Persistence study: Propagules and juvenile mangroves, St Kilda boardwalk

Green Adelaide has commissioned, and now released, the report Persistence study_propagules and juvenile mangroves at the St Kilda boardwalk by Peri Coleman, Delta Environmental Consulting and Faith Coleman.

Following the hypersalinity event (starting in December 2019) that killed Avicennia marina mangroves in South Australia’s Barker Inlet,  the consultants undertook a monitoring program at the St Kilda mangrove boardwalk between December 2021 to November 2022. The report outlines how juvenile mangroves have fared and makes observations of adult trees in the adjacent areas of stressed and healthy trees.

Quadrat along St Kilda boardwalk, photo by Faith Coleman 1.10.22

Of great concern, the report found that

• Both quantative and qualitative observations suggest that recolonisation of mangroves has been very slow in the study area and is not typical of colonisation rates in other sites in the Barker Inlet and Port River estuary
• Observations in the ‘dead’ zone included the decomposition of the standing dead trees, substantial and ongoing sediment loss from around the dead mangroves, especially along drainage lines, and dislodgement of re-established benthic mat communities.
• In the adjacent ‘live but stressed’ portions of the mangrove forest observations included reproductive failure and subsequent leaf chlorosis, leaf dieback in winter and increased leaf herbivory by opportunistic species.

Nature’s ‘green carpet’ dying at its extremities, photo by Faith Coleman 11.11.22

‘The slow rate of recovery and evidence of ongoing stress in live trees within the transition zone led the authors to consider that reducing hypersalinity inputs should be the initial focus of recovery efforts’.

This is consistent with the recent report prepared for the Department for Environment and Water, University of Adelaide, Adelaide by Leyden E, Thomas B, and Mosley LM (2022) St Kilda mangrove and saltmarsh hypersaline brine contamination 2020.

This advice also mirrors that advocated by the Save St Kilda Mangrove Alliance, since its formation in early 2020.