Mangroves might bring to mind smells of stinky mud, images of tangled roots and the sounds of mosquitos buzzing by your face. However, there is so much to love about our mangroves. These seemingly hostile habitats are nationally recognised as major contributors to both primary productivity and carbon storage in coastal marine ecosystems. The mangroves and saltmarshes of St Kilda are currently stressed and under threat from human impact, and they need our help.
Biology and extent of mangroves in SA
Australia has the third largest mangrove area in the world, with 11 000 km of mangrove coastline, while South Australian mangroves are among the most southern occurring in the world. In South Australia these are comprised of one species; the Grey mangrove Avicennia marina and occur along the sheltered shorelines of the Spencer Gulf and Gulf St. Vincent, as well as some bays on the Eyre Peninsula. These incredible trees have developed many adaptations to life in salty water and oxygen-low mud, including breathing roots that act as little snorkels growing upward out of the mud called pneumatophores, and special glands on the underside of leaves which excrete excess salts. Try giving a leaf a lick next time you are out in the mangroves; you can taste the salty crystals. Mangroves often grow in areas in conjunction with other coastal vegetation such as samphire and saltmarsh on the landward side, and seagrass meadows on the seaward side. Along with mudflats, these form a mosaic of interconnected tidal wetlands and important marine habitats for multitudes of animal species.
Importance of both of these ecosystems
The complexity of these habitats forms essential breeding grounds and juvenile nurseries for many estuarine and marine species including fisheries species like King George Whiting, Blue Swimmer Crabs, Western King Prawns and Yellow-Eye Mullet. The young of these critters love to hide away amongst the roots and saltmarshes during the incoming tide, using both the shade and structures to evade predators. Migratory shorebirds fly all the way from Alaska and Siberia to feed in these coastal wetlands, which was named the Adelaide International Bird Sanctuary/Winaityinaityi Pangkara (a place for all birds) in 2016. Both mangroves and saltmarshes play an important role in the mitigation of climate change through carbon storage. This is known as blue carbon, or carbon stored in marine systems; in these two habitats it is stored in the trees and samphires themselves, and through the burying of algal mats in the mud below. Blue carbon ecosystems, including mangroves, saltmarsh and seagrasses, bury carbon at rates 10 times greater than terrestrial forests. Mangroves also aid in the protection of our shorelines by stabilising sediments, preventing tidal and current-induced erosion and maintaining water quality by filtering and trapping sediment and excess nutrients which are recycled within the mangrove system.
The problem in St Kilda
All this information that I have learnt through lectures, talks and field work has nurtured my love for these remarkable ecosystems that do so much for our planet and for us. There has also been a lot of recent interest in the study and recognition of blue carbon, and the stabilisation of shorelines by living structures such as oyster reefs, seagrass meadows and mangrove forests. At a time when we hold on to this progress as glimmers of hope in a difficult year, the ecological disaster that began in mid-2020 comes as even more of a blow.
In general, mangroves are threatened by sea level rise, if no land is available to colonise on the landward side, as well as agricultural runoff and interruption of both river flow and ocean currents by infrastructure that reduces sediment input, and therefore the area available to be colonised by new saplings.
Currently, the mangroves and nearby saltmarshes of St Kilda are facing a different threat. Over the past 18 months, Buckland Dry Creek mining company have been moving water around the salt fields in the maintenance of a holding pattern. This involved the refilling of salt ponds with hypersaline brine, water much saltier than seawater. These pans had laid empty for the last 7 years, and due to drying out the gypsum base had cracked, and acid from beneath had been leached. The hypersaline and acidic brine began leaking into surrounding wetlands including saltmarsh and mangrove tidal areas early in 2020 and by May to July 2020, both mangroves along the boardwalk and gardens of houses in St Kilda began showing signs of stress and death. Through aerial imagery and mapping, the effects of the brine on the mangrove trees has even been likened to the effects of a bushfire, with many affected trees showing little sign of life. Through this devastation, approximately 35 hectares of saltmarsh and 10 hectares of mangrove are already dead or stressed. Peri Coleman, an environmentalist and environmental consultant, has led the charge in ensuring the relevant authorities, such as Department for Energy and Mining, Department for Environment and Water and the City of Salisbury, are aware of the issue and that action is taken swiftly to allow the mangroves and saltmarsh to recover. Pumping of the hypersaline brine out of the adjacent salt ponds began in January 2021. As vegetation dies back and waters become more saline, the reduction in bird and fish species present has meant that the mosquito population has exploded, posing a potential health issue in addition to this environmental tragedy.
Unhealthy vs. healthy mangrove habitat
Citizen science survey
In late January, I joined a group of around 40 people in a survey of the St Kilda Wetlands; assessing the health and coverage of the saltmarsh and mangrove systems near the hypersaline brine leakage, as well as some healthier places. I had not walked the boardwalk mangrove trail for a few years, and even though I had been aware of the growing issue, it was still heartbreaking to witness the devastation that awaited us. The leaves on almost every tree in the affected area were either brown and crisp or had already fallen. Similarly, the samphires that mostly make up the saltmarshes nearby looked as if they had been masked with a black and white filter. The usually vivid, motley green and red coloured fingers of the samphires protruding from the mud had lost all vibrancy.
Unhealthy vs. healthy saltmarsh habitats
In separate groups, university students, scientists, budding citizen scientists and concerned locals assessed the density and health of mangrove saplings, the health and size of mangrove trees, and the health and coverage of various plants in the saltmarshes. In the mangroves, transects took place along the boardwalk so as to not tread on any saplings or roots and further the damage. Laying down and peering over the edge of the walkway, we could see the extent of the destruction throughout the forest. Many of the saplings surveyed beneath dead mature trees were also dead or dying, as well as the lifting of algal mats caused by acidic and hypersaline brine that lay at the bottom of the water column. Surveyed areas further along the boardwalk revealed the very sudden lineation between dead and living mangrove trees; an identifiable point of no return. This is where deeper creeks are able to be flushed by the incoming tides, protecting the trees to the seaward side thus far. The good health of the mangroves beyond this point, as well as in the saltmarshes to the North of the playground, provided some hope after an upsetting start to the day. Many of the healthy mangrove trees were flowering, with delicate orange blooms attracting insects. Part of this survey was also capturing the plant and animal life as a record of biodiversity in the area using the iNaturalist application. Images of living things can be uploaded, either identified or unidentified, and added to the global biodiversity database. This not only increases our personal knowledge of the natural world and introduces us to citizen science, but also helps scientists and resource managers understand when and where organisms occur. We spotted some unidentified snails amongst the muddy snorkel roots, a gorgeous tree bug and many, many mosquitoes.
Mangrove flowers, saltmarsh survey and close-up of samphire species Sarcocornia
Hope for the future
It will take some time, but if change occurs now, we can start the slow process of rehabilitation; this species of mangrove tree is slow-growing, a tree up to around waist high takes 5 years to grow and so to replace what has been lost will take many years. But I take solace in the fact that nature will take charge and repair itself if given the right conditions. We have seen amazing concern and support for the wellbeing of these coastal wetlands, that are sure to assist in their recovery. First, in the creation of the St Kilda Mangroves Alliance. This coalition comprises of groups and individuals with backgrounds in science, industry and the community, together ensuring a remediation plan is swiftly put in place for the health and recovery of these globally important habitats. Secondly, weeks later, a vigil was held for the mangroves on the steps of Parliament House, with around 150 people in attendance, many wearing black armbands to express the grief in our loss.
You can also play a part in assessing the biodiversity of this area, or anywhere in South Australia, and become a citizen scientist using the iNaturalist app. The City Nature Challenge is happening from April 30th until May 3rd 2021, a global challenge to make the most natural observations possible.
So please, for the love of our mangroves, go and visit them and witness the beauty and magic of where the forest meets the sea. You can find out more about the issue and follow the progress of their revival at www.savestkildamangroves.com
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