Identification of processes regulating the colour and colour change in an oligotrophic,hardwater, groundwater‐fed lake,Blue Lake,Mount Gambier,South Australia

Blue Lake, located in Mount Gambier, South Australia, is the only known global example of a lake that annually changes colour in the blue end of the spectrum. A series of experiments, including Secchi depth readings, in situ spectroradiometric analysis and spectrophotometric analysis of water samples, were designed to identify the factors affecting the colour of Blue Lake and the processes that regulate these factors. Based on the results of these experiments, the processes controlling the colour and colour change in Blue Lake are described and quantified. Blue Lake colour was observed to change between AS 2700 Aqua during winter (April through to October) and Blue Bell during summer (December through to February). The colour change in Blue Lake is measurable and reflects primarily the regulation of the concentration of humic substances in the upper part of the lake by calcite precipitation. The calcite precipitation is driven by degassing of carbon dioxide from the epilimnion of the stratified lake in summer. The humic substances co‐precipitate with the calcite. Photodegradation of the humic substances may also occur.