Linking Hydrological Pathways and Organic Matter Dynamics to Water Colour in Temperate Forested Catchments

Abstract

Water colour, derived from chromophoric dissolved organic matter (cDOM), is a key indicator of water quality. Increasing water colour trends worldwide and in Australia have raised concerns, yet the drivers of colour variability in natural ecosystems remain poorly understood. We aimed to investigate the drivers of spatial and temporal variability of water colour generation across well-drained temperate forested catchments in south-eastern Australia and tested its use as a proxy for dissolved organic carbon (DOC). Baseflow colour was measured across a diverse topographical range to model spatial variability, while automatic samplers captured temporal variability during rainfall events. Our spatial model explained 87% of the variability in colour, with vegetation type, aridity index, and maximum annual temperature as significant predictors. Highly productive sites located in higher-elevation, higher-rainfall areas may produce high colour year-round due to favourable moisture conditions and high litter production supporting microbial decomposition. Lower-productivity sites may exhibit low colour during low-rainfall periods due to limited water availability for microbial decomposition but may exhibit high colour during rainy seasons due to accumulated litter decomposing with increased moisture. Hysteresis analysis indicated the majority of anticlockwise loops, suggesting that colour comes from distant sources through sub-surface flow paths, while the discharge-colour relationship showed an asymptotic relationship, indicating a finite pool of soluble organic compounds available for leaching. Overall, low colour concentrations can be expected during drought periods, followed by considerably high colour concentrations with subsequent rains. Future research should focus on modelling colour production in these catchments under future predicted climate change scenarios.

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