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Life in fresh water

by  Roger Young

At the lake’s edge, fish find cover among the rocks, while further out tiny zooplankton float and graze. In the river, mayfly nymphs drift downstream, only to be snapped up by a torrentfish. Freshwater habitats seethe with life, from tiny algae to 2-metre eels, and each species is vital to the ecosystem.


Freshwater habitats

Compared with many other countries, New Zealand is blessed with plenty of fresh water and a wide range of freshwater habitats. The wild, clear rivers and large scenic lakes attract visitors from all over the world. These freshwater habitats support a diverse range of life, much of which is not obvious to the casual observer.

Running waters

Life in the water flow of rivers and streams is challenging, and the organisms that flourish there have specific adaptations for dealing with the flow. For example, many insects that live in rivers have flattened bodies so they can seek shelter in the layer of slow-moving water near the riverbed. Other organisms have special suckers or hooks that attach them to the bottom, while some caddisflies have ‘ballast’ stones in their outer casing, making them less likely to be swept downstream.

Life in flowing water also has advantages. Aquatic insects use the flow to drift downstream and find new places to forage. Drift-feeding fish can sit in one spot and catch these tasty morsels as they go past. Some aquatic insects use a similar strategy, building nets in the current to filter food from the water as it passes downstream.

Because of New Zealand’s variable climate, the rivers tend to have very changeable flows. Organisms have to cope with large floods, and with long periods of low or intermittent flow.

Standing waters

In lakes, life is concentrated near the edges, making the most of the sunlight reaching the lake bed in the shallows. Aquatic plants often grow profusely, providing cover and habitat for other species further up the food chain.

In the middle of a lake, the community is normally fuelled by phytoplankton (floating microscopic plants) that live near the surface, where there is sufficient light for photosynthesis. Larger microscopic animals called zooplankton graze on these tiny plants. To stop them sinking below the productive sunlit zone, they have various features or behaviours: many can swim, while others use air bubbles for buoyancy, or have spines and other outgrowths to slow their sinking rate.

In the darkness near the bottom of deep lakes, there is a more limited range of organisms. Life depends on organic matter raining down from above. In this zone, low oxygen levels are common in lakes that are high in nutrients, making life even more challenging.

Wetlands and groundwater

When people think of freshwater habitats, they mostly think of rivers and lakes. But wetlands and the species found there are also reliant on fresh water. A distinct range of organisms can also be found underground. For example, tiny crustaceans known as amphipods live in alluvial groundwater beneath the Canterbury Plains.


Plants and recyclers

Plants and decomposers play an important role in freshwater ecosystems. They are the organisms that produce and recycle the organic matter used as food by other organisms.

Algae: tiny but important

Phytoplankton are algae that live in water. They are vital because they convert sunlight to organic matter through photosynthesis, and so provide the base of the food chain.

Algae are classified into several groups, often according to their colour – for example, green algae and red algae. (Cyanobacteria are often referred to as blue-green algae, but they have a primitive cell structure and are more like bacteria than true algae.) Some types of algae grow independently as single cells, while others form colonies or filaments. Some are fixed to river and lake beds, and others live suspended in the water.

Trouble comes in threes

Cyanobacteria (blue-green algae) live in fresh water. Some cyanobacteria produce powerful toxins that can cause sickness and death in people or animals exposed to them. Neurotoxins target the nervous system, hepatotoxins damage the liver, and dermatoxins irritate the skin.

Diatoms are single-celled algae enclosed in a hard shell. They live in thin layers on river and stream beds, and are an important food source for mayfly nymphs and many other grazing invertebrates. Not all algae support the food chain to the same extent. The unsightly filaments of green algae often seen in nutrient-rich streams are not very palatable and can affect water quality.

Most of New Zealand’s common algae also occur overseas. But it is possible that with better understanding, more species unique to New Zealand will be identified.

Larger plants: macrophytes

A wide array of larger plants also live in fresh water for at least part of their life cycle. These plants, known as macrophytes, include macroalgae, mosses, liverworts, ferns, and vascular plants (angiosperms).

Most are restricted to lakes or lowland rivers with relatively slow-flowing water and/or stable beds. But mosses and liverworts are often found in fast-flowing streams, although only in areas with a stable bed.

Macrophytes have a variety of growth forms. Some species such as reeds are rooted to the river or lake bed, but push up above the water. Others grow entirely beneath the surface. Some have been recorded at depths of 70 metres in very clear lakes. Several species such as duckweed (Lemna minor) and ferny azolla float on the surface.

Although macrophytes are common and abundant, only two types of invertebrates (animals without backbones) will actually eat the living plants – freshwater kōura (crayfish) and an aquatic moth (Nymphula nitens).

After they die, macrophytes begin to decompose and their nutrients become available for other plants and animals to use.

Bacteria and fungi: recyclers

Many people associate bacteria with disease. But most bacteria found in natural systems play a very important role, breaking down organic matter and recycling its nutrients – these are then available for other plants and animals to use. Although they are important, there has been a limited amount of study on their diversity. Only a fraction of those found in natural systems have been scientifically described.

Fungi break down wood and other larger pieces of organic matter that become food for many invertebrates. Over 600 different types of fungi occur in fresh water.


Invertebrates

Animals without backbones (known as invertebrates) include insects, snails and worms. They play an important role in freshwater ecosystems. They feed on living and dead plant matter, and on each other, and are an important food for fish and some birds.

A diverse group

Most invertebrate families have members that live in fresh water. There are sponges, flatworms, annelids (worms, leeches), molluscs (snails, mussels), crustaceans (crayfish, shrimps, copepods), insects (caddisflies, dragonflies, mayflies) and mites. Lesser-known groups include rotifers, hydras, hairworms, ribbon worms and bryozoans.

Freshwater invertebrates range from tiny animals that can barely be seen with the naked eye (such as planktonic rotifers) to quite large crayfish (kōura) and mussels (kākahi), a traditional food source for Māori.

Testing the water

Because they are so diverse and plentiful, invertebrates are ideal indicators of water quality and habitat health. If pollution-tolerant species are present, this shows the water is in poor health. Species that are sensitive to pollution signal water in good condition. For example, certain mayflies and stoneflies are only found in streams with high-quality water.

Many are highly mobile, while other types, such as sponges, spend much of their life attached to a single spot. Some have shells (for example, molluscs) or cases (for example, caddisflies) to protect them from predators. Others move to refuges such as the darkness of deep water, where it is more difficult for predators to see them.

Reproduction

Freshwater invertebrates have a wide range of reproductive methods:

  • Hydra and some worms reproduce asexually.
  • Leeches and flatworms are often hermaphrodites (with both male and female reproductive organs).
  • Some snails and crustaceans lay eggs that develop without fertilisation.
  • Young freshwater shrimps are initially male and then become female as they grow.
  • Sexual reproduction is common for most aquatic insects, many of which breed only once and then die.

Life cycles

Freshwater crayfish put great effort into parental care and carry their eggs and offspring under their tail for many months. Some other invertebrates rely on a numbers game, releasing thousands of eggs that each have only a limited chance of survival.

Crayfish are one of the longest-lived invertebrates, sometimes surviving for more than 16 years. Several of the larger aquatic insects live for several years, although most have a shorter span, with one or two generations per year. The tiny rotifers live for only 8–10 days.

Diet

Like most other animals, invertebrates have specialised eating habits:

  • Some species are grazers and eat phytoplankton or algae.
  • Some prey on other invertebrates.
  • Some feed on dead and decaying organic matter.
  • A group common in flowing waters have net-like appendages to catch passing food particles.

A connection with Gondwana

New Zealand’s freshwater invertebrates are quite distinctive, and many species are not found elsewhere. However, many groups are also related to similar species found in South America, New Caledonia, Australia and South Africa. This suggests that they have common ancestors that once lived on the ancient supercontinent of Gondwana, before it began to split apart some 85 million years ago.


Fish

Food for people

Fish are at or near the top of the food chain in most freshwater systems. Many species are valued by Māori as mahinga kai (traditional food sources), and the whitebait fishery is based on catching juveniles from five species of native galaxiids.

Some introduced fish are common in freshwater systems, where they compete for food – for instance, they eat invertebrates that are a food source for galaxiids. One common, introduced fish, trout, are implicated in serious damage to some native freshwater species, however, they are valued by anglers and are an important symbol for the protection and restoration of freshwater ecosystems.

Native species

The latest information indicates that New Zealand has around 40 native species of freshwater fish. This count may increase as new genetic techniques bring a better understanding of the diversity within some fish groups. Thirty-three species are known only in New Zealand. Kōaro, longfin eels and spotted eels are also found in Australia, while lamprey and īnanga also occur in Australia, Chile and Argentina.

New Zealand’s native freshwater fish belong to eight distinct families: the jawless lamprey, eels, smelts, southern graylings, galaxiids, torrentfish, bullies and flounder.

Most are relatively small, although the eels are an obvious exception – some female longfin eels are up to 2 metres long and 25 kilograms in weight.

Age

Longfin eels are the longest-living native fish, generally taking 20–30 years to reach maturity. Some large female eels stay in fresh water for over 80 years. In contrast, smaller species, such as īnanga, reach maturity in just one year and rarely live longer than three years.

Diet

Almost all native fish prey on invertebrates or other fish. However, historical accounts indicate that the now-extinct grayling (Prototroctes oxyrhynchus) was an exception and grazed on algae from river rocks.

Migration and life cycles

Nearly half the native fish species migrate to and from the sea during their life cycle. As not all species can climb rapids and waterfalls, freshwater fish are most diverse at low altitudes, closer to the coast.

  • Lampreys reproduce in fresh water but develop mostly at sea.
  • Eels reproduce at sea, but most of their growth occurs in fresh waters.
  • The five species of galaxiids that form the whitebait catch lay their eggs in fresh water, but their larvae quickly migrate to sea and spend a few months there before heading back to fresh water.

Grayling gone

Although once common in New Zealand, grayling (Prototroctes oxyrhynchus) began to decline soon after European settlement. They were last noted in the early 1920s. It is not clear what caused their disappearance, but it could be linked to land use or the introduction of trout.

Distribution

Although not found elsewhere, many of New Zealand’s freshwater fish do have close relatives in Australia, South America and Southern Africa. Two species that do exist beyond New Zealand are lampreys and īnanga. This distribution, with other evidence, indicates that migration across the oceans is an important factor explaining the global spread of these fish.


Factors affecting life in fresh water

A range of factors influence the types of species that live in different parts of New Zealand.

  • At a broad level, evolutionary history and climate determine which species are found in certain regions.
  • At a finer level, factors such as water quality, land use, channel shape, or lake type influence the range of species found in a river reach or lake.
  • Small-scale factors (light and food availability, cover, predation, competition, water depth and speed) also control which species live at certain sites within a river or lake.

All these factors can interact to shape freshwater ecosystems.

Light availability

Without light, plants cannot grow. There are big differences between the communities living in the well-lit surface and edges of lakes, and those in the depths, which receive little or no light. Turbid (muddy) water also restricts the amount of light that reaches the bed.

Lakes with large shallow areas tend to be more productive than lakes with little shallow water (because more light reaches the lake bed). A similar situation exists in river systems, where heavily shaded streams rely on leaves and other organic material from the surrounding catchment to fuel the ecosystem. In larger rivers the canopy opens up, allowing algal production to become more important.

Predators versus available food

There is debate about whether the number of invertebrates (insects and other animals without backbones) in a habitat is controlled by the number of predators that will eat them, or by the amount of food available for them.

Both factors are important, but their relative importance may vary. For example, the introduction of trout appears to have changed some streams from systems controlled by the available food to those controlled by trout as predators.

Disturbances: floods and droughts

Rivers and streams are particularly prone to disturbance, due to huge changes in flows, as in floods and droughts. Although these are often seen as harmful, it appears that a moderate level of disturbance can promote higher diversity in some systems. This makes sense: if there is some variation in an environment, more life forms may be able to live there.

Human impact

Irrigation and hydroelectricity projects

Water is currently in high demand, and the value of freshwater systems is often weighed against the potential value of using the water. Hydroelectric dams and large irrigation projects can turn running waters into lakes, affect water quality, and restrict fish movement along river systems.

Farming

Many rivers and lakes now have high concentrations of nutrients, sediment and faecal bacteria, and problems with algal blooms (a heavy growth of algae). This is because more intense farming leads to excessive runoff of sediment and fertilisers into waterways. Taking water for irrigation has also harmed the water quality of lowland streams and some lakes. The Rotorua lakes are classic examples. Too many nutrients enter the lakes, causing algal blooms, which in turn starve the water of oxygen. At times, the lakes are closed for swimming, due to low water quality.

Pests

Various pests have become established in freshwater systems. Pest fish prey on native species and can stir up mud and alter the water quality. Invasive aquatic plants grow profusely, changing the habitat and causing problems for recreational activities. For example the introduced weed Lagarosiphon major can grow so profusely that it clogs the shoreline, making it unattractive for swimming. Even tiny algae can cause problems. In 2004 outbreaks of Didymosphenia geminata smothered South Island riverbeds, affecting everything living there.


External links and sources

More suggestions and sources


How to cite this page: Roger Young, 'Life in fresh water', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/life-in-fresh-water/print (accessed 20 November 2019)

Story by Roger Young, published 24 Sep 2007