Catchment to the coast

Australian coastal ecosystems are particularly vulnerable to eutrophication and sedimentation as they evolved under very low nutrient and sediment regimes and are widely dominated by nutrient sensitive corals in the north, and seagrass in the south. Estuaries and coastal lagoons whose upper river catchments have been cleared for intensive agriculture and whose lower reaches are subject to major urban and industrial developments are at particular risk.

Zann (1995), p. 55

Although much of the Western Australian and South Australian coasts have virtually no run-off to the sea, coastal catchments discharge to the sea at least seasonally around the south-eastern, north-eastern and northern coasts of the continent (see chapter 2). The impact in the coastal zone of pollutants within this surface and subsurface flow, gathered through natural drainage networks, farm drains, and stormwater drains from rural and peri-urban catchments, is the subject of this section. Within these catchments, land-use changes alter total discharge, flow regime, sediment and nutrient discharge, salinity and trace element concentration in catchment waters.

The gathering of pollutants across catchment lands is referred to as diffuse source pollution. Within Australia this pollution load is often unknown because of limited monitoring. Smith (1998, p. 75) commented: 'Notwithstanding the almost universal warnings of declining quality from studies (mainly funded by government agencies) that cover every aspect of water quality, monitoring of the differing forms of agricultural pollution is little short of a national disgrace.'

Pollution from specific sources, such as a sewage treatment works or a quarry, is known as 'point source pollution'. The amount of pollutants from such sources are often known, although the monitoring of impacts remains sporadic to rare.

The extent to which pollution from catchments affects estuarine and nearshore waters depends on the volume of the receiving waters, seasonality, and flushing time, as well as the resilience of the biota. Localised impact may often be readily apparent after a large flow, while more widespread slow change may be much harder to detect.

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