although there were notable instances of training walls and access
roads to the river and damage was observed behind training walls
at Bulwer Island and other impounded areas. The extent of this kind
of damage increased to moderate levels by 2002. In recent years, I
have observed increasing examples of habitat damage, although the
amount varies from one shore-edge property to the next.
Spill damage was also minimal prior to the twentieth century,
when fossil fuel usage increased. Until 1946, there were no records
of spill incidents. The amount of mangrove habitat lost from spill
damage remained minor until 2002, but in April 2003 the most
severe incident recorded in Brisbane occurred when a burst pipe at
a Lytton industrial business spilled oil into a mangrove-lined channel,
causing extensive dieback.
Accumulation and erosion of sediments are natural factors
that can lead to mangrove loss. After 1860, massive clearing of
catchment vegetation, coupled with soil tillage for crop farming,
harmed mangrove growth and would have increased erosion and
run-off of sediment, although it is likely they were obscured by
nearby reclamation losses and channel dredging. By 2002, areas
around Fisherman Islands showed new mangroves, fitting the
expected pattern.
Shallow subsidence and dieback
Large patches of mangroves throughout Moreton Bay are
threatened by a combination of shallow subsidence preceded
by mangrove dieback, like that at the mouth of the Caboolture
River. The cause of subsidence is unknown, although there was
a correlation with rapid root decomposition. For the affected
sites, there has been no recovery to date. During preliminary
investigations, my colleagues and I discovered that mangrove
dieback may be of lesser concern. Dieback can be explained
mostly by long-term decreases in rainfall. Shallow subsidence and
ponding, however, occurred after significant instances of mangrove
dieback. Since shallow ponding is uncommon in pristine areas, the
subsidence of tidal wetland areas is most likely caused by associated
human activities, like excess nutrients and/or pesticides used within
or upstream of tidal mangrove wetlands.
Notable dieback and sinking centres remained more or less
absent from 1940 to 1980, after which they developed rapidly. The
2004–2007 extent of mangrove loss was 3379 ha – 19% of all tidal
wetland areas. Mangrove dieback is associated with sinking centre
ponds (around 2627 ha); and mangrove dieback without obvious
sinking (around 752 ha). Amounts of dieback and centre sinking areas
for each of the sub-regions of sunken ponds varied from 10% to 26%
of the area, compared to areas of dieback of 0.5% to 16%.
Severe storms, cyclonic winds, gusts, hail storms, strong wave
activity and high flows notably damage mangrove plants. Perhaps
the most catastrophic weather events for mangroves are hailstorms.
In 1997, around 200 ha of mangroves were destroyed during a
single storm on Cobby Cobby Island. To this day, the site remains in
a state of disturbance and its prognosis is not good. These impacts
may be exacerbated by increasing storm severity, large waves and
stronger currents associated with anthropogenic climate change.
Changes in rainfall also shift vegetation along the distinctive
‘ecotones’ – areas in which one floral or biological community
transitions into the next. One trend I observed was indicative of
a probable increase from dry conditions in the 1940s to wetter
conditions in the 1970s.
Mangroves are also sensitive to sea level changes. Shifts in
ocean sea levels around northern Moreton Bay have not yet been
recorded; however, there is evidence for localised sea level rises
throughout southern parts of the region. The opening of the
Jumpinpin and Southport seaways over the last century appears to
have allowed closer contact with open-ocean tidal conditions. The
result has seen seawater flooding into wide bands of supra-tidal
casuarina and melaleuca forests. Dead trees and stumps are now
present throughout high-water margins of the southern bay area,
like around Cobby Cobby Island. Mangrove and saltmarsh plants
have shifted upland as a result, with younger plants observed dead
among upland trees. These changes have contributed to large
increases in the area of mangroves in Moreton Bay.
READING Duke NC, Schmitt K. 2016. Mangroves: Unusual Forests at the Sea’s
Edge. Tropical Forest Handbook. L Pancel & M Köhl (eds). Berlin Heidelberg, Springer-
Verlag: 2:1693–1724. Duke NC. 2002. Sustained high levels of foliar herbivory of
the mangrove Rhizophora stylosa by a moth larva Doratifera stenosa (Limacodidae) in
north-eastern Australia. Wetlands Ecology and Management 10: 403–419. Duke NC.- Australia’s Mangroves. The authoritative guide to Australia’s mangrove plants.
University of Queensland and Norman C Duke, Brisbane. Duke NC. 2014. Mangrove
Coast. Encyclopedia of Marine Geosciences, (Eds) J Harff, M Meschede, S Petersen & J
Thiede. Springer, Netherlands. pp 412–422.
PROFESSOR NORM DUKE is a mangrove ecologist from James
Cook University TropWATER Centre. He is the CEO and director of
MangroveWatch (www.mangrovewatch.org.au) and facilitates the
Australian Mangrove and Saltmarsh Network along with being the vice
president of the International Society for Mangrove Ecosystems and a
member of the Mangrove Specialist Group of the IUCN Species Survival
Commission. His colleague, JOCK MACKENZIE, is a researcher at the
TropWATER Centre and the coordinations director of MangroveWatch.ECOLOGICA
Mudskippers, crustaceans, worms and
molluscs inhabit the mud of mangrove
stands. Photo: Eric GofreedPetroleum hydrocarbons
in mangrove sediments
contribute to leaf pigment
variegation and lethal
genetic ‘albino’ mutations
in mangrove propagules.
Albino propagules of
the grey mangrove are
distinctively yellow or
red, whereas normal
propagules are green.
Photo: Norm DukeOil spill containment in a
mangrove-lined channel
at Lytton on the Brisbane
River. Photo: Norm DukeECOLOGICA
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