Differing effects of catchment land use on water chemistry explain contrasting behaviour of a diatom index in tropical northern and temperate southern Australia

The DSIAR biotic index for freshwater diatoms, regarded as a potential indicator of impact from agricultural and urban land use on rivers in temperate south-eastern Australia, did not correlate significantly with an index of catchment condition in a tropical region of northern Australia. However, the relationships between the index and water chemistry, especially pH, salinity and concentrations of nitrogen and phosphorus, were consistent in both regions. The variable relationship between the index and catchment conditions can be explained by differing effects of catchment land use on stream-water chemistry in northern and southern Australia. In the south, land use has commonly resulted in increases in stream pH, salinity and nutrients, whereas in the north its impact on pH and salinity appears weak. These findings emphasise the need to interpret biological and ecological indices in the context of the varying causal pathways by which human activities affect stream ecosystems in different circumstances.