Investigation of a Rare Occurrence of a Diatomaceous Coating of the Cotswold Weir Wall on the Condamine River, Australia.

A white encrustation of the Cotswold Weir wall in the lower reaches of the Condamine River, a tributary of the Murray-Darling River system in semi-arid Australia, was investigated following community concern that it indicated health risks from an unknown substance in the water resulting from mining and agricultural enterprises in the catchment. The vitreous white surface consisted of closely packed frustules of diatoms, observed by scanning electron microscopy, with an underlying layer of clay particles and dried filamentous green algae. Pennate diatoms identified in the white encrustation were the benthic species (predominant), , aff. , var. , and . The centric diatom species was also present as filamentous chains of cylindrical frustules among the aggregated pennate diatom frustules. The encrustation was the remains of a periphyton (biofilm) of diatoms and green algae that had developed during protracted stream flow over the weir wall following record flooding. A dry period had resulted in the death of the diatoms and exposure of their aggregated siliceous frustules as a vitreous white coating. All diatom species identified are considered tolerant of eutrophic and mildly saline conditions. Chemical analyses of water from the Cotswold Weir, compared to long-term records, revealed higher salinity, with changes from March when the river was flowing to September when the white coating was noted, in electrical conductivity (299 to 461 µS/cm), and in sodium (26 to 43 mg/L) and chloride (26 to 75 mg/L) concentrations, respectively. Total nitrogen (0.82 to 1.6 mg/L) and total phosphorus (0.24 to 0.094 mg/L) were at mesotrophic and eutrophic concentrations, respectively, together with substantial dissolved silica concentrations (18 to 11 mg/L). Atomic ratios for total nitrogen/total phosphorus (7.6 to 37.6), nitrate-nitrogen/orthophosphate-phosphorus (2.3 to 274), and dissolved silica-silicon:orthophosphate-phosphorus (81.7 to 749) probably favoured diatoms over other photoautotrophs. While the diatomaceous encrustation indicated no health risks from the weir water, continued watch is required to avoid eutrophication and salinization of the river.