage, authors typically mention that it is considered native to the Atlantic coast of North America (Mills et al. 1993).

Map Key
This map only depicts Great Lakes introductions.

Great Lakes Nonindigenous Occurrences: The first Great Lakes record of U. intestinalis was in 1926, when it was observed near a salt plant at Wolf Creek near Silver Springs, New York, part of the Lake Ontario drainage (Muenscher 1927). It was then reported in 1951 from an upwelling spring near Elmore, Ohio, in the Portage River drainage, part of the Lake Erie drainage (Taft 1964). In 1979, it was recorded near the Ojibway Salt Mine on the Detroit River, which flows out of Lake St. Clair and into Lake Erie (Catling and McKay 1980).

Table 1. Great Lakes region nonindigenous occurrences, the earliest and latest observations in each state/province, 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 Ulva (Enteromorpha) intestinalis are found here.

State/ProvinceFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
NY192619261Upper Genesee

Table last updated 11/29/2022

† Populations may not be currently present.

Ecology: Ulva intestinalis occurs in many different habitat types and takes many different forms. It has been recorded in fresh to saline waters from ditches, pools, rockpools, canals, moorlands, and bedrock. It can grow epiphytically, epilithically, as unattached floating monostromatic sheets, detached floating ropes, attached mats of monostromatic tubes, and as proliferous bottle brush forms (Bäck et al. 2000, Bjoerk et al. 2004, Blomster et al. 1998, 2002, Hadi et al. 1989, Moss and Marsland 1976, Reed and Russell 1978, Romano et al. 2003, Simons 1994, Vadas and Beal 1987).

Ulva intestinalis has an enhanced ability to form blooms in eutrophic conditions. It exhibits rapid nutrient uptake, growth, and osmoregulation, particularly in conditions of reduced salinity and light. Optimal salinity for growth may be around 15–24‰ but varies greatly depending on the population. Ulva intestinalis populations around the world consist of various ecotypes that are somewhat genetically different from each other, each specifically adapted to grow best in a different salinity regime. Although growth is typically positively related to salinity, many populations can survive and grow in freshwater conditions, and the negative effects of low salinity can be offset by increased nutrient concentrations. Most U. intestinalis ecotypes, however, exhibit very broad salinity tolerance (Cohen and Fong 2004, Edwards et al. 1988, Kamer and Fong 2000, 2001, Martins et al. 1999, McAvoy and Klug 2005).

Ulva intestinalis has two life stages, the sexual gamete-producing gametophyte and the asexual zoospore-producing sporophyte. Gametes are biflagellate and zoospores are typically quadriflagellate. Sporophytes usually occur over a wider temperature and salinity range than gametophytes. The latter are generally not well adapted to low salinity values and extended periods of desiccation. Sporophytes are often also capable of reproducing over longer time periods than gametophytes (Cordi et al. 2001, Pringle 1986). Swarmers can survive in motile form for around 5–8 days. They disperse well, as they are positively phototactic and thus can remain high in the water column, allowing them to be carried far away from parent populations (Hoffman and Camus 1989).

In Ohio, U. intestinalis has been recorded from shady regions of the Portage River, where there is almost no flow in shallow bedrock pools created by upwelling through limestone faults (Taft 1964). On the other hand, at the Ojibway Salt Mine near the Detroit River, forms of this species have occurred in an effluent stream and lagoon on rocks subject to wave action (Catling and McKay 1980). Finally, as previously mentioned, the population originally found near a salt plant at Wolf Creek, New York (Muenscher 1927) has decreased and may no longer even be present, probably due to decreased salinity (Marcus et al. 1984).

Means of Introduction: Ulva intestinalis was accidentally released into the Great Lakes drainage (Mills et al. 1993).

Status: Established where recorded. However, populations are greatly reduced or possibly no longer present in the Wolf Creek drainage due to decreased salinity (Marcus et al. 1984).

Great Lakes Impacts:

Ulva intestinalis has a moderate environmental impact in the Great Lakes.

Ulva intestinalis has caused serious negative impacts in marine and coastal areas outside of the Great Lakes region. In these regions, U. intestinalis may form green tides and biofouling mats that cause cascading effects throughout the food web. However, the harmful bloom development seen in marine environments is rare in inland, freshwater populations (Messyasz and Rybak 2011). Large systems like the Great Lakes may experience more negative effects; U. intestinalis typically forms green tides in the Baltic Sea in eutrophic conditions (Alstroem-Rapaport and Leskinen 2002), where it may be associated with food web alterations. In such conditions, grazing pressure often cannot control massive blooms (Lotze et al. 2000, Lotze and Worm 2002).

Ulva intestinalis mats can deplete the available oxygen in the water and increase the production of hydrogen sulphide in the sediment, which can cause population declines in other fauna and flora (Bäck et al. 2000, Cummins et al. 2004, Vadas and Beal 1987). Mats can also shade out native seagrass beds (Cummins et al. 2004) and negatively impact their corresponding communities, as well disrupt feeding by wading birds (Raffaeli et al. 1998). Furthermore, Romano et al. (2003) observed in England an increase in friction drag with the presence of Ulva intestinalis mats, causing a 10% to 56% reduction in current velocities. There was also a significant reduction is sediment erosion.

Ulva intestinalis has the potential to be a superior macrophyte competitor. Lotze et al. (2000) found that this species can produce a propagule bank capable of surviving winter conditions in the Baltic Sea. Such a seed bank allowed U. intestinalis to begin growing two months earlier than many native species, enabling it to escape herbivory and nutrient competition. 

Internationally, Ulva intestinalis has also been associated directly or in part with negative impacts on diversity or specific taxa. In Indian coastal areas, filamentous forms of U. intestinalis have been associated with lower faunal community diversity than areas with more bushy algae (Yogamoorthi 1998). In European coastal waters, epiphytic benthic diatoms prefer growing on monosiphonous forms of U. prolifera to colonizing broad and flattened forms of U. intestinalis (Holt 1980). Furthermore, some marine forms of U. intestinalis are more difficult for grazers to handle and ingest than species with more frond structure (Watson and Norton 1985). Epibionts like Ulva can also exert increased drag on snails living in high flow conditions, causing them to invest more energy in foot muscles and less in growth (Wahl 1996). In the Gulf of Maine, blooms of novel floating rope forms have colonized the substrate, causing anoxia with the potential to exert negative impacts on bivalve species (Vadas and Beal 1987). Finally, in conditions of nitrogen scarcity in estuaries and lagoons on the coast of southern California, U. intestinalis can out-compete Ulva expansa (Fong et al. 1996).

There is little or no evidence to support that Ulva intestinalis has significant socio-economic impacts in the Great Lakes.
Ulva intestinalis is one of the species that contributes to the 109 kg of seaweed removed every year from recreational beaches in France (Blomster et al. 2002).

Mats of U. intestinalis in England also caused an order of magnitude decrease in abundance of the economically important bivalve Cerastoderma edule (Romano et al. 2003).

There is little or no evidence to support that Ulva intestinalis has significant beneficial effects in the Great Lakes.


Regulations (pertaining to the Great Lakes region)
There are no known regulations for this species.

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.


There are no known biological control methods for this species.

There are no known physical control methods for this species.

In locations outside of the Great Lakes, the distribution and abundance of U. intestinalis is dependent on salinity and nutrient levels (Kramer and Fong 2000, 2001;Messyasz and Rybak 2011). The reduction of pollution and nutrient run-off could decrease the viable habitat for U. intestinalis.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

Remarks: Ulva was described by Linneaus to consist of green seaweeds with distromatic blades. Link, later removed some species from Ulva and moved species that had a distinct tubular form to Enteromorpha. After conducting a phylogenetic DNA analysis, Hayden et al. (2003) found that the separation of the two genera was artificial and that species in Enteromorpha should be returned to the original Ulva classification. While ITIS lists U. flexuosa under the genus Enteromorpha, Enteromorpha species are now recognized as belonging to the genus Ulva.

References: (click for full references)

Alstroem-Rapaport, C., and E. Leskinen. 2002. Development of microsatellite markers in the green algae Enteromorpha intestinalis (Chlorophyta). Molecular Ecology Notes 2(4): 581-583.

Back, S., A. Lehvo, and J. Blomster. 2000. Mass occurrence of unattached Enteromorpha intestinalis on the Finnish Baltic Sea coast. Annales Botanici Fennici 37(3): 155-161.

Bjoerk, M., L. Axelsson, and S. Beer. 2004. Why is Ulva intestinalis the only macroalga inhabiting isolated rockpools along the Swedish Atlantic coast? Marine Ecology Progress Series 284: 109-116.

Blomster, J., C.A. Maggs, and M.J. Stanhope. 1998. Molecular and morphological analysis of Enteromorpha intestinalis and E. compressa (Chlorophyta) in the British Isles. Journal of Phycology 34: 319-340.

Blomster, J., S. Back, D.P. Fewer, M. Kiirikki, A. Lehvo, C.A. Maggs, and M.J. Stanhope. 2002. Novel morphology in Enteromorpha (Ulvophyceae) forming green tides. American Journal of Botany 89(11): 1756-1763.

Catling, P.M., and W.G. McKay. 1980. Halophytic plants in southern Ontario. Canadian Field Naturalist 94(3): 248-258.

Cohen, R.A., and P. Fong. 2004. Physiological responses of a bloom-forming green macroalga to short-term change in salinity, nutrients, and light help explain its ecological success. Estuaries 27(2): 209-216.

Cordi, B., J. Peloquin, D.N. Price, and M.H. Depledge. 2001. The influence of UV-B radiation on the reproductive cells of the intertidal macroalga, Enteromorpha intestinalis. Aquatic Toxicology (Amsterdam) 56(1): 1-11.

Cummins, S.P., D. E. Roberts, and K.D. Zimmerman. 2004. Effects of the green macroalga Enteromorpha intestinalis on macrobenthic and seagrass assemblages in a shallow coastal estuary. Marine Ecology Progress Series 266: 77-87.

Edwards, D.M., R.H. Reed, and W.D.P. Stewart. 1988. Osmoacclimation in Enteromorpha intestinalis: long-term effects of osmotic stress on organic solute accumulation. Marine Biology 98: 467-476.

Fong, P., K.E. Boyer, J.S. Desmond, and J.B. Zedler. 1996. Salinity stress, nitrogen competition, and facilitation: what controls seasonal succession of two opportunistic green macroalgae? Journal of Experimental Marine Biology and Ecology 206(1-2): 203-221.

Hayden, H.S., J., Blomster, C.A., Maggs, C. A., P.C., Silva, M.J., Stanhope, and J.R. Waaland. 2003. Linnaeus was right all along: Ulva and Enteromorpha are not distinct genera. European Journal of Phycology 38: 277–94.

Hadi, R., A.M. Hadi, K.M. Bahram, and A.A.S. Hassan. 1989. The new recorded species of Enteromorpha in Baghdad area, Iraq. Bulletin of the Iraq Natural History Museum 8(2): 163-172.

Hoffmann, A.J., and P. Camus. 1989. Sinking rates and viability of spores from benthic algae in central Chile. Journal of Experimental Marine Biology and Ecology 126: 281-291.

Holt, G. 1980. Benthic diatoms on green algae in Norway and Faeroe Islands, Scotland, UK. Blyttia 38(1): 9-18.

Kamer, K., and P. Fong. 2000. A fluctuating salinity regime mitigates the negative effects of reduced salinity on the estuarine maroalga, Enteromorpha intestinalis (L.) link. Journal of Experimental Marine Biology and Ecology 254(1): 53-69.

Kamer, K., and P. Fong. 2001. Nitrogen enrichment ameliorates the negative effects of reduced salinity on the green macroalga Enteromorpha intestinalis. Marine Ecology Progress Series 218: 87-93.

Kamer, K., P. Fong, R. Kennison, and K. Schiff. 2004. Nutrient limitation of the macroalgae Enteromorpha intestinalis collected along a resource gradient in a highly eutrophic estuary. Estuaries 27(2): 201-208.

Kapraun, D.F. 1970. Field and cultural studies of Ulva and Enteromorpha in the vicinity of Port Aransas, Texas. Contributions in Marine Science 15: 205-285.

Lotze, H.K., B. Worm, and U. Sommer. 2000. Propagule banks, herbivory and nutrient supply control population development and dominance patterns in macroalgal blooms. Oikos 89: 46-58.

Lotze, H.K., and B. Worm. 2002. Complex interactions of climatic and ecological controls on macroalgal recruitment. Limnology and Oceanography 47(6): 1734-1741.

Marcus, B.A., H.S. Forest, and B. Shero. 1984. Establishment of freshwater biota in an inland stream following reduction of salt input. Canadian Field Naturalist 98(2): 198-208.

Martins, I., J.M. Oliveira, M.R. Flindt, and J.C. Marques. 1999. The effect of salinity on the growth rate of the macroalgae Enteromorpha intestinalis (Chlorophyta) in the Mondego estuary (west Portugal). Acta Oecologica 20(4): 259-265.

McAvoy, K.M., and J.L. Klug. 2005. Positive and negative effects of riverine input on the estuarine green algae Ulva intestinalis (syn. Enteromorpha intestinalis) (Linnaeus). Hydrobiologia 545: 1-9.

Messyasz, B., and A. Rybak. 2011. Abiotic factors affecting the development of Ulva sp. (Ulvophyceae; Chlorophyta) in freshwater ecosystems. Aquatic Ecology 45(1): 75-87.

Mills, E.L., J.H. Leach, J.T. Carlton, and C.L. Secor. 1993. Exotic species in the Great Lakes: a history of biotic crises and anthropogenic introductions. Journal of Great Lakes Research 19(1): 1-54.

Moss, B., and A. Marsland. 1976. Regeneration of Enteromorpha intestinalis. British Phycological Journal 11(4): 309-313.

Muenscher, W.C. 1927. Spartina patens and other saline plants in the Genesee Valley of western New York. Rhodora 29: 138-139.

Pringle, J.D. 1986. Swarmer release and distribution of life-cycle phases of Enteromorpha intestinalis Chlorophyta in relation to environmental factors. Journal of Experimental Marine Biology and Ecology 100(1-3): 97-112.

Raffaelli, D.G., J.A. Raven, and L.J. Poole. 1998. Ecological impact of green macroalgal blooms. Oceanography and Marine Biology: An Annual Review 36: 97-125

Reed, R.H., and G. Russell. 1978. Salinity fluctuations and their influence on bottle brush morphogenesis in Enteromorpha intestinalis. British Phycological Journal 13(2): 149-153.

Romano, C., J. Widdows, M.D. Brinsley, and F.J. Staff. 2003. Impact of Enteromorpha intestinalis mats on near-bed currents and sediment dynamics: flume studies. Marine Ecology Progress Series 256: 63-74.

Simons, J. 1994. Field ecology of freshwater macroalgae in pools and ditches, with special attention to eutrophication. Netherlands Journal of Aquatic Ecology 28(1): 25-33.

Taft, C.E. 1964. The occurrence of Monostroma and Enteromorpha in Ohio. Ohio Journal of Science 64: 272-274.

Vadas, R.L., and B. Beal. 1987. Green algal ropes: a novel estuarine phenomenon in the Gulf of Maine. Estuaries 10(2): 171-176.

Wahl, M. 1996. Fouled snails in flow: potential of epibionts on Littorina littorea to increase drag and reduce snail growth rates. Marine Ecology Progress Series 138(1-3): 157-168.

Watson, D.C., and T.A. Norton. 1985. The physical characteristics of seaweed thalli as deterrents to littorine grazers. Botanica Marina 28(9): 383-387.

Yogamoorthi, A. 1998. Ecological studies on phytal fauna associated with intertidal seaweeds from south east coast of India. Journal of Ecobiology 10(4): 245-250.

Other Resources:
Author: Kipp, R.M., M. McCarthy, and A. Fusaro

Contributing Agencies:

Revision Date: 9/12/2019

Citation for this information:
Kipp, R.M., M. McCarthy, and A. Fusaro, 2022, Ulva (Enteromorpha) intestinalis Linnaeus, 1753: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI, https://nas.er.usgs.gov/queries/GreatLakes/FactSheet.aspx?NoCache=2%2F21%2F2011+7%3A06%3A11+AM&Species_ID=1714&State=&HUCNumber=, Revision Date: 9/12/2019, Access Date: 11/29/2022

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.