Physical Environment
The Kaipara Harbour is a large harbour formed from a system of drowned river valleys (Hume & Herdendorf, 1988), with the inlet bounded by the ends of two sand spits (South Head and Pouto Peninsula). The mouth of the harbour contains extensive sand bars, and these combined with the high wave action from the exposed ocean, make entrance to the harbour treacherous. Marine sands are moved through the mouth by tidal action. In general, this type of harbour mouth is characterised by pronounced lateral instability, and usually occurs on parts of the coast where the direction of littoral drift fluctuates, and where the inlet position is not stabilised by headlands (Hume & Herdendorf, 1988). This certainly appears to be the case with respect to the entrance of the Kaipara Harbour. Traditional oral accounts of the arrival of the Mahuhu canoe in the Kaipara Harbour tell of a sand bar that extended from the heads into the harbour, dividing the waters of the Kaipara and the Wairoa. The remnants of this bar are still apparent as shallow water in places. However, in earlier times this bar was raised as a peninsula of dry land on which villages and cultivations were established. Similarly, the Kaipara bar was raised above the tide level, and formed a base for fishing expeditions (Jackson, 1997). Comparison of early maps with current maps of Pouto Peninsula indicates that the southern part of the peninsula has prograded significantly in the last century.
The Kaipara Harbour is the largest harbour estuary complex in Northland, with a total surface area of 947 km 2 . The harbour is predominantly shallow, with 409 km 2 of the total surface area exposed as mudflats at low tide (Heath, 1975). The harbour perimeter is convoluted by the entry of numerous rivers and streams, and is approximately 612 km long (Heath, 1975). Major rivers entering the harbour include the Wairoa, Arapaoa, Otamatea, Oruawharo, Tauhoa, Hoteo and Kaipara.
The harbour is characterised by strong tidal currents, which may reach up to 8 knots as a high volume of water flows through restricted or narrow channels. However, a model proposed by Heath (1976) suggests that the water circulation in the harbour is not dominated by tidal flow alone (as is the case in some harbours), but is also related to other factors such as vertical and horizontal two-dimensional wind induced circulation.
The land surrounding the harbour is comprised of unconsolidated to poorly consolidated sands, silts, clays and peaty deposits. Weathering of the sand deposits at the heads has helped to stabilise them as feldspar and ferromagnesium minerals break down to clays. The clays bind the more resistant sand particles and iron oxides – these cause the yellow-brown staining evident in the substrate.
In addition to the information about sediment grain size and composition in the sand barriers at the mouth of the harbour, there is little published information about the marine sediments in the harbour itself. The sand occurring along the eastern shore of the harbour is fine to medium grained silica sands of Pleistocene age, probably deposited in a similar environment to the sands at the harbour entrance and along the west coast (Applied Geology Associates, 1982). Elsewhere, the substrate is mostly comprised of fine mud and silt characteristic of sheltered estuarine areas.
The land in the catchment of the Kaipara Harbour is predominantly pastoral. Some areas of the shoreline have been significantly modified in the south of the harbour, significant reclamation has occurred along the coast. In places these reclamations have not been completed, and this has caused silting in what were previously sandy areas of coast. Between the early 1900's and 1950's, the areas in which natural rock oyster beds occurred were extended by placing rocks along the hard shorelines to act as an additional substrate on which the oysters could grow. This rock work was very extensive, and involved a large number of areas (26 in total) including some on the eastern shore of the lower Wairoa River, on either side of the Funnel, the Arapaoa, Otamatea, Whakaki, and Oruawharo Rivers, and near Moturemo Island. It is likely that this has also resulted in some additional sedimentation along these shores.
There is a general trend of increasing sedimentation in the harbour. While this is a natural process in most harbours, there is probably an increased sedimentation rate due in part to the deforestation of the harbour catchment, both in fires that occurred on Pouto Peninsula prior to European settlement (archeological evidence, see Jackson, 1997), and subsequent clearance of forest for timber.
The average rainfall in the area is in the range of 1,200-1,400 mm, with a peak occurring in the winter months. Summers are warm and humid, and winters cool and wet. Water deficits are not uncommon in the South Kaipara area from December to April (Cromarty, 1996).
Geological features of significance in the harbour include: the Hukatere Miocene mass flow fossiliferous deposits at Coral Point on the west side of the Hukatere Peninsula; the Pahi Peninsula greensand-limestone sequences from the tip of the Pahi Peninsula for 1.5 km along the western side; the Pakaurangi-Puketi shelf sediments exposed on the south-eastern corner of the Hukatere Peninsula; and the Strawberry Bay pillow lava, which are exposed around the coast of Strawberry Bay and Papuia Island on Kukatere Peninsula in low cliffs and intertidal rocks (Kenny & Hayward, 1993).
Flora & Fauna
The Kaipara Harbour is a highly productive ecosystem, with a high diversity of habitats, and sequences of ecotones including inter-tidal mudflats, mangrove forests, swamps, salt- rush and reed swamps, sand-flats, tidal reaches, saltmeadow and maritime rushes.
The Kaipara Harbour has extensive areas of tidal flats, grading into mangroves, and salt marshes. Much of the harbour is fringed with mangroves. There are approximately 40,000 hectares of mud and sand flats, with about 12,500 hectares covered by mangroves ( Avicennia marina var. resinifera ). Amongst the mangroves live a variety of organisms, including gastropods (for example, the mud snail Amphibola crenata , Zediloma subrostrata, Zeacumantus lutulentus, Melagraphia aethiops, Lunella smaragda, Cominella glandiformis, and numerous smaller species such as Potamopyrgus antipodum ), burrowing worms (such as Nicon aestuarensis ) and crustacea (such as the tunnelling crab Helice crassa ). Oysters ( Crassostrea gigas and Saccostrea glomerata ) can also be found settling on the trunks and pneumatophores (aerial roots) of mangroves in some areas.
Numerous large illegal reclamations, mainly for the purposes of agriculture, have occurred in mangroves in the harbour in the past – 17 out of a total 42 mangrove areas, have been partially or totally reclaimed. However, increased sedimentation of the harbour is now increasing the area and quality of mangroves in the harbour. For example, in a study done by the author in the Waioneke wetland area, comparison of aerial photographs of mangroves taken at intervals over the last 30 years indicated that the area covered by mangrove is increasing, and that the mangroves are increasing in size (B. Hay, AquaBio Consultants Ltd, unpublished data ).
The value of mangrove forest is wide ranging:
• they are a highly productive estuarine ecosystem of vital importance in animal food webs. It has been estimated that mangroves produce up to 10 tonnes of organic plant debris/hectare/year, and enrich the nearshore environment by a factor of at least 10 over similar coastlines lacking mangroves (Ritchie, 1976);
• they have a function in oxygenating the water and sediments of harbours;
• they provide important ecological links between marine ecosystems and terrestrial and freshwater ecosystems;
• they protect coastlines from erosion;
• they provide refuge for large numbers of bird species, including forest birds in the canopy, and waders in the tidal flats;
Much coastal fishery in New Zealand is fundamentally dependent upon mangrove forests ‑ at least 30 species of fish use mangrove wetlands at some stage of their life cycle, including commercially important species. It has been reported that of fish caught commercially, 80% are linked to food chains dependent ultimately on mangroves. The Kaipara Harbour plays a significant role in this respect: The marine and estuarine areas are noted for the breeding of important fish species, snapper (Chrysophrys auratus), mullet, ( Mugil cephalus ), flounder ( Rhombosolea sp. ), sole (Peltorhamphus novaezeelandiae), kahawai (Arripis trutta), trevally (Caranx georgianus), g urnard (Chelidonichthys kumu), yellow‑eyed mullet (Aldrichetta forsteri) and skates, rays and sharks. Being the largest such harbour on the west coast, and containing significant large areas of suitable juvenile habitat and breeding grounds, and having less problems with water quality than the Manukau, the Kaipara Harbour is the single most significant wetland for west coast fisheries (DOC, 1990).
Landward of the mangrove swamps the flats are only inundated by the sea at spring tides. Here the vegetation consists mainly of rushes and sedges (for example, the sea rush Juncus maritimus var. australiensis , the jointed rush Leptocarpus simplex and the sedge Scirpus americanus ). Beyond this lies saltmeadow, where the margin of the land has been built up to the continued trapping of silt and is rarely inundated by tides. Here the flora consists of such species as glasswort ( Salicornia ), and shore pimpernels ( Samolus repens ).
Oysters abound on the rocky shores of the Kaipara Harbour, both the native rock oyster ( Saccostrea glomerata ) and, slightly lower in the inter-tidal zone, the introduced Pacific oyster ( Crassostrea gigas ). Other bivalves are also plentiful: cockles ( Austrovenus stutchburyi ) on the lower tidal flats, mussels ( Perna canaliculus ) from low spring on the rocks to sub-tidal beds closer to the mouth of the harbour, and scallops ( Pecten novaezelandiae ) in the tidal channels. The scallop population appears to have periodic incidences of widespread, high mortality, the causes of which have not been identified. One such incident occurred in late winter-early spring 1998, in the scallop beds off Batley. In this case, mortality was virtually 100% in scallop beds closest to Batley, with lower mortality in populations closer to the mouth of the harbour. Samples of moribund scallops were taken from these populations and sent to a pathologist at NIWA to see if the mortality were caused by a bacterial disease. The author also sent samples to scientists at HortResearch to see if viruses could be detected in the scallops (viruses have been linked to scallop mortalities elsewhere). The results of this investigation were inconclusive. No obvious protozoan or bacterial infections were found in the samples (Dr. B. Diggles, NIIWA, Wellington, pers. comm .), nor did there appear to be large numbers of viruses present, although the results of the investigation to detect viruses was somewhat inconclusive (Dr P. Scotti, HortResearch, Auckland, pers. comm .). It is possible that in this case the mortalities were caused by environmental factors such as low salinity, or high levels of suspended sediment, but these were not detectable by the time the local people became aware of the problem with the shellfish.
Within the Kaipara Harbour, there are a number of sites where rare or endangered species are known, or have been sighted. These are summarised from the Department of Conservation (1990a, 1990):
The entire Kaipara is a migratory bird habitat of international significance. Forty-two coastal species are known, with up to 45,000-50,000 birds not uncommon. Rare international species that use the harbour for feeding during summer before returning to the northern hemisphere to breed, such as the bar‑tailed godwit (Limosa lapponica baueri), lesser knot (Calidris canutus canutus), and turnstone (Arenaria interpres interpres) are frequently noted. Threatened or endangered native species, such as North Island fernbird (Bowdleria punctata vealeae), crake (Porzana spp.), Australasian bittern (Botaurus stellaris poiciloptus), banded rail (Rallus philippensis assimilis), grey‑faced petrels (Pterodroma macroptera gouldi), banded dotterel (Charadrius bicinctus bicinctus) , NZ dotterel ( Charadrius obsurus , South Island pied oystercatcher ( Haematopus ostralegus finschi ), pied stilt ( H. haematopus leucocephalus ), and wrybill (Anarhynchus frontalis) are also present . Significant local populations of black swan (Cygnus atratus), pukeko (Porphyrio porphyrio melanotis), and grey duck (Anas superciliosa superciliosa) also breed in the area.
Reclamation has reduced the number of roosting areas for birds in the Kaipara Harbour. The Tapora coast is one of the major roosts, with Jordans Farm and Oyster Point being the only roosts in the southern part of the harbour. Other important roosting areas include Tinopai, and the Otamatea River.
Botanically , there are a number of different habitats, within which rare or unusual species are found. The Kaukapakapa forest contains the rare Prasophyllum pumilium. Barrs Road Forest is an example of lowland kahikatea (Podocarpus dacrydiodis) forest uncommon in the Auckland region, and Mt Auckland forest contains Marattia salicina, and Bulbophyllum tuberculatum, both rare. Moturemu Island has the endangered Clinathis puniceus (Kaka beak) once thought extinct in Auckland (Given et al., 1987). The Hoteo River, a significant freshwater input into the Harbour, arises from a deep gorge and flows directly into the harbour with a minimal estuarine zone. Such a habitat is rare in Auckland generally, and especially for the Kaipara where broad estuarine flats are the norm.
Many of the sites within the Kaipara Harbour are noted as being representative of their respective habitats and communities. Of particular significance are the following areas: forty mangrove swamps (as discussed in Chapman, 1976) and the Arapaoa River, Te Ngaio Point, Barrs Rd Forest as examples of successional habitats; East Kaipara, Tauhoa River and South East Kaipara for mangrove and saltmarsh habitats; Kaukapakapa forest and Pukekohuhu for terrestrial coastal forests; and the Hoteo River/Atuanui for successional sequences from estuarine to freshwater terrestrial habitats.
The Kaipara Harbour is an area of high ecological significance. It is recognised as a site of international significance for its bird species and habitats. The extensive estuarine mudflats and mangrove areas comprise a richly productive primary source in the marine food chain, supporting fish and bird life, both inside and outside the harbour. It is recognised for its contribution as the spawning grounds and juvenile habitats for important west coast commercial fisheries, and is of national significance as a site of spat collection for Pacific oyster farming.
References:
Applied Geology Associates. 1982. Coastal sand and shingle resources of Auckland and Northland. Unpublished report to the Auckland Regional Authority, April 1982.
Chapman, V. J. 1976. Mangroves and salt marshes of the Kaipara harbour: a study with proposals for preservation of areas supporting the harbour ecosystem. New Zealand Department of Lands and Survey, Auckland.
Cromarty, P. 1996. A directory of wetlands in New Zealand. Compiled by P Cromarty for the NZ Department of Conservation, International Waterfowl and Wetlands Research Bureau, and the Ramsar Convention Bureau. Published by NZ Department of Conservation, Wellington.
Department of Conservation. 1990. Coastal resource inventory: First order survey - Northland conservancy. Department of Conservation, Wellington.
Department of Conservation. 1990a. Coastal wetland inventory for northern New Zealand: Volume 1: Auckland Region (Draft). Department of Conservation, Wellington.
Given, D. R., Sykes, W. R., Williams, P. A. & Wilson, C. H. 1987. Threatened local plants of New Zealand: A revised checklist. Botany Division Report. Department of Scientific and Industrial Research, Christchurch.
Heath, R. A. 1975. Stability of some New Zealand coastal inlets. New Zealand Journal of Marine & Freshwater Research. 9 (4):449-57.
Heath, R. A. 1976. Broad classification of New Zealand inlets with emphasis on residence times . New Zealand Journal of Marine & Freshwater Research. 10 (3):429-44.
Hume, T. M. & Herdendorf, C. E. 1988. A geomorphic classification of estuaries and its application to coastal resource management - a New Zealand example . Ocean and Shoreline Management 11 :249-274.
Jackson, M. A. 1997. Pouto Peninsula: An archaeological perspective; The evidence of Moira Annette Jackson, amended 10 April, 1997. Statement of Claim by Te Uri O Hau O Te Wahapu (Wai 271).
Kenny, J. A. & Hayward, B. W. 1993. Inventory of important geological sites and landforms in the Northland region. Geological Society of New Zealand Miscellaneous Publication No. 67 (2 nd Edition).
Ritchie, L. D. 1976. Fish and fisheries aspects of mangrove wetlands. In: Proceedings of Symposium: Why are mangroves important? Whangarei, June 1976. Nature Conservation Council, Wellington.
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