Biodiversity & Conservation

In the lower eulittoral zone, limpet grazing, which limits the upward extension of low-shore species in to the mid-eulittoral (e.g. Underwood, 1979; Underwood, 1980; Underwood & Jernakoff, 1981), becomes less effective and algal turfs of Himanthalia elongata, red algae and kelp species dominate the lower shore. Physical conditions in the lower eulittoral may be optimal for macroalgae. Several species of fucoids grow faster lower on the shore than higher on the shore (Schonbeck & Norton, 1980). Rapid growth to attain a size greater than 5 cm, grants immunity from limpet grazing and enhances survival (see Hawkins & Hartnoll, 1983 for review).
Brown algal canopy:Fucus serratus cannot withstand too much turbulence created by wave action, whilst Fucus serratus persists in the biotope, under wave exposed conditions the biotope is dominated by Himanthalia elongata. The cover provided by the 'straps' of Himanthalia elongata varies throughout the year (see recruitment processes) but the canopy it provides probably enhances the survival of an undergrowth of red algae by providing protection from excessive insolation, desiccation or by inhibiting the growth of potential rivals.

Himanthalia elongata supports a considerable epifauna and epiflora (see epiphytic species below) described by Kitching (1987).

Red algal turf and coralline algal crust: Red algae represent a major source of food for a variety of herbivores, including molluscs, crustaceans and sea urchins. Grazers may truncate the vertical distribution of some red algae or eliminate all but the most tolerant grazer-resistant species, but Chondrus crispus, Osmundea pinnatifida, and many calcareous crustose species in the biotope seem in some circumstances to be reliant upon browsing fauna to remove epiphytes or other competitors that might otherwise smother the plant (Kain & Norton, 1990).

Species such as Lomentaria articulata, Plumaria elegans and Cladophora rupestris prefer the shade of rocky overhangs or a screen of larger fucoids, and may become scarce if shaded surfaces are lacking. Osmundea pinnatifida can become more abundant further upshore in the eulittoral zone as long as it is shaded. Corallina officinalis, however, can rise further upshore in open sunlight than in shade provided that it is in damp depressions or runnels, indicating that strong sunlight does not harm it when desiccation is minimal (Lewis, 1964).

Epiphytic species:To obtain a better position in the 'struggle for light', smaller, non-parasitic algal species may grow as epiphytic algae on larger plants, and in most instances are only superficially attached to their host (Lüning, 1990). Epiphytic algae decrease the growth rate of their host, increase the probability of breakage, and may reduce reproductive output (d' Antonio, 1985).

On exposed shores the 'straps' of Himanthalia elongata have few epiphytes, however in slightly more sheltered locations they may develop a 'microforest' of filamentous algae, which provides a habitat for a variety of faunal epiphytes. Kitching (1987) recorded 105 species of flora and fauna on Himanthalia elongata plants in Lough Hyne. The upper surface of the buttons were very resistant to colonization by epiphytes while the underside of the buttons were usually colonized by Bryozoa and spirorbid worms. The straps were frequently covered in a microforest of filamentous algae, such as Ectocarpus siliculosus, Ceramium pedicellatum and Ulva prolifera. Fauna such as the gastropods Rissoa pavida, juvenile Helcion pellucidum and amphipods, were often associated with the filamentous epiphytes. The cushion-like brown algae Elachista scutulata occurs exclusively on Himanthalia elongata (Lüning, 1990). As a protection against epiphytes, young actively growing plants of Himanthalia elongata regularly shed off the outer layer of their outer-most cell wall (Filion-Myklebust & Norton, 1981). However, Elachista scutulata by-passes the cell shedding mechanism if its host by growing in cryptostomata (invaginations with hairs) and other natural breaks in the host thallus surface, where cells are not removed (Russell & Veltkamp, 1984).

Cell-shedding has also been detected in crustose coralline algae that grow beneath the brown algal canopy. In this instance, however, it is a case of ridding their surfaces of algal gametophytes (Masaki et al., 1981).

Where competition for space and light restricts the occurrence of Palmaria palmata on rock, the species often has an epiphytic habit on other algae, especially kelps.

Sessile and mobile epilithic species:Species such as the barnacles, Semibalanus balanoides and Chthamalus spp., limpet, Patella vulgata, gastropod molluscs, Littorina neglecta and Nucella lapillus, and the mussel, Mytilus edulis, are dominant species within the biotope, although as algal cover increases, so does competition for space.

Barnacles remain most numerous in the most wave exposed situations in the biotope, especially on vertical surfaces, steep slopes facing seawards and mounds rising above seaweed covered flats as recumbent fronds of macroalgae and red algal turf make flat surfaces less hospitable for barnacles. Algal competition for space in the lower eulittoral not only curtails the distribution of Semibalanus balanoides and Chthamalus spp. but also suppresses the penetration of Balanus perforatus up the shore (Lewis, 1964).

Density and size of Patella vulgata in the lower eulittoral is highly variable, and although a common species in the ELR.Him biotope, numbers in macroalgae dominated biotopes are usually less than on barnacle dominated biotopes but individuals can achieve a greater size (Lewis, 1964).

Nucella lapillus is an important intertidal predator and preys mainly on barnacles and mussels but may also prey on cockles, other bivalves and gastropods. Crothers (1985) suggested a mean annual consumption of 15-20 mussels per dog whelk (Largen, 1967a; Bayne & Scullard, 1978) and reported rates of 0.5 or 0.59 mussels/day or 1.1 Semibalanus balanoides /day in summer (Connell, 1961; Fretter & Graham, 1962; Anala, 1974). Dog whelks avoid dense mussel beds, preferring the diffuse margins between the mussel bed and the surrounding barnacle dominated substratum, or solitary mussels (Petraitis, 1987; Fretter & Graham, 1994; Davenport et al., 1996). This was partly because mussels can immobilise gastropods (Nucella lapillus and Littorina spp.) by entangling the gastropods with their byssus threads.

Many other animals are found in the biotope, but do not make a distinct contribution to the appearance of the shore. However, a species that may be apparent is the anemone Actinia equina that benefits from the damp conditions of pits and crevices amongst algae. Numerous small and specialised habitats exist within the biotope. On the algae or on damp rock beneath gammarid amphipods, the isopod Idotea, small hydroids and bryozoans (Dynamena pumila and Flustrellida hispida especially) and other small gastropod molluscs may be found.

Species within the biotope demonstrate seasonality in terms of growth and reproduction. For instance, germlings of Himanthalia elongata become visible on the shore in early March and form buttons with an average size of 10-25 mm by August. Those buttons which grow to 15 mm by November produce receptacles that autumn. The receptacles grow little in length during autumn and winter but increase rapidly between February and May and may reach 2 m in length. Chondrus crispus is a perennial species whose holdfasts may persist on the substratum for several years but whose growth is annual, following die-back. It is fertile throughout the winter and can therefore take advantage of surfaces that become available for colonization, after other species have died back (Kain, 1975). Storms and increased wave action are more likely to occur in the winter months and may cause physical damage to the community. Dudgeon & Johnson (1992) noted wave induced disturbance of intertidal Chondrus crispus on shores of the Gulf of Maine, USA, during winter. 25-30 % of cover of large Chondrus crispus thalli was lost in one winter. Physical disruption of the algal turf is likely to promote diversity as spaces become available for colonization.

Bedrock forms the substratum of the biotope, the pits, crevices and inclination of which create microhabitats exploitable by both mobile and sessile epilithic species. In addition the algal species of the community add considerable structural complexity to the biotope in the form of additional substratum for settlement by epiphytic species. The straps of Himanthalia elongata form a canopy over the substratum, the cover of which varies throughout the year according to the stage of growth. The straps of Himanthalia elongata grow from February onwards so that by the summer they shade the understorey vegetation and create a local microclimate while the tide is out.

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). 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. Only about 10 % of the primary production is directly cropped by herbivores (Raffaelli & Hawkins, 1996). 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. For instance, the mean release per month from each square meter of a stand of Chondrus crispus was estimated to be 961 X 10⁶ carpospores and 204 X 10⁶ tetraspores (Bhattacharya, 1985). Measurements of the productivity of benthic algae are relatively few, particularly for the Rhodophyta (Dixon, 1973). Blinks (1955) estimated the net production of red algae to be in the order of 11 to 54 g dry weight per m² per day.

• Himanthalia elongata has a life history and growth pattern unique among the Fucales. The species invests 98 % of its total biomass in reproductive rather than vegetative tissue. It is usually has a biennial lifecycle, reproducing once and then dying. The reproductive bodies or receptacles take the form of long straps, which sprout from the centre of the button. Gametes are released from June until winter. Usually germlings become visible on the shore in early March and form buttons with an average size of 10-25 mm by August. Those buttons which grow to 15 mm by November produce receptacles that autumn. The receptacles grow little in length during autumn and winter but increase rapidly between February and May. From June onwards, adult plants release gametes on a low tide by liberating them into mucus, which dribbles onto the substratum below.
• All the spores of red algae are non flagellate and dispersal is wholly a passive process (Fletcher & Callow, 1992). Spores do however vary in their sinking rate as determined by size and density. In general, due to the difficulties of re-entering the benthic boundary layer, it is likely that successful colonization is achieved under conditions of limited dispersal and/or minimum water current activity. Norton (1992) reported that although spores may travel long distances (e.g. Ulva sp. 35 km, Phycodrys rubens 5 km), the reach of the furthest propagule does not equal useful dispersal range, and most successful recruitment probably occurs within 10 m of the parent plants. It is expected, therefore, that recruitment of, for example, Chondrus crispus, Palmaria palmata and the majority of other macroalgae in the biotope would occur from local populations and that establishment and recovery of isolated populations would be patchy and sporadic. Scrosati et al. (1994) commented that viability of spores of Chondrus crispus was low (<30 %) and suggested that reproduction by spores probably does not contribute much to maintenance of the intertidal population in Nova Scotia, compared to vegetative growth of gametophytes.
• Vadas et al. (1992) reviewed recruitment and mortality of early post settlement stages of benthic algae. They identified 6 intrinsic and 17 extrinsic factors affecting recruitment and mortality. They concluded that grazing, canopy and turf effects were the most important but that desiccation and water movement may be as important for the early stages. The review indicated that recruitment is highly variable and episodic and that mortality of algae at this period is high. Chance events during the early post settlement stages are therefore likely to play a large part in survival.
• See full MarLIN reviews for specific information on reproduction and longevity: Chondrus crispus, Palmaria palmata, Corallina officinalis, Fucus serratus.
• Many animal species in the biotope, possess a planktonic larval stage which float in the plankton before settling and metamorphosing into adult form, for example; Mytilus edulis, Patella vulgata and Semibalanus balanoides. This strategy allows species to rapidly colonize new areas that become available such as in the gaps on otherwise algal dominated rocks 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. Nucella lapillus is oviparous, it lays its fertilized eggs directly on the substratum.

From the information available on recruitment of important characterizing species, it is likely that the community would reach maturity within five years.

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