Ascophyllum nodosum on very sheltered mid eulittoral rock.
Ecological and functional relationships
While physical factors, especially the degree of exposure to wave action and amount of emersion, clearly influence the distribution and abundance of species on rocky shores, biological factors are especially described here.
Ascophyllum nodosum is of great ecological importance in the North Atlantic because of its high abundance on most sheltered rocky shores, where it must be a major contributor to the oxygen budget of shallow waters to a wide range of intertidal animals (Stengel & Dring, 1997). The species, and other macroalgae, increase the amount of space available for attachment, provide shelter from wave action, desiccation and heat, and may be an important food source.
Sheltered conditions favour the growth of fucoid algae and allow the maintenance of a more or less total and permanent canopy (Hartnoll & Hawkins, 1985). Communities on sheltered shores are much more stable than those of moderately exposed shores where a mosaic of patches of fucoid cover, dense barnacles and limpets are subject to small scale temporal variations. The biotope may also be present on sheltered areas of more exposed shores.
Patella are absent in New England and it is possible that this allows Ascophyllum nodosum to extend into more exposed conditions. The exclusion of fucoids from exposed shores, and hence the presence of dense beds of fucoids on sheltered shores, results from grazing pressure on exposed shores. A dynamic balance probably exists between fucoids and limpets plus barnacles, and is mediated by wave action. In sheltered conditions the balance shifts in favour of the fucoids and in exposure the balance shifts towards limpets, barnacles and mussels. Limpet grazing normally prevents fucoids from extending onto exposed headlands, but how fucoids maintain their dominance on sheltered shores and what stops barnacles and limpets extending into sheltered conditions is unknown (Raffaelli & Hawkins, 1996).
The presence of a dense fucoid canopy inhibits the settlement of barnacles by blocking larval recruitment mainly by 'sweeping' the rock of colonizers.
There is a close association between Ascophyllum nodosum and the snail Littorina obtusata. The snail grazes away some epiphytes, thereby reducing the hydrodynamic loadings on plants and decreasing the detachment rate during storms. Although the snail also consumes the algal thallus, this does not appear to affect the performance of the plant. Ascophyllum produces noxious secondary chemical (polyphenols) which deter most grazers, but which attract L. obtusata. It is one of the few grazers to actually consume the thallus tissue, others feed on epiphytes (Norton et al., 1990 cited in Raffaelli & Hawkins, 1996). Littorina littorea may be an important grazer of small fucoid plants and germlings and where it occurs in high abundance may delay colonization of fucoids (Lewis, 1964).
Grazing on rocky shores can exert significant controlling influences on the algal vegetation, particularly by patellid limpets and littorinid snails which are usually the most prominent grazers. There are probably also significant effects caused by 'mesograzers' - amphipods such as Hyale prevostii and isopods, which are much smaller but may occur in high densities.
Seasonal and longer term change
Sheltered conditions favour the growth of fucoid algae and allow the maintenance of a more or less total and permanent canopy (Hartnoll & Hawkins, 1985) so that communities on sheltered rocky shores tend to have a high level of stability both seasonally and in the longer term. Ascophyllum nodosum
has a very long life span where individual fronds can survive for 10-15 years and the holdfast for several decades which also contributes to the stability of the biotope. Other fucoid plants found in the biotope, such as Fucus vesiculosus
, have life spans in the order of 3-5 years. However, growth rates do show seasonal changes. For example, in Strangford Lough in Northern Ireland, Stengel & Dring (1997) observed the growth 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. Of the animal species present, Hyale prevostii
may increase in numbers during the reproductive period when juveniles are released from brood pouches of the females, whilst littorinids are unlikely to show significant seasonal change. Although present in small numbers in the biotope, barnacles are likely to show increased abundance after settlement in the spring.
Habitat structure and complexity
Fucoid shores provide a variety of habitats and refugia for other species. The dense beds of Ascophyllum nodosum
and the other fucoids in the biotope increases the structural complexity of the habitat providing a variety of resources that are not available on bare rock. Fronds provide space for attachment of encrusting or sessile epifauna and epiphytic algae and provide shelter from wave action, desiccation and heat for invertebrates. For example, the immediate effects of the removal of Ascophyllum
plants are to: destroy the epifauna and flora; increase desiccation; increase predation; increase erosion and aid the settlement of other species (Boaden & Dring, 1980). Crevices in the bedrock and overhangs on fucoid rocky shores also increase habitat complexity by providing refugia for a variety of species.
Rocky shore communities are highly productive and are an important source of food and nutrients for members of neighbouring terrestrial and marine ecosystems (Hill et al
., 1998). Production rates of Ascophyllum nodosum
in Nova Scotia were estimated to be between 0.61 and 2.82 kg/m² (Cousens, 1984). Only about 10% of the primary production is directly cropped by herbivores (Raffaelli & Hawkins, 1999). Macroalgae, such as Ascophyllum nodosum
and other fucoids, 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. Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea. This may enter the food chain of local, subtidal ecosystems, or be exported further offshore. Rocky shores make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains.
Many rocky shore species, plant and animal, possess a planktonic stage: gamete, spore or larvae which float in the plankton before settling and metamorphosing into adult form. This strategy allows species to rapidly colonize new areas that become available such as gaps created by storms. For these organisms it has long been evident that recruitment from the pelagic phase is important in governing the density of populations on the shore (Little & Kitching, 1996). Both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates.
- Ascophyllum nodosum is recruited from pelagic sporelings, but recruitment is generally poor and few germlings are found on the shore. The species is extremely fertile every year and Printz (1959) suggests it must be assumed that some special combination of climatic or environmental conditions is needed for an effective colonization.
- Recruitment of Patella vulgata fluctuates from year to year and from place to place. Fertilization is external and the larvae is pelagic for up to two weeks before settling on rock at a shell length of about 0.2mm. Winter breeding occurs only in southern England, in the north of Scotland it breeds in August and in north-east England in September.
- Barnacle recruitment can be very variable because it is dependent on a suite of environmental and biological factors, such as wind direction and success depends on settlement being followed by a period of favourable weather. Long term surveys have produced clear evidence of barnacle populations responding to climatic changes. During warm periods Chthamalus spp. predominate, whilst Semibalanus balanoides does better during colder spells (Hawkins et al., 1994). Release of Semibalanus balanoides larvae takes place between February and April with peak settlement between April and June. Release of larvae of Chthamalus montagui takes place later in the year, between May and August.
Some of the species living in the biotope do not have pelagic larvae, but instead have direct development of larvae producing their offspring as 'miniature adults'. For example, Nucella lapillus
and some winkles do this, as do all amphipods. Adult populations of these species are governed by conditions on the shore and will generally have a much smaller dispersal range.
Time for community to reach maturity
The time for an Ascophyllum nodosum
community to reach maturity is likely to be many years because the species has very poor recruitment and is very slow growing. The species does not reach sexual maturity until about 5 years of age and individual fronds can live to be up to 15 years old and whole plants for several decades. In their work on fucoid recolonization of cleared areas at Port Erin, Knight and Parke (1950) observed that even eight years after the original clearance there was still no sign of the establishment of an Ascophyllum nodosum
population. There is a long-recognised shortage of sporelings (David, 1943) and the failure of the species to recolonize denuded areas for decades. However, the species is extremely fertile every year and Printz (1959) suggests it must be assumed that some special combination of climatic or environmental conditions is needed for an effective recolonization. If plants are not removed completely Ascophyllum nodosum
plants cut within 10-15cm of the base recover fully in 4-5 years (Printz, 1959).
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This review can be cited as follows:
Ascophyllum nodosum on very sheltered mid eulittoral rock..
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=4&code=1997>