Investigation of a rare occurrence of a diatomaceous coating of the Cotswold Weir wall on the Condamine River, Australia

Abstract

Abstract 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 the substance indicated health risks from the water. 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 Nitzschia palea (predominant), Eolimna subminiscula, Craticula aff. cuspidata, Navicula viridula var. rostellata, and Luticola mutica. The centric diatom species Melosira varians was also present as filamentous chains 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 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 as measured by electrical conductivity and higher sodium and chloride concentrations, total nitrogen and total phosphorus with wide atomic ratios for total nitrogen:total phosphorus, nitrate-nitrogen:orthophosphate-phosphorus and dissolved silica–silicon:orthophosphate-phosphorus that 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.