19 2 Hard Shores CONTENTS 2.1 Zonation Patterns on Hard Shores 20 2.1.1 The Shore Environment and Zonation Patterns 20 2.1.2 Zonation Terminology 20 2.1.3 Widespread Features of Zonation Patterns 23 2.2 Zonation Patterns on Representative Shores 24 2.2.1 The British Isles 24 2.2.2 The Northwest Atlantic Shores 27 2.2.3 The Pacific Coast of North America 29 2.2.4 New Zealand 31 2.2.5 South Africa 33 2.3 The Causes of Zonation 36 2.3.1 Wave Action and Zonation 36 2.3.1.1 Introduction 36 2.3.1.2 The Problem of Defining Wave Exposure 36 2.3.1.3 The General Effects of Wave Action 38 2.3.2 Tidal Currents and Zonation 41 2.3.3 Substrate, Topography, Aspect, and Zonation 42 2.3.4 Sand and Zonation 42 2.3.5 Climatic Factors and Zonation 44 2.3.5.1 Solar Radiation 44 2.3.5.2 Temperature 45 2.3.6 Desiccation and Zonation 45 2.3.7 Biotic Factors and Zonation 46 2.3.8 Factor Interactions 46 2.3.9 Critical Levels 47 2.4 Hard Shore Microalgae 51 2.5 Hard Shore Micro- and Meiofauna 53 2.6 Rocky Shore Lichens 54 2.6.1 Species Composition and Distribution Patterns 54 2.6.1.1 British Isles 55 2.6.1.2 The sub-Antarctic Region 55 2.7 Hard Shore Macroalgae 56 2.7.1 Zonation Patterns 56 2.7.2 Factors Controlling the Lower Limits of Intertidal Microalgae 56 2.7.3 Factors Controlling the Upper Limits of Intertidal Microalgae 56 2.8 Key Faunal Components 58 2.8.1 Mussels 58 2.8.1.1 Introduction 58 2.8.1.2 Factors Limiting Mussel Zonation 59 2.8.1.3 Mussels as a Habitat Structure for Associated Organisms 59 2.8.1.4 Role of Mussel Beds in Coastal Ecosystems 60 2.8.2 Limpets 61 2.8.2.1 Adaptations to Intertidal Living 61 0008_frame_C02 Page 19 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC 20 The Ecology of Seashores 2.8.2.2 Factors Controlling Vertical Distribution 61 2.8.2.3 Algal–Limpet Interactions 61 2.8.2.4 Limpet–Barnacle Interactions 62 2.8.2.5 Intra- and Interspecific Interactions 63 2.8.2.6 Limpet–Predator Interactions 63 2.8.3 Barnacles 64 2.8.3.1 Adaptation to Intertidal Life 64 2.8.3.2 Settlement 65 2.8.3.3 Factors Affecting Settlement 66 2.8.3.4 Variability in Settlement and Recruitment 67 2.8.3.5 Barnacle Distribution Patterns 68 2.8.3.6 Predation and Other Biotic Pressures 69 2.9 Special Habitats 70 2.9.1 Boulder Beaches 70 2.9.1.1 Boulder Types 70 2.9.1.2 The Boulder Environment 70 2.9.1.3 Disturbance and Boulder Community Structure 72 2.9.2 The Fauna Inhabiting Littoral Seaweeds 73 2.9.2.1 Introduction 73 2.9.2.2 Community Composition 73 2.9.2.3 Seasonal Change in Species Composition 74 2.9.2.4 Factors Influencing Community Diversity and Abundance 75 2.9.3 Rock Pools 76 2.9.3.1 Introduction 76 2.9.3.2 The Physicochemical Environment 77 2.9.3.3 Temporal and Spatial Patterns in the Tidepool Biota 77 2.9.3.4 Factors Affecting Community Organization 78 2.9.3.5 Conclusions 79 2.9.4. Kelp Beds 79 2.9.4.1 Introduction 79 2.9.4.2 Species Composition, Distribution, and Zonation 80 2.9.4.3 Kelp Bed Fauna 80 2.9.4.4 Reproduction, Recruitment, and Dispersal 82 2.9.4.5 Impact of Grazers on Kelp Communities 82 2.9.4.6 Predation 83 2.9.4.7 Growth and Production 83 2.1 ZONATION PATTERNS ON HARD SHORES 2.2.1 T HE S HORE E NVIRONMENT AND Z ONATION P ATTERNS The vertical distribution of plants and animals on the shore is rarely, if ever, random. On most shores, as the tide recedes, conspicuous bands appear on the shore as a result of the color of the organisms dominating a particular level roughly parallel to the water line (Figure 2.1). In other places, while the bands or zones are less conspicuous and less readily distinguishable, they are rarely, if ever, com- pletely absent. Stephenson and Stephenson (1949; 1972) and Southward (1958) and Lewis (1955; 1961; 1964) have summarized much of the earlier information on zonation distribution patterns of intertidal organisms, and have shown that such zones are of universal occurrence on rocky shores, although their tidal level and width is depen- dent on a number of factors, of which exposure to wave action is the most important. More recent reviews of zona- tion patterns are to be found in Knox (1960; 1963a; 1975), Newell (1979), Lobban et al. (1985), Peres (1982a,b), Norton (1985), and Russell (1991). 2.2.2 Z ONATION T ERMINOLOGY A variety of schemes have been proposed to delineate the various zones found on rocky shores, and I do not propose to review them here. Details of these schemes can be found in Southward (1958), Hedgpeth (1962), Hodgkin (1960), and Lewis (1964). Based on the work of Lewis (1964) and 0008_frame_C02 Page 20 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC Hard Shores 21 Stephenson and Stephenson (1972), who recognized three primary zones on marine rocky shores, each characterized by particular kinds of organisms, the scheme given in Table 2.1 and shown in Figure 2.2 will be used in the following discussion. In the Stephenson and Stephenson scheme, the inter- tidal zone is called the littoral zone extending from the extreme high water of spring tides (EHWS) to the extreme low water of spring tides (ELWS). A midlittoral zone extends from the upper limit of the barnacles down to the lower limit of large brown algae (e.g., laminarians). A supralittoral fringe straddles EHWS extending from the upper limit of the barnacles to the lower limit of the terrestrial vegetation of the supralittoral zone . Its upper limit often coincides with the upper limit of littorinid snails. Below ELWS is the infralittoral zone, which is the upper part of the permanently submerged subtidal or sub- littoral zone. Between the upper limits of the infralittoral zone a fringing zone, between the midlittoral and the infralittoral, the infralittoral fringe , is often distinguished. The principal difference between the Stephensons’ scheme and that proposed by Lewis is that the latter accounts for the impact of wave action in broadening and extending the vertical height of the zones. This takes into account the actual exposure time and not the theoretical time as determined from tide tables. In his scheme, Lewis extended the term littoral to include the Stephensons’ supralittoral fringe and called the latter the littoral fringe. The rest of the littoral zone down to the upper limit of the laminarians is called the eulittoral zone. Lewis did not distinguish a zone equivalent to the Stephensons’ infralit- toral fringe. In this book, cases where a fringing zone between the eulittoral and the sublittoral is recognized will be called the sublittoral fringe. As Russell (1991) points out, identification of the primary zones by inspection of a shore is necessarily influenced by the species composition of the topmost layer of the communities. He illustrates this in the diagram reproduced in Figure 2.3 of the stratification of the algal vegetation of the eulittoral zone on a Netherlands dyke as described by Den Hartog (1959). At the rock face surface, the entire extent of the zone is covered by the crustose red alga Hildenbrandia rubra . The middle stratum, also of red algae, has an upper band of Catenella caespitosa and a FIGURE 2.1 A comparison of the widespread features of zonation with an example that complicates them. A coast is shown on which smooth granite spurs are exposed to considerable wave action. On the middle spur, some of the widespread features are summarized and the following succession is shown. A , littoral fringe (= Littorina zone ), blackened below by myxophyceans; B , eulittoral (balanoid zone), occupied by barnacles above and lithothamnia below; C , sublittoral fringe, dominated in this case by laminarians growing over lithothamnia. On the other spurs ( foreground and background ) the actual zonation from the Atlantic coast of Nova Scotia is shown. Here the simplicity of the basic plan is complicated by maplike black patches in the littoral fringe, consisting of Codiolum, Calothrix , and Plectonema ; the existence of a strongly developed belt of Fucus (mostly F. vesciculosus and F. endentatus , in this example) occupying a large part of the eulittoral zone and overgrowing the uppermost barnacles; and a distinct belt of Chondrus crispus growing over the lower part of the eulittoral zone and largely obliterating the belt of lithothamnia, which, on the middle spur, extends over the laminarians. (From Stephenson, T.A. and Stephenson, A., Life Between Tidemarks on Rocky Shores, W.H. Freeman, San Francisco, 1972, 386. With permission.) 0008_frame_C02 Page 21 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC 22 The Ecology of Seashores lower band of Mastocarpus stellatus . Finally the outer canopy layer consists of large brown (fucoid) algae in four conspicuous belts, with Pelvetia caniculata at the top, followed successively by Fucus spiralis, Ascophyllum nodosum, and Fucus serratus . This demonstrates that zonation is a three-dimensional phenomenon and that the zones defined by the uppermost stratum may conceal a number of other patterns. As Lobban et al. (1985) point out, there are difficulties in defining zones on the shore in terms of the organisms TABLE 2.1 Table Showing the Principal Zones of Universal Occurrence on Hard Shores Tidal Level Zone Indicator Organisms MARITIME ZONE Terrestrial vegetation, orange and green lichens Extreme high water of spring tides LITTORAL ZONE LITTORAL FRINGE Upper limit of littorinids Melaraphe (=Littorina) neritoides Ligia, Petrobius, Verrucaria etc. EULITTORAL ZONE Upper limit of barnacles Barnacles Mussels Limpets Fucoids (plus many other organisms) Extreme low water of spring tides SUBLITTORAL ZONE Upper limit of laminarians Rhodophyceae Ascidians (plus many other organisms) FIGURE 2.2 Diagram showing the effect of exposure to wave action on the intertidal zones of shore in the British Isles. (Modified from Lewis, J.R., The Ecology of Rocky Shores , English University Press, London, 1964, 49. With permission.) 0008_frame_C02 Page 22 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC Hard Shores 23 found on them. Floras and faunas change geographically, and while a topographically uniform shore may have a uniform zonal distribution pattern, a broken shoreline of varying exposure to wave action and/or a broken substra- tum of irregular rocks and boulders can present a confusing pattern, with the zones breaking down into patches. How- ever, if comparisons of surfaces with the same exposure, slope, and aspect are compared, then like patterns emerge. In addition to variability in space, there is also vari- ability in time. There are seasonal and successional changes in the vegetation and in the timing of disturbance that make space available for settlement (Dethier, 1984). The net result is a changing mosaic pattern of distribution. Vertical limits of many species can vary from year to year (Figure 2.4), perhaps dependent on variations in emersion- submersion histories. Relative abundances and distribution of species which may be nearly equal competitors change over time (Lewis, 1982 ). Among the algae, the presence or absence of a particular species at a given locality can be interpreted to mean that conditions there have been suitable for its growth since it settled (Lobban et al., 1985). Absence, on the other hand, only indicates that at a par- ticular time conditions were unfavorable for the settlement of the reproductive bodies of that species, such as unfa- vorable currents or extreme desiccating conditions. 2.1.3 W IDESPREAD F EATURES OF Z ONATION P ATTERNS A consideration of the zonation patterns discussed above and in the next section (2.2) of this chapter reveals a number of widespread features or tendencies (Stephenson and Stephenson, 1972) as follows: 1. Near the high water mark there is a zone that is wetted by waves only in heavy weather, but affected by spray to a greater or lesser extent. The number of species is relatively small, and includes particular species adapted to semiarid conditions, and belonging to the gastropod genus Littorina , and related genera, or to genera of snails containing similarly adapted species. Semi- terrestrial crustaceans, such as isopods of the genus Ligia, are also characteristic of this zone. 2. The surface of the rock in the zone described above, especially in the lower part, is blackened by encrustations of blue-green algae, or lichens of the Verrucaria type, or both. This is a most persistent feature of the zone. Depending on the latitude and geographic location, other grey, green blue-green, and orange lichens (the latter belonging to the genus Caloplaca ) paint splashes of color on the rocks. 3. The middle part of the shore typically includes numerous balanoid barnacles belonging to gen- era such as Balamus, Semibalanus , Chthalamus, and Tetraclita . The upper limit of the zone is marked by the disappearance of barnacles in quantity. Herbivorous and carnivorous gastro- pods, especially limpets, whelks, and chitons are often abundant. On some shores, algae, espe- cially fucoids, may form conspicuous bands. FIGURE 2.3 Stratification of vegetation in the eulittoral zone of a dyke in The Netherlands. The rock surface (1) bears the encrusting red alga Hildenbrandia rubra , the second stratum (2) consists of Cantenella caespitosa and Mastocarpus stellatus, and the canopy (3) comprises, in descending order, Pelvetia canaliculatus, Fucus spiralis, Ascophyllum nodosum, and Fucus serratus . Based on a diagram in den Hartog (1959). (Redrawn from Russell, G. in Intertidal and Littoral Ecosystems , Ecosystems of the World 24, Mathieson, A.C. and Nienhuis, P.H., Eds., Elsevier, Amsterdam, 1991, 44. With permission.) 0008_frame_C02 Page 23 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC 24 The Ecology of Seashores 4. The lowest part of the shore is uncovered only by spring tides and is characterized by a diverse assemblage of species. In cold-temperate regions, it consists of a forest of brown algae (e.g., laminarians) with an undergrowth of smaller algae, especially reds, between the holdfasts. In warm-temperate regions, it may support (a) a dense covering of simple ascidians (e.g., Pyura ), (b) a dense mat of small mixed algae, primarily reds, or (c) other communities. 2.2 ZONATION PATTERNS ON REPRESENTATIVE SHORES In this section we will briefly detail the principal zonation patterns on a range of shore from both the southern and northern hemispheres. From this survey it will be seen that although there are similarities between the patterns, and while some taxa (e.g., barnacles, mussels, herbivorous and carnivorous gastropods, limpets, and some algal spe- cies) are found on most shores, there are considerable differences in the distribution patterns related to the lati- tude of the shore (affecting seasonal ranges in temperature and other climatic variables), the patterns of the tides, and in the species composition of the shore communities. 2.2.1 T HE B RITISH I SLES The British Isles are approximately 1,125 km long and are subject to cool-temperate climatic conditions. The sea- sonal variation in sea temperatures is roughly 7°C in northern parts and up to 12°C in part of the Irish Sea and the southeastern coasts. The range of spring tides varies from 0.6 m to 12 m, although ranges of between 7 and 12 m are more common. Detailed accounts of the zonation patterns on the shores of the British Isles are to be found in Lewis (1964) and Stephenson and Stephenson (1972). The general pattern of zonation is as follows: (1) a littoral fringe dominated by “black” lichens, dark micro- phytes, and littorinid snails, (2) a eulittoral zone domi- nated by various combinations of barnacles, mussels, lim- pets, snails, and brown (fucoid) and red algae; and (3) a sublittoral fringe dominated by laminarian algae. Littoral Fringe: The upper limit of the littoral fringe is placed at the junction between the black lichens and the band of orange and/or grey lichens above, although on other shores this latter zone is regarded as the upper littoral fringe. Two species of lichens dominate much of this black zone, Verrucaria throughout and Lichina confinis toward the upper limit. In wave-swept places, algal growth super- imposed on the lichens takes the form of a very fine layer dominated by cyanophyceans ( Calothrix spp. in particu- lar), and, more locally, filamentous green and red algae ( Ulothrix, Urospora, and Bangia ). Superimposed on this are the larger red alga, Porphyra umbilicalis, and species of the green algal genus, Enteromorpha . Most of these algae are seasonal in occurrence. The lower limit of the littoral fringe is taken as the upper limit of barnacles in quantity. Where Chthalamus stellatus predominates (in southwestern areas generally and exposed situations in the west and northwest), the “barnacle line” is higher than in areas where Balanus FIGURE 2.4 Year-to-year changes in upper and lower limits of two intertidal kelp species on three transects at an exposed site on the west coast of Vancouver Island, British Columbia. A gently shelving platform, a rocky point, and a narrow channel are compared. (Redrawn from Druehl, L.D. and Green, J.M., Mar. Ecol. Prog. Ser., 9, 168, 1982. With permission.) 0008_frame_C02 Page 24 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC Hard Shores 25 balanoides is present alone (on the north and east coasts, and in sheltered areas of the west and northwest). Conse- quently, some conspicuous zone-forming plants of narrow vertical range (the lichen Lichina pygmaea and Fucus spiralis ) lie largely within the eulittoral zone on “ Chthal- amus shores” and partly, or completely in the littoral fringe on “ Balanus shores” (Figure 2.5). The characteristic ani- mals in the littoral fringe are littorinid snails, Littorina neritoides and L. saxatilus . Other animals are mites, the thysanuran, Pterobius maritimus , and the eulittoral mol- luscs, Patella vulgata and Littorina littorea . Eulittoral Zone: At one extreme this zone is domi- nated by (1) barnacles or mussels (or both), and (2) at the other by exceptionally heavy growths of long-fronded fucoid algae. 1. Barnacle-dominated shores (Figure 2.6): Where they are abundant, barnacles can extend from their sharp upper limit to within a few centimeters of the topmost laminarians. Bala- nus balanoides is the most ubiquitous, while Chthalamus stellatus predominates in the southwest but is absent from North Sea coasts and the entire eastern half of the English Chan- nel. On moderately exposed sites in southwest England and Wales a third larger species, Bal- anus perforatus , occupies a belt 60 to 90 cm high immediately above the laminarians, or forms isolated patches at higher levels. Since the late 1940s, the Australasian barnacle, Elm- inius modestus , has established itself in harbors and estuaries and along the less exposed coasts, mainly at the expense of Balanus balanoides . Associated animals include limpets ( Patella depressa , P. vulgata, and P. aspersa ) and whelks ( Gibbula cineraria, G. umbilicalis, and Nucella lapillus ). 2. Fucus-dominated shores (Figure 2.7): As expo- sure decreases, there is a progressive replace- ment of barnacle- and mussel-dominated communities by fucoids, beginning with the appearance of Fucus vesiculosus f. linearis . Pel- vetia gradually appears in the littoral fringe and F. serratus begins to mingle with the low level Himanthalia . As the larger and sheltered shore form of F. vesiculosus replaces F. vesiculosus f. linearis , F. spiralis appears and F. serratus dis- places Himanthalia . Next, Ascophyllum nodosum starts to appear in the flatter and more protected places among the F. vesiculosus . This process culminates in very sheltered bays and locks with luxuriant narrow belts of Pelvetia and F. spiralis surrounding a midshore belt of long- fronded Ascophyllum, with a narrow belt of F. spiralis just above the laminarians (Figure 2.7). The relative proportions of the eulittoral zone occupied by Ascophyllum, F. vesiculosus, and F. serratus vary greatly. The shade of the fucoids enables Laurencia , Leathe- sia, and other members of the red algal belt to extend upshore, but under the dense growths of Ascophyllum and F. serratus they are replaced by lithothamnion. As the fucoids develop there is a loss of such open-coast species as Littorina littorea, Patella aspersa, P. depressa, Balanus perforatus, and Mytilus edulis , with its associated fauna. The topshell, Gibbula umbilicalis, becomes plentiful throughout the middle zone and is joined by G. cineraria FIGURE 2.5 Simplified diagram showing the littoral fringe on: A. Balanus shore; B. situations on the north-west coasts where Chthalamus is confined to exposure; and C. Chthalamus shores in the British Isles. (From Lewis, J.R., The Ecology of Rocky Shores , English University Press, London, 1964, 54. With per- mission.) 0008_frame_C02 Page 25 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC 26 The Ecology of Seashores FIGURE 2.6 A barnacle-dominated face near Hope Cove, South Devon, typical of many exposed and south-facing areas of the English Channel coast. (From Lewis, J.R., The Ecology of Rocky Shores, English University Press, London, 1964, 78. With permission.) FIGURE 2.7 Representation of a moderately sheltered Fucus -dominated shore. (From Lewis, J.R., The Ecology of Rocky Shores, English University Press, London, 1964, 119. With permission.) 0008_frame_C02 Page 26 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC Hard Shores 27 and Monodonta lineata in the lower and upper levels, respectively. Littorina saxatilus is joined by L. obtusata, mainly in the fucoids, and by large number of L. littorea. Sublittoral Fringe and Upper Sublittoral: The flora of the sublittoral fringe is characteristically dominated by laminarians. Most of the permanently submerged “forest” consists of Laminaria hyperborea. Above this species on open coasts, two species predominate — Alaria esculenta in very exposed situations, and Laminaria digitata else- where. They form a continuous narrow belt, typically not more than 30 to 60 cm deep. As L. digitata replaces Alaria the undercanopy algal growth becomes more variable and luxuriant, and commonly includes species such as Cera- mium spp., Chondrus crispus, Cladophora rupestris, Cys- toclonium purpureum, Delessaria sanguinea, Dictyota dichotoma, Membranoptera alata, Plocamium coc- cineum, Plumaria elegans, Polysiphonia spp., and Rhody- menia palmata . The fauna of this zone changes from one of relatively large numbers of a few species to one of small numbers of very many species. A few eulittoral species extend down into this zone such as Patella aspersa, Gibbula cineraria, Mytilus edulis, and barnacle (Verrucaria stro- emia and Balanus crenatus). Sublittoral species that occur, depending on the degree of wave exposure, include sponges, hydroids, anemones, tubiculous polychaetes, bryozoans, and ascidians. 2.2.2 THE NORTHWEST ATLANTIC SHORES This encompasses the North American coastline between Cape Cod/Nantucket Shoals and Newfoundland, and exhibits conspicuous regional differences in temperature, tidal fluctuation, ice scouring, wave exposure, and nutrient enrichment. This area has been extensively studied by a number of investigators (see references by A.R.O. Chap- J. Lubchenco; K.H. Mann; A.C. Mathieson; B.A. Menge; J.L. Menge; J.D. Pringle, 1987; and R.S. Steneck, 1982, 1983, 1986. Stephenson and Stephenson (1954a,b; 1972) have given accounts of the zonation patterns in Nova Scotia and Prince Edward Island, and Mathieson et al. (1991) have recently reviewed northwest Atlantic shores. Tidal ranges within the area vary considerably. Aver- age tidal ranges within the Gulf of Maine vary from 2.5 to 6.5 m (mean spring tides = 2.9 to 6.4 m), while those elsewhere vary from 2.7 to 11.7 m (mean spring tides = 3.1 to 13.3 m) in the Bay of Fundy, 0.7 to 2.2 m on the Atlantic coast of Nova Scotia, and 0.8 to 1.9 on the coast of Newfoundland. In the Bay of Fundy the annual tem- perature range is moderate, the maximum being 1.8°C in February to a maximum of 11.4°C in September. Salinities of the surface waters vary from 30 to 33. For the Atlantic coast of Nova Scotia, intertidal populations are subjected to very cold waters (sometimes below 0°C) in the winter, and relatively warm water (often near 20°C, or locally even higher) in the summer. Descriptions of zonation patterns on New England coasts can be found in J.L. Menge (1974; 1975), B.A. Menge (1976), B.A. Menge and Sutherland (1976), Lub- chenco and Menge (1978), Menge and Lubchenco (1981), Mathieson et al. (1991), and Vadas and Elner (1992). The basic zonation patterns of the New England coasts are depicted in Figure 2.8. Littoral Fringe: The littoral fringe is characterized by blue-green algae (Calothrix, Lyngbya, Rivularia, etc.) and ephemeral macrophytes (such as Bangia, Blidingia, Coliolum, Porphyra, Prasiola, Ulothrix, and Urospora, lichens (such as Verrucaria maura), and a periwinkle (Lit- torina saxatilis). Eulittoral Zone: On a typical semi-exposed rocky shore, three major zones occur (Lubchenco, 1980): (1) an upper barnacle zone with Semibalanus balanoides domi- nating; (2) a mid-shore brown algal zone with Ascophyl- lum nodosum and/or Fucus spp.; and (3) a lower red algal zone with Chondrus crispus and Mastocarpus stellatus. The S. balanoides zone exhibits a conspicuous uplift- ing with increasing wave action, while the brown and red algal zones are compressed and displaced downwards. Barnacles may also extend down into the lower eulittoral zone, particularly in extremely exposed habitats. Other species include the predatory dogwhelk, Nucella lapillus, and the periwinkle, Littorina littorea. On some exposed shores the dwarf fucoid, Fucus distichus ssp. uncaps, grows on the barnacles. Depending upon wave action and other associated physical and biological factors, either A. nodosum or Fucus spp. will dominate the mid-shore (Lub- chenco, 1980). As in Europe, A. nodosum is most abundant in sheltered sites and is replaced by F. vesiculosus and F. distichus ssp. dentatus with increasing wave exposure. Under extreme wave action the fucoids are limited and Mytilus edulis becomes the major occupier of space in the mid-shore. In the lower eulittoral zone, C. crispus and/or Mastocarpus stellatus dominate at all but the most exposed sites, where mussels are the most abundant mac- roorganism. C. crispus is found mainly on shelving and horizontal surfaces, whereas M. stellatus dominates the vertical ones (Pringle and Mathieson, 1987). Substrata with intermediate slopes are populated by a mixture of both algae. In the mid-eulittoral, competition between Mytilus edulis and Semibalanus balanoides is the dominant bio- logical interaction. Predation and herbivory are the main factors affecting space utilization (Menge and Sutherland, 1976; Menge, 1978a,b; Lubchenco, 1983; 1986). By clear- ing space, Nucella lapillus and other predators of Mytilus edulis allow the persistence of Fucus vesciculosus and Ascophyllum nodosum on semiprotected and protected sites, respectively (Keser and Larson, 1984a,b). Both fucoids are competitively inferior to many ephemeral 0008_frame_C02 Page 27 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC man, 1981, 1984, 1990; C.R. Johnson, 1985; C.S. Lobban; 28 The Ecology of Seashores algae (such as Enteromorpha spp., Porphyra spp., and Ulva lactuca.) In addition to Mytilus edulis and Semibalanus bal- anoides, numerous other invertebrate species, both sessile and motile, characterize the eulittoral zone. Several her- bivorous crustaceans and gastropods are common (Vadas, 1985), including amphipods (such as Hyale nilssoni), her- bivorous snails (Littorina littorea, L. obtusata, L. saxatilis, and Lacuna vincta), and limpets (Acmaea testudinalis). The chiton, Tonicella ruber, and the sea urchin, Strongy- locentrotus droebachiensis, graze within the lower eulit- toral and sublittoral zones. The whelk, Nucella lapillus, and two crab species, Carcinus maenas and Cancer irro- tatus, and a starfish, Asterias vulgaris, are important pred- ators in both the lower eulittoral and sublittoral zones. The abundance of these species decreases with increasing wave exposure. This allows M. edulis to achieve domi- nance over Chondrus crispus and Semibalanus balanoides FIGURE 2.8 Schematic diagram showing the vertical distribution patterns of major taxa on northwest Atlantic shores. (a) A relatively exposed shore. (b) A moderately sheltered shore. The vertical distribution is shown by the length of the arrows, while the width depicts the relative abundance or functional importance. A dashed line indicates a changing or ephemeral, seasonal pattern. (Redrawn from Vadas, R.L. and Elner, R.W., in Plant-Animal Interactions in the Marine Benthos, John, D.M. and Hawkins, S.J., Eds., Clarendon Press, Oxford, 1992, 36, 37. With permission.) 0008_frame_C02 Page 28 Monday, November 13, 2000 9:34 AM © 2001 by CRC Press LLC [...]... Figure 2. 24 plots the upper and lower limits for the dominant species on the Snares Islands shores From the plots it can be seen that there are clusters of vertical limits at a number of levels on the shore The major ones coincide with the upper limits of the bull kelp Durvillaea antarctica and the red alga Pachymenia lusoria and the lower limits of the lichens of the littoral fringe Possible causes of these... eulittoral Other large algae characteristic of the sublittoral zone are Cystophora platylobium, Sargassum sinclairii, Ecklonia radiata, and Macrocystic pyrifera 2. 2.5 SOUTH AFRICA The southern African region extends from the northern border of Namibia (17°S) to the southern border of Mozambique (21 °S) (Figure 2. 12) The overall length of the coastline is about 4,000 km The eastern coast is influenced by the. .. in the Agulhus Current ranges from 21 to 26 °C and the salinity is 35.4 Along the west coast there are regions of upwelling of cold water (8 to 14°C) On the east coast, mean monthly sea surface temperatures range from 22 °C in the winter to 27 °C in the summer, while on the south coast they range from 15 to 22 °C, respectively The entire region is subject to a simple diurnal tidal regime, with a spring-tide... seaweeds The role of light in photosynthesis and primary production will be dealt with later in the relevant sections 2. 3.5 .2 FIGURE 2. 20 Absorption of light by green, red, and brown seaweeds Green seaweeds absorb maximally in the blue and red portions of the spectrum; hence, they appear green in color The brown color of seaweeds results from absorption near the middle of the spectrum, which removes more of. .. determine the onset of breeding, or the timing of events such as spawning Radiant energy from the sun’s rays encompasses the electromagnetic spectrum from long-wave, low-energy to short-wave, high-energy rays “Light” refers to the narrow region of the spectrum visible to the human eye, plus the ultraviolet and infrared wavelengths One of the most important variables controlling plant photosynthesis is the. .. Agulhus Current, while the west- 0008_frame_C 02 Page 34 Monday, November 13, 20 00 9:34 AM 34 The Ecology of Seashores FIGURE 2. 12 Major oceanographic features around the coast of southern Africa and associated shore communities (Adapted from Branch, G.M and Branch, M.L., The Living Shores of Southern Africa, C Struik, Cape Town, 1984, 14 With permission.) ern coast is bathed by the cold Benguela Current... long-term mean Denny predicted that an increase of 1 m in yearly average significant wave height would result in a fourfold increase in the rate of patch formation in a mussel bed 2. 3.1.3 The General Effects of Wave Action The general effects of wave action can be summarized as follows: 1 A general shift of the concentration center of most of the species of the eulittoral and the littoral fringe 2 An... matter of minutes as the water reaches it 2. 3.6 DESICCATION AND ZONATION Many observations have demonstrated that desiccation effects are particularly important in setting the upper limits of the intertidal distributions of many species, such as: (1) the elevation of zones in areas of wave splash (see Figure 0008_frame_C 02 Page 46 Monday, November 13, 20 00 9:34 AM 46 The Ecology of Seashores FIGURE 2. 21... strongest 0008_frame_C 02 Page 42 Monday, November 13, 20 00 9:34 AM 42 2.3.3 The Ecology of Seashores SUBSTRATE, TOPOGRAPHY, ASPECT, ZONATION AND The nature of the substratum can influence the kinds of plants and animals that may be present on hard shores Rock surfaces may be smooth and polished or pitted and rugose This surface texture influences the settling of the larval stages of many species (see Section... variety were greater lower on the shore and at more exposed sites He suggested that the great variety of the microalgal assemblages that he found may be due to: (1) the density of Anacystis sp spores and microscopic red, green, and 0008_frame_C 02 Page 52 Monday, November 13, 20 00 9:34 AM 52 The Ecology of Seashores ᮂ FIGURE 2. 25 Seasonal patterns of vertical distribution of microalgae measured by chlorophyll . Patterns 23 2. 2 Zonation Patterns on Representative Shores 24 2. 2.1 The British Isles 24 2. 2 .2 The Northwest Atlantic Shores 27 2. 2.3 The Pacific Coast of North America 29 2. 2.4 New Zealand 31 2. 2.5. Press LLC 20 The Ecology of Seashores 2. 8 .2. 2 Factors Controlling Vertical Distribution 61 2. 8 .2. 3 Algal–Limpet Interactions 61 2. 8 .2. 4 Limpet–Barnacle Interactions 62 2.8 .2. 5 Intra- and Interspecific. 33 2. 3 The Causes of Zonation 36 2. 3.1 Wave Action and Zonation 36 2. 3.1.1 Introduction 36 2. 3.1 .2 The Problem of Defining Wave Exposure 36 2. 3.1.3 The General Effects of Wave Action 38 2. 3 .2 Tidal