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(1971) 17:137-148

137

SOME FACTORS AFFECTING ZONATION OF ROCKY SHORE ORGANISMS A T KAWERUA by B.W. Hayward *

SUMMARY The zonation patterns of the rocky shore organisms at Kawerua, on the west coast of Northland, are described. Nine transects were made at localities with varying aspect, slope and intensity of wave action. An attempt is made to assess the relative role of each of these three variables in determining zonation. INTRODUCTION "Alternating emersion and submersion are the primary causes of the zoned distribution of shore organisms. However, various modifying factors such as exposure or aspect may greatly modify these zones." (Lewis, 1964). Besides the intensity of exposure to wave action, two other physical variables aspect and slope, were studied in an attempt to determine their effect on rocky shore zonation at the Auckland University Field Club station, Kawerua (see fig. 1). Kawerua was a suitable locality for such a study with its very variable, embayed, basaltic rocky coastline. Here wave intensity grades from extremely exposed conditions at the seaward ends of long rocky headlands, to protected lagoons behind large rock masses. Kawerua is in the convergence zone between the two opposing West Coast currents and as a result is on the boundary between the warm water species of the Aupourian province and those of the intermediate Cookian biogeographic province (Morton and Miller, 1968; p. 282). The species present, and zonation, should therefore be of interest, owing to its position midway between the well documented shores around Cape Maria van Dieman in the north and Piha in the south.

FIG. 1

Location map.

* Department of Geology, University of Auckland.

138 METHOD OF STUDY Nine transects were carefully sited su that the effects of exposure, slope and aspect could be assessed independently by comparison of two or more of th?m (see fig. 2). These were: A. Sloping exposed (Slope 1 in 5 W, bearing 270°) Slightly protected by off-lying rocks and adjacent promontories; waves breaking before reaching the shore and surging well up the slope. B. Vertical, extremely exposed (Slope 8 in 1 W, bearing 230°) On the southern face of a 15 foot high promontory at the seaward end of a rocky headland; subjected to continual wave battering that explodes water high up the face. Steep slope and aspect combine to give shaded conditions most of the day. C. Vertical exposed (Slope 7 in 1 W, bearing 270°) Protected from direct wave break by large rock off seaward side, but large surging wave action may still send splash 12 feet up the face in an average sea. Exposed to wave action of similar intensity to transect A. D. E, F a n d Gare subject at all tides to the actions of waves side-on (i.e. strike of rock face parallel to axis of major water movement). D. Sloping side-on facing south (Slope 1 in 3 S, bearing 190°) Not often shaded, but the intensity of sun's rays on one spot is less than that on the north-facing transects. E. Vertical side-on facing south (Slope 6 in 1 S, bearing 195°) Shaded for much of the day. F. Vertical side-on facing north (Slope 6 in 1 N , bearing 340°) Sun's intensity greater than on D and E, but less than on G. G. Sloping side-on facing north (Slope 1 in 3 N , bearing 340°) Greatest light and heat intensity from sun's rays. H. Sloping protected (Slope 1 in 2Vz E, bearing 100°) On protected side of 10 foot high rock mass. At high tide, water surges over the top and down into quiet lagoonal waters, which at other tides arc only subject to gentle surges. I. Vertical Protected (Slope 8 in I E, bearing 100°) Subject to same wave conditions as //, but intensity of sun's rays less; shaded in the afternoon. For each transect a series of counts of the organisms within a square-loot quadrat, placed every 6" elevation from the level of mean low water spring (MLWS), t' i the top of the rocky face, was in i lo. The algae, bivalves and barnacles were recorded is percentage cover of the rock, while the numbers of other organisms were recorded from each quaJi.it (Hayward and Triggs, 1967). These figures were then converted into conventional "kite" form. The kites of all zoning organisms for each transect were then placed in planes within

139

140 block diagrams. For tigs. I and 5 the planes were divided into sloping and vertical transects and arranged in order of decieusing intensity of wave action. The side-on transect planes were arranged in order of increasing sun's intensity in tig. 6. To translate the histograms back into percentage coverage or numbers, see table I. ZONATION A f K A W E R U A (see fig. 3) The upper limit of the sublittoral fringe, which decreases from 7'6" above Ml WS in transect B to 3" above MLWS in transects H and /, is defined at this locality by the highest elevation of the dominant zoning algae, Xiphophora chondrophylla var. minor, together with the encrusting red algae, of the Corallina and 'Lithophyllum' genera. Xiphophora, however, is absent from the extremely exposed transect B. In the exposed transects, the green alga Microdictyon mutabile and the small

red algae Gymnogongrus humilis, Melanthalia abscissa, Pterocladia lucida and

Vidalia colensoi, are also found in zoning proportions. On the protected transects the brown alga Carpophyllum maschalocarpum is present within the sublittoral fringe. The limpet Patelloidea corticata Corallina is found extending into this zone from the mid-eulittoral, together with the snakeskin chiton, Sypharochiton pelliserpentis. The green mussel, Perna canaliculus attaches to the rocks in the upper portion of the sublittoral fringe on exposed transects. Off the more exposed shore, in sublittoral waters, the giant bull-kelp, Durvillea antarctica and the laminarian kelp Lessonia variegata were observed in addition to the algae of the sublittoral fringe. The eulittoral zone, defined at its upper level by the limits of barnacle colonization, has an upper limit decreasing from greater than 13'6" above MLWS on transect B to 6'3" on transect //. Protected faces and the lower portions of exposed transects are dominated by Chamaesipho columna, while the upper portions of the exposed eulittoral are dominated by the 'surf-loving' Chamaesipho brunnea. The small mussel Modiolus neozelanicus is found with the barnacles on exposed slopes in the lower eulittoral.

The two limpets Cellana ornata and Cellana radians are confined to the

eulittoral, while the limpet Notoaanea pileopsis was only recorded from the protected eulittoral zone. The cat's eye, Lunella smaragda and oyster borer Lepsiella scobina are also eulittoral dwellers found on protected transects or in cracks on the more exposed. The seasonal "gummy-weed", Splachnidium rugosum formed a belt in the lower half of the eulittoral zone on exposed faces, and another seasonal alga Apophloea sinclairii was abundant on sheltered eulittoral shores. The delicate seasonal laver, Porphyra columbina was mainly restricted to the exposed upper eulittoral. The littoral fringe, above the barnacle zone, is bare rock but for the periwinkles Melarapha oliveri and occasional Melarapha cincta.

Many other species colonize these rocks, but are in insufficient numbers to form zones as distinctive as those described.

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FIG. 3

Shore-scape showing zonation from exposed to protected on the rocky shore at Kawerua. 1. Chamaesipho brunnea 2. Porphyra columbina 3. Apophloea sinclairii 4. Modiolus neozealanicus 5. Splachnidium rugosum 6. Chamaesipho columna 7. Perna canaliculus 8. Small red algae - Gymnogongrus humilis, Melanthalia abscissa, Pterocladia lucida and Vidalia colensoi 9. Encrusting calcareous algae - 'Lithophyllum' and Corallina. 10. Xiphophora chondrophylla var. minor 11. Carpophyllum maschalocarpum 12. Microdictyon mutabilc.

FIG. 4

Comparison of histograms of zoning organisms with decreasing intensity of wave action on sloping transects.

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Algae Xiphophora chondrophylla Encrusting calc. algae Small red algae A pophloea sinclairii Carpophyllum maschalocarpum Splachnidium rugosum Porphyra columbina Microdictyon mutabilc Animals 100% Chamaesipho spp. 100% Perna canaliculus 100% Modiolus neozealanicus 100% Melarapha spp. 66% Cellana ornata 66% Cellana radians 66% Notoacmea pileopsis 66% Lunella smaragda 100% 100% 100% 50 10 10 10 10

TABLE 1:

Key to histogram conversion, giving percentage coverage or numbers of organisms per square foot quadrat, represented by one unit in figures 4, 5 and 6.

RESULTS Intensity of wave action affects the zonation of all the organisms studied (see figs. 4 and 5). All major zones were enlarged and elevated by increasing wave action. Species not found on protected transects included Microdictyon

mutabile, Melanthalia abscissa, Gymnogongrus humilis, Pterocladia lucida, Vidalia colensoi, Perna canaliculus, Modiolus neozelanicus, Splachnidium

rugosum and Porphyra columbina. Species restricted to sheltered transects

included Carpophyllum maschalocarpum, Lunella smaragda, Notoacmea pileopsis and to a greater extent Apophloea sinclairii.

The slope of a rocky substrate may affect zonation through its modification of certain physical parameters. Since it would require many chapters to consider these in full, only a few of the ways slope may affect zonation are mentioned. Water movement increases and generally becomes more turbulent with increasing slope, but not always. If the waves have not broken before reaching a vertical face they may simply slop up and down, wetting an area not much higher than the wave crests, whereas waves always break on to a sloping shore and will surge to much higher levels than the height of the wave crests. This may explain why the upper limit of Xiphophora chondrophylla and the encrusting calcareous algae is higher on the sloping transect A than the vertical C with similar intensities of wave action. This is not always the case however, as a wave that breaks on to a vertical face may explode water many feet vertically and keep a larger area, wet at low tide, thus raising the zonation levels, as in transect

B.

Slope may also affect the ability of an organism or predator to cling to steep faces. Water runoff would be more rapid from a vertical face than a sloping one and as a result may increase the rate of desiccation. The mussels Perna canaliculus and Modiolus neozelanicus were found restricted to sloping transects while Splachnidium rugosum was only recorded from vertical transects no matter what direction they faced.

147 Possibly the most significant effect of slope is its alteration of the spread of the sun's rays, producing areas of shade or with lower insolation and illumination. The sublittoral fringe was extended higher on shaded transects, reaching 4'6" above MLWS on transect E and only 1'9" above on transect /'"(see fig. 6). The tufted green alga Microdictyon mutabile and the red alga Apophloea sinclairii were restricted to only the more shaded transects. Splachnidium rugosum colonised higher on south-facing transects than north-facing and Porphyra columbina had a denser and larger zone on south-facing transects. The

gastropods Lunella smaragda, Lepsiella scobina, Cellana radians and Notoacmea

pileopsis were all higher on the shaded vertical protected transect than on the sloping one. The effects of exposure, slope and shade on the zonation of the barnacles

Chamaesipho columna and Chamaesipho brunnea can be summarised in more

detail (see fig. 7). The 'surf-loving' C. brunnea was found almost exclusively on exposed shores, being dominant in the upper half of the eulittoral. C. columna colonised all degrees of exposure and was the dominant barnacle on both protected shores and the lower half of the exposed eulittoral. The lower limit of C. columna coincides with the upper limit of the sublittoral fringe algae and is therefore higher with increased wave action or shade. The lower limit of C. brunnea appeared to be fixed at 5'6" to 6' above MLWS. The upper limit of the barnacle zone is also elevated with increasing wave action. On south-facing shores however, the upper level of barnacle colonisation is only elevated with respect to the corresponding north-facing shores on the shaded vertical faces and not on the slopes. Barnacles are more resistant to desiccation than the sublittoral algae, as it appears that only on the uppermost levels of north-facing vertical transects, where the combination of insolation and desiccation is extreme, are conditions unfavourable for the colonization by barnacles, compared with the same elevation on the shaded southern vertical faces. Barnacles generally appear to be more dense on sloping shores than vertical, this seems rather odd since on the sloping shores they would have competition for space from the mussels Perna canaliculus and Modiolus

neozelanicus.

CONCLUSIONS Kawerua, on the border between the Aupourian and Cookian biogeographic provinces, shows the effects of the converging northerly and southerly currents with the presence of species such as Xiphophora chondrophylla var. minor,

Nerita melanotragus, small numbers of Melarapha cincta, Durvillea antarctica and Lessonia variegata.

The intensity of wave action appears to be the major factor influencing zonation, other than tidal variation. Increasing water turbulence extends and elevates the zones, as well as limiting certain species to exposed or protected shores. Mussels appear to be limited to sloping substrates in this area, while dependent on the intensity of wave action. Slope also affects the elevation of zones.

148 Shade or low intensity of solar radiation also appears to affect zonation at Kawerua. The algal sublittoral fringe rises higher above MLWS on shaded south-facing shores while the eulittoral algae are more dense or even restricted to south-facing transects. Grastropods and barnacles appear to be affected by a combination of slope, shade and intensity of wave action. ACKNOWLEDGEMENTS The author wishes to thank Miss G.C. Puch for her valuable field assistance and Mr D.J. Bettesworth, Mr G.R.V. Anderson and Prof. J.E. Morton for their helpful comments on the manuscript. REFERENCES

HAYWARD, B.W., TRIGGS, C M . 1967 A comparison of the marine zonation of two rocky shores in the North Cape area. RokirokiJ: 28-34. 'The ecology of rocky shores'. English University Press, London, 321 pp. 'The New Zealand Seashore'. Collins, London, 668 pp.

LEWIS, J R .

1964

MORTON, J.E., MILLER, M.C.

1968

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