Wetlands,
Biodiversity and Salt
Responses to salinisation - an experimental approach
July 2007
Introduction
How we tested the model
A large number of wetlands are affected by, or
at risk of, secondary salinisation. This poses
challenges for wetland management, with an
urgent need to better understand the dynamics
of wetland ecosystems and the interaction of
biodiversity with secondary salinisation.
We designed a two-phase experiment to test the
predictions of the model, as we were unable to
manipulate whole wetlands (Factsheet
“Wetlands, Biodiversity and Salt - Lessons learnt
trying to initiate a management study in nonriverine wetlands” June 2007). Sediment from
two wetlands (one fresh, one saline) was
inundated at a range of salinities to establish the
impact of increase in salinity on biota (phase 1).
Salinities were then reduced to simulate a series
of salt-reducing management scenarios
(phase 2).
Our team developed a theoretical model of the
response of wetland biodiversity to secondary
salinisation (Factsheet “Wetlands, Biodiversity
and Salt - Modelling” August 2006).
Among the many predictions of this complex
model is the potential for biodiversity to exhibit a
hysteretic response to increasing and decreasing
levels of salinity. The model predicts that in
some wetlands the recovery threshold will be
significantly lower than the collapse threshold.
Alternatively, wetland biodiversity may respond
to increases in salinity in a gradual or nonhysteretic manner, with continual losses in
biodiversity as salinity increases, and may exhibit
a similar, but opposite response with decreasing
salinity.
An understanding of the nature of this response
will have substantial impacts on management
decisions regarding the protection of wetlands
from secondary salinisation and the restoration
of currently salinised wetlands.
Phase 1
was designed to emulate secondary salinisation,
and examines the nature of the response of
wetland biota to increases in wetland salinity.
This will allow us to:
•
determine if the response of wetland
biota to increasing salinity has a
hysteretic component, and
•
identify the threshold level of salinity at
which wetland biota collapses.
Phase 2
of the experiment simulates restoration efforts
after wetlands have become salinised. The
results will tell us if:
•
the recovery of wetland biota has a
hysteretic component or is simply a
linear response with decreasing salinity,
•
the recovery threshold is lower than the
collapse threshold, indicating another
hysteretic component of the relationship
between salinity and wetland biota and
•
the subsequent recovery of wetland is
influenced by the degree of secondary
salinisation.
www.dse.vic.gov.au/ari/
The Wetlands, Biodiversity and Salt project can be found by
following ‘Research Themes’ to ‘Salinity and Climate change’
A Victorian
Government
initiative
The experiment was conducted in two large
glasshouses. We collected soil sediment
containing seed banks from a freshwater
wetland (Tang Tang Swamp) and a saline
wetland (Lake Cullen). Wetland sediments were
placed in clear plastic tanks and subjected to
inundation at increased (phase 1), then
decreased (phase 2), levels of salinity.
Phase 1 sampling
At the end of phase 1 (14 weeks), five tanks at
each of the nine levels of initial salinity were
harvested for aquatic plants and invertebrates.
Phase 2 sampling
In phase 2 of our experiment, for the four
highest treatment levels (6, 7, 8 and 9), the
salinity of unharvested tanks was reduced to
each of the lower salinity levels. For example,
tanks that had been maintained at treatment
level 7 for 14 weeks were subsequently
subjected to one of the lower treatment levels:
6, 5, 4, 3, 2 or 1 for a further 13 weeks.
Therefore, each tank in phase 2 of the
experiment was subjected to two salinity
treatments (an initial treatment, treatment A,
and a final treatment, treatment B).
At the completion of phase 2, plants and
invertebrates were sampled as for phase 1.
The experiment, consisting of 370 experimental
tanks, ran for six months.
What we found
Salinity significantly influenced all of the
response variables for both wetlands: the
abundance of individuals, the number of taxa,
the biomass of plants present and the overall
species composition.
When salinities were reduced (phase 2), aquatic
communities demonstrated some recovery. This
response differed between the two wetlands;
Tang Tang Swamp showed recovery at lower
levels of salinity than that at which decline had
been observed. Our results also indicated that
the degree of initial salinisation influenced the
recovery of biota. Complex statistical analyses
are currently underway to relate our
experimental results to our predictive model.
Published by the Victorian Government Department of Sustainability and Environment Melbourne, July 2007
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A Victorian
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