TANE 26,1980 THE MACROBENTHOS OF THE CAVALLI ISLANDS, NORTHERN NEW ZEALAND by Roger V. Grace* and Bruce W. Haywardf * 274a Birkdale Road, Birkenhead, Auckland 10 TN.Z. Geological Survey, DSIR, P.O. Box 30-368, Lower Hutt SUMMARY Forty-seven dredge samples from the sea-bed to the north, west, and south of Motukawanui, the largest island in the Cavalli Group, were analysed for macrofauna, macroflora, and sediment texture. Four associations were recognised. One, inhabiting fine sand in shallow water, was characterised by Myadora boltoni and Pectinaria australis. Another, characterisec by Tawera spissa, occurred in shelly medium to coarse sands in depths to about 10 metres. A third association, which occurred in fine to very fine sand in depths greater than 30 metres, contained Nemocardium pulchellum, but was poorly sampled. In intermediate depths, an association containing Zeacolpus pagoda, Zegalerus tenuis and juvenile Tawera spissa occurred in substrates ranging from fine sand to fine shell gravel. Algae-dominated rocky substrates were mapped in the southern part of the area with the aid of aerial photographs. INTRODUCTION Motukawanui is the largest island in the Cavalli Group off the east coast of Northland at latitude 35°00'S, longitude 173°57'E, and approximately 2.4 km from the mainland. Field work was carried out during the Offshore Islands Research Group trip to the Cavalli Islands during December 1978 and January 1979.The locations of the 47 dredge stations are indicated in Fig. 1, together with bathymetry of the area, based on the Royal N.Z. Navy Hydrographic Office chart number N.Z.5121, "Cavalli Passage". METHODS Samples were collected using a new model of a small dredge described by Grace and Whitten (1974), as used by Grace and Grace (1976). The dredge held approximately 4.5 litres of sediment, and under ideal conditions sampled an area of approximately 0.075 square metre to a depth of 6 cm. The dredge was hand-hauled from a 3.8-metre aluminium dinghy with an 18 horsepower outboard motor. 189 Upon arrival of the dredge at the surface, the volume of each sample was estimated as tenths of the dredge filled, to enable crude quantitative estimates of the biota to be made. (About 100 to 150 mis of sediment was removed for textural analysis, and for analysis of populations of foraminifera (Hayward 1979, 1980). The rest of the sample was then passed over a wire mesh sieve with 2 mm openings. Organisms retained were sorted fresh, identified where possible and counted, then returned to the sea. Some specimens were retained for subsequent laboratory identification. Estimates of the volume of residue remaining after sorting were also made, to give an indication of the quantity of sediment coarser than 2 mm particle size. Grain size analyses of the sediments were carried out in the laboratory using sieves. Stations were located using a sextant to measure horizontal angles between fixed points on the shore. Rocky bottom areas were briefly investigated using snorkel and SCUBA diving techniques, and mapped in the southern part of the area with the aid of aerial photographs. RESULTS Lists of station data are presented in Appendices 1 and 2. Sediments (Fig. 2) Fine sands are present in the shallow Papatara and Waiiti Bays on the south-west side of Motukawanui, where maximum shelter from the prevailing east or north-east swell occurs. The study area is, however, dominated by coarse and very coarse shelly sands to the south, west, and north of Motukawanui, with even coarser shell gravel in the channel immediately south of Motukawanui. These are relatively wave-exposed areas, and with the large numbers of shells of Tawera spissa produced here, heavy deposits of shells have accumulated. Around the small islands to the south, the sediments are mainly medium sands. The finest sediments are found in deep water to the northwest, where very fine sands have accumulated in quiet conditions at over 40 metres depth. Small pockets of very fine sand also occur in shallow, very sheltered inshore parts of Papatara Bay. Flora An important component of the "sediments" between Motukawanui and the small islands to the north, and more especially between Motukawanui and the small islands to the south, is the large number of "rhodoliths" of both living and dead coralline algae (Fig. 1). Each "rhodolith" consists of a small central nucleus of rock or shell about which has grown encrusting coralline algae in the form of numerous narrow fingers projecting radially. The diameter of the 191 "rhodoliths" measures approximately 20 to 30 mm, with each finger- like projection about 2 to 3 mm in diameter (Fig. 3). These "rhodoliths" are heavier in structure than those noted from Great Mercury Island (Grace and Grace 1976), but finer than those at Army Bay north of Auckland. Similar "rhodoliths" occur in parts of the Bay of Islands (personal observations), and "rhodoliths" have recently been found to occur extensively in deeper water off the Northland coast (K. Grange, Fig. 2. Sediments 192 NZOI, pers. comm.). The heaviest accumulations of shells and rhodoliths impart a certain degree of stability to the sediments, allowing the development of algae attached to the larger particles. These algae include Ulva lactuca, sometimes large sheet-like plants of Gigartina circumcincta, and a variety of smaller red algae. Most of these would be temporary in occurrence, being easily destroyed in storms, but re-developing fairly rapidly during extended calm periods. Fauna The analysis of the fauna has been approached from the classical point of view of selecting the most obvious, most numerous, or characteristic species to define benthic communities. Here the term "community" is used to apply to a recurring combination of species of organisms, in a characteristic biotope, and with a considerable degree of continuity in space. Four communities of organisms inhabiting sediments have been recognised from field observations and an examination of the data. An additional community is found on the rocky seabed. These communities are mapped in Fig. 4, and discussed below. Only those species present in more than 25% of the samples assigned to a particular community are listed, although complete data is available in Appendix 2. 1. Myadora boltoni - Pectinaria australis community. Stations : 16, 20, 21, 22, 23, 24, 25, 27, 28 Marginal stations : 15, 26 Biotope : Sediment: fine sand to very fine sand. Depth : low tide to 5 metres. Wave energy : low to moderate. Characterising species : No. of stations % of occurrence Myadora boltoni 6 55 Pectinaria australis 9 82 Associated species : Polychaetes 11 100 Owenia fusiformis 6 55 Amalda novaezelandiae 6 55 Myadora striata 6 55 Zegalerus tenuis 6 55 Scalpomactra scalpellum 5 45 Zeacolpus pagoda 5 45 Amphipoda 5 45 Zostera muelleri 4 36 Duplicaria tristis 4 36 Trochodota sp. 4 36 Cominella quoyana 3 27' Soletellina nitida 3 27 193 Fig. 3. Calcareous rhodoliths of algae are common in the Cavalli Passage between Motukawanui and the mainland. This community occurs in Papatara and Waiiti Bays on the sheltered south-west side of Motukawanui, from low water to about 5 metres depth. The sand-mason worm Pectinaria australis is widespread within the community. The bivalve Myadora boltoni occurs in only 55% of the stations and reaches a maximum density of 5 in the sample at station 20. Myadora striata also occurs, but is even less dense. Other species typical of fine sediments include the bivalves Scalpomactra scalpellum and Soletellina nitida. More tolerant of a wider range of sediment types are the epifaunal gastropods Amalda novaezelandiae, Zegalerus tenuis (where shells provide a hard attachment surface), Duplicaria tristis and Cominella quoyana. A bed of the eel grass Zostera muelleri occurs in part of this community in Papatara Bay, but is not well developed. Reference to aerial photographs taken at different times over the past 30 to 40 years indicates that the Zostera bed has undergone various changes in density and extent, and was at a low ebb at the time of sampling. Had this bed been well developed, it may have been appropriate to subdivide this community, distinguishing the Zostera community as a separate entity, probably with characteristic fauna. A SCUBA dive on the densest part of the bed revealed a low incidence of the following fauna often characteristic of Zostera beds, in addition to species collected in the dredge samples: Venerupis largillierti, Dosinia 194 subrosea, Pecten novaezelandiae, Struthiolaria papulosa, Echinocardium cordatum, Astropecten polyacanthus and Squilla armata. 2. Tawera spissa community Stations: 2,3,4,14,19,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44 Marginal stations : 1,45 Biotope : Sediment: fine sand to very coarse shelly sand and shell gravel. Depth : 2 to 15 metres. Wave energy : moderate to high. Characterising species : No. of stations % occurrence Tawera spissa 21 92 Associated species : Polychaetes 18 78 Amphipoda 18 78 Rhyssoplax stangeri 17 74 Zegalerus tenuis 17 74 Hermits 16 70 Amalda novaezelandiae 14 61 Cominella quoyana 14 61 Small red algae 13 57 Notoacmea helmsi 12 52 Membraniporiform bryozoa 11 48 Serpulidae 11 48 Micrelenchus rufozonus 11 48 Owenia fusiformis 10 43 Isopoda 10 43 Rhodoliths 9 39 Dosinia maoriana 8 35 Glycymeris modesta 8 35 Trochodota sp. 8 35 Ulva lactuca 7 30 Marginella pygmaea 7 30 Zeacolpus pagoda 7 30 Trachelochismus sp. 7 30 Corbula zelandica 6 26 Liocarcinus corrugatus 6 26 Amphipholis squamata 6 26 This community is best developed between Motukawanui and Piraunui Islands, although a smaller area is occupied by a less well developed example between Motukawanui and the islands to the north. The northern area contains mainly juvenile Tawera spissa about 10 to 15 mm long, to a density of about 14 000 per square metre (station 3).