Seasonal responses of macroinvertebrate assemblages to magnesium in a seasonally flowing stream.

Macroinvertebrates can be highly sensitive to elevated salinity in freshwater environments, and are known to respond to saline discharges. Magnesium (Mg) is a mine-related contaminant and is a potential environmental risk to a seasonally-flowing, receiving water stream in Kakadu National Park, located in the wet-dry tropics of Australia. The macroinvertebrate assemblage in the stream in the was characterised at four hydrographic phases, from early wet season flow to early dry season pools at flow cessation. On each of the four occasions representing the respective phases, individuals from the most abundant macroinvertebrate species present were collected and acutely exposed to a range (up to 19) of Mg concentrations under laboratory conditions. Sensitivity of taxa to Mg ranged between 39 mg/L Mg (Caenidae: Tasmanocoenis spp.) and 4400 mg/L Mg (Dytiscidae: Clypeodytes feryi), based on the 50% Lethal Concentration (LC50). Characterisation of the macroinvertebrate assemblage at each hydrographic phase indicated the seasons when Mg-sensitive species were present. Whilst no statistical differences in measures of seasonal sensitivity were found, the macroinvertebrate assemblages present during the early flow period had higher Mg-sensitivity than the assemblages present during other hydrographic phases. This could be attributed to the greater relative proportions of Mg-sensitive taxa (e.g. Ephemeroptera) present at early flow compared to greater relative proportions of more Mg-tolerant taxa (C. feryi and Hydacarina spp.) present during later hydrograph phases, especially periods of lower, or no, flow.