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Brief Summary

It's not hard to see where this seaweed got its name. The large, sturdy single bladders filled with gas in the middle of the fronds look like large knots. These 'airbags' help the plant to stand up straight under water. Knotted wrack is found in the North Sea and along the Atlantic coasts. The species attaches itself to rocks and stones in the middle of the tidal region. It can survive for a long time after breaking off from its base. All sorts of other seaweed species grow underneath knotted wrack, species that without protection would normally dry out during ebbing water. This seaweed also provides shelter for small marine animals.
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Comprehensive Description


 A common large brown seaweed, dominant on sheltered rocky shores. The species has long strap like fronds with large egg-shaped air bladders at regular intervals. The fronds of Ascophyllum nodosum are typically between 0.5 and 2m in length. The species often bears tufts of the small reddish-brown filamentous epiphytic algae Polysiphonia lanosa. Ascophyllum nodosum occurs on the middle of the shore, often with Fucus vesiculosus. The species grows slowly and plants can live to be several decades old. Individual fronds can become up to 15 years old before breakage.Detached forms of Ascophyllum nodosum are known from several habitats. Ascophyllum nodosum var. mackaii is found on very sheltered shores, in sea lochs and is sometimes common on the west coasts of Ireland and Scotland. The frond has extensive dichotomous branching and bears few air bladders. The plants drift in large, spherical masses in sheltered waters. Ascophyllum nodosum var. scorpioides, which is abundant in New Hampshire (U.S.A.), is often associated with the marsh grass Spartina alterniflora. According to Gibb (1957) the major difference between the ecads mackaii and scorpioides is the proportion of apical to lateral branching. If branching is both 'apical and lateral' the algae would be designated as mackaii while if it is 'almost entirely lateral' it would be designated as scorpioides. Unattached forms arise when detached fragments of Ascophyllum nodosum are deposited onto the shore where they continue to multiply and branch independently of the original fragment (Chock & Mathieson, 1976).

 Chock & Mathieson (1979) demonstrated the physiological responses of Ascophyllum nodosum and its detached ecad scorpioides were similar under varying conditions of light intensity, temperature and salinity.

 Ascophyllum nodosum var. mackaii:
The presence of the ecad in any particular situation depends on the combination of a number of conditions applying at a tide level between high and low water neaps:
  • frequent alternation of high and low salinities so a supply of freshwater is of primary importance;
  • good shelter from wave action because of the unattached state of the ecad;
  • absence of fast moving water, whether caused by freshwater streams or tidal conditions;
  • flat, undulating or slightly sloping shore profile where stability is high, and
  • substratum type, the porosity of which affects the conditions of salinity and also influences, to some extent, the development of the ecad.
 Very sheltered conditions are often found at loch heads on the west coast of Scotland and in these situations the ecad is sometimes present in great abundance. Sheltered or land-locked bays or situations in the lee of small islands are other favourable positions (Gibb, 1957).
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Depth range based on 124 specimens in 2 taxa.
Water temperature and chemistry ranges based on 9 samples.

Environmental ranges
  Depth range (m): 0 - 34.75
  Temperature range (°C): 11.855 - 12.348
  Nitrate (umol/L): 4.729 - 6.151
  Salinity (PPS): 35.184 - 35.363
  Oxygen (ml/l): 6.128 - 6.151
  Phosphate (umol/l): 0.351 - 0.418
  Silicate (umol/l): 2.578 - 3.285

Graphical representation

Depth range (m): 0 - 34.75

Temperature range (°C): 11.855 - 12.348

Nitrate (umol/L): 4.729 - 6.151

Salinity (PPS): 35.184 - 35.363

Oxygen (ml/l): 6.128 - 6.151

Phosphate (umol/l): 0.351 - 0.418

Silicate (umol/l): 2.578 - 3.285
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.


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 The species attaches to rocks and boulders on the middle shore in a range of habitats, from estuaries to relatively exposed coasts. It occupies a similar shore height as Fucus vesiculosus. Subtidal populations have been reported, for example in the very clear waters of Rhode Island, USA. However, an intertidal habit is more usual.
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©  The Marine Biological Association of the United Kingdom

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Plant / epiphyte
Bowerbankia imbricata grows on Ascophyllum nodosum
Other: major host/prey

In Great Britain and/or Ireland:
Foodplant / parasite
immersed pycnidium of Mycosphaerella ascophylli parasitises live receptacle of Ascophyllum nodosum
Remarks: season: (11-)1-5

Foodplant / parasite
Orcadia ascophylli parasitises old, worn thallus of Ascophyllum nodosum

Foodplant / parasite
Trailia ascophylli parasitises Ascophyllum nodosum
Remarks: Other: uncertain


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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage: Ascophyllum nodosum

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 0
Specimens with Barcodes: 1
Species With Barcodes: 1
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© Barcode of Life Data Systems

Source: Barcode of Life Data Systems (BOLD)


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Ascophyllum nodosum is a large, common brown alga (Phaeophyceae) in the family Fucaceae, being the only species in the genus Ascophyllum. It is seaweed of the northern Atlantic Ocean, also known as rockweed, Norwegian kelp, knotted kelp, knotted wrack or egg wrack. It is common on the north-western coast of Europe (from Svalbard to Portugal) including east Greenland[1] and the north-eastern coast of North America.[2]



Ascophyllum nodosum has long fronds with large egg-shaped air-bladders set in series at regular intervals in the fronds and not stalked. The fronds can reach 2 m in length and are attached by a holdfast to rocks and boulders. The fronds are olive-brown in color and somewhat compressed but without a mid-rib.[3]

Life history is of one diploid plant and gametes. The gametes are produced in conceptacles embedded in yellowish receptacles on short branches.[2][4]

Varieties and forms[edit]

Several different varieties and forms of this species have been described.

There are free floating ecads of this species such as Ascophyllum nodosum mackaii Cotton, which is found at very sheltered locations, such as at the heads of sea lochs in Scotland and Ireland.[6][7]


Ascophyllum nodosum is found mostly on sheltered sites on shores in the mid-littoral where it can become the dominant species in the littoral zone.[8][9]

The species is found in a range of coastal habitats from sheltered estuaries to moderately exposed coasts, often it dominates the inter-tidal zone (although sub-tidal populations are known to exist in very clear waters). However it is rarely found on exposed shores, and if it is found the fronds are usually small and badly scratched. This seaweed grows quite slowly, 0.5% per day; carrying capacity is about 40 kg wet weight per square meter and it may live for 10–15 years. It may typically overlap in distribution with Fucus vesiculosus and Fucus serratus. Its distribution is also limited by salinity, wave exposure, temperature, desiccation and general stress. These, and other attributes of the algae are summarized in Schonbeck & Norton (1980).[10][11][12] It may take approximately five years before becoming fertile.

Phlorotannins in Ascophyllum nodosum act as chemical defenses against the marine herbivorous snail, Littorina littorea.[13]

Vertebrata lanosa (L.) T.A. Christensen is a small red alga, commonly found growing in dense tufts on Ascophyllum whose rhizoids penetrate the host.[14] It is considered by some as parasitic, however as it only receives structural support from Knotted Wrack (i.e. non-parasitically), it acts as an epiphyte.


Recorded in Europe from: Faroe Islands,[15] Norway,[16] Ireland, Britain and Isle of Man,[17] Netherlands,[18] North America: Bay of Fundy, Nova Scotia, Baffin Island, Hudson Strait, Labrador and Newfoundland.[1][2] It has been recorded as an accidental introduction to San Francisco, California, and as a potentially invasive species eradicated.[19]


Ascophyllum nodosum is harvested for use in alginates, fertilisers and for the manufacture of seaweed meal for animal and human consumption.[20] It has long been used as an organic and mainstream fertilizer for many varieties of crops due to its combination of both macronutrient, (e.g. N, P, K, Ca, Mg, S) and micronutrients (e.g. Mn, Cu, Fe, Zn, etc.). It also host to cytokinins, auxin-like gibberellins, betaines, mannitol, organic acids, polysaccharides, amino acids, and proteins which are all very beneficial and widely used in agriculture.[21] Ireland, Scotland and Norway have provided the world's principal alginate supply.[22][23]

Ascophyllum nodosum is frequently used as packaging material for baitworm and lobster shipments from New England to various domestic and international locations.[24] Ascophyllum itself has occasionally been introduced to California, and several species frequently found in baitworm shipments, including Carcinus maenas and Littorina saxatilis, may have been introduced to the San Francisco Bay region this way.[24]

Toxicological uses[edit]

Because the age of the different parts of A. nodosum can be identified by its shoots, A. nodusum has also been used to monitor concentrations of heavy metals in sea water. A concentration factor for zinc has been reported to be of the order 10 to the fourth.[25][26]


A. nodosum contains the phlorotannins tetraphlorethol C and tetrafucol A.[27]

Harvesting controversy[edit]

There is controversy over impacts of commercial harvesting of Ascophyllum nodosum for use in garden or crop fertilizers and livestock feed supplements in North America and Europe. Some research has been focused on by-catch and impact on Intertidal zone communities. [28] Opponents of wild Ascophyllum harvests point to the algae's high habitat value for over 100 marine species,[29] including benthic invertebrates, [30] commercially important fish,[31] wild ducks,[32] shorebirds,[33] and seabirds.[34] Shoreland owners in Maine, as well as federal, state, and local agencies in the United States have placed their conservation lands off limits to Ascophyllum removal.[35][36] Rockweed harvesters point to the value of the seasonal jobs created by the harvest operation.[37]


This article incorporates CC-BY-2.5 text from the reference[24]

  1. ^ a b M. D. Guiry & Wendy Guiry (2006-11-23). "Ascophyllum nodosum (Linnaeus) Le Jolis". AlgaeBase. 
  2. ^ a b c W. R. Taylor (1962). Marine Algae of the Northeastern Coast of North America. Ann Arbor, University of Michigan Press. ISBN 0-472-04904-6. 
  3. ^ S. Hiscock (1979). "A field key to the British brown seaweeds (Heterokontophyta)". Field Studies 5: 1–44. 
  4. ^ H. Stegenga, J. J. Bolton & R. J. Anderson (1997). Seaweeds of the South African West Coast. Bolus Herbarium Humber 18, University of Cape Town. ISBN 0-7992-1793-X. 
  5. ^ M. J. Lynn (1949). "A rare alga from Larne Lough". Irish Naturalists' Journal 9: 301–304. 
  6. ^ D. C. Gibb (1957). "The free-living forms of Ascophyllum nodosum (L.) Le Jol". Journal of Ecology (British Ecological Society) 45 (1): 49–83. doi:10.2307/2257076. JSTOR 2257076. 
  7. ^ O. Morton (2003). "The marine macroalgae of County Donegal, Ireland". Bulletin of the Irish Biogeographical Society 27: 3–164. 
  8. ^ O. Morton (1994). Marine Algae of Northern Ireland. Ulster Museum, Belfast. ISBN 0-900761-28-8. 
  9. ^ J. R. Lewis (1964). The Ecology of Rocky Shores. English Universities Press, London. 
  10. ^ Schonbeck, M. W.; Norton, T. A. (1980). "Factors controlling the ower limits of fucoid algae on the shore". J.exp.mar.biol. ecol. 43: 131–150. 
  11. ^ Seip, K. L. (1980). "A mathematical model of competition and colonization in a community of marine benthic algae". Ecological modelling 10: 77–104. 
  12. ^ Seip K. L. "Mathematical models of rocky shore ecosystems". In: Jørgensen S. E. & Mitch W J. (Eds.) Application of ecological modelling in environmental management, Part B, Chap 13, pp 341-433.
  13. ^ Polyphenols in brown algae Fucus vesiculosus and Ascophyllum nodosum: Chemical defenses against the marine herbivorous snail, Littorina littorea. J. A. Geiselman and O. J. McConnell, Journal Of Chemical Ecology,1981, Volume 7, Number 6, pages 1115-1133, doi:10.1007/BF00987632
  14. ^ C. A. Maggs (1993). Seaweeds of the British Isles. Vol. I: Rhodophyta. Part 3A. Natural History Museum, London. ISBN 0-11-310045-0. 
  15. ^ F. Børgesen (1903). Botany of the Færöes Part II, pp. 339-532. Det nordiske Forlag Ernst Bojesen, Copenhagen. 
  16. ^ F. E. Round (1981). The Ecology of Algae. Cambridge University Press Cambridge. ISBN 0-521-22583-3. 
  17. ^ F. G. Hardy & M. D. Guiry (2006). A Check-list and Atlas of the Seaweeds of Britain and Ireland. British Phycological Society, London. ISBN 3-906166-35-X. 
  18. ^ H. Stegenga, I. Mol, W. F. Prud'homme van Reine & G. M. Lokhorst (1997). "Checklist of the marine algae of the Netherlands". Gorteria. supplement 4: 3–57. 
  19. ^ A. W. Miller, A. L. Chang, N. Cosentino-Manning & G. M. Ruiz (2004). "A new record and eradication of the north Atlantic alga Ascophyllum nodosum (Phaeophyceae) from San Francisco Bay, California, USA". Journal of Phycology 40 (6): 1028–1031. doi:10.1111/j.1529-8817.2004.04081.x. 
  20. ^
  21. ^ J. Norrie & D. A. Hiltz (1999). "Seaweed Extract Research and Applications in Agriculture". Agro food Industry hi-tech. 
  22. ^ L. G. Lewis, N. F. Stanley & G. G. Guist (1988). "Commercial production and applications of algal hydrocolloides". In C. A. Lembi & J. R. Waaland. Algae and Human Affairs. Cambridge University Press, Cambridge. ISBN 0-521-32115-8. 
  23. ^ M. D. Guiry & D. J. Garbary (1991). "Geographical and Taxonomic guide to European Seaweeds of Economic Importance". In M. D. Guiry & Blunden. Seaweed Resources in Europe: Uses and Potential. John Wiley & Sons, England. ISBN 0-471-92947-6. 
  24. ^ a b c Chang A. L., Blakeslee A. M. H., Miller A. W. & Ruiz G. M. (2011). "Establishment Failure in Biological Invasions: A Case History of Littorina littorea in California, USA". PLoS ONE 6(1): e16035. doi:10.1371/journal.pone.0016035.
  25. ^ Seip,K.L. 1979. A mathematical model for the uptake of heavy metals in benthic algae. Ecological modelling 6: 183-197
  26. ^ Melhus, A.,KL Seip, HM Seip and S. Myklestad. 1978. A preliminary study of the use of benthic algae as biological indicators of heavy metal pollution in Sørfjorden, Norway. Environ. pollut (15):101107
  27. ^ Effects of phlorotannins from Ascophyllum nodosum (brown seaweed) on in vitro ruminal digestion of mixed forage or barley grain. Y. Wang, Z. Xu, S.J. Bach and T.A. McAllister, Animal Feed Science and Technology, 14 August 2008, Volume 145, Issues 1–4, Pages 375–395, doi:10.1016/j.anifeedsci.2007.03.013
  28. ^ T. Trott & P.F. Larsen (2008) Evaluation of short-term changes in rockweed (Ascophyllum nodosum) and associated epifaunal communities following cutter rake harvesting in Maine. Maine Department of Marine Resources Retrieved 2011-07-13
  29. ^ "Six plans approved for Cobscook Bay rockweed harvest". The Quoddy Tides. March 25, 2011. Retrieved 2011-03-23. 
  30. ^ P. Larsen (2010). "The Macroinvertebrate Fauna of Rockweed (Ascophyllum nodosum)–Dominated Low-Energy Rocky Shores of the Northern Gulf of Maine". Journal of Coastal Research. doi:10.2112/JCOASTRES-D-10-00004.1. 
  31. ^ "Gulf of Maine Rockweed Conference Notes". Retrieved 2011-03-23. 
  32. ^ B. M. Blinnabd, A. W. Diamonda, D. J. Hamiltonac (2008). "Factors Affecting Selection of Brood-rearing Habitat by Common Eiders (Somateria mollissima) in the Bay of Fundy, New Brunswick, Canada". Waterbirds. 31(4): 520–529. doi:10.1675/1524-4695-31.4.520. 
  33. ^ L. Tudor (2000). Migratory Shorebird Assessment. Maine Department of Inland Fisheries and Wildlife. 
  34. ^ "Life Within the Rockweed". The Quoddy Tides. September 22, 2000. Retrieved 2011-03-24. 
  35. ^ "Rockweed harvest in question this year". Bangor Daily News. June 29, 2010. Retrieved 2011-03-24. 
  36. ^ "The Rockweed Coalition, Maps". Retrieved 2011-03-24. 
  37. ^ Acadian Seaplants Limited. "2011 Rockweed Harvest Plan for Cobscook Bay, Maine". Retrieved 2011-03-23. 
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