An Intertidal Zostera Pool Community at Kawerua, Northland and Its Foraminiferal Microfauna

TANK 25, 1979 AN INTERTIDAL ZOSTERA POOL COMMUNITY AT KAWERUA, NORTHLAND AND ITS FORAMINIFERAL MICROFAUNA by B.W. Hayward New Zealand Geological Survey, P.O. Box .10-368. Lower Hull SUMMARY An unusual occurrence of the sea grass, Zostera capricorni, living in an intertidal pool on the storm-swept west coast of Northland is described. The macrofauna associated with the Zostera is similar to that found associated with it elsewhere in its more normal sheltered subtidal and intertidal sandy mud Hat habitat. The pool has a sandy hot lorn with rocks scattered throughout. The rocks support a rich macrofauna and flora both over their sides and beneath them and this is briefly described. Living foraminiferal microfaunas were found in the sandy sediments that cover the pool bottom, as well as on Corallina turf, but were not found on the laminae of Zostera or on the brown alga Hormosira banksii. The domi• nant foraminifera on Corallina turf are Rosalina bradyi and Ammonia beccarii with a number of less numerous species. The bottom sands are dominated by live Florilus parri (27-42%), Ammonia beccarii (7-28%) and Evolvocassidulina orientalis, Quinqueloculina seminulum, Rosalina bradyi, Cassidulina carinata, Elphidium charlottensis and Discorbinella bertheloti (all 3-10%). At suborder level the fauna consist of about 90% Rotalina, 9% Miliolina and less than 1% Textulariina. Counts of stained (live) foraminifera indicate standing crop abundance values of 10 000-16 500 liv• ing foraminifera per square metre of pool bottom. The faunas contain an unusually high number of planktonics (13-17% of total) for an intertidal situation, but this is thought to be due to the prevalence of onshore winds that drive oceanic water in close to the west coast of New Zealand. INTRODUCTION The Auckland University Field Club Scientific Station is situated at Kawerua on the exposed west coast of Northland, 14km south of the mouth of the Hokianga Harbour (Fig. 1). Directly in front of the station, the coast consists of a number of basaltic reefs that extend out into the sea for up to 100m. They enclose lagoons and numerous large and small intertidal pools, providing a diversity of habitat types and wealth of marine life available for study. This article describes the macroflora and fauna, and foraminiferal microfauna of only one of the many types of intertidal pool present. 173 Fig. 1. The locution of the Zostera pool al Kawerua on the west coast of Northland, New Zealand. The Zostera Pool The pool studied is situated 50m north of the track which leads down to the beach from the Kawerua Station (Fig. 1). It is at approximately mid- tidal level and has no connection with the sea for about 6 hours during every low tide. The pool is protected from the fierce Tasman Sea waves by a solid reef across its western side (Fig. 1). It is enclosed by low piles of boulders covering flat rock shelves to the north and south, and by a quartz sand beach on the eastern, shoreward side. The pool itself is approximately 30 x 15m in size and has a water depth of 5-25cm when not submerged. The pool contains numerous rocks (10-30cm diameter) scattered throughout. Much of its floor is covered by a l-15cm thick layer of shelly, quartz sand, but in places where there is an abundance of rocks, the sediment is fine gravel. Small clumps (up to 30cm diameter) of the sea grass, Zostera capricorni, occur sporadically in parts of the pool and around their roots the sediment is dark muddy sand. INTERTIDAL ZOSTERA POOL COMMUNITY 1. Zostera and its epifauna The sea grass, Zostera, is characteristic of sheltered coasts and well pro- 174 tected situations and grows in shallow subtidal or intertidal localities which do not dry out completely at low tide (Armiger 1965). Armiger (1965) says "On the exposed west coast of the Auckland Province, characterised by rocky surf beaches and sand dune formations, Zostera is absent, except at the mouth of the Waikato River and Manukau Harbour". Its presence at Kawerua is therefore all the more unusual, for it manages to survive (cer• tainly not thrive) in this very localised habitat sheltered by the rock reefs around it. At Kawerua, small clumps of Zostera also grow in a sandy substrate just below spring low tide level, in several small sheltered embayments between the reefs. In the intertidal pool, the Zostera laminae support many of the epifaunal species that occur on them in the more usual, well protected situations. The most common organisms found clinging to the Zostera (Fig. 2) are the small, browsing, herbivorous gastropods Zeacumantus subcarinatus, Cantharidella tessellata, Estea zosterophila and the minute limpet Notoacmea helmsi scapha. Also quite common is the small gastropod Eatoniella olivacea, which lives by scraping detritus and diatomaceous film from the surface of the laminae. Other organisms occasionally present are the small bivalve Notolepton antipodum and the large ostracod Cyclasterope zelandica. On one occasion, a sea horse Hippocampus abdominalis was found with its tail entwined in the Zostera laminae. 2. Infauna around Zostera The well sorted, medium-coarse sand that covers most of the pool floor is devoid of any infaunal macrofauna, but the muddy sand substratum held firmly in place by the Zostera roots supports an infauna (Fig. 2) similar to that found in Zostera beds elsewhere. The most common organisms present are the sand mason worm Pectinaria australis, the tube-building sabellid worm Owenia fusiformis and the deposit-feeding bivalves Macomona liliana and Nucula hartvigiana. Also occasionally found is the small, pink holothurian Trochodota dunedinensis. 3. Epifauna beneath Zostera Another group of organisms (Fig. 2) live predominantly in the shelter provided by the stable sedmiment substrate around the Zostera clumps. Here, most common are the bullomorph Haminoea zelandiae, which feeds on Zostera and the organic muds that accumulate around its base, the scavenging hermit crab Pagurus and omnivorous cushion star Patiriella regularis. Less common are the carnivorous scavenging gastropods Cominella adspersa and C. glandiformis, the boring gastropod Xymene plebejus, the hermit-like tanaid Pagurapseudes and hairy pillbox crab Hymenicus pubescens. 4. Epifauna and flora on or beneath rocks (Fig. 2) Subrounded to rounded rocks occur throughout the pool, generally held 175 Fig. 2. Schematic diagram of the intertidal Zostera pool habitat at Kawerua. Upper part illustrates the macrofauna associated with clumps of Zostera and lower part illustrates the macrofauna and flora associated with rocks. Drawings of organisms adapted from Morton and Miller (1968). Aca = Acanthochiton zelandicus, Act = Actinia olivacea. Amp = Amphiura sp., Apo = Apophloea sinclairii, Can = Cantharidella tessellata, Cel = Cellana radians, Cha = Chamaesipho columna, C. off - Corallina officinalis. Col = Colpomenia sinuosa. Com = Cominella adspersa, Cor = Coryphellina sp., Cra = Crassostrea glomerata. Eat = Eatoniella olivacea. Est = Eslea zosterophila, Ham = Haminoea zelandiae, Hip = Hippocampus abdominalis, Hor = Hormosira banksii, Hyd = Hydroides norvegicus, Hym = Hymenicus pubescens, Mac = Macomona liliana, mem = membraniporiform bryozoa, N. hel = Notoacmea helmsi scapha. Not = Notolepton antipodum, Oka = Okadaia cinnebareus. Owe = Owenia fusiformis, Ozi = Ozius truncatus, Pag = Pagurus sp., Pal = Palaemon affinis, Pat = Patelloidea corticata, 176 in a stable position by the sand around their base. The tops of the larger rocks are exposed when the tide recedes and these upper surfaces support dark brown mats of the encrusting alga Apophloea sinclairii, together with the shells of the rock oyster Crassostrea glomerata and small barnacle Chamaesipho columna. Attached to rocks around the level of the water in the pool are numerous plants of Neptune's necklace Hormosira banksii and the seasonal brown alga Colpomenia sinuosa. Beneath water level and attached to the rocks near the water surface are the red algae Pterocladia capillacea and P. lucida, the seasonal, green, sea lettuce Viva lactuca and clumps of the coralline algae Corallina officinalis and Amphiroa anceps. Deeper in the water, occasional plants of Vidalia colensoi are attached to rocks as are mats of the green alga Enteromorpha. The limpets Cellana radians, C. ornata, Patelloidea corticata, the chitons Acanthochiton zelandicus, Sypharochiton pelliserpentis and the catseye Lunella smaragda are generally found grazing on the sides of rocks below water level but they also move over the top surfaces when these are submerged at high tide. A number of encrusting organisms occur low on the sides of the rocks, often partly sheltered beneath slight overhangs. These include the serpulids Hydroides norvegicus, Pomatoceros caeruleus and Spirorbis, membraniporiform bryozoa and the byssally attached small mussel Xenostrobus pulex. A diverse fauna lives around the lower margins of the rocks and beneath them. Attached to the undersides of rocks are the sea anemone Actinia olivacea, the barnacle Tetraclita purpurascens and the athecate, encrusting hydrozoan Syncoryne. Also often found clinging to the rocks' undersurface are the brittle star Amphiura, polyclad flat worm Stylchoplana, the small herbivorous gastropod Rissoina anguina, the red and white sea-slug Coryphellina and the pure red sea-slug Okadaia cinnebareus. The most common crabs beneath rocks are the blue-green half crab Petrolisthes elongatus and black-finger crab Ozius truncatus, but also present are the big-hand crab Heterozius rotundifrons and the pill-box crab

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