Reefs

There are two key types of reefs in the bays:

• Subtidal
  • sheltered, shallow reefs of Port Phillip Bay that are never exposed to the air from tidal influences, generally covering depths of 2.5-20 m.
• Intertidal
  • periodically exposed to air at low tide but are submerged or directly influenced by sea water at high tide. Intertidal reefs are also known as rocky shores.

Shallow subtidal reefs are an important component of the marine environment because of their high biological complexity, species diversity and productivity.  Shallow reef habitats cover extensive areas along the Victorian coast. The majority of subtidal reefs are exposed to strong winds, currents and large swell.

Seaweeds

Seaweeds provide important structures to support the habitat of the reef.

Tall vertical structures in the water column are formed by giant string kelp. Other seaweeds, such as common kelp, crayweed and bull kelp form a canopy of about 0.5-2 m above the rocky shore.

Lower layers are formed by:

• macroalgae that are typically 10-30 cm high
• seabed covering mats formed by green algae
• turfs (to 10 cm high) of red algae species, and
• encrusting layers such as pink coralline algae.

Reef fish include:

• roaming predators such as blue throat wrasse
• herbivores such as herring cale
• planktivores such as sea sweep, and
• picker-feeders such as six-spined leatherjacket.

The numbers of reef fish in the north of Port Phillip Bay are generally low but variable. The exception to this is snapper, which is declining. Limited monitoring occurs in the north.

Pope’s Eye has been protected from all forms of fishing since 1978. The area now shows a great amount of density and diversity. As an example of the improving ecosystem, the numbers of western blue groper, which used to be abundant at Nepean Bay and Point Lonsdale, are increasing.

Grazing and predatory mobile invertebrates are prominent animal inhabitants of the reef. Common grazers include:

• warrener
• sea urchins, and
• black-lip and green-lip abalone.

These species can influence the growth and survival of organisms that help form the reef habitat. For example, sponges and foliose seaweeds are often prevented from growing on encrusting coralline algae surfaces through the grazing actions of abalone and sea urchins.

Predatory invertebrates include:

• dogwhelks 
• southern rock lobster
• Maori octopus, and
• a wide variety of seastar species.

Other large reef invertebrates include:

• feather stars
• sponges
• corals
• hydroids,
• ascidians, sac-like filter feeders, and
• bryozoans, which are important filter feeders.

The last decade demonstrates that the southern reefs are healthy – with the exception of a decreasing numbers of seastars.

What we need to know

Healthy reefs depend on depth, geology, geomorphology, sediment and wave exposure. New technology for monitoring reefs can revolutionise how reefs (and other habitats) are mapped, understood and managed. The new technologies describe the biotopes (the regions of a habitat associated with a particular ecological community) of the reefs and adjacent areas. It improves monitoring into the future but also allows us to reclassify old video footage to provide better trend and historical data.

There is a healthy abundance of urchins in the south of the bay. However, there is an overabundance in the north and urchin numbers are increasing.

Most reefs in the north have low wave energy and have been permanently changed by native urchins and the highly-invasive Japanese kelp, which take advantage of the disturbance caused by the urchins.

Urchin barrens and sessile sponge and coral communities benefit from the carpets of Caulerpa, sediment-covering brown algae, and the wide range of red algae.

Studies suggest that 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. Deteriorating macroalgae allowed the introduced Japanese kelp to get a foothold – this particular type of kelp is very resilient and can create communities on any type of surface. Over-grazing of sea urchins leads to loss of fish habitat that was formed by the dense macroalgae.

Urchins will eat anything – algae, drift weed and even sand is found inside dissected urchins.

The urchins that dominate Port Phillip Bay prefer to eat drift algae – which can float on to, and smother, the reef and sustain urchins. The western area of the bay has experienced urchin barrens since the mid-1980s, but urchins have few predators now. We know little about which animals eat small urchins or urchin eggs.

Intertidal reefs in Victoria are generally restricted to points and headlands, and are often isolated from each other by stretches of sandy intertidal habitats. Victorian intertidal reefs vary in structure from steep sloping rock faces to relatively flat or gently sloping boulder fields and rock platforms.

These areas are subject to changes in environmental conditions such as temperature, salinity and drying out through exposure to air.

Fishes move in over the reef as the tide rises and can be important structuring components of intertidal reef communities.  

Water quality is an issue for intertidal habitats near Point Cooke, close to the Western Treatment Plant and storm water inflows.

Molluscs tend to be the dominant fauna in intertidal reefs.

Herbivorous species include:

• limpet
• top shells, and
• conniwinks.

Small mussels and tubeworms create encrusting mats on the surface of the reef. Other invertebrates on intertidal reefs include:

• small crustaceans such as crabs, and
• sessile animals including anemones.

Mollusc predators include marine snails such as the spotted cominella.

Neptune’s necklace is an intertidal algal that forms large beds. It provides habitats for other flora and fauna including macroinvertebrate grazers, predators, scavengers and microfauna that live on the algae.

Data for macroalgae, sessile and mobile invertebrate indicators from the Intertidal Reef Monitoring Program (IRMP) indicates these reef communities have remained in good condition since 2003 with Hormosira cover increasing steadily since 2009.

Immobile invertebrates include species such as:

• bryozoans
• tubeworms
• mussels, and
• barnacles.

All provide habitats for other invertebrates, like limpets. The sessile invertebrates also provide feeding areas for predatory gastropods.

A pressure on mobile invertebrates is the illegal collection of sea snails by humans, generally as a food source. Education is required to inform the population that it is prohibited to collect from intertidal zones around the bays.

What we know

Intertidal reefs are highly accessible and subject to trampling; human pressures, including collection of animals for food and fishing bait; and pollution from catchment discharges.

To help effectively manage and conserve these habitats, the Victorian Government, through Parks Victoria, has established the Intertidal Reef Monitoring Program (IRMP). 

Data for macroalgae, sessile and mobile invertebrate indicators from the IRMP shows that these reef communities have remained in good condition since 2003.

Rocky reefs occupy only a very small part of Western Port, but three areas are particularly special;

• Crawfish Rock: an unusual habitat with very high biodiversity.
• a small reef near San Remo that is significant for its sea slugs and snails, and
• intertidal reefs along the south-western coast, particularly Honeysuckle Reef, all of which have high biodiversity.

Intertidal reefs in Western Port are likely to be very vulnerable to sea level rise.

There is some evidence that there has been a loss of diversity at Crawfish Rock, most likely a result of high turbidity in the North Arm.

The Victorian Subtidal Reef Monitoring Program (SRMP) has surveyed a small reef near San Remo off Phillip Island. Those surveys revealed a new species of opisthobranchs (sea slugs and snails).

San Remo is a listed reef community under the Flora and Fauna Guarantee Act 1988 and therefore requires considered monitoring.

Understanding intertidal reefs in Western Port requires further study, particularly:

• assessing the risks from sea level rise, and

• improving our knowledge about poor water quality and its effects on algae on reefs in the North Arm.

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.

The indicators were chosen because they best ‘indicate’ whether that key topic is healthy.

Here's how reefs measured up*:

* The main indicators for this section are from Port Phillip Bay because there isn't consistent monitoring for reefs in Western Port.