Mangrove Recovery 10 Years after a Category 5 Cyclone

During the early hours of the morning on 6 February 2011, the most powerful cyclone in Queensland’s recorded history struck the coast.  Cyclone Yasi crossed the coast as a Category 5 storm between Cardwell and Tully Heads.   Where it crossed the coast, the storm surge was said to have been 7 m high.  Fortunately, the cyclone crossed the coast in an unpopulated area of swampy Girrimay National Park.  Several kilometres to the south saltwater inundated the Cardwell esplanade and adjacent town up to the ceilings of low set houses.   

Immediately after the cyclone, it is hard to inspect the impacted area.  Low clouds and rain prevent aviation and aerial imagery.  On the ground, power lines are down and often areas are closed to the public to prevent looting.  It was not until the middle of April that I could go in and record the damage.  My role was to assess the level of damage to trees and coastal reserves.  Some of the records that I collected are presented in previous posts including Environmental Damage from Cyclone Yasi  and Legacy of Cyclone Yasi.

By April, the mangroves were completely dead.  Wherever there were deep green mangrove forests, there were now broad grey fringes of dead forest.  Something similar happened in Darwin after Cyclone Tracey.  However, neither the process of mangrove death or the gradual recovery of the mangrove ecosystems were recorded.  Exactly why the mangroves die is not known to me.  I suspect that the mangroves are shaken so hard by waves that the soil releases its poison gasses into the roots of the mangroves and that the trees are poisoned.  Coastal trees growing on thin fingers of sand within the mangroves survived and had partially regenerated their crowns within only a few months of the cyclone.  Whatever kills the mangroves is mangrove specific and extremely lethal.  

Dead mangroves on Cardwell foreshore, April 2011

The best area to observe the long-term impact of the mangroves is near the boat ramp at Meunga Creek, which is about 2 km north of Cardwell town.  In the ten years since Cyclone Yasi, mangrove forests that were approximately 20 m high have recovered to about 5 m tall.  Log piles formed from toppled adn washed-up mangroves there lined the seaward fringe are now gone.  Gaps in seaward stands where trees once stood remain open as there is very little regeneration within the remnants of these once dense stands.  Sonneratia mangrove seedlings formed a band in front of the tall stilt mangroves but ten years on, they remain shrubby and wind burned.

Seaward mangroves in 2014

Seaward mangroves in 2018

Seaward mangroves in 2021

On the sandy levee which runs through the mangroves, there is now a 5 m tall stand of a type of Orange Mangroves - Brugiera parviflora.  These trees can grow taller than 25 m, so they are still young.  After the cyclone, it was difficult to walk here as fallen mangroves were so densely packed that they almost formed a deck that could be walked on.  The maelstrom of crashing waves and grinding logs would have cleared a patch of ground on which these mangroves could regenerate.  Mangrove propagules would have been washed in and could start growing.  

Orange mangrove stand in 2014

Same stand in 2018

Inside stand in 2021

Looking up inside stand in 2021

In the dense stand behind the levee the mangroves were not toppled but died off in droves for reasons unknown.  There was so much fallen timber that very few mangrove propagules would have been washed in.  Also as very few mangrove trees survived, local production of propagules was almost non-existent. Regeneration in these back swamps has a slow start.

The back swamp was mostly dead in 2014

By 2018 some new mangroves had established

In 2021, the mangroves are starting to close the gaps

It has taken 10 years for mangrove regeneration to fill most of the gaps.  Based on gaps created by Cyclone Winifred in 1986, which were visible from the now closed mangrove boardwalk at Cairns Airport, it will take approximately 30 years before the regenerating mangroves start to blend in with the trees that survived.  It might take twice as long for the forest to recover its original stature.

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

Volcanic Reefs of Mission Beach

At Mission Beach are some unique rocky reefs of volcanic origin. Local volcanoes produced a very fluid lava that spread across the landscape in long streams. One of these streams created Clump Point, a finger of land which juts almost a kilometre into the sea. On aerial photos, other lava flows are easily identified including a few that are partially or fully submerged. Old lava flows are also hidden two metres below the tourist trampled white sand of Mission Beach.

Pelicans resting on a lava flow, a few hundred metres from shore

This post looks at a pillow lava reef which was five kilometres out to sea. This rocky reef probably breaks the surface far more than it is submerged. There is no beach and no terrestrial vegetation, only a hectare sized patch of black rocks. It is very hot and not very interesting unless you are a biologist. Even then, you want to study this place properly the first time so that you do not have to come back. Surrounding the reef is a sandy seabed and the margins of the reef have a hedge of sargassum. Coral is not to be seen.

This lava reef is five kilometres from shore

The edge of the old lava flow has sargassum and is surrounded by a sandy bottom

When you stand and stare long enough, you realise that this reef has a rich diversity of algal turf grazers. Fish, crabs and snapping shrimp battle for territories of turf. Blennies would attack and drive off much larger crabs. Crabs with heavy nippers would become frustrated with trying to clip the turf and would push their faces into the turf to chew it off directly. Even snapping shrimps would graze the turf using their larger nippers like shears with long, thin, lightly built arms darting over the surface picking up bits and pieces.

This snapping shrimp was cropping algal turf with the big nipper and picking at it with a skinny green nipper

A common hairy crab - Pilumnus vespertilio

A swimming crab grazing with its mouth

Another type of hairy crab grazing with its nippers

Large, heavily built crabs consume molluscs, both oysters and many gastropods that would normally assume the grazing role. Oysters were limited to exposed edges where water flow would be better and predator access more limited. Only a few neritas and cowries were on the basalt reef.

A black finger crab is able to crush strong sea shells

On a nearby reef trochus shells were present. The nearby reef was a different geology, possibly a contact zone rock which was melted by the lava flow, then re-solidified. Corals also performed much better on this substrate.

Trochus shell in pocket in rocky reef

The underside of the same shell

A rocky reef that is probably contact zone rock, rather than basalt.

Volcanic features such as dykes and cliffs of solidified ash are present on nearby islands and on the mainland. At the bottom of one of these cliffs was a rock shelf which was only just inundated at high tide. On this shelf was a very shallow pool and a few rocks, fallen from the cliff. It was a particularly nasty place, intensely hot and saline. Yet when I looked, most of the stones were surrounded by a wide black halo. I also had the feeling that something was hiding behind one of the stones. I walked around the stone about five times and didn’t see anything but could hear frantic splashing. The only way to find out was to lift and remove the rock. A few crabs ran off but there were several large gobies stranded in a film of water too shallow for them to swim in. Apparently this species chooses to live in this habitat, a tiny area of the very worst aquatic habitat I could find. I love finding exotic creatures in places were nothing should be able to survive.

Dusky Frill Gobies in very shallow water on a rock platform 

The rock I lifted to see the fish

Wider view of platform of compressed volcanic ash