potential-impacts-of-climate-change-on-the-swan-and-canning-rivers

Table 17 Ecological <strong>impacts</strong> on fringing vegetation due to predicated increases in sea level
and decreases in river run<strong>of</strong>f
Impact
Increase in sea level rise
Decrease in river run<strong>of</strong>f
Increase in sea level rise
Decrease in river run<strong>of</strong>f
Invasion by transition vegetation
Invasion <strong>of</strong> dryland species at the
landward edge
Reduced groundwater flows may
also contribute to this pattern.
Reduced run<strong>of</strong>f in the freshwater
reaches <strong>of</strong> the river.
Decrease in river run<strong>of</strong>f
Community structure
Response
Zones <strong>of</strong> saltmarsh, Casuarina/Melaleuca forest and Melaleuca/Juncus
(tree/sedge) will retreat from the shore and
extend their distribution landwards, and also extend their
distribution further upstream.
Transition vegetation will retreat from the shore and extend
its distribution landwards. It will also retreat from its downstream
extent but extend its distribution further upstream.
Some areas <strong>of</strong> Eucalyptus/Melaleuca forest will be invaded
by more salt tolerant species such as Casuarina, and may
become Casuarina/Melaleuca forest (this has already happened
in some parts <strong>of</strong> the Swan Canning).
The downstream extent <strong>of</strong> freshwater riparian vegetation
will be reduced.
Freshwater riparian vegetation will extend further into the
(retreating) river bed, and the width <strong>of</strong> riparian vegetation
may be reduced overall
During transition in the fringing vegetation there is the <strong>potential</strong>
for large areas <strong>of</strong> tree dieback or loss <strong>of</strong> tree health
in areas <strong>of</strong> fringing vegetation.
The effective width <strong>of</strong> any vegetated zone along the freshwater
reaches <strong>of</strong> the river will be retained, but will consist
<strong>of</strong> more dryland species on the outer edge.
Climate <strong>change</strong> is expected to result in a slightly larger and deeper lower estuary that has essentially
marine conditions for most if not all <strong>of</strong> the year, with water quality similar to or possibly better than
at present. In contrast, <strong>climate</strong> <strong>change</strong> is expected to result in reduced water quality, increased low
DO events, more din<strong>of</strong>l agellate blooms and possibly more fi sh kills in the middle and upper reaches.
The <strong>change</strong>s in community structure that are expected to occur are as follows.
In the lower estuary, there will be limited <strong>change</strong> in productivity and a possible slight increase in species
diversity due to the recruitment and retention <strong>of</strong> more marine species <strong>of</strong> plankton, macroalgae,
invertebrates and fi sh (see Trophic Dynamics in Detail – Birds and Fish). These <strong>change</strong>s in productivity
are based on the assumption that the proportions <strong>of</strong> seagrass, mudfl at and fringing vegetation
habitats in the lower estuary do not <strong>change</strong> signifi cantly.
In the middle and upper estuary productivity will be high, but there will be a reduction in the diversity
<strong>of</strong> plankton, invertebrates and fi sh. Increased dominance by small, fecund and fast growing opportunistic
species is likely. This is a classic symptom <strong>of</strong> nutrient enrichment (Pearson and Rosenberg
1978).
An extension <strong>of</strong> seagrasses into shallow water habitats may take place, as could the growth <strong>of</strong> free
fl oating algal species such as Gracilaria and Hincksia, leading to <strong>change</strong>s in total habitat areas (see
Seagrass and Macro-algae below).
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