Biodiversity & Conservation

This biotope is predominantly of algae which dominate the rock surface and canopy. Macroalgae provide habitats for many species of invertebrates and fish and also provide shade under their canopy.

Rock type and sand scour effects are of critical importance to the development of this biotope. Sand-binding algal species are able to colonize soft or crumbly rock more successfully than fucoids (Lewis, 1964). Where sand scour is severe, fucoids and Rhodothamniella floridula tend to be absent while ephemeral green algae dominate the substratum and a different biotope will be present (Connor et al., 1997b).

No information was found specifically on this biotope. However, some general observations from rocky shore communities are relevant.

• Ephemeral green algae may show a peak in abundance during the spring.
• Winter storms will reduce or damage fucoids and macroalgal cover.
• Crab and fish tend to move to deeper water in the winter months, so that predation is probably reduced.
• Corallina officinalis may be overgrown by epiphytes, especially during summer. This overgrowth regularly leads to high mortality of fronds due to light reduction (Wiedemann pers comm. to Tyler-Walters, 2000).
• At least in northern Britain, Littorina littorea migrates down shore as temperatures fall in autumn (to reduce exposure to sub-zero temperatures) and up shore as temperatures rise in spring; migration depends on local winter temperatures.
• The upper limit of distribution of Patella vulgata on a shore is increased by shade and exposure. In some situations seasonal variations in sunshine causes a downward migration in spring/summer and an upward migration in autumn/winter (Lewis, 1954).

Bedrock and boulders form the substratum in this biotope; the pits, crevices and inclination of which create microhabitats exploitable by both mobile and sessile epilithic species. In addition, the macroalgal species of the community add considerable structural complexity to the biotope in the form of additional substratum for settlement by epiphytic species. The sand scour tolerant species, Rhodothamniella floridula, enhances the structural complexity by binding sand within a mat over the rocky substratum into which polychaetes and amphipods can burrow. There is likely to be considerable structural heterogeneity over a small scale within the biotope. For instance, although barnacles may form a dense layer over the substratum that largely excludes other species, the gaps created by dead barnacles may be exploited by small invertebrates.

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 is 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 larva 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 in the gaps often 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.

• Community structure and dynamics are strongly influenced by larval supply. Annual variation in recruitment success, of algae, limpets and barnacles can have a significant impact on the patchiness of the shore.

• The propagules of most macroalgae tend to settle near the parent plant (Schiel & Foster, 1986; Norton, 1992; Holt et al., 1997). For example, red algal spores and gametes are immotile and the propagules of Fucales are large and sink readily. Norton (1992) noted that algal spore dispersal is probably determined by currents and turbulent deposition (zygotes or spores being thrown against the substratum). For example, spores of Ulva spp. have been reported to travel 35km. The reach of the furthest propagule and useful dispersal range are not the same thing and recruitment usually occurs on a local scale, typically within 10m of the parent plant (Norton, 1992). Vadas et al. (1992) noted that post-settlement mortality of algal propagules and early germlings was high, primarily due to grazing, canopy and turf effects, water movement and desiccation (in the intertidal) and concluded that algal recruitment was highly variable and sporadic. However, macroalgae are highly fecund and widespread in the coastal zone so that recruitment may be still be rapid, especially in the rapid growing ephemeral species such as Ulva spp. and Ulva lactuca, which reproduce throughout the year with a peak in summer. Similarly, Ceramium species produce reproductive propagules throughout the year (Dixon & Irvine, 1977; Burrows, 1991; Maggs & Hommersand, 1993).
• Gastropods exhibit a variety of reproductive life cycles. The common limpet Patella vulgata and the periwinkle Littorina littorea have pelagic larvae with a high dispersal potential, although recruitment and settlement is probably variable.
• Barnacles such as Semibalanus balanoides have a planktonic nauplius larva, which spends about 2 months in the plankton, with high dispersal potential. Peak settlement in Semibalanus balanoides occurs in April-May in the west and May-June in the east and north of the British Isles, However, settlement intensity is variable, subsequent recruitment is inhibited by the sweeping action of macroalgal canopies (e.g. fucoids) or the bulldozing of limpets and other gastropods (see MarLIN review for details).
• Many species of mobile epifauna, such as polychaetes that may be associated with patches of mussels or rock crevices, have long lived pelagic larvae and/or are highly motile as adults.

The MLR.Rho biotope consists mainly of algal species, with high spore production and dispersal potential, enabling rapid colonization and recolonization. The development of the community from bare or denuded rock is likely to be similar to that occurring after an oil spill. Recovery of rocky shore populations was intensively studied after the Torrey Canyon oil spill in March 1967. Areas affected by oil alone recovered rapidly, within 3 years. If rocks or boulders are present with sand in suspension, it is likely that recovery of the MLR.Rho biotope would take approximately the same amount of time.

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