South-east Queensland has a globally unique concentration of sand dunes lakes. These are situated on the largest sand islands in the world including K’gari and Minjerribah (North Stradbroke Island), and the mainland Cooloola sand mass. The largest lake population is the World Heritage-listed lakes on K’gari (colonial name: Fraser Island); these lakes are highly significant to the Butchulla Traditional Owners. Despite their importance, little is known about the lakes on K’gari. This study focuses on water quality characteristics and their drivers. Data from K’gari are complemented by long-term research from three Minjerribah lakes. This work is part of a broader program to understand lake vulnerability to present and future climate.
The relationship to groundwater and climate variability have a fundamental influence on lake water quality on Minjerribah. In particular, those lakes that are “perched” above the regional water table are generally lower in pH and are more deeply coloured. In perched lakes reduced rainfall results in reduced colour, higher pH and lower nutrient concentrations likely as a result of reduced delivery of organic material. During periods of higher rainfall, this situation is reversed. By contrast, at times of high rainfall, Blue Lake (a “window” lake that intersects the regional water table) has lower nutrient concentrations, lower colour and higher pH. These results indicate that a mixed population of perched and window lakes may experience fundamentally different responses to future climate drying and, or extreme rainfall events.
This observation highlights a critical need to understand whether these processes (observed in a small number of Minjerribah lakes) are also a feature of lakes on K’gari. To underpin this, we assessed the connection between lakes and the regional water table by comparing the isotopic composition and water quality of lakes to local rainfall and regional groundwaters and identifed window and perched lakes.
K'gari water quality since late 2023, and earlier research, suggests that the patterns observed on Minjerribah are seen on K’gari, and will facilitate identification of their vulnerability to climate change and tourism. A notable exception is Deepwater Lake, which has recently experienced a local fish extirpation. We identified Deepwater Lake as a window lake, yet it has some of the lowest pH recorded on K’gari (< 4 pH). It appears that this is due to acid sulphate soils being exposed around the lake edge as a result of climate driven reduced lake levels which in turn has driven lake acidification.