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 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 F. 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,. It may take approximately five years before becoming fertile.
Polysiphonia lanosa (L.) Tandy 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 "P. lanosa" 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 eradicated as a potential invasive species there. 
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, (eg. N, P, K, Ca, Mg, S) and micronutrients (eg. 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.
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.
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