Port Phillip Bay
Recent studies on the state of Port Phillip Bay show that despite over 4 million people living within Port Phillip Bay’s catchment, including two major cities: Melbourne and Geelong, the bay is generally in good health. However, human impacts are still locally significant around the mouths of creeks and drains, and in the northern part of the bay adjacent to the Yarra River and the Western Treatment Plant (WTP) at Werribee.
Between 1990 and 2011, Port Phillip Bay experienced two major ecological disturbances: a prolonged drought from 1997–2010; and the introduction of the invasive northern Pacific seastar – a native of the northwest Pacific and a voracious predator which peaked in population at 56% of the resident fish biomass in 2000.
Size and volume
Length: 58 kilometres (Altona to Rye)
Width: 41 kilometres (Portarlington to Seaford)
Depth: the average depth is 13 metres. The greatest depth is 24 metres. Nearly half the bay is less than eight metres deep.
Volume: 25 cubic kilometres
Coastline: 333 kilometres (Point Lonsdale to Point Nepean)
Total area: 1,930 square kilometres
Catchment area: 9,790 square kilometres
Population: over 4 million people live around the bay.
Physical processes
Salt content
Despite input of freshwater from the Yarra River and other streams, about 660 km³ of ocean water from Bass Strait enters the bay every year through tidal action, making the bay a marine system. Variation in freshwater input contributes to variations in salinity, such as the rise in salinity recorded during the millennium drought (1997–2009).
Catchment inputs
Eight major creeks and rivers flow into the bay. The Yarra River is the largest single source of freshwater. Treated effluent from the WTP is the largest source of nutrients. On average, freshwater inputs to Port Phillip Bay are equivalent in volume to evaporation from the bay, which is conducive to a healthy marine environment.
Water movement
Several natural forces drive circulation in the bay: tidal movement, wind generated waves and surface currents, and salinity and changes in temperature.
Mixing
Port Phillip Bay is well-mixed, reflecting its shallow nature and large surface area, which allow winds and currents to mix the water column. Waters in the bay have a long residence time – greater than 400 days. This is a significant difference between Port Phillip Bay and Western Port.
Sedimentation
Silt, mud and organic debris enters the bay from the rivers, creeks and drains. The total input of sediment has been estimated at 15,000 to 80,000 t/year, with an average of 30,000 t/year. Wave action drags finer, lighter material and transports it further offshore to the deep central basin. Chemical analysis of the bottom sediments shows that the central basin contains mostly mud and silt (silica particles) with about 5% organic matter.
Bacteria compose a negligible fraction of the sediment.
For more information see Water quality.
Nutrients
Nutrients, particularly nitrogen, are important for marine ecosystems to function, but they have a negative effect when levels are too high (eutrophication), which can lead to algal blooms and drift algae.
Despite its proximity to Victoria’s largest cities, the nutrient levels of Port Phillip Bay are low and the nutrient cycle demonstrates a generally healthy system. However, there are seasonal variations in the nitrogen and chlorophyll concentrations because the freshwater input volume varies with rainfall.
Water has more information and explains denitrification efficiency – an important process in Port Phillip Bay’s nitrogen cycle.
The WTP and catchment inflows (from both rural and urban land) are the major sources of nutrients for the bay.