Estuarine Life

Click to enlarge

Map of Waitakere waterways showing Henderson Creek.

These pages have been developed to introduce the animals found at two estuarine sites of the Henderson Creek as shown by the blue stars on this map.

The Lower Oratia Stream site is just below the falls in Henderson and the Taipari Strand site is downstream where the Henderson Creek enters the Waitemata Harbour.

Important features of estuaries

Estuaries are amazing places. This is "where the river meets the sea". This web resource describes the biology of the tidally affected habitats of the Henderson Creek catchment, on the Te Atatu Peninsula, Waitakere City. River estuaries are fish highways because many of our freshwater fish have a seawater phase in their life cycle and many seawater fish enter the lower reaches of rivers to feed. The Henderson Creek Estuary has many typical features of upper North Island estuaries:

• The lower reaches are the widest and muddiest part of the river system. The water is primarily seawater partially diluted by fresh water from the river. The mangroves on the fringe, the abundance of mud crab holes in the banks and the presence of oysters and other marine shellfish, reflect the strong seawater influence. The great width and low gradient results in slow current speeds and this allows fine sediment to settle to create a constantly muddy bed. Wind and wave action often stir up the exposed, shallow water, resulting in the typically muddy-brown colour of lower estuaries. All creatures found in these habitats are adapted to living in seawater. The Taipari Strand site is an example of this part of the estuary.
• Slightly further upstream, at the Central Park Drive site, the water is more significantly diluted by fresh water, especially at low tide. These reaches are strongly tidally affected and muddy banks and areas of riverbed are exposed at low tide. Some mangroves and mud crab holes can be found, but they might not be as abundant as they were further downstream. Many creatures struggle to live in this zone for long periods because of the extremes in salinity as seawater dominates at high tide and freshwater dominates at low tide.
• Still further upstream is a zone of tidally affected river beyond the reach of seawater but where the river level rises and falls with the tides. This can be a tricky zone for many types of freshwater life because of the changing current speeds (fast at low tide but almost stagnant at high tide). Many stream invertebrates are not well suited to these variations and seawater species cannot live for long in the fresh water. The weedy or grassy margins of this zone are where inanga spawn and therefore, they are important habitats that affect the viability of these native fish populations. The Lower Oratia site is an example of this part of the estuary.

Check out a Waitakere estuary to see the interesting creatures that can be found.

The Lower Oratia Site.	Central Park Drive site at high tide.	Central Park Drive site, Henderson Creek at low tide	Lower reaches - Taipari Strand
Video showing sediment in water in Lower Oratia Site.	Video showing sediment in water in Henderson Creek at Central Park Drive.		Video showing sediment in water in Henderson Creek at Taipari Strand.

A Waitakere estuary

Visit the animals found at the places below.

More information about estuaries

What are estuaries like to live in?

Estuaries are harsh environments for many aquatic invertebrates because of the habitat variability associated with the tides.

Rising tides bring increases in salinity, changes in current speed or direction and flooding of the estuary. Falling tides cause salinity to decline, a return of "downstream flow", a drop in water level and often exposure of large areas of mudflat habitats to the air. These extremes mean that many freshwater macroinvertebrate species are simply unable to survive, and as a result, the estuarine biota has a more marine (seawater) look to it.

Estuarine communities tend to be dominated by crustaceans (particularly crabs, amphipods and isopods), molluscs (snails and bivalves), and worms (particularly polychaetes). Insects dominate most freshwater habitats.

What are estuarine macroinvertebrates?

Estuaries are homes to lots of tiny animals. Some swim in the open water but many others live on or in the bottom sediments or submerged water plants and other things. These animals include crustaceans, molluscs, worms, and insects. Many are tiny, only a few millimetres in length, and they may be difficult to see at first glance. Others are larger but might be hidden in burrows for much of the time (mud crabs) or they may be almost transparent (shrimps) or just not very active (snails). Collectively these animals are called macroinvertebrates, creatures without backbones, which can be seen without the help of a magnifying glass or microscope.

In some estuary habitats there may be many different types of macroinvertebrates in one small area and many thousands of individuals per square metre. Find out the names of some of the creatures that belong to these groups.

What can macroinvertebrates tell us about estuaries?

When people start looking at estuaries (including tidally affected streams) to see how healthy they are, they sometimes get a good idea about the water or habitat quality from testing for things like salinity, temperature, amounts of dissolved oxygen, nutrients and the amount of certain contaminants. Important information also comes from analysis of the type of sediments, the effects of tidal flows and river flows and the type of vegetation in the estuary.

To get more information about the ecology of the estuary, scientists can also collect macroinvertebrate samples and then sort them to get an idea of the range of creatures present and which groups occur most frequently. This is useful because particular estuarine macroinvertebrates have specific habitat requirements. Some species are associated with particular types of sediments, others might be tolerant of certain types of contaminants, some might only be found in oxygen-rich habitats while others might be indicators of a lack of oxygen.

The variability of estuarine habitats makes it hard to generalise about the numbers or types of species that should occur in estuaries, but a total lack of macroinvertebrates would be a strong sign of a significant habitat or water quality problem.

Why are estuarine macroinvertebrates important?

As food for people

Several estuarine macroinvertebrate species are important to humans for their food value.

Cockles, oysters and various species of snails commonly found in the lower reaches of estuaries were historically important food resources to Maori (and still are in many places). Some water managers and researchers monitor such edible species to check if there are sustainable populations in key estuaries and to ensure that edible species are not dangerously contaminated by various pollutants, making them unsafe to eat.

Because they clean up the estuary

As in freshwater habitats, many estuarine macroinvertebrates feed on the green and brown slimy algae growing on the sediments. This means that without these invertebrates our estuaries might be much more slimy.

Some macroinvertebrates filter tiny swimming or floating animals and plants (zooplankton and phytoplankton) and it is possible that filter feeders can help to reduce the occurrence of "blooms" of some types of plankton. Scavenger species, including most crustacea and some of the snails will also consume dead animals left by the falling tide helping to keep our estuaries cleaner.

Omnivores and various plant-eating species do a similar job by helping to break down dead plant matter that may have come from the land, the river or the sea.

As they provide food for fish

Many marine fish species feed on estuarine macroinvertebrates and healthy macroinvertebrate populations may enhance the productivity of our coastal fisheries.

Several of our native freshwater fish species spend part of their lives in estuaries and estuarine macroinvertebrates may make up an important part of their diet while they are living there.

How do people affect the macroinvertebrates in our estuary?

Harmful effects

By causing sudden influxes of sediment from earthworks and erosion which

• Bury macroinvertebrates that cannot move or burrow quickly.
• Bury the food that many species rely on.
• Overload filtering species with too much inedible material for them to get a decent meal.
• May contain high concentrations of contaminants that could be harmful.
• May fill in areas of the estuary if the mud settles out of the water. This means that more of the estuary is exposed at low tide decreasing the area suitable for species that need to remain submerged.

By increasing contaminants from urban areas

• Urban stormwater tends to contain high concentrations of metals, particularly copper, zinc and lead, and these metals could be toxic to some species.
• Urban stormwater may contain high concentrations of toxic organic compounds, including pesticides, herbicides and poly aromatic hydrocarbons (PAH's) which are toxic to some species.
• Urban stormwater and sewage discharges may contain high concentrations of nutrients that encourage algae growth and increase rates of decomposition, decreasing oxygen levels.
• Industrial areas may be sources of many toxic contaminants.

Studying estuarine macroinvertebrates

We do not have the expertise to carry out surveys so have provided a link to Environment Waikato's guidelines below.

Note: You will need to have Adobe Acrobat Reader installed on your computer in order to view and print this document.  For help opening PDF files or tips on copying information see Helpful Tips.

"Guidelines for community focused monitoring of mangrove habitats in estuaries."