Biodiversity & Conservation

Fucoids and kelps in deep eulittoral rockpools

LR.LR.Rkp.FK


LR.FK

Image Jon Davies - Fucoids and kelps in deep eulittoral rockpools. Image width ca XX m.
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Distribution map

LR.LR.Rkp.FK recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)



Ecological and functional relationships

The rockpool environment varies depending on factors such as pool depth, surface area, volume, orientation to sunlight, shading, internal topography, sediment content and type, together with wave exposure, shore height, and hence flushing rate, and the presence of absence of freshwater runoff. As a result, no two rockpools exhibit exactly the same physio-chemical conditions and exhibit large spatial variation in community structure, even between adjacent pools at the same shore height (Ganning, 1971; Metaxas & Scheibling, 1993; Metaxas et al., 1994). Any given rockpool is effectively unique (Metaxas & Scheibling, 1993). Therefore, while the organisms detailed below have been recorded in this rockpool biotope, not all of the organisms, and hence the relationships between them, may occur in any one rockpool.

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, the rockpool environment differs from that of the surrounding emergent rock surfaces, so that not all species that thrive on the surrounding rock occur abundantly in rockpools and much of the lower shore open rock fauna is absent from rockpools (Lewis, 1964). Rockpools constitute a distinct environment for which physiological adaptations by the flora and fauna may be required (Lewis, 1964; Metaxas & Scheibling, 1993). The following description is based on reviews by Lewis (1964), Ganning (1971) and Metaxas & Scheibling (1993), the species listed in the MNCR database (JNCC, 1999) and additional references as cited. Macroalgae such as kelps, fucoids, red algae, green algae, encrusting red algae, erect and encrusting corallines provide primary productivity either directly to grazing invertebrates and fish or indirectly to detritivores and decomposers, in the form of detritus and drift algae or as dissolved organic material and other exudates. Benthic microalgae, periphyton and phytoplankton (e.g. diatoms) also add to primary productivity.

Where present, large macroalgae such as the laminarians (e.g. Laminaria digitata, Saccharina latissima and occasional Laminaria hyperborea and Saccorhiza polyschides), Halidrys siliquosa and fucoids (e.g. Fucus serratus, Fucus vesiculosus) shade the substratum (depending on density) so that understorey plants tend to be shade tolerant red algae and corallines. Understorey algae, by effectively restricting access to the substratum, may also inhibit or restrict recruitment of other species of macroalgae (Hawkins & Harkin, 1985; Hawkins et al., 1992).

Macroalgae provide substrata and refuges for a variety of invertebrates and epiphytic algae. The stipes and lamina of Laminaria spp. may support bryozoans (e.g. Membranipora membranipora or Electra pilosa) and grazing blue-rayed limpets (Helcion pellucida), while their holdfasts provide additional refuges for meiofauna and small invertebrates. If present, the stipes of Laminaria hyperborea may support numerous epiphytes such as Palmaria palmata, Phycodrys rubens and Cladophora rupestris (Goss-Custard et al., 1979). Where present, Halidrys siliquosa provide substratum for epiphytes, depending on location, including microflora (e.g. bacteria, blue green algae, diatoms and juvenile larger algae), Ulothrix sp. and Ceramium sp., hydroids (e.g. Obelia spp.), bryozoans (e.g. Scrupocellaria spp.), and ascidians (e.g. Apilidium spp., Botryllus schlosseri, and Botrylloides leachi) ( Lewis, 1964; Moss, 1982; Connor et al., 1997b).

The macroalgae provide refuges for small invertebrates, such as isopods, amphipods, ostracods and copepods. Corallina officinalis provides a substratum for small spirorbids e.g. Spirorbis corallinae, which is only found on Corallina officinalis. Increasing density of Spirorbis corallinae was shown to increase the species richness of the epiphytic fauna. The invertebrate fauna of Corallina officinalis is detailed in £LR.Cor£

Amphipods (e.g. Gammaridae), isopods (e.g. Idotea spp. and Jaera sp.) 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 also graze the macroalgae but do not normally adversely affect the canopy (Brawley, 1992b).

Grazers of periphyton (bacteria, blue-green algae and diatoms) or epiphytic algae include harpacticoid copepods, the limpets Patella vulgata and Patella ulyssiponensis, the blue-rayed limpet Helcion pellucidum, and gastropods such as Gibbula cineraria, Gibbula umbilicalis, Littorina saxatilis, Littorina littorea, Littorina obtusata and Rissoa spp. Limpets and littorinids also graze macroalgal sporelings and green algae especially.

Coralline algae are probably relatively grazing resistant (Littler & Kauker, 1984) and few species graze the corallines directly except perhaps chitons (e.g. Acanthochitona spp. and Lepidochitona cinerea) and limpets of the genus Tectura.

Grazing by littorinids and gammarid amphipods has been shown to significantly affect macroalgal abundance and diversity. For example in cage experiments in littoral fringe pools, Parker et al. (1993) found that gammarid amphipods significantly reduced the erect macroalgal canopy, while littorinids grazed microalgae and macroalgal sporelings, and prevented the establishment of erect and encrusting algal canopies. Both groups reduced the species richness of the algal canopy (Parker et al., 1993). In tidepools in Nova Scotia, Chapman (1990) and Chapman & Johnson (1990) reported that grazers (especially littorinids) reduced the abundance of Fucus spp. sporelings and juveniles but increased the abundance of ephemeral algae, while having no effect on the encrusting red alga Hildenbrandia rubra (Metaxas & Scheibling, 1993). Conversely, Lubchenco (1978) noted that the addition of littorinids to mid-shore pools in Massachusetts decreased the abundance of dominant Ulva spp. (as Enteromorpha spp.) in favour of Chondrus crispus (Metaxas & Scheibling, 1993) a less palatable red alga. Wolfe & Harlin (1988a) noted that Rhode Island tidepools with the highest littorinid densities had the lowest abundance of macroalgae. Similarly, removal of the limpet Patella vulgata from high tidal pools at Lough Ine resulted in an increased abundance of Ulva sp. (as Enteromorpha) (Goss-Custard et al., 1979). Where present, suspension feeders include the barnacles (e.g. Semibalanus balanoides, Balanus crenatus and Verruca stroemia), the mussel Mytilus edulis, hydroids (e.g. the epiphytic Dynamena pumila), tubeworms ( e.g. Spirorbis spp. and Pomatoceros spp.), ascidians (e.g. Botrylloides leachi, Botryllus schlosseri, and Dendrodoa grossularia), bryozoans (e.g. Membranipora membranacea and Electra pilosa) and sponges (e.g. Halichondria panicea and Hymeniacidon perleve). Deep low shore pools may also support the plumose anemone Metridium senile. However, the abundance of barnacles and mussels in rockpools is usually low (Lewis, 1964), presumably due to heavy predation on juveniles by the dog whelk Nucella lapillus and crabs (e.g. Carcinus maenas and Cancer pagurus) and poor survival of spat or juveniles.

Macroalgae compete for space with sessile invertebrates such as sponges, hydroids, ascidians and bryozoans. However, sessile epifauna predominate on vertical rock surfaces, overhangs and under boulders within the pool.

Scavengers include shrimps (e.g. Palaemon spp.), brittlestars (e.g. Amphipholis squamata and Ophiothrix fragilis), cushion stars (Asterina gibbosa), gastropods (e.g. Hinia spp.), hermit crabs (e.g. Pagurus bernhardus) and crabs (e.g. Carcinus maenas and Cancer pagurus).

Invertebrate predators include turbellarians, nemerteans and polychaetes (e.g. Eulalia viridis and Harmothoe spp.) feeding on small invertebrates such as copepods and small gastropods. Lower to mid shore rockpools provide refugia for dog whelks Nucella lapillus feeding on barnacles and small mussels within the rockpool and /or leaving the rockpool to forage at high tide. Similarly, crabs such as Carcinus maenas and Cancer pagurus are generalist predators of gastropods (e.g. littorinids) and bivalves as well as scavengers. Passive carnivores include sea anemones such as Anemonia viridis, Actinia equina, Cereus pedunculatus, Sagartia elegans and Urticina felina. In addition, intertidal fish such as the shanny Lipophrys pholis and gobies prey on small invertebrates such as copepods, amphipods and isopods.

As with grazing, predation pressure is potentially higher in mid to low shore rockpools, since predators can continue feeding irrespective of the state of the tide. For example, in New South Wales, whelks were shown to reduce the abundance of barnacles, tubeworms and limpets (Fairweather, 1987; Metaxas & Scheibling, 1993). Dethier (1984) concluded that harpacticoid copepod abundance in mid to low shore pools was low due to the presence of predators such as fish and to a lesser extent anemones. The reduced abundance of barnacles and mussels observed in rockpools (Lewis, 1964) is probably partly due to increased predation pressure.

Seasonal and longer term change

Conditions within rockpools are the consequence of prolonged separation from the main body of the sea, and physico-chemical parameters within them fluctuate dramatically (Pyefinch, 1943; Ganning, 1971; Daniel & Boyden, 1975; Goss-Custard et al., 1979; Morris & Taylor, 1983; Huggett & Griffiths, 1986; Metaxas & Scheibling, 1993; Metaxas et al., 1994). 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.

Temperature
The temperature of pools fluctuates with air temperature and sunlight, and tends to warm throughout the day, especially if in direct sunlight (Daniel & Boyden, 1975; Goss-Custard et al., 1979). Shaded pools exhibit less dramatic changes in temperature (Daniel & Boyden, 1975). For example, the temperature of a high shore pool exposed to direct sunlight rose quickly in the morning to a maximum of 25 °C, while a shaded high shore pools only rose by 1 °C, even though air temperatures reached 20 °C (Daniel & Boyden, 1975). In addition, deeper pools may become stratified, with warmer water near the surface and cooler near the bottom (Daniel & Boyden, 1975), primarily due to sunlight. They noted that pool temperatures remained almost constant at night and suggested that pool temperatures would fluctuate slowly during the day under overcast conditions. In deeper pools, the vertical temperature gradation present in summer may reverse during winter owing to density stratification, so that ice may form (Naylor & Slinn, 1958). Morris & Taylor (1983) reported warmer water at the bottom of the pools and cooler at the surface, which they attributed to cooling of the surface water by wind. Examples of temperature ranges reported for mid to low shore pools include annual maxima and minima of 1-25 °C and 2-22 °C (Morris & Taylor, 1983), a diurnal range of 24 °C (day) and 13 °C (night) for a mid shore pool (Daniel & Boyden, 1975), and surface water temperature ranges of 14-19.25 °C and 15.5-20.75 °C in mid shore pools (Pyefinch, 1943). Smaller (or shallower pools) are more affected by climatic change in temperature than larger and/or deeper ones (Ganning, 1971). Morris & Taylor (1983) noted that temperature showed the greatest seasonal variation of all the physical parameters examined. In summer, the minimum recorded temperatures were greater than the maximum temperatures recorded in winter, and the daily temperature ranges were greater in summer than in winter, in both high and low shore pools (Morris & Taylor, 1983).

Salinity
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. In addition, freezing of surface water increases the salinity of the underlying water (Ganning, 1971; Metaxas & Scheibling, 1993). Daniel & Boyden (1975) and Morris & Taylor (1983) reported little variability in salinity over one tidal cycle, and Ganning (1971) suggested that changes in salinity were of limited importance. However, heavy rain resulted in a layer of low salinity water on the surface of pools. Morris & Taylor (1983) stated that a low salinity layer of 2-10 mm was normal but after one storm the low salinity layer increased in depth, eventually resulting in a homogeneous pool of brackish water. But the deep pools characterized by this biotope are unlikely to become brackish. Morris & Taylor (1983) reported an annual salinity range in mid to low shore pools of 26-36.5 ppt.

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). 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).

Oxygenation
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 (Pyefinch, 1943; Ganning, 1971; Daniel & Boyden, 1975; Goss-Custard et al., 1979; Morris & Taylor, 1983; Metaxas & Scheibling, 1993). In summer, rockpools are likely to be supersaturated with oxygen during the day (Pyefinch, 1943). For example, the greatest range of oxygen saturation of 101.7 % occurred in a seaweed dominated, sediment floored pool, which reached over 190 %saturation on some days (Pyefinch, 1943). Daniel & Boyden (1975) noted that a mid shore, seaweed dominated pool reached 194% saturation (ca 15 mg O2/l) but that oxygenation was also marked in shaded pools. A pool with dense fauna exhibited a maximum saturation of 210 % (Pyefinch, 1943). 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 as respiration utilizes much of the available oxygen and pH decreases. Daniel & Boyden (1975) reported oxygen depletion at night, with mid to low shore pools reduced to 8-44 % saturation. They noted that the crab Carcinus maenas leaves the pools at night, and that other species with the ability to air-breathe could also do so, e.g. limpets, littorinids, and the shanny Lipophrys pholis. They also observed that shrimps gathered at the edge of high shore pools at night, presumably to take advantage of the better oxygenated surface layer (Daniel & Boyden, 1975). Daily fluctuation in oxygen concentration and pH also vary seasonally, and with the height of the pool on the shore or pool depth (Daniel & Boyden, 1975; Morris & Taylor, 1983; Metaxas & Scheibling, 1993). Morris & Taylor (1983) noted variation in oxygen concentration with pools with pool depth and proximity to pool algae. Again, inundation by the rising tide causes sudden changes in oxygenation, either sudden drops during the day or increases at night.

Ganning (1971) noted that the temperature of sediment at the bottom of pools showed little variation in temperature, and noted that diurnal fluctuations of greater than 1°C were rarely observed in bottom sediments of pools deeper than 0.5 m, although a temperature of fall 0.5-1 °C was observed at the water/sediment interface. The presence of sediment in pools increases the risk of scour, which is likely to vary seasonally, increasing in winter storms. In addition, anoxic conditions within the sediment could potentially cause significant and sudden decreases in oxygen levels if the sediment was stirred up by wave action, although no evidence of this effect was found.

Seasonal change in communities
Tidepool community structure has been show to vary markedly over time, particularly with season (Metaxas & Scheibling, 1993). However, most studies have examined tidepools overseas, or different tidepools communities to those that occur in LR.FK (Dethier, 1984; Wolf & Harlin, 1988a,b; Kooistra et al., 1989; Metaxas et al., 1994) so that the species concerned are very different. Seasonal changes in macroalgal cover and diversity were associated with changes in water temperature, light intensity and day length, and reduced grazing pressure from littorinids in winter, although the community types were relatively stable over time in Rhode Island pools (Wolfe & Harlin, 1988a,b). Metaxas et al. (1994) noted that sheet forming algae (e.g. Ulva lactuca) were found throughout the year, filamentous forms (e.g. Cladophora and Ceramium) were present in late spring and summer (although other studies found them to be present from late spring to late autumn) while thick leathery and encrusting forms did not vary seasonally. However, macroalgal diversity was lowest in summer and autumn, especially in mid shore pools, probably due to grazing. Metaxas et al. (1994) also noted that the abundance of mussels, littorinids and whelks in Rhode Island peaked in summer due to recruitment but varied significantly between pools.

Many of the macroalgae within the biotope are perennial. However, they show seasonal variation in reproduction, with Halidrys siliquosa releasing spores in the winter months, potentially enabling them to colonize free space opened up by increased wave action in winter storms and the dying back of annual species (see Kain, 1975). Growth rate of Laminaria digitata is seasonally controlled with a period of rapid growth from February to July and one of slower growth from August to January. Saccharina latissima grows fastest from late winter to spring then growth declines from June onwards and may cease in late summer. Increased wave exposure and storms in winter months are likely to erode Laminaria blades so that they appear tattered in winter months and overall standing biomass is reduced. Annual species, e.g. Chorda filum are likely to proliferate in spring, reaching maximum abundance in summer (high insolation and temperature). In Fucus serratus reproduction commences in late spring/early summer and continues through summer and autumn, peaking in August - October, and in the summer months it can reach 95% cover on emergent rock (see review). In England, Fucus vesiculosus has a protracted reproduction period of about six months which varies only slightly in timing between a population at Wembury on the south coast of Devon and one at Port Erin, Isle of Man (Knight & Parke, 1950). Gametes may be produced from mid winter until late summer with a peak of fertility in May and June.

Red algae exhibit seasonal variation in growth and reproduction and red algal turf declines in abundance during the winter months, partly due to die back and abrasion during winter storms. Although protected from wave action in deep pools, macroalgae will be susceptible to damage and abrasion by wave action and winter storms near the surface or in shallower pools. For example, Delesseria sanguinea exhibits a strong seasonal pattern of growth and reproduction. New blades appear in February and grow to full size by May -June becoming increasing battered or torn and the lamina are reduced to midribs by December (Maggs & Hommersand, 1993). The maximum growth of Furcellaria lumbricalis occurs in March/April (Austin, 1960b) and release of carpospores and tetraspores occurs in December/January (Bird et al., 1991). Reproductive bodies are present on the gametophytes of Ahnfeltia plicata between July and January and mature carposporophytes occur between October and July (Maggs & Paschal, 1989). Dickinson (1963) reported that Chondrus crispus was fertile in the UK from autumn to spring, but that the exact timing varied according to local environmental conditions. In the Bristol Channel, Bamber & Irving (1993) noted that the biomass of Corallina officinalis increased steadily through spring and summer and began to decline after July. Mastocarpus stellatus (as Gigartina stellata) was reported have a perennial holdfast, losing many erect fronds in winter, which grow back in spring (Dixon & Irvine, 1977). Osmundea pinnatifida also shows seasonal variation in growth, expanding its perennial holdfast in June to September, and producing erect fronds from October onwards reaching a maximum in February to May (Maggs & Hommersand, 1993). Maggs & Hommersand (1993) reported that spermatangia of Ceramium virgatum (as nodulosum) are recorded in January, March-April, June and August-September; cystocarps in January-February and April-September; tetrasporangia in February-September. The advantage of being fertile through the winter, as in the case of Ahnfeltia plicata, Furcellaria lumbricalis and Chondrus crispus, is the availability of substrata for colonization as other annual species die back (Kain, 1975)

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 depending on grazing pressure. In summer, erect and encrusting corallines may be bleached (especially in shallow pools or near the surface or deep pools) and loose their pink pigment but in some species, e.g. Phymatolithon sp., this does not necessarily result in death of the plant and pigment may be re-synthesized (Little & Kitching, 1996).

Habitat structure and complexity

As already noted, no two rockpools exhibit exactly the same physio-chemical conditions and exhibit large spatial variation in community structure, even between adjacent pools at the same shore height (Ganning, 1971; Metaxas & Scheibling, 1993; Metaxas et al., 1994). Habitat complexity will vary with macroalgal cover, the dominant macroalgal species, shore height, pool depth and topography. Deeper pools are likely to be more complex than shallower pools. This biotope resembles the mid and low tidal pools described by Goss-Custard et al. at Carrigathorna near Lough Ine, Ireland.

In deep pools the underlying rock is likely to be covered by erect and encrusting corallines. Large macroalgal species (e.g. kelps and fucoids) may dominate the surface of the pool. Their depth within the pool (vertical zonation) is limited by self-shading so that only corallines and red algae occur beneath them. The upper limit of some species of algae within the pool may be limited by the summer surface water temperatures e.g. Bifurcaria bifurcaria (Kooistra et al., 1989), and or desiccation after evaporation (e.g. corallines). For example, Goss-Custard et al. (1979) noted that Laminaria spp. and Halidrys siliquosa formed a margin in deep mid shore pools of 0.5-0.6 m deep, with Laminaria hyperborea below them, wherever the substratum was stable enough. The Corallina officinalis supported colonies of bryozoans, encrusting corallines and spirorbids and provided refuge for a wide variety of fauna including polychaetes, isopods, rissoid gastropods, amphipods, and Hiatella arctica (Goss-Custard et al., 1979; and 'ecological relationships' above). The Laminaria spp. provide substratum for specialist grazers such as Helcion pellucidum and epiphytic bryozoans and algae (see 'ecological relationships' above). Goss-Custard et al. (1979) noted that the edge of pools that lost water slowly due to leakage during emersion was dominated by ephemeral green algae, e.g. Ulva spp. Grazing intensity due to littorinids may also affect the abundance of fleshy macroalgae, so that the pools may be dominated by less palatable red algae (e.g. Chondrus crispus and Mastocarpus stellatus). In deep pools the understorey is likely to be dominated by coralline turf and red algae. For example, Goss-Custard et al. (1979) reported that, in their deepest pool, the laminarians thinned below 0.5 m and the brown algae Dictyota dichotoma, Desmarestia ligulata appeared, together with Dilsea carnosa, Delesseria sanguinea, while Chondrus crispus and Corallina officinalis remained. Below 2m, the laminarians were absent and replaced by a mixed algal shrub of Dictyota dichotoma, Phyllophora crispa, Nitophyllum punctatum and Cryptopleura ramosa (Goss-Custard et al., 1979)

Vertical surfaces within deep pools, and crevices or overhangs present, are likely to be dominated by encrusting fauna e.g. the sponges Halichondria panicea and Hymeniacidon perleve, tubeworms (Pomatoceros spp.) and anemones. For example, Goss-Custard et al. (1979) noted Corynactis viridis and Sagartia elegans on the sides of deep mid shore pools.

The presence of boulders and large stones at the bottom of the pools provides additional niches. The surface of larger stones and pebbles may support tubeworms, spirorbids and the holdfasts of kelps or Chorda filum, if stable enough and not too deep. The undersides support typical underboulder communities (see MLR.Fser.Fser.Bo; Gordon, 1972), likely to include encrusting sponges, bryozoans and tubeworms, small ascidians, the saddle oyster Anomia ephippium, and the porcelain crabs Porcellana platycheles and Pisidia longicornis. Underboulder habitats also provide refugia for crabs (e.g. Carcinus maenas and Necora puber) and fish.

The holdfasts of kelps and fucoids, and fronds of filamentous species and erect corallines provide refuges for small invertebrates (e.g. amphipods, isopods and small gastropods) or meiofauna (e.g. copepods) (see ecological relationships above). In addition, the shade of macroalgae provide refuges for shrimps (e.g. Palaemon spp.) and intertidal fish (e.g. blennies and gobies), while crevices provide additional refuges for crabs.

Rockpool species also display zonation patterns, similar to the emergent species. For example, brown algae and corallines are usually dominant in mid to low shore pools, while green algae tend to dominate high shore pools (Metaxas & Scheibling, 1993). Kooistra et al. (1989) noted vertical zonation within pools and found that macroalgal communities could be allocated to different depths within pools in the lower or higher parts of the shore. However, the communities studied in Brittany differed markedly from those found in this biotope. Similarly, zonation patterns have also been reported in flatworms, rotifers, oligochaetes, cladoceans, copepods, ostracods, barnacles, amphipods, isopods, chironomid larvae and fish (see Metaxas & Scheibling, 1993). Littorina littorea, mussels, whelks, limpets and sea urchins tend to dominate in lower shore pools, while other littorinids dominate higher on the shore (Metaxas & Scheibling, 1993). Nevertheless, Metaxas et al. (1994) noted that horizontal spatial variability between pools within the same shore height appeared to be as great as variability along the intertidal gradient, and suggested that the physical setting of the pool may be of primary importance in determining the macroalgal abundance. Dethier (1984) examined the effect of natural disturbance rockpool communities in the coast of Washington State. She noted that disturbance such as heat stress in summer and wave action in winter occurred regularly (ca 1.6 times per year per pool). The observed disturbances affected dominant species, so that no one dominant species could occupy all the pools within the tidal range at any one time. None of the tidal pool assemblages observed were stable over many generations and disturbances resulted in a mosaic of species assemblages within pools in any one region (Dethier, 1984).

Productivity

Although little information on rockpool productivity was found, rocky shore communities are highly productive and are an important source of food and nutrients for members of neighbouring terrestrial and marine ecosystems (Raffaelli & Hawkins, 1999; Hill et al., 1998). Macroalgae exude considerable amounts of dissolved organic carbon which are absorbed 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, 1999). 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. Accumulations of algal debris are also likely in rockpools and such detrital material contributes to overall productivity.

Information specific to the community was not found but Workman (1983) gave an estimate of primary production by microalgal films on the lower shore in the British Isles to be in the region of 100 g C/m²/yr, much of which will be utilized directly by grazers, while primary productivity for fucoids on sheltered shores was estimated to be 1250 g C/m²/yr (Hawkins et al., 1992) and for encrusting corallines to be 1000 g C/m²/yr (Dawes et al., 1991; Raffaelli & Hawkins, 1999). Ganning & Wulff (1970) reported primary productivity values in terms of gross photosynthesis of between 2 and 3.5 O2/m3/ hr in brackish water rock pools dominated by green algae. Overall, deep rockpools with abundant macroalgae are likely to be highly productive mesocosms on the shore.

Recruitment processes

As with the emergent rock surfaces, variation and unpredictability in recruitment of species contributes to the variation in dominant species within rockpool communities and hence spatial variation between rockpools (Dethier, 1984). Recruitment processes of characteristic or dominant species are described here.

Flora
  • Vadas et al. (1992) reviewed recruitment and mortality of early post settlement stages of benthic algae. 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.
  • Laminaria hyperborea and Laminaria digitata produce vast numbers of spores, however they need to settle and form gametophytes within about 1 mm of each other to ensure fertilization and therefore may suffer from dilution effects over distance. However, Chapman (1981) demonstrated that substantial recruitment of Laminaria digitata plants to areas barren of kelp plants was possible up to 600 m away from reproductive plants. Saccharina latissima (studied as Laminaria saccharina), is short-lived (2-4 years), reaches maturity quickly and colonizes bare substratum rapidly (ca 6 months) (Kain, 1975; Kain, 1979; Birkett et al., 1998b; see MarLIN review) and may be regarded as relatively opportunistic.
  • Fucoids (e.g. Fucus serratus and Fucus vesiculosus) recruit readily to cleared areas, especially in the absence of grazers (Holt et al., 1997). However, fucoid propagules tend to settle near to the parent plants, due to turbulent deposition by water flow. Within monospecific stands recruitment of conspecifics is most likely, and community recovery is likely to be rapid. However, in cleared areas, recruitment is likely to be rapid but recovery of the original community structure is likely to take some years (Holt et al., 1997). Sousa et al. (1981) reported that experimental removal of sea urchins significantly increased recruitment in long-lived brown algae. In experimental plots cleared of algae and sea urchins in December, Halidrys dioica colonized the plots, in small numbers, within 3-4 months. Plots cleared in August received few , if any recruits, suggesting that recolonization was dependant on zygote availability and therefore the season. Halidrys dioica did not colonize plots grazed by urchins in their experiments (Sousa et al., 1981). Wernberg et al. (2001) suggested that the lack of long range dispersal success in Halidrys siliquosa was responsible for its regional distribution in the north east Atlantic.
  • Settlement of algal spores is partly dependant on their motility (if any) and adhesive properties together with preferences for topography (surface roughness), the chemical nature of the substratum and water movement (Norton 1992; Fletcher & Callow 1992). As with all red algae, the spores of Chondrus crispus, Palmaria palmata and Ceramium virgatum are non-flagellate and therefore dispersal is a wholly passive process (Fletcher & Callow, 1992). 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 occurs within 10 m of the parent plants. It is expected, therefore, that recruitment of red algae, e.g. Chondrus crispus, Palmaria palmata, Furcellaria lumbricalis, Mastocarpus stellatus and the majority of other macroalgae in the biotope would occur from local populations on the surrounding emergent rock but 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. 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., which reproduce throughout the year with a peak in summer. Similarly, Ceramium species produce reproductive propagules throughout the year, while Mastocarpus stellatus produce propagules from February to December, and exhibit distinct reproductive papillae in summer (Dixon & Irvine, 1977; Burrows, 1991; Maggs & Hommersand, 1993).
  • Lithophyllum incrustans, representing the encrusting corallines in the biotope, 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). Dispersal is likely to be in excess of 5 km and spores will settle and new colonies will arise rapidly on bare substratum, although the growth rate is slow (2-7 mm per annum - see Irvine & Chamberlain, 1994). The erect coralline Corallina officinalis has isomorphic sexual (gametophyte) and asexual (sporophyte) stages (see MarLIN review). 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).
Fauna
  • Gastropods exhibit a variety of reproductive life cycles. The common limpets Patella vulgata and Patella ulyssiponensis, the top shell Gibbula umbilicalis, and Littorina littorea have pelagic larvae with a high dispersal potential, although recruitment and settlement is probably variable. However, Littorina obtusata lays its eggs on the fronds of fucoids form which hatch crawl-away miniature adults. Similarly, the dog whelk Nucella lapillus lays egg capsules on hard substrata in damp places on the shore, from which crawl-always emerge. Therefore, their dispersal potential is limited but probably designed to colonize an abundant food source. In addition, most gastropods are relatively mobile, so that a large proportion of recruitment of available niches would involve migration. Nucella lapillus is an exception, as they generally do not move far, averaging 100 mm /tidal cycle, or between 30 cm or 10 m per year when in the vicinity of an abundant food source (see MarLIN reviews for details; Fish & Fish, 1996).
  • Many species of mobile epifauna have long lived pelagic larvae and/or are highly motile as adults. 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 for example). Similarly, isopods such as Idotea species brood their young. Idotea species are mobile and active swimmers and probably capable to recruiting to new habitats from the surrounding area by adult migration. 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 form 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).
  • Epiphytic and sessile fauna, such as sponges, hydroids, bryozoans and ascidians, have pelagic but short lived larvae with relatively short effective dispersal ranges, depending on the local hydrography. However, most epiphytic species are widespread and ubiquitous and would probably recruit rapidly from adjacent or nearby populations.

Time for community to reach maturity

Development of the community in this biotope, from bare or denuded rock, is likely to follow a similar successional pattern to emergent sheltered rocky shore communities. The loss of grazing species results in an initial proliferation of ephemeral green then fucoid algae, which then attracts mobile grazers, and encourages settlement of other grazers. Limpet grazing reduces the abundance of fucoids allowing barnacles to colonize the shore. After the Torrey Canyon oil spill in March 1967, recovery rates were dependant on local variation in recruitment and mortality so that sites varied in recovery rates, for example maximum cover of fucoids occurred within 1-3 years, barnacle abundance increased in 1-7 years, limpet number were still reduced after 6-8 years and species richness was regained in 2 to >10 years (Southward & Southward, 1978; Hawkins & Southward, 1992; Raffaelli & Hawkins, 1999). Where only the rockpool was affected, recruitment is potentially good, especially in motile species. However, variability in rockpool communities and their response to disturbance (recovery) has been attributed to variation and seasonality in recruitment (Metaxas & Scheibling, 1993).

Dethier (1984) noted that few rockpool populations, even of dominant species, remained static over time, based on long-term observations over several years. On the coast of Washington State, partial disturbance (a reduction in abundance or cover) resulted in relatively rapid recovery of the community, for examples encrusting corallines recovered in over 2 months, erect corallines attained 87% of its original cover in 2 years. The red algae Rhodomela sp. exhibited 39% recovery from total loss after 2 years and Cladophora sp. exhibited 77% recovery after total loss. Dethier (1984) concluded that disturbance was a factor that developed a mosaic of different communities in rockpools within an area and that, at any point in time, rockpool communities were probably in different stages of recovery.

Additional information

None entered

This review can be cited as follows:

Tyler-Walters, H. 2005. Fucoids and kelps in deep eulittoral rockpools. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 31/10/2014]. Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=282&code=1997>