ECOLOGY OF WAIMAMAKU RIVER ESTUARY, NORTH OF KAWERUA, NORTH AUCKLAND. by Bruce W. Hayward and Christopher J. Hollis SUMMARY

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1 ECOLOGY OF WAIMAMAKU RIVER ESTUARY, NORTH OF KAWERUA, NORTH AUCKLAND by Bruce W. Hayward and Christopher J. Hollis Auckland Institute and Museum, Private Bag 92018, Auckland SUMMARY The 3 to 3.5 km long Waimamaku River estuary, as defined by the limits of tidal influence and saline water penetration, is 30 to 80 m wide and 2 to 4.5m deep at high tide. Upper reaches of the estuary have a subtidal faunal association dominated by pipi (Paphies australis), together with common Pacific oyster (Crassostrea gigas) and mud whelk (Cominella glandiformis). Intertidally the margins of the upper reaches are characterised by abundant tunnelling mud crabs (Helice crassa), freshwater snail (Potamopyrgus estuarinus) and clumps of sedge (Cyperus ustulatus) and extensive sea rush (Juncus maritimus var australiensis). Lower reaches of the estuary have a richer and more diverse subtidal fauna still dominated by pipi (up to 90 per litre of sediment) but with common cockle (Austrovenus stutchburyi), wedge shell (Macomona liliana), heart urchin (Echinocardium cor datum) and sand mason worm (Pectinaria australis). The lower reaches have an intertidal fringe of jointed rush (Leptocarpus similis) interspersed with basalt boulders and the fauna is dominated by the hairy-handed crab (Hemigrapsus crenulatus), the sand hopper (Talorchestia tumida) and the long-legged isopod (Ligia novaezelandiae). The smooth shore crab (Cyclograpsus lavauxi) and a number of other normal marine intertidal organisms (e.g. acorn barnacle, rock oyster, periwinkle, green-lipped mussel) are present on and under cobbles and boulders near the mouth. INTRODUCTION Location Waimamaku River estuary lies 5 km north of the Auckland University Field Club's field station at Kawerua on the west coast of Northland (Fig. 1). The estuary is at the mouth of the 20 km Waimamaku River which drains the largely pastoral Waimamaku Valley. Methods Waimamaku River estuary was studied during a two day visit in April Marginal and intertidal sediments were sampled by hand and the biota qualitatively recorded. Subtidal biota and sediments were sampled using a 5 litre 69 Tane, VoL 34, 1993

2 capacity bucket dredge, hand-hauled from a 4 m dinghy powered by a 7.5 horse power outboard motor. Approximately 200 cc of sediment from each dredge sample was retained for later study and the remainder was passed over a sieve with 2 mm openings and live organisms identified and counted. Sampling stations are shown in figure 1. Waimamaku River estuary The lower Waimamaku River meanders through alluvial flats for 5 km and then straightens to pass through a m wide, 1.3 km long gorge to reach the Tasman Sea. During periods of normal flow, the meandering alluvial path of the river is largely confined within a m wide entrenched channel with 1-2 m high banks carved out of the alluvial plain. The gorge has cut through a 250 m high barrier of coastal hills composed of early Miocene conglomerate and basalt flows (Hayward 1973). Large blocks, boulders and cobbles of basalt line some of the margins of the estuary in the vicinity of the gorge (Fig. 2). The river mouth is partially blocked by a 200 m wide stretch of mobile beach and dune sands. The river outlet is never completely blocked, although sand build up during periods of low river flow and its subsequent removal by floods, has a strong influence on the tidal range within the estuary. When sand has built up in the outlet channel, the estuary tidal range may be restricted to less than 0.5m. When the channel is recently cleared of sand the tidal range may be in excess of 1 m. For many years the outlet channel has been located along the northern side of the valley mouth and windblown and storm-driven sand has been advancing into the estuary in the centre of the mouth. At the present time this has produced an anaerobic backwater (station 22) along the southern side of the valley mouth where a former outlet channel used to flow. The upstream limit of the Waimamaku River estuary is 3 to 3.5 km from its mouth. Tidal effects extend approximately 3.5 km up the river and the presence of the mud crab Helice crassa suggests that brackish water penetrates about 3 km upstream (to station 1) during periods of low river flow. BATHYMETRY The upstream reaches of the estuary (upstream of stn 7) are largely bordered by steep alluvial banks and have little in the way of intertidal flats or beaches. Further downstream the estuary has a number of areas that are bordered by narrow intertidal beach and several areas of extensive intertidal muddy sand flat (around stations 8-9, 14, 22-23). Subtidally within the estuary there is a 5-15 m wide main channel that meanders from one bank to the other, usually hugging the outside of bends. At high tide this channel is 2 to 4.5m deep (average 3-70 fane, VoL 34, 1993

3 3.5m) and is 0.5 to 1 m shallower at low tide. The two deepest parts of the channel occur on straights (4 m at stations 4 and 5; 4.5m at station 16). SEDIMENT The sediment grain size recorded in the samples shows little consistent or mappable pattern. Most of the subtidal and intertidal sediment is fine sandy mud or muddy fine sand. Several dredge samples showed a layering of channel bottom sediments with coarse sand overlain by mud. This layering is probably a result of mud sedimentation during periods of low and normal flow and coarse sediment transport and fine sediment erosion during flood events. Pebble gravel deposits occur intertidally on the inside of several bends in the upstream part of the estuary and boulder and cobble deposits occur intertidally where the estuary passes through the gorge. Clean, well-sorted, medium-grained beach and dune sand partially blocks the mouth of the estuary and also forms the bottom sediment of the outlet channel (stations 23, 24). 71

4 MARGINAL VEGETATION (Fig. 2) Salt marsh vegetation within the upper part of the tidal zone on the margins of Waimamaku River estuary exhibits salinity-related zonation. Clumps of sedge (Cyperus ustulatus) occur adjacent to the upper reaches of the estuary (statio 1 to 3) but not nearer the sea. The sea rush (Juncus maritimus var australien dominates extensive areas of salt marsh in the upper and middle reaches of the estuary (stns 2, 3, 10, 14) but is less common near the mouth. The dominant plant in the fringing salt marsh of the middle and lower reaches of the estuary is the jointed rush (Leptocarpus similis). Raupo (Typha orientalis) occu several large clumps in low lying backwaters at the mouths of small tributary streams in several places (near stns 1, 3, 17) and salt-marsh ribbon wood (Plagianthus divaricatus) is common in the higher parts of the salt mar adjacent to stn 3. The reason for the absence of mangroves (Avicennia marina from Waimamaku River estuary is not clear. They are certainly abundant in the Hokianga and Kaipara harbours to the north and south. A diverse ground cover flora (e.g. Lilaeopsis novae-zelandiae, Sellie radicans) occurs sporadically at or above high tide level within or adjacent to the taller salt marsh vegetation. Fruticose lichens (Teloschistes chrysophthalmus SALT MARSH r -i T sea rush, Juncus maritimus australiensis \^=>\ intertidal boulders L jointed rush, Leptocarpus similis *'.[ intertidal pebbly sand M sedge, Cyperus R raupo, Typha ustulatus orientalis intertidal sand 500m < common Leptocarpus common Cyperus ustulatus similis common Juncus maritimus australiensis Fig. 2. Distribution of intertidal environments and salt marsh vegetation along the margin of Waimamaku River estuary. 72

5 Ramalina celastri) grow on the branches and twigs of the ribbon wood and a mixture of crustose (Buellia sp.) and foliose lichens (Dirinaria applanata, Parmotrema chinense, Physcia tribacioides, Xanthoria ligulata, X. parietina) cover some of the larger basalt boulders on the margins of the lower estuary where it passes through the gorge. SUBTIDAL BENTHOS (Fig. 3) The subtidal sediments of Waimamaku River estuary have a fauna dominated by pipi (Paphies australis) and cockle (Austrovenus stutchburyi). Two associations can be recognised: a. An upper estuary association (stations 2-13), characterised by common pipi together with less abundant Pacific oyster (Crassostrea gigas) and mud whelk (Cominella glandiformis). Also occasionally present are cockle, the wedge shell (Macomona liliana) and estuarine mussel (Xenostrobus securis). b. A lower estuary association (stations 15-23), characterised by even more abundant pipi (up to 90 per litre of sediment, stn 15), together with common cockle (up to 6 per litre), and frequent heart urchin (Echinocardium cordatum) and sand mason worm (Pectinaria australis). This association is generally richer and more diverse than that in the upper estuary and also includes frequent horn shell (Zeacumantus lutulentus), and the polychaete worms Platynereis australis and Glycera lamellipodia. Pacific oysters were also seen in this association, although none were taken in the dredge samples. The pill-box crab (Halicarcinus whitei) and stalk-eyed mud crab (Macropthalmes hirtipes) were recorded in subtidal sediment in the middle reaches of the estuary, straddling the two associations. The only fish life seen during our visits was an adult mud flounder (Rhombosolea retiaria). Of interest and perhaps some concern is the extent of penetration almost into freshwater of the recent New Zealand immigrant, the Pacific oyster. INTERTIDAL BENTHOS (Fig. 4) Crabs are the dominant animals of the intertidal margins of the Waimamaku River estuary. The dominant crab in the upper and middle reaches of the estuary (stations 1-14) is the tunnelling mud crab (Helice crassa). Their tunnels riddle the near vertical soil banks and areas of intertidal flat. Common under stones and wood debris in the middle and lower reaches of the estuary (stations 14, 24) is 73

6 the hairy-handed crab (Hemigrapsus crenulatus), with the smooth shore cra (Cyclograpsus lavauxi) only present beneath rocks around the lowest, near normal salinity, reaches of the estuary (station 21). The estuarine form of the freshwater snail (Potamopyrgus estuarinus) is common grazer on the muddy sand in salt marsh on the fringes of the upper estuary (stations 1-3). The mud snail (Amphibola crenata) is extremely rare in this estuary and was only found in very small numbers on the intertidal mudflats around station Platynereis australis Cominella glandiformis SUBTIDAL Fig. 3. Distribution of dominant subtidal benthic animals in Waimamaku River estuary. 74

7 Other Crustacea that occur in abundance in the intertidal zone in the middle and lower reaches of the estuary fringe, beneath stranded driftwood and seaweed, are the sand hopper (Talorchestia tumida) and the fast-running isopod (Ligia novaezelandiae). Organisms living intertidally on rocks just inside the mouth of Waimamaku River estuary (station 24) indicate that near normal salinity conditions prevail here at high tide. Present in numbers are the acorn barnacle (Chamaesipho columna), the rock oyster (Crassostrea glomerata), the green-lipped mussel (Perna canaliculus) and the periwinkle (Littorina unifasciata antipodum). Five bird species were seen feeding in the estuary during our visits. The most common were Grey duck (Anas superciliosa), Pied shag (Phalacrocorax varius) and Pied stilt (Himantopus leucocephalus), with only a single Whitefaced heron (Ardea novaehollandiae) and Black shag (Phalacrocorax carbo) being seen. Potamopyrgus estuarinus <^ Littorina unifasciata antipodum Cyclograpsus lavauxi Hemigrapsus crenulatus Helice crassa ^> Talorchestia tumida ^> Ligia novaezelandiae Fig. 4. Distribution of dominant intertidal benthic animals in Waimamaku River estuary. ACKNOWLEDGEMENTS We thank Mike Eagle for company in the field and Brett Stephenson for identification of the crabs. Anthony Wright and Brett Stephenson read the manuscript and suggested improvements. 75

8 REFERENCE Hayward, B.W. 1973: Miocene stratigraphy of the Hokianga-Waimamaku Coastline, :rt of Kawerua, North Auckland. Tone 19: APPENDIX I. Waimamaku Estuary station data. Stn Depth Sediment Auckland Museum number 1 HT sand L m sandy mud LI HT slightly muddy sand L m mud L m pebbly mud L m mud over sand L m pebbly mud L LT muddy fine sand LI MT muddy sand LI HT soil LI m muddy sand L m muddy sand LI m slightly sandy mud LI MT muddy sand LI m muddy sand L m muddy fine sand L HT fine sandy mud L m mud Li m muddy fine sand L lm fine sandy mud L MT fine sand L m black mud L lm medium sand L MT medium sand L1176 APPENDIX II. Species counts for Waimamaku Estuary dredge stations. For each tax on. the stations at which it occurs alive is given, followed in brackets by the number of live individuals present in the sample. When no stations are given, the record is qualitative for the estuary. PLANTS LICHENS Buellia sp. Dirinaria applanata Parmotrema chinense Physcia tribacioides 76

9 Ramalina celastri Teloschistes chrysophthalmus Xanthoria ligulata Xanthoria parietina VASCULAR PLANTS Juncus maritimus australiensis Leptocarpus similis Lilaeopsis novae-zelandiae Cyperus ustulatus Phormium tenax Plagianthus divaricatus Selliera radicans Typha orientalis ANIMALS POLYCHAETA Glycera lamellipodia 22(1) Pectinaria australis 15(1),16(2),18(2),19(4),20(5) Platynereis australis 4(1),19(2),22(2) indet. 15(2),18(1),19(4) GASTROPODA Amphibola crenata 9(1) Cominella glandiformis 7(1),11(7),13(2),20(2) Littorina unifasciata antipodum 24 Philine sp. 15(1) Zeacumantus lutulentus 19(1),20(2) Potamopyrgus estuarinus 3,14 BIVALVIA Austrovenus stutchburyi 5(1 ),6(1), 13(6), 15(25), 16(6), 18(1), 19(4),20(30),22(1) Crassostrea gigas 4(2),5(1), Crassostrea glomerata 21,24 Macomona liliana 7(1),16(3),19(1),20(3),22(3) Paphies australis 2(1),5(1),6(29),11(3),12(19),13(11),15(450),16(10),18(6),19(25),20(75),22(3), 23(35) Perna canaliculus 24 Xenostrobus securis 4(7), CRUSTACEA Chamaesipho columna 24 Cyclograpsus lavauxi 21(8) Halicarcinus whitei 12(1),15(1),20(1) Helice crassa 1(12),14(7) Hemigrapsus crenulatus 13(1),14(6),24(4) Ligia novaezelandiae 14 77

10 Macrophthalmus hirtipes 13(1), 16(1) Talorchestia tumida 14 ECHINODERMATA Echinocardium cordatum 15(1),16(2),18(1),19(4),22(1) AVES Phalacrocorax varius (Pied shag) 3 Phalacrocorax carbo (Black shag) Anas superciliosa (Grey duck) 10 Himantopus leucocephalus (Pied stilt) 4 Ardea novaehollandiae (Whitefaced heron) FISH Rhombosolea retiaria (mud flounder) 14 78