Freshwater Communities

There is a wide variety of biota that makes up freshwater ecosystems and that are dependent on freshwater bodies for their survival. This includes fish, birds, and invertebrates which depend in turn on plants and communities of microscopic organisms such as algae, bacteria and fungi. All these organisms combine to form complex communities and food webs, which encompass both the aquatic environment itself, and the adjacent land and vegetation.

The primary producers of energy in fresh water communities are bacteria and algae as well as mosses and liverworts, which all utilise sunlight. These are then consumed by primary consumers which typically include insects, molluscs such as snails, crustaceans such as freshwater crayfish and worms. Primary consumers also include microscopic zooplankton that graze on the phytoplankton, particuarly in lakes or large, slow-moving river systems. 

These primary consumers are the food source for secondary consumers which are mainly fish but also include the larvae of insects such as the dobsonfly and stonefly and some birds. These secondary consumers can also be eaten by tertiary consumers such as eels, trout and birds. In addition, bacteria and fungi may also act as decomposers, breaking down dead leaves and wood in the water.

There is a close ecological linkage between the aquatic environment and adjacent land. Dead leaves and branches, which fall into the water from terrestrial plants, along with terrestrial invertebrates, provide an important source of energy and nutrients. Pieces of wood also provide important habitat for invertebrates and fish. Stones and boulders become coated with microscopic layers of organisms (biofilms) which provide a food source for other stream life. Many aquatic insects have a larval stage in freshwater but with the adult forms living in terrestrial systems. Some fish spawn in vegetation or gravels growing along the waters’ edge, known as the riparian margin. This means that land use adjacent to rivers and lakes can have a major impact on freshwater life. 2001

Most aquatic life in New Zealand’s waterways developed in heavily shaded forest conditions, meaning that the ongoing clearance of native forests and riparian habitat has had a significant impact on freshwater biodiversity. Deterioration in water quality is also a major threat to freshwater ecosystems. New Zealand’s freshwater bodies have been increasingly invaded by a large number of aquatic plants, diatoms such as 'didymo' and pest fish. Many human activities within the catchments of waterways continue to have a major impact on indigenous freshwater biodiversity.

The following sections describe indigenous freshwater species.

Freshwater invertebrates

There is are a multitude of invertebrates (animals without a backbone) which can be found in New Zealand’s freshwater habitat: freshwater crayfish, shrimp, crab, worms, flatworms, leeches, snails, bivalves, molluscs, amphipods, water fleas, seed shrimps (ostracods), larvae of beetles, midges, cranefly, caddisfly, mayfly, stonefly and sandfly. Many of these species are only found in New Zealand.

Freshwater crayfish (koura)

New Zealand has two native freshwater crayfish: the northern koura (Paranephrops planifrons) is found throughout the lakes, streams and wetlands of the North Island and northern and west coasts of the South Island. It grows to around 7 centimetres long. The southern koura (Paranephrops zealandicus) is found in the east and south of the South Island and on Stewart Island. They are slightly larger than their northern cousins, and have hairy pincers, but in other respects are very similar. Koura are unable to survive in polluted water, and as a result, their absence or presence can be used as an indicator of the health of a waterbody.  Southern koura are considered ‘at risk’ of becoming a threatened species and are declining in population, mainly as a result of degradation and loss of their habitat.

Koura are mainly nocturnal and shelter between rocks and stones. In lakes they burrow into the muddy bottom during the day, emerging to feed at night. They can also burrow into the bottom of temporarily dry water bodies to await the return of the water. Koura feed on aquatic insects and vegetation, and tend to remain in the same water body their whole lives.They are an animal of high importance to Māori as a food resource.

 

Freshwater shrimp

New Zealand’s only native species of freshwater shrimp, Paratya curvirostris, is found in lowland streams, living amongst vegetation. It feeds on detritus such as algal films and bacteria which it scrapes off rocks and wood with its pincers. These shrimp go through a male phase of development before turning into adult females. Similar to many indigenous fish species, the freshwater shrimp is diadromous (migratory) and includes a marine stage in its lifecycle. The shrimp is common around New Zealand and is fairly tolerant of the degraded condition of many lowland streams.

Freshwater crab

There is also only one species of freshwater crab (Amarinus lacustris) in New Zealand which is found in a variety of habitats including lakes and slow-flowing streams and rivers. The crab feeds on animal and vegetable detritus found in the silt in which they live, and grow to around 10 millimetres wide. They are also found in South Eastern Australia and the Norfolk Islands. It is likely that many populations of this crab have not yet been discovered, but nevertheless they are considered to be threatened.

Freshwater mussel (kākahi)

There are three species of freshwater mussel in New Zealand with Hyridella menziesii being the most common. Freshwater mussels can be found in lakes, rivers and streams. They are filter feeders, siphoning water and filtering out small algae and bacteria. This filtering activity can help increase water clarity. The mussels are slow growing and can reach sizes of up to 8 centimetres and ages of up to 50 years.

Freshwater mussels once formed extensive beds in many lakes and were harvested by Ma-ori for food. More recently they have been in decline and are now considered threatened. The reasons for this decline are not fully understood. One theory focuses on the close relationship between the mussels and indigenous fish species. The life cycle of the mussels includes a larval stage, with the larvae attaching themselves to fish such as kōaro. Populations of indigenous fish species have been in sharp decline and this may explain the reduction in mussel populations. Another possible reason for the mussel’s decline is the inundation of its preferred spring-fed habitats with sediment and invasive aquatic weeds.

Other invertebrates

There are over 200 species of smaller macroinvertebrates (invertebrates which can be seen without a microscope) in New Zealand’s freshwaters, with still more species yet to be discovered. Some species, such as mayflies, stoneflies and caddisflies, are only usually found in healthy streams. Other species such as worms, midges, snails and leeches are more tolerant of degraded conditions and can be found in muddy and polluted streams. This means that the presence or absence of particular species in a waterbody can be used as an indicator of water quality.

Macroinvertebrate water quality measures

Because some species of macroinvertebrates are very sensitive to pollution, while others are more tolerant, the presence of absence of particular species can be used as an indicator of overall water quality. Two common measures of macroinvertebrates are used to indicate water quality.

The Macroinvertebrate Community Index (MCI) looks at the entire macroinvertebrate population structure and provides a score that indicates overall water quality. Generally, an MCI score of less than 80 indicates poor water quality and a score of greater than 119 indicates excellent water quality.

The Percentage of the total abundance comprising Ephemeroptera, Plecoptera, and Trichoptera taxa (%EPT) measures the contribution to the total abundance of macroinvertebrates belonging to the sensitive mayfly, stonefly and caddisfly species. A low %EPT indicates degraded water quality whereas a high %EPT indicates good water quality.

Freshwater fish

There are around 38 species of indigenous freshwater fish in New Zealand, many of which are endemic (found nowhere else in the world). Less than five percent of our rivers and streams have been properly sampled and new species are still being discovered. Many species are very poorly understood by scientists, but it is thought that only one species has become extinct – the Southern grayling (Prototroctes oxyrhynchus) which was last seen in the 1920s.

Indigenous New Zealand freshwater fish are different from those in other countries in that they mostly live on the bottom (benthic), are active during darkness hours (nocturnal), and have a migratory marine stage in their life-cycles (diadromous). These characteristics evolved in response to the peculiarities of the local environment. Before human settlement, the streams and wetlands in New Zealand were heavily shaded by indigenous evergreen forest which produced relatively little forest litter and the waterbodies were unstable due to high rainfall and short catchments.

Because indigenous fish are cryptic, secretive, benthic and nocturnal, they are not easy to see. This meant that they were not very obvious to the early European settlers, who concluded that there was a lack of native sports fish, decided to introduce fish species such as trout into New Zealand waters.

Around 60 percent of native New Zealand fish have a migratory marine stage in their life cycles. Either the adult goes to sea to spawn or the larvae are washed out to sea. This characteristic has an evolutionary advantage in New Zealand’s unstable physical environment. It enables species to recolonise rivers and streams from the sea, over wide geographical areas. Diadromy can have considerable disadvantages however, in human-modified systems, because structures such as dams and culverts and heavily polluted areas of rivers can form barriers preventing fish from travelling between rivers and the sea. Some diadromous species are able to vary their behavior, and remain inland, if geographical constraints prevent them from acessing marine areas.

Indigenous fresh water fish include: galaxiids, bullies, lamprey, torrentfish, smelt, freshwater flounder and eels.

Galaxiids

A family of fish called Galaxiids make up a large proportion of the native freshwater fish in New Zealand. There are probably at least 25 species of Galaxiids in New Zealand of which around 80 per cent are threatened. The juveniles of five species giant kōkopu (Galaxias argenteus), kōaro (Galaxias brevipinnis), banded kōkopu (Galaxias fasciatus), short jaw kōkopu (Galaxias postvectus) and inanga (Galaxias maculatus) make up the whitebait fishery, along with common smelt (Retropinna retropinna).

The immature forms are caught when the young swim back inland after being washed out to sea as larvae. Galaxiids take their name from the little silvery gold spots, that many of them have on their bodies, which were thought to resemble stars in a galaxy. They have no scales and a dorsal fin that is positioned quite far back on their bodies. Some galaxiids are good climbers, (kōaro, banded and shortjaw kokopu) particularly when they are small, and use their fins to hold onto rocks. They can climb waterfalls using surface adhesion as well as their pectoral and pelvic fins to wriggle up wet surfaces. They mainly eat insects.

The most common of the Galaxiids in the whitebait catch is the inanga. They live in shoals in open pool areas of rivers, growing to around 10 centimetres long, and living only for about a year. Mostly during the Autumn, large shoals of adults migrate downstream to the coast, spawning when spring tides flood the grasses that line lower river reaches and some estuary shores. The adults then die. The eggs remain in the grasses, and hatch when inundated one month later by the next spring tide. The young then swim out to sea. They remain in the sea over the winter and return as adults to a river the following spring. Modification and degradation of lowland tidal rivers and estuaries mean inanga habitat is under threat. 

Another native New Zealand Galaxiid, the kōaro (sometimes known as ‘mountain trout’) are found in fast flowing mountain streams. These fish prefer streams with some native bush cover, because they use the damp edges of the streams to spawn. They are long and slender and covered in a mottled gold and brown pattern. Kōaro are generally diadromous, and travel between fresh and saltwater, but have at developed landlocked populations in some lakes in the Tarawera, Taupo, Manapouri and Wanaka.

The banded kōkopu is covered in thin pale vertical bands and lives in small river tributaries, in areas where there is vegetation cover giving complete shade. They are common in the coastal areas of the west of the country. They like small streams and are even found in the urban areas of Auckland and Wellington.

Giant kōkopu mostly live in slow moving lowland rivers and coastal lakes close to the coastline. They are also found in landlocked inland lakes. Although most Galaxiids are very small, these fish can grow to nearly half a metre long, although specimens of around 200-300 millimetres are far more common. Giant kōkopu are found mostly in coastal areas, and prefer slow moving or still waters such as lakes or swamps. They tend to spend much of their time hidden beneath debris or overhanging vegetation, awaiting their insect prey.

Other members of the Galaxiid family include the pencil galaxiids, which are pencil shaped fish with small fins. They live in the gravels of stream beds, feeding on aquatic insects from under stones. Many of these small galaxiids are non-migratory and most are threatened due to habitat loss or predation by trout.Mudfish are also galaxiids, with five species present in New Zealand. They live among debris in wetlands, and are able to spend some time lying inactive in the mud, if the water disappears.

Bullies

Bullies are the second largest group of native New Zealand fish with seven species. They are mostly small, reaching about seven centimetres long. Four of the species have some individuals which spend their first few months at sea and then move into freshwater. Consequently, adult bullies can be found in a wide range of New Zealand’s freshwater bodies including places accessible from the sea as well as landlocked water bodies. All bully migratory species are becoming rare with three species (redfin, bluegill and common bully) undergoing significant declines.

Lamprey (piharau/kanakana)

The lamprey (Geotria australis) is another of New Zealand’s native freshwater fish. Although they spend most of their lives at sea, they spawn in fresh water, forested catchments. They look similar to eels, but have a circular sucker armed with rows of horny teeth instead of a mouth.

Lamprey has an interesting life cycle. The first life stage is as larvae when the young have no eyes, nor mouth, but a hood that can be orientated upstream to filter feed. At about one year of age the larvae metamorphoses into the adult form. The adults feed by attaching their sucker mouth to other animals and rasping holes in their flesh. Lampreys continue to be an important food source for Māori, and a traditional fishery still occurs on a small scale in the Whanganui River near Pipiriki and in other parts of Taranaki. They are also important to Southland Māori.

Torrentfish

Torrentfish (Cheimarrichthys fosteri) are the only member of their family Cheimarrichthidae globally. Related to the blue cod, torrentfish are small fish that live in the fast flowing rapids of stony rivers and streams. They have dark bands along their sides, a flattened head and large pectoral fins, which they use to anchor themselves to the riverbed. Torrentfish spend part of their lives at sea, and juveniles enter river systems in spring and autumn, gradually moving upstream. Torrentfish are not good climbers but are excellent swimmers and will penetrate inland rivers where the gradient is relatively low. Torrentfish require good flows and are affected by hydrological modification abstraction of water from rivers. The are found in most parts of New Zealand, but rare in Otago and Southland and are considered at risk and declining in population.

Smelt

There are two species of smelt native to New Zealand. Smelt have distinctly forked tails, and an extra fin – a fleshy lobe between their dorsal fin and tail. They have scales that are easily dislodged, and give off a distinct cucumber smell. They live in the upper water layers and are frequently seen in rivers and lakes feeding on drifting food organisms. Common smelt (Retropinna retropinna) are found throughout New Zealand, whilst Stokells smelt (Stokellia anisodon) are found only in parts of the east coast of the South Island, not far from the sea. Smelt are a prized member of the whitebait catch for Māori. 

Eels (tuna)

There are two types of native eel in New Zealand: the long fin eel (Anguilla dieffenbachia) and short fin eel (Anguilla australis). The long fin eel is one of the largest freshwater eels in the world and is found only in New Zealand. It is declining in abundance and is classified as at risk of becoming a threatened species. A recent arrival, the Australian spotted eel (Anguiilla reinhardtii), is found only in northern rivers. This eel was first spotted in New Zealand in 1997. It has a mottled or blotched back and yellow pectoral fins.

Eels live mainly in rivers and lakes but can be found in all types of New Zealand’s waterways, usually well inland from the coast. Longfin eels prefer clean, inland waters while shortfin eels prefer lowland lakes and swamps. Eels are extremely good climbers and can climb waterfalls and dams by leaving the water and wriggling over damp areas. They are slow growing fish, beginning life at one millimetre long but growing to up to two metres in length over 60 years.

Long-fin eels have an unusual life cycle. The adults breed only once at the end of their life. In order to do so, they swim around five thousand kilometres to spawning grounds in the tropics, thought to be in deep ocean trenches near Tonga. The adult eels die after they have spawned and never return to New Zealand. The fertilised eggs develop into larvae which drift back to New Zealand on ocean currents. The larvae develop into small ‘glass’ eels (elvers) which swim up rivers and slowly grow into adults.

In earlier times,anglers used to regularly catch huge eels weighing up to 40 kilograms. These would have been more than 100 years old. But since commercial eel fishing became popular in the 1960s numbers have declined. Today, although it is still possible to spot eels, there are very few large ones remaining. They are considered a taonga species by Māori.

Conservation status

There has been a significant decline in freshwater biodiversity since 1970, with the biggest decline occurring during the past decade. Catchments where the land cover is predominantly pasture, urban or exotic forest have significantly less indigenous  fish than catchments with native forest cover. 

More than 74 percent of freshwater fish species in New Zealand have been identified as threatened and this proportion has climbed in recent years. So it is somewhat surprising that most of these species are not classified as protected wildlife and some are still legally harvested including the longfin eel, giant kōkopu, īnanga, kōaro and shortjaw kōkopu.

Freshwater birds

Over 160 species of birds have been recorded being present on New Zealand’s rivers and estuaries and many of these are unique to New Zealand. They inhabit  a wide variety of different freshwater habitats ranging from deep open water, to mountain torrents, to swampy vegetation. Five endemic species spend all or crucial parts of their life cycle on rivers and are particularly adapted to them – blue duck, wrybill, black stilt, black-billed gull and black-fronted tern. Many species of freshwater bird are threatened and a number have become extinct during human occupation, including a pelican, a swan and a little bittern.

Blue duck (whio)

The whio (Hymenolaimus malacorrhynchos) forages in the fast flowing, turbulent waters of high country streams. It mainly feeds on the larvae of caddisfly. The ducks have some unusual features which make them particularly adapted to their habitat. They have a streamlined head and large webbed feet which enable them to manoeuvre in fast moving water. The ducks have a thick, fleshy ‘lip’ that overlaps their lower bill and allows them to scrape off insect larvae from rocks without damaging their bills.

The duck is endemic to New Zealand and has no close relative anywhere else in the world. Only around 640 pairs remain in the North Island and just under 700 pairs remain in the South Island, a total population of between 2,000 and 3,000 individuals. The species is classified by the Department of Conservation as nationally endangered. One of the major causes of decline in the population is predation. Habitat loss from forest clearance for agriculture and poor riparian management are also problems. In addition, changes to river flows from the operation of hydro-electric power stations and irrigation have made some rivers unsuitable for blue duck.

Wrybill (ngutu pare)

Wrybills (Anarhynchus frontalis) breed on braided rivers in Canterbury and inland Otago and then fly to Northland and harbours in the Auckland and Nelson region during the summer. They are found in large numbers in the Firth of Thames. The wrybill is particularly adapted to feeding on braided rivers as it has a beak which curves to the right, allowing it to more easily catch insects hidden under river stones. It was this unique feature which led the birds to be collected for use as museum specimens in the late 1800s and early 1900s.

The wrybill population has been declining over the past few decades and there are only 4,000 to 5,000 left in the wild. This decline is probably due to the loss of habitat in braided rivers caused by hydro-electric power development, extraction of water for irrigation, and the invasion of weeds into shingle riverbeds. The Department of Conservation has classified the species as nationally vulnerable.

Black stilt (kaki)

Black stilts (Himantopus novaezelandiae) wade out into deep slow moving water and reach down to capture mayfly and caddisfly larvae from the river bottom. The species used to be widely spread around the entire South Island and lower North Island but it has declined drastically since the 1960s. Black stilt now only breed on the braided rivers and wetlands of the Mackenzie Basin.

By 1981 the population of black stilt had declined to a low of just 23 birds and the population has been identified as nationally critical. This drastic decline was a result of predation by feral cats and mustelids and loss of habitat through hydro-electric development, the drainage of wetlands and the spread of willows and lupins onto river beds. Because the population of black stilt reached such low levels, individuals have also started breeding with the more numerous pied stilt resulting in hybrid offspring. A captive breeding programme, commenced during the 1980s, has led to an increase in the population to around 95 birds.

Black-fronted tern (tarapirohe)

Black-fronted terns (Sterna albostriata) breed in shallow depressions on braided rivers on the eastern side of the South Island and in the Nelson region. They feed on rapidly flowing rivers, touching the surface of the water with their bill, to capture emergent mayflies and stoneflies. Black-fronted terns also eat earthworms and grass grub larvae from farmland. Most birds move to the coast during autumn and winter to feed on planktonic crustaceans in the sea. There are only around 5000 black-fronted terns surviving and the species has been classified as nationally endangered by the Department of Conservation.

Other Freshwater communities

There are a wide variety of different freshwater wildlife communities in New Zealand. The following sections describe some of the communities associated with braided rivers, lakes and groundwater.

Braided river wildlife

As well as making them aesthetically dramatic, the multidimensional and dynamic structure of braided rivers makes them very important habitats for a variety of wildlife. The extended floodplains which typically surround the rivers create a myriad of localised environments, ranging from large unstable channels to well-established spring complexes, quiet side braids and more turbulent main braids. Together they create a mosaic of aquatic habitats for small organisms.

The instability of the floodplain environment means that organisms found there tend to have a high turnover. But the communities survive, because whilst a particular type of habitat may be destroyed in one part of the river, it will remain intact, or be forming, in others. This ensures that the relative proportion of each habitat in any particular floodplain remains roughly constant over time and allows mobile species to move to find suitable new habitat.

Braided rivers support diverse communities of plants, invertebrates, fish and birds. For example, pied and black stilts, banded dotterels, oystercatchers, plovers and some gulls all use braided rivers at some point during their life cycles.

Many bird species migrate to the braided rivers in late winter and early spring to coincide with increased river flows generated by snow melt. The greater volume of water inundates previously dry channels and this increases the habitat for aquatic invertebrates. This, in turn, increases the availability of food for the birds which enables them to recover from their migration and to breed and raise chicks successfully.

Braided rivers support a wide variety of fish species, including many of New Zealand’s native galaxiids, a large proportion of which are endangered. Introduced species of trout and salmon can also be found there in large numbers.

Lake biota

In lakes, the area near the shore (littoral zone) is the most productive area and is usually rich with life. These are the shallow areas where light can penetrate down to the lake bed, and therefore where plants can grow. The plants provide important habitat for algae, invertebrates and fish.

Most lake plants grow near the water’s surface, either rooted near the shoreline or floating. Some plants however can survive at considerable depths – the deepest known plants are some species of bryophyte which have been found at depths of more than 70 metres in Lake Coleridge in Canterbury, where the water is particularly clear.

Many aquatic animals live on phytoplankton. These are tiny floating organisms such as algae and cyanobacteria that grow where there is sufficient light for them to carry out photosynthesis. The phytoplankton are eaten by zooplankton (such as water fleas and copepods) and juvenile forms of other animals. Invertebrates that prefer water with little movement live in lakes, such as freshwater crayfish and insects such as caddisflies stoneflies, midges mosquitos and pondskaters.

Many lakes contain a variety of native fish such an inanga, kōaro and kōkopu. They also contain several introduced fish including ‘sport fish’ such as brown and rainbow trout as well as ‘pest fish’ such as koi carp.

New Zealand lakes also support water fowl, including some found only around lakes. These include grey teal, grey ducks, scaup, New Zealand dabchick (on the North Island) and crested grebe (South Island). Introduced species include Canada geese, mallard ducks and black swans. There are also Australian coots and a number of species of shag.

Wetland biota

The particular environmental features created by a wetland give rise to a large number of species which rely on these characteristics for their survival. Wetlands are a harsh and stressful environment for plants. Therefore, those that grow there have had to adapt to living in the specific conditions that the wetland offers. Kahikatea and Pukatea, which are tall trees found throughout New Zealand, possess buttress roots and therefore are stable in wet soils. Many smaller plants have spongy tissues in their leaves that act as a reservoir and passageway for oxygen to move down to the roots in wet, oxygen-deprived soils. Some plants have underground storage organs that help them survive and regrow rapidly after the sudden floods and prolonged droughts common in wetlands.

Wetlands support a high diversity of smaller plants including 47 species of rush and 72 species of native sedge. Some endangered plant species depend totally on wetlands. In the Waikato region, for example, the remaining wetlands make up only five per cent of the region but support almost a quarter (13) of the region’s threatened plant species. Threatened wetland plants include a swamp helmet orchid, a clubmoss, a hooded orchid and a carnivorous  bladderwort.

Today wetlands cover less than two per cent of New Zealand’s land area but support 22 per cent of native bird species (more types of native bird life than any other habitat in New Zealand). For example, the Australasian bittern, brown teal, fernbird, marsh crake and white heron all rely for their continued survival on New Zealand’s wetlands. In addition, 30 per cent of New Zealand’s native freshwater fish are found in wetlands, including inanga, shortfinned eels, kōkopu, bullies and mudfish. Wetlands are also very important for a range of invertebrate species including freshwater crayfish (koura).

Groundwater biota

Although on the face of it, groundwater aquifers might appear to constitute the type of environment that should be devoid of life, they actually support important ecosystems and life known as stygofauna that play a significant role in the maintenance of healthy groundwater resources. The ecosystems are based on organic carbon brought in when surface waters recharge the aquifer.48 The carbon gets bound into thin layers of slime (biofilms) that form on all surfaces, whatever their size.

Groundwater crustaceans, commonly called well shrimps, feed on the biofilms, keeping them in check and preventing them from clogging up the fine pore spaces in the aquifer, thereby maintaining water quality and flow.

In recent years, scientists have found that groundwater crustaceans are widespread throughout the groundwaters of New Zealand and they are very diverse. Many species lack scientific names and are endemic to particular areas of New Zealand. More are still unknown to science. Crustaceans living in groundwater are generally colourless and blind, and can be up to 25 millimetres long, although some are much smaller, so that they fit into the tiny crevices within aquifers.

Scientists have found that groundwater invertebrates become more abundant in areas close to a pollution source (for example, areas where land irrigation of waste water is used, causing more organic matter to runoff into the groundwater) in response to a corresponding increase in their biofilm food.49 This abundance of invertebrates is able to keep the biofilms in check, maintaining water flow through the aquifer and protecting water quality.

However, the system cannot cope with endless amounts of pollution. When very large amounts of organic matter enter the aquifer, it overwhelms the invertebrates, and pores in the aquifer become clogged. High bacterial populations can use up all the available oxygen, killing the invertebrates. This results in reduced flow through the aquifer and low water quality. Introduction of other toxic materials to the aquifer, including nitrate and ammonia in high concentrations and organic ions such zinc, arsenic, nicol, cadmium and copper, can also affect these invertebrates adversely.

2589

  1. Winterbourn M, 2004, 13.8 –13.9, , in J Harding, P Mosley, C Pearson and B Sorrell (eds), Freshwaters of New Zealand, New Zealand Hydrological Society Inc and New Zealand Limnological Society Inc

  2. Raewyn Peart, Kate Mulcahy and Natasha Garvan. (2010). Managing Freshwater: An EDS Guide. Environmental Defence Society Incorporated.

  3. Stark and Maxted (2007) https://www.mfe.govt.nz/sites/default/files/mci-user-guide-may07.pdf

  4. Goodman et al. (2014) http://doc.org.nz/documents/science-and-technical/nztcs7entire.pdf

  5. Joy (2009) https://www.mfe.govt.nz/sites/default/files/media/Environmental%20reporting/Temporal%20and%20land-cover%20trends%20in%20freshwater%20fish%20communities%20in%20New%20Zealand%27s%20rivers%20an%20analysis%20of%20data%20from%20the%20New%20Zealand%20Freshwater%20Database%20-%201970%20-%202007.pdf

  6. Goodman et al. (2014) http://doc.org.nz/documents/science-and-technical/nztcs7entire.pdf

Last updated at 5:09PM on November 17, 2017