Soft Bottom Macrobenthos and Sediments Off the Broken Islands, Northern New Zealand
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SOFT BOTTOM MACROBENTHOS AND SEDIMENTS OFF THE BROKEN ISLANDS, NORTHERN NEW ZEALAND by Bruce W. Hayward,1 Roger V. Grace2 and John McCallum3 'New Zealand Geological Survey, P O Box 30368, Lower Hutt 2274A Birkdale Road, Birkenhead, Auckland 10 382 Exmouth Road, Northcote, Auckland 9 SUMMARY The soft bottom macrobenthos (exceeding 2 mm in size) and sediments were studied from 34 dredge stations located in a 5 km2 area of sea bed (1-59 m depth) around the Broken Islands, west of Great Barrier Island, northern New Zealand. Six benthos associations and three subassociations are recognised and mapped. An infaunal bivalve Scalpomactra scalpellum-Dosinia subrosea asso• ciation occurs in clean, well-sorted fine to medium sand in shallow (5-9 m), moderately sheltered bays. Slightly coarser sediment in more exposed portions of bays (5-11 m) has another infaunal bivalve associa• tion, Felaniella zelandica-Talabrica bellula. In areas of greater wave exposure or strong currents (10-15 m), the sandy, shelly gravel substrate is characterised by a rhodolith — "Cucumaria" (holothurian) — Glycymeris laticostata (bivalve) association and rich subsidiary epifauna. Slightly deeper (11-31 m) in quieter conditions, a slightly shelly, fine sand substrate has a Pupa kirki (gastropod) - Echinocardium cordatum (heart urchin) - Myadora boltoni (bivalve) association. The most prevalent association in the area is that with common Corbula zelandica (bivalve). Within this a bivalve subassociation of Corbula-Limaria orientalis occurs in shelly fine to very coarse sand at 10-20 m depths in moderately exposed portions of two open bays. Two further infaunal bivalve subassociations, Corbula-Venericardia purpurata and Corbula- Pleuromeris zelandica, occur in shelly, fine to coarse sand in quieter conditions at slightly greater depths (23-25 m and 26-32 m). An association characterised by the ophiuroid, Amphiura and the bivalves Saccella bellula, Notocallista multistriata and Cuspidaria willetti occurs in muddy, slightly shelly, fine to medium sand in moderately quiet bottom conditions at 31-59 m depth. INTRODUCTION This paper documents the results of an inshore marine dredging programme undertaken during the Offshore Islands Research Group TANE 31, 1985-86 85 trip to the Broken Islands Group from 28 December 1984 to 5 January 1985. The Broken Islands Group (latitude 36°15'S, longitude 175°25'E) consists of three larger islands (Motutaiko, Rangiahua, Mahuki) and many smaller ones (including the Junction Islands in the east), that lie close to the west coast of Great Barrier Island (Fig. 1). Great Barrier Fig. 1. Lower. Location of study area, around the Broken Islands, west Great Barrier, northern New Zealand. Upper. Station locations and bathymetry (in metres). 86 Island is situated 80 km northeast of Auckland City at the entrance to the Hauraki Gulf. The 34 dredge stations sampled were located in the large, open bay on the southwest side of the Broken Islands, in the moderately deep, narrow bay between Mahuki and Junction Islands and in the shallow narrow channels between Rangiahua and Motutaiko, Rangiahua and Mahuki and Rangiahua and Great Barrier Islands. The stations sample an area of 5 km2 of sea floor, at depths of 1-59 m (Fig. 1, Appendix I). Bathymetry is based on depths measured in the field with a lead line and adjusted to mean low water spring tide level. METHODS Samples were collected using a 4.5 litre bucket dredge (described by Grace and Whitten 1974), hand-hauled from a 4 m dinghy powered by a 7.5 horse power outboard motor. Under ideal conditions the dredge sampled an area of 0.075m2 of sea floor to a depth of 6 cm. Approximately 200 cm3 of each sample was taken for later sedimentary analysis. The remaining bulk of the sample was passed over a sieve with 2 mm openings and live organisms retained on it were sorted fresh, identified, counted and returned to the sea. Three stations struck rock (18, 24, 27) and no sediment was obtained. Insufficient time at the end of the trip prevented full faunal analysis of stations 32 and 34. Sediment grain size analyses were carried out in the laboratory using sieves (Folk 1968, p. 34). BATHYMETRY (Fig. 1) In the large, open bay on the southwest side of Broken Islands, the seafloor drops rapidly away to 40 m, then slopes more gently down to an almost flat area at 55-60 m depth. Charts indicate that this flat area extends some distance offshore and is the maximum depth reached anywhere in the Hauraki Gulf inside Great Barrier Island. The narrow bay between Mahuki and Junction Islands has a gently sloping floor 25-35 m deep; the bay between Rangiahua and Motutaiko Islands has a gently sloping floor 20-35 m deep; and the bay on the southwest side of Rangiahua Island slopes gently out from 0 to 20 m, before dropping off more rapidly. The narrow channel between Rangiahua and Mahuki Islands is mostly 5-6 m deep, increasing to 10 m at the northeast end; the 1 km long channel between Rangiahua and Great Barrier is consistently 9-10 m deep in most parts. 87 SEDIMENTS Grain size terminology follows Folk (1968, p. 26-30). Sediment types at dredge stations are listed in Appendix I and their distributions shown in Fig. 2a. The shores of all the islands are fringed almost continuously at intertidal and shallow subtidal depths by a 20-100 m wide zone of rock or of bouldery gravel. Off more exposed parts of the coast on the southwest side of Broken Islands, this zone is fringed by areas of pebble gravel and pebbly coarse sand, down to depths of 40 m. In the mouths of more sheltered bays the sediment is fine sand, whereas the shallower, wave-influenced inshore parts of three bays consists of moderately to well-sorted medium sand. Offshore, away from the rocky island coasts and below wave base, the seafloor is covered in slightly shelly to shelly, slightly muddy to muddy fine sand. The channels between the islands are floored by various sediment types depending on tidal current strengths and depth. The shallows (less than 5 m) of the current-swept channel between Rangiahua and Motutaika are composed of coarse gravel and boulders, fining to coarse sand by 20 m depth. Most of the floor of the narrow channel between Rangiahua and Great Barrier, with its very strong tidal currents, is covered in algal rhodolith gravel, fining to coarse and medium sand by 20 m depth at the southern end. The sinuous, shallow channel between Rangiahua and Mahuki has much weaker currents and is floored by clean, well sorted medium and fine sand, becoming course sand at the west entrance and pebbly medium sand at the east entrance. The map (Fig. 2b) showing the distribution of shell, pebble and rhodolith gravel (greater than 2 mm) clearly shows the influence of strong currents and wave erosion in producing coarse sediment in the channels and around the shore lines. The map (Fig. 2c) showing the distribution of mud also clearly shows the effect of current activity and waves in preventing mud from settling in shallow water. MACROBENTHIC ASSOCIATIONS In analysing the live benthic biota (Appendix II) we have attempted to subjectively recognise recurrent sets of taxa, or "associations", that have considerable spatial continuity or have been recognised elsewhere. The associations are named after the dominant or most characteristic taxa. We recognise and map the distribution of six associations and three subassociations within the study area (Fig.3). 88 fine sand medium sand coarse sand - shell gravel pebble gravel rhodolith gravel rock and boulders muddy ^xjfk, mud) >500/, ^ 25-50°/, 15-25% 5-15% 0.5-5% Fig. 2. a. Distribution of seafloor sediments, Broken Islands. b. Percentage of pebble-size clasts, including shells (larger than 4 mm) in sediments. c. Percentage of mud (finer than 0.0625 mm) in sediments. l. Scalpomactra scalpellum-Dosinia subrosea association Stations: 5, 21, 25 Sediment: clean, fine to medium sand 89 Fig. 3. Distribution of macrobenthos associations, Broken Islands. Depth: 5-9 m Wave energy: moderately low Characterising taxa: BIVALVES. Scalpomactra scalpellum (5, 21, 25), Dosinia subrosea (5, 21). Common associated taxa: GASTROPODS. Zeacolpus pagoda (21, 25), 90 Cominella quoyana (5, 21;, Pupa kirki (5, 25), Epitonium minora (5, 21). This association is confined to sheltered areas on the south side of Rangiahua Island — in a narrow enclosed bay and in the central part of the narrow channel between Rangiahua and Mahuki. The infaunal bivalves that characterise this association are not recorded elsewhere in the study area. Other taxa, occurring in fewer numbers, not found outside this association are the infaunal bivalve, Gari lineolata and the gastropods, Zeacolpus pagoda and Antisolarium egenum. The gastropod, Epitonium minora is also almost entirely confined to this association, occurring elsewhere only in station 2, which is difficult to place in any association. Species diversity and abundance is low in this association with very little other than molluscs present. 2. Felaniella zelandica-Talabrica bellula association Stations: 20, 22 Sediment: clean to slightly muddy, shelly, medium sand Depth: 5-11 m Wave energy: moderate to moderately low Characterising taxa: BIVALVES. Felaniella zelandica (20, 22), Talabrica bellula (22). Common associated taxa: GASTROPODS. Cominella quoyana (20, 22), Xymene plebeius (22); BIVALVES. Glycymeris laticostata (22), Tawera spissa (20). This association occurs in moderately exposed, shallowish entrances to bays or channels on either side of Mahuki Island. The sediment is slightly coarser and exposure slightly greater than in the areas occupied by Scalpomactra-Dosinia association. Both characterising infaunal bivalves of this Felaniella-Talabrica association are not recorded elsewhere in the study area. Species diversity and abundance is low in this association with very little other than molluscs present.