Biodiversity & Conservation

Ascophyllum nodosum ecad mackaii beds on extremely sheltered mid eulittoral mixed substrata



Image Sue Scott - A dense mat of Ascophyllum nodosum mackaii. Image width ca 40 cm.
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Distribution map

LR.SLR.FX.AscX.mac recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats
  • UK_BAP

Ecological and functional relationships

An ecad is a distinctive form of a species which develops in response to environmental conditions rather than genotypic differences. Ascophyllum nodosum ecad mackaii arises when detached fragments of the species are deposited onto sheltered shores where they continue to multiply and branch independently of the original fragment (Chock & Mathieson, 1976). The frond has extensive dichotomous branching and bears few air bladders. The plants drift in large, spherical masses in sheltered waters.

Extensive beds sometimes develop in appropriate conditions. However, more often beds are very local, often only a few metres across, and typically in small bays between rock outcrops (Anonymous, 1999(t)).

The presence of the Ascophyllum nodosum ecad mackaii 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. Very sheltered sea loch shores where freshwater runs or seeps across the shore can provide suitable conditions. The freshwater forms a brackish layer at the loch surface over the saline waters beneath, which moves up and down with the tides and subjects the shores to regularly fluctuating salinities. Therefore, 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

substratum type, the porosity of which affects the conditions of salinity also influences, to some extent, the development of the ecad.

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.

The loose mats of Ascophyllum nodosum ecad mackaii provide a sheltered and humid habitat for many mobile mid-shore animals which would otherwise be unable to live on open sediments or shingle. Gammarid amphipods, shore crabs and littorinid snails hide and feed amongst the weed, while barnacles and mussels are often attached to stones beneath. Fish such as young common eels Anguilla anguilla and viviparous blennies Zoarces viviparus may also shelter in the weed.

Seasonal and longer term change

In Strangford Lough in Northern Ireland, Stengel & Dring (1997) observed the growth of the attached form of Ascophyllum nodosum to be highly seasonal with low growth rates during November and December, and highest growth rates in late spring and early summer. A decline in growth in mid-summer was observed at all shore levels. Chock & Mathieson (1979) demonstrated the physiological responses of Ascophyllum nodosum and its ecad scorpioides were similar under varying conditions of light intensity and temperature. Therefore, it seems likely that growth rate patterns of the mackaii ecad will be similar to the attached plant under the same conditions and the ecad will show clear seasonal changes in growth rate. Gibb (1957) found growth of the ecad to be very slow during the winter. There was no other information found on seasonal or temporal changes in the biotope.

Habitat structure and complexity

The unattached mackaii ecad is generally found on flat, undulating or slightly sloping shores of mud, muddy sand or small stones with high stability so there is low habitat complexity. However, the presence of plants of the free living ecad itself add complexity to the habitat because plant masses provide shelter for other species that would otherwise be unable to live on open sediments or shingle.


Most productivity in the biotope will be primary due to the photosynthetic activity of Ascophyllum nodosum. However, there will also be secondary productivity in the biotope. For example, macroalgae exude considerable amounts of dissolved organic carbon which are taken up readily by bacteria and may even be taken up directly by some larger invertebrates. Detrital input may also be important for the deposit and suspension feeders, such as Arenicola marina and Lanice conchilega, which may also be present in the biotope.

Recruitment processes

Formation of the mackaii ecad of Ascophyllum nodosum is dependant on a supply of fragments from the attached form of the species. Once formed, the ecad can proliferate itself vegetatively from its own broken fragments which continue to divide forming new plants. Ascophyllum nodosum is recruited from pelagic sporelings, but recruitment is generally poor with few germlings found on the shore so recruitment of the ecad will also probably be poor. A supply of pelagic larvae is likely to be important in the recruitment of many of the macrofaunal species in the biotope. Some species, such as amphipods, have direct development and so recruitment will be mainly from local populations although some 'rafting in' of individuals does occur.

Time for community to reach maturity

Ascophyllum nodosum is a slow growing species. If growth rate of the unattached mackaii ecad is also low then the time for the community to reach maturity is likely to be several years.

Additional information

This review can be cited as follows:

Hill, J.M. 2001. Ascophyllum nodosum ecad mackaii beds on extremely sheltered mid eulittoral mixed substrata. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 19/04/2014]. Available from: <>