Structure of a rocky intertidal community in New South Wales: Patterns of vertical distribution and seasonal changes

J. exp. tnar. Biol. Ecol., 1981, Vol: 51, pp. 57-85

0 Elsevier/North-Holland

Biomedical Press

STRUCTURE OF A ROCKY INTERTIDAL COMMUNITY IN NEW SOUTH WALES: PATTERNS OF VERTICAL DISTRIBUTION AND SEASONAL CHANGES

A. J. UNDERWOOD Department of Zoology, School of Biological Sciences, Australia

University of Sydney, Sydney N.S. W. 2006,

Abstract: Patterns of vertical distribution of common intertidal animals and plants were sampled in transects and groups of replicated quadrats on a sandstone rock-platform (Green Point, New South Wales) from October 1972 to October 1976. Zones corresponding to those described in previous qualitative studies were consistent throughout the study. The bottom of the shore was dominated by 100% cover of foliose macroalgae and there were few animals present. Mid-shore levels were dominated by grazing molluscs, sessile animals (notably barnacles and tubeworms) and/or encrusting algae. At the upper levels of the shore was a zone of littorine gastropods of three species. In mid-shore areas, foliose algae were sparse except in pools and were positively correlated with the abundance of sessile animals. The upper limits of vertical distribution of dense cover of foliose algae, the height of peak abundance of mid-shore grazers and the upper limits of these grazers were at higher levels on the shore where exposure to wave-action was greater. There was considerable patchiness in the occupancy of primary substratum from one part of the shore to another, and no clear trends of diversity of species with the gradient of exposure to wave-action were evident. There were, however, clear seasonal trends in the vertical distributions of some algae, which extended to higher levels on the shore during colder months than during the summer. In addition, some species of algae were only present during some seasons of the year, and others showed marked seasonal variability in frequency of occurrence in quadrats. These observations’are discussed with respect to known aspects of the ecology of some of the organisms, and provide a background for experimental tests of some hypotheses raised about the structure of this community.

INTRODUCTION

Despite the enormous literature concerning the zonation of intertidal animals and algae (e.g. Lewis, 1964; Ricketts et al., 1968; Stephenson 8z Stephenson, 1972) there are few quantitative studies conducted over a long period. In eastern Australia, this is particularly true. There have been numerous detailed descriptions of patterns of zonation, the most important of which are those by Dakin et al. (1948), Endean et al. (1956), and Dakin (1969). These qualitative studies provide a useful framework of reference in terms of major zones, dominant organisms at different levels on the shore, geographical changes of fauna, etc., but do not record quantitatively the nature of organization of intertidal communities, nor do they indicate any dynamic processes which occur. Elsewhere, there have been detailed experimental analyses of the interactive nature of organization of intertidal communities (see particularly Dayton, 1971; Paine, 1974; Menge, 1976; Lubchenco & Menge 1978). 57

58

A.J. UNDERWOOD

An understanding of the processes affecting patterns of distribution and abundance of intertidal organisms can only be gained by experimental manipulations to test hypotheses derived from well-documented observations (see particularly Connell, 1974). This background of observattis is sparse in published accounts of seashores in eastern Australia. The present paper is an account of the patterns of vertical distributions of animals and algae along a gradient of exposure to wave-action on a rock-platform at Green Point (Broken Bay, New South Wales), and the seasonal changes in these patterns during 1972-1976. It is intended that this, and subsequent publications (in prep.) on the seasonal patterns of abundance of major organisms will provide an observational basis for the more detailed experimental work already completed (Underwood, 1976, 1978a; Denley & Underwood, 1979) and currently in progress to evaluate recently proposed general accounts of the structure of intertidal communities (e.g. Dayton, 1971; Paine, 1974; Connell, 1975; Menge, 1976). Eventually, such a study could be used as the background for an investigation of the long-term changes of structure of communities as proposed by Lewis (1976). In any study of this nature, some compromises have to be made within the constraints of time and effort (see discussion by Lewis, 1976). For example, with a particular method of sampling, less information can be gained about rare species of algae than about very abundant ones. The strategy adopted here was to yse a simple method of quadrat sampling, which would enable sufficient data to be gathered under all weather conditions. Under most circumstances, this gains little detailed information on any particular species. In the present paper, only general trends of distribution will be discussed. Nevertheless, this represents the first long-term quantitative account of the patterns of distribution of intertidal species on a rock-platform in eastern Australia. MATERIALSAND METHODS AREASTUDIED The Green Point rock-platform on the northern side of Broken Bay (N.S.W.) is about 30 km due north of Sydney. It was chosen for this study because the shore is typical of the range of sandstone coastal rock-platforms in the area, in terms of fauna and algae, and because the sheltering effect of Lion Island (see Fig. 1) causes a gradient of increasing exposure to wave-action from south to north along the platform. Wave-exposure was not quantified on an absolute scale during this study, but the relative forces of wave-action during different stages of the tide and at different times of the year were calculated from the width of white splash on photographs taken from the top of a cliff above the platform during 1972-1973. The rank order of these observations is in Table I. Except during periods of extremely rough southerly weather, the southern end of the platform experiences

A

_

BROKEN BAY

-

Fig.

HEAD

TRUE

Kibmetrcr

d

0

NQRTH

PACIFIC OCEAN

_..

1. Maps of the site studied.

BARRENJOEY

_ ._.

LION ISLAND

0 Kibmetres

0.2

72 GREEN POINT

A. J. UNDERWOOD

60

very little to no wave-action, with small waves gently rolling over the shore until the tide drops below the seaward edge of the platform. More northerly parts of the TABLE I Transects sampled along the shore:

in rank order of wave-action were equal.

1 is most exposed;

Height Transect ll0.

1 2 3 4 5 6

Transects

4 and 5

(m above I.L.W.S.)

Rank order of wave-action

Length (m)

Top

Bottom

Range

3

14 37.5 30 40 29 43

3.83 4.73 3.69 3.93 3.93 2.36

1.04 -0.90 0.56 0.02 0.49 0.33

2.79 5.63 3.13 3.91 3.44 2.03

1 2 4= 4= 6

shore experience increasing wave-height, and presumably wave-shock, at all stages of the tide and times of the year, except when the sea is very calm. METHODS

OF SAMPLING

Quadrat sampling was done in two different ways, using transects and replicated quadrats at particular heights. Six transects were surveyed perpendicular to the water. These were from the lowest level reached by the tide on the day of sampling to an arbitrarily chosen point just above the highest marine organism (usually Littorina unifasciata Gray). The top position of each transect was used for all subsequent times of sampling (and represents 0 m horizontal distance in Tables and Figures). Heights on the shore were surveyed using a dumpy level and a Survey Benchmark on the shore (Gosford-Kincumber Sewerage Survey Datum). All heights are presented here relative to chart datum, which is detined as Indian Low Water Springs (I.L.W.S.) at Fort Denison (Sydney). On each occasion, the entire transects were sampled using contiguous 0.25m2 quadrats. The total numbers of most species of gastropods, barnacles, and starfish were recorded, with percentage cover of algae and sessile animals such as the mussel Trichomya hirsuta (Lamarck) and the bryozoan Watersipora cucullata (Busk). Percentage cover was estimated either from a grid of a hundred points regularly arranged across the quadrat, or using a scale of ranges (+ = O-l%; 1 = l-IO%; 2 = l&25%; 3 = 25-50x; 4 = N-75%; 5 = 755lOOo/,). The latter scale was always independently determined by at least two observers. When discrepancies arose, the percentage was estimated directly from the grid, or assessed again by the observers. For all but the rarest species, the scale proved accurate when compared against readings from the grid. This was tested several times throughout the study.

INTERTIDAL COMMUNITY

STRUCTURE

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61

Each quadrat was then thoroughly searched for species of algae which may have been missed before. This method of sampling does not distinguish completely between primary and secondary or “understorey” space (see Dayton, 1971, 1975; Menge, 1976) because individual holdfasts of some erect algae could not be sampled without destroying the algal beds. For many foliose algae, for example C~railina spp., there is no primary space between holdfasts in a dense clump, and the cover of secondary space is equal to that of primary space. For other species with long thin thalli (e.g. Sargassum spp.) the cover of secondary space was estimated, and the thalli pulled aside to measure cover of species underneath. A complete list of those species which occupied only primary space (that is encrusting algae and sessile animals), where secondary and primary space are effectively the same (e.g. Coraflina spp.) and which were sampled as secondary space (e.g. Sargass~~) is given in Table Il. On some occasions, in between main periods of sampling, the percentage TABLEII Categorization of algal species in samples. A. Encrusting: sampled as primary space Abundant ~ifdeizbrandiff profot~pus Nardo

B. Foliose: sampled as primary and secondary space Abundant Corullina oj~icinalis l.. Gelidium pusilfum (Stack.) Le Jol. Grucifariu lichenoides (L. ex Turn.) Harv. Luurencia pjnnat~~~d~Lamour.

C. Foiiose: sampled as secondary space Abundant Cofpomenia sinuosa (Roth.) Derb. & Sol. Hormosira banksii (Turn.) Decne. Ulva lactuca Linn. Wrangelia plumosa Harv.

Common eo{fiuF?~fucasii Setchell Litllop~~~lf~rn sp. Phil. Peyssoneli<~gunnicrnu J. Ag.

Common Enteromorpha sp. Link. Pacfina,fraseri (Grev.) J. Ag. Zonaria Iurneriana J. Ag.

Common Fikdmentous reds Ectocarpus sp. Lyngb. fleafascia (Muell) Fries. Pterocladia capillacea (Gmel.) Born. & Thur. Sqassum spp. (Decne.) D. T.

Restricted common Nemafion rn~~l~~~durn (Weber & Mohr) J. Ag. Porphyru umbilicalis Lucas

cover of different organisms occupying primary substratum was estimated using the grid of points in quadrats in the usual transects and elsewhere, In addition to these samples, some smaller animals (e.g. ~~~~~r~~~ spp.) were too numerous to count in 0,2.5-m* quadrats. These were always counted in 25-cm? quadrats placed at five random points inside the larger quadrats.

62

A. J. UNDERWOOD

Intertidal communities usually contain a very large number of species of animals and plants. In the present study, decisions had to be made about the design of sampling with respect to the time available during low tide, and the information which could be acquired about various organisms. As in other studies (e.g., Paine, 1974; Underwood, 1978b) it was decided to concentrate on common species, common either in terms of percentage cover or density, and those which could be reliably identified and quantified in quadrats. Some organisms were not sampled because of rarity, very rapid mobility (i.e. they left the quadrats before they could be counted) or because of great taxonomic uncertainty (so that no reliable identification of each specimen was possible in the field). These include amphipods (too mobile), crabs (too scarce) and sponges (scarce in most parts of the area studied and because of great taxonomic uncertainty). No attempt was made to sample encrusting organisms on the thalli or holdfasts of algae. Several small epiphytic species of red algae (e.g. of the genera Ceramium, Polysiphonia, and Centroceras) were indistinguishable in the field without examination by a hand-lens. This proved too time-consuming for the present sampling programme and such species were not estimated separately and were lumped as filamentous algae. Furthermore in this study, rock-pools were not sampled. The organisms in deep pools, particularly algae, are often different from those on surrounding surfaces (the “emergent substrata” of Lubchenco & Menge, 1978) and many pools contain species commonly found at lower levels on the shore (see Lewis, 1964). Organisms in pools are not, of course, emersed during low tide, and pools do not represent an intertidal habitat. The presence of a pool in a transect can cause major irregularities in the data on vertical distribution of a species, because, for example, a quadrat covering a pool at high levels on the shore may contain species which are not encountered again’until much lower levels are reached. The vertical distribution of those species would then appear to extend to high levels simply because of the presence of a pool. Pools were found scattered at random on the shore studied, and the true pattern of vertical distribution of species would not be sampled if pools were included. Whenever a pool deeper than 20 cm was encountered, quadrats were moved sideways from the line of the transect to avoid it. Initial observations on the rock-platform indicated that there were different groups of organisms at different levels on the shore. These could be broadly categorized as a high-shore animal community, a mid-shore mixed community, and a low-shore algal community, as illustrated in Fig. 2. It was originally intended to sample each of these three major groups with a series of replicates in each transect at two levels, but rough weather and the concomitant dangers of the lower shore made this impossible for all times of the year. Consequently, only the two levels at each of the upper two groups were continued throughout the study (see Fig. 2 and Table III). For the uppermost group of organisms, the upper level chosen was near the horizontal centre (Level l), and the lower level (Level 2) towards the bottom of the distribution of the community. For the mid-shore community, the

INTERTIDAL

COMMUNITY

STRUCTURE

IN NEW SOUTH

63

WALES

upper level (Level 3) was near the top, and the lower level (Level 4) was near the horizontal centre of the distribution of the group of species. These levels are illustrated in Fig. 2 and the details are in Table III. In Transect 1, the very steep slope of the shore prevented sampling of any level lower than the third (Table III), as the change in height across a 50 cm wide quadrat was great. TABLE

Levels sampled

with replicated

III

quadrats on each transect: height (H; m above distance from top of transect (D; m).

I.L.W.S.)

and horizontal

Level 1 Transect no.

4 ______~

3

2

H

D

H

D

H

D

I

2.50

4

1.84

II.5

1.04

13.5

2 3 4 5 6

3.59 2.56 2.55 2.52 1.99

2 4 10 IO 6.5

1.82 1.73 2.37 2.07 1.93

6 7.5 18.5 16.5 8.5

1.45 0.48 1.52 1.40 1.14

12 11 21 21 15

H

D

_ 1.26 0.64 I.42 1.30 0.90

17 16 24 23 37

At each of these levels, a series of randomly placed, replicate quadrats was sampled within 3 m on either side of the transects (three quadrats during the first year of sampling and five afterwards). These quadrats gave replicated data on abundances of animals and plants (which will be described elsewhere) and provided additional data on uncommon species to those gained from the permanent transects. All organisms were identified using the reference collections of the Australian Museum and the National Herbarium, Sydney. The taxonomic status of many intertidal animals and plants in Australia is uncertain, and considerable help was gained from the local experts (see Acknowledgements).

RESULTS

GENERAL

PATTERNS

OF ZONATION

In general, the organisms on the shore were distributed as three major groups. At the top of the shore was a zone predominantly occupied by animals, notably the littorinid snails Littorina unifasciata, L. acutispira Smith, and Nodilittorina pyramid&s (Quoy and Gaimard). This corresponds to the littorinid zone of Dakin et al. (1948). Much of the substratum towards the very top of the shore was covered at all times by a thin film of the maritime lichen Calothrix crustacea (?) Schousb. & Thur. (see also Womersley & Edmonds, 1958). At the bottom of this upper zone were the barnacles Chthamalus antennatus (Darwin) and Chamaesipho columna

A. J. UNDERWOOD

64

(Spengler), which extended down into the mid-shore zone. This is illustrated for one of the transects in Fig. 2, where the upper zone extends to about 19 m horizontally from the top of the shore. The only algae found in this upper zone were the blue-green Rivularia australis

4On

LEVEL1

LEVEL 2

HORIZONTAL DISTANCE (M)

Ncdllittorina pyramidalis I Littorina untfasciata Ltttorina acutispira ~tha~lus antennatus Chamaesipho columna Tesseropora rosea

-

I8 -m-

Cellana tramosenca Montfortula rugosa Bembmmm nanum Nerda atramentosa Austrocochlea constrrcta Dicathais orbita Morula marginalba Galeolaria caesprtoss Subninetla undulata Patirietla calcar Siphonda denhculata Chdon septentnones Megabalanus mgrescens Waterslpora cucullata

I m

Htldenbrandia prototypus Rivulana australis Caloglossa adnata Lit~yllum sp. Ilea fascia Corallina officinaks Hormosua banks.11 Ulva lactuca Colpomenia sinuosa Gekdium puslllum Wrangelia plumosa Laurencla pinnatiftda Codwm lucaw Sargassum sp. Pteroctadta copillacea Dictyota dichotoma Rodymenia australis Zonarla turneriana Gracilana kchenoldes Filamentous red algae Gracllarla secundata

-m

Fig. 2. Profile of Transect 4 during October 1972 showing ranges of distribution of common species of algae and animals: arrows indicate the heights chosen for replicate quadrat samples; dashed fines indicate the approximate heights of mean high and low water of Spring and Neap tides, as predicted from tide tables for the Sydney region; generally, the levels of high tide were observed to be higher than those predicted in tables (b-y z 0.5 m). except during periods of very calm weather.

INTERTIDAL

COMMUNITY

STRUCTURE

IN NEW SOUTH WALES

65

Engl. and the red Culuglossa adnata Zan. which were found only amongst barnacles, and usually occupied l-2% and rarely up to 5% of the substratum. On Transect 4 during October 1972 (Fig. 2), these algae extended only to 20 m horizontal, but on other transects and at other times, they were found as high on the shore as the barnacles. Occasional tiny patches (much less than 1% of the area) of several unidentified blue-green algae were found on various transects. Below this region was the mid-shore mixed community of animals and algae (extending to 30 m horizontal in the transect illustrated in Fig. 2). Much primary space in this region was occupied by encrusting macroalgae (predominantly Hildenbrandia prototypus) and sessile animals, notably barnacles and the serpulid tube-worm Galeolaria cuespitosa (Lamarck). Small areas of dry rock were covered by mats of the foliose red alga Gelidium pusillum, but the only other foliose algae in this region were patches of Corullina officinalis, occasional scattered thalli of Zlea fascia, which was seasonal (see below), and a number of species in shallow pools (< 5-10 cm deep). Notable among the latter was the brown Hormosiru banksii which was abundant in the more sheltered transects (5 and 6) and mostly found in and at the edges of shallow pools. Throughout this zone, there was always some bare rock, and many grazing animals (gastropods, chitons, and the starfish Patiriella exigua (Lamarck)) were present on bare rock and on the encrusting algae. Where there were foliose macroalgae in shallow pools, there were also macroalgal grazers such as the turbinid snail Subninella undulata (Solander) and the starfish Patiriella calcar (Lamarck) which was most often found in pools where there was Corallina. In this zone but patchily distributed was the predatory whelk Morula marginalba Blainville, found feeding on barnacles, or aggregated in crevices and depressions. At the bottom of the shore, most space was taken over by a profusion of foliose macroalgae (Fig. 2). Few animals were present in these lower regions, and the only species found regularly were the macroalgal grazers (Patiriella calcar, Subninella undulata, the chiton Plaxiphora paeteliana (Thiele), some fissurellid limpets Montfortula rugosu (Quoy and Gaimard) although these occurred in greater densities in shallow pools at higher levels), the barnacle Megabalanus nigrescens (Lamarck) and the whelk Dicathais orbita (Gmelin). There were some obvious departures from this general pattern along the shore. For example, in some areas, the mussel Trichomya hirsuta occurred in dense patches in the mid-shore zone. On Transects 2 and 5, at the very bottom of the shore were stands of the ascidian Pyura praeputialis (Heller), but these were not well-developed on this rock-platform. VERTICAL

DISTRIBUTION

OF MAJOR GROUPS OF ORGANISMS

The general pattern described above can be much better perceived by examination of the vertical distribution of the major groups of organisms. To do this, the densities of Littorina unifasciata and the mid-shore microalgal grazers (which

TRANSECT

-60

2

***P . . . . . . . . .O..* .u*

P...

a.

-20 -20 *...

z

**.. I%.-..

3.0-3.2

TRANSECT

40-4.2

4

--9--9---L 80 5

4iL-4L 1.0-1.2

--_-_-_i-a-*

80

1

0-l

o-0.2

\

I

(

\

\

‘W.. ‘.O. . ..^

3.0-3.2

.A

TRANSECT

PO 5

)

T-4 , ;...*- \ Y I

1.0-1.2

-80

J lRANSECT

6 -60

HEIGHT

ON

SHORE

(M ABOVE

LMS.1

Fig. 3. Vertical distribution of major groups of organisms: the presence of sessile animals is indicated by bars above each graph; for other groups of organisms, the number or percentage cover in each interval of height in each transect is shown as the mean of all quadrats in that transect that were within that height-interval; n , percentage cover of foliose macroalgae; left density axis is for mid-shore microalgal grazers per 0.25-m2 quadrats (0); right density axis is for Littorirza unz$zsciata per 125 cm2 (0).

INTERTIDAL

COMMUNITY

STRUCTURE

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WALES

67

comprised mostly the gastropods Austrocochlea constricta (Lamarck), Bembicium nanum (Lamarck), Nerita atramentosa Reeve, Cellana tramoserica (Sowerby), the small starfish Patririella exigua and the polyplacophoran Chiton septentriones (Ashby)) were plotted against the height of each quadrat in each transect. The percentage cover of foliose algae (i.e. not including encrusting algae) was also plotted. Several striking patterns emerge, as illustrated in Fig. 3. At the bottom of the shore in all transects, there was lOOo/;,cover of foliose algae, and there were no microalgal grazers. At higher levels, the cover of algae declined, and the density of microalgal grazers rose rapidly to a maximum. Although the abundance of sessile animals is not indicated in Fig. 3, they were present mostly above the level where the cover of algae declined. At increasingly higher levels on the shore, the density of most species of grazers declined, and the density of Littorina un(fksciata rose to a peak and then fell again towards the highest level of the shore. This pattern was repeated in all transects, with one major exception at low levels in Transect 3 (Fig. 3) where there was a discontinuity of cover of foliose algae at about 0.2-0.8 m above I.L.W.S. In this area, the substratum was divided by channels with algae on the sides, and loose sand on the base. As the tide rose across this area, the sand was scoured across the ridges between the channels, and this apparently abraded away many foliose algae, reducing the algal cover in the quadrats sampled. On these ridges were a number of gastropods, mostly the limpet Cellana tramoserica which may have contributed to the reduced cover of algae. At slightly higher levels (1.0 m above I.L.W.S., Fig. 3), the channels ended, there was no more sand, and the algae again occupied virtually all the space. Thus, in all transects, there was a clear pattern of algal dominance at the bottom end of the shore, with a major peak of density of grazing animals above this level. There was a clear trend for the distribution of grazers and of sessile animals to be negatively correlated with high percentage cover of algae. Finally, there was a general upward trend of distribution of algae, grazers, and Littorina unifasciata with increasing exposure to wave-action (Fig. 3; note relative exposure of transects in Table I). Of these trends, the rank order of upper limit of dense cover of foliose macroalgae, the height of greatest abundance of mid-shore grazers, and the upper limit of grazers were significantly positively correlated with the rank order of exposure to wave-action (coefficients of concordance, 1, P < 0.01; 0.87, P < 0.01; 0.73, P < 0.05, respectively). The trend for Littorina was not significant, but the winkles extended to higher levels at the more wave-exposed parts of the platform. These general patterns of vertical distribution did not alter throughout the study, and showed no seasonal changes. This is illustrated for some of the transects in Table IV, where the heights of important boundaries (as seen in Fig. 3 and discussed above) are given for each of the major groups of organisms (foliose algae, microalgal grazing gastropods, sessile animals, and Littorina unifasciata). Although there are fluctuations in abundance of many of these organisms, the heights are fairly uniform

TABLEIV

1.5 1

Littorina unifasciata Lowest level found No. per 125 cm2

4.3 4

2.2 4

Highest level found % cover

Highest level found No. per 125 cm2

1.3 I

Sessile animals Lowest level found % cover

2.2 37

1.8 44

Highest level found No. per 0.25 m2

Height of greatest density No. per 125 cm*

1.7 79

1.1 1

Grazers Lowest level found No. per 0.25 m2

Height of greatest density No. per 0.25 m*

2.2 5

1.3 100

Highest level found y0 cover

Foliose macroalgae Highest level where cover > 90% % cover

72

Oct.

4.4 13

2.2 45

1.6 1

2.2 I

1.2 1

1.8 50

1.8 50

1.1 3

2.2 1

1.2 100

Jan. 73

4.4 17

2.5 62

1.8 35

I

2.5

1.3 5

1.8 3

1.6 50

1.1 10

2.2 5

1.3 100

Apr. 73

Transect 2

4.6 2

2.2 76

1.6 40

2.2 6

1.2 4

1.8 4

1.6 66

1.1 11

2.0 5

1.4 90

Jul. 73

3.6 15

2.5 35

2.0 7

2.5 2

0.9 1

2.5 I1

1.5 115

0.9 I

2.4 10

0.9 100

3.6 12

2.5 87

1.9 2

2.4
0.9 2

2.5 7

1.4 58

0.9 2

2.4
0.9 100

3.6 12

2.5 83

1.9 IS

2.5 1

13

1.0

2.6 4

1.4 38

5

1.0

2.3 10

90

1.0

Transect 4 -______ Jan. Apr. Oct.

3.6 I1

2.5 72

2.0 I

2.4 15

0.9 3

2.5 1

1.4 40

0.9 4

2.4 15

1.0 95

Jul.

3.0 2

2.5 45

,I.7 24

2.5 1

1.2 54

1.9 1.5

1.5 117

1.2 39

1.9 5

0.9 100

-Oct.

3.0 3

2.4 99

1.5 15

2.5 1

3.0 I

2.2 78

1.5 4

2.4 3

1.2 32

I 1.0 41

2.0

1

1.5 90

1.2 4

1.9 20

0.8 100

Apr.

2.1

1.5 137

1.2 8

1.9 10

I.0 100

Jan.

Transect 5

2.9 19

2.4 71

I.5 2

2.4 3

1.2 42

1.9 14

1.4 76

1.2 13

2.1 10

0.9 90

Jul.

2.2 26

2.1 35

1.9 2

1.9 4

0.6 2

1.9 4

1.2 147

0.9 4

1.2 7

0.8 95

Oct.

2.4 1

2‘0 35

1.9 32

1.9 2

0.8 I

1.9 3

1.2 135

0.8 2

1.2 1

0.6 100

Jan.

2.4 2

2.1 42

1.9 7

2.0 3

0.7 3

1.9 2

1.2 88

0.9 3

1.2 5

0.8 95

Apr.

Transect 6

2.4 3

2.1 28

1.9 II

1.9 10

0.8 5

2.0 5

1.4 114

0.9 3

1.2 10

0.7 100

Jul.

Vertical distribution of major groups of organisms (as in Fig. 3) in different seasons during 1972-1973: height (m above I.L.W.S.) for each quadrat, and the density or percentage cover of the organisms in that quadrat are presented.

INTERTIDAL

COMMUNITY

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69

during different seasons of the year. The other transects and the data for other years show the same consistency. The general pattern of vertical distribution illustrated in Fig. 3 was typical for the shore at any time during the four years of the study. PATCHINESS

IN THE OCCUPATION

OF PRIMARY

SPACE

The general trends of vertical distribution discussed above tend to mask the spatial variability of horizontal distribution of major groups of organisms. To illustrate this, data from nine extra transects were collected during two days in June 1976. These were grouped as three sets of transects, the first set near Transect 2, the second near Transect 4, and the third near Transect 6. In each group, the tops of 3 transects were spaced 10 m apart, and each transect was divided, after surveying with a dumpy level, into one-tenth vertical intervals from the top of the Littorina zone, to the lowest level reached by the tide on the day of sampling. This was done to avoid having transects of different lengths, and with different ranges of absolute height to be sampled. On and adjacent to each of these nine transects, the position of each height interval was found by levelling, and marked. Two more points at the same height, but separated by 50 cm, were found within 2 m horizontally from the first point. These were also marked. Quadrats were centred on each of these three points on each transect at each level. These provided replicate estimates of the percentage cover of sessile animals, encrusting algae and foliose algae occupying primary substratum. The means of the three quadrats are shown in Fig. 4, where the amount of bare space is 100% minus the covers of organisms shown. The same general pattern of vertical distribution as discussed previously is evident, but there were considerable differences among the transects within each group. On all transects, there was no bare space at the bottom of the shore and all the space was taken over by the hold-fasts or densely-packed thaili of foliose algae (mostly Corallina, Pterocladia, Gracilaria, and Sargassum). Somewhat higher on the shore the foliose algae declined in abundance, and space was occupied by a mixture of sessile animals and encrusting algae (notably Hildenbrandia, but with some coralline species in small patches). The only foliose algae were found amongst and around the sessile animals or in shallow poo f s. There was a highly significant positive correlation between the cover of sessile animals and the cover of these algae, which mostly consisted of Rivularia austraiis, Calogiossa adnata, Gelidium pusillum and mixtures of small amounts of Ulva and several stunted species of red algae (correlation between cover of sessile animals and cover of foliose algae above the lower limit of sessile animals, r = 0.78, n = 36, P < 0.001). The only foliose algae at these levels not amongst sessile animals were Z&rmosira banksii and Ilea fascia. There was considerable amounts of bare space at these mid-shore levels (Fig. 4) and no constancy in the mixture of sessile animals

A. J. UNDERWOOD

70

and encrusting algae. For example, consider Transects A in Fig. 4; there were no encrusting algae in Transect Al (left), few in A2 (middle), yet nearly 100% cover of Hifdenbrandia in Transect A3 (right, Fig. 4A). Similarly, at the other end of the shore, there was marked variability in the cover of the barnacle Chamaesipho columna in transects only 10 m apart (Transects C, sessile animals, Fig. 4 and Table V). -

FOLIO.% ALGAE

c--o ENCRUSTING ALGAE

m--m SESSILE ANIMALS

100 80 60

B

40 20 0

C

--ii--i

2

4

6

8

HEIGHT INTERVAL (1 = bottom

10 of shore)

Fig. 4. Percentage occupancy of primary substratum in nine transects: A, at the exposed end of the shore, near Transect 2; B, near Transect 4; C, at the sheltered end of the shore near Transect 6; in each area, the three transects sampled are shown as 1 to 3 from left to right: the means of three adjacent quadrats on each transect at each level are plotted; SE’S pooled from all transects and levels (variances homogeneous, Bartlett’s tests, P >0.20)were not plotted, for convenience, but for foliose algae, encrusting algae and sessile animals were 11.8”/,, 8.6%, and 6.0:<, respectively; height intervals were one-tenth of the vertical range of each transect (see text for further details).

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71

72

A. J. UNDERWOOD

Although there were major variations in the occupancy of mid-shore levels from transect to transect, there were some consistent trends in the composition of the sessile animals (see Table V). On Transects Cl to C3, towards the sheltered end of the shore (near Transect 6) the majority (% 95%) of the cover of sessile animals consisted of the small barnacle Chamaesipho cofumna, although in rock-pools this was not true, and much of the space was covered by the mussel Trichomya hirsuta. As discussed previously, however, pools were not sampled during this study. Towards the middle of the platform (Transects B in Fig. 4 and Table V) the major sessile animals were the barnacles Tesseropora rosea (Krauss) (Levels 3 or 4 up to 7 or 8) with the tube-worm Galeolaria caespitosa at the lower levels on the shore (Levels 2-4). Finally, at the most wave-exposed end of the platform, near Transect 2 (Transects Al to A3 in Fig. 4 and Table V), the major species was Tesseropora rosea, with Galeolaria at lower levels and the barnacle Chthamalus antennatus dominating at higher levels (7-8). Although all these species were present on all transects, this shift of dominance with increasing exposure to wave-action from Chamaesipho to Tesseropora at mid-shore levels, and increasing importance of Galeolaria at low levels and Chthamalus at high levels is fairly typical for rock-platforms in this part of New South Wales (see Dakin et al., 1948). The patchiness of occupancy of mid-shore levels is obvious in Table V. Note that major differences occur in the cover of different species in transects only 10 m apart (e.g. the % cover of Chamaesipho columna in Levels 3-6 of Transects Cl-C3; of Tesseropora rosea in Levels 4-7 in Transects Al-A3 and of Chthamalus antennatus in Levels 68 of Transects Al-A3 in Table V). This scale of patchiness was a notable feature of the shore during all seasons and all years of the study, and the data in Table V are typical of those from other seasons and years. SEASONAL

CHANGES

IN DISTRIBUTION

OF ALGAE

The species of algae sampled during this study fall into four categories of abundance. Abundant algae, e.g. Ulva lactucu (see Table II), were found throughout the transects, and occupied considerable amounts of space on the shore ( > 30% of many quadrats and > 50% of some areas). A detailed analysis of seasonal changes in abundance of these species will be described elsewhere, but seasonal fluctuations in the patterns of vertical distribution are discussed below. The second group are here defined as common (see Table II), and were present in at least 3% of all quadrats sampled on the lower levels of the shore, but never covered more than 5-10x of the majority of these quadrats. The number of quadrats in which these species occurred is discussed below, and no further analysis of their abundance is warranted from these data. A third group of algae was also common, but restricted to certain parts of the shore. Thus, Porphyra umbilicalis was found in x 25% of quadrats in four sampling

INTERTIDAL

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73

WALES

areas, but was virtually absent from other areas (see below, Table VII). Seasonal patterns of these restricted common algae are discussed below, and the species in this group are listed in Table II. Finally, there were numerous rare algae, which occurred in only a few quadrats during the study and/or were found to occupy minute percentage cover of the shore. These include ephemeral species such as the green algae Cludophora spp. Kuetz. and Chaetomorpha spp. Kuetz. and a number of perennial red algae (e.g. Rhodymenia spp. Harv.) and the brown algae (e.g. Dict~ota dichotoma Lenorm). These occurred in such small quantities in the areas sampled that no analysis of their distribution or abundance is attempted. It should be noted, however, that they were not necessarily rare on the shore as a whole, but were rare in the areas sampled. For example, several species of Ralfsia were present in some parts of the shore after 1975, but not in the sampling areas of this study. FREQUENCY

OF OCCURRENCE

IN QUADRATS

The seasonal changes in abundance of common algae is examined by comparison of the frequency of occurrence of these species in the quadrats sampled in the replicated quadrats in Levels 3 and 4 of Transects 2-6. Transect 1 was excluded because the lower levels did not properly sample the main algal zone (see p. 63) and the higher levels (1 and 2) were not included because few foliose macroalgae were present in these areas. The number of quadrats in which each species was found, and the total number of quadrats sampled in each season were pooled for the years 1972-1976 (Table VI). TABLE VI

Seasonal

occurrence

of common algae: in this and subsequent Tables ** P < 0.01; *** P < 0.001; ns is non-significant,

SeZI%Xl Months sampled

No. quadrats sampled No. quadrats alga present: Filamentous reds Lithophyllum sp. Ectocarpus sp. llea fbseia Wrung& plumosu Codium lucasii Lauren& pinnatfida Colpomenia sinuosa Sargassum sp. Enrrromorpha sp. Pqmonelia gunniana Zonaria rurnrriana Pad&a fraseri Ptrrocladiu capillmeu

Spring Oct. 72. 73. 14. 15

I80 55 63 44 56 20 25 13 51 14 11 IO 14 5 6

Summer Jan. 73, 14. 75. 16

200 94 90 41 23 27 38 21 17 2s 31 12 I4 IO

Autumn Apr. 73. 75, May 76

* denotes significance P > 0.05. Winter Jul. 73. Jun. 74, Aug. 75.16

Total

140

190

710

18 60 50 31 42 I8 26 6 20

63 74 44 42 37 36 36 16 I8

8 x I0 6

13

290 281 I19 152 126 117 96 90 77 56 43 41 30 21

5 5 14

P < 0.05;

2 3 d.f.

28.5*** 4.4 ns 11.0* 2l.8*** 22.1-q 4.0 ns 15.5** 55.0*** 4.4 “S 55.1*** 0.3 ns 5.3 ns 5.4 ns 12.8;’

A. J. UNDERWOOD

74

It is apparent that a number of species show a decline in occurrence during the spring and/or summer and an increase during the autumn and/or winter (Pterocladia, Wrangelia, filamentous red algae, Laurencia, Ectocarpus; see Table VI). These algae, with the exception of Ectocarpus, all showed signs of physical damage, such as severe bleaching or complete desiccation, during the summer months. Other algae, however, showed an increase in occurrence in spring followed by a decline throughout summer (Ilea) or by the end of summer (Enteromorpha, Colpomenia; Table VI). Except for Enteromorpha, these species showed severe bleaching and drying out during low tide periods in summer. The remaining common species showed no evidence of seasonal variation in occurrence (Table VI), but without exception all were recorded in quadrats adjacent to pools or were in small water-filled depressions in areas with drainage of water during low tide. It seems that the physical conditions prevailing during the summer are an important determinant of the prevalence of many algae on the shore. TABLE VII Seasonal Season

occurrence

of restricted

Summer

Spring

common

Autumn

Winter

Porphyra umbilicalis Locations sampled No. quadrats sampled No. quadrats present

Transect 72 46

2,

Level 3; 80 0

Transect 55 0

3,

Nemalion multifidum Locations sampled No. quadrats sampled No. quadrats present

Transect 54 0

1, Level 2;

Transect 40 0

3,

Codium decorticatum Locations sampled No. quadrats sampled No. quadrats present

Transect 18 8

3,

60 39

Level 4 20 14

10 4

algae. Total

2 3 d.f.

Levels 2,3; 75 24

Transect 282 70

5, Level 3

Level 3; 55 0

Transect 209 39

5, Level 3

15 9

63 35

105.5***

119.1***

3.7 ns

Two of the restricted common species showed striking patterns of seasonal distribution. Porphyra appeared in many quadrats towards the top of the main zone of algae, and in some areas above this level (e.g. at Level 2, Transect 3) during winter and spring, and then completely disappeared during summer (Table VII). During the summer, the plants faded from their original deep red to an olive-brown colour and were finally bleached white before they disappeared. Nemalion, in complete contrast, appeared in some areas only during the summer (Table VII).

INTERTIDAL PATTERNS

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75

DISTRIBUTION

For a number of common and abundant species, it was possible to examine seasonal trends in vertical distribution in the transects sampled from October 1972 to October 1973. Complete data were available for Transects 2, 4, 5, and 6; Transect 1 was not used for reasons given above. Data from Transect 3 were incomplete because bad weather prevented sampling during July 1973. The upper limits of distribution of nine species of algae which were present at all times of the year in all four transects examined were analysed as follows. The transects were considered as replicates, and the uppermost quadrat in which a given species could be found was noted. These uppermost quadrats for a given transect for five times of sampling were ranked in order from the lowest to the highest. The summed ranks for the four replicate transects in each of the five periods were then compared by the analysis of variance analogue described by Winer (1971, p. 301) so that differences were detected by significant values of x2. The results are presented as summed ranks in Table VIII, so that significant trends are easily seen: low summed rank scores TABLE VIII Seasonal Summed

rank scores

Ilea fascia Codium lucasii Laurencia pinnatijida Colpomenia sinuosa Pterocladia capillacea Ulva lactuca Corallina qfficinalis Hormosira banksii Celidium pusilhm

changes

in upper limits of distribution

of algae.

Oct. 1972

Jan. 1973

Apr. 1973

Jul. 1973

Oct. 1973

x’ 4 d.f.

14.5 9 10 16 13.5 12.5 9 13 8.5

5 9 4.5 4 4.5 5 10 10.5 12.5

9 15.5 13 9 10.5 14 14.5 10.5 8

14.5 12 18 13 14 17.5 12 15.5 13.5

17 14.5 14.5 18 17.5 11 14.5 10.5 17.5

10.2* 5.5 ns 13.1* 13.3* 10.0* 10.6* 5.7 ns 3.5 ns 6.6 ns

indicate relatively low positions on the shore. This statistical procedure overcomes problems caused by the different lengths of the four transects. A number of species showed no seasonal change in vertical distribution, but others, such as Ilea fascia and Ulva lactuca showed striking downward shifts in their limit to distribution during summer (Table VIII). In all cases, there was an upward return of the upper limit of distribution by autumn (Ulva and Laurencia) or by winter (Ilea, Colpomenia, and Pterocladia). DIVERSITY

OF SPECIES

There was a slight trend for increased species diversity with decreased waveexposure along the shore (except for Transect 1: see Table IX), when the total

16

A. J. UNDERWOOD

number of species found in each transect was averaged over the five periods sampled from October 1972 to October 1973. Transect 1 sampled very few species of foliose macro-algae and grazing animals when compared with the other transects, but this is attributable to the few (only 4) quadrats which could be sampled in the levels below the Littorina zone, because of the steep slope of the shore. Presumably so few quadrats did not contain many species, as most of these are patchily distributed. In general, however, there appeared to be somewhat greater numbers of species of grazing and sessile animals towards the more sheltered end of the platform, but no clear pattern in the numbers of foliose and encrusting algae, nor in the number of species of predatory whelks. To examine this further, particularly because the transects were all of different lengths and thus consisted of different numbers of quadrats, the numbers of species found per quadrat on each transect were analysed. Quadrats were picked at random in the lower algal-dominated areas, and in the mid-shore areas, from those available in each transect at each time of sampling, so that balanced sets of equal numbers of quadrats could be analysed. This gave eight quadrats per transect for each time of sampling for the mid-shore areas (with sessile and grazing animals) and five quadrats for the lower algal areas. From two-factor analyses of variance of these data, some differences between transects, or between seasons were identified. Only the results of analyses showing significant differences are summarized in Table IX. There was no difference in TABLE Diversity

of species

A. Mean no. of species per transect in 5 samples from October

Transect no. Rank order of decreasing wave-action Mean no. of species of: Encrusting algae Foliose algae Sessile animals Grazing animals Predatory whelks Total

Sessile animals Grazing

animals

Month Foliose algae

in transects.

1972LOctober

1973

2

3

I

4

5

6

I

2

3

4=

4=

6

5.8 17.2 5.0 il.2 2.0 41.2

5.0 19.3 3.5 10.5 2.2 40.5

5.0 I I.4 4.8 6.4 1.0 28.6

5.3 20.2 6.2 15.2 2.2 49.0

5.2 18.4 7.0 12.0 2.2 44.8

B. Mean no. (and range) of species present per quadrat. Transect Foliose algae

IX

2 5.7 (69) I.2 (O-4) 3.1 (O-6) Otto ber 72 6.2

October

1972ZOctober

3 5.8 (3-9) 0.9 (O-3) 3.0 (l-6) January 73 4.5

April 73 5.4

5.8 21.8 7.6 17.4 2.4 55.0

1973

4 5.5 (2-Q 1.7 (O-4) 3.1

5 5.8 (3-10) ?.I (O-4) 3.0

(O-6)

(O-5) July 73 5.6

6 5.8 (3-10) 1.8 (o-4) 5.5 (2-8) October 73 6.7

n 25

St 0.30

40

0.13

40

0.16

n 20

0.34

SE

INTERTIDAL

COMMUNITY

STRUCTURE

IN NEW SOUTH WALES

II

number of species of algae per quadrat from transect to transect, but there was a significant decrease in the algal diversity during spring and summer (analysis of variance: P > 0.05 and P < 0.01, respectively). Diversity in January was less than at all other times of the year (S.N.K. tests, P < 0.05). There was an obvious trend for increasing diversity from January to October (see means in Table IX: the means and SE in the table are pooled from all transects, as is appropriate because there were no significant interactions in the analysis of variance). For sessile and grazing animals, there was no seasonal change in diversity (analyses of variance, P > 0.05 and P > 0.5, respectively) but for both groups, there were significant differences among the transects (P < 0.01, P < 0.001, respectively). Mean numbers of sessile species per quadrat in the more wave-exposed areas (Transects 2 and 3) were approximately half the numbers in more sheltered areas (Transects 4 to 6) (S.N.K. tests, P < 0.05: Table IX, means and SE pooled across times of sampling). There were more species of grazers in the most sheltered Transect (6) than in the others, which did not differ (S.N.K. tests, P < 0.01; Table IX). Some caution is needed in the interpretation of these analyses. For foliose algae, the decrease in diversity during summer was attributable to the marked reduction in abundance of several species, and the change in vertical distribution of some species already discussed (see Tables VI and VIII). For the animals, however, the differences in diversity per quadrat and per transect summarized in Table IX were not a function of absence of certain species from areas with greater exposure to wave-action. For example, all the species of barnacles usually recorded on Transect 6 (Chthamalus antennatus, Chamaesipho columna, Tesseropora rosea, Tetraclitella purpurascens (Wood), and Megabalanus nigrescens) were also found on all the other transects, or in their immediate vicinity. Similarly, the tube-worm Galeolaria caespitosa was present in some quadrats in all transects. Thus, the differences in diversity among transects are attributable to differences in density or spatial dispersion of these species. For example, the gastropod Bembicium nanum was much more abundant in Transect 6 than elsewhere, and thus occurred in more quadrats than on transects where it was sparse. The analysis of numbers of species of animals in mid-shore areas was repeated in August 1975 using 1 m x 1 m quadrats (four times the area of those normally used) and the number of species of sessile and grazing animals per quadrat did not differ among transects (analysis of variance, P > 0.05) but using 50 cm x 50 cm quadrats, the same differences between transects were obtained as found during 1972-1973. This confirm? that differences in diversity of species along the shore are primarily a function of the density and dispersion of different species and not a result of the absence of some species from some places. For comparison with many of the seasonal patterns shown by the algae, some aspects of weather during this study are summarized in Fig. 5. Air temperature shows a rapid rise during spring and a peak in summer which is clearly correlated

A. J. UNDERWOOD

78

with the seasonal variation of many of the algae discussed above. Rainfall is not particularly seasonal, but is illustrated, with a measure of coastal swell, because these two components of weather can ameliorate the effects of low-tidal air temperature and desiccation during summer. The swell data in Fig. 5 indicate periods

0

Jy 1972

0

Ja

Ap

Jy 1973

0

Jo

Ap

Jy 0 1974

Jo

Ap

Jy 0 1975

Jo

Ap

Jy 0 1976

Fig. 5. Weather data from Bureau of Meteorology records for Sydney and Gosford: mean rainfall and air temperature per IO-day periods are plotted; the proportion of coastal swell greater than low to moderate in each IO-day period is also plotted; this level of swell at least causes splash and spray on the rock-platform, but can range to severe storms,

when waves and spray affect the rock-platform. There is no seasonal trend in these data, and periods of rough seas can occur at any time of the year, with or without increased rainfall. These data suggest that seasonal effects of air temperature are likely to be varied by uncorrelated fluctuations in waves and rainfall. It is possible for effects which might be attributable to air temperature to be negated by patterns of rainfall and wave-action.

INTERTIDALCOMMUNITYSTRUCTUREIN NEWSOUTHWALES

19

DISCUSSION MAJORPATTERNSOF VERTICALDISTRIBUTION The results of this study, as might be expected, substantially confirm, for this rock-platform, the major findings of the detailed descriptive accounts of Dakin et al. (1948) and are very similar to the descriptions of Endean et al. (1956) for shores in southern Queensland. Some striking correlations of limits to distribution are, however, evident here which were not so obvious in previous published accounts. Furthermore, there are clear seasonal patterns of change which were not discussed by earlier workers and which are now considered. In general, the bottom of the shore is characterized by a complete cover of foliose macroalgae, with few sessile animals and few grazers. The grazers present, apart from occasional individuals, are those which feed on macroalgae (such as the starfish Patiriellu calcar). This complete occupancy of primary substratum by algae is a marked contrast to other areas such as the shores studied by Dayton (1975) where bare space was available in considerable quantities at low levels amongst the algae. On this particular shore, beds of the large ascidian Pyuru praeputialis are not well-developed, but these are common sessile animals in these low levels on other shores (Dakin et al., 1948). Here, there were small patches of bryozoans and sponges, but never more than !Z 1% of primary substratum, and a few scattered small patches of Galeolaria caespitosa. The large barnacle, Megabalanus nigrescens, was found in low numbers in the upper part of the algal zone. Endean et al. (1956) commented that the presence of large beds of algae at low levels on the shore probably prevented larvae of Galeoluriu from settling there. Algae rapidly grew at low levels in the algal zone (Denley & Underwood, 1979) a phenomenon well known on other shores (e.g. Hatton, 1938; Barnes, 1955). The main reason for the lack of barnacles at low levels was, however, the lack of space for settlement because of the dense cover of algae (Denley & Underwood, 1979). Throughout the present study, the cover of foliose macroalgae in the algal zone on all transects was virtually 100’~ at all times of the year. This would account for the complete lack of most species of barnacles where there was a complete cover of foliose macroalgae. Lack of space for larval settlement of Galeolariu is probably the cause of its lower limit, as suggested by Endean et al. (1956). Nothing is yet known about the causes of limits to the distribution of Pyura praeputialis on shores in New South Wales, so an explanation for its ability to survive and grow amongst macroalgae must await experimental investigation. Similarly, it is not yet known how Megubalunus nigrescens manages to avoid overgrowth by algae, but it has very rapid growth, reaching about 5 cm height in <8 months (pers. obs.) and this may enable it to escape from overgrowth by algae. The presence of predatory whelks, Dicathais orbita, at low levels on the shore may also be involved in the absence of some sessile species, but this seems of little significance here, because of the lack of settlement of most species discussed above.

80

A. J. UNDERWOOD

The reasons for the abrupt decline of mid-shore grazers at low levels are not obvious. Connell (1975) suggested that the activities of large, predatory starfish or other predators which eat sessiie and grazing animals, may be responsible for the dominance of foliose algae at low levels on seashores in many parts of the world. Throughout this study, no predatory starfish were found in 4 yr of sampling, and octopuses and crabs were rare; the impact of predatory fish was not examined. Dicathais orhita has occasionally been seen eating mid-shore gastropods (pers. obs.) but this is not a frequent occurrence, and the whelks eat barnacles and mussels in preference. Moran (pers. comm.) has demonstrated, by experimental removal of predatory whelks, that their impact on the abundance of mid-shore gastropods is negligible. Underwood & Jernakoff (in prep.) have found no predation on grazing gastropods when they are experimentally transplanted to low levels on the shore amongst foliose algae. Underwood (1979) has suggested that the algae themselves may be responsible for the lower limit of the majority of microalgal grazing gastropods, and experimental investigations of this will be described elsewhere. Above this alga-dominated zone, there was a rapid increase in density of microalgal grazing gastropods, and in the cover of sessile animals and/or encrusting algae. Fohose macroalgae were sparse above their major zone and, apart from rock-pools, were mostly confined to areas where there were sessile animals, notably barnacles. Various factors have been discussed as causing upper limits of foliose algae on shores in other parts of the world. These include the effects of physical factors, such as desiccation, during low tide (e.g. Schonbeck & Norton, 1978; Hruby & Norton, 1979; Hay, 1979), and some effects of grazing by herbivorous molluscs (e.g. Castenholz, 1961; Southward, 1964; Lubchenco, 1978). The effects of these factors will be described elsewhere, but unpublished experiments indicate that grazing by gastropods on the spores of the algae causes the upper limit of the algal zone as already discussed by May et al. (1970). When grazers are removed, the growth and abundance of algae above this level are influenced by physical factors (unpubl. data). The virtual absence of foliose algae on dry rock at mid-shore levels is a striking contrast to the situation in many parts of the world (see Lewis, 1964; Stephenson & Stephenson, 1972) but is similar to the situation in parts of New Zealand (e.g. Morton & Milier, 1973). The lack of large algae, particularly fucoids, has little or no apparent effect on the overall species composition of the animal community in that there is, in New South Wales, a similar range of species of gastropods (trochids, patellid limpets and the littorinid Bembicium nanum) and a variety of species of barnacles similar in size and vertical distribution to those found on British shores, where fucoid algae form dense beds at mid-shore levels (Lewis, 1964). The widespread presence of encrusting algae, in great abundance in some areas, at mid-shore levels is a very obvious feature of the shore. This is not mentioned by Dakin et al. (1948) nor Endean el al. (19.56), but they were primarily concerned with the distributions of animals. How algae such as Hildenbrandia prototypus manage to grow in

INTERTIDAL

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81

profusion in areas where grazing by gastropods is intense will be discussed elsewhere. A parallel situation apparently exists on the eastern coast of the United States of America where the perennial alga Chondrus crispus is a dominant occupier of primary substratum in some areas, and is resistant to grazing by gastropods (Lubchenco, 1978; Lubchenco & Menge, 1978). At high levels on the shore, as is commonly found on shores in other parts of the world, the numerically dominant species are small littorinid gastropods, of which there are three species on many shores in New South Wales. In some areas, the limpet Notoacmaea petterdi is very abundant on vertical surfaces at high levels on the shore (Creese, 1978), but not in the transects sampled here. The causes of the upper and lower limits of distribution of littorinids in New South Wales are not yet known. Although the whelk, Morula marginalba, eats them at mid-shore levels in some places, this predation cannot be the sole cause of their lower limit. These whelks are very patchily distributed and prefer other prey at most times of the year (Moran, pers. comm.). HORIZONTAL

PATTERNS

OF DISTRIBUTION

AND PATCHINESS

There were some trends in the patterns of distribution of species with increasing wave-exposure. First, there was a general tendency for the vertical distributions of major groups of species to be shifted upwards with increasing exposure to waveaction. Thus, the upper limits of foliose algal beds, mid-shore grazers, and the distribution of littorinids were all at higher levels on more wave-exposed transects. This is not surprising, and has been widely documented for shores in other parts of the world (e.g. Lewis, 1964; Stephenson & Stephenson, 1972) and for New South Wales (Dakin et al., 1948). The increased splash and spray at wave-exposed sites is generally thought to raise the height on the shore to which various species can extend, because it reduces the physical harshness at higher levels during low tide, and increases the period of submersion during which the animals at the higher positions can feed. Other less direct effects of wave-action have been described in other parts of the world, notably the decrease in intensity of predation on barnacles and mussels with increasing wave-exposure (Dayton, 1971; Menge, 1976). Little can be said about this from the present data because the range of wave-exposure across the shore does not represent the full range that can be found in N.S.W. For example, the extremely exposed shores in New South Wales are characterized by dense stands of large Tesseropora rosea and the surf-barnacle, Catomerus polymerus (Dakin et al., 1948; pers. obs.). On the shore studied here, Catomerus was never abundant, and the abundance and extent of Tesseropora were not as great as in more exposed sites. At the other end of the wave-exposure gradient, there are sheltered areas in New South Wales where barnacles are sparse or absent, and gastropods are very abundant and the encrusting algae clearly dominate the primary substratum (pers.

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obs. and Endean et al., 1956). These areas are not well represented on the rockplatform studied here. Thus, both extremes of sheltered and exposed conditions are lacking on this shore. There were, however, well-represented intermediate situations which are marked by changes in dominance of the sessile animals from sheltered to exposed areas. This was described in detail by Dakin et al. (1948) and the present observations were consistent with their conclusions. Briefly, at the more sheltered transects, there was a wide band of Chamaesipho cofumna stretching from the top of the algal zone to the littorinid zone. There were sparse Chthamalus antennatus at the top of the barnacle zone, and only occasional Tesseropora rosea and a few Galeolaria caespitosa at low levels above the algal zone. On more exposed transects, the tubeworm Galeolaria caespitosa occurs at and above the top of the algal zone, and above it are Tesseropora rosea and then Chamaesipho columna in smaller numbers than in more sheltered sites, and Chthamalus antennatus. These general patterns are not, however, as invariant as the accounts of Dakin et al. (1948) suggest. There was an enormous variability from place to place at a scale of a few metres horizontal distance (Table V). In some areas, at sheltered and more exposed sites, encrusting macroalgae, particularly Hildenbrandia prototypus, were major occupiers of primary substratum at mid-shore levels. On adjacent transects, however, sessile animals were more numerous. This patchiness occurred all along the shore. A wide variety of physical factors and biological interactions, which operate after recruitment of dispersive larvae has occurred, can lead to variations in abundances of intertidal organisms (see particularly Dayton, 1971; Connell, 1972). The variability within sites and along the wave-exposure gradient in the abundances of different species of barnacles on the shores in N.S.W. is also very much a result of variations in intensity and patterns of larval settlement (pers. obs. and Denley, pers. comm.). Temporal and spatial variations in recruitment of animals on this shore will be discussed elsewhere. Related to this type of patchiness of dominance by different types of animals along the shore, there was an increased diversity of species of sessile animals per quadrat at the most sheltered area compared with other areas (Table IX). It is apparent, however, that all species found in any transect could be found on or near all the other transects and, in terms of simple numbers of species present, no change in fauna1 or algal composition occurs along the shore. There were, however, greater densities and/or less scattered dispersion of sessile animals at the sheltered end of the shore. The reasons for such variation, within sites of similar exposure to waveaction must be investigated further. SEASONAL

CHANGES

IN PATTERNS

OF DISTRIBUTION

Some species of algae showed marked seasonal changes in pattern of distribution. This seasonal variability varied from complete disappearance of visible thalli in

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some seasons (e.g. Porphyra umbilicalis and Nemalion multifidum) to changes in the upper limit of distribution (e.g. Ulva lactuca and Zlea fascia). With some notable exceptions, such as the appearance of seasonal species in summer, many of the components of seasonal variability are clearly correlated with the increased physical harshness of the shore during low tide in summer. There are probably less direct effects of seasonal patterns of weather than simply mortality caused by increased insolation and desiccation. One less direct effect on algae of seasonal patterns of weather concerns reproduction and recruitment. Clearly, if recruitment of a given species of alga were continuous, and sufficient at all times of the year to replace individual plants that die, the species would show no seasonal pattern of distribution. If, however, reproduction and/or recruitment were itself seasonal, there could be a seasonal change in the pattern of distribution without any change in the rate of mortality of individual plants. Seasonal variability of recruitment and growth of most intertidal algae in New South Wales has not yet been investigated, and the life-histories of the species are very poorly known. There are apparently no general effects of temperature inducing seasonal patterns of reproduction, although some algae are known to cease reproduction at high and low temperatures, and when light intensity is very great (Gessner, 1970; Hellebust, 1970). Matsuura (1958) however, found that reproduction of many species of red algae occurred mostly in spring and was correlated with rising temperatures of the sea, but reproductive activity decreased as water temperature reached its summer maximum. During the period of increased reproduction, more species of red algae were present in his study site than at other times of the year. Their disappearance was related to the cessation of breeding and both were correlated with maximal sea temperatures. A second direct effect could be through the effects of seasonal patterns of weather on the activities of grazers. For example, if grazers were in greater abundance, or were more active during warmer months of the year, they could cause seasonal changes in the distribution of algae. In the absence of experimental tests designed to distinguish between alternative hypotheses, it is pointless to speculate further on the causes of changes in patterns of distribution which are correlated with seasonal changes in environmental conditions. It is not wise to assume, however, that increased physical harshness in summer is the direct cause of all observed seasonal variability in the distribution of algae. The possible relationships between seasonal patterns of distribution, the direct effects of weather, and indirect effects correlated with seasonal patterns of weather must await detailed investigation for these algae. Finally, it must be noted that some aspects of the seasonal differences in algae may be variable from year to year. The patterns of rain and coastal swell in New South Wales (see Fig. 5) indicate that in some years (e.g. 1976) there is considerable rainfall during summer and this would tend to reduce the degree of desiccation during low tides. Furthermore, periods of moderately rough weather (as indicated by coastal swell in Fig. 5) can occur at any time of the year, Rough seas, with concomitant increased splash and spray, and reduced period of emersion will reduce

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the effects of physical factors during low tide. There is no obvious seasonal pattern in the rough weather, as described by Dayton (1971) and Menge (1976) who reported severe rough weather during winter, which had important effects on the intertidal communities they studied. Periods of rough weather and storms can occur at any time of the year in New South Wales and will not, therefore, be particularly correlated with seasonal patterns of distribution of intertidal species. The general patterns of distribution described here lead to a number of experimental approaches to determine the factors governing the structure of this intertidal community. Some of these are at present being investigated, and will be described elsewhere using this descriptive series of observations as a background.

ACKNOWLEDGEMENTS

This study was supported by a University of Sydney Postdoctoral Fellowship, a Queen Elizabeth Fellowship in Marine Science, and the Australian Research Grants Committee. I thank Mr. P.A. Cameron and Dr. P. A. Underwood for considerable assistance with field-work. Dr. W. F. Ponder provided much assistance with identification of molluscs. Much help with identification of algae was given by Drs. R. Hinde, A. Larkum, V. May, and Professor B. Womersley; any errors are my fault. Many colleagues discussed this work, and some improved the manuscript.

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