The effect of water and sediment quality on platypus numbers has been studied in creeks near Melbourne (Serena and Pettigrove 2005). Three factors have been found to be adversely related to platypus population status: (1) nutrient enrichment (especially elevated levels of total phosphorus in water), (2) the amount of suspended solids in surface water after storms, and (3) the concentration of metal contaminants (lead, zinc and cadmium) in bottom sediment.
The maximum average total phosphorus concentration associated with healthy platypus populations (defined by one or more animals consistently being captured at every survey site) has been found to be approximately 0.06 mg/litre. A negative impact of phosphorus on platypus numbers makes sense – excessive nutrients are known to promote algal blooms and trigger low levels of dissolved oxygen, causing many aquatic invertebrates to decline or disappear. Increased algal growth may also hamper platypus foraging by physically impeding prey detection and capture.
Given that a platypus normally shuts its eyes underwater and often feeds at night, the adverse impact of suspended solids is unlikely to be due to poor water clarity. Instead, it probably reflects the fact that suspended solids eventually settle on the channel bed, and unconsolidated fine sediment provides a poor habitat for most aquatic macroinvertebrates. In addition, high levels of suspended solids can trigger downstream drift by aquatic insects, sometimes reducing bottom-dwelling populations by more than 50% in just 24 hours (Culp et al. 1986).
Exposure to high levels of lead, zinc and cadmium in sediment can be lethal to freshwater fish and invertebrates, and has been linked to reduced macroinvertebrate diversity and abundance in Australian streams. Although elevated metal concentrations in urban creeks most plausibly contribute to low platypus numbers due to reduced prey abundance, it’s possible (though by no means proven) that direct toxic effects could occur – for example, through bioaccumulation. Because heavy metals bind to bottom sediment, they can remain in the environment long after the primary contamination source has been eliminated.
What can be done to help the platypus?
- Monitor and, if necessary, take action to improve water quality in platypus habitats located downstream of urban or industrial wastewater discharge points.
- Manage livestock access to platypus habitats so minimal animal waste enters the water either directly or indirectly (through rain run-off).
- Apply chemical fertiliser sparingly and at the optimum time of year to crops, golf courses or lawns, so it’s absorbed efficiently by growing plants.
- Avoid washing motor vehicles on impermeable surfaces (such as driveways or carparks), especially if soapy water runs to a concrete gutter or stormwater drain.
- Use low phosphate or phosphate-free detergents to wash dishes and clothes.
- If your home has a septic system, don’t overload it (for example, by doing too many loads of washing in one day) and arrange for the tank to be cleaned out at appropriate intervals.
- Take action to reduce ongoing or potential soil erosion on both public and private land, e.g. by adopting the measures recommended in Managing stormwater drainage and Protecting and improving bank habitats.
- Direct downspouts from galvanized metal roofing to grassed lawns (or a rainwater tank) as opposed to delivering stormwater directly to concrete gutters or drains.
- Car exhaust, motor oil and wear from car tires, brake pads and engine parts collectively contribute a large proportion of the heavy metal contaminants typically found in urban waterways. Reduce your personal use of cars by sharing rides, using public transport or walking or riding a bike whenever possible.
Photo courtesy of A. Dickins (below), APC (above)
Culp JM, Wrona FJ and Davies RW (1986) Response of stream benthos and drift to fine sediment deposition versus transport. Canadian Journal of Zoology 64, 1345-1351.
Serena M and Pettigrove V (2005) Relationship of sediment toxicants and water quality to the distribution of platypus populations in urban streams. Journal of the North American Benthological Society 24, 679-689.