How the bays work


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.


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.


Despite the sand barrier presented at the southern entrance and continual input of fresh water by the Yarra River and other streams, about 660 cubic kilometres of ocean water enter The Rip every year through tidal action, making the bay a marine system.


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, 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.

Marine snails. Image credit - Julian Finn, Museums Victoria

Our natural environment is a treasure trove. Protecting  and restoring our bays, shores and the marine environment delivers a wealth of economic, social and cultural benefits. By translating nature’s benefits into a common language that allows for a dollar value to be placed on our ecosystems can transform the way policy makers, and the community, account for the environment.

Port Phillip Bay


Port Phillip Bay hosts a diversity of ecosystems and habitats, including sandy beaches, rocky shores, saltmarsh, mangroves, seagrass, sand beds, rocky reefs and open water.

The bay is home to thousands of species of marine flora and fauna.

Within Port Phillip Bay are four of Victoria’s marine protected areas (a marine national park and three marine sanctuaries) that were established in 2002 and are managed by Parks Victoria. The marine national parks and sanctuaries are highly protected no-take areas which cover approximately 63,000 hectares or 5.3% of Victorian state marine waters.

The areas in Port Phillip Bay are:

  • Port Phillip Heads Marine National Park
  • Jawbone Marine Sanctuary
  • Point Cook Marine Sanctuary
  • Ricketts Point Marine Sanctuary.

Plant life

There are four main kinds of aquatic plant species living in the bay, all of which use light energy from the sun to transform carbon dioxide, water and elements such as nitrogen and phosphorous into organic matter and oxygen (the process called photosynthesis).

  • Seagrasses

Seagrass beds provide food and shelter for many marine animals. About 95% of the bay’s seagrasses occur in waters shallower than five metres (typically on the low energy western and southern coastal fringes), reflecting their reliance on access to sunlight and low energy conditions.

Sediments can adversely affect seagrass by blocking light in the water column or through movement of sand.

  • Phytoplankton

Single-celled algae, which include diatoms and dinoflagellates.

  • Microphytobenthos (MPB)

Microscopic algae living on the seafloor grow over the surface of the sediment, and play a critical role in processing nitrogen inputs for the bay.

  • Seaweeds (macroalgae)

Large, multi-celled algae generally known as ‘seaweeds’.

Over 60 species of green algae, nearly 100 species of brown algae and about 260 species of red algae have been recorded.

Animal life

The breadth of animal life in the bay ranges from microscopic single-celled organisms through to sharks, as well as marine mammals such as seals and dolphins.

  • Zooplankton

These are the small invertebrate animals living in the waters of the bay. They range in size from microscopic single-celled protozoa and include all the invertebrates with armoured bodies and jointed legs (like crabs, prawns and lobsters).

It has been calculated that the free-swimming zooplankton alone probably filter the volume of the bay about twice a month, and the benthic (seafloor) filter feeders are similarly active. Hence the whole volume of the Bay passes through the total invertebrate population on something like a weekly basis.

  • Zoobenthos

These are the invertebrate animals which live in or on the sediments. They vary in size from protozoa to quite large shellfish such as scallops. There are about 700 species present, such as sea-urchins and starfish, sea squirts, roundworms, flatworms, sponges, sea spiders, bryozoans, sea anemones and corals.

  • Fish

The depth distribution of fish seems to be related to their diet. Most fish consume invertebrate animals living on the bottom of the bay, and these in turn are distributed by depth, mainly because they prefer different kinds of bed sediment (sand, silt or clay).

Fish is an important topic in the state of the bays reporting, and is discussed in the Fish section.

  • Marine mammals

St Kilda has a small colony of iconic little penguins. The WTP and nearby Ramsar sites have an abundance and diversity of shorebirds, while Pope’s Eye has an Australasian gannet colony.

The bay has a healthy population of resident dolphins with smaller populations of large whales, including humpbacks, southern rights and also killer whales, which have been sighted at various times, but don’t stay permanently.

A number of seals have also taken up residence, making use of shipping buoys and beacons.

Malurus cyaneus: superb fairy-wren
Citizen science

King tides are changing. You can help map the changes in king tides to form a critical photographic record.

Witness King Tides is a community photography project that helps visualise the potential future impacts of sea level rise. The programme asks people to visit the beach when king tides are coming to take photos of before, during and after as part of an impressive long-term monitoring project into the impact of king tides and climate change.


What we know

Developed in 2001, the first Port Phillip Bay Environmental Management Plan (EMP) has been an important guide for managing the bay. The EMP specified a set of environmental objectives to manage two priority risks to the bay: nutrients (see Water quality) and marine pests. One of the major objectives of the 2001 EMP was to reduce the amount of nitrogen flowing into the bay: this included upgrades to the Western Treatment Plant and improved stormwater management in the catchments.

What we need to know

With significant progress made towards existing targets and the scientific understanding of Port Phillip Bay much greater than in 2001, a new EMP is due to be finalised in 2017. It will provide an overarching framework for government, community and business to work together to address key issues that impact the health of the bay.


Over 160 introduced marine species have been recorded in Port Phillip Bay, many introduced by international shipping traffic in ballast or fouling on ship hulls. Marine pests are species that significantly harm biodiversity and habitats, shipping, coastal infrastructure, seafood industries and coastal communities, and can have economic impacts. Notable pest species include:

  • the northern Pacific seastar (Port Phillip Bay),
  • the Japanese kelp (Port Phillip Bay), and
  • the European fan worm (both Port Phillip Bay and Western Port).

Studies suggest that native urchins pose the biggest threat to reefs in Port Phillip Bay.

A sea urchin infestation in northern Port Phillip Bay was first noticed during the Millennium drought (1997-2009). In drought conditions there is less sediment and nutrients, so hungry urchins turn to algae as a primary food source. Over-grazing of sea urchins leads to loss of fish habitat.

The western area of the bay has experienced urchin barrens since the mid-1980s, but urchins have few predators now.

Western Port


Cardinia and Casey shires are experiencing remarkably fast population growth – estimated to be 14% from the years 2005 to 2015. The Victorian Planning Authority reported in September 2016 that Wyndham was the fastest growing municipality in Melbourne, followed by Casey and Whittlesea. Along with this have been changes to land use in the mixed urban-agricultural land around Western Port. 

Western Port and its islands are criss-crossed by seven seismically active fault lines and the Western Port area experiences numerous minor earthquakes every year.

Western Port and its surrounds have also been recognised internationally, with UNESCO’s Man and the Biosphere Programme designating it as one of just over 500 Biosphere Reserves around the world.

Western Port was listed as a Ramsar site in 1982. One of the largest breeding populations of Australian pied oystercatchers is found on French Island. The site is also listed as part of the Western Port Biosphere Reserve.

Within Western Port are three of Victoria’s marine national parks that were established in 2002 and are managed by Parks Victoria:

  • Churchill Island Marine National Park
  • French Island Marine National Park
  • Yaringa Marine National Park.

Western Port has also hosted an important commercial shipping port (Hastings) for many years.

Size and volume

Maximum length: 295 kilometres

Depth: the greatest depth is six metres. The average depth is three metres.

Coastline: 263 kilometres (including islands)

Total area: 680 square kilometres

Physical processes

Western Port is a large shallow bay that is divided into five basins or segments by large islands and mudflats and several narrow constrictions. The water flows in Western Port include catchment inflows and resuspended bay sediments, resulting in poorer water quality (and higher residence times) in the east. There is also short-term variation in water quality over tidal cycles, most likely from an interplay between the mudflats and incoming ocean waters.

Western Port has an extraordinary diversity of habitats, from rocky shores to deep channels with strong currents, mangroves, saltmarshes, seagrass beds, intertidal mudflats that are so important to shorebirds and subtidal soft sediments that harbour a diverse invertebrate fauna.

Western Port’s geography generates complex relationships within the bay, especially because its strong currents move sediments, nutrients and toxicants around, and provide a path for plants and animals to disperse.

The water column

The water column in Western Port is inhabited by microscopic single-celled organisms (phytoplankton), small animals that drift passively with the currents (zooplankton), and larger, passively drifting animals such as jellyfish. The phytoplankton are important indicators of environmental impacts such as elevated nutrients.


Drifting sediments are the most important aspect of water quality – causing turbidity and reducing water clarity - and have been a target of management action for some time. Nutrients are an issue in some parts of Western Port (around Watsons Creek and in the Corinella segment) and toxicants, particularly those associated with the eastern catchments.


While there are similarities to Port Phillip Bay, Western Port is more complex than its western neighbour.

Key habitats include:

  • extensive mudflats
  • two large islands (Phillip Island and French Island)
  • tidal flats that are cut by deep channels
  • mangroves and saltmarshes that fringe most of Western Port’s coastline
  • extensive seagrass meadows on mudflats and below the low-tide level
  • scattered rocky reefs
  • extensive habitat for shorebirds, with much of Western Port’s shoreline being important for international migratory birds.

Western Port receives inflows from several catchments which results in complex oceanographic circulation.


Mud covers about two thirds of Western Port. The unvegetated area has increased following the loss of seagrass beds. The extensive intertidal flats are important feeding  grounds for shorebirds. The mud contains a high diversity of ghost shrimps, brachiopods (marine animals that are considered living fossils), rare rhodoliths (a type of algae) and various species that are listed as endangered.


There was extensive loss of seagrasses in the 1970s, followed by some recovery, but large areas that lost seagrass have not recovered, and recovery has been poor in areas where water quality is a concern. 


Western Port harbours some of the southernmost mangroves in the world, and these mangroves line much of the shore. Historical comparison shows some loss, especially near Hastings. Localised disturbances and changes in the sediment of the bay have contributed to changes in mangrove distribution.


There is about 1,000 ha of saltmarsh in Western Port, and much of this is likely to be very vulnerable to sea level rise and other consequences of climate change, especially rising air and water temperatures. Saltmarshes have been progressively lost already, mostly because of development for agriculture and industry, around the western and northern shores of Western Port.

Animal life


Western Port has a high diversity of fish, including;

  • Small fish which live in  the extensive seagrass beds.
  • Open sea species such as Australian anchovy

The bay is also a breeding habitat for species such as elephant fish and school shark.

The greatest threat to fish in Western Port is the loss of their habitat, in particular the potential loss of seagrass habitat.

Birds and marine mammals

Western Port is home to the iconic little penguins of Phillip Island. But there are an abundance and diversity of bird populations, and it is an important refuge against drought. Western Port is also a non-breeding area for migrant shorebirds from the northern hemisphere and New Zealand.

This means that Australia has obligations under a suite of international treaties and agreements. It is also designated as part of the global network of Birdlife International’s Important Bird Areas.

The greatest threats to birds in Western Port are loss of habitat, reductions in food supply through extraction (particularly fish-eating birds) and seagrass loss (most species) and high levels of disturbance from human recreational activity (shorebirds). 

Over the past 200 years, humans have altered the Western Port environment through clearing vegetation, draining of the expansive Koo Wee Rup swamp and the progressive growth of agriculture, industry and residential areas. Given the close connection between the health of the catchment and the health of the bay, dramatic changes such as these are expected to put considerable pressure on the marine and coastal environment.

What we don’t know

Greater understanding of the impact of climate change on Western Port is required. Sea levels are expected to rise, which will affect Western Port’s gently sloping shoreline. Climate change could also impact catchment discharges, alter bay water chemistry, and change wind and storm patterns. Increased temperature and evaporation can cause desiccation on exposed mudflats, leading to loss of species including those that provide food for birds.

What's the state of our bays?

Despite their proximity to Victoria’s largest cities, Melbourne and Geelong, Port Phillip Bay and Western Port are quite healthy overall.

To measure the environmental health of the bays, the Commissioner for Environmental Sustainability worked with marine scientists who identified 36 indicators across the six key topics you see on this website:

  • Water quality (and nutrients)
  • Mangroves, saltmarsh
  • Seagrass
  • Reefs
  • Fish
  • Birds

The indicators were chosen because they best ‘indicate’ whether that key topic (seagrass, fish etc) is healthy. Read more about the indicators in each of the key topics.