How waterways work in the Albany area
Figure 2-6: Albany waterways management area
There are two major types of waterway ecosystems in the Albany Waterways Management Area.These include the:
Princess Royal Harbour is a bay (also called a 'marine embayment'). It is not, strictly speaking, an estuarine ecosystem because a river doesn't flow directly into it. However, it does display some of the characteristics of an estuarine ecosystem. So, in this book it is considered an estuarine ecosystem.
The land within the catchment area is important to how Princess Royal Harbour works, so it has to be considered, too. However, the adjacent marine ecosystems of King George Sound and the Southern Ocean are not discussed in this book.
The King and Kalgan rivers are the two major riverine ecosystems within the Albany Waterways Management Area. Numerous smaller creeks and streams feed into these rivers from throughout the catchment. The King and Kalgan rivers drain into Oyster Harbour on its northern shore. They have the characteristics of an estuarine ecosystem for seven and nine kilometres upstream respectively. The tides affect the two rivers in these reaches.
The riverine section of the Kalgan River winds through agricultural land stretching northward to the southern face of the Stirling Ranges. The King River extends in a westerly direction draining areas south of Mount Barker.
|For more about stratification see section 2.1|
|For more about this see section 9.1|
The main estuarine ecosystems of Albany's waterways are in Oyster and Princess Royal harbours and the tidal reaches of the King and Kalgan rivers. Princess Royal Harbour is not a true estuarine ecosystem because it doesn't have a river flowing directly into it with a strong enough flow of fresh water to dilute the harbour's salt water. Despite this, the harbour displays characteristics which are typical of estuarine ecosystems.
Figure 2-7: Waterway ecosystems of the Albany area
On average, the salinity levels of the waters of both Princess Royal Harbour and Oyster Harbour are close to that of marine waters (around 35 parts per million). Salinity levels tend to be higher in the harbours in summer owing to increased evaporation and less fresh water flowing into them than in winter. These seasonal changes aren't as big in Princess Royal Harbour owing to its small catchment and the lack of fresh river water.
|For more information about stratification see Circulation and Tidal Flow|
This stratification probably is important to bottom-dwelling macroalgae because it controls the availability of nutrients.
|For more information about this see section 9.2|
In the past, there has been more algal growth in Princess Royal Harbour than in Oyster Harbour owing to the:
The amount of nutrients coming into Oyster Harbour depends on the season. The nutrients come from the King and Kalgan rivers, the Yakamia Drain and leaching from sandy soils around the harbour. Most nutrients come into the harbour when there is a lot of run off, after rain. Much of the nutrients are carried straight out into King George Sound, in the almost-fresh water flowing from the river on the surface of the harbour, before it can be mixed into the harbour's waters and fuel algal growth.
The sediments of both the harbours store a lot of their nutrients. The sediments in Oyster Harbour are the largest pool of both phosphorus and nitrogen. They contain at least 80% of the harbours total nitrogen and total phosphorus. Under certain conditions these stored nutrients become available for algae to use for growth. This store of nutrients must be carefully considered when developing management strategies for the harbours.
Circulation and tidal flow
The waters of Princess Royal Harbour have good circulation and are generally well-mixed. The wind, and tidal currents close to the harbour's mouth, cause most of the circulation.
Water circulation within Oyster Harbour varies from season to season. In winter, brackish, nutrient-laden water from the King and Kalgan rivers and other sources flows across the surface of the harbour into King George Sound. It hardly mixes at all with the other waters in the harbour. This surface water flow is important because it carries nutrients out of the harbour and into the open sea, so they don't get into the harbour's ecoystems. In the summer months, less fresh water flows into the harbour and the sun heats the water. Stratification occurs: warmer, less dense water is on the surface and colder, denser water below. These waters mix a little, caused by the summer sea breezes.
As Princess Royal Harbour and Oyster Harbour are both permanently open to the marine waters of King George Sound, flushing rates are generally high. Under average conditions, surface waters remain in Oyster Harbour for two days or less, and bottom waters for 10-20 days. It takes about 20 days for Princess Royal Harbour to flush 90%. In contrast, other estuaries on the southern coast of Western Australia take months or years to flush.
Oyster Harbour and Princess Royal Harbour provide lots of sheltered areas where creatures can breed and feed. Because the Southern Ocean has strong, mainly south-westerly winds, these harbours are safe havens for many organisms. The southern and eastern shores of Princess Royal Harbour are sheltered from south-westerly winds because they lie in the lee of the Vancouver Peninsula. The south-eastern shore of Oyster Harbour is also sheltered because it lies in the lee of Mount Martin.
Oyster Harbour has lots of shallows. About half of the harbour has water less than 1 m deep. Shallows are found along the entire eastern side of the harbour. There are similar areas in the south-western and north-western corners, making up about 40% of the harbour's total area. Sediment carried in river floods is deposited in the shallows, particularly near the King and Kalgan river mouths.
Princess Royal Harbour is a deep basin bordered by shallow sandflats. About half of the harbour is less than 2 m deep. The shallow sandflats are most extensive off the western and southern shores and along the Vancouver Peninsula.
Although seagrass meadows have declined greatly over recent years, these areas still cover a substantial proportion of both Oyster Harbour and Princess Royal Harbour. Three principal marine species of seagrass are found in the harbours: Posidonia australis, Posidonia sinuosa and Amphibolis antarctica. Small areas of two estuarine species, Ruppia megacarpa and Halophila ovalis, are also found in Oyster Harbour.
There are shallow sandbank and mudflat areas on the fringes of both Princess Royal and Oyster harbours. In Princess Royal Harbour the most extensive areas are off the western and southern shores. In Oyster Harbour the entire eastern shoreline is shallow sandflats. There are similar areas at the mouths of the King and Kalgan rivers.
Macroalgae are usually found in the shallow sandbank areas of the two harbours. The free-floating algae accumulate in these areas, especially on the eastern shores of Oyster Harbour and the south-eastern shores of Princess Royal Harbour, because of prevailing winds and currents. The macroalgae species include the common estuarine species of green algae: Cladophora and Chaetomorpha.
The open waters of Oyster Harbour and Princess Royal Harbour are a habitat for many fish species. Common species include the black bream, sea mullet, yellow-eyed mullet, flathead and cobbler. Most commercial species found in the harbours are common coastal marine fish which spawn at sea and use the harbours as nursery feeding areas. Most of the fish species found in the harbours move up the Kalgan and King rivers as far as the rock bars, but only black bream and sea mullet are found further upstream. Mulloway is also found upstream in the Kalgan River. Several species of birds, including pelicans and cormorants, are also found fishing in the open waters of the harbours. Many species of waterbirds also use the open water of river pools in the King and Kalgan rivers.
Low-lying areas around Oyster Harbour support saltmarsh fringing forests. These wetland areas are extremely productive parts of the estuarine ecosystem because they are a major source of detritus. Detritus is a major food source for other organisms in the ecosystem.
Three main areas in Oyster Harbour support these sorts of habitats. These include:
These areas are important feeding and roosting areas for waterbirds, including waders and waterfowl. Migratory waders also feed in these areas, putting on fat during the summer months to prepare for their long journey to their breeding grounds in the northern hemisphere. Groups of migratory waders observed in these areas include sandpipers, plovers, stilts and stints. Many of these birds are protected by international agreements.
In estuarine regions of the rivers the saltwater paperbark Melaleuca cuticularis is the dominant tree, usually growing among dense stands of the fringing rush Juncus kraussii and the tall tufted sedge Gahnia trifida. As the rivers become fresh further upstream, the saltwater plant species are replaced by a fringing forest of Eucalyptus calophylla (marri), Eucalyptus rudis (flooded gum) and Melaleuca rhaphiophylla (swamp paperbark), over a variety of freshwater sedges, including Lepidosperma effusum and Baumea juncea, and the tall shrub Astartea fascicularis. Eucalyptus rudis and M. rhaphiophylla dominate in swampy areas.
A number of tree species or tall shrubs, including Eucalyptus occidentalis (swamp yate), Banksia seminuda (river banksia), Oxylobium lanceolatum, Trimalium floribundum and Hakea oleifolia, are also found along freshwater parts of the river. The upper parts of the Kalgan River become increasingly saline and estuarine species, typical of Oyster Harbour, dominate the floodway of the river along with a number of Melaleuca species.
In the freshwater, high rainfall sections of the rivers, the upper embankments of the river valleys are dominated by forests of Eucalyptus calophylla and E. marginata (jarrah). On the Kalgan River this forest gives way to heaths of Calothamnus quadrifidus, Thryptomene saxicola and Darwinea citriodora, broken in places by scrub of E. marginata and E. decipiens, on rocky ground. Further upstream where the country becomes drier and the river valley shallower, the surrounding woodland of Eucalyptus wandoo and E. occidentalis comes to dominate the upper river embankments.
In the freshwater areas of the rivers the fringing vegetation is mostly healthy, but where fences haven't been maintained livestock have grazed and trampled out native species. This degradation, along with frequent fires, has encouraged the growth of introduced grasses. The loss of deep-rooted native vegetation has led to the erosion and subsidence of river embankments in places. In the upper region of the Kalgan River, increased salinisation has killed native fringing forests over large sections. In some cases, salt-tolerant tree species, such as the saltwater paperbark M. cuticularis, are present to replace the less tolerant ones. But where they are absent, the forest is simply replaced by rushes or, worse, annual grasses which provide little support to the river embankments.