Physicochemical stream response to diurnal, seasonal, and synoptic drivers on sub-Antarctic Marion Island

The hyper-oceanic climate of sub-Antarctic Marion Island renders its freshwater systems highly sensitive to meteorological drivers. The physicochemical dynamics of stream water were determined by analysing high-frequency (5-min) data collected over a 2-year period (May 2015 – April 2017) in the perennial Soft Plume River, which drains the largest catchment on the island. Diurnal, seasonal, and synoptic-scale variations in stream temperature, pH, dissolved oxygen (DO), and specific conductivity (EC) were examined in relation to rainfall and air-surface temperature. Distinct diurnal patterns were evident, with stream temperature, pH, and DO reaching their highest levels around midday, while EC peaked at midnight. Seasonally, summer conditions produced higher stream temperatures, pH, and EC, but lower DO concentrations compared to winter, reflecting the influence of elevated air temperatures and increased evaporation during the warmer months. Four significant synoptic weather events were identified, each characterised by distinct physicochemical responses in the river. In general, increased precipitation produced a dilution effect on solutes, resulting in reduced pH and EC. DO consistently exhibited a strong negative correlation with stream temperature, which itself showed a delayed peak following maximum rainfall. An exception occurred after an extended dry period, when the first rainfall event triggered a sharp rise in EC. Such responses were associated with specific synoptic patterns that are reportedly becoming more frequent under current climate change. Overall, the findings indicate that the physicochemical characteristics of Marion Island’s streams are strongly influenced by both short-term weather events and long-term climatic shifts, particularly rising mean annual temperatures and declining precipitation.