Fig 1.
Our investigations of the 2015 mass dieback of mangroves in Australia’s Gulf of Carpentaria revealed an additional threat from greenhouse warming [14, 15].
The three images of shoreline mangroves near Limmen Bight River in the Gulf highlight the situation: A) prior to 2015, healthy canopies of largely Avicennia marina trees; B) in late 2018, standing dead trees killed after the 2015 desiccation event; and C) in late 2019, catastrophic scouring caused by Category 3 Tropical Cyclone ‘Owen’ (Image source: photographs by NCD).
Fig 2.
A. Locations of sites across northern Australia both west and east of the main study area of the Gulf of Carpentaria (GOC). See Table 2 for location coordinates. Instances of 2015 dieback of shoreline mangroves occurred from Mangrove Bay near Exmouth to the Gulf. B. Gulf sites were spread across six sub-regional areas. Severe damage (75–100% loss of shoreline mangroves) was recorded mostly at four sites (GOC3, GOC4, GOC6 & GOC7) further assessed in the field in 2018, while less severe dieback (25–75%) was observed in 2017 and 2019 during aerial shoreline surveys from Weipa (QLD) to Blue Mud Bay (NT) (Image source: illustration by NCD using basemap from NESP NA Hub).
Table 1.
Longer term (25–50 year) climate and sea level averaged conditions for sites across northern Australia.
SLSI is the sea level stress index described in the Methods. Data sources include the: Permanent Service for Mean Sea Level (PSMSL)–(https://www.psmsl.org/) and the, Australian Bureau of Meteorology (http://www.bom.gov.au/). Sites in bold are those located in Australia’s Gulf of Carpentaria (for site location details, see Fig 2; Table A in S1 Text). The WCI (% Wetland Cover Index) describes the ratio of mangrove area to total area of tidal wetlands (the combination of mangroves, tidal saltmarsh and saltpan) in local estuarine systems [3].
Table 2.
The sites (see Table A in S1 Text) where a changepoint is likely to occur (probability >0.66) in the fractional canopy cover anomaly (1987–2020) at a time proximal to the Taimasa occurring in 2015, including the credible date interval, as predicted by Bayesian time series decomposition [46].
ND indicates no likely changepoint.
Fig 3.
Timeseries of the green fraction anomaly in 14 sites across northern Australia (see Fig 2) from 1987–2020.
These plots depict changes in mangrove canopy densities and their relationship with each site, particularly in showing the 2015 mass dieback event (vertical shaded bars) as synchronous in all sites from W2 to E1 across most of tropical, northern Australia (Image source: illustration by NCD).
Table 3.
Comparison between extreme lows in port sea level records and nearby mangrove canopy condition between 1987 and 2019.
The table shows key instances of low sea level stress index (SLSI) recorded in sea level stations in Gulf of Carpentaria (GOC) ports of Milner Bay (NT), Weipa and Karumba (Qld) (see Figs 2 & 4). Karumba was a site of notably severe mangrove dieback in 2015 while no dieback was observed there in 1997, and less so at the other locations or dates.
Fig 4.
Timelines of influencing variables shown in climate (A & B), sea level change anomaly (C) and the SLSI (D) compared to the SOI anomaly and changes observed in green fraction (= mangrove canopy condition) for the severely impacted site near Karumba in the Gulf of Carpentaria (E).
Vertical red lines indicate major desiccation events in 1982 and in 2015 (Image source: illustration by NCD).
Fig 5.
Relationships between the SLSI for Karumba in Australia’s Gulf of Carpentaria between July 1987 and December 2018 and measures of mangrove canopy condition (Site 9—GOC6, see Tables 1–3), in A. as green fraction (%), and B. the green fraction anomaly (r2 = 0.227, n = 251, P<0.001; see Methods). For A, the red arrow line depicts the loss in canopy condition between January 2015 to June 2016. Note the point of inflection of canopy decline corresponds to minimum negative values of SLSI ~-400 mm SLSI, marking the major desiccation event in October 2015. For B, numbers in circles show ensemble averaged data for each month of the year (1 = Jan to 12 = Dec) during the period (r2 = -0.500, n = 12, P<0.02); noting March and April (shaded orange, peak wet season) and October and November (shaded yellow, peak dry season). Refer to ‘S1 Text’ for records from all 14 study sites (Image source: illustration by ADC).
Fig 6.
Plot of recovery times deduced from notable declines in canopy condition (impacted) observed in green fraction plots of shoreline mangrove sites across northern Australia (see Fig 3).
Whilst recovery times were mostly independent of the type of impact, they were dependent on the severity of canopy loss. Storms and ‘desiccation dieback’ caused the most severe damage, but storms were considered localised impacts affecting up to 100–500 kilometres of shorelines on each occasion (Image source: illustration by NCD).
Fig 7.
The collapse-recovery cycle affecting shoreline mangroves of Australia’s Gulf of Carpentaria derived from various observations, and based on the 1987–2020 timeseries of canopy condition of site GOC6 Karumba (cs. Fig 4E).
Note the three cyclical phases of canopy condition from 1) collapses in 1982 and 2015 (vertical red lines), 2) recovery, to 3) maximal canopy density (Image source: illustration by NCD).
Fig 8.
The relationships between canopy condition of tropical, semi-arid shoreline mangroves and mean sea level (MSL) identified during this study.
When conditions exceeded the mangrove Goldilocks zone (central green shaded block) of normally moderate annual oscillations in mean sea level, severe destructive impacts occurred as a result of high or low extreme events (drowning or desiccation dieback respectively; pink shaded blocks). Moderate oscillations appear to drive natural seasonal cycles of leafing and leaf fall where low levels (less inundation) correspond with leaf fall (maximal in Sept-Nov), and high levels (more inundation) with new leaf production (maximal in Mar-May) (Image source: illustration by NCD).