Biodiversity & Conservation

The coralline algae are the dominant species in this biotope. To a great extent the rockpool biotope is an upward extension of £ELR.Coff£, although the rockpool biotope has its own characteristics. Corallina officinalis and various lithothamnia are successful in the upper half of the eulittoral zone, especially in shallow, well-lit rockpools (Lewis, 1964). In this zone, some limitation on species develops and not all lower littoral species of the open rock surface can colonize upper shore in rockpools. For example, Fucus serratus can do so but Laurencia pinnatifida, Lomentaria articulata and Rhodymenia become much less plentiful, almost to the point of exclusion (Lewis, 1964).

Other filamentous and foliose red algae found in the pools include Dumontia contorta, Mastocarpus stellatus, Ceramium nodulosum and Chaetomorpha, Ectocarpus, Polysiphonia and Scytosiphon species. The green seaweeds Cladophora rupestris, Ulva spp. and Ulva lactuca can also occur in high abundance.

Seaweeds provide primary productivity either directly to grazing fish and invertebrates or indirectly, to detritivores and decomposers, in the form of detritus and drift algae or as dissolved organic material and other exudates.

The faunal communities of coralline turfs are described in detail by Hagerman (1968), Dommasnes (1968, 1969), Hicks (1985), Grahame & Hanna (1989), Crisp & Mwaiseje (1989), Bamber (1988) and Bamber & Irving (1993). (see ELR.Coff for details). Corallina officinalis provides substratum for spirorbid worms (e.g. Spirorbis corallinae), epiphytes, periphyton, microflora (e.g. bacteria, blue green algae, diatoms and juvenile larger algae), and interstices between the fronds provide refuges from predation for a variety of small invertebrates.

Amphipods (e.g. Parajassa pelagica and Stenothoe monoculoides), isopods (e.g. Idotea pelagica and Jaera albifrons) and other mesoherbivores graze the epiphytic flora and senescent macroalgal tissue, which may benefit the macroalgal host, and may facilitate dispersal of the propagules of some macroalgal species (Brawley, 1992b; Williams & Seed, 1992). Mesoherbivores may also graze the macroalgae but do not normally adversely affect the canopy (Brawley, 1992b). Grazing is likely to be advantageous to encrusting corallines owing to the removal of epiphytes.

Foliose seaweeds are grazed by large numbers of molluscs, especially the winkle, Littorina littorea, the limpet, Patella vulgata and top shell, Gibbula cineraria. Littorinids show definite preferences for particular algal foods. Littorina littorea tends to prefer the green algae such as Ulva to perennial red algae (Little & Kitching, 1996). Thin filamentous or membranous seaweeds, such as Ulva, Ceramium and Polysiphonia, are likely to more edible than tougher leathery forms. Some red seaweeds such as Corallina officinalis and coralline crusts (Lithothamnion, Lithophyllum) protect their thalli with a coating of calcium carbonate and are probably relatively grazing resistant (Littler & Kauker, 1984). Ephemeral algal species may be able to escape herbivory in time and space, owing to the fact that they are less predictable for herbivores, occurring at different times and in different places, usually as a result of disturbance (Raffaelli & Hawkins, 1999). The chiton, Lepidochitona cinerea probably grazes the corallines directly.

Grazers of periphyton (bacteria, blue-green algae and diatoms) or epiphytic algae include harpacticoid copepods, small gastropods (e.g. Rissoa spp. and Littorina neglecta.

Within the pools, pits and crevices are likely to be occupied by the beadlet anemone, Actinia equina and small mussels, Mytilus edulis. The food of anemones consists of a wide variety of crustaceans, molluscs, worms, other invertebrates and even fishes, caught using nematocysts borne on its tentacles.

The barnacle Semibalanus balanoides may be found over the rock surface. It and small mussels, are preyed upon by the whelk, Nucella lapillus.

As communities in rockpools remain constantly submerged and the danger of desiccation is absent, it might be expected that rockpools form an easier environment in which to live for marine life than drying rock surfaces, and that species from regions lower on the shore would be able to extend much further up the shore. However, much of the lower shore open rock fauna is absent from rockpools. Rockpools constitute a distinct environment for which physiological adaptations by the flora and fauna may be required (Lewis, 1964). Conditions within rockpools are the consequence of prolonged separation from the main body of the sea, and physico-chemical parameters within them fluctuate dramatically (Huggett & Griffiths, 1986). In general, larger and deep rockpools low on the shore tend to correspond to the sublittoral habitat with a more stable temperature and salinity regime. In contrast, small and shallow pools higher on the shore are especially influenced by insolation, air temperature and rainfall, the effects of which become more significant towards the high shore, where pools may be isolated from the sea for a number of days or weeks (Lewis, 1964).

• Weather conditions exert a considerable influence on temperature and salinity. Water temperature in pools follows the temperature of the air more closely than that of the sea. In summer, shallow pools or the surface waters of deeper pools are warmer by day, but may be colder at night, and in winter may be much colder than the sea (Pyefinch, 1943). In deeper pools, the vertical temperature gradation usually present in summer reverses during winter owing to density stratification, so that ice may form (Naylor & Slinn, 1958).
• High air temperatures cause surface evaporation of water from pools, so that salinity steadily increases, especially in pools not flooded by the tide for several days. Alternatively, high rainfall will reduce pool salinity or create a surface layer of brackish/nearly fresh water for a period. The extent of temperature and salinity change is affected by the frequency and time of day at which tidal inundation occurs. If high tide occurs in early morning and evening the diurnal temperature follows that of the air, whilst high water at midday suddenly returns the temperature to that of the sea (Pyefinch, 1943). Heavy rainfall, followed by tidal inundation can cause dramatic fluctuations in salinity, and values ranging from 5-30 psu have been recorded in rockpools over a period of 24 hrs (Ranade, 1957). Rockpools in the supralittoral, littoral fringe and upper eulittoral are liable to gradually changing salinities followed by days of fully marine or fluctuating salinity at times of spring tide (Lewis, 1964).
• Other physico-chemical parameters in rockpools demonstrate temporal change. The biological community directly affects oxygen concentration, carbon dioxide concentration and pH, and are themselves affected by changes in the chemical parameters. Throughout the day, algae photosynthesize and produce oxygen, the concentration of which may rise to three times its saturation value, so that bubbles are released. During photosynthesis algae absorb carbon dioxide and as concentrations fall, the pH rises. Morris & Taylor (1983) recorded pH values >9 in rockpools on the Isle of Cumbrae. At night changes occur in the opposite direction a respiration utilizes much of the available oxygen and pH decreases.

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.). The ephemeral green seaweeds Ulva intestinalis and Ulva lactuca are likely to be more abundant in summer. In summer, corallines may be bleached and loose their pink pigment but in some species, e.g. Phymatolithon, this does not necessarily result in death of the plant and pigment may be re-synthesized (Little & Kitching, 1996).

Bedrock forms the substratum of the biotope. Rockpools vary greatly in their physical features. Pools may be shallow and well-lit or deep and shaded with overhanging sides and vertical surfaces. Algae growing within provide additional surfaces for colonization and there is also a tendency for loose substrata (sand, stones, rocks) to accumulate in pools, the instability of which may affect species diversity. Within rockpools, crevices and pits may be found and exploited by species such the mussel Mytilus edulis and the beadlet anemone, Actinia equina, while the underside of stones and boulders support underboulder communities (see MLR.Fser.Fser.Bo for example).

Little information concerning the productivity of coralline turf communities was found. The red algae, algal epiphytes and periphyton provide primary productivity to grazers, while their spores and phytoplankton provide primary productivity to suspension feeders. For instance, spore production by the encrusting 'coralline' algae, Lithophyllum incrustans may be up to 18 million m²/yr (Edyvean & Ford, 1986).

Secondary productivity of the invertebrate fauna may be high and coralline turf may support high abundances of invertebrates. For example, Choat & Kingett (1982) recorded the following numbers of epiphytic fauna: amphipods 1038 / 0.01m²; ostracods 219 / 0.01m².

Recruitment processes of some of the characterizing species of the biotope are given below:

• Corallina officinalis has isomorphic sexual (gametophyte) and asexual (sporophyte) stages (see MarLIN review). Settled tetraspores develop into a perennial crustose base, from which the upright, articulate fronds develop. Sporelings formed within 48 hrs, a crustose base within 72 hrs, fronds being initiated after 3 weeks and the first intergeniculum (segment) formed within 13 weeks of settlement (Jones & Moorjani, 1973). Settlement and development of fronds is optimal on rough surfaces but settlement can occur on smooth surfaces (Harlin & Lindbergh, 1977; Wiedeman pers. comm.). Corallina officinalis settled on artificial substrata within 1 week of their placement in the intertidal in New England summer suggesting that recruitment is high (Harlin & Lindbergh, 1977).
• Besides having a meristem, Lithophyllum incrustans has its conceptacles (reproductive organs) buried in its calcified thallus, and connected to the exterior by canals (Edyvean & Ford, 1986). Reproductive types (gametangial and tetrasporangial plants) occur from October to April but decline into summer although some conceptacles are present throughout the year (Irvine & Chamberlain 1994). It has been calculated that 1 mm x 1mm of reproductive thallus produces 17.5 million bispores per year with average settlement of only 55 sporelings/year (Edyvean & Ford 1984).
• All the spores of red algae are non flagellate and dispersal is wholly a passive process (Fletcher & Callow, 1992). Spores 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 the reach of the furthest propagule does not equal its useful dispersal range, and most successful recruitment probably occurs within 10m of the parent plants. It is expected, therefore, that recruitment of foliose macroalgae in the biotope would occur from local populations and that establishment and recovery of isolated populations would be patchy and sporadic.
• Littorina littorea is an iteroparous breeder with high fecundity (up to 100,000 for a large female (27 mm shell height)) that lives for several (at least 4) years. Littorina littorea can breed throughout the year but the length and timing of the breeding period are extremely dependent on climatic conditions. Littorina littorea sheds egg capsules directly into the sea and release is synchronized with spring tides on several separate occasions. Larval settling time or pelagic phase can be up to six weeks (Fish, 1972).
• Recruitment of Patella vulgata fluctuates from year to year and from place to place (Bowman, 1981). Fertilization is external and the larvae are pelagic for up to two weeks before settling on rock at a shell length of about 0.2 mm. Winter breeding occurs only in southern England, in the north of Scotland it breeds in August and in north-east England in September.
• Gammarid amphipods brood their embryos and offspring but are highly mobile as adults and probably capable of colonizing new habitats from the surrounding area (e.g. see Hyale prevostii review). Similarly isopods such as Idotea species and Jaera species brood their young. Idotea species are mobile and active swimmers and probably capable of recruiting to new habitats from the surrounding area by adult migration. Jaera albifrons, however, is small and may take longer to move between habitats and Carvalho (1989) suggested that under normal circumstances movement was probably limited to an area of less than 2 m. Hicks (1985) noted that epiphytic harpacticoid copepods lack planktonic dispersive larval stages but are active swimmers, which is therefore the primary mechanism for dispersal and colonization of available habitats. Some species of harpacticoids are capable to moving between low and mid-water levels on the shore with the tide, while in others, colonization rates decrease with increasing distance from the resident population. Overall immigration and in situ reproduction were thought to maintain equilibrium populations exposed to local extinction, although there may be local spatial variation in abundance (see Hicks, 1985).

The epiphytic species diversity of the coralline turf in the rockpool is dependant on the Corallina officinalis cover and its growth form (Dommasnes, 1968, 1969; Seapy & Littler, 1982; Crisp & Mwaiseje, 1989). Corallina officinalis was shown to settle on artificial substances within 1 week of their placement in the intertidal in New England summer suggesting that recruitment is high (Harlin & Lindbergh, 1977). New fronds of Corallina officinalis appeared on sterilized plots within six months and 10% cover was reached with 12 months (Littler & Kauker, 1984). In experimental plots, up to 15% cover of Corallina officinalis fronds returned within 3 months after removal of fronds and all other epiflora/fauna (Littler & Kauker, 1984). Bamber & Irving (1993) reported that new plants grew back in scraped transects within 12 months, although the resistant crustose bases were probably not removed. New crustose bases may recruit and develop quickly but the formation of new fronds from these bases and recovery of original cover may take longer. Once a coralline turf has developed it will probably be colonized by epiphytic invertebrates such as harpacticoids, amphipods and isopods relatively quickly from the surrounding area. Therefore, the biotope would be recognizable once the coralline turf has regrown, which is likely to be quite rapid if the resistant crustose bases remain. Recruitment of red algae probably equally rapid, and once the algal turf has developed most of the epiphytic invertebrates would colonize quickly, although some species e.g. small brooding gastropods would take longer.

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