Diatom–water quality thresholds in South Australian streams indicate a need for more stringent water quality guidelines

Abstract

Water quality guidelines are an important tool that guide stream protection. In South Australia, as in other Australian states, guidelines have been set using statistical properties of physical and chemical stressors in reference streams. Given that water quality guidelines are designed to protect ecosystems, biologically based guidelines are more desirable. In this context, we investigated diatom–water quality relationships in South Australian streams. Our analysis focused on electrical conductivity (EC) and total phosphorus (TP), which explained significant variance in diatom assemblages. Threshold indicator taxa analysis was conducted to determine thresholds of diatom community change along EC and TP gradients. There were significant declines in the relative abundance of sensitive species at an EC of ~280μScm–1 and a TP concentration of 30μgL–1. The TP threshold is considerably lower than the trigger value in South Australia’s guidelines (100μgTPL–1). The change in species composition in relation to EC is considerably lower than the upper limit of the water quality guidelines (which range from 100 to 5000μScm–1). Hence, particularly in the case of TP, but also for EC, the current water quality guideline trigger values are too high in South Australia, and indeed in other temperate Australian states.