Seafloor

The seafloor is a fascinating underwater environment, home to unique plant and animal species.

Our report explores seagrass, nitrogen cycling, macroalgae and shellfish reefs across the assessed regions. We also assess the conservation of marine ecosystems in protected areas.

Seagrass meadows

Seagrass meadows are a critical habitat for many marine species, including fish targeted by commercial and recreational fishers. They provide shoreline protection and store significant amounts of carbon. Changes in their condition can have environmental, social and economic effects.

Considerable losses in seagrass have been observed in:

• Port Phillip Bay (in conjunction with the Millennium Drought from 1997 to 2009)
• Western Port (in the mid-1970s and early 1980s)
• Corner Inlet (a slow decline from 1965 to 2013).

Macroalgae on intertidal and subtidal reefs

Macroalgae are an important source of food and habitat for many marine species. Macroalgae are seaweeds and other benthic (attached to the seafloor) marine algae generally visible to the naked eye.

Subtidal reefs are sheltered, shallow reefs that are never exposed to the air from tidal influences. They generally cover depths of 2.5 to 20 metres. Intertidal reefs are 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.

Parks Victoria monitor and report on macroalgae on intertidal and subtidal reefs. They periodically publish technical reports focussing on marine protected areas. We have assessed the condition and extent of macroalgae on subtidal reefs in Port Phillip Bay as:

• poor for Point Cooke and Jawbone Marine Sanctuaries
• fair for Ricketts Point Marine Sanctuary
• good for Port Phillip Heads Marine National Park.

In 2020, Parks Victoria studied golden kelp in Point Addis Marine National Park. Once a dominant species, it has shown an alarming decline since 2012. Some other canopy-forming brown algae have increased since then. In a 2018 survey, canopy-forming algae fell below the lower control limit. Macroalgal beds have been under threat in Cape Howe Marine National Park and Beware Reef Marine National Parks. Parks Victoria advises there has been a dramatic increase in urchin barrens. By overgrazing the seaweed on a reef, urchins leave nothing but bare rock, which supports far less marine life diversity.

Substantial losses of giant kelp have been seen this century in marine areas off the coast of south-eastern Australia. These losses are not restricted to Victoria’s marine protected areas. There are concerns for the future of this species as a major habitat-forming kelp in Australia.

Shellfish reefs

Shellfish reef ecosystems support unique assemblages of creatures and valuable ecosystem services. These include fish production, coastal protection, erosion mitigation and nutrient cycling. Historically, large areas of native flat oyster reefs existed in Port Phillip Bay, Western Port and Corner Inlet. Large areas of blue mussel reefs were also present in Port Phillip Bay. The extent of these reefs is now minimal. We have rated the status of the shellfish reefs indicator as poor for these regions. Large mussel reefs can still be found in the entrance region of the Gippsland Lakes. The status is fair in this region despite the extent of shellfish reefs declining during the 20th century.

Conservation of marine ecosystems in protected areas

Victoria’s marine protected areas cover 106,106 hectares or 10.4% of state waters. Based on 10.4% of all Victoria’s marine coastal waters being covered by marine protected areas, Victoria does satisfy an international target for at least 10% marine protected area coverage. However, only 5.2% of Victoria’s state waters are no-take zones where removing animals and plants is banned. Victoria has the second smallest proportion of no-take areas of any Australian state or territory. Parks Victoria reports that the condition of natural values is good or very good in 93% of marine parks.