Alaria esculenta on exposed sublittoral fringe bedrock
Image Roger Mitchell - Exposed sublittoral fringe bedrock with Alaria esculenta, Isles of Scilly. Image width ca 1 m.
Image copyright information
Ecological and functional relationships
Alaria esculenta is able to out compete other Laminarians in wave exposed sublittoral fringe due to its rapid growth rate and ability to withstand wave exposed conditions. It is particularly successful in exposed areas due to its flexible stipe and narrow, streamlined, blade.
Grazers are relatively rare in the sublittoral fringe and canopy interactions may be the most important structuring agency (Hawkins & Hartnoll 1985). The sweeping action of Laminaria digitata, and presumably Alaria esculenta, in wave exposed conditions prevents colonization by ephemeral algae by abrasion (Hawkins & Hartnoll 1985).
The understory is dominated by encrusting corallines and Corallina officinalis turf. In wave exposed sites the erect coralline turf grows very compactly reducing the interstitial space (exacerbated by presence of small Mytilids) and affecting the interstitial fauna. Dommanses (1968) showed that the coralline turf fauna varied with wave exposure. Wave exposed sites were dominated by amphipods and short legged isopods capable of grasping the fronds firmly. Interstitial fauna consists of grazers and suspension feeders (Dommaneses 1968) that probably remove ephemeral algae and epiphytes from Corallina officinalis.
On Rockall, the extreme wave exposure results in Alaria esculenta forest dominating both the sublittoral and infralittoral zone in the deeper areas. In the Rockall Alaria forest the holdfasts of Alaria esculenta become covered in a thick layer of encrusting coralline algae and, after the death of the alga, the holdfast rots leaving a space under the coralline crust that is a habitat for several mobile species that would most likely not survive on the open rock in such a wave exposed situation (K. Hiscock, pers. comm.).
Barnacle species are suspension feeders, that probably take algal spores and larvae that would otherwise settle within the community. Birkett et al. (1998b) point out that active suspension feeding by Mytilus edulis probably removes large numbers of settling algal spores and also compete for space. However in the wave exposed sublittoral fringe, the small size of individual mytilids suggests that larger individuals are removed by wave action.
Patellids are active grazers, however, they lack the necessary enzymes to digest Laminarian tissue (Birkett et al. 1998b) and probably do not graze Alaria esculenta extensively. However, they probably graze other algae and are important in keeping coralline turf and encrusting corallines free of ephemeral algae. Grazing activity of patellids probably remove Alaria esculenta and other kelp germlings as the limpets pass over them; a bulldozing effect (T. Hill pers. comm.).
Helcion pellucidum may graze Alaria esculenta but prefers Laminaria digitata when present.
Chitons were reported to be an important controlling agency in Alaria sp. populations allowing a otherwise weaker competitor to dominate in the north east Pacific (Paine 1980).
Seasonal and longer term change
is quite ephemeral in nature and will settle on bare surfaces, including mobile boulders and in deeper water than the infralittoral fringe. Alaria esculenta
releases spores between November and March with new sporophytes appearing in early spring (Birkett et al.
Growth of Alaria esculenta
slows in June and July and the lamina becomes eroded and torn in the following months. In extremely wave exposed conditions, especially in winter months, the blade may be reduced to just the midrib. Similarly, in wave exposed areas Laminaria digitata
is stunted and may be removed by storms and appears tattered in the winter months.
Habitat structure and complexity
dominates the sublittoral fringe in areas exposed to severe wave action or where water surges along the sides of gullies (Lewis, 1964). Such wave exposed shores are also characterized by pink encrusting and erect corallines, including Corallina officinalis
. This biotope (EIR.Ala) can be divided into two sub-biotopes depending on the degree of wave exposure EIR.Ala.Myt and EIR.Ala.Ldig. Alaria esculenta
support few epizoics (e.g. Audouinella alariae
on older specimens) and holdfasts yield few species (Lewis 1964; Birkett et al.
In areas of extreme wave exposure, on steeply sloping rock, exposed to oceanic swell EIR.Ala may extend up to 3 m above and below chart datum (Hiscock 1983).
As the wave exposure decreases the width of the Alaria esculenta
zone narrows and Laminaria digitata
is able to invade the sublittoral fringe resulting in a mixed macroalgal stand and increased species richness, characteristic of EIR.Ala.Ldig.
EIR.Ala.Ldig usually occurs above Laminaria hyperborea
forest although a narrow zone of Laminaria digitata
(MIR.Ldig) may intervene. EIR.Ala.Ldig biotopes probably represents and intermediate community between very wave exposed shores dominated by EIR.Ala.Myt and more wave sheltered shores dominated by MIR.Ldig.
On the extremely wave exposed steep and vertical rock faces of Rockall Alaria esculenta
replaces Laminaria hyperborea
as the dominant kelp and extends to down to 35m in depth. Above 13m the biotope resembles EIR.Ala.Myt. However, from 14 - 35 m the understory is covered by a dense turf of anemones and encrusting sponges. Cryptopleura ramosa
dominates horizontal surfaces. Between 30 -35m the Alaria esculenta
thins and the rock surface bears a dense red algal turf.
Kelp biotopes are highly productive and contribute up to 90 percent of their primary productivity to the detrital food webs of coastal areas in the form of drift algae, particulate and dissolved organic matter. For example primary productivity of Laminaria hyperborea
kelp beds were estimated at 1225 gC/sq. meter/year (Mann 1982 cited in Raffaelli & Hawkins 1999). However, no estimates were found for Alaria esculenta
biotopes, though figures are likely to be similar for all kelps (Tim Hill pers. comm.).
Laminarians, such as Alaria esculenta
, exhibit alternation of generations. The sporophytes releases meiotic haploid spores from sori located on sporophylls (born at the top of the short stipe) between November and March. Laminarian spores need to settle at high density so that the resultant gametophytes are close enough to cross fertilise. Laminarians produce vast numbers of spores and are expected to disperse over considerable distances. However, Sundene (1962) noted that germlings colonized within 10 m of adults in a Norwegian fjord, and Norton (1992) suggested that Alaria esculenta
would exhibit less dispersal than other kelp species due to the basal location of the sporophylls. Experiments on algal re-colonization in kelp beds (Kain 1975; Hawkins & Harkin 1985; Hill 1993) show that Alaria esculenta
is an opportunistic colonizing species, appearing early in the algal succession (c. 3 months after clearance of dominant algae; especially in areas cleared in the winter months) before being out competed by other kelp species (on moderately exposed shores). Corallina officinalis
produces spores over a protracted period and can colonize artificial substratum within one week in the intertidal (Harkin & Lindbergh 1977; Littler & Kauker 1984). The crustose base enables Corallina officinalis
to survive extreme exposure and damage (loss of fronds), and to take advantage (colonize) of space left after winter storms have removed competing macroalgae (Littler & Kauker 1984).
The mobile interstitial fauna of the coralline turf is reduced by trampling (Brown & Taylor 1989) but is likely to recruit to or recolonize the turf from the surrounding communities. Encrusting and erect corallines are also known to stimulate the settlement of a variety of marine invertebrate larvae and algal spores (e.g. Helcion pellucidum
Time for community to reach maturity
is an opportunistic and rapidly colonizing species (see above) capable of growing 20 cm/month in optimal conditions. In canopy removal experiments in the Isle of Man, Hawkins & Harkin (1985) found that areas cleared of Laminaria digitata
(moderately exposed) Alaria esculenta
became the dominant canopy algae within 9 months (October - June). Corallina officinalis
is capable of colonizing new substratum rapidly. In experimental plots 15 percent cover of fronds returned within 3 months (Littler & Kauker 1985) and Brown & Taylor (1999) noted that the articulated coralline algal turf community on a New Zealand shore returned to normal levels within 3 months of trampling events, although they suggested that a return to its previous cover may take longer.
Little information on the sublittoral fringe communities was available. Therefore the material presented in based on the general ecology of the key and important characterizing species.
This review can be cited as follows:
Alaria esculenta on exposed sublittoral fringe bedrock.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=165&code=1997>