Low tide, intertidal flats (King George Sound)
Habitats
Low tide, intertidal flats (King George Sound)
Understanding how a waterway functions can be approached in two ways. The first is to look at the various plants and animals and see how they use the waterway. The second is to study the various habitats within the waterway and look at the physical factors which make the habitats and how various plants and animals use them.
A habitat is a particular environment inhabitated by organisms. A number of different habitats have been identified which are common in most estuarine and riverine ecosystems. These areas provide certain living conditions for flora and fauna and play an important role in how an ecosystem works. A brief description of each habitat and its associated biota is given below. Some habitats are found in both the estuarine and riverine environments, while others are specific to a particular ecosystem type.
Seagrass meadows are important to estuarine systems because they provide food, shelter, breeding grounds and nurseries for many marine invertebrates and some fish species. Also, the complex root and rhizome system of seagrasses helps to keep the estuary floor stable. It also provides a stable habitat for molluscs, worms and other invertebrates found in the estuarine sands. Seagrasses are also extremely important to the productivity of the estuarine system. The decomposition of seagrass leaves produces a large amount of organic matter. This adds to the formation of detritus, which is a major source of food for a variety of fish and invertebrates which are valued by commercial and recreational fishers.
Shark Bay has the largest reported seagrass meadows in the world. They provide food for dugongs and turtles, and food and shelter for fish, crustaceans and molluscs, many of which are important for commercial and recreational fishing.
Shallow sandbanks and mudflats (also called `intertidal flats') are located on the fringes of waterways. These areas are rich feeding grounds for waterbirds, particularly after the tide has flooded them. Invertebrate fauna (such as worms and shellfish) are found in large numbers in these areas. They are an important food for fish and birds.
Mangroves grow in intertidal areas of sheltered marine shores, estuaries and creeks, mostly in the macrotidal areas north of Shark Bay. Some of Australia's most vauable commercial fisheries, such as the banana prawn, are directly linked to these habitat areas.
The open waters of waterways, especially estuaries, are a habitat for many different fish. They also provide pathways for fishes, shrimps and crabs migrating between the estuary and the open sea. Many marine fish species are dependent on estuarine waters as spawning and nursery areas for their young, such as tailor in the Swan River Estuary. Some of these species, such as the blue manna crab, spawn in marine waters and their tiny young find their way into the protected estuarine waters. Other fish come into the estuary to spawn. Different species of birds fish in the open waters of the estuaries and river pools. For example, darters, grebes, pelicans, cormorants and terns forage, mainly for fish, in the deeper waters of the Swan River Estuary.
The fringes of the estuaries and rivers support a variety of vegetation. Fringing vegetation is very important for the natural workings of a waterway. The vegetation stabilises the banks and reduces the chances of erosion. It provides wildlife habitats. It traps sediment, nutrients and pollutants draining from surrounding land. Also, marshes and fringing vegetation are rich feeding grounds, roosts and breeding sites for waterbirds. Organic detritus (such as leaves from trees and water plants) breaking down in the water also provides food for invertebrates and fish.
The Peel-Harvey Estuary is an outstanding example of a shallow estuary. It is the largest in south-western Australia, with substantial remnant areas of saltmarsh. The estuary is the principal migration stop-over area, and probably the main drought refuge area, for waterbirds in south-western Australia. The highly productive saltmarsh areas provide feeding grounds while open scrub and low, open forest are good roosting and breeding habitats.
These areas are important roosting and breeding sites for waders and other waterbirds. They also provide a refuge for the birds away from people, dogs and boats.
The floodplains of rivers and estuaries have a particular ecological character. A floodplain follows the shoreline of a waterway and includes many of the important areas already discussed. A floodplain acts as a buffer between the land and water environments, protecting the waterways from run off surge (after heavy rain), heavier loads of sediment and other pollutants. Floodplain vegetation protects and stabilises the waterway's fringes.
Floodplains can be protected by carefully controlling the types and extent of development. This prevents erosion and general urban pollution, diversion or disruption of the natural drainage patterns and loss of critical waterway habitats.
Open, shallow marshes and paddocks around harbours and rivers provide seasonally abundant food (crustacea, insects, frogs) for flocks of heron, ibis, swans and ducks.
The low-lying areas around the Swan River in Bassendean and Guildford would be familiar to many people as a seasonally inundated floodplain.
Natural habitats around waterways may be disturbed or totally detroyed by human activities such as clearing, grazing and development.
People are attracted to living near waterways. To allow agricultural and urban development, low-lying land is often drained. In the past, natural creeks and rivers were often altered to improve drainage. This involved clearing natural debris such as logs and branches, straightening the course, and even running the river or creek through concrete pipes.
Clearing and building has altered run off and streamflow patterns. Urban, industrial and rural land uses have degraded water quality. Many of our South-West streams have become saltier. Most now carry excessive quantities of plant nutrients and sediments and, in some cases, pesticides and other pollutants. The streams carry the pollution downstream to estuaries and inshore oceanic waters. This causes environmental problems. Natural habitats have been altered, and this has not always been good for the creatures and plants which once prospered there.
One of the major impacts on waterway habitats has been the loss of natural fringing vegetation. Once foreshore vegetation is cleared, stock or humans can easily get to a waterway's banks. Erosion and undermining of trees can soon follow. Then the waterway's scenic, recreational and habitat values can quickly be lost.
There are many areas within waterways ecosystems which are of particular importance to certain species or to the working of the overall ecosystem. Protection of these areas may involve restricting human use of the area, not allowing pollution and refusing other external sources of disturbance. This section describes these important areas, their ecological roles and their vulnerability to disturbance.
Different species of birds and other creatures find different places the best for breeding. Saltmarshes and fringing reeds around waterways are usually important areas for waterbirds. Shallows near the banks are excellent breeding areas for some species of fish. These areas can be identified and protected.
Many species of aquatic fauna (such as worms, molluscs and fish) spawn in specific parts of a waterway. Often these areas are very limited in size. A major disturbance to a spawning area could kill all the spawn and even lead to the extinction of that species in the area. Other animals that were dependent on them for food would then start to die, too.
The young of many marine and estuarine species, when they are several weeks old, congregate in special areas called nurseries. In these places juveniles prosper because they have the right food, there aren't many predators and other conditions are most suitable for their survival. These areas may overlap with other important places such as breeding areas or migration pathways.
Different species require different sorts of nursery areas. Each species has its own particular way of reproducing and needs special conditions for fast growth. Many marine fish species depend on estuarine waters as nurseries for their young. Some of these species spawn in the ocean and their tiny young find their way into the protected estuarine waters. Others come into the estuary to spawn.
It's often easy to find the feeding grounds of mobile species such as waterbirds. They often like tidal mudflats. Other creatures, such as certain fish, may feed in seagrass meadows. Feeding areas are important ecosystems that need to be protected for lots of reasons, especially because they are breeding and nursery places. Other places, which are not so obvious, may not be very big but could still be important feeding areas. It's important to protect these key areas from disturbance.
When animals migrate, they usually travel along specific, restricted pathways. Flight paths of migratory wader birds are easy to identify, but the underwater pathways of fish are much more difficult to locate. Fish move along these pathways for reasons that are fixed by the nature of the environments they pass through. These reasons (such as water temperature and flow, and the instinct to spawn) remain essentially the same from year to year. In general, most inlets from the ocean to estuaries are routes of migration for fishes, shrimps or crabs. These areas, together with the resting places of migratory birds on the shorelines of estuaries and rivers, need to be protected to keep the ecosystem healthy.
Certain species of animals and plants within a waterway's environment can be useful indicators of environmental health. Such species are either especially sensitive to changes in the environment (they die quickly if there is a certain type of pollutant, for example) or are particularly easy to work with (they're easy to reach and test).
An indicator species is used to monitor the effects of pollution or other degradation of an environment. The current numbers and condition of the species is compared with its previous, undisturbed condition.
Non-mobile benthic invertebrate species (creatures such as certain molluscs that live on the bottom or in its sediments and don't move much, if at all) are preferred indicators because they stay in one place and are sensitive to disturbances.