Water flows in a constant cycle from ocean to sky and back again, often taking an overland route on its return journey to the sea (see Figure 7.1). In a complete cycle water evaporates from the oceans and is carried in moist air currents which are eventually forced up into cooler altitudes when they pass over landmasses. As it cools the air's moisture condenses into droplets that are visible as clouds. With further cooling the droplets become raindrops or snowflakes which are too heavy to remain in the air and fall to the ground as precipitation. In New Zealand, precipitation mainly occurs as rain, but snow is also common in mountainous areas.
Some precipitation is absorbed by plants and animals and is re-evaporated back into the atmosphere, some flows back to the sea through streams, rivers, lakes, wetlands and estuaries (known as surface waters), and some seeps down into underground sediments, rock fractures, and caves (known as groundwaters or aquifers). Groundwater may flow back into surface waters, lie immobile in confined aquifers, or flow underground through unconfined aquifers to the sea.
The main factors influencing New Zealand's water cycle are the prevailing westerly winds and ocean currents (which bring us our rain), the country's latitudinal position (which extends from warm subtropical waters to cool subantarctic waters), the size and position of our mountains (which determine where the rain falls and where the rivers run), and the amount of forest cover (which determines how much water is returned to the sky as evaporation and how much runs back to the sea as surface and ground water).
New Zealand's natural water environment, therefore, consists of four main components: rainfall (which also includes other forms of precipitation, such as snow and hail); surface water (which includes streams, rivers, lakes and wetlands); groundwater (which includes both ambient and geothermal underground water); and the marine water into which they all drain (which includes estuaries, coastal waters and the open sea) . Interwoven with these is an unnatural, but highly important, fifth component - the piped (or reticulated) drinking water and drainage systems that supply clean water to, and remove dirty water from, most of New Zealand's households.
The total annual amount of precipitation in New Zealand is anywhere between 300,000 million and 600,000 million cubic metres (Mm3). No one knows for sure because yearly rainfall has not been evenly recorded throughout the country. In most parts of the North Island rainfall averages between 1,200 and 2,400 millimetres (see Figure 7.2). In the South Island rainfall shows greater regional variation. Generally, the west is wetter and the east is drier, with the most extreme variations in the far south.
The driest area, Central Otago, is east of the Southern Alps and has an average yearly rainfall of around 350 mm per year, sometimes dipping to 300 mm, while Fiordland and Westland, west of the Alps, have an average rainfall of more than 6,000 mm (or 6 metres) with some sites occasionally exceeding 13,000 mm. The record of more than 14 metres of rain was set in 1982 at Waterfall Creek in Westland (Statistics New Zealand, 1995).
The reason for the uneven rainfall is the high mountain backbone of the country. This forces the moist westerly winds from the Tasman Sea to rise, get colder, and release most of their moisture as rain and snow west of the main divide. The dried-out winds then sweep eastwards leaving a dry 'rain shadow' east of the mountains. One result of these differences in rainfall is that areas with the least population (e.g. Fiordland, Westland, and Tasman) have the most water, while areas with the greatest demand sometimes have water shortages.
Rainfall also varies with the seasons. Most areas receive more rain during winter and spring than during the summer and autumn. In many areas these variations can result in floods (see Box 7.2). In the rain shadow areas they can also result in prolonged dry spells which affect farmers and townspeople alike by reducing pasture growth, urban water supplies and hydro-electricity supplies (see Figure 7.3 and Box 7.3).
Flooding has been the most common reason for declarations of civil defence emergency in New Zealand. In the 19th Century it was dubbed 'the New Zealand Death' (McConchie, 1992). Flooding can occur in any season and in all regions, although the steep, mountainous, catchments of the South Island's West Coast have the highest frequency. Even drought-stricken Auckland was afflicted by flood damage in the midst of its 1994 water supply crisis. It is likely that flooding increased after New Zealand's two main episodes of forest clearance following Māori and European settlement. Because most land clearance occurred before 1920, when few flood records were kept, the full impact on flooding is unknown. However, modern comparisons of forested and deforested catchments suggest that it must have been considerable (e.g. Dons, 1987; Smith, 1987; Duncan, 1994).
Flood trends since 1920 are also difficult to establish, partly because the definition of a flood event often depends on whether serious property damage occurred. Even if rainfall and river flows had been constant since 1920, we might expect the amount of property damage to increase simply because of population growth and urban and farm development in flood-prone areas. A recent review of river flow data found no overall trends in the frequency of high or low flows (Pearson, 1992). Nor has it been possible to draw a firm conclusion about national trends in damaging floods (Ericksen, 1986).
A total of 820 damaging flood events were recorded from 1920 to 1953 (averaging 25 events per year), but only 118 were recorded from 1955 to 1985 (averaging 4 per year). The two sets of records are not comparable, however, because the latter refers only to floods affecting towns. The Ministry of Civil Defence has listed 10 major flood 'disasters' in the period 1986 to 1991, but these excluded floods affecting less than 100 people (Ministry of Civil Defence, 1994).
An indication of extreme flood trends in some catchments is available from historical records, such as those for the Wairau River in Marlborough in the north east of the South Island, and the Waipaoa River in Gisborne in the east of the North Island. The Wairau catchment was cleared and settled well before the end of the last century, and has had at least one damaging flood every decade since European settlement (Williman, 1995). Floods were so frequent in the early days that Blenheim was known as Beavertown. Prior to 1920, a total of 30 years were spent on the construction of river control works. Flood records from 1920 to 1990 show a decline in small floods but little change in the frequency of extreme floods. Eleven floods had flows greater than 3,000 cumecs (3 million litres per second). Six of these occurred in the 35 year period prior to 1955, and five occurred in the 35 year period since, two in 1983 (Williman, 1995).
Gisborne's Waipaoa River catchment, on the other hand, was still being cleared of forest up to the 1920s. Extreme floods in this catchment therefore became more numerous in the ensuing decades. Between 1900 and 1990 the Waipaoa river had 29 extreme floods, in this case defined as flows greater than 1,500 cumecs (1.5 million litres per second) (Kelliher et al., 1995). Of these, 10 occurred in the 45 years up to 1945, and 19 occurred in the 45 years since. The six largest floods (i.e. those with flows greater than 3,000 cumecs) occurred in 1906, 1910, 1948, 1950 and March and September of 1988.
Across the nation, the yearly cost of flood damage was estimated in 1986 at $90 million (around $125 million in today's terms). In 1988 this doubled as a result of Cyclone Bola. These costs were in addition to the $30 million spent annually on flood protection, and millions more spent on insurance (Ericksen, 1986; Ministry for the Environment, 1992; Ministry of Civil Defence, 1994). The current national cost of flooding is unknown but is unlikely to have diminished. Typical flood damage includes erosion, deposits of sediment over vegetation, reduced farm production, and damaged crops, roads, bridges and buildings. Flooding also causes ecological damage, eroding river banks and depositing sediment which can destroy coastal seaweed and shellfish communities as well as freshwater fish habitat and populations (Jowett, 1992; McDowall, 1993). Flood control efforts also have ecological impacts. Channelisation and stopbanks have degraded the natural character of most lowland rivers and streams in New Zealand.
In 1941 the New Zealand Government passed the Soil Conservation and Rivers Control Act to limit soil erosion and the effects of flooding. This established catchment boards to coordinate erosion and flood control within regions and it also set up a national coordination and research body. A considerable amount of money was spent on flood protection and river control works, including the construction of flood protection embankments (or stopbanks) and drainage works. From 1941 to 1963, 25 large-scale river control schemes were initiated (Roche, 1994). This figure excludes the largest flood control scheme in New Zealand, the Waihou Valley Scheme in the Thames Valley, which was conceived in the 1960s with construction running from the 1970s into the 1990s.
Despite the legislation and massive investment in prevention measures, flood losses have continued to rise. Part of the reason is increasing urbanisation. Many studies have shown that paving and drainage systems in urban areas increase flooding, particularly as many urban areas are located on floodplains and former wetlands (McConchie, 1992). A study of Auckland's North Shore found that: average annual floods are nearly doubled by the provision of stormwater drains and sewers, which concentrate flows; the peak flow of a two-year flood increases by more than 4 times when a catchment is fully urbanised; the peak flow of large floods, such as 50 and 100-year events, more than doubles; floods rise to a peak and fall away more rapidly than in rural areas; and the number of bank 'overflows' increases-perhaps doubling where a fifth of the catchment has storm sewers and paving (Auckland Regional Authority, 1983).
The suggestion has been made that, in some areas, the river control works themselves have increased the amount of flood damage by attracting urban developers and farmers into flood-prone areas that are not safe from extreme events (Ericksen, 1986). Nearly 100 communities in New Zealand are currently prone to flooding. Of 17 urban areas flooded since 1970, most (80 percent) had some form of flood control. Two-thirds of our cities with more than 20,000 people have river flood problems. While entire urban areas are not likely to be inundated, a 1981 study of 18 communities found that an average of 20 percent of the built-up area lay within the historical flood zone (Ericksen, 1986; McConchie, 1992).
Periods of low rainfall in New Zealand are shorter and less widespread than those experienced in such places as Australia or parts of Africa, but they can have significant impacts on urban and rural water supplies, hydro electricity generation, and agricultural production. The areas most prone to periods of water shortage are in the east of the country and in some 'rain shadow' enclaves on both main islands (see Figure 7.3). Over a dozen significant drought events occurred between 1978 and 1994. In some cases only 50 to 100 farms were affected. In others, thousands of farms and hundreds of thousands of urban households were affected. In one case, the entire country experienced electricity cuts. The impacts on farms are sometimes made worse by the style of farm management. Hill country pastoral properties in drought-prone areas, for example, can be severely affected because they tend to be marginally economic and so carry the maximum possible stock numbers.
Some dry spells are also made worse by the climatic phenomenon known as El Niño, which alters weather patterns over large areas of the Southern Hemisphere (see Chapter 5). El Niño brings more frequent south-westerly winds to New Zealand, intensifying the rain shadow effect so that eastern and northern parts of New Zealand become drier than normal, and the country as a whole becomes a little cooler. In the Southern Alps 'drought' of 1992, El Niño reduced precipitation on the eastern side of the Southern Alps, parching the headwaters of the South Island hydro lakes. The Electricity Corporation of New Zealand was managing the lake levels on the assumption that normal rainfall patterns would replenish them during the winter. When the rain did not come the result was a national power shortage which required 20 percent reductions in power usage. The Government introduced legislation to lower the water level in Lake Pukaki to increase hydroelectricity generation during this period.
In 1993 and 1994, it was Auckland's turn to be caught out by El Niño. Aucklanders' demand for water had increased steadily throughout the 1980s, putting strain on the available supplies. A successful water conservation strategy had begun to reduce water use, but it was based on the assumption that normal rainfall levels would keep the reservoirs topped up. By mid-1993, however, Auckland's rainfall had been below average for 15 of the past 18 months and the usual winter rains had not come. Lake levels peaked at 75 percent of their total capacity in September 1993, then started to drop.
In May 1994, Aucklanders were told they would have to reduce their water consumption by 25 percent. By early June, the lake levels were down to 31 percent of total capacity. In July a bill was introduced to Parliament which would allow water to be piped from the Waikato River without going through the normal consent process of the Resource Management Act. However, just as the politicians were debating the need for this, a late rainfall made the decision for them. The bill was withdrawn on 5 October 1994 as water storage climbed back to 78 percent of total capacity.
Auckland's reprieve is only temporary. Conservation measures reduced water consumption from the 1988 level of around 340 million litres per day to a 1995 level of around 300 million litres, but population and economic growth in the region are expected to force total consumption up. By the end of this decade, demand is expected to exceed supply, so plans are underway for a new pipeline. Water supply authorities in other drought-prone parts of the country are also anticipating future thirsty periods. Many are putting greater emphasis on water conservation and metering to reduce unnecessary water use. What is not known is whether El Niño-induced dry spells will become more or less frequent in future. Some scientists are now beginning to suspect that the El Niño phenomenon is a direct symptom of rising global temperatures and will become more frequent as the 'greenhouse effect' intensifies (Kerr, 1994; Wuethrich, 1995).
|River||Catchment area (hectares)||Mean annual flow (cumecs)|
Source: Mosley (1993)
Surface water includes streams, lakes, rivers and wetlands. New Zealand's rivers and lakes provide about 60 percent of the water we consume (the other 40 percent comes from underground). They also provide 75 percent of our electric power, and they are home to more than 30 species of native fish and 20 species of introduced ones, including trout and salmon, as well as freshwater invertebrates and plants.
The mountainous terrain of both main islands has produced many comparatively small catchments with fast-flowing stony streams and rivers. The South Island has 40 major river catchments and the North Island has 30 (see Figures 7.4a and 7.4b). Over 180,000 kilometres of rivers have been mapped on 1:250,000 scale maps, but many tens of thousands of small streams are too small to be shown. The total area of riverbed has been estimated at 204,000 hectares (Molloy, 1980).
The largest river is the South Island's Clutha which drains a catchment area of 2,058,000 hectares (or 20,580 square kilometres). This is more than 13 percent of the South Island. The Clutha has an average flow of 570 cubic metres per second or cumecs (570,000 litres per second). The longest river, at 425 kilometres, is the Waikato River in the North Island. It has a catchment area of 1,140,000 hectares and an average flow of 327 cumecs (see Table 7.3).
Seasonal variations in rainfall cause river flows to change through the year (i.e. lower flows in summer and autumn and higher flows in winter and spring). This variation is more extreme in the east coast of both islands where summers are relatively dry. In some snow-fed South Island rivers, however, the pattern is reversed, with the lowest flows occurring in the winter and early spring, before the snow melts, and increasing in late spring and summer (Figure 7.4b). In some rivers, such as the Rakaia and the Ahuriri, the summer flows are relatively high because water from the melting snow is augmented by increased rainfall from the seasonal northwest winds (Duncan, 1992).
In some places, river flow is also augmented by groundwater which may enter from springs and openings in hillsides and cliffs or seep from sloping ground. In some of the central North Island rivers, the contribution from groundwater is large enough to reduce the seasonal flow variation caused by rainfall, as is demonstrated by the relatively constant flow of the Hamurana Stream near Rotorua.
Main river catchments in the North Island:
- 013 Awanui
- 037 Waitangi
- 091 Piako
- 092 Waihou
- 146 Kaiuna
- 153 Tarawera
- 154 Rangitaiki
- 155 Whakatane
- 159 Waioeka
- 165 Motu
- 183 Waiapu
- 189 Hikuwai
- 197 Waipaoa
- 214 Wairoa
- 218 Mohaka
- 230 Tutaekuri
- 231 Ngaruroro
- 232 Tukituki
- 243 Porangahau
- 259 Wharema
- 292 Ruamahanga
- 298 Hutt
- 318 Otaki
- 325 Manawatu
- 327 Rangitikei
- 330 Turakina
- 331 Whangaehi
- 333 Whanganui
- 339 Waitotara
- 343 Patea
- 395 Waitara
- 407 Mokau
- 408 Awakino
- 434 Waikato
- 457 Hotea
- 466 Wairoa
Typical flow patterns:
- Tarawera River, 1946 to 1987: flow peaks in August and September; average flow 320 cumecs; low flow 24.3 cumecs; flood flow 55 cumecs.
- Motu River, 1958 to 1990: flow peaks in June to August; average flow 92 cumecs; low flow 14.2 cumecs; flood flow 1590 cumecs.
- Waikato River, 1958 to 1990: flow peaks k in July to September; average flow 311 cumecs; low flow 177 cumecs; flood flow 790 cumecs.
- Manawatu River, 1972 to 1987: flow peaks in July; average flow 102 cumecs; low flow 14.3 cumecs; flood flow 1450 cumecs.
Adapted from Duncan (1992)
Main river catchments in the South Island:
- 520 Aorere
- 529 Takaka
- 570 Motueka
- 575 Waimea
- 589 Pelorus
- 601 Wairau
- 602 Awatere
- 621 Clarernce
- 643 Conway
- 646 Waiau
- 651 Hurunui
- 659 Waipara
- 662 Ashley
- 664 Waimakariri
- 680 Selwyn
- 685 Rakaia
- 688 Ashburton
- 693 Rangitata
- 695 Orari
- 709 Opihi
- 711 Waihao
- 726 Waitaki
- 731 Shag
- 743 Taieri
- 753 Clutha
- 775 Mataura
- 786 Oreti
- 789 Aparima
- 797 Waiau
- 809 Wairaurahiri
- 813 Waitutu
- 847 Cleddau
- 851 Hollyford
- 863 Arawata
- 868 Haast
- 906 Hokitika
- 911 Taramakau
- 914 Grey
- 932 Buller
- 951 Karamea
Typical flow patterns:
- Buller River, 1964 to 1990: flow peaks in September to November; average flow 425 cumecs; low flow 108.5 cumecs; flood flow 4720 cumecs.
- Ahuriri River, 1964 to 1990: flow peaks in November and December; average flow 23 cumecs; low flow 8.6 cumecs; flood flow 240 cumecs.
- Rakaia River, 1960 to 1990: flow peaks in December; average flow 207 cumecs; low flow 91.8 cumecs; flood flow 2510 cumecs.
- Hakataramea River, 1964 to 1990: flow peaks in August; average flow 60 cumecs; low flow 0.9 cumecs; flood flow 180 cumecs.
- Clutha River, 1955 to 1986: flow peaks in November and December; average flow 563 cumecs; low flow 268 cumecs; flood flow 1730 cumecs.
Adapted from Duncan (1992)
New Zealand has at least 770 lakes with a combined surface area of some 334,000 hectares (Molloy, 1980). The vast majority are small with surface areas of less than 50 hectares (or half a square kilometre). Most are shallow lakes only several metres deep and surrounded by farmland. Only 40 lakes have areas greater than 50 hectares (Lowe and Green, 1987).
New Zealand's lakes were formed by three broad processes. Volcanic eruptions created the hollows for most of the larger North Island lakes. Glacial ice gouged out the basins for most of the South Island lakes. Land barriers formed by accumulated sediment, sand bars, and earth movements, have blocked off river channels, creating a large number of shallow lakes in floodplains and coastal areas.
Lake Taupo in the North Island is New Zealand's largest lake, with an area of about 62,000 hectares and a maximum depth of 163 metres. Its size is the legacy of several vast ancient eruptions. The next largest North Island lake is the shallow Lake Wairarapa, with an area of almost 8,000 hectares and a maximum depth of less than 3 metres. Drainage for farmland has greatly reduced Lake Wairarapa's area. The deepest (and probably least modified) North Island lake is Waikaremoana, which formed when a landslide blocked a valley 2,000 years ago. Its maximum depth is 248 metres. Apart from Taupo and Waikaremoana, only one other North Island lake (Rotomahana) is deeper than 100 metres (Spigel and Viner, 1991).
The South Island's largest lakes are the glacial Lake Te Anau, with an area of around 35,000 hectares and a maximum depth of 417 metres, and Lake Wakatipu, with an area of 29,000 hectares and a maximum depth of 380 metres. The third largest South Island lake, Ellesmere (or Waihora), is also the country's largest shallow lake with an area of 18,000 hectares
(reduced from 30,000 hectares) and a maximum depth of less than 3 metres. The deepest South Island lake, and also the deepest lake in New Zealand, is the glacial Lake Hauroko in Southland which has a maximum depth of 462 metres. Apart from Te Anau, Wakatipu and Hauroko, 12 other South Island lakes have depths greater than 100 metres (Spigel and Viner, 1991).
At least 16 artificial lakes have been created for hydro power stations, the largest of which is the South Island's Lake Benmore (6,900 hectares) and the smallest of which is the North Island's Lake Aratiatia (34 hectares).
Wetlands are areas of shallow water containing specially adapted plant and animal communities (e.g. rushes, sedges, reeds, flax, water birds, eels, mudfish, aquatic invertebrates etc.). They occur on land-water margins, or on land that is temporarily or permanently wet. Although they are found at all altitudes, from coastal estuaries and sand dunes to alpine tarns, wetlands mainly occur in valley floors and on flood plains, often in association with former river courses and ponding areas, lake margins and dune hollows. In estuarine areas and in inland Otago's salt pans their water may be brackish or salty.
New Zealand's wetlands are as varied as the terrain that shapes them, but they can be grouped into three broad categories reflecting their water quality and typical vegetation. Eutrophic mires have high nutrient levels and are dominated by the native reed, raupo (Typha orientalis). Mesotrophic wetlands have moderate nutrient levels and are dominated by rushes, sedges and the native flax, harakeke (Phormium tenax). Oligotrophic bogs have very low nutrient levels and are dominated by spaghnum moss, rush-like sedges (e.g. S choenus, Baumea, and Tetraria spp.) and restiad rushes (e.g. Empodisma and Sporodanthus) (Newsome, 1987). The oligotrophic wetlands often have no significant surface water. Some wetlands were dominated by kahikatea forests, others by pukatea and swamp maire.
An additional category which is often not considered to be true wetland is pakihi, a term coined by West Coast miners last century to describe land which was formerly covered in tall podocarp forest but became water-logged when the water table rose after deforestation (Mew and Johnston, 1988). Pakihi areas are characterised by ferns, mosses and rushes, and sometimes manuka, gorse, and bracken (Newsome, 1987). Many have been sustained by fire and stock grazing and have shown no substantial changes in the last 100 years.
Wetlands are a major habitat for at least eight species of indigenous freshwater fish as well as frogs, birds and invertebrates. Coastal wetlands are more biologically productive than virtually any other ecosystem, providing habitat, breeding areas, and food for shellfish, crustaceans, inshore fish and birds. A fifth of New Zealand's indigenous birds use wetlands as their primary habitat. Wetlands also support other ecosystems by absorbing flood waters and filtering waste water. They regulate water flows, recharge ground aquifers, maintain water quality, and limit coastal erosion.
Freshwater wetlands covered at least 670,000 hectares before European settlement, but have now been reduced by drainage for pasture to around 100,000 hectares. Although several thousand wetlands still survive, most are very small and have been modified by human activities and invasive species. It is likely that, in the last 100 years, some characteristic New Zealand wetland types have been lost completely, while very few examples are left of others, such as kahikatea swamp forest and some kinds of flax swamp and salt marsh (Cromarty and Scott, 1996).
After rainfall soaks into the ground as soil moisture it may seep into streams as delayed flow, it may be absorbed by plants, or it may seep further into the ground and become groundwater. The underground areas in which groundwater collects are called aquifers . Aquifers take many geological forms, including caverns, deep rock fissures, and porous gravel beds. The most extensive New Zealand aquifers are porous sediments lying on top of harder rock layers.
New Zealand's groundwaters range in temperature from less than 10° C to more than 300° C, depending on how close they are to active faultlines or volcanic zones.
The Resource Management Act defines groundwater of 30° C or more as geothermal, whereas groundwater cooler than this is considered to be within the range of ambient land and surface water temperatures.
Extensive aquifers of 'ambient' groundwater occur in many parts of the country, including the Canterbury Plains, Marlborough and Tasman Districts, Hutt Valley, Manawatu, Hawke's Bay, the Bay of Plenty, the Waikato and Hauraki Lowlands, and South Auckland. In areas prone to surface water shortages or seasonal fluctuations in river flows and rainfall, these groundwater reserves have become an important source of supply. Aquifers can satisfy heavy summer demand with water stored during the winter and can smooth out the effects of wet and dry years. Approximately 40 percent of New Zealand's freshwater supplies are now drawn from groundwater.
Aquifers vary in their depth and in the extent to which they are 'confined' or 'unconfined'. Unconfined aquifers are surrounded by porous rock or sediment, while confined ones are surrounded by impermeable materials. Water flows easily through unconfined aquifers, but is often trapped or reduced to very low flows in confined aquifers. Because they are more exposed to surface water and leachate, unconfined aquifers are more vulnerable to pollution, particularly if they are shallow and close to the land surface.
Places with unconfined aquifers include parts of the Aupouri peninsula in Northland, the Pauanui spit on the eastern Coromandel coast, the Hamilton basin, and much of the Wairarapa and Canterbury plains. Areas with confined aquifers include the Kaawa shellbeds between Manukau harbour and Pukekohe, the Rangitaiki Plain, and much of the Heretaunga Plain of Hawke's Bay (Thorpe, 1992).
Geothermal groundwater occurs where aquifers have been heated in volcanic zones, along faultlines, or in deep fissures (5 kilometres or more underground). These heated groundwaters are usually referred to as geothermal fields or systems. New Zealand's geothermal fields are classified as low temperature if they are below 100°C, and high temperature if they are above 100°C (Hunt and Bibby, 1992).
Low temperature geothermal systems are generally associated with faultlines or deep groundwater circulation (Cave et al., 1993). They are found in the centre and north of the North Island from Taranaki and Hawke's Bay northwards. In the South Island they are associated with the Alpine and Hope faultlines, and so run in a band from Hanmer Springs to the Copland River in Westland and on down to western Fiordland. When low temperature geothermal systems breach the surface, they appear as hot springs. Often, however, they remain underground until discovered by drilling.
High temperature geothermal systems are associated with volcanic activity. All 24 high temperature fields are in the North Island, where their total surface area has been estimated at around 3,000 to 4,000 hectares (Cave et al., 1993; Bibby, 1995). Nine have temperatures ranging from 100°C to 180°C, and most of the rest are in the 200°C to 300°C range. The high temperature fields display themselves at the surface in a variety of ways, ranging from warm ground to spectacular geysers of steam and boiling water. In between these extremes are hot springs, sinter deposits, fumaroles, and hot mud pools.
Except for one high temperature field at Ngawha, in Northland, which is not associated with recent volcanism, all the others occur in the Taupo Volcanic Zone. This zone extends from Mount Ruapehu in the centre of the North Island to the Bay of Plenty. It incorporates Lakes Taupo, Rotorua and Tarawera, the upper reaches of the Waikato and Tarawera Rivers, and the significant townships of Taupo and Rotorua.
New Zealand may have the eighth longest coastline of any nation, although its exact length is not known because of all the twists and turns it takes around inlets, headlands, spits, bays, harbours, fiords, sounds and estuaries. Depending on how detailed the map is, estimates range from 10,000 to as many as 15,000 kilometres. A more solid statistic is that we have the fourth largest maritime area, with an exclusive economic zone (EEZ) of some 483 million hectares. Only the United States, Indonesia, and French Polynesia have larger maritime areas.
The coastal marine environment forms three temperature-related bioregions: northern, central and southern (Tortell, 1981). The northern bioregion covers the north-east of the upper North Island, from Northland Peninsula through to East Cape. Its warm temperate shores are influenced by the subtropical East Auckland Current (see Figure 7.5). The central bioregion covers the large area from East Cape to Otago on the east coast and virtually all of the west coast from the western tip of Northland down to Fiordland. The cool southern bioregion is influenced by the Southland Current and the West Wind Drift.
For these reasons, our coastal waters vary from warm, salty, sub-tropical currents in the north and west, to cold, less saline, sub-antarctic currents in the far south and south-east. The warmer currents come from the coastal waters around Australia. They swirl around most of the North Island and the northern and western part of the South Island and meet the cold sub-antarctic waters in a zone called the Sub-tropical Convergence, which extends from Banks Peninsula to Milford Sound. Summer water temperatures on the continental shelf range from about 21°C in the north to 14°C in the south. Although the typical seasonal range is 5°C, significant local variations occur from time to time as a result of upwelling caused by onshore winds and freshwater inflows.
Apart from the variations of current and temperature, the marine environment also varies in other ways. Underwater habitats range from monotonous plains of mud to occasional wonders, such as the volcanic vents near White Island, whose micro-organisms "breathe" sulphur rather than oxygen, or the great, coral-festooned, seamounts of the deep ocean (see Box 7.3) or even the rocky reefs of our coastal waters. However, the greatest variation is probably between the intermittently wet tidal environment along the shoreline and the sea environment beyond this.
About two-thirds of our coastline is hard rocky shore while soft shores of sand or gravel cover about one third. Some 80 percent of the coast is directly exposed to the sea, with the remainder sheltered in harbours and estuaries. The rocky shores contain three tidal zones: littoral; eulittoral; and sublittoral. The littoral fringe is well up the beach, beyond the reach of all but the highest spring tides. It is climatically harsh and sometimes dry for days at a time. Only a few well-adapted pioneer species are found there, such as yellow and grey lichens and periwinkles.
Figure 7.5: New Zealand's marine environment, showing the 200-mile Exclusive Economic Zone, the main ocean currents and the 200-metre depth contour.
New Zealand's 200 mile Exclusive Economic Zone extends around New Zealand, the Kermadec Islands, Chatham Islands, Bounty Island, Antipodes Island, Campbell Island, Auckland Island and Snares Island.
The Subtropical convergence runs below Stewart Island, along the east coast of the South Island until Banks Peninsula and then out east under the Chatham Islands.
The current flows are:
- The Tasman Current, from across the Tasman towards the west coast of New Zealand.
- The Westland Current, from south to north along the west coast of the South Island.
- The Southland Current, from south to north along the east coast of the South Island, then east along the subtropical convergence.
- The East Auckland Current, from north to east along the north-east coast of the North Island.
- The East Cape Current, from north to south along the east coast of the North Island, then east along the subtropical convergence.
Source: Francis (1996)
The much larger eulittoral zone is bathed twice daily by the tide and is home to acorn barnacles, mussels, and serpulid tubeworms (Pomatoceros). The lower third of the zone has a seaweed turf of the calcareous red alga Corallina officinalis, accompanied by the bladdered fucoid seaweed, Hormosira banksii, and sometimes by the green alga, Codium convolutum. The sublittoral fringe is underwater most of the time and is richly clad in half a dozen or more species of brown algae, better known to most people as kelp seaweed. It also supports a small but diverse assortment of bullies and other rockpool fish (see Chapter 9).
The soft shore beaches are made of gravel or sand or mixtures of these. Gravel beaches tend to be steep and tiered, but sand beaches have a low, gentle gradient. They are porous, firm underfoot, and the open beach sand is free of silt, even-grained, and subject to constant wave action. These conditions favour shellfish, such as the toheroa (Paphies ventricosum), which forms its zone in the mid-beach, and a profusion of other molluscs just below wave-break. The open beaches of the east coast are composed of relatively coarse to medium sand. Onshore surf, common on the west coast, is here confined to occasional periods of easterly wind. Toward low water the dominant bivalve mollusc is the tuatua (Paphies subtriangulatum).
Much of the coastline is made up of river-fed estuaries, whose wide, shallow, waters are permanently protected from ocean waves by sand or shingle bars or offshore islands (see Figure 7.6). Such shores include drowned harbours or, more generally, wide, level flats of sand (which are often miscalled mudflats). New Zealand's estuaries cover a total area of at least 100,000 hectares. Most have developed where coastal sand bars at river mouths have caused the rivers to spread out. In total, 164 estuaries are bar-built, 56 are drowned river valleys, 65 are lagoons and 16 are fiords (McLay, 1976).
The sandflats and wetlands associated with shallow estuaries are the most productive ecosystems on Earth, growing three or four times more plant and animal matter per hectare than the land or sea to either side of them (Knox, 1980). The rich assortment of burrowing sandflat animals includes many bivalve molluscs, notably pipi (Paphies australis) and crowded beds of the cockle (Chione stutchburyl), and a rich wealth of worm species, including the pencil-sized yellow proboscis worm (Balanoglossus australiensis). Various echinoderms are also common in the sandflats, such as the comb star (Astrpecten polyacanthus), the brittle star (Amphiura aster), the worm-like holothurian (Trochodota dendyi) and sometimes the burrowing urchins Echinocardiun australe and Arachnoides novaezelandiae .
In the upper reaches of estuaries, soft deposits accumulate and form habitat for both bivalve and gastropod (snail) molluscs. Tiny crustaceans called copepods are also common. Pauatahanui inlet, near Wellington, has the highest recorded density of copepods in the world. Though partially degraded by pollution and sediment, part of the estuary became a wildlife management reserve in 1985, in recognition of its importance to wading birds (Forlong and Kirkland, 1993).
Some estuarine ecosystems are based on plants. Seagrass (Zostera) can form wide green swards, such as in the Manukau Harbour where scallops (Pecten novaezelandiae) are abundant and form large beds around low water (Tortell, 1981). Mullets and flatfish (flounders and sole) share the rich feeding grounds with wading birds as the tide ebbs and floods. Another plant-based ecosystem is formed by the mangrove tree (Avicennia marina resinifera), which flourishes in the warm harbour and estuarine waters of the northern third of the North Island (Hackwell, 1989). Though low in species diversity, mangrove ecosystems are a haven for young flatfish and provide habitat for more than 30 fish species in all. The most common of these is the yellow-eyed mullet (Aldrichetta forsteri). The seagrass and mangrove ecosystems have declined this century as a result of widespread modifications to estuaries caused by such as activities as infilling for agriculture, rubbish disposal, and commercial land development.
The nature of our sea environment is determined by several factors, including our remote location in the South Pacific Ocean, the wide latitudinal range of our marine zone stretching from sub-antarctic to sub-tropical waters, the predominant westerly weather pattern, and the extensive continental shelf surrounding New Zealand which, at 24 million hectares, is only slightly smaller than New Zealand's total land area of 27 million hectares.
New Zealand's coastal waters support hundreds of fish species, a variety of marine mammals, and many marine invertebrates. Some of the unique marine ecosystems include The Gut, in Fiordland's Doubtful Sound, famous for its red and black corals and sea pens, and Nugget Point on the South Otago coast which is the only place on the mainland where New Zealand fur seals, New Zealand sea lions, and elephant seals co-exist (Forlong and Kirkland, 1993). Banks Peninsula near Christchurch was made a marine mammal reserve to protect the world's rarest and smallest marine dolphin, Hector's dolphin, while, in other parts of the coast, fur seals, dolphins and whales have become popular tourist attractions. The most popular of all are probably the sperm whales which feed in the deep water just off the Kaikoura coastline.
The waters around New Zealand have many temperate reefs. These are rocky ridges which range from warm temperate in the north through to cool temperate in the south. Warm temperate reefs are characterised by the kelp, Ecklonia radiata, or large, grazed areas of the urchin Evechinus chloroticus. Common fish include the seaweed grazers-butterfish (Odax pullus), parore (Girella tricuspidata), silver drummer (Kyphosus sydneyanus), marblefish (Aplodactylus arcticdens), and black angelfish (Parma alboscapularis). Other common reef fish include the spotty or paketi (Notolabrus celidotus), banded wrasse (Notolabrus fucicola), hiwihiwi or kelpfish (Chironemus marmoratus ), red moki (Cheilodactylus spectablis), blue cod (Parapercis colias), goatfish (Upeneichtys lineatus), and snapper (Pagrus auratus).
The deeper reef areas are covered with sponges, ascidians and other colourful encrusting animals. In the cooler southern waters, butterfish and marblefish are the main seaweed grazers and blue moki (Latridopsis ciliaris), copper moki (L. forsteri), and tarakihi (Nemadactylus macropterus) feed on the sea floor. Generally, algal species diversity is greater in the cool temperate waters. The dominant kelp varies depending on exposure-the bull kelp (Durvillaea willana) is present in exposed conditions and Macrocystis pyrifera in more sheltered areas.
Seabed communities vary from those of the muddy shelves off Westland, Hawkes Bay and Wairarapa, to those of the sandy and gravelly shelves off Northland, Otago and Southland.
They include many species of true crab, hermit crab, sea snail, octopus, starfish and brittlestar, and two species of rock lobster, the commonest of which, the red rock lobster (Jasus edwardsii) is the basis of a commercial fishery. Seabed invertebrates are also important in the diet of many species of fish that live on or close to the bottom, such as dogfish, carpet shark, skate, elephant fish, gurnard, tarakihi, flatfish and our most commercially important inshore fish, snapper (Pagrus auratus), which eats a wide range of seabed invertebrates and small fish.
Some of the least known parts of the marine environment are the deep water ecosystems beyond the continental shelf. Two thirds of New Zealand's Exclusive Economic Zone (EEZ) consists of deep water, much of it barren of fish and extending over vast areas of featureless, muddy, seafloor. Compared with coastal waters, the deep sea is a constant environment. Even the effect of latitude is relatively small, so that whereas many marine organisms of surface waters occur only in the north or south of our Exclusive Economic Zone, those of the deep sea (e.g. orange roughy) are often found throughout the EEZ.
The expanses of ocean mud are inhabited by a variety of marine invertebrates, such miniature worms, crustaceans, and molluscs, but their total biomass is believed to be relatively low-even compared to desert ecosystems on land. However, the deep oceans are not total deserts. On the Chatham Rise, for instance, at depths of about 400 metres, small spiny nodules projecting from the mud are festooned with bonsai-like colonies of soft corals and bryozoans. And marine life flourishes on and around the great mountain ranges and peaks which occasionally rear up from the muddy plains. These seamounts, as they are called, are very rich fishing grounds and are also havens of marine biodiversity (see Box 7.4).
New Zealand's major estuaries and harbours in the North Island:
- Bay of Islands
- Waitangi National Reserve
- Waitangi National Reserve
New Zealand's major estuaries and harbours in the South Island:
- Whanganui Inlet
- Waimea Inlet
- Nelson Haven
- Lake Forsyth
- Lake Ellesmere
- Aramoana Tidal Flats
- New River
- Waituna Wetlands
Source: Crisp and Walsby (1986)
The great coral reefs found in shallow tropical waters are well known as gardens of marine biodiversity. Formed by the accumulation of coral skeletons over hundreds or thousands of years, these reefs support a wide variety of species within their nooks and crannies. A myriad of fish, invertebrates and seaweeds dwell there, some for their entire lives, others for key phases of their life cycle. The abundance of reef life is mainly fueled by algae living inside the corals. These capture energy from the sun shining overhead. New Zealand has no coral reefs and so is not directly affected by the fact that many of the world's reefs are now under threat. But we do have something similar - coral-dominated seamounts. Out in the deeper waters of our Exclusive Economic Zone, tall mountains rear up from the mud-covered abyssal plains which dominate 80 percent of the ocean floor. Living on their peaks and ridges, half a kilometre or more below the surface, are dense communities of marine invertebrates and fish. They receive their primary energy supply not from the sun, but from nutrient-rich water currents which well-up around them and a daily 'catch' of tiny invertebrate prey (zooplankton) that get trapped on the seamounts each morning as they descend from their night-time forages near the surface. The seamount communities are also fed by a constant 'rain' of detritus and faecal pellets from organisms which live closer to the surface.
The seamount ecosystems are complex and varied. Many harbour their own unique endemic species. As on the reefs, the dominant animals are corals - not the typical reef species, but groups such as black corals and horny corals (or gorgonians). 'Thickets' of tree-like and bush-like coral colonies festoon seamount ridges and pinnacles wherever they can find exposed rock surfaces to cling to. For them, the seamounts are like desert oases. The seamounts are also places of shelter or sustenance for other small marine animals, such as anemones, sponges, echinoderms (e.g. brittle stars, snake stars, sea stars), bryozoans, worms, molluscs (e.g. snails, bivalves) and crustaceans (e.g. crabs). These attract large congregations of fish, which, in turn, attract other species to the surrounding waters, such as sperm whales and sea birds.
The biodiversity of New Zealand's seamounts is just beginning to be understood. Most of the invertebrates found in a recent study of trawler bycatch on seamounts in the Chatham Rise had not previously been scientifically identified (Probert, 1996; Probert et al., in press). But scientists are not the only ones to recognise the seamounts' high productivity. Many seamounts are targeted by trawlers seeking deep water fish, such as orange roughy. Anecdotal evidence from trawler crews and fishery scientists suggests that large numbers of corals and other marine invertebrates are killed when the trawlers first move onto a seamount and then populations decline markedly as fishing progresses (Jones, 1992). Scientific investigations are still at an early stage, but the Chatham Rise survey appears to confirm the anecdotal reports. Deep-water coral banks, large gorgonians, sea pens and sponges are particularly vulnerable as they are fragile and slow-growing. Once destroyed, coral formations appear to need 200-400 years to fully recover, and will never be quite the same if their endemic species have been lost (Probert, 1996; Probert et al., in press).
Given the relative scarcity of seamount habitats, their biodiversity and their vulnerability, some marine scientists are now voicing concern about their lack of protection (Jones, 1992; Probert et al., in press). The scientists are also beginning to wonder if seamounts play key roles in the life cycles of commercially important deep water fish (e.g. orange roughy). It is known that some coastal fish use bryozoan mats and coral thickets as spawning grounds and nurseries for their young. If the seamounts play a similar role, their destruction may have a long-term impact on our commercial fisheries. Until more is known about this, a precautionary approach to seamount fishing would seem the safest strategy.
To date, the conservation of marine ecosystems has focused on shallow water ecosystems (the Marine Reserves Act 1971 applies to areas within the 12 mile limit). Although the Territorial Sea and Exclusive Economic Zone Act 1977 provides for measures to protect and preserve the marine environment throughout the Exclusive Economic Zone, it has not been used to protect any deep water areas from trawling. The recently passed Fisheries Act 1996 requires the Minister to (among other things) avoid, remedy or mitigate any adverse effects of fishing on the aquatic environment, maintain the viability of associated or dependent species, maintain aquatic biodiversity, and protect any habitat which has particular significance for fisheries management. While the Fisheries Act does not empower the Minister to establish reserves as such, it does enable him to impose a range of 'sustainability measures' to stop environmentally harmful fishing. The Marine Reserves Act enables the Minister of Conservation to establish such reserves for scientific purposes. The recent extension of the Wildlife Act to the 200 mile limit also enables the Minister of Conservation to prepare Population Management Plans for protected coral species which may be at risk of 'fishing related mortality'. With the passing of these measures, and the adoption of an ecosystem management approach in the Ministry of Fisheries' proposed Fisheries 2010 strategy, the infrastructure now exists to ensure that our fascinating 'reefs' of the deep do not go the same way as the ill-fated reefs of tropics.