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

Trophic dynamics
Trophic dynamics are determined by trophic structure, which in turn is determined by the number <strong>of</strong>
trophic levels in existence. Trophic levels are amalgamations <strong>of</strong> species that have similar feeding
habits (for example, plants, herbivores, and carnivores). Therefore, trophic dynamics refers to the
feeding relationships <strong>of</strong> organisms in communities and ecosystems.
The typical estuarine food web is a complex interaction <strong>of</strong> food chains involving the following trophic
groups:
• plants (phytoplankton, macroalgae, seagrasses, fringing vegetation);
• herbivores (zooplankton, some fi sh, ducks and swans);
• planktivores (small fi sh such as pilchard, sprat, hardyheads, anchovies);
• detritivores (benthic invertebrates, Perth herring, sea mullet);
• omnivores (benthic invertebrates, black bream, cobbler, yellow eye mullet, waders);
and
• carnivores (mulloway, tailor, cormorants).
These trophic groups are represented in the lower estuary where major <strong>climate</strong> <strong>change</strong> <strong>impacts</strong> are
not anticipated. In the middle and upper estuary the trophic groups are also represented at present,
but by a reduced number <strong>of</strong> species. With the predicted decline in water quality and increased
level <strong>of</strong> low DO events in the middle and upper estuary there is the <strong>potential</strong> for trophic dynamics to
become even simpler (involving fewer species) with periods dominated by planktonic food webs due
to loss <strong>of</strong> benthic invertebrates.
This loss in biodiversity may also cause a ’feedback loop’ that exacerbates the effects <strong>of</strong> nutrient enrichment
as follows: fewer large predatory and omnivorous fi sh ==> increased abundance <strong>of</strong> small
fi sh that eat zooplankton ==> decreased abundance <strong>of</strong> herbivorous zooplankton ==> increased
abundance <strong>of</strong> phytoplankton. Depending on its spatial and temporal extent, this loss in biological
diversity and trophic structure will result in an overall loss in the ‘resilience’ (i.e. ability to cope with
<strong>change</strong> and recover afterwards) <strong>of</strong> the middle and upper Swan estuary.
Trophic dynamics in detail – birds and fish
A detailed review <strong>of</strong> the <strong>impacts</strong> <strong>of</strong> <strong>climate</strong> <strong>change</strong> may have on trophic elements <strong>of</strong> the Swan Canning
river system is beyond the scope <strong>of</strong> this background report. Consequently, birds and fi sh have
been selected as the key priority species due to their economic and social value.
Impacts on birds
The Swan and Canning rivers are <strong>of</strong> recognised importance for birds, both for birds that use the
aquatic environments (waterbirds) and birds that use terrestrial environments (landbirds).
Higher water levels, leading to stronger tidal infl uences upstream, reduced freshwater infl ow (reduced
rainfall) and increased temperatures are the key <strong>climate</strong> <strong>change</strong> <strong>impacts</strong> that will affect birds
in the Swan and Canning rivers. Further, the effects <strong>of</strong> increased water levels and tidal infl uences
will be enhanced by the reduced freshwater infl ow. These climatic variations will result in <strong>change</strong>s
in salinity levels; sedimentation; nutrient levels; vegetation; trophic structure <strong>of</strong> marine species; and
pH levels. Further, <strong>change</strong>s in development patterns surrounding the river, as a consequence <strong>of</strong>
increased residential population and <strong>climate</strong> <strong>change</strong>, will impact Swan and Canning river birds.
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