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 and the north-eastern coast of North America.
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.
Varieties and forms
Several different varieties and forms of this species have been described.
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). It may take approximately five years before becoming fertile.
Vertebrata lanosa (L.) T.A. Christensen is a small red alga, commonly found growing in dense tufts on Ascophyllum whose rhizoids penetrate the host. 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, Norway, Ireland, Britain and Isle of Man, Netherlands, North America: Bay of Fundy, Nova Scotia, Baffin Island, Hudson Strait, Labrador and Newfoundland. It has been recorded as an accidental introduction to San Francisco, California, and as a potentially invasive species eradicated.
Ascophyllum nodosum is harvested for use in alginates, fertilisers and for the manufacture of seaweed meal for animal and human consumption. 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. Ireland, Scotland and Norway have provided the world's principal alginate supply.
Ascophyllum nodosum is frequently used as packaging material for baitworm and lobster shipments from New England to various domestic and international locations. 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.
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.
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.  Opponents of wild Ascophyllum harvests point to the algae's high habitat value for over 100 marine species, including benthic invertebrates,  commercially important fish, wild ducks, shorebirds, and seabirds. 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. Rockweed harvesters point to the value of the seasonal jobs created by the harvest operation.
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