The effect of water and sediment quality on platypus numbers has been studied in creeks near Melbourne by Serena and Pettigrove (2005). Three factors were found to have a significant negative relationship with platypus occurrence, including (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.
Nutrient enrichment
A negative impact of dissolved phosphorus on platypus numbers makes sense: excessive nutrients are often associated with low levels of dissolved oxygen, causing many of the aquatic invertebrates eaten by platypus to decline or disappear. Increased algal growth may also hamper platypus foraging by physically impeding prey detection and capture. The maximum average total phosphorus concentration associated with healthy platypus populations (defined by one or more animals consistently being captured at every survey site) was found to be approximately 0.06 mg/litre, with few or no females detected at sites where total average phosphorus concentration exceeded approximately 0.09 mg/litre.
Suspended sediment
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).
Toxic metals
Exposure to high levels of heavy metals can harm and even be lethal to freshwater fish and invertebrates (e.g. Calamari et al. 1980; Hatakeyama 1989; Clements et al. 2000; Kiser et al. 2010; Liess et al. 2017), and has been linked to reduced macroinvertebrate diversity and abundance in Australian streams (Norris et al. 1982). 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?
Nutrient enrichment
- 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.
Suspended sediment
- 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.
Toxic metals
- 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.
Addressing the bigger picture
Water quality often declines during drought, as cease-to-flow events cause dissolved oxygen to drop and toxins to become more concentrated. More generally, water contamination from point sources – including sewage treatment plants, industrial releases and irrigation returns – is expected to have a greater impact on waterway health when flow is low or nonexistent. Ironically, high rainfall and flooding can also contribute to poor water quality by triggering channel erosion, promoting diffuse runoff from normally dry land and causing sewage overflows (especially at sewage treatment plants that have been designed to deal with stormwater runoff from urban areas). Careful planning and adequate investment by management agencies is needed, along with a fit-for-purpose regulatory structure, to deal with these eventualities and ensure that water quality does not decline progressively with human population growth and/or climate change. For a detailed discussion about how water quantity and water quality interact in various Australian contexts, click here.
Photo courtesy of A. Dickins (below), APC (above)
LITERATURE CITED
Calamari D, Marchetti R and Vailati G (1980) Influence of water hardness on cadmium toxicity to Salmo gairdneri Rich. Water Research 14, 1421-1426.
Clements WH, Carlisle DM, Lazorchak JM and Johnson PC (2000) Heavy metals structure benthic communities in Colorado mountain streams. Ecological Applications 10, 626-638.
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.
Hatakeyama S (1989) Effect of copper and zinc on the growth and emergence of Epeorus latifolium (Ephemeroptera) in an indoor model stream. Hydrobiologia 174, 17-27.
Kiser T, Hansen J and Kennedy B (2010) Impacts and parthways of mine contaminants to bull trout (Salvelinus confluentus) in an Idaho watershed. Archives of Environmental Contamination and Toxicology 59, 301-311.
Liess M, Gerner NV and Kefford BJ (2017) Metal toxicity affects predatory stream invertebrates less than other functional feeding groups. Environmental Pollution 227, 505-512.
Norris RH, Lake PS and Swain R (1982) Ecological effects of mine effluents on the South Esk River, northeastern Tasmania. III. Benthic macroinvertebrates. Australian Journal of Marine and Freshwater Research 33, 789-809.
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.