63 TANE (1966) 12 : 63-70

THE BOTTOM COMMUNITIES OF THE ENTRANCE TO THE WHANGATEAU HARBOUR

by Roger V. Grace

INTRODUCTION Situated on the Northland east coast about 60 miles north of Auckland City is the Whangateau Harbour. The entrance to this harbour is particularly interesting ecologically because of the large changes in the types of marine bottom communities over very short distances. Within a few hundred yards there is a complete change from harbour to offshore communities, and the special conditions found in the entrance have resulted in a variety of communities different from those found inside or outside the harbour.

THE AREA The harbour entrance is quite narrow, bounded on the western side by the Maungatawhiri spit, which encloses the Whangateau Harbour, and on the eastern and northern shores by Ti Point. The northern end of the Maungatawhiri sand spit is unstable and much movement of material occurs, particularly subtidally, in the vicinity of the harbour entrance channel. The shore of Ti Point consists of large boulders of basalt which are resistant to erosion and protect the softer sandstone underlying them. These boulders extend to a point just inside the harbour entrance, beyond which a sandstone shore continues. From the shore Ti Point rises fairly steeply to hills about 200 feet in height, and affords good shelter to the harbour. Very clean white silica-sand slopes gently out into Omaha Bay from Maungatawhiri spit, which is exposed to the east. Just outside the harbour entrance the bottom rises to a little over one fathom, but in the entrance channel drops to over four fathoms. Inside the harbour extensive sand and mud flats occur, large areas of which are exposed at low tide. The bottom of the channel is complex, the basalt boulders on the east• ern shore extending to the middle of the channel, and coarse shelly material rising up towards the tip of the sand spit. The strong tidal current (up to 4.5 feet per second) in the channel is broken up into eddy currents over the boulders on the bottom, thus preventing the full force of the current from being exerted up on material between the boulders. Consequently fairly fine material has collected between these boulders. On the western side of the channel however, where there are no boulders and the full force of the current is exerted upon the sediments, the finer material has been removed and very coarse shell-gravel has accumulated. This is quite stable towards the bottom of the channel, but in shallower water nearer the sandspit much movement of sand and shell occurs at every tide . In the harbour entrance there is a marked change in the transparency of the water at different stages of the tide. On the outgoing tide a large amount of suspended matter is removed from the harbour, and clear oceanic water returned on the incoming tide. The weather conditions have a con• siderable effect on the water transparency. An onshore wind tends to stir up sediment in Omaha Bay reducing the transparency of the incoming water.

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After rain, mud and clay from the surrounding land is washed into the harbour and this also cuts down the transparency and light penetration. Fig. 1 is a map of the harbour entrance indicating the water depth in fathoms.

SURVEY METHOD The area was surveyed and samples were collected using diving gear. There are several advantages to be gained from diving rather than using the more conventional dredging method for collecting marine bottom samples. 1. Samples can be taken in rocky areas where a dredge is unsatis• factory. 2. The nature of the bottom can be assessed first hand rather than relying entirely on material brought up from the bottom. 3. The use of a boat is unnecessary when working close to the shore. 4. Using special samplers quantitative results can be obtained if desired .

THE COMMUNITIES The bottom fauna has been classified into a number of communities, whose distribution is shown in Fig. 2. The community boundaries shown are very approximate since the communities grade one into the other. No distinct natural boundaries exist but artificial boundaries are used for convenience. 1. Inside the harbour, normal harbour communities are found, typified by the Chione-Nucula-Polychaeta community. Adult Chione stutchburyi (N.Z.cockle)are found on the harbour mudflats, but close to the entrance of the harbour only juvenile specimens are found. Nucula hartvigiana (nut shell) is very common even close to the harbour entrance. Various burrowing and tube-building polychaete worms are common on the harbour flats. 2a. Close to the entrance of the harbour where salinity is high and the sediment is sandy mud, Echinocardium australe (heart urchin) occurs, but Chione stutchburyi is no longer present. The result is the Nucula-PolychaetaEchinocardiu mcommunity .

2b. Similar to the above community, but in slightly shallower water and a little further into the harbour is the Nucula-Polychaeta-Echinocardium- Paphirus-Atrina community. Echinocardium adults are common here and in places the empty tests litter the bottom. Low salinity prevents Echinocardium from penetrating far into the harbour since the osmotic problems arising would upset the delicate water-vascular system found in all echinoderms. Large numbers of adult Paphirus largillierti (oblong venus shell) are found, and the large horse mussel Atrina zelandica is present at a density of about 2 per square metre. Many of these have the green alga Codium adhaerens encrusting the protruding end of the shells. The sea slug Bursatella glauca is occasionally found in large numbers in this area. 3a. In the entrance to the harbour and extending in to a point where presumably salinity becomes intolerably low for the animal, Amphidesma australe (pipi) is abundant. In places large numbers of juveniles are present, and some of these produce a byssus thread to attach to shell 66 particles which can then act as an anchor for holding the animal more firmly in the unstable sand and shell substrate. In this area the animals are sometimes disturbed by tidal scour and may be found lying exposed on the bottom amidst shell debris. They are then open to attack from the carnivore Coscinasterias calamaria and numbers of large specimens of this starfish can often be found feeding on the Amphidesma. 3b. In the main entrance channel itself near the bottom of the shell bank where the coarse shelly substrate is relatively stable, the Amphidesma atts£raZe-£s£ea-Ophiuroidea-Amphipoda-Anomura. community occurs. Adult and juvenile Amphidesma (Pipi) are present but the juveniles here do not produce a byssus thread presumably because the substrate is more stable since large dead pipi shells are packed firmly together. The minute gastropod Estea zosterophila (a "Rissiod") is very abundant, grazing on the microscopic algal film on the dead shells. The Ophiuroids (brittle stars) are small and have not been positively identified, but may be Amphipholis squamata. Very large numbers of Amphipoda ("sand-hoppers' belonging to the family Gammaridae are present, and many small hermit crabs (Anomura) are found inhabiting mostly Zethalia and Zeacumantus shells. 4. On the eastern side of the channel, the barnacle Balanus trigonus is extremely abundant attached to the basalt boulders on the bottom. The sediments between the boulders are inhabited by a number of Polychaete worms, and juvenile specimens of the purple sunset shell Gari stangeri. The result is the Balanus -Gari-Polychaeta community. 5. Along the eastern margin of the channel the large brown alga Carpophyllum flexuosum grows profusely attached to the subtidal boulders. This supports a number of animals including the gastropod Lunella smaragda (cat's eye), the pipe-fish Stigmatophora longirostris, and the sea• horse Hippocampus abdominalis. Found abundantly living in the shelter of the weed is the herbivorous fish Girella tricuspidata, better known as the Parore. 6. Outside the harbour on subtidal boulders along the coast there is a number of algae characteristic of semi-exposed shores. The most prominent of these is the brown seaweed, Carpophyllum maschalocarpum. 7a. Further along the Ti Point coast where the water is deeper, the large alga Ecklonia radiata is common, associated with the normal rich fauna of rocky bottom kelp beds, including the sea-egg Evechinus chloroticus. 7b. A small area of boulders just outside the harbour entrance supports the kelp Ecklonia radiata and its associated fauna on the south-western side, but on the north-eastern side of the boulder patch, no large algae occur. The boulders here are largely covered by a small cream-coloured sea-anenome. 8. In the intertidal zone on the rocky eastern shore of the channel, the Elmineus-Crassostrea community of sheltered rocky shores occurs. The small barnacle Elmineus modestus and the rock oyster Crassostrea glomerata are the dominant species, but many associated animals are found including Lepsiella scobina (oyster borer), Melagraphia aethiops (spotted top shell), Lunella smaragda (cat's eye), Nerita melanotragus (black Nerita), Cellana ornata and C. radians (limpets), Melarhapha oliveri (periwinkle), Sypharochiton pelliserpentis (snake's skin chiton), and the 67

red sea-anenome Actinia tenebrosa. The typically northern encrusting red alga Apophloea sinclairii is also plentiful, looking like reddish-brown areas of congealed blood stuck to the mid-tidal rocks. 9. In the intertidal zone on the ricky shore outside the harbour, the normal community of semi-exposed rocky shores occurs, including Chamaesipho columna (a small barnacle), a few C. brunnea (a larger barnacle more common in extreme exposure), Cellana ornata, C. radians, Notoacmea pileopsis (limpets), Nerita melanotragus, Lepsiella scobina, and the read alga Apophloea sinclairii. 10a. Outside the harbour entrance, and extending about 100 yards offshore on a clean sandy bottom is the Tawera-Glycymeris modesta-Dosinia maoriana community Tawera spissa (morning star shell) is the most common of the three large bivalves and lives buried about %" below the sand surface. Glycymeris modesta (small dog cockle) lives immediately below the sand surface, and Dosinia maoriana lives under 2 to 3 inches of sand. This area is one of the few localities where Dosinia maoriana has been found living abundantly. Associated with this community is the cephalochordate, Heteropleuron hectori, better known as Amphioxus. 10b. A little further offshore, Dosinia maoriana is no longer found, the community being characterised by Tawera spissa and Glycymeris modesta, with Heteropleuron hectori also being present. 10c. Still further offshore and extending well out into Omaha Bay, is the Tawera-Glycymeris laticostata-Astropecten community. The smaller Glycymeris modesta has been replaced by the larger dog cockle G. laticostata. The sand-burrowing starfish, Astropecten poly acanthus (comb star) is occasionally found closer inshore as well as in this area. Heteropleuron hectori is presumably again present. 11. At the southern end of the channel the fauna consists of a mixture of types found in the channel itself and outside the harbour, though no single species is particularly abundant. However, juvenile specimens of Evechinus chloroticus (sea egg) about % inch in diameter are found on the fairly coarse shell-gravel substrate. 12. The main entrance channel sweeps around the tip of the sands pit and continues into the harbour following the western side of the sandspit. On the western side of the channel in this area a substrate of fine sand occurs, which supports the Baryspira-Arachnoides-Notoplax community . Baryspira australis (southern olive shell) is a slug-like mollusc whose body completely envelopes the shell, and it moves along just below the sand surface. The extremely flattened cake urchin Arachnoides zelandiae cannot tolerate low salinity so this community does not extend far into the harbour. Notoplax cuneata is an unusual chiton in that it is found on or just under the sand surface and is not attached to a solid object as are other chitons. 13. Very few animals are found on the seaward side of the sandspit. Dosinia maoriana occurs occasionally and further to the south Amphidesma subtriangulatum (tuatua) appears.

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Fig. 2. Dominant or characteristic organisms.

Key:-

I. Chione (cockle), Nucula (nutshell), Polychaeta. (bristle worms). 2a. Nucula, Polychaeta, Echinocardium (heart urchin). 2b. Nucula, Polychaeta, Echinocardium, Paphirus (oblong venus shell), Atrina (horse mussel). 3a. Amphidesma australe (pipi). 3b. Amphidesma australe, Estea ("rissoid") Ophiuroidea (brittle star), Amphipoda ("sand hopper"), Anomura (hermit crab) 4. Balanus (barnacle), Gari (purple sunset shell), Polychaeta. 5. Carpophyllum flexuosum (brown seaweed). 6. C. maschalocarpum (brown seaweed).. 7a. Ecklonia (kelp). 7b. Ecklonia, Actiniaria (sea-anenome). 8. Elmineus (barnacle), Crassostrea (rock-oyster). 9. Chamaesipho (barnacle). 10. Tawera (morning star), Glycymeris modesta (small dog cockle), Dosinia maoriana. 10b. Tawera, Glycymeris modesta. 10c. Tawera, Glycymeris laticostata (large dog cockle), Astropecten (comb star). II. Mixed 3, 4, and 10. 12. Baryspira (southern olive), Arachnoides (cake urchin), Notoplax cuneata (sand chiton) 13. Very few animals: Amphidesma subtriangulatum (tuatua), Dosinia maoriana.

SUMMARY In general the type of community found in a particular part of the harbour entrance seems to be governed by an interacting complex of several physical, and to a lesser extent biological factors. In some cases one of these factors seems to be over-riding. Perhaps the most obvious of these is the substrate. The nature of a certain substrate will exclude animals which are not ecologically suited to living in such a deposit; for example barnacles are adapted for living fixed to solid objects like rocks and are in the main restricted to rocky areas, as in communities 4, 8, and 9. The heart urchin is adapted for a burrowing life and functions most satisfactorily in the sandy mud of communities 2a and 2b, so it would 70 be unreasonable to expect to find it in the coarse shelly substrate of community 3b, whereas the "rissiod" Estea in community 3b is found only in this type of deposit. The substrate is largely determined by wave action or current speed in the channel and surrounding area. Moderate wave action outside the entrance prevents the deposition of mud, but allows coarse clean sand to accumulate. Inside the harbour there is little wave action and mud and sandy mud flats occur. In the channel itself, the tidal current is so strong that only very coarse shell material can settle. Current speed itself can prevent certain organisms from settling or growing to maturity and this may be the reason why large algae are not found on the boulders in community 4. In the intertidal zone wave action is particularly important in determining the community structure, thus communities 8 and 9 are different, the former being characterised by organisms thriving on sheltered rocky shores, the latter by organisms adapted to life on semi-exposed rocky shores. Salinity can have a marked effect in controlling the range of some animals. This can be particularly well shown with the echinoderms, where reduced salinity excludes them from the inner harbour. This is seen in communities 2a, 2b and 12. The lower tide level can limit the upward extension of the range of some of the subtidal organisms since most of them cannot tolerate dissication to any extent. This is well shown by the large brown algae of communities 5, 6 and 7, while the lower limit of intertidal organisms is usually determined by competition. In this harbour entrance where there are marked changes in the environmental conditions over short distances there are correspond• ing differences in the communities. CONCLUSION There are several small harbours on the Northland and Coromandel east coasts which have similar entrances to that found at Whangateau. It is hoped that this survey may help to predict with some degree of accuracy the general types of bottom communities which will be found in these harbour entrances. Some localities which could be investigated along similar lines are Taipa, Ngunguru, Mangawhai heads, Puhoi river entrance, Waiwera, Whangapoua, Tairua and Whangamata. ACKNOWLEDGMENTS I am very grateful to several people for assistance with the collection and identification of some of the material, and I particularly wish to thank Mr T.P. Warren, who knows the area well, for discussion and assistance with parts of the community distribution map. REFERENCES Hedgepeth, J. (ed.) 1957 Treatise on Marine Ecology and Palaeoecology. Vol. 1. Ecology. Geol. Soc. Amen. Memoios. 67 Jones, N. 1950 Marine Bottom Communities. Biol. Rev. Vol. 25. Powell, A.W.B. 1937 Animal Communities of the Sea-bottom in Auckland and Manukau Harbours. Trans. Roy. Soc. N.Z. Vol. 66: 354-401 Ralph, P.M. and Yaldwin, J.C. 1956 Sea floor Animals from the Region of Portobello Marine Biological Station, Otago Harbour. Tuatara. 6: 57-85 Thorson, G. 1955 Modern Aspects of Marine Level-bottom Animal Communities. Sears . Found. Journ. Mar. Res.