Southernmost Mangroves in the World (almost)

In Australia, mangroves reach as far south as Corner Inlet at the very bottom of Victoria and are almost the southernmost mangroves in the world. The mangroves described in this post, were only 40 km further north at Western Port Bay which is an internationally important wetland. 

I wanted to know what mangroves are like at the extreme end of their distribution and how they compare to tropical mangroves? Which mangrove fauna and flora species are missing and which remain at the geographical limits of existence? 

I found the mangrove ecosystem in Western Port Bay to be as simple as a mangrove ecosystem can get. However, the saltmarsh behind the mangroves is sensational. There is a riot of colour and visual texture.

The saltmarsh stretches to the horizon

Jellybeans (Disphyma crassifolium)

Selligera

Triglochin

Samolus repens

The saltmarsh in early summer is a vibrant carpet of green as it is really a seasonal freshwater wetland that is watered by groundwater. In winter the rainfall is slightly more than summer and the evaporation rate is low, so groundwater rises to the surface. At least twenty species of succulents and grasses contest every patch of space. In the saltmarsh, there is even an understorey of miniature species growing below the canopy of small shrubs and herbs. Between the inner saltmarsh and the mangroves is a wide band of tall succulent bushes belonging to the saltbush family.

Metre high thicket of Tecticornia arbuscula

Close up of succulent stems of Tecticornia arbuscula

Tiny flowering daisies grow between triglochin plants

Other tiny daisies are almost all flowerhead

The best place to see mangroves and saltmarsh in Victoria is at Hastings on the Mornington Peninsula. There is a saltmarsh and mangrove boardwalk which is a few kilometres long and there is also a carefully maintained coastal grassland which also has tracks. Having boardwalks is fortunate as the mangroves are shrubby and as thick as a hedge.

Native coastal grassland

Warringine Park Boardwalk

As this part of the world has cool wet winters, there is a season excess of groundwater that flows through the sandy soil and emerges within the saltmarsh. In summer, the groundwater levels fall and the regular 3 m tides can flood the area with seawater. Hence the plants that are present all need to be tolerant of salinity. There were no salt pans, however, which makes this ecosystem markedly different from saltmarshes or samphire flats in Queensland.

With 3 metre tides, the creeks are quite deep yet narrow

In shallow lagoons, black swans would feed on aquatic plants. Wrens and honeyeaters would flitter around in the succulents. Birdlife reaches its peak on the mudflats and seagrass bed beyond the mangroves and thousands of ducks feed in the shallows.

Black swans feeding on aquatic vegetation (Lepilaena sp.)

White ibis

White-eared honeyeater

Crabs species are very few. There appears to be only 4 species of crab, of which only one species is abundant. This main species is the semiphore crab and it occupies all of the niches from sand flats beyond the seaward fringe to the upper intertidal. It only needs regular seawater inundation to thrive. It is a surface feeder like fiddler crabs. Fiddler crabs are conspicuous by the absence and the Queensland mud crab is also. The other mangrove crabs are opportunistic feeders and would feed on algae, dead insects and possibly mangrove propagules. In the adjacent forest, I found fox poo full of crab shells, so the crabs need to be fast at escaping predators including both native ones like ibis and introduced like foxes.

Adult semephore crab  (Heloecius cordiformis)

Red-fingered marsh crab (Parasesarma erythrodactylum)

Fox poo with crab shells in it

Common fish within the mangroves seem to be limited to yellow-eye mullet and smooth toad fish. Tiny gobies are present in the creeks but as herons are abundant, they zoom of rapidly when they see you. Molluscs were also limited to two species of pea-sized snail that feed on the mud surface.

Smooth toadfish

Salinator snails

Ophicardelus ornatus snails

In the seaward mangroves, the lower trunks have barnacles and support a species of gastropod, which feeds on the algal turf growing at the base of the trunk. The mangroves look healthy although some places had sooty mold infestations on the leaves and scale insects. That is a possible sign of environmental stress.

At the seaward fringe, the Avicennia marina mangroves reach about 3-5 m

Barnacles on a seaward mangrove

Sooty mold on mangrove leaves, white dots are scale insects

Beyond the mangroves are yabby flats and sea grass beds. Wading birds were not present when I was there in November. Wading birds actually take time to move down the Queensland coast so appear in Victoria much later than they do in Queensland. Instead, ducks, herons, cormorants and sea gulls and black swans were present in numbers.

Seagrass beds are exposed at low tide

Ducks grazing in seagrass meadows

The sand flats are soft and you quickly find yourself knee deep in sediment. Dead bivalve shells are abundant within the ground and over the surface. The seagrass beds trap a layer of warm water at low tide and I wonder if this helps the biota escape from the cold for time. Staying upright whilst moving through the soft ground of the seagrass beds was difficult and in the few metres I could survey, I did not find any fauna.

The marine yabbies are so abundant, they made the ground soft

Stingrays bring old shells to the surface when the make hollows

Underwater photos of the seagrass bed

Deeper in Western Port Bay, there must be substantial populations of fish as near the boat ramps, were dozens of large fish skeletons. Giant seagulls pick away at them.

Juvenile Pacific Gull - these things are seriously over-engineered