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.
Life history is of one diploid plant and gametes. The gametes are produced in conceptacles embedded in yellowish receptacles on short branches.
Varieties and formsEdit
Several different varieties and forms of this species have been described.
- Ascophyllum nodosum var. minor has been described from Larne Lough in Northern Ireland.
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.
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.
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. It may take approximately five years before becoming fertile.
Recorded in Europe from: Faroe Islands, Norway, Ireland, Britain and Isle of Man, Netherlands, North America: Bay of Fundy, Galicia, 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.