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SUBLITTORAL ZONATION OF DOMINANT ORGANISMS

AT RED MERCURY ISLAND, NORTH-EASTERN NEW ZEALAND.

by Roger V . Grace*

S U M M A R Y

Diagrammatic profiles of the rocky seabed at two localities are presented, showing the distribution of the major fauna and algae. Zonation is discussed in relation to exposure and similarity with areas studied elsewhere. Probable fungal or bacterial damage to the large brown alga Ecklonia radiata is also discussed.

I N T R O D U C T I O N

A large proportion of the coastline of Red Mercury Island consists of boulder beaches, which continue below the tide marks for varying distances offshore. At Rolypoly Bay on the western shore of the island, a boulder bottom extends from the shore for a distance of about 70 metres, where the sea floor becomes sandy at a depth of 12 metres. A t a more exposed site on the northern side of Mokomoko Rock, a small rocky island half a mile west of Red Mercury, the bottom is much steeper and of solid rock to a depth of about 11 metres, then large blocks or boulders continue to the sand at 21 metres depth.

During the Auckland University Field Club Scientific Camp in August 1971, observations were made at both these sites using S C U B A diving techniques. Only the broad distribution patterns of a few species were determined, because rough seas made diving impossible during most of the camp, preventing a more detailed analysis. Notes were made on the distribution of the larger algae, and some of the more obvious animals, along transect lines perpendicular to the shore. These observations are presented in diagrammatic form in Figs. 1 and 2. The zonation scheme follows that of Shepherd and Womersley (1970), although the "lower sublittoral" is missing at Red Mercury because of the relatively shallow depth of the change from a rocky to a sandy bottom.

R E S U L T S

Transect 1. Rolypoly Bay, Red Mercury Island.

The western side of Red Mercury Island is partly protected by the other islands of the Mercury group. A gently shelving bottom of basalt boulders up to 2.5 metres in diameter continues from the shore to a distance of 45 metres. At this point there is an abrupt change in the angle of slope, the bottom dropping away more steeply to the sand at a depth of 12 metres (see Fig. 1).

•Department of Zoology, University of Auckland.

Mid subi i t tora l

z o n e

Haliotis i r i s

Ecklonia radiata

15- Lessonia variegata Ecklonia radiata (juvenile1

0 10 2 0 3 0 4 0 5 0 6 0

DISTANCE O F F S H O R E ( M E T R E S ) Fig. 1. Diagrammatic profiles of transect 1, Rolypoly Bay, Red Mercury Island.

7 0 8 0

5')

Algae.

In the shallowest area about ELWS (corresponding roughly to the sublittoral ringe), the brown alga Carpophyllum angustifolium is dominant, attached to the upper surfaces of the boulders. Xiphophora chondrophylla is also common. On he sides of the boulders several red algae are found, including Pterocladia lucida, °. capillacea, Melanthalia abscissa, and Vidalia colensoi. Wave action here is at rimes severe, and is always greater than at any point further out from the shore.

Further seawards, at the top of the upper sublittoral zone, C. angustifolium is replaced by C. maschalocarpum and a few small specimens of Lessonia variegata on the upper parts of the boulders, with the four red algae mentioned above on the sides of the boulders and amongst the hold-fasts of the larger brown algae. A few small specimens of Ecklonia radiata first appear at about this level. Any "bare" areas of rock are covered by "coralline paint".

Further seawards C. maschalocarpum is less important, with a few small specimens of C. plumosum on the tops of the boulders. This species is typically found in calmer waters than the other two Carpophyllum species, which relates to its position here in deeper water, where turbulence from wave action is less severe. At this level Ecklonia radiata becomes dominant, marking the top of the mid sublittoral zone, and the red algae mentioned above are becoming less frequent.

At about 45 metres offshore (c. 5 metres depth), Ecklonia radiata is the only large brown alga remaining, and red algae, apart from a few turfing and en­crusting corallines, are much less important than in shallower water. With increas­ing depth the length of the stipe of Ecklonia radiata increases to about 1.2 metres at 12 metres depth. A large proportion of the adult Ecklonia radiata plants have the thallus and part or all of the stipe missing (see discussion). From about 5 to 10 metres depth, large numbers of juvenile Ecklonia plants from 10 to 20 cm high are found attached to the rock surface between the holdfasts of the adults.

Benthic fauna.

The paua, Haliotis iris, is particularly common from ELWS to a depth of about 4 metres. It is found attached to the lower parts of the sides of the boulders, usually aggregated in groups of up to 20 individuals. The average length of the pauas is about 10 cms.

The sea-urchin Evechinus chloroticus is common in the upper sublittoral from about 3 to 6 metres depth (20 to 40 metres offshore), below which it is absent. It usually occurs attached to the top or the sides of the boulders.

The large turbinid Cookia sulcata is found throughout the rocky areas surveyed, from ELWS to 12 metres depth, but is most abundant about 6 metres depth. It is found on the tops or the sides of the boulders.

On the steep slope of the mid sublittoral the feather-star Comanthus novaezelandiae is common, attached in crevices between the boulders. The steep boulder slope supports a few of the massive sponges more common in deeper water, including Ancorina alata and Polymastia granulosa, as well as smaller

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encrusting species. The rocky mid sublittoral and its associated biota is cut off at the base of the

boulder slope by the change to a sandy bottom.

Epifauna on the sand.

Coarse clean sand continues offshore from the rocky bottom, the depth increasing only very gently. The following epifauna was readily visible on the sediment surface.

Echinodermata:

Astropecten polyacanthus

Mollusca: Cominella adspersa C. quay ana Pecten novaezelandiae

Crustacea:

unidentified hermit crab.

Fish.

A preliminary list of the marine fishes of Red Mercury Island is presented elsewhere (see Grace, 1972). The common fish species found along this transect fall into 2 major zones according to depth and habitat.

Upper sublittoral zone:

From the sublittoral fringe to about 6 metres depth. This area is characterised by a gently shelving boulder bottom and extends to a distance of 45 metres offshore. The dominant fish species in this zone are: —

Parma microlepis (black angelfish) Pseudolabrus celidotus (paketi or spotty) Pseudolabrus fucicola (banded parrotfish) Coridodax pullus (butterfish or greenbone) Aplodactylus arctidens (marblefish) Cheilodactylus spectabilis (red moki) Unidentified tripterygiid (5cm long, golden yellow with

black stripe along each side)

At the time of these observations (late August), the black angelfish were beginning to establish nesting sites on the near-vertical sides of large boulders.

Mid sublittoral zone:

From about 6 metres depth to the sand at 12 metres. This area is characterised by a steeply sloping boulder bottom, and extends from about 45 metres to 70 metres offshore. The dominant fish species are: —

Chromis dispilus (demoiselle) Caesioperca lepidoptera (butterfly perch) Unidentified tripterygiid (as in upper sublittoral)

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A common fish on the sand close to the base of the boulder slope is Upeneichthys porosus, the red mullet. Also noted on the sand close to the rocks was the snake eel, Leptognathus novaezelandiae, which lives in a horizontal burrow about 15cm beneath the sand surface, with only the head projecting above the sand.

Transect 2 . Mokomoko Rock, half a mile west of Rolypoly Bay.

The northern side of this small rocky islet is exposed to the open ocean. A steep solid rock cliff extends below the tides to a depth of 7 metres, beyond which a fairly steep slope of irregular fixed rocky bottom and large stable boulders about 3 metres in diameter continues to the level sand bottom at 21 metres depth (see Fig. 2).

On the steep rock face Carpophyllum angustifolium is dominant in the sub­littoral fringe and into the shallowest part of the upper sublittoral zone to a depth of about 3 metres. Beneath the tresses of C. angustifolium several species of red algae are found, the most important species being Pterocladia capillacea, P. lucida, and Vidalia colensoi, the last two species being common to a depth of 6 metres. Below the C. angustifolium, a sparse growth of C. maschalocarpum and Xiphophora chondrophylla occurs to a depth of 6 or 7 metres, where it gradually gives way to an increasing density of Ecklonia radiata indicating the top of the mid sublittoral. In the shallowest part of the range occupied by E. radiata, juvenile plants 10 to 20cm high are common, but are absent from deeper water. There is a weak trend of increasing stipe length in E. radiata with increasing depth, with stipes a little over 1 metre long at 21 metres. A t all depths where E. radiata occurs, a proportion of the Ecklonia radiata plants have the thallus and part of the stipe missing, as in transect 1. Encrusting and turfing coralline algae are found at all depths, but reach their peak density in the upper sublittoral.

Benthic fauna

The sea-urchin Evechinus chloroticus occurs from ELWS to a depth of about 12 metres, below which it is absent. Towards the upper and lower extremes of its range it is not common. Cookia sulcata occurs throughout the upper and mid sublittoral zones. There are no other large sedentary invertebrates common on this transect, although a number of small crayfish Jasus edwardsi were seen in a hole at 15 metres depth.

Below about 10 metres, sponges become increasingly important, probably indirectly influenced by the reduced light intensity under the Ecklonia canopy. Most of the sponges are encrusting forms, but in the deeper parts large massive sponges, such as Ancorina alata and Polymastia granulosa, are impressive because of their size.

Fish.

There is no clearcut division into the shallow and deep zones as there was in transect 1, however a general trend in abundance of certain species can be correlated with depth. In the shallow parts of the transect characterised by the steep rock face with Carpophyllum spp., Pseudolabrus celidotus (paketi) and

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Aplodactylus arctidens (marblefish) arc the commonest fish. In the deepest parts of the transect, clouds of Chromis dispilus (demoiselle), Scorpis aequipinnis (blue maomao), and Caesioperca lepidoptera (butterfly perch), accompany the diver. Upeneichthys porosus (red mullet) is also common on the sand. In the middle depth zones, overhangs and small caves amongst the large rocks and boulders are frequently occupied by Cheilodactylus spectabilis (red moki).

A comparison between the two Transects.

The major differences between the two transects appear to be related to the horizontal and vertical extent and relative substrate stability of the sublittoral zone. These zones continue deeper on transect 2 than on transect 1, probably because of greater exposure and surge on transect 2.

In transect 1 these zones cover a horizontal distance (perpendicular to the shoreline) of about 45 metres to a depth of about 5 or 6 metres. Because of the variety of habitats offered by the irregular large boulder cover, a corresponding diversity of life-forms and species is present. Further contributing to the variety of species in this area is the frequency with which boulders may be moved or overturned, thus presenting fresh surfaces for colonisation.

Frequent overturning of smaller boulders is inferred from their lack of large algae compared with the relatively more stable boulders at the same level. Quite large boulders are sometimes moved considerable distances on this boulder beach, as indicated by the presence, high in the intertidal zone, of boulders up to a metre or so in diameter, with a coating of dead coralline paint and remains of low tidal or subtidal encrusting animals, e.g. polyzoa and Balanus trigonus.

In transect 2, the sublittoral fringe and upper sublittoral zone occupy a horizontal range (perpendicular to the shoreline) of about 9 metres to a depth of 8 or 9 metres. Because of a lack of an extensive suitable habitat, this very steep rock face precludes colonisation by organisms such as Carpophyllum maschalocarpum, C. plumosum, and the paua Haliotis iris, which were common on the boulders in the upper sublittoral of transect 1. In transect 2, the very steep upper sublittoral zone is also an unsuitable habitat for the large variety of fish species common on transect 1.

The mid sublittoral zones of both transects appear to be fairly similar, except that transect 2 has a better development of sponges because of the greater depth. A notable difference between the mid sublittoral zones of the transects is the absence of Comanthus novaezelandiae in transect 2, which is at present unex­plained. (C. novaezelandiae is known to have a localised irregular distribution at the Poor Knights Islands, possibly indicating an inherent patchy distribution, not necessarily directly associated with physical spatial variations in the habitat).

Comparison with sublittoral boulder beaches at Little Barrier Island.

A similar but more detailed study was carried out on the boulder beaches of Little Barrier Island in the Hauraki Gulf by Dromgoole (1964a), with particular emphasis on the algae. Although the Little Barrier situation is very similar to that at Rolypoly Bay on Red Mercury, the absence of Carpophyllum angustifolium and Lessonia variegata at the Little Barrier localities investigated by Dromgoole

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which were smaller at Little Barrier than at Red Mercury, was thought to be responsible for the absence of Cookia sulcata and Evechinus chloroticus on the boulder beach at Little Barrier, since they were present on a more consolidated shore nearby. Although not noted by Dromgoole (1964a), the feather star Comanthus novaezelandiae is known to occur in the deeper parts of the boulder beaches at Little Barrier (personal observation) in a similar situation to that in

F O R K E Y T O S Y M B O L S S E E H E . 1 .

0 10 2 0 3 0 4 0 D I S T A N C E O F F S H O R E ( M E T R E S )

Fig. 2. Diagrammatic profile of transect 2, Mokomoko Rock, Red Mercury island.

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which it occurs at Red Mercury. Dromgoole (1964a) distinguished a zonation sequence at Little Barrier based

on the larger brown algae, but used the term "lower sublittoral" to refer to the zone dominated by Ecklonia radiata. This usage of the term was also employed by Bergquist (1960) working in the same area. The more recent work of Shepherd and Womersley (1970) in South Australia suggests that the term "lower sublittoral" as used by Dromgoole (1964a) and Bergquist (1960) should be replaced by mid sublittoral, and the term lower sublittoral used to refer to the zone below the Ecklonia belt, where sponges, gorgonians, and small red algae dominate. The lower sublittoral so defined does not occur at Red Mercury and Little Barrier Islands, since the rocky bottom zonation sequence is cut off in the mid sublittoral by the change to a sandy bottom.

A comparison is given below between the zonation patterns at Rolypoly Bay on Red Mercury and a consolidated shore at Little Barrier, with the zone terminology of Dromgoole (1964a) for Little Barrier modified to fit the scheme outlined above.

Zone

S U B L I T T O R A L F R I N G E

UPPER S U B L I T T O R A L

MID S U B L I T T O R A L

Little Barrier

Xiphophora chondrophylla Cystophora torulosa Carpophyllum maschalocarpum Ecklonia radiata

Ecklonia radiata

Red Mercury

Carpophyllum angustifolium Xiphophora chondrophylla Carpophyllum maschalocarpum Carpophyllum plumosum Ecklonia radiata Ecklonia radiata

At Little Barrier Island on boulder beaches with frequent boulder movement, the upper sublittoral is largely characterised by Carpophyllum plumosum and Cystophora retroflexa.

The differences between the Little Barrier and Red Mercury zonation patterns are clearly correlated with the differences in exposure and degree of boulder movement.

Comparison With Other Localities.

In parts of north-eastern New Zealand where the rocky bottom continues much deeper than at Red Mercury and Little Barrier, Ecklonia disappears at a depth apparently determined by light intensity, and is thus related to the clarity of the water. At the Poor Knights Islands, Ecklonia disappears somewhere between 25 and 50 metres, depending on the topography of the bottom. Beyond this depth the lower sublittoral zone is dominated by a luxuriant growth of sponges and gorgonians (Doak, 1971), although red algae are still present beyond safe diving limits, until a depth is reached where there is insufficient light for any algae to survive.

In South Australia, Shepherd and Womersley (1970) described the rocky sublittoral ecology of West Island. They recognised the following zones, based primarily on the larger algae.

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S U B L I T T O R A L F R I N G E

U P P E R S U B L I T T O R A L

MID S U B L I T T O R A L L O W E R S U B L I T T O R A L

Zone

II I!

Characteristic species

Cystophora intermedia Melanthalia spp. Sargassum bracteolosum Scytothalia dorycarpa Ecklonia radiata Sponges

Gorgonians

Red Algae

Although most of the species are different from those found in New Zealand, hey may be regarded as "ecological homologues" (Udvardy, 1959) to New

Zealand species. The similarity with zonation patterns in northern New Zealand is striking. There appears, however, to be a marked difference in the depth of the boundary between the mid sublittoral and lower sublittoral zones, this boundary being shallower in Australian than in northern New Zealand. At West Island, Shepherd and Womersley (1970) noted the boundary at 19 metres on the exposed side of the island, rising to about 12 metres on the sheltered side. A profile of a typical portion of the underwater coast of New South Wales, presented by Deas and Lawler (1970), shows an apparent boundary between the mid sublittoral and lower sublittoral at only 4 to 5 metres.

Hedgpeth (1957) uses the term "inner sublittoral" to apply to the seabed and biota from the lowest intertidal (littoral) to the maximum depth at which attached algae occur. The terms used in this paper are thus subdivisions of the "inner sublittoral" of Hedgpeth (1957), and appear to be widely applicable in southern temperate and warm^errrperate seas.

Damage to Ecklonia radiata.

In the mid sublittoral zone of both transects studied at Red Mercury, a large proportion (up to 70 percent) of the adult Ecklonia radiata plants have the thallus and a varying amount of the stipe missing. Dromgoole (1964b) has observed Evechinus chloroticus feeding on Ecklonia, and suggested that E. chloroticus may at times be responsible for large scale destruction of beds of Ecklonia. In the present situation, however, it is unlikely that Evechinus is responsible for the observed condition of the Ecklonia plants.

Two lines of evidence lead to this conclusion. Firstly, the distribution of Evechinus and damaged Ecklonia plants appears inconsistent with the suggestion that Evechinus could be responsible for the damage, since Evechinus is found not where damaged plants occur, but in shallower water. Secondly, the condition of the damaged plants differs from that described by Dromgoole (1964b), but appears to be consistent with the type of damage likely from a fungal or a bacterial infection. The last 10 centimetres or so of the remaining stipe is greyish in colour and ragged in form and appears to be rotting. This type of damage to Ecklonia radiata was common along the western side of Red Mercury Island, and on the northern side of Mokomoko Rock, and has been observed by the present

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author at other localities, e.g. Ti Point and Takatu Point in the Hauraki Gulf. At various localities in the Hauraki Gulf, Ecklonia beds have been known to

decline over the years and disappear (Dromgoole, 1964b). This phenomenon has been attributed in the past to predation by Evechinus, but it is likely that in some cases a fungal or bacterial disease is the cause.

The disappearance of Ecklonia from previously dense beds has profound ecological effects. Not only is a possible important food species removed, but also several physical factors, notably light intensity and water movement, acting on the organisms beneath the kelp, are altered. The whole character of the mid sublittoral seabed community can thus be changed, which could have far reaching effects on commercially important species such as the crayfish and rocky bottom fishes. The dynamics of the interrelationships between Evechinus, Ecklonia, and the fungus or bacteria affecting Ecklonia should be understood before ecologicically meaningful legislation can be proposed, in the name of conservation, for the taking of Evechinus for human consumption, or the removal of Ecklonia for fertilizer production.

A C K N O W L E D G E M E N T

1 wish to thank Mr. Ron Vautier for accompanying me on S C U B A dives at Red Mercury Island.

R E F E R E N C E S

The marine algal ecology of some islands of the Hauraki Gulf. Proc. N.Z. Ecol. Soc. 7:43-5.

"Beneath Australian Seas." Reed, Sydney. 112pp. "Beneath New Zealand Seas." Reed, Wellington, 113pp.

The ecology of the sublittoral boulder beaches at Little Barrier Island. Tane 10:70-8. The depredation of Ecklonia radiata beds by the sea urchin Evechinus chloroticus. Tane 10:120-22.

A preliminary checklist of marine fishes at Red Mercury Island, north-eastern New Zealand. Tane 18: (this issue). Classification of marine environments. Geol. Soc. America, Mem.67, 1: 17-28.

The sublittoral ecology of West Island, South Australia. I. Environmental features and the algal ecology. Trans. R. Soc. S. Aust. 94:105-38.

Notes on the ecological concepts of habitat, biotope and niche. Ecology 40(4): 725-28.

Bergquist, P .L. 1960

Deas,W., 1970 Lawler, C.

Doak, W. 1971

Dromgoole, F.I. 1964a

1964b

Grace, R . V . 1972

Hedgpeth, J.W. 1 9 5 7

Shepherd, S.A., 1970 Womersley, H.B.S^

Udvardy, M . F . D . 1959