Mangroves

Left: Mangroves at low tide. Photo: I. Morgans, PP&WCMA.
Right: Mangroves at high tide. Photo: I. Morgans PP&WCMA.

Western Port has one species of mangrove, the white mangrove, Avicennia marina.

Mangroves are unique in being the only trees capable of living in the tidal zone along coastal or estuarine shorelines. They are advantaged by having no competitors for the niche they occupy but seem unable to compete outside this unique range. These flowering trees have many special features that enable them to survive and take advantage of these extremely harsh conditions.

Mangrove Habitat & Distribution

The mangroves of Western Port are at their most southern limit of distribution. In the cold winters of this southern climate the seedlings are subject to frost damage and death, whilst adult trees have reduced annual growth and fruiting frequency. Ideally mangroves prefer sheltered gently sloping muddy coastlines where seeds can germinate and establish without being undercut by strong wave action. For this reason mangroves are not found along Western Port’s oceanic beaches, but are prolific along the sheltered northern parts of Phillip Island, around French Island and most of the northern segment of the bay.

Left: Mangroves in Rhyll Inlet – ideal sheltered habitat. Photo: G. Rooney Melbourne Water.
Right: Saltmarsh behind protective mangrove fringe Tooradin. Photo: T. Ealey WPSP.

Whilst preferring fully sheltered environs, with defined tidal zone preferences, established mangroves are capable of withstanding significant storm and tidal events. Mangroves are very beneficial in providing natural coastal erosion barriers and are renowned as coastal colonisers due to their ability to trap sediments and extend shorelines. The regions of Western Port with established mangroves exhibit very stable coastlines, typically with mature saltmarsh communities behind them. Areas devoid of mangroves, particularly those subject to historic clearing, are highly unstable and eroding with consequent loss or deterioration on adjacent saltmarsh habitat. Once destroyed, natural mangrove recovery can be very slow especially if conditions are hostile to seed establishment.

Saltmarsh being eroded away along northern coastline of Western Port due to absence of mangroves. Photo: T. Ealey WPSP.

The mangrove stands of Western Port also represent a major store and supply of biomass and nutrients. The leaves that drop into the bay are a valuable source of food and slow release nutrients. Like seagrasses, mangroves are the energy power houses for the Bay’s ecosystem – supplying the raw material at the base of the food chain, which is then utilized by the bacteria, algae, mollusks, crustaceans, fish, birds and mammals.

Mangrove Root System

Left: Young mangroves showing cable roots radiating from base. Photo: T. Ealey WPSP.
Right: Pneumataphores at low tide. Photo: T. Ealey, WPSP

The extensive cable root system of mangroves anchors them into the thick muddy substrates that they prefer and enables them to withstand severe winds and tidal impacts.

The cable roots have holding roots that penetrate vertically downwards into the mud and finer root networks expanding outwards. Cable roots also have root extensions called pneumataphores, which protrude vertically upward out of the mud and into the water and air. These structures enable the roots to obtain oxygen which is not available in the anaerobic mud in which they grow. Mangroves often die when their pneumataphores are destroyed or covered in oil.

Mangrove Salt Control

Right: Mangrove leaves with crystals of excreted salt. Photo: T. Ealey WPSP.

To survive in salt water, mangroves have developed three methods of regulating their internal salt levels. The first method is exclusion, whereby special membranes in the roots effectively block salt uptake and only allow water to enter into the plant. The second method is to secrete salt via special glands in the leaves. Finally salt can be concentrated in bark and leaves which the plant later drops. Avicennia leaves turn yellow and drop off when loaded with salt.

Mangrove Leaves

Avicennia leaves. Photo: T. Ealey WPSP.

Mangrove leaves are strong and leathery, ideal to withstand winds, sea-spray and saltwater inundation. Water loss through evaporation and wind is minimized by their thick waxy cuticle and sunken stomata (pores). These slightly succulent leaves also store water.

Mangrove Reproduction

Left: Avicennia flowers Photo: T. Ealey WPSP.
Right: Mature mangrove fruits. Photo T. Ealey WPSP.

Avicennia marina has small, pale orange flowers that are pollinated by bees and other insects. The fruits are almond sized, green and slightly furry. The fruit ripens in summer and takes two months to mature. Inside the fruit, a single seed develops to an advanced state. When ripe the fruit drops into the sea or onto the mud. The seeds float until their coat is shed, the timing of which is linked to bay salinity levels.

Left: Mangrove seeds showing rapid germination and root development after losing their protective coats. Photo: T. Ealey, WPSP.
Right: Young seedlings growing at edge of the saltmarsh.  Photo: T. Ealey, WPSP.

Seeds can stay alive in water for three to four days. Only a small number of seeds succeed in anchoring themselves in the mud, with others stranded at high tide mark or eaten by crabs. Those that establish under existing mangroves will eventually be shaded out and die.

Link – More information about mangroves