Saline mine water discharges from mines are a common global water quality issue. Typically, saline constituents of mine waters are Na2+, Cl-, K+, Ca2+, Mg2+, HCO3- and SO42-, but the relative composition of these can differ between mines. For Ranger Uranium Mine in tropical northern Australia, the saline constituents of concern are Mg2+ and SO42-. To determine the effects of these constituents on ecological communities present in the creeks adjacent to Ranger Mine, a mesocosm experiment was undertaken. The experiment consisted of 25 mesocosms (2700 L tubs), placed in the creek bed during the recessional flow and dry season periods of 2002. Tubs were filled with sand, leaf litter and water from the adjacent creek. The sand and leaf litter provided habitat for colonising fauna and also seeded the tubs with faunal and floral communities. The tubs were left for four weeks to allow for aquatic communities to establish. Diatoms, zooplankton and macroinvertebrate communities were then sampled (pre-treatment) and the tubs were randomly assigned and spiked with one of four MgSO4 treatments (2.5 to 68 mg/L Mg) and a control. After MgSO4 addition, the same communities were sampled again after four and eight weeks. Phytoplankton and zooplankton communities responded to MgSO4 exposure, but no community-level response was detected for macroinvertebrates or diatoms. Phytoplankton response, as measured by chlorophyll a, decreased by 10% at concentrations of 2 mg/L Mg and 5 mg/L Mg at four and eight weeks, respectively. Changes to the zooplankton community were measured by modelling 95% extirpation of individual species in a SSD, resulting in a 99% species protection value of 5 mg/L Mg. Results from this study will be used as a supporting line of evidence for a site-specific magnesium standard for water bodies surrounding Ranger Mine.