Why waterways are important for animals

A lot of different animals are found in a waterway. Apart from the fish and crustacea caught by amateur and professional fishers, a waterway supports lots of other vertebrates, invertebrates, insects and birds.

For more about food webs see section 2.3

Many of these animals live in the waterway all the time while others are migrants which come to nest or to grow, drawn by the abundant food. Estuaries, in particular, usually provide a lot of food for animals. Certain areas are particularly important to aquatic animals: the shallow banks covered by seagrasses, the foreshores and the places where plants grow around the edges. In these places, where plants are most abundant, the animals gather and together form highly complex food webs.

Waterways are the breeding and nursery grounds for many aquatic animals. They also provide places to hide and rest for land animals. When plants along the shoreline decompose, large amounts of material (called 'detritus') is produced. This detritus is the basis of all the food chains in an estuary. Small creatures feed on the detritus, which are then eaten by larger creatures. This is a simple example, and an estuary's food web is very complex.

Young fish (called 'juveniles') can hide in some waterways, keeping them safe from other animals that want to eat them. A waterway can provide food for juveniles and adult fish, and be a breeding area, too. Waterways are also important feeding, roosting, breeding and rest areas for birds and other animals. Many species of migratory waders spend their non-breeding period in Australia building up fat reserves before flying to the northern hemisphere to breed.

To keep these places safe and healthy, they must be properly managed. Habitats and food sources need to be protected. This means that water plants need to be protected, and the whole food web.

Not all species are regarded as an asset, as useful. Certain insects, such as the mosquito, are nuisances, and their numbers need to be controlled. Any methods of controlling mosquitoes need to ensure that other animals and plants are not harmed as well.

Human activities which affect life in waterways

Human activities have been harmful to the habitats and ecosystems of the waterways in south-western Western Australia. Some activities have a direct impact on fish and some affect the habitats, water quality and breeding grounds.

Dredging and reclamation

Dredging removes sediment to deepen a waterway. Reclamation fills in the shallows to make extra land and remove `swamps'. Dredging and reclamation usually kills the invertebrates on the spot. The quality of the neighbouring deeper water is no longer good enough for many plants and inverterbrates and they either leave, fall ill, or die. This reduces the amount of food for fish, especially juveniles.

Dredging may be necessary to provide recreation facilities or to improve the tidal exchange between an estuary and the ocean. The most commonly dredged areas are the river and estuary shallows. Before any dredging occurs a thorough environmental investigation is required to ensure that the plants and animals will not be harmed.

Short-term effects of dredging include:

• Deoxygenation of the water column. This happens because organic matter (such as dead and decaying plants and animals) on the bottom is dredged up and suspended in the water. Bacteria increase in number to feed on the matter, which increases the biochemical oxygen demand (BOD), leaving less dissolved oxygen for animals. Plants slow or stop their photosynthesis because of the turbidity of the water (it's muddy and cloudy and light can't get through to the plants). If the estuary is stratified, with fresh water flowing on top of sea water, then there will be even less oxygen available in the deeper sea water. If the levels are less than 3 mg/L then fish will die

• Suspension of fine particulate matter. This is even more serious than deoxygenation of the water. As well as making it hard for plants to survive, fine particles of sand, silt and clay floating in the water clog the feeding parts of many animals which filter their food from the water (such as zooplankton, bottom-dwelling worms and molluscs, and some fish). The particles can also clog gills and have a harmful affect on spawning, egg and larvae development

• Release of pesticides, nutrients, heavy metals and other toxic substances from the sediments. These materials can harm animals in the waterway, and humans, too, if the animals containing some of these products are eaten

The long-term effects of dredging and reclamation are difficult to identify and not able to be readily quantified. Particular organisms may become extinct in the area. There might be changes to the bottom contours of a waterway, an increase in the depth of the water and changes to the composition of the sediment. These types of changes affect how easily invertebrates can return to live in the disturbed area. Sometimes the aquatic community can be just like it was before the dredging (such as in the Walpole Inlet). In other places (such as Pelican Rocks in the Swan River Estuary), the community still hasn't recovered after 20 years. Invertebrates will come back to live in an area if the new substrate (the sand or soil on the bottom) is coarse (like sand), the water is less than two metres deep, and the bottom is level without deep holes. Deep holes fill up with fine sediment and are not easily flushed by the water flow, which can lead to oxygen depletion in the holes and sand.

How changing a catchment affects waterways

For more about this see section 4

Clearing trees from a catchment encourages soil erosion and leaching. This leads to more sand and soil getting into the river, increasing the sediment load. The effects of this are similar to those from increased suspended particulate matter from dredging. More sediment in the water also helps nutrients, pesticides and heavy metals be carried downstream to the estuary and its animals, plants and sediments.