Physical Environment
Erosion of the exposed rocky headlands, and deposition of sand in the embayments, has resulted in a relatively straight shoreline on this coast.
The South Head of the Hokianga Harbour is the end of a long ridge system formed of sandstone and mudstone, with imbedded conglomerates. The western coastline of the Head has steep cliffs and isolated beaches. To the south of the head, the coast consists of mobile sand dunes, often backed by consolidated dunes and cliffs, and interspersed with outcrops of basaltic and andesite rock (and associated intertidal reefs). The shoreline is intersected by several rivers: Waimamaku, Wairau, Ohae, Waipoua, and Muriwai, and many smaller streams. Geological features of international significance in determining Pleistocene and Holocene global sea level changes occur at Kawerua. These include paleobeach and sub-tidal deposits, and marine cut surfaces (DOC, 1990). At Muriwai, mobile sand dunes along the coast are backed by a long shallow dune lake with consolidated/weathered sands further inland, and a small wetland. To the south, Maunganui Bluff provides an outstanding feature, with the bluff and associated ridges rising to 400 m from a long stretch of unbroken coast.
Wave action on the shore is high, since the coast is exposed to the prevailing ocean swells from a southwesterly direction. This causes almost continuous turbulence, turbidity and sediment movement in the shallow coastal waters and inter-tidal areas.
The beaches are composed of clean quartz-based sand, with a median grain size of 0.26 mm. The sand is firmly compacted with little or no silt or clay. The quartz content of the sand averages about 56 % of the sediment (with the shell removed), and has a low heavy mineral content of about 5 % (Applied Geology Associates, 1982).
Erosion of the cliffs on the coast provides continuing sediment source for progradation of the beach (i.e. sand increase). There is some long-shore movement of sediment southwards along the shore, but overall the shore is considered to be in a state of equilibrium in the long term i.e. not eroding or building up sediment (Applied Geology Associates, 1982).
Offshore from the coast is the Tasman Sea. The sea floor consists of well-sorted sandy sediments sloping evenly to a depth of 40 m at about 1.5 to 2.0 km offshore. There are very few areas of reef or foul ground off this coast. The predominant offshore current is the West Auckland Current which moves southwards down the coast.
Flora & Fauna
Flora and fauna occurring on the reefs on this coastline are typical of northern exposed hard shorelines. The species diversity is high, often with distinct zones of plants and animals, ranging from the lichens, periwinkles, and limpets at the top of the intertidal zone, through barnacles, turfing algae and numerous species of mollusca in the mid-tidal zone, to dense kelp beds with associated populations of mollusca, kina, crabs and other crustacea. Paua and rock lobster occur sub-tidally, and at the low tide mark.
Off the coast, a diversity of fish species occurs, including snapper, trevally, gurnard, kahawai, kingfish and barracouta. Mullet also occur seasonally closer to the shore. Beds of mussels ( Perna canaliculus ) are found subtidally outside the harbour mouth.
The flora and fauna of the beaches are typical of exposed northern sand beaches. On such sandy beaches, the inter-tidal animals are robust and capable of moving quickly to avoid being dislodged or smothered by sand. Species diversity is comparatively low, and usually dominated by small crustacea (i.e. amphipods ("sea slaters") and occasionally isopods (e.g. "sand hoppers"), and by bivalve mollusca (for example, tuatua).
Common on the upper beach are the sand hopper Talorchestia quoyana and the sea slater Scyphax ornatus . The dominant shellfish, tuatua ( Paphies subtriangulata ) and toheroa ( Paphies ventricosa ) are located in the mid to low inter-tidal zone of the shore. Both species are patchy in distribution,and local knowledge is usually necessary to locate the denser beds. On the west coast of Northland the toheroa has been exploited to such an extent that harvesting is highly restricted, but tuatua, which occur lower down the beach, are still present in numbers that make the species attractive to gather.
Further up the shore, marram, spinifex and pingao occur on the sand dunes, often with manuka shrubland further back. Fuschia procumbens occurs on coastal rocks at Waipoua. Vegetation in wetland areas (for example, Muriwai) includes raupo, rushes ( Leptocarpus species) and flax species. The coastal vegetation on South Head comprises low windswept manuka and flax with coprosma and cabbage trees, and a small area of coastal hardwood forest. Adjacent to the west coast are areas of grassland with flax. Coastal forest of national significance is present on Maunganui Bluff, with flax on steep slopes, and ice plant on the sea cliffs (DOC, 1990).
Coastal shore birds are common, with wetlands forming an important habitat for wetland birds. Rare bird species found on the coast include the NZ dotterel. Kawerua is an important little blue penguin breeding area. Maunganui Bluff itself is regarded as a high value habitat with pied tit, kiwi and kauri snails (DOC, 1995).
References:
Applied Geology Associates. 1982. Coastal sand and shingle resources of Auckland and Northland. Unpublished report to the Auckland Regional Authority, April 1982.
Department of Conservation. 1990. Coastal Resource Inventory - First order survey: Northland Conservancy. Compiled by Tim Shaw & Joan Maingay.
Department of Conservation. 1995. Draft conservation management strategy for Taitokerau, Northland. Volume 1 .
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