Northern Ireland's Priority Species

Ascophyllum nodosum var mackayi – free-floating knotted wrack

Ascophyllum nodosum var mackayi

Ascophyllum nodosum var mackayi (Turner)
Family: Fucaceae

Ascophyllum nodosum ecad mackayi is an olive-brown seaweed. It is an ecotype that is an ecological form of the species Ascophyllum nodosum , adapted for living in sheltered waters. It is an ecological form rather than a genetic form. The epithet "mackaii" is here corrected to "mackayi". It is one of several forma (ecads) of Ascophyllum nodosum.

In brief

  • In Northern Ireland it is found only in certain areas in Strangford Lough

  • Prefers only land-locked bays with very sheltered muddy shores

  • Can be found anytime

  • Listed as a UK Priority Species

  • It is a rare ecad of the species

  • The main threat is any change to its habitat, for example, any alteration to the shoreline or drainage from agricultural land and use of pesticides.

Species description
The seaweed Ascophyllum nodosum ecad mackayi is olive-brown in colour with dichotomous branching. It is free-living, arising from detached fragments in sheltered shores and growing to form detached ‘cushions’ up to about 50cm in diameter. The fronds may, or may not, show air bladders.

Life cycle
The plants are usually sterile and direct reproduction is vegetative; fragments become detached on sheltered shores and grow unattached.

Similar species
Other ecads of Ascophyllum, such as A. nodosum ecad scorpioides are also loose-lying, are similar, but smaller, being up to about 30cm and without vesicles or air-bladders.

How to see this species
It is found in parts of Strangford Lough. It is usually subject to variable salinity due to freshwater run-off forming a brackish layer on a lough surface. The plants drift in spherical masses in very sheltered waters and form an ankle-deep carpet, covering shingle and mud. The species can be found at any time of the year. Relevant access permissions should always be sought prior to visiting any sites.

Current status
In Northern Ireland it is found only in certain areas in Strangford Lough. It has also been recorded from Counties Donegal and Galway and the west coast of Scotland.

Why is this species a priority in Northern Ireland?

  • Listed as a UK Priority Species

  • It is generally rare and only found in specific sites.

Threats/Causes of decline
Any alteration to the shoreline or drainage from agricultural land and use of pesticides will be of concern and will endanger the plants.

Conservation of this species

Current action
There is a UK Species Action Plan which was published in 1999.

  • The site for this species, Strangford Lough, is designated as a Ramsar site, SPA, SAC, Marine Nature Reserve and ASSI.

Proposed objectives/actions

  • Maintain the number of viable populations of the species.

What you can do
Report any records to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU Tel. 028 9039 5251, [at]

Further information



Gibb, D.C. (1957). The free-living forms of Ascophyllum nodosum (L.) Le Jol. J. Ecol. 45: 49–83.

Hardy, F.G. and Guiry, M.D. (2003). A Checklist and Atlas of the Seaweeds of Britain and Ireland. British Phycological Society, London.

Lewis, J.R. (1964). The Ecology of Rocky Shores. London.

Lynn, M.J. (1935). Rare algae from Strangford Lough. – part 1. Irish Nat. 5: 201–208.

Morton, O. (1994). Marine Algae of Northern Ireland. Ulster Museum.

Morton, O. (2003). The Marine Macroalgae of County Donegal, Ireland. Bull. Ir. Biogeog. Soc. 27: 3 -164.

Moss, B. (1971). Meristems and morphogenesis in Ascophyllum nodosum ecad machaii (Cotton). Br. Phycol. J. 6: 187–193.

Text written by:
Osborne Morton